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Catalytic abatement of CO, HCs and soot emissions over spinel-based catalysts from diesel engines: An overview
Journal Pre-proof Catalytic abatement of CO, HCs and soot emissions over spinel-based catalysts from diesel engines: An overview Neha, Ram Prasad, Satya Vir Singh
PII:
S2213-3437(19)30750-X
DOI:
https://doi.org/10.1016/j.jece.2019.103627
Reference:
JECE 103627
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
22 September 2019
Revised Date:
22 November 2019
Accepted Date:
19 December 2019
Please cite this article as: Neha, Prasad R, Vir Singh S, Catalytic abatement of CO, HCs and soot emissions over spinel-based catalysts from diesel engines: An overview, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2019.103627
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Catalytic abatement of CO, HCs and soot emissions over spinel-based catalysts from diesel engines: An overview
Nehaa*, Ram Prasada and Satya Vir Singha
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Department of Chemical Engineering &Technology, Indian Institute of Technology (BHU),
Varanasi 221005, India
Corresponding author email: [email protected] ; Mobile. +91-7860991830
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*
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Abstract
The incomplete combustion from diesel-fuel engines generates airborne species such as CO,
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HCs, NOx and soot of health concern necessitate the need of understanding of work done in the area of emission control systems. Among noble metal free catalysts, spinel oxides have
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been found to be highly efficient for CO, HCs and soot oxidation. Here, we review the main fundamental understanding of the correlations between the catalytic activity of spinel structures and their chemical composition, and morphology. In addition, the key factors and essential strategies to improve their activity and stability are discussed in detail. Future
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research pathways and perspectives on the progress of the spinel formulations are provided with
a focus on high performance and cost-effective spinel formulation having good
potential of replacing commercially noble metals in use. It is envisioned that further investigations should focus on high potential multicomponent spinel oxide solid solutions,
accelerated deactivation tests to examine applied application, and integrating conductive matrixes to avoid structural lapses under practical conditions. Keywords: Emission control system; Spinel preparation; Morphology; Support; Calcination; Deactivation. 1. Introduction
The diesel engines are gaining more importance as compared to gasoline engines
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owing to its better fuel economy [1], less maintenance, high durability [2] and favourable fuel tax compliance [3] regime in some countries including India. Therefore, diesel engines are used in various fields like transportation, agriculture,
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construction, forestry, marine, mining, defence, power generation, and so on. Broad applications of diesel engines imply that the economics of a country is very much
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depended on them. With the growing economy, an exponential increase in the use of diesel-fuelled vehicles is emerging. Despite various advantages of diesel engines, there
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is serious emissions concern of primary pollutants such as carbon monoxide (CO), unburned hydrocarbons (HCs), particulate matter (PM) or soot, nitrogen oxides (NOx)
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and sulfur dioxide (SO2) from diesel engines. These emissions have a negative effect on the health as well as environment. Moreover, these primary pollutants react with atmospheric constituents of gases in the presence of sunlight to form more toxic
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pollutants like (PAN), (PAH), smog, acid rain, ground-level ozone. Alarmed due to odd effects of pollutants, various governments around the globe
have made strict legislations to regulate these emissions. Therefore, to adhere to these regulations, the research on various techniques such as engine modifications, fuel treatment, fuel modification, fuel alternatives, better tuning of the combustion process and
tailpipe
exhaust
treatment
is
ongoing.
In
fuel
treatment,
catalytic
hydrodesulphurization (HDS) is the most widely used commercial technology to remove the sulphur present in the fuel [4][5]. Among better fuel alternatives, Dimethyl ether (DME) synthesized from syngas came out as a potential alternative to the conventional diesel fuel used in the diesel engines owing to its high cetane number, low NOx emission, and nearly zero exhaust generation [6]. In tailpipe exhaust treatment, catalysis indeed plays a crucial role by oxidizing combustible pollutants (CO, HC, and Soot) and reducing NOx [7]. Besides exhaust treatment, other methods
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of COx minimization includes In general, two separate catalytic systems for the oxidation of pollutants and reduction of NOx are used. Various kinds of catalysts have been investigated for CO,
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HC and soot oxidation. Among noble metal free catalysts, substantial literature is available on catalytic oxidation of soot, CO, and HC on spinel catalysts. Despite being
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an important step for practical environmental applications, till date, very few papers
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are available on simultaneous catalytic oxidation of pollutants [8][9][10]. There is only single experimental paper [17] available in literature on simultaneous catalytic oxidation of CO, HC, and soot. Moreover, there is no review paper present in the
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literature on simultaneous catalytic oxidation of concerned pollutants. Besides having an applied interest, some researches have also reported that the simultaneous removal of pollutants causes synergistic effect, and so the overall removal efficiency of
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pollutants gets improved [11][12]. Singh and co-workers [12] have found that the CO as a co-feed gas enhances the activity of NiCo2O4 catalyst for HCs oxidation (LPG in this case). The explanation given behind this effect was that the oxidation of CO elevates the local catalyst bed temperature, which facilitates the activation of HC combustion reaction. A recent work by Genty et al. [11] also observed that the enhancement in the performance of spinel cobalt based catalysts during the
simultaneous oxidation of CO and toluene. The oxidation temperature for toluene gets lowered in the presence of CO as a reactant. The conclusion obtained based on hypothesis and observation is that this improvement is originated from the formation of easily oxidizable by-products in the presence of CO. The first by-product formed when the CO is present as the reactant is benzaldehyde, whereas, in absence CO, the first product formed is benzylic alcohol. The benzaldehyde is more easily oxidizable than benzylic alcohol.
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The present emission control system and the prospects of single [DPF] configuration instead of present [DOC+DPF] in use are discussed. Following this, the evaluation of catalytic CO, HC and soot oxidation over spinel based catalysts is made by discussing
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the main factors affecting its catalytic activity. The work done by various researchers on improving the intrinsic activity of the spinel catalysts through tuning its chemical
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composition is thoroughly examined. Further, the work done in the area of improving
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morphological characteristics of the spinel is discussed with focus on the routes leading to novel spinel formulations. The work reporting spinel formulations having
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applied interest is focused in the present manuscript.
2. Emission control systems
In order to abide by the stringent regulations on diesel emissions, many R&D activities
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are carried out scientifically or commercially to reduce the emissions from diesel-fuel engines completely. The general technological approaches to control emissions are divided into pre-treatment techniques and post-treatment techniques. In pre-treatment techniques: electronic fuel injection (EFI) [13], engine modification [14], precise fuel metering, better fuel-air mixing, emulsified fuel [15], enhancement of fuel properties by using fuel additives [16], fully computerized fuel management etc. are used for
controlling the emissions. In case of post-treatment techniques, also known as exhaust gas after treatment, the emissions like CO, HCs, NOx, and soot underwent catalytic oxidation and reduction processes inside the tailpipe before releasing the exhaust into the atmosphere. In this section, emission control systems such as DOC and DPF under post-treatment techniques are discussed for diesel engines specifically. 2.1 Diesel oxidation Catalyst (DOC) DOC (Fig. 1) is an after-treatment device, which reduces the emissions of the
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soluble organic fraction (SOF) of the PM, CO and HC effectively. Though, it does not oxidize soot fraction of the PM. As it is a flow-through device, it does not cause any back-pressure on the engine, thereby, doesn’t affect the engine efficiency [26]. DOCs
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are generally used in combination with Selective catalytic reduction (SCR) to abate NOx by oxidizing NO into NO2 and increase the NO2: NOx ratio. The standard
(1)
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CO + 1/2𝑂2 → 𝐶𝑂2
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reactions that take place in DOCs are as follows [17]:
𝐶3 𝐻6 + 9/2𝑂2 → 3𝐶𝑂2 + 3𝐻2 𝑂
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𝑁𝑂 + 1/2𝑂2 ↔ 𝑁𝑂2
(2)
(3)
CO and HC are oxidized to give end non-toxic end products CO2 and H2O [Equations
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(1) and (2)]. As diesel engine always operates under lean-burn conditions, their exhausts usually contain 2 to 17% by volume O2 unreacted. This unreacted O2 is steadily consumed in DOC. Another reaction that takes place is oxidation of NO to form NO2 as represented by Equation 3. The high concentration of NO2 in the NOx increases the DPF efficiency. In the untreated tailpipe emission, the concentration of
NO2 in the NOx is only about 10% at most operating points. With the function of the DOC, NO2: NO rate is increased by inducing thermodynamic equilibrium. Moreover, the heat released during the oxidation of HCs and CO elevates the temperature of exhaust-gas in the downstream of DOC. This increased exhaust temperature helps in the DPF regeneration. In DOC, the exhaust gas temperature increases nearly by 90⁰ C for every 1% vol. of oxidation of CO. This rapid increase in temperature results in a steep temperature gradient in DOC [18].
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DOC is generally a monolith honeycomb carrier structure made of metal or ceramic. Besides its carrier structure, it consists of an oxide mixture (washcoat) composed of alumina (Al2O3), cerium oxide (CeO2), zirconium oxide (ZrO2), and
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active catalytic components such as Pt, Pd and Rh. The washcoat facilitates large surface area to for better dispersion of the catalyst and slow down its sintering at
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elevated temperatures.
Fig. 1 Schematic diagram of the diesel oxidation catalyst (DOC).
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Since its first introduction in the 1970s, DOCs continues to be a fundamental technology for diesel engines until nowadays. All-new diesel engines of passenger cars, LD as well as HD diesel vehicles are now fitted with DOCs. The estimated reductions in emission from DOC are to be over 90% for CO and more than 70-80% for total HC over a wide range of operations [19]. Therefore, DOCs are widely used
for heavy-duty vehicles (HDVs) as well as light-duty vehicles (LDVs) in various countries. Pt and Pd containing oxidation catalysts are the most common catalysts in the world market. One of the main disadvantages associated with the use of these noble metal-based catalysts is that they oxidize the SO2 to SO3, which consequently react with water and produces forms of sulfates and sulfuric acid. This causes negative effects like reducing the efficiency of after-treatment emission control system and harmful biological impact [20] .
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2.2 Diesel Particulate Filter DPF is used to trap particulates present in the exhaust gas by the physical filtration and usually made of either cordierite (2MgO-2Al2O3-5SiO2) or silicon carbide (SiC)
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honeycomb structure monolith with the channels blocked at alternate ends. DPF is an extruded usually cylindrical structure, usually made, with a large number of small
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parallel channels placed in the longitudinal direction of the exhaust system. The
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adjacent channels in the wall-flow filters are alternatively plugged at each end, thus forcing the soot particles to flow through the porous substrate walls, which act as the
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filter medium [21] (Fig. 2). As the soot particles pass through these walls, they get deposited there. The optimum porosity of the filter walls enables the exhaust gases to pass through their walls without considerable hindrance while being effectively impermeable to soot particles. As the filter becomes increasingly saturated with soot, a
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layer of soot is observed on the surface of channel walls. This formed layer of soot facilitates highly efficient surface filtration for the operating phase, leading to a trapping efficiency of more than 95% [22]. However, excessive saturation causes the clogging of filter, which severely affects the engine output, increases consumption of fuel [22] and causes stress in the filter [23] in the DPF. Sivaram et al. [24] reported the reduction in engine efficiency and an increased amount of emissions during severe
clogging condition of DPF. Thus, to overcome these adverse effects, the DPF has to regenerate periodically by oxidizing the soot trapped in the DPF.
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Fig. 2. The functioning of typical DPF.
Active and passive regeneration [25] are the two principle regeneration techniques employed to oxidize the PM and decrease the undesired clogging. Active regeneration
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can be intermittently applied to DPFs in which captured soot is removed by controlled combustion with oxygen [26]. In an active regeneration of DPF, heat is given from
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external sources such as fuel burner or electrical heater to raise the filter’s inlet
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temperature to the 550⁰ C or higher [22]. The combustion of soot, trapped in the filter, occurs once the particulate loading in the filter touches set limit (around 45%) shown
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by the pressure drop across the DPF. This technique suffers from the disadvantage of additional energy costs, complex control system, and the creation of thermal stress for the filter element. Therefore, this method is not preferred, generally. In case of active regeneration, the combustion of PMs took place at the exhaust gas
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temperature by catalytic oxidation promoted by depositing suitable catalysts such as Pt at the site of combustion itself. Unlike in active regeneration, no additional fuel is needed to carry out the reaction process over the catalyzed DPF. Under a temperature range of 200 and 450⁰ C, small amounts of NO2 facilitates the continuous combustion of the trapped soot particles. This forms the basis of the continuously regenerating trap
(CRT) which uses NO2 continuously to oxidize soot within relatively low temperatures over a DPF. The DOC upstream of DPF increases the ratio of NO 2 to NO in the exhaust and lowers the PM oxidation temperature. NO 2 coming out from DOC serves as better oxidant than O2 and so gives better passive regeneration efficiency [27]. Besides the expensive nature of noble metal-based catalysts, their activities are found to be strongly inhibited in the presence CO as co-feed [28] which limits its
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potential for simultaneous oxidation of CO, HC and soot. In order to overcome these drawbacks, alternative catalytic systems having the significant potential for catalyzing the full oxidation of CO, HC and soot must be developed. Moreover, such catalytic
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systems should be able to perform appreciable CO, HC and soot oxidation independent of the concentration of NO2 emission from DOC. Therefore, the
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replacement of the above mentioned [Pt-DOC+DPF] emission removal device by a
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single [noble metal free catalyst-DPF] configuration seems promising, but it needs be examined first whether catalysts made from alternative materials are able to oxidize soot, HCs, and CO simultaneously, as Pt catalyst does, or not, or if further
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enhancements are needed. For achieving this configuration, we need to evaluate the work done in area of CO, HCs and soot oxidation over the alternative catalytic systems. The focus on the methodologies leading to highly active and stable catalytic
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formulations can provide direction in carrying out the work in the simultaneous oxidation of the concerned pollutants. 3. Comparison of spinels and other potential catalysts As mentioned above, noble metal-based catalysts have been reported to be highly successful in the abatement of the concerned pollutants, but suffer from certain disadvantages. Besides being expensive, the compounds of noble metals have negative
impacts on health as well as environment. They also suffer from the problems of sintering and sublimation at high temperatures [29]. The deactivation of noble catalysts also takes place in case of VOCs and soot because of sulfur and chlorine poisoning [30]. Hence, driven by the necessity to find alternatives transition metal oxide (TMO) based catalysts went under investigation in recent years. Spinels form an important class of TMOs with appealing catalytic activity for oxidation reactions owing to their desirable activity, widespread availability, low cost,
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easy synthesis, thermodynamic stability, and environmental friendliness. In the spinel structure, the coexistence of tetrahedral and octahedral sites provides multiple sites to accommodate different transition-metal cations with a wide range of valence states to
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form a large number of oxides. Spinel oxides have been found to be highly efficient for CO, HCs as well as soot oxidation. One interesting factor in favour of
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simultaneous oxidation over spinel oxides is that unlike noble metal catalysts, cobalt-
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based spinel catalysts are reported in the literature for maintaining high activity for HC oxidation even in the presence of CO as reactant [31]. It is noted that Layered double hydroxides (LDHs) also know like hydrotalcites
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demonstrated to be promising supports and catalyst precursors for heterogeneous catalysis owing to their wide variety of features, such as a large number of hydroxyl groups, tunable surface basicity and acidity as well as high adsorption capacity for the
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immobilization of active species. Prominent hydrotalcite such as Mg-Al hydrotalcitesupported Pd [32], Hydrotalcite-derived Co-Mn-Al mixed oxide catalyst [33], and Kpromoted CoMgAlO [34] have found to be promising catalyst for CO, VOCs (toluene here), and soot oxidation respectively. However, the requirement of very strict experimental conditions during hydrotalcite synthesis in order to avoid chemical segregations and improve homogeneities is very demanding. In general, there still
exist some technological-economic problems in LDHs preparation which limit its wide applicability [35]. Another class of TMOs, perovskite oxides (ABO3) has received significant attention as alternative catalysts for oxidation reactions owing to their abundant tunability of composition and structure, thermal stability, and excellent catalytic activity. Perovskite oxides containing transition metals such as Mn and Co at B sites have high oxidation ability toward CO [36], VOCs [37], and diesel soot [38]. The catalytic properties of
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perovskite oxides can be controlled by substitution of A site cations by other kinds of metals. In the case of LaMnO3 and LaCoO3 perovskite oxides, for instance, the substitution of La3+ ions by Sr2+ ions gives rise to the increase in the oxidation state of
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B site cations, such as Co3+ to Co4+ and Mn3+ to Mn4+, which improves the oxidizing ability of the oxides [36]. The synthesis temperature of perovskite oxides is relatively
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high (600-900⁰ C). Moreover, it has relatively low specific surface areas which limit
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its catalytic activity. In general, the catalytic activity of perovskites for the oxidation reactions is not as good as those of spinels. However, given its high thermal stability, the prospects of using it as a support for the highly active species such as copper are
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being explored recently, where it has been found to highly effective support [39]. Various mixed metal oxides (CeO2:ZnO [40] and Co-MgO [41]) other than spinel have also been reported to be effective for CO, HCs and soot oxidation. Interestingly,
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in simultaneous oxidation, CeZrNd mixed oxide catalyst was able to accelerate simultaneously soot, propylene and benzene oxidation. Though it was able to accelerate CO oxidation in a certain extent, but there was a net production of CO during soot combustion because the oxidation capacity of these oxides was not high enough to oxidize all CO released as soot combustion product [42]. In mixed metal oxides, the combination of two or more metal oxides produces a complex system with
multiple functions stemming from each oxide, often resulting in desired catalytic properties. 4. Factors affecting the spinel catalytic activity for CO, HCs and soot oxidation 4.1. Tuning the spinel composition The single spinel oxide such as Co3O4 has attracted huge research enthusiasm due to its low cost, environment-friendly nature and high catalytic activity [43]. Intrinsically, the spinel structure of Co3O4, sometimes expressed as CoCo2O4, is that
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the Co2+ and Co3+ cations occupy the Td and Oh sites, respectively. Both theoretical predictions, as well as experimental results, have shown that the Co3+ cations on the Oh sites are the reactive ones for many oxidation reactions (e.g. CO oxidation, VOCs
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combustion, water oxidation and, soot combustion) whereas the Co 2+ cations on the Td sites are almost inactive [44][45]. Yi et al. [46] attained excellent catalytic activity for
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cations on the exposed planes.
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CO using Co3O4 nanowires, attributed to the formation of abundant reactive Co 3+
Many researches in this field have [47] demonstrated that the intrinsic activity of
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single oxide spinels such as Co3O4, can be further enhanced by substituting Co2+ cations on the reactive Td sites through other transition metal cations (e.g. Mn2+, Ni2+, Zn2+ and Cu2+), forming spinel mixed metal oxides or spinel bi-metal oxides. Moreover, the pure single oxide spinels such as Co3O4 have disadvantages like weak
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thermal stability at elevated temperatures and water vapor inhibition at low temperatures. The deactivation of CO3O4 powder is reported in the presence of water vapor, the conversion of CO to CO2 significantly gets decreased, and the formation of carbonates is facilitated [48]. This cation substitution enhances the catalytic activity due to the formation of highly oxidized redox couples resulting in synergism between
cations A and B [49]. This increase in catalytic performance is due to synergism between different ions is invariably reported for all the oxidation reactions of our interests, such as in CO oxidation, HCs oxidation, and soot combustion. In the case of CO oxidation, Kouotou et al. [50] have reported the promoting influence of the substitution of the Fe by Co in Fe3O4 spinel oxide. Co-Fe mixed oxides exhibited high catalytic activity than the single α-Fe3O4 spinel oxide. Trivedi et al. [51] examined the promoting effect of Ni substitution in Co3O4 spinel oxide. The
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total oxidation in case of cobalt oxide was observed at 160⁰ C whereas, for nickel cobaltite spinel it was observed at 150⁰ C. It was also observed that the NiCo2O4 spinel lattice maintained its catalytic performance for a long run while the activity of
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its individual metal oxides (NiO and Co3O4) lost rapidly because of sintering and carbonaceous deposition.
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In HCs oxidation, chi et al. [52] boosted the total oxidation of VOCs over cobalt
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oxide spinel by cation-substituting effect and reported that the hollow mesosphere structures (HMS) of mixed spinel cobaltites (CuCo2O4 and NiCo2O4) performs better than its single oxide (Co3O4) performs better than its single oxide Co3O4 HMS. Similar
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observations were made by Wenfeng and co-workers [53] for soot oxidation where the CuCr2O4 activity was reported to be better than its single oxides CuO, Cr 2O3 and their mechanical mixture.
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The degree of synergism between the two cations (A and B) is also dependent on the extent of the interactions between the cations A and B. The nature of A cations also influences the catalytic activity of the formed spinel structure. This extent of interaction was observed to be dependent on the nature of the constituent cations as well as the coordination state of cations. Different metal cations on A site has different influence on the same metal cation B. Hence, various researches have examined the
influence of cation A on the parent spinel lattice. Trivedi et al. [47] studied the activity over various spinel cobaltites and reported the catalytic performance order for CO oxidation as follows: NiCo2O4>Co3O4>MnCo2O4>CuCo2O4>FeCo2O4. The best catalytic activity of NiCo2O4 spinel catalyst was attributed to its very small size crystallites formation and better interaction between Ni and Co cations. In VOCs oxidation, Hosseini et al. [54] prepared the series of various nanocrystalline manganite spinels (A = Co, Ni, Cu) among which the NiMn 2O4 spinel
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was found as the best catalyst for the removal of 2-propanol and toluene. This high activity is because of the prominent synergistic interaction between Mn and Ni on the oxide, having a high concentration of active Ni2+ and Mn3+ sites. Jiang and co-workers
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[55] also found that 3-D ordered mesoporous spinel catalysts exhibit catalytic activities in benzene total oxidation depending on the concentration of surface-active
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oxygen species in the catalysts. The different cobaltites exhibit a different relative
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amount of surface oxygen species, therefore, exhibit different catalytic activity. Chi et al. [52] observed that the acetone complete oxidation order as follows: CuCo 2O4 (260⁰ C) > NiCo2O4 (220⁰ C) > CoCo2O4 (200⁰ C). This reason behind this behaviour
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is the cation-substituting effect, which resulted in the different concentrations of surface Co3+ cations, active oxygen species, defective sites and reducible capabilities in the synthesized catalysts.
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In soot combustion, Zhang and co-workers [56] have reported that the effective interactions between Zn and Co cations led to the high surface area, high amount of surface-active Co3+ and more chemisorbed oxygen species, thereby enhanced catalyst performance. Shangguan et al. [57] have also observed that the catalytic performance exhibited by the catalysts is exclusively dependent on their constituent metal content, where CuFe2O4 was reported to be the most efficient catalyst among the various
spinels (ferrites) tested for soot combustion. Interestingly, the coordination state of metal cations, the substitution of metal cation (Cu here) in the octahedral site or in the tetrahedral plays very little role in determining its catalytic performance. In soot combustion, Zhang et al. [56] have reported that that the substitution of A site of CO3O4 spinel compound with other cations (Co, Ni, Cu, Zn) caused the change of Co3+/(Co2+ + Co3+) ratios as well as the oxygen mobility in the spinel structure. Both Co2+ and Co3+ species were found on the surface of the catalyst, and Co3+ species got
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easily reduced to lower states, thus improving the catalyst activity. Among these spinels, ZnCO2O4 exhibited the best performance owing to its good oxygen mobility and the presence of high concentration of surface-active Co3+ species in zinc cobaltite
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spinel lattice. Besides the intrinsic ability of the prepared catalysts, the use of oxidants such as NOx and O2 plays a critical role in soot combustion. Zawadzki et al. [58] have
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studied the role of both NOx and O2 in soot combustion over CoAl2O4 spinel and
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attributed the high soot oxidation activity of the catalyst to its high NO x chemisorption capacity, what allows fast NO oxidation to NO2. As NO2 is much more oxidising than NO and O2, it helps in the fast combustion of soot. However, high NO2 production
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capacity is not enough to ensure effective soot oxidation, and the temperature where NO2 is yielded must be high enough for NO2 to oxidise soot. If NO2 is generated below a certain temperature, the oxidising gas will not able to oxidise the soot surface.
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When x in the non-stoichiometric spinels of AXB3-XO4 is varied, different catalytic, electrical or magnetic properties can be realized, thereby improving the intrinsic activity of the catalyst. The non-stoichiometric spinel contributes to much more crystal defects and oxygen vacancies that form active sites for reactant molecules during the reactions and substantially improve the catalytic activity. Working towards the sole objective of improving the intrinsic activity of spinel, various researches have
prepared the bi-metal or mixed metal spinel-type oxides in varying atomic molar ratios of cations A and B in to examine the effect of metal cations molar ratio on activity. Faure et al. [59] have examined the activity of Co-Mn mixed oxides in the varied molar ratios of Co to Mn for CO as well as HC (propane here) oxidation. The mixed oxide with a composition as Co2.3Mn0.7O4 exhibited the best performance owing to its exceptionally high surface area. Similarly, Junhua et al. [60] also observed that CoMn mixed oxide catalysts with a Co/Mn atomic ratio of 5:1 shows better performance for
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CO oxidation among other CoMn catalyst with varied atomic ratio. The high intrinsic activity in the case of Co/Mn (5:1) having was attributed to the prevalence of crystal defects which help in creating more number of octahedrally coordinated reactive
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divalent cobalt cations. Although, as mentioned in the above observations, the composition of a catalyst has a strong positive effect on the activity, this effect of the
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composition may or may not be varied for different oxidation processes. Eric and co-
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workers [16] have reported that the aluminium cobaltite, with a molar ratio of Co2+/Al3+ of 6/2, contributes in CO as well as toluene oxidation. Whereas, Ivanov et al. [61] have reported that the CO oxidation rate rapidly decreases with increasing of
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Cr content in the active component whereas, for the dimethyl ether (DME), the reverse trend was reported. It was concluded that the best compromise is the spinels with Cu/(Mn + Cr) molar ratio 1:5 and Mn/Cr molar ratio from 1:3 to 1:4. Based on the
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results from XPS analysis, the increased catalytic activities could be due to the synergistic effect of the cation distribution ratio in the O d/Td sites and the oxygen vacancies on the surface. Based on above various observations made by various researches, it can be concluded that the intermediate compositions are the main active one and the phenomena of synergism are most prominent in the case of intermediate compositions.
Optimization of composition of spinel in use will be the key factor in carrying out the simultaneous oxidation of CO, HC, and soot effectively. 4.2. Doping in the spinel compositions Doping is a well-established factor in enhancing the intrinsic activities of the catalysts. The replacement of an appropriate fraction of the ions in a host spinel metal oxide lattice with other metal ions can enhance its activity. The doping cause distortions in spinel structure, which change the concentration of oxygen vacancies
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and the active sites thus form catalysts with novel morphologies and enhanced catalytic properties. In general, metal ions with variable oxidation states have been considered as active modifiers that can tune the chemical bond or surface state of
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metal oxide catalysts, thus enhancing the catalytic oxidation activity. For example, vanadium (V), variable valence metal, is suitable for doping Co2+ in the Co3O4 lattice
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as the V2+ cation has an appropriate ionic radius for doping similar to the Co2+ cation
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(V: Co = 0.063 nm: 0.065 nm). The doping of spinel oxides with alkali metals ( K +, Na+, Rb+, Cs+), alkaline earth metals (Mg2+), platinum group metals (Ag+, Pt+), or
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lanthanide oxides (CeO2, Sm3+) is also reported in the literature. Among alkali metal effect, Min and Zheng [62] studied the influence of K and Al over NiCo2O4 on its physio-chemical characteristics and VOCs removal efficiency. Aldoped NiCo2O4 exhibited poor physio-chemical characteristics, thereby, exhibits poor
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catalytic performance for VOCs oxidation. The reverse trend was observed in K doped NiCo2O4. The K doping was found to be significantly efficient in promoting NiCo 2O4 spinel catalyst in terms of reducibility and textural characteristics. In catalytic soot oxidation, the alkali metal itself can take part in the oxidative process through the formation of carbon-oxygen-metal complexes (C-O-M, with M alkali metal) as the active sites and further increases the soot oxidation rate. Legutko et al. [63] have
studied the positive influence of K+ on the activity of transition metal (Mn, Fe, Co) spinels in case of soot oxidation. The results revealed that K+ doping could significantly enhance the catalyst activity in soot combustion; though, the influence strongly depends in each case on the doping concentration and on its site (bulk or surface). On the contrary, alkali metals show poisoning effect in certain reaction systems such as in CO oxidation. The alkali metals (Na, K, and Li) were promoted onto a cobalt oxide nanocatalyst to examine their poisoning influence on CO as well as
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VOC (propane) oxidation. About 1% alkali metal dopant onto cobalt oxide was reported to raise the light-off temperature by 50°C for the oxidation of CO and over 160⁰ C for the oxidation of propane. The reason given behind this poisoning effect is
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the observation of ‘locking-effect’ on the oxygen of Co3O4 by alkali metals. This leads to weak oxygen mobility of alkali metal promoted catalysts. Another reported reason
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for the alkali metals poisoning effect on Co3O4 catalysts for CO and HC oxidation is
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that the alkali metals considerably facilitate the adsorption of CO 2 to form surface carbonate species even at elevated temperature [64]. Based on the aforementioned reported results, it is concluded that the alkali metal as a dopant is undesirable when it
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comes to simultaneous catalytic oxidation of CO, HCs, and soot over spinels. Lanthanide oxides such as ceria are widely reported effective catalyst dopants. Wide literature is available on using CeO2 to modify cobaltite based spinel catalysts,
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or using spinel cobaltites to modify CeO2-based catalysts [65]. The high oxygen storage capacity (OSC) of cerium makes it well known doped material in catalytic exhaust systems. The strong oxidative property of spinel based metal oxides together with the OSC of ceria makes ceria-doped spinel based metal oxides, group of low-cost and highly active spinel doped catalysts for oxidation reactions. Xiaowei and coworker [66] have reported that the substitution of in CoCr2O4 spinel lattice by ceria in
the appropriate concentration results in improved activity for the simultaneous abatement of NO and soot. The doping of Ce leads to the deformations in the CoCr 2O4 spinel lattice which results in crystal defects and an increase in the oxygen vacancy concentration, which favours oxygen mobility and the redox behaviour of the cerium modified cobalt chromite spinel. Dey et al. [65] have also examined the effect of CeOx addition on the activity of the CuMnO4 spinel for CO oxidation. It was reported that the CuMnOx promoted with (1.5 wt.%) CeOx enhances the activity of spinel for CO
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oxidation owing to the formation of new surface sites upon doping. Another lanthanide, Samarium (Sm3+) is also studied by Xu and co-workers [67] to examine its promotional effect on Co3O4 spinel lattice for Co oxidation and reported that the
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sample having Co/Sm molar ratio of 0.90/0.10 showed the best activity. The addition of an appropriate concentration of Sm resulted in the formation of spinel Co 3O4 and
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amorphous SmCoO3, hence increasing the number of reactive Co3+ and the active
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surface oxygen species, which was responsible for the enhancement in the catalytic activity.
In noble metal doping, Dey et al. [65] examined the influence of doping on the
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activity of CuMnO4 spinel for the oxidation of CO. It was reported that the CuMnO x promoted with (1.5 wt. %) CeOx, (1.0 wt. %) AgOx, and (0.5 wt. %) AuOx enhances the activity of spinel for CO oxidation owing to the formation of new surface sites
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upon doping. In soot oxidation, Huangrong et al. [68] have observed that the substitution of Ag+ in manganese cobaltite in various molar ratios and observed that the soot oxidation temperature decreases with increase in the concentration of Ag. The Ag phase besides the spinel phase of manganese cobaltite was observed by XRD on increasing the substitution amount of Ag above 0.1. The Mn 0.6Ag0.4Co2O4 catalyst
showed efficient catalytic performance for soot oxidation, as a result of the synergism between chemisorbed oxygen species (O2-, O-), and metallic Ag. 4.3. Controlling the morphology It is to be noted that other than tuning the chemical composition of the spinel as reported above, the catalytic activity of the spinels can also be modified by making various morphological and structural changes in the spinel. The improvement in the morphology of the catalytic structures through various ways can lead the way to the
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development of highly active catalysts. Various spinels with unique structural formations have been prepared and examined in the heterogeneous catalysis, and their structure-activity
relationship
has
been
readily
through
many
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characterization and experimental results.
confirmed
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4.3.1. Effect of synthesis approaches on spinel morphology
Indeed, various methods have been developed to synthesize spinel oxides, such as,
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amongst others, solid-state synthesis (reactive grinding), and wet chemical routes: solgel, coprecipitation, wet impregnantion (WI), alginate route, reverse micelles,
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hydrothermal/solvothermal synthesis, template synthesis, and others (Table 1). The choice of the synthesis technique is generally driven by the stability of the specific spinel composition required and by the need of obtaining desired physio-chemical characteristics for the final catalysts.
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In wet chemistry approach, the preparation of the spinel particles is controlled by a combination of various parameters such as pH, temperature, reaction time and calcination conditions. Thus, the primary objective of the preparation is an optimization of these dependent parameters to obtain oxide catalysts with desired morphological and structural characteristics. Sajad and co-workers [69] have studied
the effect of various preparation parameters such as pH, ageing time, autoclave and calcination temperature on the physio-chemical properties of the nanocrystalline CuCr2O4 spinel catalysts prepared by via surfactant-assisted hydrothermal route. The results showed that the catalysts prepared under optimized parameters (pH =12, 4h ageing period, 180⁰ C autoclave temperature and calcination at 500⁰ C) showed relatively high surface area (38.5 m2g-1) and almost pure spinel phase. Both of these characteristics favoured the CO conversion efficiency. Further, various calcination
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parameters such as temperature, duration and atmosphere can also plays very important role in the synthesis of effective spinel oxides. The optimizations of the calcination parameters can results into favorable spinel oxide morphology, thereby,
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improving its catalytic activity. Guilin et al. [70] have studied the effect of calcination temperature on catalytic activity of the prepared CoMn/AC spinel oxide catalysts at
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different calcination temperatures. The order of activity as per the toluene conversion
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efficiency was reported as follows: CAT250
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characterization results such as XPS, TPR, and SEM. The low calcination temperature (250⁰ C) found to be not favourable to the decomposition of the nitrate precursor to form active species as well as to the activation of the supported active species. Though
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high temperature calcination (400⁰ C) is also not favorable as led to the growth of the grains that decrease the dispersion and increase the sintering of catalyst as can be seen in SEM micrographs (Fig. 3a). Abolfazl and co-workers [71] have also studied the effect of calcination temperature on the CO oxidation activity of iron-cobalt spinelbased bi-metal oxides. It was reported that the calcination performed at high temperature (>400⁰ C) results in the increase of the crystallite size and decrease of the
specific surface area, which gives a negative effect on the catalytic activity (Fig. 3b). Fresh catalysts are readily calcined at different temperatures to induce favourable morphological changes in them. Other than calcination temperature, the calcination process performed in the different atmosphere (oxidative, reductive, inert, and reactive) can also modify the morphology of the catalyst. Spinel cobalt oxide pretreated in a non-reducing conditions mainly in N2 and CO/air, at moderate temperatures could attain complete CO oxidation at an exceptionally low temperature
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of -80⁰ C [72]. Another study performed on spinel cobalt oxide also observed that the calcination of cobalt oxide in inert conditions (in He) at 350⁰ C led to its surface reconstruction which led the formation of weakly bound oxygen species. These
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weakly formed species facilitates the CO oxidation (Fig. 4a) [73]. Abolfazl and coworkers [71] examined the influence of pretreatment conditions on the Co-Mn spinel
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based bi-metal and reported that the catalyst pretreated under oxidative conditions
as inert conditions (Fig. 4c).
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exhibited the better catalytic performance than those pretreated under reductive as well
In general, the prevalence of surface or crystal defects on the catalyst surface acts as
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active sites for the oxidation reaction to occur. Such crystal defect-rich nanocrystalline structures could be obtained by following the exotic calcination or thermal treatment routes in a controllable manner. Such an exotic route is followed by Paldey and co-
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workers, where the fresh spinel catalyst is heated in a reducing atmosphere followed by controlled oxidation. Such controlled decomposition environment led to the formation of fine clusters or domains of around 5 nm size on the particle’s surface with abundant lattice defects. The abundance of crystal defects results into the fast CO oxidation. Interestingly, a similar kind of route is also followed by Trivedi and Prasad [49], where the calcination of a basic hydroxycarbonate precursor takes place in the
presence of a chemically reactive-CO-air mixture. The calcination performed in such environment is known as reactive calcination (RC). The simultaneous occurrence of multifarious phenomenon of oxidation-decomposition-redox-surface-reactions during this process produced a synergistic effect which helps in the creation of oxygen deficient active phases in the catalyst. The morphological characteristics attained by catalyst at low temperature found to remain intact at high temperature. Pratichi et al. [12] compared the catalytic activity and morphological characteristics of the spinel
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nickel cobaltite prepared under reactive calcination route with catalysts prepared using conventional routes. It was observed that the spinel prepared using RC route exhibit much better activity (for CO and HCs) and morphology as compare to spinel calcined
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in stagnant and flowing air atmosphere (Fig. 4b). The remarkable activity of the reactively calcined catalyst for CO-HC mixture oxidation is because of the formation
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of partially reduced NiCo2O4−δ oxygen-deficient structure exhibiting exceptionally
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high surface area, very small crystallites, and highly dispersed homogeneous morphology. The spinel synthesis by following microwave assisted hydrothermal and combustion synthesis routes can also led to the formation of nanocrystalline defect-
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rich structures. The short reaction time in such routes favours the incomplete crystallinity and the surface roughness. The abundance of crystal defects and oxygen vacancies is seen on such rough surface, which acts as favourable sites for oxidation
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reaction to occur. The general understanding says that the interface of two mixed phases can be a very active site for reaction to occur owing to the prevalence of defects at the interfaces. To understand the interfacial structure of the CuO/MnOx mixed
phase,
layered
copper
manganese
oxide
(LCMO)
with
a
bridged
monoclinic/tetragonal phase structure was synthesized via one-step hydrothermal redox-precipitation approach by Wang and co-workers [74]. It was revealed that the
formation of the monoclinic-tetragonal phase interface with abundant defects inhibited the growth of nanoparticles which helped in keeping crystal size small and surface area high. Characterization results revealed that the interfacial structure of mixed phases induced the formation of the Cu2+-O2-Mn4+ entity, leading to the abundance of surface oxygen species and oxygen vacancies, thus improving the catalytic activity. In advanced preparation techniques, the main focus is to improve the morphology of the structures by offering more number of reactive sites for reactant adsorption and
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increased reactant-catalyst contact frequency. This can be seen in the case of mesoporous spinels with large surface-to-volume ratios, fast diffusion, and uniformity in porosity. Therefore, various approaches have been developed for the synthesis of
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mesoporous spinels. Liu et al. [75] have successfully synthesized mesoporous Co3O4 through a facile soft reactive grinding route at room temperature using the
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polyethylene glycols as an assisted agent. The obtained catalyst exhibited very high
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catalytic activity for CO oxidation (T50 = -101⁰ C) under a stream of normal feed gas containing moisture. This high activity is attributed to the exceptionally high surface area (124 m2 g-1) of mesoporous cobalt oxide. Zhang and co-workers [52] worked one
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step further and they integrated the inner cavity with mesoporous metal oxide shells of various dimensions into one unique nanostructure having very high surface area and good reactant-catalyst contact frequency. Carreon et al. [76] have reported the
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synthesis of mesoporous nanocrystalline cobalt nickel oxide spinels by self-assembly of heterometallic alkoxides in the presence of supramolecular liquid crystalline phases. The formed mesoporous nickel cobalt oxides showed better propane conversion to CO and CO2 as compared to a conventional dense phase formed spinel oxides. However, the hydrolysis and condensation of transition-metal alkoxides are not easy to control precisely in this wet chemistry approach. The resultant metal oxides attain poor
structural ordering and weak thermal stability after removal of the surfactant templates. As an alternative, the use of hard-templates like SBA-15 in nanocasting method effectively overcomes the above-mentioned problem of weak structural stability faced by wet-chemistry approach. The hard templates like SBA-15 with topologically robust frameworks resist the local strain caused during the precursor’s crystallization process, thereby, providing better characteristics (e.g. mechanical and thermal stability) desired for practical applications. Moreover, multi-component metal
homogeneity and stoichiometry of the resultant replicas.
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oxides prepared by this route as solution-based precursors could effectively control the
Over the years various researchers have studied the effect of various preparation
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methods on the spinel morphology and catalytic activity. Trivedi and Prasad [47] studied the effect of various preparation methods on spinel nickel cobaltite and
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reported that the catalyst synthesized via co-precipitation (CP) exhibited the best
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activity for the CO oxidation in comparison to other preparation methods. The order reported as per catalysts performance was as follows: CP>Reactive grinding (RG) of nitrates>RG of oxides>SG. Maria et al. [77] have also reported that the CP method got
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the most active oxide for VOC mixture oxidation due to the improved mobility of oxygen and ability to favour redox processes in the material structure. Wenfeng and co-workers [78] compared the conventional acetate process with the citric acid-aided
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process and reported that the oxides synthesized by the latter were more active as they form monophasic CuCr2O4 spinel at lower temperatures. Seyed and co-workers [54] observed that the copper magnetite catalysts prepared via SGC (Fig. 5a) showed higher SBET, higher wt.% of the normal-type spinel phase and higher amount of reactive Mn3+cations than the same catalyst prepared by conventional co-precipitation. Qian and co-workers [79] have reported that the lattice distortion induced by
prolonged citrate-precursor grinding in case of SRG helps in creating and keeping a high density of surface defects, which leads to good activity and stability of the cobaltbased spinel catalyst. The synthesis techniques of spinels largely differ by their economics, their energy requirement and their environmental impact. With the rational design of composition, structures, defects, and morphology, desired crystal defect-rich nanocrystalline catalysts can be obtained in a controllable manner. Classical methods such as co-
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precipitation can be hampered by the need of operating in the high-pH field in which all the concerned cations are out of their solubility domain, with consequent rejection of alkaline wastewaters. The techniques facilitating the precise control of particle size
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by confinement of the precursors, as the emulsion or template methods, involve the use of relatively costly organic additives. Same drawback is seen in techniques such as
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the sol-gel, Pechini or alginate methods, in which the homogeneous dispersion of the
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precursor cations in a matrix favours the formation of spinels with compositions difficult to form by co-precipitation. Some energy-intensive methods, like flux growth, results in low surface area catalysts which is undesirable characteristic. Table 1
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summarize the primary methods reported to fabricate spinel compounds along with their overviews, characteristics and drawbacks.
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Table1Summary of the methods reported in literature for preparing spinels
Methods Overview Sol-gel (SG) chelating agents, sol [47] solution formation, gel formation after losing solvent, calcination Hydrotherma l [74] [80]
hydrothermal high-pressure
Characteristics mild and large scale, nanoparticles (common), composition regulable, easily scable reactor, small crystallites, high SBET, no high temperature
Drawbacks time consuming, not efficient for multicomponent compositions, large waste relatively high cost of equipment and the inability to monitor
calcination
the growing crystals in the autoclave
transfer gaseous reactant high-purity, thin film to solid thin film
Microwave [58]
microwave energy fast and (molecular friction); temperature microwave-assisted below 100⁰ C)
Thermal decompositio n [59]
oxalate decomposition
Reactive grinding
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low not scalable, costly (even
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Solution combustion synthesis (SCS) [85]
complex reactions need to be well controlled to obtain the desired product and precursors are limited to thermally stable materials for pyrolysis High substrate temperature, no codeposition, precursors not existing for every Element
metal very low only suitable for VOC temperature oxidation; high (∼200⁰ C), cost operating temperature in exhaust require effective and easy high temperature calcination transfer solid-solid to relatively low low surface area not solid-liquid reaction temperature and short favourable for using reaction time, high catalysis, energymolten salts purity intensive process calcination of xerogel formation from alginate solution decomposition a material (nitrate precursors) in the presence of a reducing agent (organic fuel as glycine or urea) grinding of stoichiometric amounts
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Alginate route [84]
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Chemical vapour deposition (CVD)[82][8 3]
Flux grown
moisture sensitive crystallites, multistep processing, delicate temperature control, large waste
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Precipitation[ controlled by various compositions 49] precipitants, pH, and morphologies, solvents and calcination spinel hybrids, rich parameters oxygen deficiencies, cost-effective, easily scalable Spray aerosol reaction from fast, nanostructures, pyrolysis[81] liquid reactants porous, low crystalline, better control over morphology, low cost
low cost natural biopolymer precursor; easy catalyst synthesis high surface area and high-purity foamy material, predominantly single phase
Cu-Mn-O system for toluene oxidation evolution of hazardous gases, yield loss, safety issues due to exothermic reaction
high surface area metallic impurities simpler, waste-free, from the machinery
of precursors planetary mill
in easily scalable, energy efficient, costeffective
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synthesis [75]
Fig. 3. The effect of calcination temperature (a) SEM images of samples calcined at different
-p
temperature [70]; (b) The activity of Fe-Co mixed oxide catalysts calcined at different temperatures, reaction condition: 2%CO, 20%O2and 10%N2 balanced with He, calcined in
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oxidative environment [71].
Fig. 4. The effect of calcination atmosphere (a) The effect of pretreatment environment over cobalt oxide, reaction conditions: 0.15 g catalyst, 1.0 vol% CO in air, 50 ml min-1. In all the cases pretreatment was done at 150⁰ C for 40 min in dry air (▵ ), wet air (▴ ), 1.0 vol% CO/air (◊), N2 (○), 1.0 vol% CO/N2 (□), or 5.0 vol% H2/N2 (▿ ) [72]; (b) Effect of calcination environment on CO oxidation over nickel cobaltite in stagnant air (▵ ), flowing air (◊), or
reactive air(□) [12]; (c) Effect of calcination atmosphere over CuMn catalyst in oxidative(□), none pretreatment (○), inert, or reducing atmosphere [71]. 4.3.2. Effect of different supports The main drawback of the spinel-based catalysts is their deactivation as a result of an aggregation. The deactivation problem could be overcome by applying novel preparation methods and application of supports with high surface areas. Catalysts
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attain their proper shape, particle size, and uniform dispersion of active species because of supports, allows the prepared catalysts to achieve a larger specific surface area. Moreover, the interaction between the catalyst and the support can also greatly influence the properties of the supported catalysts. The conventional powdered form
-p
catalysts are generally dispersed onto the surface or mixed into the channel walls of
re
structured support by wash-coating, WI or extrusion. The supports generally used for this purpose are oxides such as Al2O3, SiO2, and TiO2. Loading a catalyst on such inert
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but high surface area support improves the accessibility of the active phase. Wang and co-workers [86] discussed the role of inert supports (Al2O3, SiO2, and TiO2) for CO
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oxidation and found out that the catalytic activity of supported cobalt oxide depends on the kind of the support being used. The results showed that the interaction of cobalt oxide with supports was much stronger in the case of Al2O3 and TiO2, while no conclusive evidence of any interaction was observed for SiO2. The degree of
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interaction between cobalt oxide and the support not only affected the surface area and reducibility of the catalyst, its activity toward the CO oxidation also gets affected. Besides the nature of support, the technique used in the dispersion of the active
species is also a key factor in determining the activity and the selectivity of the supported catalysts. Therefore, the maximization of the dispersion and the
achievement of proper characteristics for the active species are the main challenges. A reasonable approach towards the solution of this problem is related with the selection of an appropriate methodology of preparation. The deposition of the active species on the surface of catalytic supports, the most critical preparation step, usually takes place from electrolytic solutions. The pore volume impregnation (pvi), the wet impregnation, the deposition-precipitation and the equilibrium deposition filtration (edf), otherwise called equilibrium adsorption are the widely reported techniques.
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Ataloglou et al. [87] studied influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the ‘cobalt oxide’/γ-alumina catalysts for complete oxidation of benzene.
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Three series of catalysts of varying Co content (up to 21 wt.% Co) were prepared using three preparation methods: pvi, edf and pore volume impregnation adding
re
nitrilotriacetic acid (nta) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta–
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pvi ≫ pvi ≫ edf, nta–pvi ≫ edf ≈ pvi and edf > nta–pvi > pvi. However, the interaction achieved after impregnation between the positively
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charged alumina carrier and the cobalt cations using electrolytic solutions is not strong enough. This weak interaction results in the formation of undesirable large crystallite sizes, thereby, the low surface area. Therefore, various efforts have been made in the
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last few decades by several researchers to prepare highly porous alumina with high surface area by techniques such as hard templating (via carbon template), sol-gel based assembly, and microwave irradiation in the presence of surfactants. Such techniques have been used to prepare well-ordered mesoporous alumina materials with high surface area and a narrow pore size distribution. However, the process found to be very time consuming and not suitable for scaling in industrial applications.
In general, ordered mesoporous materials are promising catalyst supports due to their favourable structural characteristics as high surface area, good mechanical stability, easy modification, and uniform porous channels with controllable pore diameters and pore lengths. The presence of surface hydroxyl groups offers the opportunity to support metals and metal oxides with catalytic activity. Moreover, due to the large pores, they provide less diffusion limitation for the reactants and products. These characteristics facilitate the formation of uniformly dispersed and stable metal
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particles. In mesoporous supports, SBA-15, KIT-6, MCM-41, and MCM-48 are generally used. SBA-15 has attracted much interest as it possesses a regular hexagonal array of pores with uniform diameter, high surface area, and high pore volume. It is
-p
also inert and stable at elevated temperature and exhibit good mechanical stability. Moreover, its surface can be easily functionalized. Bordoloi et al. [88] applied
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evaporation-induced self-assembly (EISA) process to synthesize mesoporous mixed
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oxides, which consist of cobalt ions highly dispersed in an alumina matrix. The obtained mixed cobalt-aluminum oxides were studied for CO oxidation and reported to be highly active. The EISA technique can be considered liquid crystal template
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strategy. In this process, a homogeneous precursor solution is dispersed into fine droplets and subsequently dried and calcined, which results in the formation of mesostructured materials.
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The use of supports other than traditional alumina is also being reported. Nguyen et al. [89] compared the use of active CeO2-ZrO2 support with other popular nonactive but high surface area and cheap supports as Al2O3, SiO2. It was reported that the cobalt oxide supported on CeO2-ZrO2 was reported to be the more active than supported on inert Al2O3 and SiO2 supports for the catalysts for the complete oxidation of propylene since Ce0.9Zr0.1O2, itself, was also a good active phase for the reaction. 30%
Co3O4/Ce0.9Zr0.1O2 catalyst was able to oxidize 100% propylene to pure CO 2 from 250⁰ C; 30% CeO2-Co3O4 catalysts on Al2O3 and SiO2 supports were able to do the same at higher temperatures (400⁰ C). However, catalysts on Ce0.9Zr0.1O2 exhibited much lower surface areas and much less thermal stability. In carbon based supports, the use of activated carbon (AC) as support over CuMn spinel based catalysts provide loose and porous structure which facilitates the adsorption and activation ability of the reactant molecules, as well as promote the
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diffusion and migration ability of the reactant and product molecules in the prepared catalyst. Meanwhile, the formation of the loose-porous structure and the accumulation of the metal oxide grains can form the alleged lattice defects, which can provide more
-p
active sites for activating reactant molecules [70]. 5. Deactivation studies
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The water poisoning problem is generally faced by spinel oxides greatly limited their
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practical application. A reduction in catalyst activity with time-on-stream is observed due to spinel deactivation. Addition of water beyond an optimum level results in a loss of activity due to sintering of catalyst [90]. Sintering is the loss the active surface of
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the catalyst due to crystal growth of either the bulk material or the active phase [91]. A wide number of characterization techniques including WDXRF, XRD, N 2 physisorption, Raman spectroscopy, XPS, H2-TPR, TEM and TGA-MS are used to
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characterise the post-run samples. The morphological and structural changes induced in the used catalyst due to thermal and chemical deactivation gets revealed in the characterizations. Wang and co-workers [74] characterized the used spinel layered copper manganese catalyst (after 2500 min toluene oxidation reaction at 220⁰ C) by XRD, XPS and TEM. The observance of no changes in the crystallization or diffraction peaks of the used catalyst in XRD pattern (Fig. 5a) reveals the stability of
the spinel phase of the catalyst. The analysis of surface compositions of used catalyst by XPS (Fig. 5b), no deposition of carbon species was found from C 1s XPS spectra. After the catalytic enduring test, the changes of Cu 2p and Mn 2p XPS spectra were negligible, which showed that the LCMO catalyst was stable and not deactivated. Interestingly, the content of surface adsorbed oxygen (Oβ) after 2500 min reaction at 220⁰ C increased to some extent compared with fresh catalyst, implying gaseous O 2 molecules are preferentially adsorbed on the oxygen vacancies of fresh catalyst under
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an oxygen-rich atmosphere, which was beneficial to the complete oxidation of CO and VOCs and the better stability of catalyst. Moreover, as seen in TEM images (Fig. 5ce), no morphological change is seen when we compare TEM images of used catalyst
-p
with fresh catalyst. This shows that the textural characteristics of the catalyst remain intact indicating the stable nature of the catalyst. In compliance with favorable
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to H2O, SO2, and CO2 deactivation.
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characterization results, this spinel based LCMO catalyst exhibited excellent resistance
ro of -p re
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Fig. 5. Characterizations of LCMO catalysts (a) XRD patterns of used LCMO; (b) XPS of used LCMO; (c-d) TEM images of the used LCMO catalyst; (e-f) TEM images of the fresh
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LCMO [74].
In order to meet future expectations favoring diesel engines, it is necessary to develop catalysts with high performances and robustness but low costs. Therefore, efforts should be addressed towards the development of novel spinels that decrease, or
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eliminate, the use of commercially used noble metals. Mobini and co-workers [69] carried out recycle experiments on CuCr2O4 catalyst prepared via surfactant assistant hydrothermal synthesis for five subsequent runs and the results showed no deactivation of the catalyst. Moreover, the stability of this catalyst is observed in the presence of water and CO2 at high temperature (400⁰ C). Though, a pilot scale study
under real exhaust conditions over such highly active catalysts is needed to examine their applicability. A very recent work by Taiki and co-workers [92] developed a very efficient Cu0.05Ni0.95Al1.8Cr0.2O4 multicomponent spinel oxide solid solution as candidates for noble metal free TWC. Even after thermal aging at 900⁰ C for 25 h, this quaternary system achieved high phase stability as well as high catalytic activity towards CO, C3H6 and NO in a wet gas stream similar to real TWC conditions. Its high activity is attributed to combination four selected metal elements showing
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different site preferences. In simultaneous oxidation, bench scale tests on a wall-flow ceramic filter catalysed with the effective nanostructured CoCr2O4 spinel obtained by in situ combustion synthesis gave promising results, as it entailed a about four fold
-p
reduction of the time required for trap regeneration as well as a much more complete regeneration compared to that of a non-catalytic trap [85]. Though, experimental test
re
campaign is required to verify this potential at a catalytic trap level on real exhaust
lP
gases (includes CO, HCs other than soot and NOx). Such bench scale tests are necessary to evaluate possible candidates for real application in as proposed single [DPF].
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Some supplementary results and conclusions obtained in significant studies regarding the performance of spinel structures for CO oxidation, HC oxidation, and soot oxidation are summarized in Table 2, 3 and 4, respectively.
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Table 3 Summary of the catalytic performances reported over various spinel catalysts for CO oxidation Catalyst synthesis
Reaction Conclusions conditions OP: Co3O4; WHSV= Alkali metal doping on spinel impregnation: Na, 120000 mL g-1 h-1 total oxidation; doping reduced K and Li doped mobility of spinel by locking Co3O4 from desorption and promoting carbonate species formation
Ref. delayed [64] oxygen oxygen surface
1 vol. % CO, 10 vol. % O2 and He for balance to 100 vol. %, GHSV: 6000 h-1
Single oxide Co/SBA-15 most active [93] (T100= 140⁰ C): presence of only reactive Co3+ species on surface; SBA-15 stable after reaction tests
PSE-CVD: Co-Fe 1% CO, 10% O2 mixed oxides in Ar; TFR: 15 ml min-1; WHSV =45000 mLg-1cat h-1. Decomposition of 0.8% CO ,20% oxalates: Co-Mn O2 in Ar. mixed oxides
Co-Fe synergism; αFe2O3
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Hydrothermal RP: layered CuMn oxide (LCMO), granulated oxide (GCMO); RP: rod oxide (RCMO) Heating of Co 1.0 vol.% foilCo3O4 CO,16.0 vol.% nanowires air, 83.0 vol.% N2, TFR:100 ml min-1 WI: Cu-Mn-Cr 0.9-1.0% CO, mixed oxides 3.0-3.2% H2O, GHSV:10000 h-1
-p
Surfactant-assisted hydrothermal: CuCrxCo2-xO4(x=0 to 2)
Low temperature spinel synthesis, Co-Mn [59] synergism; Co2.3Mn0.7O4 most active and stable: SBET; similar SEM images of oxalates before and after decomposition 2% CO, 20% O2 CuCr1Co1O4>CuCr1Co1O4>CuCr2O4>Cu [69] balanced with Co2O4>CuCr1.5Co0.5O4; CuCr1Co1O4: Ar; TFR: 100 ml high TOF, low activation energy, high min-1;4 vol.% reusability and thermal stability steam 1000 ppm CO in Cu-Mn synergism: Cu2+-O2-- Mn4+ entity [74] humid air; formation; LCMO>RCMO>GCMO; TFR:100mLmin- LCMO: most active and stable as H2O, 1 ; GHSV=50000 SO2, CO2 and other VOCs resistant, high h-1 surface O2 species and O2 vacancies Ultrathin Co3O4 nanowires (3mm): T100= [46] 120⁰ C and no deactivation (60 h) due to high SBET and abundant reactive Co3+ cations on the exposed planes (011)
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SG: SBA-15; supported catalyst: Two solvent xCoMnxSBA-15 where x= moles
UHP- MxCo3-xO4 Diffuse (M=Co, Ni, Zn) reflectance nanoarrays infrared Fourier transform (DRIFT) study
Monolithically integrated spinel [96] nanoarrays exhibit tunable activity by selective cation occupancy and concentration, which lead to controlled adsorption-desorption behaviour and
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20% O2, 2% CO balanced with Ar ;TFR: 100 ml min-1 CP: Mg1-xCux 9333 ppm CO, Al2O4 (x = 0-1.0); 1900 ppm O2, Mg0.7M0.3Al2O4 8% CO2. (M= Mn, Co, and Ni) Nanocasting:MCo 1.0 vol.% CO, 100 mL 2O4 (M = Cu, Mn TFR: and Ni) min-1; SV: 120 000 mL h-1g-1
T100 = ~220⁰ C; spinel composition affected activity; increasing Cr content: CO and CH3OH conversion increase; optimized composition: Cu/(Mn + Cr) in 1:5 molar ratio and Mn/Cr molar ratio from 1:3 to 1:4 Synthesis parameters influenced activity; optimized parameters: 12 pH, 4 h ageing duration, 180⁰ C autoclave temperature and 500⁰ C calcination temperature activity spinel composition dependent: Cu-Mg highest synergism at x=3 in Mg1xCux Al2O4; cation substituting effect: Cu > Co > Mn > Ni; Mg0.7M0.3Al2O4 most active: high reducibility Structural stability owing to mesoporous structure formation during synthesis; CuCo2O4 > MnCo2O4 >NiCo2O4; NiCo2O4 poor performance in life test
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surfactant-assisted hydrothermal: CuCr2O4
[61]
[69]
[94]
[95]
CP: Co-Sm varied Co/Sm Ratios
in 1% CO, 21% O2, 78% N2, 4% O2, 95% N2
USP:CuxMn3xO4 with varied x
CP: xCuxMn2O4
Ni1-
1 vol% CO, 20 vol% O2, 79 vol% in He; TFR: 10 ml min1 ; GHSV: 6000 ml g-1h-1 5% CO, 5% O2 in N2
[71]
[97]
[67]
[81]
[98]
[47]
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CP: MCo2O4 0.5 g catalyst (M=Co, Mn, Cu, ;1.5% CO in air ; Fe and Ni) TFR: 100 mL min-1.
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DRIFT and quadrupolar mass spectrometry (QMS) study.
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Precipitation: Co3O4 powder
surface defect population Influence of calcination temperature and pretreatment environment on structureactivity: Cat300> Cat400> Cat500> Cat600 and oxidative> none> inert>reductive Co3O4: water poisoning effect at low temperatures, no significant effect on activation with O2; Co3O4 surface interaction with O2 originate activated surface O2 species Sm doping in Co/Sm<1 increased activity and Co/Mn>1 decreased activity; doping increases reactive Co3+ and active surface O2 concentration; Co0.95Sm0.10 most active: favourable co-existence of crystalline Co3O4 and amorphous SmCoO3; water vapour deactivation Cu-Mn synergism; activity dependent on synthesis parameters and spinel composition; Cu1Mn2O4-550 most active: poor crystalline spinel phase, surface defects, vacancies and reactive Cu (II) ions Cation substituting effect; spinel composition influence activity, Ni0.5Cu0.5Mn2O4 most active: highest Ni2+ ions on octahedral sites NiCo2O4>CoCo2O4>MnCo2O4>CuCo2O4 >FeCo2O4; small crystallites and high SBET favoured conversion
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CP: Fe-Co mixed 2% CO, 20% O2, oxide 1/5 ratio 10% N2 balanced; GHSV :60,000ml g-1 h-1
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Table 4 Summary of the catalytic performances reported over various spinel catalysts for HC (VOCs) oxidation. Catalysts synthesis Reaction conditions Conclusions Ref. CP: NiCo2O4, WI: K- SV: 5000 h-1. NiCo2O4, and AlNiCo2O4
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citric acid complex: Co- 800 ppm propane, Ce oxides 10% O2 in N2; TFR: 300 ml min-1; GHSV: 150000 h-1. SG: SBA-15; Co- 2000 ppm C3H8, 1000 Mn/SBA-15: two ppm C6H14 in air; solvent GHSV: 60000 h-1 GC, SRG, CC, SG- 1vol.% C3H8, 10 Co3O4 vol.% O2, 89 vol.%
Activity decreased on Al addition and increased on K addition in NiCo2O4; activity order: K-NiCo2O4> NiCo2O4> Al-NiCo2O4, SBET of NiCo2O4 increased on K+ doping favoring activity. CeO2 < Co3O4 < Co1Ce2Ox
[62]
[52]
[93]
[99]
N2; TFR: 100 mL min-1 Solvothermal 1000 ppm acetone, alcoholysis (SA): 20%O2/N2; TFR : 155 Hollow mesospheres ml min-1; WHSV: structures (HMS) 93,000 mL g-1 h-1 MCo2O4 (M = Co, Ni, Cu)
Co3O4 performance (T100= 240⁰ C)
Co3O4: nanosheets (NS), nanowires (NW), nanoclusters (NC) and coral-like microspheres
Hierarchical Co3O4 nanostructures in-situ [100] grown on 3D nickel foam; NC > NW > NS; Co3O4 NC most active: high surface adsorbed O2 species, structural defects and reactive Co3+, reducibility; high regenerability and stability High concentration of superficial [101] electrophilic O2 species and lattice distortion induced by prolonged grinding during synthesis favored activity
1000 ppm toluene balanced with air; TFR :100 ml min-1 GHSV:20,000 h-1
C2H5OH:O2:He = 1:20:79 and C4H8O2:O2:He = 1:20:79; TFR: 100 ml min-1 1000 ppm o-xylene, Cu-Ce-Mn synergism, optimized synthesis: TFR: 200mL min-1, 11.4 wt.% loading on cordierite, calcined at GHSV: 10,000h-1 500⁰ C for 3h. MnCeCu0.4/cordedite (T100= 340⁰ C) > MnCeCu0.4/monolith (T100= 370⁰ C) Impregnation: 1.0 vol.% toluene in calcination effect: CAT250 Co3O4 > 20/80 vol. % O2 /N2; CoCr2O4; high temperature increase Co and B = Co, Cr) GHSV: 30,000 h-1; followed by fast cooling during synthesis TFR: 100 mL min-1. led to defect-rich nanocrystalline structural formation favoring activity. SGC and CP: nano 0.2 conc. % propanol NiMn2O4 > CuMn2O4 > CoMn2O4; crystalline AMn2O4 (A and toluene in full NiMn2O4: High Ni-Mn synergism and = Co, Ni, Cu) stream active Ni2+and Mn3+sites.
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Citrate method: Fe-Mn oxide with varied Fe: Mn ratio; mechanical mixture using Fritsch ball mill SG: Mn-Ce-M (M= Ni, Cu or Co) mixed oxide
Cation substituting effect: CuHMS < [52] NiHMS < CoHMS; CuCo2O4 HMS most active, stable, water resistant: high reactive Co3+ concentration, surface active O2, defective sites and reducibility; Mars-Van Krevelen (MVK) mechanism followed
[102]
[70]
[103]
[54]
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Alginate route: Cu-Mn 1000 ppm toluene in Environment friendly synthesis using [84] oxides with varied air; TFR: 100 ml min- natural polymer, Cu-Mn synergism, T100: 1 Cu/Mn ratios Cu1.5Mn1.5O4 (315⁰ C) > Mn3O4 (325⁰ C) > CuO (535⁰ C). Modified SA: hollow 1000 ppm acetone, T100: CuCo2O4(260⁰ C) > NiCo2O4 (220⁰ C) [52] mesoporous spherical 20% O2 / N2 and > CoCo2O4 (200⁰ C) ; Cation-substituting structures (HMS) WHSV= 93,000 mL effect; HMS: stable and water resistant MCo2O4 (M = Co, Ni, g-1 h-1 owing to stable spinel phase and robust Cu) structure
Table 5 Summary of the catalytic performances reported over various spinel catalysts for soot oxidation
Conclusions
K+ doping improves activity; K+ doping bulk more [63] effective than K+ surface doping; bulk doping T100= ~250⁰ C. K doping to CuFe2O4 promoted activity; No [104] promotional effect with the Li, Na, Cs, V and Pt; K and Cu-Fe spinel synergism at K (x= 0.05) concentration negligible role of cation coordination state in [57] activity, cation substituting effect; CuFe2O4 most active: highest selective N2 formation, lowest selectivity to N2O and intermediate Tig of soot; high SBET; primitive reaction mechanism followed
C/S ratio:80/20, 5%Cu/ZnAl2O4> ZnAl2O4> Zn0.95Cu0.05Al2O4; [105] Loose contact, 500 5%Cu/ ZnAl2O4 most active: high reducibility of ppm NOx, 5% O2, Cu catalyses the NO oxidation to NO2, high N2 balanced; TFR: selectivity towards CO2 150 ml min-1. Ag doping enhances activity; [68] MnCo2O4
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Catalyst/soot ratio: 10:1; Loose contact; 0.2% NO, 5% O2 with He.
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glycothermal followed by substitution and impregnation ZnAl2O4, Zn0.95Cu0.05Al2O4 and 5% Cu/ZnAl2 O4. SG: Mn1-x AgxCo2O4 (x= 0, 0.1, 0.2, 0.3, 0.4)
Zn>Cu>Co > Ni, ZnCo2O4 most active: high O2 [56] mobility and surface-active Co3+; activity inhibited in SO2 and steam
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Citric acid SG: ACo2O4 Catalyst (C)/soot (A = Co, Ni, Cu, Zn) (S) wt. ratio 95:5 and 66:33, loose contact; 10 vol.% O2 , TFR:300 mL min-1 WI: K+ doping, solid 5 % O2 in He; 3.3 state reaction: K+- % O2, 0.3 % NO spinel (Mn, Fe, Co) in He; TFR: 60 ml min-1 Citric acid aided (CIT)- 0.5% NO, 5% O2 in He CuFe2O4, ImpregnationCu1xAxFe2O4 (A: Li, Na, K, Cs), V and Pt CIT: ACr2O4 (A = Cu, C/S =0.33 g; 10% Mg, Co, Mn); O2 in He and 1% CoMn2O4 and AFe2O4 NO in He (A = Cu, Co, Ni)
Ref.
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Reaction conditions
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Catalysis synthesis
Without 3DOM MCr2O4<3DOM MCr2O4; [106] 3DOMMn <3DOM-Co<3DOM-Zn<3DOM-Ni, and BK-NiMnCr2O4>CoFe2O4; CoCr2O4: fine [85] crystals (<20 nm) and foamy structural formation of its agglomerates maximized catalyst-soot contact frequency microwave assisted C/S= 90/10; tight CoAl2O4>ZnAl2O4; CoAl2O4: high NOx [58] glycolthermal: contact; NOx +O2 chemisorption capacity facilitate NO oxidation which helps soot conversion CoAl2O4, ZnAl2O4 stream and WI: Pt/Al2O3
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Colloidal crystal template (CCT) and SG: MCr2O4 (M = Co, Ni, Zn, Mn) with 3DOM and without 3DOM SCS: AB2O4 (where A = Co and Mn, and B = Cr and Fe)
5.0 vol% O2, 10 vol% H2O, 2500 ppm NO, N2 balanced ; TFR :80 ml min-1, GHSV:6760 h-1 1000 ppm NO, 10 vol% O2 , balance He; tight contact
[66] SCS: CexCo1-xCr2O4 C/S: 1/9; loose Ce0.1Co0.9Cr2O4>Ce0.05Co0.95Cr2O4 (x= 0, 0.05, 0.1, 0.15) contact; 1000 ppm >Ce0.15Co0.85Cr2O4>CoCr2O4; Ce0.1Co0.9Cr2O4 (T90 of NO, 10 vol% = 446⁰ C); Substitution of Ce: favourable
O2 with N2; TFR: distortions in spinel structure and increased crystal 50 ml min-1. defects. 6. Conclusions This review validates that spinel catalysts have the ability to perform well in the oxidation of all the three pollutants (CO, HC and soot) of concern. A number of spinels including those based on cobalt, nickel, nickel, zinc, iron and several other mixed-metal oxide spinels have been designed for the catalytic oxidation of the concerned pollutants. It is observed that the appropriate
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compositions of cations and their positive synergism for oxidation reaction are desirable factors for the formulation of the most active catalysts. Besides composition, improvement in the morphological characteristics of the spinel could also enhance the spinel activity. Future development is expected to focus on multicomponent spinel oxide formulations having
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compositions tuned in such a way each metal element of the formulation can afford high
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oxidation activity. A positive synergism could be expected between the pollutants in such cases, thereby increasing the overall removal efficiency. The achievement of this synergism could be
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perfect research niche for developing noble metal free TWC.
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Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal
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relationships that could have appeared to influence the work reported in this paper.
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PII:
S2213-3437(19)30750-X
DOI:
https://doi.org/10.1016/j.jece.2019.103627
Reference:
JECE 103627
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
22 September 2019
Revised Date:
22 November 2019
Accepted Date:
19 December 2019
Please cite this article as: Neha, Prasad R, Vir Singh S, Catalytic abatement of CO, HCs and soot emissions over spinel-based catalysts from diesel engines: An overview, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2019.103627
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Catalytic abatement of CO, HCs and soot emissions over spinel-based catalysts from diesel engines: An overview
Nehaa*, Ram Prasada and Satya Vir Singha
a
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Department of Chemical Engineering &Technology, Indian Institute of Technology (BHU),
Varanasi 221005, India
Corresponding author email: [email protected] ; Mobile. +91-7860991830
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*
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Abstract
The incomplete combustion from diesel-fuel engines generates airborne species such as CO,
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HCs, NOx and soot of health concern necessitate the need of understanding of work done in the area of emission control systems. Among noble metal free catalysts, spinel oxides have
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been found to be highly efficient for CO, HCs and soot oxidation. Here, we review the main fundamental understanding of the correlations between the catalytic activity of spinel structures and their chemical composition, and morphology. In addition, the key factors and essential strategies to improve their activity and stability are discussed in detail. Future
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research pathways and perspectives on the progress of the spinel formulations are provided with
a focus on high performance and cost-effective spinel formulation having good
potential of replacing commercially noble metals in use. It is envisioned that further investigations should focus on high potential multicomponent spinel oxide solid solutions,
accelerated deactivation tests to examine applied application, and integrating conductive matrixes to avoid structural lapses under practical conditions. Keywords: Emission control system; Spinel preparation; Morphology; Support; Calcination; Deactivation. 1. Introduction
The diesel engines are gaining more importance as compared to gasoline engines
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owing to its better fuel economy [1], less maintenance, high durability [2] and favourable fuel tax compliance [3] regime in some countries including India. Therefore, diesel engines are used in various fields like transportation, agriculture,
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construction, forestry, marine, mining, defence, power generation, and so on. Broad applications of diesel engines imply that the economics of a country is very much
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depended on them. With the growing economy, an exponential increase in the use of diesel-fuelled vehicles is emerging. Despite various advantages of diesel engines, there
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is serious emissions concern of primary pollutants such as carbon monoxide (CO), unburned hydrocarbons (HCs), particulate matter (PM) or soot, nitrogen oxides (NOx)
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and sulfur dioxide (SO2) from diesel engines. These emissions have a negative effect on the health as well as environment. Moreover, these primary pollutants react with atmospheric constituents of gases in the presence of sunlight to form more toxic
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pollutants like (PAN), (PAH), smog, acid rain, ground-level ozone. Alarmed due to odd effects of pollutants, various governments around the globe
have made strict legislations to regulate these emissions. Therefore, to adhere to these regulations, the research on various techniques such as engine modifications, fuel treatment, fuel modification, fuel alternatives, better tuning of the combustion process and
tailpipe
exhaust
treatment
is
ongoing.
In
fuel
treatment,
catalytic
hydrodesulphurization (HDS) is the most widely used commercial technology to remove the sulphur present in the fuel [4][5]. Among better fuel alternatives, Dimethyl ether (DME) synthesized from syngas came out as a potential alternative to the conventional diesel fuel used in the diesel engines owing to its high cetane number, low NOx emission, and nearly zero exhaust generation [6]. In tailpipe exhaust treatment, catalysis indeed plays a crucial role by oxidizing combustible pollutants (CO, HC, and Soot) and reducing NOx [7]. Besides exhaust treatment, other methods
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of COx minimization includes In general, two separate catalytic systems for the oxidation of pollutants and reduction of NOx are used. Various kinds of catalysts have been investigated for CO,
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HC and soot oxidation. Among noble metal free catalysts, substantial literature is available on catalytic oxidation of soot, CO, and HC on spinel catalysts. Despite being
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an important step for practical environmental applications, till date, very few papers
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are available on simultaneous catalytic oxidation of pollutants [8][9][10]. There is only single experimental paper [17] available in literature on simultaneous catalytic oxidation of CO, HC, and soot. Moreover, there is no review paper present in the
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literature on simultaneous catalytic oxidation of concerned pollutants. Besides having an applied interest, some researches have also reported that the simultaneous removal of pollutants causes synergistic effect, and so the overall removal efficiency of
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pollutants gets improved [11][12]. Singh and co-workers [12] have found that the CO as a co-feed gas enhances the activity of NiCo2O4 catalyst for HCs oxidation (LPG in this case). The explanation given behind this effect was that the oxidation of CO elevates the local catalyst bed temperature, which facilitates the activation of HC combustion reaction. A recent work by Genty et al. [11] also observed that the enhancement in the performance of spinel cobalt based catalysts during the
simultaneous oxidation of CO and toluene. The oxidation temperature for toluene gets lowered in the presence of CO as a reactant. The conclusion obtained based on hypothesis and observation is that this improvement is originated from the formation of easily oxidizable by-products in the presence of CO. The first by-product formed when the CO is present as the reactant is benzaldehyde, whereas, in absence CO, the first product formed is benzylic alcohol. The benzaldehyde is more easily oxidizable than benzylic alcohol.
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The present emission control system and the prospects of single [DPF] configuration instead of present [DOC+DPF] in use are discussed. Following this, the evaluation of catalytic CO, HC and soot oxidation over spinel based catalysts is made by discussing
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the main factors affecting its catalytic activity. The work done by various researchers on improving the intrinsic activity of the spinel catalysts through tuning its chemical
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composition is thoroughly examined. Further, the work done in the area of improving
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morphological characteristics of the spinel is discussed with focus on the routes leading to novel spinel formulations. The work reporting spinel formulations having
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applied interest is focused in the present manuscript.
2. Emission control systems
In order to abide by the stringent regulations on diesel emissions, many R&D activities
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are carried out scientifically or commercially to reduce the emissions from diesel-fuel engines completely. The general technological approaches to control emissions are divided into pre-treatment techniques and post-treatment techniques. In pre-treatment techniques: electronic fuel injection (EFI) [13], engine modification [14], precise fuel metering, better fuel-air mixing, emulsified fuel [15], enhancement of fuel properties by using fuel additives [16], fully computerized fuel management etc. are used for
controlling the emissions. In case of post-treatment techniques, also known as exhaust gas after treatment, the emissions like CO, HCs, NOx, and soot underwent catalytic oxidation and reduction processes inside the tailpipe before releasing the exhaust into the atmosphere. In this section, emission control systems such as DOC and DPF under post-treatment techniques are discussed for diesel engines specifically. 2.1 Diesel oxidation Catalyst (DOC) DOC (Fig. 1) is an after-treatment device, which reduces the emissions of the
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soluble organic fraction (SOF) of the PM, CO and HC effectively. Though, it does not oxidize soot fraction of the PM. As it is a flow-through device, it does not cause any back-pressure on the engine, thereby, doesn’t affect the engine efficiency [26]. DOCs
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are generally used in combination with Selective catalytic reduction (SCR) to abate NOx by oxidizing NO into NO2 and increase the NO2: NOx ratio. The standard
(1)
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CO + 1/2𝑂2 → 𝐶𝑂2
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reactions that take place in DOCs are as follows [17]:
𝐶3 𝐻6 + 9/2𝑂2 → 3𝐶𝑂2 + 3𝐻2 𝑂
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𝑁𝑂 + 1/2𝑂2 ↔ 𝑁𝑂2
(2)
(3)
CO and HC are oxidized to give end non-toxic end products CO2 and H2O [Equations
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(1) and (2)]. As diesel engine always operates under lean-burn conditions, their exhausts usually contain 2 to 17% by volume O2 unreacted. This unreacted O2 is steadily consumed in DOC. Another reaction that takes place is oxidation of NO to form NO2 as represented by Equation 3. The high concentration of NO2 in the NOx increases the DPF efficiency. In the untreated tailpipe emission, the concentration of
NO2 in the NOx is only about 10% at most operating points. With the function of the DOC, NO2: NO rate is increased by inducing thermodynamic equilibrium. Moreover, the heat released during the oxidation of HCs and CO elevates the temperature of exhaust-gas in the downstream of DOC. This increased exhaust temperature helps in the DPF regeneration. In DOC, the exhaust gas temperature increases nearly by 90⁰ C for every 1% vol. of oxidation of CO. This rapid increase in temperature results in a steep temperature gradient in DOC [18].
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DOC is generally a monolith honeycomb carrier structure made of metal or ceramic. Besides its carrier structure, it consists of an oxide mixture (washcoat) composed of alumina (Al2O3), cerium oxide (CeO2), zirconium oxide (ZrO2), and
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active catalytic components such as Pt, Pd and Rh. The washcoat facilitates large surface area to for better dispersion of the catalyst and slow down its sintering at
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elevated temperatures.
Fig. 1 Schematic diagram of the diesel oxidation catalyst (DOC).
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Since its first introduction in the 1970s, DOCs continues to be a fundamental technology for diesel engines until nowadays. All-new diesel engines of passenger cars, LD as well as HD diesel vehicles are now fitted with DOCs. The estimated reductions in emission from DOC are to be over 90% for CO and more than 70-80% for total HC over a wide range of operations [19]. Therefore, DOCs are widely used
for heavy-duty vehicles (HDVs) as well as light-duty vehicles (LDVs) in various countries. Pt and Pd containing oxidation catalysts are the most common catalysts in the world market. One of the main disadvantages associated with the use of these noble metal-based catalysts is that they oxidize the SO2 to SO3, which consequently react with water and produces forms of sulfates and sulfuric acid. This causes negative effects like reducing the efficiency of after-treatment emission control system and harmful biological impact [20] .
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2.2 Diesel Particulate Filter DPF is used to trap particulates present in the exhaust gas by the physical filtration and usually made of either cordierite (2MgO-2Al2O3-5SiO2) or silicon carbide (SiC)
-p
honeycomb structure monolith with the channels blocked at alternate ends. DPF is an extruded usually cylindrical structure, usually made, with a large number of small
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parallel channels placed in the longitudinal direction of the exhaust system. The
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adjacent channels in the wall-flow filters are alternatively plugged at each end, thus forcing the soot particles to flow through the porous substrate walls, which act as the
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filter medium [21] (Fig. 2). As the soot particles pass through these walls, they get deposited there. The optimum porosity of the filter walls enables the exhaust gases to pass through their walls without considerable hindrance while being effectively impermeable to soot particles. As the filter becomes increasingly saturated with soot, a
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layer of soot is observed on the surface of channel walls. This formed layer of soot facilitates highly efficient surface filtration for the operating phase, leading to a trapping efficiency of more than 95% [22]. However, excessive saturation causes the clogging of filter, which severely affects the engine output, increases consumption of fuel [22] and causes stress in the filter [23] in the DPF. Sivaram et al. [24] reported the reduction in engine efficiency and an increased amount of emissions during severe
clogging condition of DPF. Thus, to overcome these adverse effects, the DPF has to regenerate periodically by oxidizing the soot trapped in the DPF.
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Fig. 2. The functioning of typical DPF.
Active and passive regeneration [25] are the two principle regeneration techniques employed to oxidize the PM and decrease the undesired clogging. Active regeneration
-p
can be intermittently applied to DPFs in which captured soot is removed by controlled combustion with oxygen [26]. In an active regeneration of DPF, heat is given from
re
external sources such as fuel burner or electrical heater to raise the filter’s inlet
lP
temperature to the 550⁰ C or higher [22]. The combustion of soot, trapped in the filter, occurs once the particulate loading in the filter touches set limit (around 45%) shown
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by the pressure drop across the DPF. This technique suffers from the disadvantage of additional energy costs, complex control system, and the creation of thermal stress for the filter element. Therefore, this method is not preferred, generally. In case of active regeneration, the combustion of PMs took place at the exhaust gas
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temperature by catalytic oxidation promoted by depositing suitable catalysts such as Pt at the site of combustion itself. Unlike in active regeneration, no additional fuel is needed to carry out the reaction process over the catalyzed DPF. Under a temperature range of 200 and 450⁰ C, small amounts of NO2 facilitates the continuous combustion of the trapped soot particles. This forms the basis of the continuously regenerating trap
(CRT) which uses NO2 continuously to oxidize soot within relatively low temperatures over a DPF. The DOC upstream of DPF increases the ratio of NO 2 to NO in the exhaust and lowers the PM oxidation temperature. NO 2 coming out from DOC serves as better oxidant than O2 and so gives better passive regeneration efficiency [27]. Besides the expensive nature of noble metal-based catalysts, their activities are found to be strongly inhibited in the presence CO as co-feed [28] which limits its
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potential for simultaneous oxidation of CO, HC and soot. In order to overcome these drawbacks, alternative catalytic systems having the significant potential for catalyzing the full oxidation of CO, HC and soot must be developed. Moreover, such catalytic
-p
systems should be able to perform appreciable CO, HC and soot oxidation independent of the concentration of NO2 emission from DOC. Therefore, the
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replacement of the above mentioned [Pt-DOC+DPF] emission removal device by a
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single [noble metal free catalyst-DPF] configuration seems promising, but it needs be examined first whether catalysts made from alternative materials are able to oxidize soot, HCs, and CO simultaneously, as Pt catalyst does, or not, or if further
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enhancements are needed. For achieving this configuration, we need to evaluate the work done in area of CO, HCs and soot oxidation over the alternative catalytic systems. The focus on the methodologies leading to highly active and stable catalytic
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formulations can provide direction in carrying out the work in the simultaneous oxidation of the concerned pollutants. 3. Comparison of spinels and other potential catalysts As mentioned above, noble metal-based catalysts have been reported to be highly successful in the abatement of the concerned pollutants, but suffer from certain disadvantages. Besides being expensive, the compounds of noble metals have negative
impacts on health as well as environment. They also suffer from the problems of sintering and sublimation at high temperatures [29]. The deactivation of noble catalysts also takes place in case of VOCs and soot because of sulfur and chlorine poisoning [30]. Hence, driven by the necessity to find alternatives transition metal oxide (TMO) based catalysts went under investigation in recent years. Spinels form an important class of TMOs with appealing catalytic activity for oxidation reactions owing to their desirable activity, widespread availability, low cost,
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easy synthesis, thermodynamic stability, and environmental friendliness. In the spinel structure, the coexistence of tetrahedral and octahedral sites provides multiple sites to accommodate different transition-metal cations with a wide range of valence states to
-p
form a large number of oxides. Spinel oxides have been found to be highly efficient for CO, HCs as well as soot oxidation. One interesting factor in favour of
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simultaneous oxidation over spinel oxides is that unlike noble metal catalysts, cobalt-
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based spinel catalysts are reported in the literature for maintaining high activity for HC oxidation even in the presence of CO as reactant [31]. It is noted that Layered double hydroxides (LDHs) also know like hydrotalcites
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demonstrated to be promising supports and catalyst precursors for heterogeneous catalysis owing to their wide variety of features, such as a large number of hydroxyl groups, tunable surface basicity and acidity as well as high adsorption capacity for the
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immobilization of active species. Prominent hydrotalcite such as Mg-Al hydrotalcitesupported Pd [32], Hydrotalcite-derived Co-Mn-Al mixed oxide catalyst [33], and Kpromoted CoMgAlO [34] have found to be promising catalyst for CO, VOCs (toluene here), and soot oxidation respectively. However, the requirement of very strict experimental conditions during hydrotalcite synthesis in order to avoid chemical segregations and improve homogeneities is very demanding. In general, there still
exist some technological-economic problems in LDHs preparation which limit its wide applicability [35]. Another class of TMOs, perovskite oxides (ABO3) has received significant attention as alternative catalysts for oxidation reactions owing to their abundant tunability of composition and structure, thermal stability, and excellent catalytic activity. Perovskite oxides containing transition metals such as Mn and Co at B sites have high oxidation ability toward CO [36], VOCs [37], and diesel soot [38]. The catalytic properties of
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perovskite oxides can be controlled by substitution of A site cations by other kinds of metals. In the case of LaMnO3 and LaCoO3 perovskite oxides, for instance, the substitution of La3+ ions by Sr2+ ions gives rise to the increase in the oxidation state of
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B site cations, such as Co3+ to Co4+ and Mn3+ to Mn4+, which improves the oxidizing ability of the oxides [36]. The synthesis temperature of perovskite oxides is relatively
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high (600-900⁰ C). Moreover, it has relatively low specific surface areas which limit
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its catalytic activity. In general, the catalytic activity of perovskites for the oxidation reactions is not as good as those of spinels. However, given its high thermal stability, the prospects of using it as a support for the highly active species such as copper are
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being explored recently, where it has been found to highly effective support [39]. Various mixed metal oxides (CeO2:ZnO [40] and Co-MgO [41]) other than spinel have also been reported to be effective for CO, HCs and soot oxidation. Interestingly,
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in simultaneous oxidation, CeZrNd mixed oxide catalyst was able to accelerate simultaneously soot, propylene and benzene oxidation. Though it was able to accelerate CO oxidation in a certain extent, but there was a net production of CO during soot combustion because the oxidation capacity of these oxides was not high enough to oxidize all CO released as soot combustion product [42]. In mixed metal oxides, the combination of two or more metal oxides produces a complex system with
multiple functions stemming from each oxide, often resulting in desired catalytic properties. 4. Factors affecting the spinel catalytic activity for CO, HCs and soot oxidation 4.1. Tuning the spinel composition The single spinel oxide such as Co3O4 has attracted huge research enthusiasm due to its low cost, environment-friendly nature and high catalytic activity [43]. Intrinsically, the spinel structure of Co3O4, sometimes expressed as CoCo2O4, is that
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the Co2+ and Co3+ cations occupy the Td and Oh sites, respectively. Both theoretical predictions, as well as experimental results, have shown that the Co3+ cations on the Oh sites are the reactive ones for many oxidation reactions (e.g. CO oxidation, VOCs
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combustion, water oxidation and, soot combustion) whereas the Co 2+ cations on the Td sites are almost inactive [44][45]. Yi et al. [46] attained excellent catalytic activity for
lP
cations on the exposed planes.
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CO using Co3O4 nanowires, attributed to the formation of abundant reactive Co 3+
Many researches in this field have [47] demonstrated that the intrinsic activity of
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single oxide spinels such as Co3O4, can be further enhanced by substituting Co2+ cations on the reactive Td sites through other transition metal cations (e.g. Mn2+, Ni2+, Zn2+ and Cu2+), forming spinel mixed metal oxides or spinel bi-metal oxides. Moreover, the pure single oxide spinels such as Co3O4 have disadvantages like weak
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thermal stability at elevated temperatures and water vapor inhibition at low temperatures. The deactivation of CO3O4 powder is reported in the presence of water vapor, the conversion of CO to CO2 significantly gets decreased, and the formation of carbonates is facilitated [48]. This cation substitution enhances the catalytic activity due to the formation of highly oxidized redox couples resulting in synergism between
cations A and B [49]. This increase in catalytic performance is due to synergism between different ions is invariably reported for all the oxidation reactions of our interests, such as in CO oxidation, HCs oxidation, and soot combustion. In the case of CO oxidation, Kouotou et al. [50] have reported the promoting influence of the substitution of the Fe by Co in Fe3O4 spinel oxide. Co-Fe mixed oxides exhibited high catalytic activity than the single α-Fe3O4 spinel oxide. Trivedi et al. [51] examined the promoting effect of Ni substitution in Co3O4 spinel oxide. The
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total oxidation in case of cobalt oxide was observed at 160⁰ C whereas, for nickel cobaltite spinel it was observed at 150⁰ C. It was also observed that the NiCo2O4 spinel lattice maintained its catalytic performance for a long run while the activity of
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its individual metal oxides (NiO and Co3O4) lost rapidly because of sintering and carbonaceous deposition.
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In HCs oxidation, chi et al. [52] boosted the total oxidation of VOCs over cobalt
lP
oxide spinel by cation-substituting effect and reported that the hollow mesosphere structures (HMS) of mixed spinel cobaltites (CuCo2O4 and NiCo2O4) performs better than its single oxide (Co3O4) performs better than its single oxide Co3O4 HMS. Similar
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observations were made by Wenfeng and co-workers [53] for soot oxidation where the CuCr2O4 activity was reported to be better than its single oxides CuO, Cr 2O3 and their mechanical mixture.
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The degree of synergism between the two cations (A and B) is also dependent on the extent of the interactions between the cations A and B. The nature of A cations also influences the catalytic activity of the formed spinel structure. This extent of interaction was observed to be dependent on the nature of the constituent cations as well as the coordination state of cations. Different metal cations on A site has different influence on the same metal cation B. Hence, various researches have examined the
influence of cation A on the parent spinel lattice. Trivedi et al. [47] studied the activity over various spinel cobaltites and reported the catalytic performance order for CO oxidation as follows: NiCo2O4>Co3O4>MnCo2O4>CuCo2O4>FeCo2O4. The best catalytic activity of NiCo2O4 spinel catalyst was attributed to its very small size crystallites formation and better interaction between Ni and Co cations. In VOCs oxidation, Hosseini et al. [54] prepared the series of various nanocrystalline manganite spinels (A = Co, Ni, Cu) among which the NiMn 2O4 spinel
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was found as the best catalyst for the removal of 2-propanol and toluene. This high activity is because of the prominent synergistic interaction between Mn and Ni on the oxide, having a high concentration of active Ni2+ and Mn3+ sites. Jiang and co-workers
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[55] also found that 3-D ordered mesoporous spinel catalysts exhibit catalytic activities in benzene total oxidation depending on the concentration of surface-active
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oxygen species in the catalysts. The different cobaltites exhibit a different relative
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amount of surface oxygen species, therefore, exhibit different catalytic activity. Chi et al. [52] observed that the acetone complete oxidation order as follows: CuCo 2O4 (260⁰ C) > NiCo2O4 (220⁰ C) > CoCo2O4 (200⁰ C). This reason behind this behaviour
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is the cation-substituting effect, which resulted in the different concentrations of surface Co3+ cations, active oxygen species, defective sites and reducible capabilities in the synthesized catalysts.
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In soot combustion, Zhang and co-workers [56] have reported that the effective interactions between Zn and Co cations led to the high surface area, high amount of surface-active Co3+ and more chemisorbed oxygen species, thereby enhanced catalyst performance. Shangguan et al. [57] have also observed that the catalytic performance exhibited by the catalysts is exclusively dependent on their constituent metal content, where CuFe2O4 was reported to be the most efficient catalyst among the various
spinels (ferrites) tested for soot combustion. Interestingly, the coordination state of metal cations, the substitution of metal cation (Cu here) in the octahedral site or in the tetrahedral plays very little role in determining its catalytic performance. In soot combustion, Zhang et al. [56] have reported that that the substitution of A site of CO3O4 spinel compound with other cations (Co, Ni, Cu, Zn) caused the change of Co3+/(Co2+ + Co3+) ratios as well as the oxygen mobility in the spinel structure. Both Co2+ and Co3+ species were found on the surface of the catalyst, and Co3+ species got
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easily reduced to lower states, thus improving the catalyst activity. Among these spinels, ZnCO2O4 exhibited the best performance owing to its good oxygen mobility and the presence of high concentration of surface-active Co3+ species in zinc cobaltite
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spinel lattice. Besides the intrinsic ability of the prepared catalysts, the use of oxidants such as NOx and O2 plays a critical role in soot combustion. Zawadzki et al. [58] have
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studied the role of both NOx and O2 in soot combustion over CoAl2O4 spinel and
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attributed the high soot oxidation activity of the catalyst to its high NO x chemisorption capacity, what allows fast NO oxidation to NO2. As NO2 is much more oxidising than NO and O2, it helps in the fast combustion of soot. However, high NO2 production
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capacity is not enough to ensure effective soot oxidation, and the temperature where NO2 is yielded must be high enough for NO2 to oxidise soot. If NO2 is generated below a certain temperature, the oxidising gas will not able to oxidise the soot surface.
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When x in the non-stoichiometric spinels of AXB3-XO4 is varied, different catalytic, electrical or magnetic properties can be realized, thereby improving the intrinsic activity of the catalyst. The non-stoichiometric spinel contributes to much more crystal defects and oxygen vacancies that form active sites for reactant molecules during the reactions and substantially improve the catalytic activity. Working towards the sole objective of improving the intrinsic activity of spinel, various researches have
prepared the bi-metal or mixed metal spinel-type oxides in varying atomic molar ratios of cations A and B in to examine the effect of metal cations molar ratio on activity. Faure et al. [59] have examined the activity of Co-Mn mixed oxides in the varied molar ratios of Co to Mn for CO as well as HC (propane here) oxidation. The mixed oxide with a composition as Co2.3Mn0.7O4 exhibited the best performance owing to its exceptionally high surface area. Similarly, Junhua et al. [60] also observed that CoMn mixed oxide catalysts with a Co/Mn atomic ratio of 5:1 shows better performance for
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CO oxidation among other CoMn catalyst with varied atomic ratio. The high intrinsic activity in the case of Co/Mn (5:1) having was attributed to the prevalence of crystal defects which help in creating more number of octahedrally coordinated reactive
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divalent cobalt cations. Although, as mentioned in the above observations, the composition of a catalyst has a strong positive effect on the activity, this effect of the
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composition may or may not be varied for different oxidation processes. Eric and co-
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workers [16] have reported that the aluminium cobaltite, with a molar ratio of Co2+/Al3+ of 6/2, contributes in CO as well as toluene oxidation. Whereas, Ivanov et al. [61] have reported that the CO oxidation rate rapidly decreases with increasing of
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Cr content in the active component whereas, for the dimethyl ether (DME), the reverse trend was reported. It was concluded that the best compromise is the spinels with Cu/(Mn + Cr) molar ratio 1:5 and Mn/Cr molar ratio from 1:3 to 1:4. Based on the
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results from XPS analysis, the increased catalytic activities could be due to the synergistic effect of the cation distribution ratio in the O d/Td sites and the oxygen vacancies on the surface. Based on above various observations made by various researches, it can be concluded that the intermediate compositions are the main active one and the phenomena of synergism are most prominent in the case of intermediate compositions.
Optimization of composition of spinel in use will be the key factor in carrying out the simultaneous oxidation of CO, HC, and soot effectively. 4.2. Doping in the spinel compositions Doping is a well-established factor in enhancing the intrinsic activities of the catalysts. The replacement of an appropriate fraction of the ions in a host spinel metal oxide lattice with other metal ions can enhance its activity. The doping cause distortions in spinel structure, which change the concentration of oxygen vacancies
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and the active sites thus form catalysts with novel morphologies and enhanced catalytic properties. In general, metal ions with variable oxidation states have been considered as active modifiers that can tune the chemical bond or surface state of
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metal oxide catalysts, thus enhancing the catalytic oxidation activity. For example, vanadium (V), variable valence metal, is suitable for doping Co2+ in the Co3O4 lattice
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as the V2+ cation has an appropriate ionic radius for doping similar to the Co2+ cation
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(V: Co = 0.063 nm: 0.065 nm). The doping of spinel oxides with alkali metals ( K +, Na+, Rb+, Cs+), alkaline earth metals (Mg2+), platinum group metals (Ag+, Pt+), or
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lanthanide oxides (CeO2, Sm3+) is also reported in the literature. Among alkali metal effect, Min and Zheng [62] studied the influence of K and Al over NiCo2O4 on its physio-chemical characteristics and VOCs removal efficiency. Aldoped NiCo2O4 exhibited poor physio-chemical characteristics, thereby, exhibits poor
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catalytic performance for VOCs oxidation. The reverse trend was observed in K doped NiCo2O4. The K doping was found to be significantly efficient in promoting NiCo 2O4 spinel catalyst in terms of reducibility and textural characteristics. In catalytic soot oxidation, the alkali metal itself can take part in the oxidative process through the formation of carbon-oxygen-metal complexes (C-O-M, with M alkali metal) as the active sites and further increases the soot oxidation rate. Legutko et al. [63] have
studied the positive influence of K+ on the activity of transition metal (Mn, Fe, Co) spinels in case of soot oxidation. The results revealed that K+ doping could significantly enhance the catalyst activity in soot combustion; though, the influence strongly depends in each case on the doping concentration and on its site (bulk or surface). On the contrary, alkali metals show poisoning effect in certain reaction systems such as in CO oxidation. The alkali metals (Na, K, and Li) were promoted onto a cobalt oxide nanocatalyst to examine their poisoning influence on CO as well as
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VOC (propane) oxidation. About 1% alkali metal dopant onto cobalt oxide was reported to raise the light-off temperature by 50°C for the oxidation of CO and over 160⁰ C for the oxidation of propane. The reason given behind this poisoning effect is
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the observation of ‘locking-effect’ on the oxygen of Co3O4 by alkali metals. This leads to weak oxygen mobility of alkali metal promoted catalysts. Another reported reason
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for the alkali metals poisoning effect on Co3O4 catalysts for CO and HC oxidation is
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that the alkali metals considerably facilitate the adsorption of CO 2 to form surface carbonate species even at elevated temperature [64]. Based on the aforementioned reported results, it is concluded that the alkali metal as a dopant is undesirable when it
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comes to simultaneous catalytic oxidation of CO, HCs, and soot over spinels. Lanthanide oxides such as ceria are widely reported effective catalyst dopants. Wide literature is available on using CeO2 to modify cobaltite based spinel catalysts,
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or using spinel cobaltites to modify CeO2-based catalysts [65]. The high oxygen storage capacity (OSC) of cerium makes it well known doped material in catalytic exhaust systems. The strong oxidative property of spinel based metal oxides together with the OSC of ceria makes ceria-doped spinel based metal oxides, group of low-cost and highly active spinel doped catalysts for oxidation reactions. Xiaowei and coworker [66] have reported that the substitution of in CoCr2O4 spinel lattice by ceria in
the appropriate concentration results in improved activity for the simultaneous abatement of NO and soot. The doping of Ce leads to the deformations in the CoCr 2O4 spinel lattice which results in crystal defects and an increase in the oxygen vacancy concentration, which favours oxygen mobility and the redox behaviour of the cerium modified cobalt chromite spinel. Dey et al. [65] have also examined the effect of CeOx addition on the activity of the CuMnO4 spinel for CO oxidation. It was reported that the CuMnOx promoted with (1.5 wt.%) CeOx enhances the activity of spinel for CO
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oxidation owing to the formation of new surface sites upon doping. Another lanthanide, Samarium (Sm3+) is also studied by Xu and co-workers [67] to examine its promotional effect on Co3O4 spinel lattice for Co oxidation and reported that the
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sample having Co/Sm molar ratio of 0.90/0.10 showed the best activity. The addition of an appropriate concentration of Sm resulted in the formation of spinel Co 3O4 and
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amorphous SmCoO3, hence increasing the number of reactive Co3+ and the active
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surface oxygen species, which was responsible for the enhancement in the catalytic activity.
In noble metal doping, Dey et al. [65] examined the influence of doping on the
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activity of CuMnO4 spinel for the oxidation of CO. It was reported that the CuMnO x promoted with (1.5 wt. %) CeOx, (1.0 wt. %) AgOx, and (0.5 wt. %) AuOx enhances the activity of spinel for CO oxidation owing to the formation of new surface sites
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upon doping. In soot oxidation, Huangrong et al. [68] have observed that the substitution of Ag+ in manganese cobaltite in various molar ratios and observed that the soot oxidation temperature decreases with increase in the concentration of Ag. The Ag phase besides the spinel phase of manganese cobaltite was observed by XRD on increasing the substitution amount of Ag above 0.1. The Mn 0.6Ag0.4Co2O4 catalyst
showed efficient catalytic performance for soot oxidation, as a result of the synergism between chemisorbed oxygen species (O2-, O-), and metallic Ag. 4.3. Controlling the morphology It is to be noted that other than tuning the chemical composition of the spinel as reported above, the catalytic activity of the spinels can also be modified by making various morphological and structural changes in the spinel. The improvement in the morphology of the catalytic structures through various ways can lead the way to the
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development of highly active catalysts. Various spinels with unique structural formations have been prepared and examined in the heterogeneous catalysis, and their structure-activity
relationship
has
been
readily
through
many
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characterization and experimental results.
confirmed
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4.3.1. Effect of synthesis approaches on spinel morphology
Indeed, various methods have been developed to synthesize spinel oxides, such as,
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amongst others, solid-state synthesis (reactive grinding), and wet chemical routes: solgel, coprecipitation, wet impregnantion (WI), alginate route, reverse micelles,
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hydrothermal/solvothermal synthesis, template synthesis, and others (Table 1). The choice of the synthesis technique is generally driven by the stability of the specific spinel composition required and by the need of obtaining desired physio-chemical characteristics for the final catalysts.
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In wet chemistry approach, the preparation of the spinel particles is controlled by a combination of various parameters such as pH, temperature, reaction time and calcination conditions. Thus, the primary objective of the preparation is an optimization of these dependent parameters to obtain oxide catalysts with desired morphological and structural characteristics. Sajad and co-workers [69] have studied
the effect of various preparation parameters such as pH, ageing time, autoclave and calcination temperature on the physio-chemical properties of the nanocrystalline CuCr2O4 spinel catalysts prepared by via surfactant-assisted hydrothermal route. The results showed that the catalysts prepared under optimized parameters (pH =12, 4h ageing period, 180⁰ C autoclave temperature and calcination at 500⁰ C) showed relatively high surface area (38.5 m2g-1) and almost pure spinel phase. Both of these characteristics favoured the CO conversion efficiency. Further, various calcination
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parameters such as temperature, duration and atmosphere can also plays very important role in the synthesis of effective spinel oxides. The optimizations of the calcination parameters can results into favorable spinel oxide morphology, thereby,
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improving its catalytic activity. Guilin et al. [70] have studied the effect of calcination temperature on catalytic activity of the prepared CoMn/AC spinel oxide catalysts at
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different calcination temperatures. The order of activity as per the toluene conversion
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efficiency was reported as follows: CAT250
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characterization results such as XPS, TPR, and SEM. The low calcination temperature (250⁰ C) found to be not favourable to the decomposition of the nitrate precursor to form active species as well as to the activation of the supported active species. Though
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high temperature calcination (400⁰ C) is also not favorable as led to the growth of the grains that decrease the dispersion and increase the sintering of catalyst as can be seen in SEM micrographs (Fig. 3a). Abolfazl and co-workers [71] have also studied the effect of calcination temperature on the CO oxidation activity of iron-cobalt spinelbased bi-metal oxides. It was reported that the calcination performed at high temperature (>400⁰ C) results in the increase of the crystallite size and decrease of the
specific surface area, which gives a negative effect on the catalytic activity (Fig. 3b). Fresh catalysts are readily calcined at different temperatures to induce favourable morphological changes in them. Other than calcination temperature, the calcination process performed in the different atmosphere (oxidative, reductive, inert, and reactive) can also modify the morphology of the catalyst. Spinel cobalt oxide pretreated in a non-reducing conditions mainly in N2 and CO/air, at moderate temperatures could attain complete CO oxidation at an exceptionally low temperature
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of -80⁰ C [72]. Another study performed on spinel cobalt oxide also observed that the calcination of cobalt oxide in inert conditions (in He) at 350⁰ C led to its surface reconstruction which led the formation of weakly bound oxygen species. These
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weakly formed species facilitates the CO oxidation (Fig. 4a) [73]. Abolfazl and coworkers [71] examined the influence of pretreatment conditions on the Co-Mn spinel
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based bi-metal and reported that the catalyst pretreated under oxidative conditions
as inert conditions (Fig. 4c).
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exhibited the better catalytic performance than those pretreated under reductive as well
In general, the prevalence of surface or crystal defects on the catalyst surface acts as
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active sites for the oxidation reaction to occur. Such crystal defect-rich nanocrystalline structures could be obtained by following the exotic calcination or thermal treatment routes in a controllable manner. Such an exotic route is followed by Paldey and co-
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workers, where the fresh spinel catalyst is heated in a reducing atmosphere followed by controlled oxidation. Such controlled decomposition environment led to the formation of fine clusters or domains of around 5 nm size on the particle’s surface with abundant lattice defects. The abundance of crystal defects results into the fast CO oxidation. Interestingly, a similar kind of route is also followed by Trivedi and Prasad [49], where the calcination of a basic hydroxycarbonate precursor takes place in the
presence of a chemically reactive-CO-air mixture. The calcination performed in such environment is known as reactive calcination (RC). The simultaneous occurrence of multifarious phenomenon of oxidation-decomposition-redox-surface-reactions during this process produced a synergistic effect which helps in the creation of oxygen deficient active phases in the catalyst. The morphological characteristics attained by catalyst at low temperature found to remain intact at high temperature. Pratichi et al. [12] compared the catalytic activity and morphological characteristics of the spinel
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nickel cobaltite prepared under reactive calcination route with catalysts prepared using conventional routes. It was observed that the spinel prepared using RC route exhibit much better activity (for CO and HCs) and morphology as compare to spinel calcined
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in stagnant and flowing air atmosphere (Fig. 4b). The remarkable activity of the reactively calcined catalyst for CO-HC mixture oxidation is because of the formation
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of partially reduced NiCo2O4−δ oxygen-deficient structure exhibiting exceptionally
lP
high surface area, very small crystallites, and highly dispersed homogeneous morphology. The spinel synthesis by following microwave assisted hydrothermal and combustion synthesis routes can also led to the formation of nanocrystalline defect-
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rich structures. The short reaction time in such routes favours the incomplete crystallinity and the surface roughness. The abundance of crystal defects and oxygen vacancies is seen on such rough surface, which acts as favourable sites for oxidation
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reaction to occur. The general understanding says that the interface of two mixed phases can be a very active site for reaction to occur owing to the prevalence of defects at the interfaces. To understand the interfacial structure of the CuO/MnOx mixed
phase,
layered
copper
manganese
oxide
(LCMO)
with
a
bridged
monoclinic/tetragonal phase structure was synthesized via one-step hydrothermal redox-precipitation approach by Wang and co-workers [74]. It was revealed that the
formation of the monoclinic-tetragonal phase interface with abundant defects inhibited the growth of nanoparticles which helped in keeping crystal size small and surface area high. Characterization results revealed that the interfacial structure of mixed phases induced the formation of the Cu2+-O2-Mn4+ entity, leading to the abundance of surface oxygen species and oxygen vacancies, thus improving the catalytic activity. In advanced preparation techniques, the main focus is to improve the morphology of the structures by offering more number of reactive sites for reactant adsorption and
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increased reactant-catalyst contact frequency. This can be seen in the case of mesoporous spinels with large surface-to-volume ratios, fast diffusion, and uniformity in porosity. Therefore, various approaches have been developed for the synthesis of
-p
mesoporous spinels. Liu et al. [75] have successfully synthesized mesoporous Co3O4 through a facile soft reactive grinding route at room temperature using the
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polyethylene glycols as an assisted agent. The obtained catalyst exhibited very high
lP
catalytic activity for CO oxidation (T50 = -101⁰ C) under a stream of normal feed gas containing moisture. This high activity is attributed to the exceptionally high surface area (124 m2 g-1) of mesoporous cobalt oxide. Zhang and co-workers [52] worked one
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step further and they integrated the inner cavity with mesoporous metal oxide shells of various dimensions into one unique nanostructure having very high surface area and good reactant-catalyst contact frequency. Carreon et al. [76] have reported the
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synthesis of mesoporous nanocrystalline cobalt nickel oxide spinels by self-assembly of heterometallic alkoxides in the presence of supramolecular liquid crystalline phases. The formed mesoporous nickel cobalt oxides showed better propane conversion to CO and CO2 as compared to a conventional dense phase formed spinel oxides. However, the hydrolysis and condensation of transition-metal alkoxides are not easy to control precisely in this wet chemistry approach. The resultant metal oxides attain poor
structural ordering and weak thermal stability after removal of the surfactant templates. As an alternative, the use of hard-templates like SBA-15 in nanocasting method effectively overcomes the above-mentioned problem of weak structural stability faced by wet-chemistry approach. The hard templates like SBA-15 with topologically robust frameworks resist the local strain caused during the precursor’s crystallization process, thereby, providing better characteristics (e.g. mechanical and thermal stability) desired for practical applications. Moreover, multi-component metal
homogeneity and stoichiometry of the resultant replicas.
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oxides prepared by this route as solution-based precursors could effectively control the
Over the years various researchers have studied the effect of various preparation
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methods on the spinel morphology and catalytic activity. Trivedi and Prasad [47] studied the effect of various preparation methods on spinel nickel cobaltite and
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reported that the catalyst synthesized via co-precipitation (CP) exhibited the best
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activity for the CO oxidation in comparison to other preparation methods. The order reported as per catalysts performance was as follows: CP>Reactive grinding (RG) of nitrates>RG of oxides>SG. Maria et al. [77] have also reported that the CP method got
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the most active oxide for VOC mixture oxidation due to the improved mobility of oxygen and ability to favour redox processes in the material structure. Wenfeng and co-workers [78] compared the conventional acetate process with the citric acid-aided
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process and reported that the oxides synthesized by the latter were more active as they form monophasic CuCr2O4 spinel at lower temperatures. Seyed and co-workers [54] observed that the copper magnetite catalysts prepared via SGC (Fig. 5a) showed higher SBET, higher wt.% of the normal-type spinel phase and higher amount of reactive Mn3+cations than the same catalyst prepared by conventional co-precipitation. Qian and co-workers [79] have reported that the lattice distortion induced by
prolonged citrate-precursor grinding in case of SRG helps in creating and keeping a high density of surface defects, which leads to good activity and stability of the cobaltbased spinel catalyst. The synthesis techniques of spinels largely differ by their economics, their energy requirement and their environmental impact. With the rational design of composition, structures, defects, and morphology, desired crystal defect-rich nanocrystalline catalysts can be obtained in a controllable manner. Classical methods such as co-
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precipitation can be hampered by the need of operating in the high-pH field in which all the concerned cations are out of their solubility domain, with consequent rejection of alkaline wastewaters. The techniques facilitating the precise control of particle size
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by confinement of the precursors, as the emulsion or template methods, involve the use of relatively costly organic additives. Same drawback is seen in techniques such as
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the sol-gel, Pechini or alginate methods, in which the homogeneous dispersion of the
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precursor cations in a matrix favours the formation of spinels with compositions difficult to form by co-precipitation. Some energy-intensive methods, like flux growth, results in low surface area catalysts which is undesirable characteristic. Table 1
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summarize the primary methods reported to fabricate spinel compounds along with their overviews, characteristics and drawbacks.
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Table1Summary of the methods reported in literature for preparing spinels
Methods Overview Sol-gel (SG) chelating agents, sol [47] solution formation, gel formation after losing solvent, calcination Hydrotherma l [74] [80]
hydrothermal high-pressure
Characteristics mild and large scale, nanoparticles (common), composition regulable, easily scable reactor, small crystallites, high SBET, no high temperature
Drawbacks time consuming, not efficient for multicomponent compositions, large waste relatively high cost of equipment and the inability to monitor
calcination
the growing crystals in the autoclave
transfer gaseous reactant high-purity, thin film to solid thin film
Microwave [58]
microwave energy fast and (molecular friction); temperature microwave-assisted below 100⁰ C)
Thermal decompositio n [59]
oxalate decomposition
Reactive grinding
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low not scalable, costly (even
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Solution combustion synthesis (SCS) [85]
complex reactions need to be well controlled to obtain the desired product and precursors are limited to thermally stable materials for pyrolysis High substrate temperature, no codeposition, precursors not existing for every Element
metal very low only suitable for VOC temperature oxidation; high (∼200⁰ C), cost operating temperature in exhaust require effective and easy high temperature calcination transfer solid-solid to relatively low low surface area not solid-liquid reaction temperature and short favourable for using reaction time, high catalysis, energymolten salts purity intensive process calcination of xerogel formation from alginate solution decomposition a material (nitrate precursors) in the presence of a reducing agent (organic fuel as glycine or urea) grinding of stoichiometric amounts
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Alginate route [84]
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Chemical vapour deposition (CVD)[82][8 3]
Flux grown
moisture sensitive crystallites, multistep processing, delicate temperature control, large waste
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Precipitation[ controlled by various compositions 49] precipitants, pH, and morphologies, solvents and calcination spinel hybrids, rich parameters oxygen deficiencies, cost-effective, easily scalable Spray aerosol reaction from fast, nanostructures, pyrolysis[81] liquid reactants porous, low crystalline, better control over morphology, low cost
low cost natural biopolymer precursor; easy catalyst synthesis high surface area and high-purity foamy material, predominantly single phase
Cu-Mn-O system for toluene oxidation evolution of hazardous gases, yield loss, safety issues due to exothermic reaction
high surface area metallic impurities simpler, waste-free, from the machinery
of precursors planetary mill
in easily scalable, energy efficient, costeffective
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synthesis [75]
Fig. 3. The effect of calcination temperature (a) SEM images of samples calcined at different
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temperature [70]; (b) The activity of Fe-Co mixed oxide catalysts calcined at different temperatures, reaction condition: 2%CO, 20%O2and 10%N2 balanced with He, calcined in
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oxidative environment [71].
Fig. 4. The effect of calcination atmosphere (a) The effect of pretreatment environment over cobalt oxide, reaction conditions: 0.15 g catalyst, 1.0 vol% CO in air, 50 ml min-1. In all the cases pretreatment was done at 150⁰ C for 40 min in dry air (▵ ), wet air (▴ ), 1.0 vol% CO/air (◊), N2 (○), 1.0 vol% CO/N2 (□), or 5.0 vol% H2/N2 (▿ ) [72]; (b) Effect of calcination environment on CO oxidation over nickel cobaltite in stagnant air (▵ ), flowing air (◊), or
reactive air(□) [12]; (c) Effect of calcination atmosphere over CuMn catalyst in oxidative(□), none pretreatment (○), inert, or reducing atmosphere [71]. 4.3.2. Effect of different supports The main drawback of the spinel-based catalysts is their deactivation as a result of an aggregation. The deactivation problem could be overcome by applying novel preparation methods and application of supports with high surface areas. Catalysts
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attain their proper shape, particle size, and uniform dispersion of active species because of supports, allows the prepared catalysts to achieve a larger specific surface area. Moreover, the interaction between the catalyst and the support can also greatly influence the properties of the supported catalysts. The conventional powdered form
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catalysts are generally dispersed onto the surface or mixed into the channel walls of
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structured support by wash-coating, WI or extrusion. The supports generally used for this purpose are oxides such as Al2O3, SiO2, and TiO2. Loading a catalyst on such inert
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but high surface area support improves the accessibility of the active phase. Wang and co-workers [86] discussed the role of inert supports (Al2O3, SiO2, and TiO2) for CO
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oxidation and found out that the catalytic activity of supported cobalt oxide depends on the kind of the support being used. The results showed that the interaction of cobalt oxide with supports was much stronger in the case of Al2O3 and TiO2, while no conclusive evidence of any interaction was observed for SiO2. The degree of
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interaction between cobalt oxide and the support not only affected the surface area and reducibility of the catalyst, its activity toward the CO oxidation also gets affected. Besides the nature of support, the technique used in the dispersion of the active
species is also a key factor in determining the activity and the selectivity of the supported catalysts. Therefore, the maximization of the dispersion and the
achievement of proper characteristics for the active species are the main challenges. A reasonable approach towards the solution of this problem is related with the selection of an appropriate methodology of preparation. The deposition of the active species on the surface of catalytic supports, the most critical preparation step, usually takes place from electrolytic solutions. The pore volume impregnation (pvi), the wet impregnation, the deposition-precipitation and the equilibrium deposition filtration (edf), otherwise called equilibrium adsorption are the widely reported techniques.
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Ataloglou et al. [87] studied influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the ‘cobalt oxide’/γ-alumina catalysts for complete oxidation of benzene.
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Three series of catalysts of varying Co content (up to 21 wt.% Co) were prepared using three preparation methods: pvi, edf and pore volume impregnation adding
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nitrilotriacetic acid (nta) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta–
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pvi ≫ pvi ≫ edf, nta–pvi ≫ edf ≈ pvi and edf > nta–pvi > pvi. However, the interaction achieved after impregnation between the positively
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charged alumina carrier and the cobalt cations using electrolytic solutions is not strong enough. This weak interaction results in the formation of undesirable large crystallite sizes, thereby, the low surface area. Therefore, various efforts have been made in the
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last few decades by several researchers to prepare highly porous alumina with high surface area by techniques such as hard templating (via carbon template), sol-gel based assembly, and microwave irradiation in the presence of surfactants. Such techniques have been used to prepare well-ordered mesoporous alumina materials with high surface area and a narrow pore size distribution. However, the process found to be very time consuming and not suitable for scaling in industrial applications.
In general, ordered mesoporous materials are promising catalyst supports due to their favourable structural characteristics as high surface area, good mechanical stability, easy modification, and uniform porous channels with controllable pore diameters and pore lengths. The presence of surface hydroxyl groups offers the opportunity to support metals and metal oxides with catalytic activity. Moreover, due to the large pores, they provide less diffusion limitation for the reactants and products. These characteristics facilitate the formation of uniformly dispersed and stable metal
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particles. In mesoporous supports, SBA-15, KIT-6, MCM-41, and MCM-48 are generally used. SBA-15 has attracted much interest as it possesses a regular hexagonal array of pores with uniform diameter, high surface area, and high pore volume. It is
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also inert and stable at elevated temperature and exhibit good mechanical stability. Moreover, its surface can be easily functionalized. Bordoloi et al. [88] applied
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evaporation-induced self-assembly (EISA) process to synthesize mesoporous mixed
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oxides, which consist of cobalt ions highly dispersed in an alumina matrix. The obtained mixed cobalt-aluminum oxides were studied for CO oxidation and reported to be highly active. The EISA technique can be considered liquid crystal template
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strategy. In this process, a homogeneous precursor solution is dispersed into fine droplets and subsequently dried and calcined, which results in the formation of mesostructured materials.
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The use of supports other than traditional alumina is also being reported. Nguyen et al. [89] compared the use of active CeO2-ZrO2 support with other popular nonactive but high surface area and cheap supports as Al2O3, SiO2. It was reported that the cobalt oxide supported on CeO2-ZrO2 was reported to be the more active than supported on inert Al2O3 and SiO2 supports for the catalysts for the complete oxidation of propylene since Ce0.9Zr0.1O2, itself, was also a good active phase for the reaction. 30%
Co3O4/Ce0.9Zr0.1O2 catalyst was able to oxidize 100% propylene to pure CO 2 from 250⁰ C; 30% CeO2-Co3O4 catalysts on Al2O3 and SiO2 supports were able to do the same at higher temperatures (400⁰ C). However, catalysts on Ce0.9Zr0.1O2 exhibited much lower surface areas and much less thermal stability. In carbon based supports, the use of activated carbon (AC) as support over CuMn spinel based catalysts provide loose and porous structure which facilitates the adsorption and activation ability of the reactant molecules, as well as promote the
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diffusion and migration ability of the reactant and product molecules in the prepared catalyst. Meanwhile, the formation of the loose-porous structure and the accumulation of the metal oxide grains can form the alleged lattice defects, which can provide more
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active sites for activating reactant molecules [70]. 5. Deactivation studies
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The water poisoning problem is generally faced by spinel oxides greatly limited their
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practical application. A reduction in catalyst activity with time-on-stream is observed due to spinel deactivation. Addition of water beyond an optimum level results in a loss of activity due to sintering of catalyst [90]. Sintering is the loss the active surface of
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the catalyst due to crystal growth of either the bulk material or the active phase [91]. A wide number of characterization techniques including WDXRF, XRD, N 2 physisorption, Raman spectroscopy, XPS, H2-TPR, TEM and TGA-MS are used to
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characterise the post-run samples. The morphological and structural changes induced in the used catalyst due to thermal and chemical deactivation gets revealed in the characterizations. Wang and co-workers [74] characterized the used spinel layered copper manganese catalyst (after 2500 min toluene oxidation reaction at 220⁰ C) by XRD, XPS and TEM. The observance of no changes in the crystallization or diffraction peaks of the used catalyst in XRD pattern (Fig. 5a) reveals the stability of
the spinel phase of the catalyst. The analysis of surface compositions of used catalyst by XPS (Fig. 5b), no deposition of carbon species was found from C 1s XPS spectra. After the catalytic enduring test, the changes of Cu 2p and Mn 2p XPS spectra were negligible, which showed that the LCMO catalyst was stable and not deactivated. Interestingly, the content of surface adsorbed oxygen (Oβ) after 2500 min reaction at 220⁰ C increased to some extent compared with fresh catalyst, implying gaseous O 2 molecules are preferentially adsorbed on the oxygen vacancies of fresh catalyst under
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an oxygen-rich atmosphere, which was beneficial to the complete oxidation of CO and VOCs and the better stability of catalyst. Moreover, as seen in TEM images (Fig. 5ce), no morphological change is seen when we compare TEM images of used catalyst
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with fresh catalyst. This shows that the textural characteristics of the catalyst remain intact indicating the stable nature of the catalyst. In compliance with favorable
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to H2O, SO2, and CO2 deactivation.
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characterization results, this spinel based LCMO catalyst exhibited excellent resistance
ro of -p re
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Fig. 5. Characterizations of LCMO catalysts (a) XRD patterns of used LCMO; (b) XPS of used LCMO; (c-d) TEM images of the used LCMO catalyst; (e-f) TEM images of the fresh
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LCMO [74].
In order to meet future expectations favoring diesel engines, it is necessary to develop catalysts with high performances and robustness but low costs. Therefore, efforts should be addressed towards the development of novel spinels that decrease, or
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eliminate, the use of commercially used noble metals. Mobini and co-workers [69] carried out recycle experiments on CuCr2O4 catalyst prepared via surfactant assistant hydrothermal synthesis for five subsequent runs and the results showed no deactivation of the catalyst. Moreover, the stability of this catalyst is observed in the presence of water and CO2 at high temperature (400⁰ C). Though, a pilot scale study
under real exhaust conditions over such highly active catalysts is needed to examine their applicability. A very recent work by Taiki and co-workers [92] developed a very efficient Cu0.05Ni0.95Al1.8Cr0.2O4 multicomponent spinel oxide solid solution as candidates for noble metal free TWC. Even after thermal aging at 900⁰ C for 25 h, this quaternary system achieved high phase stability as well as high catalytic activity towards CO, C3H6 and NO in a wet gas stream similar to real TWC conditions. Its high activity is attributed to combination four selected metal elements showing
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different site preferences. In simultaneous oxidation, bench scale tests on a wall-flow ceramic filter catalysed with the effective nanostructured CoCr2O4 spinel obtained by in situ combustion synthesis gave promising results, as it entailed a about four fold
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reduction of the time required for trap regeneration as well as a much more complete regeneration compared to that of a non-catalytic trap [85]. Though, experimental test
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campaign is required to verify this potential at a catalytic trap level on real exhaust
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gases (includes CO, HCs other than soot and NOx). Such bench scale tests are necessary to evaluate possible candidates for real application in as proposed single [DPF].
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Some supplementary results and conclusions obtained in significant studies regarding the performance of spinel structures for CO oxidation, HC oxidation, and soot oxidation are summarized in Table 2, 3 and 4, respectively.
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Table 3 Summary of the catalytic performances reported over various spinel catalysts for CO oxidation Catalyst synthesis
Reaction Conclusions conditions OP: Co3O4; WHSV= Alkali metal doping on spinel impregnation: Na, 120000 mL g-1 h-1 total oxidation; doping reduced K and Li doped mobility of spinel by locking Co3O4 from desorption and promoting carbonate species formation
Ref. delayed [64] oxygen oxygen surface
1 vol. % CO, 10 vol. % O2 and He for balance to 100 vol. %, GHSV: 6000 h-1
Single oxide Co/SBA-15 most active [93] (T100= 140⁰ C): presence of only reactive Co3+ species on surface; SBA-15 stable after reaction tests
PSE-CVD: Co-Fe 1% CO, 10% O2 mixed oxides in Ar; TFR: 15 ml min-1; WHSV =45000 mLg-1cat h-1. Decomposition of 0.8% CO ,20% oxalates: Co-Mn O2 in Ar. mixed oxides
Co-Fe synergism; αFe2O3
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Hydrothermal RP: layered CuMn oxide (LCMO), granulated oxide (GCMO); RP: rod oxide (RCMO) Heating of Co 1.0 vol.% foilCo3O4 CO,16.0 vol.% nanowires air, 83.0 vol.% N2, TFR:100 ml min-1 WI: Cu-Mn-Cr 0.9-1.0% CO, mixed oxides 3.0-3.2% H2O, GHSV:10000 h-1
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Surfactant-assisted hydrothermal: CuCrxCo2-xO4(x=0 to 2)
Low temperature spinel synthesis, Co-Mn [59] synergism; Co2.3Mn0.7O4 most active and stable: SBET; similar SEM images of oxalates before and after decomposition 2% CO, 20% O2 CuCr1Co1O4>CuCr1Co1O4>CuCr2O4>Cu [69] balanced with Co2O4>CuCr1.5Co0.5O4; CuCr1Co1O4: Ar; TFR: 100 ml high TOF, low activation energy, high min-1;4 vol.% reusability and thermal stability steam 1000 ppm CO in Cu-Mn synergism: Cu2+-O2-- Mn4+ entity [74] humid air; formation; LCMO>RCMO>GCMO; TFR:100mLmin- LCMO: most active and stable as H2O, 1 ; GHSV=50000 SO2, CO2 and other VOCs resistant, high h-1 surface O2 species and O2 vacancies Ultrathin Co3O4 nanowires (3mm): T100= [46] 120⁰ C and no deactivation (60 h) due to high SBET and abundant reactive Co3+ cations on the exposed planes (011)
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SG: SBA-15; supported catalyst: Two solvent xCoMnxSBA-15 where x= moles
UHP- MxCo3-xO4 Diffuse (M=Co, Ni, Zn) reflectance nanoarrays infrared Fourier transform (DRIFT) study
Monolithically integrated spinel [96] nanoarrays exhibit tunable activity by selective cation occupancy and concentration, which lead to controlled adsorption-desorption behaviour and
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20% O2, 2% CO balanced with Ar ;TFR: 100 ml min-1 CP: Mg1-xCux 9333 ppm CO, Al2O4 (x = 0-1.0); 1900 ppm O2, Mg0.7M0.3Al2O4 8% CO2. (M= Mn, Co, and Ni) Nanocasting:MCo 1.0 vol.% CO, 100 mL 2O4 (M = Cu, Mn TFR: and Ni) min-1; SV: 120 000 mL h-1g-1
T100 = ~220⁰ C; spinel composition affected activity; increasing Cr content: CO and CH3OH conversion increase; optimized composition: Cu/(Mn + Cr) in 1:5 molar ratio and Mn/Cr molar ratio from 1:3 to 1:4 Synthesis parameters influenced activity; optimized parameters: 12 pH, 4 h ageing duration, 180⁰ C autoclave temperature and 500⁰ C calcination temperature activity spinel composition dependent: Cu-Mg highest synergism at x=3 in Mg1xCux Al2O4; cation substituting effect: Cu > Co > Mn > Ni; Mg0.7M0.3Al2O4 most active: high reducibility Structural stability owing to mesoporous structure formation during synthesis; CuCo2O4 > MnCo2O4 >NiCo2O4; NiCo2O4 poor performance in life test
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surfactant-assisted hydrothermal: CuCr2O4
[61]
[69]
[94]
[95]
CP: Co-Sm varied Co/Sm Ratios
in 1% CO, 21% O2, 78% N2, 4% O2, 95% N2
USP:CuxMn3xO4 with varied x
CP: xCuxMn2O4
Ni1-
1 vol% CO, 20 vol% O2, 79 vol% in He; TFR: 10 ml min1 ; GHSV: 6000 ml g-1h-1 5% CO, 5% O2 in N2
[71]
[97]
[67]
[81]
[98]
[47]
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CP: MCo2O4 0.5 g catalyst (M=Co, Mn, Cu, ;1.5% CO in air ; Fe and Ni) TFR: 100 mL min-1.
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DRIFT and quadrupolar mass spectrometry (QMS) study.
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Precipitation: Co3O4 powder
surface defect population Influence of calcination temperature and pretreatment environment on structureactivity: Cat300> Cat400> Cat500> Cat600 and oxidative> none> inert>reductive Co3O4: water poisoning effect at low temperatures, no significant effect on activation with O2; Co3O4 surface interaction with O2 originate activated surface O2 species Sm doping in Co/Sm<1 increased activity and Co/Mn>1 decreased activity; doping increases reactive Co3+ and active surface O2 concentration; Co0.95Sm0.10 most active: favourable co-existence of crystalline Co3O4 and amorphous SmCoO3; water vapour deactivation Cu-Mn synergism; activity dependent on synthesis parameters and spinel composition; Cu1Mn2O4-550 most active: poor crystalline spinel phase, surface defects, vacancies and reactive Cu (II) ions Cation substituting effect; spinel composition influence activity, Ni0.5Cu0.5Mn2O4 most active: highest Ni2+ ions on octahedral sites NiCo2O4>CoCo2O4>MnCo2O4>CuCo2O4 >FeCo2O4; small crystallites and high SBET favoured conversion
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CP: Fe-Co mixed 2% CO, 20% O2, oxide 1/5 ratio 10% N2 balanced; GHSV :60,000ml g-1 h-1
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Table 4 Summary of the catalytic performances reported over various spinel catalysts for HC (VOCs) oxidation. Catalysts synthesis Reaction conditions Conclusions Ref. CP: NiCo2O4, WI: K- SV: 5000 h-1. NiCo2O4, and AlNiCo2O4
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citric acid complex: Co- 800 ppm propane, Ce oxides 10% O2 in N2; TFR: 300 ml min-1; GHSV: 150000 h-1. SG: SBA-15; Co- 2000 ppm C3H8, 1000 Mn/SBA-15: two ppm C6H14 in air; solvent GHSV: 60000 h-1 GC, SRG, CC, SG- 1vol.% C3H8, 10 Co3O4 vol.% O2, 89 vol.%
Activity decreased on Al addition and increased on K addition in NiCo2O4; activity order: K-NiCo2O4> NiCo2O4> Al-NiCo2O4, SBET of NiCo2O4 increased on K+ doping favoring activity. CeO2 < Co3O4 < Co1Ce2Ox
[62]
[52]
[93]
[99]
N2; TFR: 100 mL min-1 Solvothermal 1000 ppm acetone, alcoholysis (SA): 20%O2/N2; TFR : 155 Hollow mesospheres ml min-1; WHSV: structures (HMS) 93,000 mL g-1 h-1 MCo2O4 (M = Co, Ni, Cu)
Co3O4 performance (T100= 240⁰ C)
Co3O4: nanosheets (NS), nanowires (NW), nanoclusters (NC) and coral-like microspheres
Hierarchical Co3O4 nanostructures in-situ [100] grown on 3D nickel foam; NC > NW > NS; Co3O4 NC most active: high surface adsorbed O2 species, structural defects and reactive Co3+, reducibility; high regenerability and stability High concentration of superficial [101] electrophilic O2 species and lattice distortion induced by prolonged grinding during synthesis favored activity
1000 ppm toluene balanced with air; TFR :100 ml min-1 GHSV:20,000 h-1
C2H5OH:O2:He = 1:20:79 and C4H8O2:O2:He = 1:20:79; TFR: 100 ml min-1 1000 ppm o-xylene, Cu-Ce-Mn synergism, optimized synthesis: TFR: 200mL min-1, 11.4 wt.% loading on cordierite, calcined at GHSV: 10,000h-1 500⁰ C for 3h. MnCeCu0.4/cordedite (T100= 340⁰ C) > MnCeCu0.4/monolith (T100= 370⁰ C) Impregnation: 1.0 vol.% toluene in calcination effect: CAT250
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Citrate method: Fe-Mn oxide with varied Fe: Mn ratio; mechanical mixture using Fritsch ball mill SG: Mn-Ce-M (M= Ni, Cu or Co) mixed oxide
Cation substituting effect: CuHMS < [52] NiHMS < CoHMS; CuCo2O4 HMS most active, stable, water resistant: high reactive Co3+ concentration, surface active O2, defective sites and reducibility; Mars-Van Krevelen (MVK) mechanism followed
[102]
[70]
[103]
[54]
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Alginate route: Cu-Mn 1000 ppm toluene in Environment friendly synthesis using [84] oxides with varied air; TFR: 100 ml min- natural polymer, Cu-Mn synergism, T100: 1 Cu/Mn ratios Cu1.5Mn1.5O4 (315⁰ C) > Mn3O4 (325⁰ C) > CuO (535⁰ C). Modified SA: hollow 1000 ppm acetone, T100: CuCo2O4(260⁰ C) > NiCo2O4 (220⁰ C) [52] mesoporous spherical 20% O2 / N2 and > CoCo2O4 (200⁰ C) ; Cation-substituting structures (HMS) WHSV= 93,000 mL effect; HMS: stable and water resistant MCo2O4 (M = Co, Ni, g-1 h-1 owing to stable spinel phase and robust Cu) structure
Table 5 Summary of the catalytic performances reported over various spinel catalysts for soot oxidation
Conclusions
K+ doping improves activity; K+ doping bulk more [63] effective than K+ surface doping; bulk doping T100= ~250⁰ C. K doping to CuFe2O4 promoted activity; No [104] promotional effect with the Li, Na, Cs, V and Pt; K and Cu-Fe spinel synergism at K (x= 0.05) concentration negligible role of cation coordination state in [57] activity, cation substituting effect; CuFe2O4 most active: highest selective N2 formation, lowest selectivity to N2O and intermediate Tig of soot; high SBET; primitive reaction mechanism followed
C/S ratio:80/20, 5%Cu/ZnAl2O4> ZnAl2O4> Zn0.95Cu0.05Al2O4; [105] Loose contact, 500 5%Cu/ ZnAl2O4 most active: high reducibility of ppm NOx, 5% O2, Cu catalyses the NO oxidation to NO2, high N2 balanced; TFR: selectivity towards CO2 150 ml min-1. Ag doping enhances activity; [68] MnCo2O4
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Catalyst/soot ratio: 10:1; Loose contact; 0.2% NO, 5% O2 with He.
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glycothermal followed by substitution and impregnation ZnAl2O4, Zn0.95Cu0.05Al2O4 and 5% Cu/ZnAl2 O4. SG: Mn1-x AgxCo2O4 (x= 0, 0.1, 0.2, 0.3, 0.4)
Zn>Cu>Co > Ni, ZnCo2O4 most active: high O2 [56] mobility and surface-active Co3+; activity inhibited in SO2 and steam
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Citric acid SG: ACo2O4 Catalyst (C)/soot (A = Co, Ni, Cu, Zn) (S) wt. ratio 95:5 and 66:33, loose contact; 10 vol.% O2 , TFR:300 mL min-1 WI: K+ doping, solid 5 % O2 in He; 3.3 state reaction: K+- % O2, 0.3 % NO spinel (Mn, Fe, Co) in He; TFR: 60 ml min-1 Citric acid aided (CIT)- 0.5% NO, 5% O2 in He CuFe2O4, ImpregnationCu1xAxFe2O4 (A: Li, Na, K, Cs), V and Pt CIT: ACr2O4 (A = Cu, C/S =0.33 g; 10% Mg, Co, Mn); O2 in He and 1% CoMn2O4 and AFe2O4 NO in He (A = Cu, Co, Ni)
Ref.
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Reaction conditions
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Catalysis synthesis
Without 3DOM MCr2O4<3DOM MCr2O4; [106] 3DOMMn <3DOM-Co<3DOM-Zn<3DOM-Ni, and BK-Ni
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Colloidal crystal template (CCT) and SG: MCr2O4 (M = Co, Ni, Zn, Mn) with 3DOM and without 3DOM SCS: AB2O4 (where A = Co and Mn, and B = Cr and Fe)
5.0 vol% O2, 10 vol% H2O, 2500 ppm NO, N2 balanced ; TFR :80 ml min-1, GHSV:6760 h-1 1000 ppm NO, 10 vol% O2 , balance He; tight contact
[66] SCS: CexCo1-xCr2O4 C/S: 1/9; loose Ce0.1Co0.9Cr2O4>Ce0.05Co0.95Cr2O4 (x= 0, 0.05, 0.1, 0.15) contact; 1000 ppm >Ce0.15Co0.85Cr2O4>CoCr2O4; Ce0.1Co0.9Cr2O4 (T90 of NO, 10 vol% = 446⁰ C); Substitution of Ce: favourable
O2 with N2; TFR: distortions in spinel structure and increased crystal 50 ml min-1. defects. 6. Conclusions This review validates that spinel catalysts have the ability to perform well in the oxidation of all the three pollutants (CO, HC and soot) of concern. A number of spinels including those based on cobalt, nickel, nickel, zinc, iron and several other mixed-metal oxide spinels have been designed for the catalytic oxidation of the concerned pollutants. It is observed that the appropriate
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compositions of cations and their positive synergism for oxidation reaction are desirable factors for the formulation of the most active catalysts. Besides composition, improvement in the morphological characteristics of the spinel could also enhance the spinel activity. Future development is expected to focus on multicomponent spinel oxide formulations having
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compositions tuned in such a way each metal element of the formulation can afford high
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oxidation activity. A positive synergism could be expected between the pollutants in such cases, thereby increasing the overall removal efficiency. The achievement of this synergism could be
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perfect research niche for developing noble metal free TWC.
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Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal
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relationships that could have appeared to influence the work reported in this paper.
References [1]
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