Synthesis of highly active Cobalt catalysts for low temperature CO oxidation

Chemical Data Collections 24 (2019) 100283

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Synthesis of highly active Cobalt catalysts for low temperature CO oxidation Subhashish Dey a,∗, Ganesh Chandra Dhal a, Devendra Mohan a, Ram Prasad b a b

Department of Civil Engineering, IIT (BHU), Varanasi, India Department of Chemical Engineering and Technology, IIT (BHU), Varanasi, India

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 18 January 2019 Revised 17 September 2019 Accepted 18 September 2019 Available online 21 September 2019 Keywords: CO oxidation Cobalt precursors Reactive calcination Low temperature

Cobalt-oxide catalysts has shown a huge potential for CO oxidation in a catalytic converter for their high thermal stability and tailoring flexibility. A novel route of reactive calcination (RC) of Co-salts (oxalate = O, oxalate synthesize by reactive grinding = ORG , carbonate basic = C, acetate = A, nitrate = N) for the preparation of highly active catalysts was studied. The route concerned feeding a low concentration of chemically reactive CO–Air mixture over the cobalt salts at a low temperature. The RC of the precursor produced cobalt species (Co3 O4 and CoO) in thermodynamic equilibrium, with the major nano-size Co3 O4 phase having a large surface area, while applying in a stagnant air at 400 °C resulted in a CoO phase of better crystallites with the poorer surface area. The amazing performances of novel catalysts over conventional ones (obtained by calcination of the precursors in stagnant air and flowing air) in CO oxidation was associated with the presence of Co3 O4 and its unusual morphology as evidenced by XRD, SEM-EDX, XPS and FTIR characterization. The catalysts obtained by RC of various precursors showed activities for CO oxidation in the following order: Cat-ORGr >Cat-Or >Cat-Cr > Cat-Ar >Cat-Nr between (60–130 °C) while the traditional route of catalysts followed the same activity order but at the higher temperatures (75–170 °C). Further, the activity order of the catalysts obtained by various calcination conditions was as follows: RC> flowing-air>stagnant-air. The current research focused highly active Co3 O4 catalysts would be applied in wide range of reactions for commercial importance, as well as those associated with environmental cleanliness and production of clean energy sources etc. © 2019 Elsevier B.V. All rights reserved.

Specifications Table

Subject area Compounds Data category Data acquisition format Data type

Catalysis, Inorganic Chemistry, Carbon monoxide, Reactive calcination, Computational Chemistry Co3 O4 , CoO Synthesis, Spectral data, Images, graphs, modification procedure Gas chromatograph data, XRD, XPS and SEM-EDX data Experimental and Theoretical analysis (continued on next page)

∗

Corresponding author. E-mail addresses: [email protected], [email protected] (S. Dey).

https://doi.org/10.1016/j.cdc.2019.100283 2405-8300/© 2019 Elsevier B.V. All rights reserved.

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S. Dey, G.C. Dhal and D. Mohan et al. / Chemical Data Collections 24 (2019) 100283

Procedure

Data accessibility

The activity of the (Co3 O4 , CoO) catalysts for air oxidation of CO was measured under the following reaction conditions: 100 mg of catalyst, feed consisting of a lean mixture of 2.5% CO in the air maintained at a total flow rate of 60ml.min−1 . The reaction temperature ranged from ambient to the value when 100% CO conversion was achieved at atmospheric pressure. The reactants and products were analyzed under the steady-state conditions for CO and CO2 with the help of an online gas chromatograph equipped with a merchandiser, a pore pack-Q column and FID detector. The catalytic activity was expressed by the conversion of CO calculated. Data is with this article

1. Rationale The Co3 O4 , a unique spinel-structure transition metal oxide finds various applications because of its excellent properties in different fields such as, catalysis [1], super-capacitors [2], micro-batteries [3], rechargeable lithium-ion batteries [4], electrode materials [4], gas sensors [5], electronic ceramics [6], solar energy absorbers [7], electrochromic devices [3], for glass and porcelain colorants [8], enamels [9], pigments [10], specific alloy [11], production of clean energy sources and conversion methods [12,13] etc. Co3 O4 as catalysts of environmental impact assessment for total conversion of CO [14], oxidation of hydrocarbons [15], oxidation of volatile organic compounds (VOCs) [16], NO and SO2 reduction [17,18], three-way catalytic conversion [19], diesel soot oxidation [20], phenol oxidation [6] etc. Co3 O4 as catalysts related to the production of clean energy sources include the manufacture of hydrogen by various processes [21], such as steam reforming of methanol and ethanol [22,23], the water–gas shift (WGS) reaction [12] and refinement of hydrogen by preferential oxidation of carbon monoxide (CO-PROX) [24], for fuel cell applications [25] etc. Co3 O4 as catalysts of commercial importance include oxidation [26] hydrogenation [27], oxidative dehydrogenation [28], hydrogenolysis of esters [29] etc. Recently, quite a few review papers on the applications, synthesis methods and reaction mechanism of Co3 O4 based catalysts have been published [26,30,31]. Various universities have accepted several PhD thesis [32,33], many research projects have been certified [34,35], several patents have been granted [12,36,37] and several studies have been conducted on the preparation methods and utilizations of Co3 O4 catalysts [31,32,38,39]. However, the scope of novel preparation methods of Co3 O4 catalysts still exists, and present work represents a unique route of reactive calcination of cobalt salts for creation of highly active catalysts for CO oxidation at ambient conditions. Carbon monoxide (CO) is a poisonous gas mainly to human beings and in general, to all life forms that respire. It is a colourless, odourless, tasteless and non-irritating gas, which makes it very difficult for humans to detect [40]. CO is a product of the partial oxidation of carbon-containing compounds. Huge amounts of CO are emitted in the world (≈1.09 billion tons in 20 0 0), mainly from transportation, industrial, power plants and domestic activities. The ambient temperature catalytic oxidation of CO is very important reaction, and it’s crucial in many applications such as in automotive and residential air cleaning technologies, gas masks for firefighters, CO detectors, mining application and selective oxidation of CO in reformer gas for fuel cell applications [41–43]. Commercial catalysts for CO oxidation in exhaust gas clean-up are mainly noble metals. However, they are active at a high temperature above 100 °C and susceptible to poisoning at a low temperature [44]. Further, noble metals are too expensive to be used widely. Therefore, more attention has been played to developing efficient, lowcost oxidation catalyst active and stable at low temperature. Since the standard work of Langmuir CO oxidation on platinum group catalysts, many researchers have investigated this reaction over cheaper materials [45,46,74–76]. Cobalt catalyst is considered as an attractive alternative to the noble metals because of its lower cost, abundant availability and higher activity at low temperature for total oxidation of CO [47]. Co3 O4 is the most active catalyst among transition metal oxides for CO oxidation and shows extraordinary activity and stability far below the ambient temperature under dry conditions [48]. Presence of moisture in the reactants feeds severely deactivates Co3 O4 catalyst, especially at low temperature, which regains activity for CO oxidation above 100 °C [49]. The Co3 O4 crystal has a spinel structure of general formula AB2 O4 and can be written as CoCo2 O4 . It has a mixed-valence oxide couples Co2+ /Co3+ with Co3+ cations occupies 16 octahedral sites, and Co2+ cations occupy eight tetrahedral sites, and O2− ions occupy 32 sites. The Co3+ is regarded as the active site for CO oxidation, whereas the Co2+ is almost inactive in this reaction [14,50]. CO oxidation is a structure-sensitive reaction, i.e. exposer of different crystal planes of Co3 O4 spinel shows different activity in CO oxidation [51]. For example in CO oxidation, the Co3 O4 nano-rods shows much higher activity and improved durability due to exposer of (110) planes rich in Co3+ sites than the conventional Co3 O4 nanoparticles having exposer of less active (001) and (111) planes containing inactive Co2+ sites [3,52]. Further, Liu et al. 2012 synthesized mesoporous Co3 O4 catalyst possessing large surface area of 124 m2 /g and narrow pore size distribution centered at 3.8 nm by reactive grinding a mixture of 2CoCO3 ·3Co(OH)2 ·H2 O, H2 C2 O4 ·2H2 O and polyethene glycols, followed by calcination, which improved total CO oxidation at −90 °C [52,53]. In addition, Yu et al. showed that in situ pretreatment of Co3 O4 in dry air, N2 or CO/air at 150 °C led to the formation of surface oxygen vacancies which oxidized total CO at −80 °C [54,77,78]. Thus, the steps involved in the preparation of Co3 O4 such as the choice of precursor, its calcination, followed by in situ activation in various environments have important roles in the performance of final catalyst was formed [55,79,80]. Generally, agglomerated particles are produced as a result of calcination of precursor above its decomposition temperature followed by activation steps. The recent work of our laboratory demonstrated that the two-step processes of the calcination of the precursors and ensuing activation could be reduced to a single step of reactive calcination (RC) in a reactive CO-air mixture at low temperature ∼160 °C [56,57]. The RC process not only minimized a process step but also produced catalysts with improved performances for CO oxidation. The present investigation also proves that the superiority of RC route

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Table 1 Cobalt precursors and nomenclature of the corresponding catalysts formed by RC. Catalyst resulted under different calcination strategies∗ Cobalt precursor Co(NO3 )2 ·3H2 O Co(CH3 COO)2 .H2 O CoCO3 . H2 O PPT:CoC2 O4 ·H2 O RG: CoC2 O4 ·H2 O

Stagnant air

Flowing air

RC

Cat-Na Cat-Aa

Cat-Nfa Cat-Afa

Cat-Nr Cat-Ar

Cat-Ca Cat-Oa Cat-ORGa

Cat-Cfa Cat-Ofa Cat-ORGfa

Cat-Cr Cat-Or Cat-ORGr

∗ Calcination strategies are represented as suffixes; a, fa and r referring stagnant air, flowing air and RC respectively.

of Co3 O4 production from its various precursors to their counterpart usually prepared catalysts (by calcination of the precursors in stagnant air and flowing air) for CO oxidation [58]. To the best of our knowledge, there is no published literature on the RC route of preparation of Co3 O4 catalysts. The effect of preparation conditions including metal ions concentration, aging time, pH, drying temperature and calcination conditions is highly effective on the activity of resulting catalyst. The current seminal investigation of facile synthesis of highly active Co3 O4 catalysts would be useful for a broad range of reactions of commercial importance, as well as those associated to environmental cleanliness, production of clean energy sources, energy conversion methods, etc. [59,60,81–84]. 2. Procedure All the cobalt salts precursors used in the present study were of analytical grade or synthesized in the laboratory using AR grade chemicals to make their purity. The AR grade cobalt oxalate was not commercially available; therefore, it was synthesized by the precipitation of cobalt nitrate with oxalic acid. Oxalic acid was taken in excess (5%) than required stoichiometrically. The precipitate formed was filtered, washed free of anions and dried in an oven at 105 °C overnight [63–65]. A cobalt oxalate precursor was also prepared by soft reactive grinding method followed by mixing 2CoCO3 ·3Co(OH)2 ·H2 O with (20%) excess of oxalic acid in a zirconia jar. Then PEG-20 0 0 0 was added into the mixture with weight ratio 1:10 [57]. The mixture was ground by planetary mill having zirconia balls of 10 mm diameter, and the weight ratio of balls in the mixture was 10:1, at a speed of 600 rpm for 2 hr. The ground material, resulting cobalt oxalate was washed with distilled water and ethyl alcohol and then dried at 100 °C [66,67]. All the cobalt precursors were calcined in three different ways; first following the traditional method of calcination in stagnant air at 400 °C just above the decomposition temperatures of the precursors for 2 h in a muffle furnace, second in situ calcination in flowing air at a rate of 32.5 ml.min−1 at 400 °C for 2 h [68–70,85,86]. The third-way calcination was carried out under in situ reactive calcination (RC) as described below. 2.1. Reactive calcination of the precursors The reactive calcination of the cobalt salt precursors was carried out in situ in a downflow bench-scale tubular reactor with a definite amount of precursor was diluted with α -alumina to make a total volume of 1 ml at atmospheric pressure. The amount of each precursor was taken in the reactor is equivalent to 100 mg weight of the Co3 O4 catalyst. The reactor was placed vertically in a split open microprocessor-based temperature-controlled furnace. The heating rate of the bed was 2 °C min−1 with temperature control of ±0.5 °C. Reactive calcination of the precursors was carried out by the introduction of low concentration of chemically reactive CO–Air mixture (4.6% CO) at a total flow rate of 32.5 ml.min−1 over the hot precursors. The digital gas flow meters were used to measure the flow rates of CO and air to feed the mixture (dried and CO2 free) in the required proportion at the reactor. A thermocouple placed in the thermo-well of the reactor in contact with the precursor bed calculated the temperature of the bed [71]. The temperature of the bed was increased from room temperature to 160 °C where CO conversion has started. This temperature was maintained for a defined period and CO concentration was measured in the exit stream of the reactor at regular intervals until 100% CO conversion was achieved. After achieving total CO oxidation, the resultant catalyst was annealed for half an hour at the same temperature then the temperature was increased up to 400 °C and upheld for an hour followed by cooling to room temperature in the same environment. For comparative studies, the catalysts were prepared by usual calcination of the various precursors in the air at 400 °C for an hour [61,62]. The nomenclature of the resulting unsupported catalysts thus formed was given by the first capital letter of the corresponding cobalt precursor used and the suffixes ‘r’, ‘a’ and ‘fa’ represent whether there were obtained by calcination in air, flowing air or by RC respectively, as presented in Table 1. 2.2. Characterization The catalysts with the maximum activity synthesized via the novel route and traditional way were characterized to mark the differences in the properties of catalysts. The X-ray measurement of the catalyst was done by Rigaku D/MAX-2400

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Fig. 1. Schematic diagram of experimental set up.

diffractometer by using Cu Kα radiation (100 mA, 40 kV). The crystallite size (d) of the catalyst was calculated from the Scherrer Eq.(1).

d=

0.89λ β cosθ

(1)

˚ Fourier Where d = the mean diameter of crystallite, the Scherrer constant = 0.89, the X-ray wavelength (λ = 1.54056 A). transforms spectra (FTIR) of the prepared catalysts were recorded in the range of 40 0–40 0 0cm−1 on a Shimadzu 840 0 FTIR spectrometer with KBr pellets at room temperature. Scanning electron micrographs (SEM) and (SEM-EDX) were recorded on the Zeiss EVO 18 instrument. The accelerating voltage was used 15 kV and magnification of 10 0 0X were applied. Xray photoelectron spectroscopy (XPS) was used to monitor the chemical states and surface compositions of the constituent elements. It was performed on an Amicus spectrometer equipped with Mg Kα X-ray radiation. For typical analysis, the source was operated at a voltage of 15 kV and current of 12 mA. The binding energy of the element was calibrated by the setting of main C(1 s) line of adventitious impurities at 284.7 eV.

2.3. Activity measurement The activity of catalysts for air oxidation of CO was measured under the following reaction conditions: 100 mg Co3 O4 catalyst, feed consisting of a lean mixture of 2.5% CO in air maintained at a total flow rate of 60 ml.min−1 . The reaction temperature ranged from ambient to the value when 100% CO conversion was achieved at atmospheric pressure. The air feed was made free of moisture and CO2 by passing through it CaO and KOH pellet drying towers. The catalytic experiments were carried out under steady-state conditions. The reactants and products were analyzed under steady-state conditions for CO and CO2 with the help of an online gas chromatograph equipped with a methaniser, a porapack-Q column, and FID detector, as shown in Fig. 1. The catalytic activity was expressed by the conversion of CO calculated by the following Eq. (2):

(XCO ) = [(CCO )in − (CCO )out ]/[(CCO )in ] =

[(ACO )in − (ACO )out ] [(ACO )in ]

(2)

The conversion of CO at any instant was calculated on the basis of values of the concentration of CO (CCO )in in the feed and the concentration of CO2 (CCO )out in the product stream by the following Eq. (2). The concentration of CO was proportional to the area of chromatogram ACO (ACO )in at any instant was proportional to the area of chromatogram of CO2 (ACO )out formed. Where the change in the concentration of CO due to oxidation at any instant [(CCO )in − (CCO )out ] was proportional to the area of chromatogram of CO2 formed at that instant [(ACO )in − (ACO )out ] and the concentration of CO in the inlet stream (CCO )in was proportional to the area of chromatogram of CO2 formed (ACO )out by the oxidation of CO.

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Fig. 2. XRD analysis of different Co-oxide catalysts.

3. Data, value and validation 3.1. Catalyst characterization The characterization of Co3 O4 catalyst by various techniques represents phase identification, material structure analysis and binding energy identification etc. The following techniques did characterization of all cobalt oxide catalysts prepared by various precursors in reactive calcination conditions and activity of the catalyst for CO oxidation was discussed below.

3.1.1. X-ray diffraction (XRD) X-ray diffraction (XRD) measurements of the catalysts were carried out to identify the phases and crystallite size present in the catalysts. The structure, phase, crystal orientation, lattice parameters and crystallite size get from the XRD analysis of various catalysts. XRD pattern of the Cat-Nr catalyst has shown that the diffraction peak at 2θ of 24.54 corresponds to its lattice plane (101), (121), (111), (122), (113) and (132) hexagonal primitive cubic Co3 O4 form (PDF-74–1032 JCPDS file) and crystallite size of the catalyst was 13.28 nm. XRD pattern of the Cat-Ar catalyst has shown that the diffraction peak at 2θ of 36.74 corresponds to its lattice plane (111), (100), (122), (101) and (012) hexagonal primitive CoO form (PDF-72–1034 JCPDS file) and crystallite size of the catalyst was 10.52 nm. XRD pattern of the Cat-Cr catalyst has shown that the diffraction peak at 2θ of 28.64 corresponds to its lattice plane (121), (111), (110), (012), (113) and (001) hexagonal primitive Co3 O4 form (PDF-72–1042 JCPDS file) and crystallite size of the catalyst was 8.56 nm. In Cat-Or catalyst has shown that the diffraction peak at 2θ of 28.64 corresponds to its lattice plane (113), (131), (122), (112), (001) and (131) hexagonal primitive Co3 O4 form (PDF-72–1054 JCPDS file) and crystallite size of the catalyst was 7.42 nm. In Cat-ORGr catalyst diffraction peak at 2θ was 37.60 corresponding to its lattice plane was (131), (132), (111), (101) and (011) was hexagonal primitive Co3 O4 form and crystallite size of the catalyst was 5.70 nm. The Co3 O4 nano-sheet shows much higher activity and improved durability due to the expose of {110} planes rich in Co3+ sites than the conventional Co3 O4 nanoparticles having much exposed of less active {001} and {111} planes containing inactive Co2+ sites. The CO oxidation is a structure-sensitive reaction, i.e. expose of different crystal planes of Co3 O4 spinel shows that the different activity in CO oxidation. In the Cat-CRGfa catalyst diffraction peak at 2θ was 37.60 corresponding to its lattice plane was (131), (122), (001), (111) and (013) was hexagonal primitive Co3 O4 form and crystallite size of the catalyst was 6.25 nm. XRD study of the cobalt catalysts was shown in Fig. 2 and discussed in Table 2. The Co3 O4 with the octahedrally coordinated Co2+ had unexpectedly high activity due to the simple surface oxidation of Co2+ to Co3+ . As Co3+ surface enrichment and Co3+ /Co2+ ratio have been affecting the specific reaction rates and stabilities for many catalytic applications. The

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Structure

Cat-Nr Cat-Ar Cat-Cr Cat-Or Cat-ORGr Cat-CRGfa

Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal

primitive primitive primitive primitive primitive primitive

Phase

Crystallite size

Co3 O4 CoO Co3 O4 Co3 O4 Co3 O4 Co3 O4

13.28 nm 10.52 nm 8.56 nm 7.42 nm 5.70 nm 6.25 nm

Fig. 3. FTIR spectra of the catalysts.

octahedrally coordinated Co2+ is located in more open framework site than the tetrahedrally coordinated Co2+ . The valence state of cobalt species is more important for catalytic activity [72]. The crystallite size of the particles present in catalyst surfaces obtained by RC conditions was as follows: Cat-Nr > Cat-Ar > Cat-Cr > Cat-Or > Cat-CRGfa > Cat-ORGr . The relative intensities of the XRD peak of Cat-ORGr were very low as compared to other catalysts. The crystallites size of Cat-Nr was almost higher, in comparison to other catalysts. The crystallite size of the particles present in catalyst was analysis by the XRD technique was matched with the particle size calculated by the SEM characterization. Calcination of cobalt precursors in the air caused highly agglomeration of Co3 O4 particles due to sintering, whereas reactive calcination using a gas flow containing a small concentration of CO resulted in relatively smaller crystallite size. 3.1.2. Fourier transforms IR spectroscopy (FTIR) The FTIR transmission spectra of the catalysts was shown in Fig. 3. In the invested region (40 0 0–50 0 cm−1 ) to obtain the whole absorption spectra peaks to indicates that the presence of different elemental groups in the catalysts. The two strong bands at 1320 cm−1 and 1510 cm−1 in the cobalt oxide catalyst indicates that the presence of stretching vibrations of the metal-oxygen bond and confirm that the presence of Co3 O4 spinel phases. The IR band (3340 cm−1 and 3180 cm−1 ) has shown that the presence of O–H group and (1140 cm−1 and 540 cm−1 ) shown CoO group vibration mode for Cat-Ar , which was due to the Co–O vibration stretching and weak band 1510 cm−1 has shown CO3 2- group. The infrared bands observed at 2340 cm−1 were assigned to -NH stretching vibration group and weak band 1480 cm−1 has shown that the presence of COO group. The infrared bands observed at 680 cm−1 were assigned to C = O. The FTIR peaks has shown that the presence of CoO group and O–H group impurities at 1480 cm−1 and 3340 cm−1 decreases in the following order: Cat-Nr > Cat-Ar > CoOr > Cat-Cr > Cat-CRGfa > Cat-CRGr . The Cat-CRGr catalyst was highly pure as compared to other catalysts. The peak of Co-O bond was not observed that the dispersion of Co in the catalysts, which was in a good harmony of XRD data. Using FTIR technique, confirm that the carbonates and other species associated with catalyst exposed to a mixture containing CO in the

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Fig. 4. SEM image of (A) Cat-Nr, (B) Cat-Ar, (C) Cat-Or, (D) Cat-Cr, (E) Cat-ORGr and (F) Cat-ORGfa .

air at 300 °C. The lowest adsorption spectrum of Cat-Nr catalyst has shown that the CO adsorption ability of Cat-Nr catalyst was lower than the other catalysts. 3.1.3. Scanning electron microscopy analysis The SEM micrographs analysis has shown that the morphological images of cobalt catalysts. It represents that the microstructure of cobalt oxide catalyst in RC conditions. The SEM micrographs for various cobalt oxide catalysts prepared with different cobalt precursors show different granule sizes of 20–110 μm. The size range of all granular particles was varied between (0.5 and 0.78 μm) calculated by “Image J software” as shown in Fig. 4. As seen in SEM micrograph, the particles size of catalysts were comprised of more coarse, coarse, fine, more fine and finest size grains resulted by RC of Cat-Nr , Cat-Ar , Cat-Or , Cat-Cr and Cat-ORGr precursors respectively. The particles of CatORGr were smaller, less agglomerated and uniform in nature. The particle size of Cat-CRGfa was coarser as compared to Cat-ORGr particle. The particle size of catalysts produced by RC of Cat-Nr was very large and agglomerated than the catalyst produced by RC of cobalt acetate and cobalt oxalate. It was also observed that the cobalt oxide to well dispersed in the catalyst granule. The different cobalt catalysts have resulted in different crystallite size of cobalt oxide species was formed. The size orders of different catalysts were as follows: Cat-Nr > Cat-Ar >Co-Or >Cat-Cr >Cat-CRGfa >Cat-CRGr . The Cat-CRGr catalyst particle size was the smallest as compared to other catalysts. Decreasing the particles size was a key factor to improve the catalytic activity of CO oxidation. The smaller size particles present in Cat-CRGr catalyst has more efficient for CO oxidation on their surfaces, it results improved their catalytic activity. The shape and homogeneity of particles have been changed with changing the various compositions present in the catalysts. An amorphous structure was detected in all catalysts sample after calcination at a higher temperature. 3.1.4. Elemental analysis After the SEM micrographs were taken, the elemental mapping was performed to determine the elemental concentration distribution of the catalyst granules by using Isis 300 software. The SEM-EDX analysis was performing on different crosssectioned symbols of catalyst granules to determine the concentration of cobalt at different locations on the cobalt oxide catalyst granular surfaces, and also observed that the high concentration of cobalt placed on the exterior surface of CatORGr catalyst. It was clear from the results of SEM-EDX analysis has shown that all catalyst samples were pure due to the presence of Co and O peaks only. The SEM-EDX analysis also confirms that the mostly cobalt located on the pores of cobalt oxide catalysts as shown in the Fig. 5. The atomic and weight percentage of different elemental groups’ presence in the catalyst surfaces represented in Table 3. The cobalt surfaces reconstruction behavior during the prolonged exposure of CO gas or aggregation of smaller nanoparticles into the bigger size particles. The atomic percentage of cobalt was also higher than oxygen in Cat-Ar , Cat-Or , Cat-ORGr , Cat-ORGfa and Cat-Cr catalyst. The atomic percentage of cobalt in Cat-Nr , Cat-Ar , Cat-Or , Cat-Cr , Cat-ORGfa and Cat-ORGr catalyst was 28.89%, 58.87%, 71.23%, 65.20%, 79.40% and 85.40% respectively and weight percentage of cobalt in Cat-Nr , Cat-Ar , Cat-Or , Cat-Cr , Cat-ORGfa and CatORGr catalyst was 27.65%, 55.65%, 73.35%, 78.45%, 79.65% and 87.90% respectively. The atomic percentage of oxygen in Cat-Nr , Cat-Ar , Cat-Or , Cat-Cr , Cat-ORGfa and Cat-ORGr catalyst was 71.11%, 41.13%, 28.77%, 34.80%, 20.60% and 14.60% respectively and weight percentage of oxygen in Cat-Nr , Cat-Ar , Cat-Or , Cat-Cr , Cat-ORGfa and Cat-ORGr catalyst was 72.35%, 44.35%, 26.65%, 21.55%, 20.35% and 12.10% respectively. The oxygen present in the Cat-ORGr catalyst was lower as compared to other catalysts. It shows that there will be an oxygen deficiency in the Cat-ORGr catalyst

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Fig. 5. SEM-EDX image of (A) Cat-Nr, (B) Cat-Ar, (C) Cat-Or, (D) Cat-Cr (E) Cat-ORGr and (F) Cat-ORGfa .

Table 3 The atomic and weight abundance of elements measured over different catalysts.

Catalyst Cat-Nr Cat-Ar Cat-Or Cat-Cr Cat-ORGr Cat-ORGfa

Elements atomic (%)

Elements Weight (%)

O

Co

O

Co

71.11 41.13 28.77 34.80 14.60 20.60

28.89 58.87 71.23 65.20 85.40 79.40

72.35 44.35 26.65 21.55 12.10 20.35

27.65 55.65 73.35 78.45 87.90 79.65

so that it creates a high density of active sites as a result of improving the activity of catalyst. Therefore the Cat-ORGr catalyst has shown that the best catalytic performance for CO oxidation at the low temperature. Based on the SEM-EDX analysis, it can be concluded that the redispersion of agglomerated Co particles into the Co3 O4 catalyst can be realized in a flow of CO as a partial oxidation stream. 3.1.5. X-ray photoelectron spectroscopy (XPS) With the help of XPS analysis, we can get that the binding energy, chemical state and surface valence state of cobalt oxide catalysts. The peak at 287.3 eV BE was attributed to C = O. The carbon peak located at 284.6 eV was applied to calculate the BE displacement caused by differential charging phenomena. The higher binding energy was preferably for CO oxidation. In Fig. 6 display the Co2p XPS spectra obtained from the different types of cobalt precursors after RC conditions. In Table 4, observe that the Co ions in the Cat-Ar catalyst was a CoO phase and the Co ions in the (Cat-Nr , Cat-Or , Cat-Cr and Cat-ORGr ) was the octahedral symmetry as Co3 O4 form. The full-width half maximum of the Co2p3/2 peak was found to be higher (4.3 eV) in the octahedral form. The Co2p core-level spectrum as shown in Fig. 6 of Cat-ORGr shows that three peaks located at 782.62 eV, 784.15 eV and 797.52 eV, which correspond to Co(2p3/2 ) and Co(2p1/2 ), respectively, with a splitting of ∼15.2 eV. These values match well with the data reported for Co2p in Co3 O4 .The binding energy of Co2p in CoOx catalyst at Cat-Nr , Cat-Ar , Cat-Cr , Cat-ORGr , Cat-ORGfa and Cat-Or catalysts were (782.45 eV and 797.42 eV), (782.52 eV, 783.85 eV and 797.50 eV), (782.58 eV, 786.25 eV and 797.52 eV), (782.62 eV, 784.15 eV and 797.52 eV), (783.45 eV, 784.22 eV and 797.60 eV) and (780.15 eV, 786.32 eV and 797.60 eV) respectively. From Fig. 6, it was clear that the binding energy value of Co(2p) in Cat-ORGr catalyst was higher as compared to Cat-ORGfa catalyst. The prominent peak of Cat-ORGr (Co2p3/2 ) level was deconvoluted into two peaks centered at 783.45eVand 798.92 eV respectively. Although, it can be proposed that the high binding energy was preferably for CO

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Fig. 6. XPS peaks of Co(2p) in catalysts.

Table 4 The chemical state and binding energy of cobalt oxide catalysts in XPS analysis.

Sample Cat-Nr Cat-Ar Cat–Or Cat–Cr Cat–ORGr Cat–ORGfa

Chemical state of Elements

Binding energy of Elements

Co

O

Co

O

Co3 O4 CoO Co3 O4 Co3 O4 Co3 O4 Co3 O4

C=O C-O C-O C-O C-O C-O

797.42eV 797.50eV 797.60eV 797.52eV 797.52eV 797.60eV

531.85eV 531.83eV 531.80eV 531.80eV 531.45eV 531.45eV

oxidation and its match with the results and discussions. The binding energy of O(1 s) was displayed in Fig. 7. In general, there were two different types of oxygen present in the catalysts with the binding energy (531.2eV–531.85 eV) and (531.3eV– 532.82 eV) which could be ascribed as the chemisorbed oxygen (denoted as Oa , such as O2 2− , O− , OH− , CO3 2− , etc.) and lattice oxygen (denoted as Ol , such as O2− ), respectively. The O(1 s) core level spectrum of Cat-ORGr shows a broad Gaussian peak that was deconvoluted at the 532.806 eV due to the oxygen in the Co3 O4 crystal lattice, which corresponds to the O–Co bond, with the lattice oxygen species (O2− , O− ), which reflect the redox behavior of Co oxide. In the present study, the oxygen with the binding energy of 531.80 eV was the main form and could be ascribed to the chemisorbed oxygen (Oa ). The huge amount of surface chemisorbed oxygen (most active oxygen) was preferable for improving the catalytic activity of CO oxidation. The one CO molecule adsorb on one cobalt sites; therefore, the bridged bond accounted for highest in cobalt oxalate precursor prepared catalyst. The presence of oxygen in the Co3 O4 crystal lattice, which corresponds to the O–Co bond, whereas the peak of Cat-ORGr at the binding energy of 531.45 eV corresponds to the lattice oxygen species (O2− , O− ), which reflect the redox behavior of Co-oxide. The content order of Oa /(Oa +Ol ) ratio was shown as following: Cat-Nr < Cat-Ar < Co-Or < Cat-Cr
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Fig. 7. XPS peaks of O(1 s) in the catalysts. Table 5 Textural property of catalysts. Catalyst

Surface Area (m2 /g)

Pore Volume (cm3 /g)

˚ Ave. Pore Size (A)

Cat–ORGr Cat-Or Cat-Cr Cat-Ar Cat-Nr

121.74 114.28 102.46 95.45 87.38

0.734 0.702 0.674 0.652 0.638

78.45 72.54 67.38 63.45 58.76

affect the catalytic activity for CO oxidation. The isotherm gave useful information on the mesopores structure through its hysteresis loop. The textural property of different cobalt oxide catalysts was discussed in Table 5. The specific surface area was measured by BET analysis and also followed the SEM and XRD results. The surface area of Cat-ORGa , Cat-Oa , Cat-Ca , CatAa and Cat-Na catalysts are 121.74, 114.28, 102.46, 95.45 and 87.38m2 /g respectively. The surface area, pore volume and pore size of Cat-ORGa catalyst were much superior to the other catalysts. Order of surface area of the catalysts was as follows: Cat-ORGa > Cat-Oa > Cat-Ca > Cat-Aa > Cat-Na . The increase in pore volume and pore size of catalyst is more active for CO oxidation at a low temperature. A large number of more pores present in catalysts means a higher number of CO molecules captured on the catalyst surfaces; therefore, it has to shows that the better catalytic activity. Changes occurring during the catalyst life are almost in the direction of catalyst surface area loss. The effect of catalyst surface area on the catalytic behavior because it has been impossible to control the catalyst surface area by using a conventional method for catalyst preparation. The mesopores cobalt oxide catalysts synthesized by reactive calcination conditions have displayed excellent activity in CO oxidation reactions. 3.2. Catalyst performance and activity measurement Activity measurment of the catalyst was carried out to evaluate the effectiveness of cobalt oxide catalysts as a function of temperature. It was measured in different calcination conditions like stagnant air, flowing air and reactive calcination. The activity was increased with the increasing of temperature from room temperature to a certain high temperature for full conversion of CO. The light-off characteristics were used to evaluate the activity of resulting catalysts with the increasing of temperature. The characteristic temperature T10 , T50 and T100 correspond to the initiation of oxidation, half conversion and full conversion of CO respectively. 3.2.1. Reactive calcination The RC of the cobalt precursors was carried out by passing an Air-CO mixture over the precursors at 160 °C temperature. In the start, very intentional exothermic oxidation of CO over the precursor’s crystallites started causing a small rise in the

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Fig. 8. Conversion of CO over various catalysts with time. Table 6 Activity test of various cobalt precursors obtained by RC conditions with time. Time (CO Conversion%) Catalyst Cat-Nr Cat-Ar Cat-Or Cat-Cr Cat-ORGr

10 min

20 min

30 min

40 min

50 min

60 min

13.75 16.70 21.70 17.85 25.8

29.80 49.90 42.40 34.75 72.45

45.65 82.45 65.90 54.79 100

68.30 100 90.70 75.65

89.65

100

100 96.70

100

local temperature, resulting in the decomposition of precursor also. The temperature was maintained for a definite period of time which 100% CO conversion was achieved. The RC phenomena of the precursors were shown in Fig. 8 and discussed in Table 5. The rate of CO oxidation was increases with time and flattened at the end. This may be due to the synergistic effect of synchronized diverse phenomenon of exothermic oxidation–decomposition– redox-surface-reactions resulting in the production of various cobalt oxide phases (CoO and Co3 O4 ) in thermodynamic equilibrium. The characterization by various techniques (SEM-EDX, XRD, XPS and FTIR) of the catalysts formed by RC shown that the presence of major Co3 O4 with minor CoO phases. It was apparent from Fig. 8 and Table 6 that the RC period i.e. time from the initiation to 100% CO conversion, varied for the different precursors. During the RC period at 160 °C, the percentage of CO conversion indicates that the degree of calcination of the precursor. The overall rate of decomposition for carbonate-RG precursor was the fastest resulted in the shortest time of 30 min to reach the complete calcination, while the rate for nitrate precursor was slowest, taking the longest period of 55 min. The reactive calcination onset temp. around 160 °C varied a little for the precursors studied in the current analysis. 3.2.2. Choice of precursor The activity tests were carried out to evaluate the effectiveness of five different cobalt oxide catalysts as a function of temperature for the oxidation of CO. The activity test was done in different calcination conditions like reactive calcination (RC), flowing air calcination (FAC) and stagnant air calcination (SAC) conditions into the laboratory. The catalysts obtained after SAC of the cobalt precursors were examined for CO oxidation at various temperatures, as shown in the Fig. 9. The results confirm that the catalysts prepared by SAC were more active for total oxidation of CO at the temperatures ranging between 100 and 170 °C depending upon the precursor used. It was apparent from the Fig. 9 that the oxidation of CO was initiated at 41 °C, 30 °C, 25 °C, 32 °C and below ambient temperature over Cat-Na , Cat-Aa , Cat-Oa , Cat-Ca and Cat-ORGa respectively. The full conversion of CO was achieved at 100 °C over Cat-ORGa catalyst, which was lowered by 70 °C, 40 °C, 20 °C and 30 °C over than that of Cat-Na , Cat-Aa , Cat-Oa and Cat-Ca respectively. Among all the catalysts, the Cat-CRGa prepared from cobalt carbonate by reactive grinding showed that the best activity with light-off temperatures T50 and T100 at 30 °C and 100 °C, respectively. Thus, the cobalt oxalate (RG) precursor was the best choice of precursor for facile synthesis of highly active catalysts for CO oxidation. The light-off characteristics for CO oxidation in stagnant air calcination conditions were discussed in Table 7. It was clear from Table 7 and Fig. 9 that the

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Fig. 9. CO conversion over various catalysts produced by stagnant air calcination.

Table 7 Light-off characteristics of catalysts obtained by SAC of various cobalt precursors. Characteristic Temperature (°C)

Cat-Na

Cat-Aa

Cat-Oa

Cat-Ca

Cat-ORGa

T10 T50 T100

41 90 170

30 80 140

25 65 120

32 70 130

< ambient 30 100

Table 8 Light-off characteristics of catalysts obtained by FAC of various cobalt precursors. Characteristic temp.

Cat-Nfa

Cat-Afa

Cat-Ofa

Cat-Cfa

Cat-ORGfa

T10 (°C) T50 (°C) T100 (°C)

30 80 140

30 70 130

22 45 100

22 55 110

22 25 75

Cat-ORGa has shown that the best activity than the other catalysts. From Fig. 9 confirmed that the activity of Cat-Ca lies in between the activities of Cat-Aa and Cat-Oa . Thus, the order of activity of different cobalt oxide catalysts for CO oxidation obtained by the stagnant air calcination was as follows: Cat-ORGa > Cat-Oa > Cat-Ca > Cat-Aa > Cat-Na . The Co ions naturally present on the surface of Co3 O4 {111} and Co3 O4 {100} thin films or artificially created on the CoO {111} surface is extremely important for chemical properties of the catalyst surface. The Co3 O4 has an ideal spinel structure in which one-eighth of the tetrahedral sites are occupied by the Co2+ cations while half of the octahedral sites are occupied by the Co3+ cations. In flowing air calcination conditions, the CO oxidation was initiated at 30 °C, 30 °C, 22 °C, 22 °C and 22 °C over Cat-Nfa , Cat-Afa , Cat-Ofa , Cat-Cfa and Cat-ORGfa catalysts respectively. The total oxidation of CO was achieved at 75 °C over Cat-ORGfa , which was lowered by 65 °C, 55 °C, 25 °C and 35 °C over than that of Cat-Nfa , Cat-Afa , Cat-Ofa and Cat-Cfa respectively. The light-off characteristics for CO oxidation were discussed in Table 8. It was clear from Fig. 10 and Table 8 that the Cat-ORGfa has shown the best activity than the other catalysts. As the calcination conditions of stagnant air and flowing air affects the activity of resulting catalysts. In comparison between stagnant air and flowing air calcination conditions, the flowing air calcination produced more active cobalt oxide catalysts for CO oxidation at low temperature as compared to stagnant air calcination conditions. From Fig. 10 observed that the activity of Cat-ORGfa catalyst has shown that the best as compared to other catalysts. The activity order of catalysts obtained by various calcination conditions was as follows: Flowing-air> Stagnant-air. All the cobalt oxide catalysts obtained from different cobalt precursors showed similar trends in Fig. 10 produced by stagnant air and flowing air calcination conditions. The order of activity of the various cobalt oxide catalysts obtained by flowing air calcination conditions was as follows: Cat-ORGfa > Cat-Ofa > Cat-Cfa > Cat-Afa > Cat-Nfa .

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Fig. 10. CO conversion over various catalysts produced by flowing air calcination.

Fig. 11. CO conversion over catalysts produced by RC of Co-precursors in a 2.5% CO in air.

3.2.3. Comparison of reactive calcination with traditional calcination Fig. 11 shows a comparison of the CO oxidation catalytic activities of various cobalt oxide catalysts produced by reactive calcination conditions. The complete conversion of CO over Cat-Nr was observed at 70 °C higher than the Cat-ORGr catalyst. In reactive calcination conditions the CO oxidation was initiated at 30 °C, 28 °C, 22 °C, 25 °C and 22 °C over Cat-Nr , CatAr , Cat-Or , Cat-Cr and Cat-ORGr catalysts respectively. The full conversion of CO was achieved at 60 °C for Cat-ORGr , which was lowered by 70 °C, 50 °C, 20 °C and 30 °C over than that of Cat-Nr , Cat-Ar , Cat-Or and Cat-Cr respectively. Thus, it was observed from Fig. 11 that the catalysts produced following the novel route of RC of the precursors were more active for CO oxidation than the ones prepared by the traditional method of calcination of the same precursors in the air. It was clear from the Table 9 and Fig. 11 that the Cat-ORGr has shown that the best activity than the other catalysts. The total oxidation of CO has occurred at 60 °C over Cat-ORGr , which was lowered by 20 °C and 70 °C over than that of Cat-ORGfa and Cat-ORGa , respectively. The activity order of catalysts obtained by various calcination conditions irrespective of the salts for the oxidation of CO in the decreasing sequence was by characterization, which was as follows: Reactive Calcination > Flowing-air >Stagnant-air. The reason for high activity of RC produced catalysts as compared to the catalysts obtained by traditional calcination conditions may be due to the combined effect of Co3 O4 phase with high surface area and smallest crystallite size. The order

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S. Dey, G.C. Dhal and D. Mohan et al. / Chemical Data Collections 24 (2019) 100283 Table 9 Light-off characteristics of catalysts obtained by RC of various cobalt precursors. Characteristic temp.

Cat-Nr

Cat-Ar

Cat-Or

Cat-Cr

Cat-ORGr

T10 (°C) T50 (°C) T100 (°C)

30 75 130

28 65 110

22 30 80

25 55 90

22 20 60

Fig. 12. Activity test of Cat-ORG catalyst with different calcination conditions.

of activity of different cobalt oxide catalysts obtained by reactive calcination conditions was as follows: Cat-ORGr > Cat-Or > Cat-Cr > Cat-Ar > Cat-Nr . The surface restoration behavior of metal surface such as cobalt metal has been observed in the presence of adsorbing CO gas. In these studies, we can also get that the even small changes in lattice constant can significantly affect the binding energy of surface species. The experimental result follows with a two-step of reaction mechanism in which the CO first reacts on the surface to obtain the OCO intermediate followed by the dissociation to form the CO2 in the gas phases. The other factors affect the reaction rates are total number of active sites which will depend on the exposed facets, the corner atoms, and the step edges present on the catalyst surface. The improved catalytic activity of Cat-ORGr can be ascribed to the unique structural and textural characteristics as the smallest crystallites of Cat-ORGr . The highly dispersed and highest specific surface area which could expose more active sites for catalytic oxidation and relatively open textured pores which will favor for the adsorption of reactants and desorption of products and thus facilitate the oxidation process. Moreover, the presence of partly reduced phase provides an oxygen-deficient defective structure which can create high density of active sites as a result of reactive calcination, consequently the Cat-ORGr turn into the most active catalyst.

3.2.4. Comparison of Cat-ORG produced in different calcination conditions A comparative study of CO oxidation over Cat-ORG , produced under different calcination conditions of stagnant air, flowing air and RC was shown in the Fig. 12 and observed that the calcination strategy has an extreme effect on the activity of resulting catalyst. The activity of Cat-ORGfa was lies in between the activities of Cat-ORGr and Cat-ORGa catalysts. It can be visualized that 72% CO conversion has occurred at ambient temperature 18 °C over Cat-ORGr catalyst. The full conversion of CO over Cat-ORGa was observed at 35 °C higher than Cat-ORGr catalyst. Thus, it was apparent from Fig. 12 that the catalysts produced following the novel route of RC of the precursors were more active for CO oxidation than the ones prepared by the traditional methods of calcination of the same precursors in the stagnant and flowing air. The light-off characteristics of Cat-ORG obtained by SAC, FAC and RC conditions were discussed in Table 10. The Co3 O4 , a distinctive spinel-structure transition metal oxide finds numerous applications because of its outstanding performance in various fields. The nature of cobalt oxides especially in the proportion of Co2+ , Co3+ and Co4+ was highly used for CO oxidation. Among all available cobalt oxide, the most active form would be Co3 O4 in which cobalt is present in two valence states +2 and +3.

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Table 10. Light-off characteristics of Cat-ORG obtained by SAC, FAC and RC. Characteristics Temperature (°C) T10 T50 T100

Catalyst Cat-ORGa

Cat-ORGfa

Cat-ORGr

< ambient 36 95

< ambient <18 75

< ambient <<18 60

Thus, the activity order of catalysts obtained by various calcination conditions was as follows: RC > flowing-air > stagnant-air. Finally, we confirmed that the RC produced catalysts has shown that the best catalytic activity than the other catalysts produced by conventional methods of calcination, whether it was in stagnant or flowing air. 3.2.5. Comparison of stagnant air, flowing air and reactive calcination of various cobalt catalysts As a comparison, the light-off characteristics of all catalysts produced by RC and traditional way of calcination was given in Table 11. It was observed that the RC produced catalysts was better than the catalysts produced by conventional methods of calcination, whether it was in stagnant or flowing air. In general, the catalyst affects the chemical reactions mostly by its exposed active site and surface properties. A novel route of single-step reactive calcination of cobalt precursors far below their decomposition temperatures for the preparation of active catalysts for CO oxidation has been investigated. The catalysts produced following the novel route of RC was more active than the ones prepared by the traditional methods. The novelty of catalysts produced by RC was associated with the presence of nano-sized major Co3 O4 phase and unusual morphology. This method was simple, reliable, cost-effective and easy to carry out on a large scale. It could be extended and expanded to provide a convenient strategy for the preparation of oxide nanostructures for several reactions in a single step minimizing the drawbacks of conventional methods of two-step processes of calcination and activation, bypassing the reduction step. The order of activity of various cobalt oxide catalysts obtained by various calcination conditions was as follows: Cat-ORG > Cat-O> Cat-C> Cat-A> Cat-N. Among all the prepared catalysts, the Cat-ORGr obtained from cobalt oxalate precursor showed that the best activity with light-off temperatures, T50 and T100 at 20 °C and 65 °C respectively. This result showed that the Cat-ORGr catalysts synthesized by RC conditions were more active for CO oxidation at low temperature. Therefore, the cobalt oxalate was the best choice of precursor for facile synthesis of highly active catalysts for CO oxidation. The active form of cobalt catalyst for CO oxidation was the cobalt oxide (Co3 O4 ) metal. The reduction of cobalt oxide species was essentially performed into the calcination process of active cobalt atoms for catalyzing the reaction. Among the five types of cobalt compounds used in this study, cobalt oxalate seems to be the best (optimum) cobalt precursors to prepare cobalt catalyst with major CO oxidation at commercially relevant synthesis conditions. 3.3. Blank experiment A blank experiment was carried out with alpha-alumina only in place of the catalyst. At bed temperature increase up to 200 °C, almost no oxidation of CO has been observed under the experimental conditions. From the blank test, confirmed that the performance of reactor in the absence of catalyst for CO oxidation and increasing of temperature does not show any activity for CO oxidation. Thus, the catalytic effect of the reactor wall and alumina used as diluents can be neglected within the experimental conditions. 3.4. Stability test The stability test of Cat-ORGr catalyst was conducted at 60 °C for the oxidation of CO in a continuous running for 48 h under the earliest mentioned experimental conditions. The results shown that the practically no deactivation of Cat-ORGr catalyst has occurred in the experiments, as shown in the Fig. 13. The amazing performance of Cat-ORGr catalyst produced by RC for CO oxidation was associated with the modification in intrinsic textural, morphological characteristics such as crystallite size, surface area and particle size of the catalyst. The performance of Cat-ORGr catalyst was judge by their activity, selectivity and stability. These studies evaluate the stability of Cat-ORGr catalyst as well as their importance for CO2 formation. The performance of catalysts for CO oxidation was associated with the modification in intrinsic textural, morphological characteristics such as crystallite size, surface area and particle size of the catalyst. The dynamics of catalyst surface during the reaction was essential to an understanding of the deactivation mechanism of cobalt catalysts, as well as the role of Co in stabilizing the catalyst. The Co loading reduces the surface oxygen content. When Co loading was increased, therefore, the concentration of remaining protons was get reduced. 3.5. Applications of cobalt oxide catalysts for future developments The recent progress of cobalt oxide catalysts for future developments in catalytic centers as the highly useful towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), oxidation of hydrocarbons and carbon monoxide

16

Characteristics Temp. T10 (°C) T50 (°C) T100 (°C)

Cat-N

Cat-A

Cat-O

Cat-C

Cat-ORG

Cat-Na

Cat-Nfa

Cat-Nr

Cat-Aa

Cat-Afa

Cat-Ar

Cat-Oa

Cat-Ofa

Cat-Or

Cat-Ca

Cat-Cfa

Cat-Cr

Cat-ORGa

Cat-ORGfa

Cat-ORGr

41 90 170

30 80 140

30 75 130

30 80 140

30 70 130

28 65 110

25 65 120

22 45 100

22 30 80

32 70 130

22 55 110

25 55 90

22 30 100

22 25 75

22 20 60

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Table 11. Light-off characteristics of catalysts obtained by SAC, FAC and RC of various cobalt catalysts.

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Fig. 13. Stability test of Cat-ORGr catalyst for oxidation of CO.

in fuel cell and hydrogen evolution reaction (HER) in acid and alkaline media. A series of cobalt-based high-performance catalysts have been designed and synthesized including cobalt oxides for low temperature CO oxidation and stability of cobalt oxide catalysts also improved for applications in long-time. Cobalt is the 32nd most abundant element in the Earth’s crust, has emerged as an attractive to the non-precious metal for chemical reactions due to its catalytic performance [73– 75]. The surface chemistry of Co oxide films depends much more on the Co surface atoms present in the Co3 O4 or CoO nature of the film. The molecularly adsorbed CO desorbs at temperatures between −173 °C and −90 °C, whereas the surface carbonate decomposes in a broad temperature range up to 127 °C. The CO molecules bind to the surface of Co2+ ions, while a compressed phase is formed at higher coverage with additional of CO molecules located at sites in between the Co2+ ions. The recent research results have shown that the cobalt containing compounds supported onto conducting carbonaceous materials [76–78]. The cobalt oxide catalyst deposited on highly active supporting material is very important to improve the catalytic activity. The strong metal–support interaction is an important ingredient for development of highly active catalysts in futute applications. The structure sensitivity of cobalt oxide catalyst is high depending upon the preparation conditions. The intensive efforts have been made to fabricate various surface-tuned cobalt oxides materials with different nanostructures and morphologies for future developments in different areas [85–87]. 4. Conclusion The calcination strategies of the precursor have a great influence on the activity of resulting catalysts. The RC route was the most appropriated calcination strategy for the production of highly active Co3 O4 catalyst for CO oxidation. The calcination order with respect to the performance of cobalt salts catalysts for the oxidation of CO was as follows: reactive calcination> flowing air> stagnant air. The extraordinary performance of Cat-ORGr produced by reactive grinding followed by RC in CO oxidation was associated with the modification in intrinsic textural and morphological characteristics such as surface area, crystallite size, particle size and oxygen-deficient defective structure which can creates the high density of active sites. The catalysts produced following the novel routes of RC were more active than the ones prepared by the traditional one. The novelty of catalysts produced by RC was associated with the presence of nano-sized major Co3 O4 phase and unusual morphology. The RC route was the simple, cost-effective and easy to carry out on a large scale. The cobalt oxide catalysts are one of the best transition metal oxide catalysts for low-temperature CO oxidation. The novelty of cobalt oxide catalyst is that the present of nano-sized major Co3 O4 phase and unusual morphology. The addition of appropriate promoters, supports, pretreatment and advanced preparation methods would lead to improvements in the activity of Co3 O4 catalyst towards CO oxidation. Declaration of Competing Interest The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript. No animals and humans are harm in this research work. References [1] C. Zhang, L. Zhang, G. Xu, X. Ma, Y. Li, C. Zhang, D. Jia, Metal organic framework-derived Co3 O4 microcubes and their catalytic applications in CO oxidation, New J. Chem. 41 (2017) 1631–1636.

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