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Low-temperature complete oxidation of CO over various manganese oxide catalysts
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Low-temperature complete oxidation of CO over various manganese oxide catalysts Subhashish Deya,∗, Ganesh Chandra Dhala, Devendra Mohana, Ram Prasadb 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
Keywords: Mn-acetate Mn-nitrate MnOx Precursors Calcination and characterization
The low temperature catalytic oxidation of CO conceivably the most extensively studied reaction in the history of heterogeneous catalyst and it is important in the context of cleaning the air and lowering the automotive emissions. Among a variety of manganese oxides catalyst, the Mn2O3 is considered to be the most favorable with respect to the catalytic activity. A novel route of reactive calcination (RC) of Mn-salts (Mn-Acetate and MnNitrate) for the synthesis of highly active catalysts was studied. The calcination of the precursor produced manganese oxide species (Mn2O3 and MnO2) in thermodynamic equilibrium, with the major nano-size Mn2O3 phase having a large surface area, and lower crystallinity. The amazing activity of the novel MnOx catalysts over the conventional ones obtained by calcination of the precursors in stagnant air, flowing air and reactive calcination (4.5%CO in air) in CO oxidation was associated with the presence of Mn2O3 and its unusual morphology as evidenced by XRD, SEM-EDX, XPS and FTIR characterization. The catalysts obtained by calcination of various MnOx precursors showed activity for the CO oxidation in the following order: Mn-Acetate > Mn-Nitrate and all the traditional route of calcination was also followed the same activity order. Further, the activity order of the catalysts obtained by various calcination conditions was as follows: RC > flowing-air > stagnant-air.
1. Introduction The catalytic oxidation of carbon monoxide (CO) receives a considerable attention due to its applications in many different fields like automobile vehicles, industry and environmental fields, such as personal respiratory protective devices, CO2 laser gas generation, proton exchange membrane fuel cells and automobile emission controls etc (Njagi et al., 2010; Didenko et al., 2011; Cai et al., 2012). It is widely used in the automobile vehicles to reduce CO from the environment, therefore it has been attracted much attention (Lee et al., 2016). CO has been termed as the silent killer for the 21st century (Xie et al., 2009). It is also called the carbonous oxide is a colorless, odorless, tasteless and nonirritating gas, which makes it difficult for humans to detect (Cholakov, 2000). This is a poisonous and life-threatening gas to humans and all other forms of air-breathing life, as inhaling even relatively small concentration of it can lead to serious injury, neurological damage and possibly death (Clarke et al., 2015). When CO enters the bloodstream it combines with hemoglobin and forms carboxy-hemoglobin, which reduces the oxygen-carrying ability of the blood. In the past 35 years, the catalytic converters have been installed on more
than 1000 million vehicles around the world (Ghaffari et al., 2008). The choice of appropriate catalyst is an important step for reducing the atmospheric pollution. After research for decades, two main types of catalysts for CO oxidation near room temperatures have been developed, the first type includes high surface area precious metal (Pt, Pd, Rh, Au, etc.) based catalysts and the second one transition metal (Cu, Mn, Cr, Co, Ni, Fe etc) based catalysts (Kanungo, 1979). The noble metals are widely used as catalysts for a long duration but due to its high price and sulfur poisoning, we have to search other substitute catalysts like mixed metal oxides and transition metal oxides for CO oxidation (Tang et al., 2006). The manganese oxide catalyst is also able for the oxidation of CO at a low temperature due to the presence of lattice oxygen on their surfaces (Dong et al., 2015). The size of catalysts has also been several advantages for CO oxidation purposes, because it's small size particles have a high surface area it causes the number of active sites presence per unit mass of the catalyst is increase (Halim et al., 2007; Hu et al., 2007). (see Table 5) The present work discussed about the experimental procedures to understand the characterization techniques applied to gain information about the manganese oxide catalyst structure and their activity for CO
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. E-mail address: [email protected] (S. Dey). https://doi.org/10.1016/j.apr.2018.01.020 Received 31 August 2017; Received in revised form 31 January 2018; Accepted 31 January 2018 1309-1042/ © 2018 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V.
Please cite this article as: Dey, S., Atmospheric Pollution Research (2018), https://doi.org/10.1016/j.apr.2018.01.020
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and Prasad, 2017). The RC process minimized a process step by converting two steps processes into single step process in a reactive CO-air mixture at a 300 °C (Trivedi and Prasad, 2016). Such single step thermal treatment of the precursor was called “RC method” by the authors (Dey et al., 2017). It is also produced MnOx catalysts with improved performance for CO oxidation as compared to the traditional method of calcination. It was also postulated that the RC method shows the concurrent multifarious phenomena for CO oxidation and precursor decomposition, cause a synergistic effect in the formation of the oxygendeficient catalyst surface at a low temperature.
oxidation. However, the relatively few comprehensive characterization studies of supported manganese oxide catalysts have appeared in comparison to those who devoted to the other transition metal oxides (Lee et al., 2001). This may in part, due to the fact that manganese forms very complex species with oxygen, especially in the presence of other cations and water, make a characterization of supported manganese oxide catalysts difficult (Roy et al., 2016). In addition, several manganese oxide catalysts can exist in different crystalline or pseudocrystalline forms, and a number of non-stoichiometric mixed-valence compounds are known, further complicating their characterization (Kapteijn et al., 1994). Emphasis has been put on the textural properties of the catalysts, there nature and distribution on the supported manganese oxide species (Jiao and Frei, 2010). The MnOx catalyst is preparing from the manganese nitrate precursor have obtained the high CO conversion efficiency by the addition of gold (Au/MnOx) catalyst (Jiao and Frei, 2010; Kapteijn et al., 1994). Among all the transition metal oxides, the Mn2O3 exhibits stable performance for CO oxidation. The reaction of Mn2O3 and O2, forming MnO2, is influenced by the oxygen concentration and temperature. Higher the oxygen concentration and lower the temperature are, the more conducive to the formation of MnO2 (Li et al., 2007). On the contrary, lower the oxygen concentration and higher the temperatures are, the slower the generation rate of MnO2 and Mn2O3 gradually forms as the reaction proceeds (Kondrat et al., 2011). Manganese oxide catalysts analysis by the XRD and FTIR is consistent with the presence of (Mn2O3, MnO2, and Mn3O4) phases, with the composition of strongly dependent on the calcination temperature between 200 and 300 °C (Jiao and Frei, 2010). When the catalyst is mainly MnO2, the hydroxyl radical oxidation mechanism is dominant and the content of Mn2O3 is high in the equilibrium phase of MnO2–Mn2O3 and the oxygen anion in oxidation mechanism is dominant (Najafpour et al., 2012). The oxidation state of manganese is highly influential when the using of manganese oxide as a support. The MnOx compounds have a typical benthollide structure containing the labile lattice oxygen (Balgooyen et al., 2016). The labile oxidation state facilitates Mn to act either as a reducing agent or an oxidizing agent. The oxygen storage capacity in the crystalline lattice and the capability of manganese to form the oxides with variable oxidation states determines its catalytic property (Ilton et al., 2016). The Au and MnOx interaction synergistically, exhibiting long-term CO oxidation at a low temperature with negligible activity decays (Solsona et al., 2004). The MnOx has high oxygen storage along with faster oxygen adsorption and oxide reduction rates, which can be used to reduce the amount of CO in the exhaust gas or natural gas vehicle (Manceau et al., 2012; Villalobos et al., 2003). Among the transition metals supported on manganese oxides are good substitute catalysts because of their low price and widespread use. The manganese or manganese oxide can exhibit higher activities per unit surface area than those of noble metal catalysts. The CuMnOx catalyst being produced under the oxygen-rich atmosphere and its confirmation of the retardation of copper oxide and reduction of manganese phases under the oxygen-deficient conditions, to produce residual Cu2O and Mn2+/3+ oxide phases (Jones et al., 2008). Several studies have shown that the preparation conditions and calcination strategy highly influenced on the activity of resulting catalyst. The success of MnOx catalysts has prompted a big deal of fundamental work devoted to the instructive role played by the each element and the nature of active sites (Zhanga et al., 2011; Solsona et al., 2004). The effect of surface area of the catalyst is the main reason for their high catalytic activity is also discussed in this research work. From the experimental results, we can get that the MnOx catalyst prepared by reactive calcination conditions is more active for the complete oxidation of CO as compared to the stagnant air and flowing air calcination conditions. The reactive calcination (RC) of the precursor is carrying out by the introduction of a low concentration of chemically reactive CO–Air mixture (4.5% CO) at a total flow rate of 32.5 ml min−1 over the hot precursor in a down flow bench-scale tubular reactor (Trivedi
2. Experimental 2.1. Catalyst preparation The manganese oxide catalysts were prepared from the precursors of manganese nitrate or manganese acetate separately. All the chemicals used for the preparation of catalyst they were of analytical grade. Ammonia (Verbièse, 25%) was added drop-wise to an aqueous solution of the Mn nitrate precursor (Mn(NO3)3H2O, 99.98%,Aldrich) or (Mn (CH3COO)2·4H2O, 99.98%, Aldrich) under constant stirring until the pH reached to a value of 10.0. The resulting solid was kept under stirring for 1 h. After filtration and washing, the solid was dried overnight at 110 °C and subsequently calcined in a similar procedure as above to yield the MnOx catalyst. The molecular weight of manganese nitrate [Mn(NO3)2·3H2O] and manganese acetate [Mn(CH3COO)2·4H2O] precursor was 178.95 g and 245.09 g respectively and converted into Mnoxide with the molecular weight (MnO2 = 87 g and Mn2O3 = 158 g). The 1 g MnO2 was equivalent to (87/178.95) for Mn(NO3)2·3H2O and 1 g Mn2O3 was equivalent to (158/245.09) for Mn(CH3COO)2·4H2O respectively (Dinh et al., 2015). The calcination of the precursor was done just before the activity measurement of the catalyst. It was carried out under the following three different conditions: (I) Stagnant air calcination: The calcination of the precursor was carried out in a furnace in the presence of stagnant air at 300 °C for 2 h to produce the catalyst. The calcined catalyst was stored in an air tight glass bottle. (II) Flowing air calcination: The calcination of the precursor was performed in situ under flowing air in the reactor at 300 °C for 2 h, just before the activity measurement experiment. (III) Reactive calcination: The calcination of the precursor was performed in situ under a flowing reactive mixture of 4.5% CO-Air at 300 °C for 2 h in a compact bench scale of fixed bed tubular reactor, just before the activity measurement experiment. The reactive calcination of differently prepared catalyst samples was carried out by passing a CO-Air mixture over the precursors at 160 °C for 30min and 300 °C for 1hr for the total decomposition of the catalyst. The RC process minimized a process step by converting two steps processes into single step process. It was also produced catalysts with improved performance for CO oxidation (Dey et al., 2017). 2.2. Characterization The X-ray diffraction (XRD) measurement of the catalyst was carried out by using Rigaku D/MAX-2400 diffractometer with Cu-Kα radiation at 40 kV and 40 mA. The mean crystallite size (d) of the catalyst was calculated from the line broadening of the most intense reflection using the Scherrer Equation. It provides information about the structure, phase, crystal orientation, lattice parameters, crystallite size, strain and crystal defects etc. d = 0.89λ/βcosθ
(1)
Where d is the mean crystallite diameter, 0.89 is the Scherrer 2
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min. To monitor the flow rate of CO and air passing through the catalyst presence in the reactor was done by the digital gas flow meters. The CO oxidation was analysis by the gas chromatogram to measure the activity of the resulting catalyst. Pure α–alumina spheres were used in the preheating section and the section after the catalyst bed. Eq. (2) can be representing the air oxidation of CO over the catalyst. For controlling the heating temperature of the catalyst present in a reactor was done by a microprocessor based temperature controller (Berger et al., 2002).
constant, l is the X-ray wave length (1.54056 Å), and b is the effective line width of the observed X-ray reflection, calculated by the expression β2 = B2-b2 (where B is the full width at half maximum (FWHM), b is the instrumental broadening) determined through the FWHM of the X-ray reflection at 2θ of crystalline SiO2. The Fourier transform infrared spectroscopy (FTIR) analysis was done by the Shimadzu 8400 FTIR spectrometer in the range of 400–4000 cm−1 and the wavelength range of Shimadzu 8400 FTIR was (700 nm - 25 μm). To calibrate the optical path length of a white cell of FTIR L = 21 m by means of a reference gas cell of an optical path length of Lref = (0.2003815 ± 0.000062) m. For the mid-IR region, 2−25 μm (5000–400 cm−1), the most common source is a silicon carbide element heated to about 1200 K. It provides information about the kind of materials present in a catalyst sample by their peak values. The scanning electron microscope (SEM) used a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. It was produced the topographical image of a catalyst by an electron beam and the image of catalyst was recorded on Zeiss EVO 18 (SEM) instrument. The accelerating voltage was used 15 kV and magnification of the image 5000X was applied. Areas ranging from approximately 1 cm to 5 μm in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50–100 nm). It provides information about the average aggregate size, crystallite degree and the microstructures of the catalyst. The best resolution is about 2–5 nm but many routine studies are satisfied with a lower value and exploit the case of image interpretation and the extraordinary depth of field to obtain a comprehensive view of the specimen. The SEM has more than 300 times the depth of field of the light microscope. The resolution we choose to image will obviously affect the number of pixels per row as well as the number of rows that constitute the scanned area. Magnification will increase if we reduce the size of the area scanned on the specimen. Magnification area scanned on the monitor/area scanned on the specimen. The X-ray photoelectron spectroscopy (XPS) analysis of the catalyst was measured with Amicus spectrometer equipped with Al Kα X-ray radiation at a voltage of 15 kV and current of 12 mA to measure the binding energy used for the calibration of adventitious carbon C(1s) present in the catalyst. The C(1s) peak is often used as an internal standard for the calibration of the binding energy scale. When X-ray photons strike a surface, they cause the emission of electrons. It provides information about the surface compositions and chemical states of the different constituent elements present in a catalyst. The features of charge compensation for the amicus spectrometer are patented ‘in the lens’ multi-mode electrostatic flood source, 0–5eV (mode 1) charge compensation, 0–1000eV (mode 2) reels/imaging/alignment, up to 500 μA electron emission, additional dual beam (ion electron) flood source. The evolution of XPS, the ability to compensate for surface charging and accurately determine the binding energies, particularly with electrically in homogenous samples. The Brunauer Emmett Teller Analysis (BET) provides information about the specific surface area, pore volume and pore size of the catalyst. The isotherm was recorded by micromeritics ASAP 2020 analyzer and the physical adsorption of N2 at the temperature of liquid nitrogen (−196 °C) with a standard pressure range of 0.05-0.30 P/Po.
2CO + O2→2CO2
(2)
The gaseous products were produced after the oxidation reaction in a reactor was analysis by an online gas chromatogram (Nucon series 5765) equipped with a porapack q-column, FID detector and a methaniser for measuring the concentration of CO and CO2. Where, the concentration of CO was proportional to the area of chromatogram ACO. The overall concentration of CO in the inlet stream was proportional to the area of CO2 chromatogram. (XCO) = [(CCO)in - (CCO)out] / [CCO]in = [(ACO)in - (ACO)out] / [ACO]in (3) The conversion of CO at any instant was calculate 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. (3). 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 the chromatogram of CO2 formed (ACO )out by the oxidation of CO. 3. Results and discussions The characterization of differently prepared catalyst samples in RC conditions was done by the following techniques and their activity for CO oxidation was discussed below. 3.1. Catalyst characterization The characterization of catalyst was carried out information about the morphology, surface area, binding energy, pore volume, pore size, chemical state, material composition and the percentages of different materials present on the surface of catalyst (see Figs. 1, 3 and 5). 3.1.1. Scanning electron microscopy analysis Morphology of the prepared catalyst samples in RC conditions was done by the scanning electron microscopy analysis. It was shown (Fig. 2) that the large differences in the surface texture and the other property of various MnOx catalysts. The SEM image clearly shows that the using of different precursors in the preparation of various catalysts makes a large difference in their morphology. In addition, the smaller size particles and the good distribution of active phase presence on the surface layer of catalyst, which causes a significant increasing of the effective surface area of the catalyst (Njagi et al., 2011). As seen in the SEM micrograph, the particles were comprised of coarse and fine by RC of Mn-N and Mn-A catalyst respectively. The particle size of Mn-N and Mn-A catalyst was 1.245 μm and 0.345 μm respectively. The particles present in the Mn-A catalyst was smaller size, less agglomerated and homogeneous as compared to the other catalyst sample (Jones et al., 2008). The particle size of the catalyst was also confirmed by the SEM image analysis and it was also observed that the particle size of the catalyst was increased in the following order: MnN > Mn-A. With the decreasing of particle size of the catalyst, therefore, more and more CO dispersed on their surfaces, it causes activity of the catalyst has been increased. The surface reconstruction behavior of
2.3. Catalytic activity measurement The oxidation of CO was carried out under the following reaction conditions: 100 mg of catalyst was diluted to α-alumina with feed gas consisting of a lean mixture of (2.5 vol% CO in air) and the total flow rate was maintained at 60 mL min−1. The. The air feed into the reactor was made free from moisture and CO2 by passing through it CaO and KOH pellet drying towers. The catalytic experiment was carried out under the steady state conditions and the reaction temperature was increased from room temperature to 300 °C with a heating rate of 2 °C/ 3
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Fig. 1. Schematic diagram of Experimental Set up.
Prasad, 2016). The decreasing of oxygen concentration in the surface layer of catalyst, therefore, the activity of the catalyst has been increased. Due to the presence of high oxygen deficiency in the Mn-A catalyst, therefore the activity of the catalyst has been increased (Dey et al., 2017). The high level of oxygen deficiency was created the high density of active sites present on the catalyst surfaces. It was also confirmed that the presence of pure oxides phases in the surface of catalyst was a good harmony with the XRD and FTIR results also.
the different size of particles presents in a catalyst surfaces during the prolonged exposure of CO gas (Mirzaei et al., 2003) (see Table 1).
3.1.2. Elemental analysis It was very clear from the SEM-EDX analysis that all the samples of catalysts were pure due to the presence of their respective element peaks only. After the SEM micrography was taken, the elemental mapping of different catalysts sample was analyzed to determine the elemental concentration distribution of the catalyst surface. The SEMEDX was performed on the different spots of cross-sectioned of the catalyst granules to determine the concentration of different elemental groups present at different locations on the catalyst surfaces. It was very clear from the SEM-EDX analysis that the entire catalyst sample was pure and there was not present any types of impurities in the catalyst surfaces (Mirzaei et al., 2003). In Table 2 we have to discuss about the relative atomic percentage and weight percentage of different elemental groups' present on the catalyst surfaces (Dey et al., 2017). The atomic and weight percentage of oxygen present in the surface layer of MnOx catalysts were decreased in the following order: Mn-N > Mn-A. The presence of high concentration of oxygen in the surface layer of catalyst, it causes activity of the catalyst has been decreased (Dey et al., 2017; Boreskov, 1966). This is the reason for Mn-N catalyst has shows a poor performance for the CO oxidation. It was very clear from the table and figure that the atomic and weight percentage of manganese present in a Mn-N and Mn-A catalyst was higher than oxygen (Trivedi and
3.1.3. X-ray diffractogram XRD analysis of the catalysts (Fig. 4) was used to determine the final phases after heat treatment at 300 °C in RC conditions. It was carried out to identify the crystallite size and coordinate dimensions present on the surface layer of the catalysts. The phase analysis of the differently prepared catalyst samples was done by the XRD studies (Villalobos et al., 2003). In the Mn-N catalyst, their diffraction peak at 2-Theta (2θ) was 37.25 and their corresponding lattice plane (h k l) value of largest peak was (1 2 1). The structure was end centered; monoclinic MnO2 phase at the JCPDS reference no. (89–2545) and the crystallite size of the catalyst was 7.19 nm. In the Mn-A catalyst, their diffraction peak at 2-Theta (2θ) was 36.95 and their corresponding lattice plane (h k l) value of largest peak was (1 1 1). Structure was end centered; monoclinic Mn2O3 phase at the JCPDS reference no. (89–2530) and crystallite size of the catalyst was 6.89 nm. The crystallite size of the catalyst Fig. 2. SEM image of the catalysts (A) Mn-N and (B) Mn-A.
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Fig. 3. The SEM-EDX image of the catalysts (A) Mn-N and (B) Mn-A.
highly active for the complete oxidation of CO at a low temperature. The activation or deactivation periods on the catalytic reaction and the loss or gain of catalytic activity on the reaction may be related to the appearance or loss of specific bulk phases. After the SEM and XRD analysis, we have to get that the crystallite size and particle size of the catalyst was followed the same order.
Table 1 Nomenclature of the catalyst samples in this study was as follows. Catalyst Name
Nomenclature
Mn(NO3)2·3H2O Mn(CH3COO)2·4H2O
Mn-N Mn-A
3.1.4. Fourier transforms infrared spectroscopy The identification of the metal-oxygen bonds present in the catalyst surfaces was made by the Fourier transform infrared spectroscopy (FTIR) analysis. In the invested region (4000-400 cm−1) to obtain the entire absorption spectra of different peaks to indicates the presence of different elemental groups (bond) in the catalyst samples. All the catalysts were prepared in RC conditions before applying in different characterization work. In the Mn-N catalyst at the transmittance conditions, there were seven peaks we obtained. The IR band (3590 cm−1) has shown the presence of -OH bond, (2350 cm−1 and 679 cm−1) has shown the presence of COO bond, (1640 cm−1 and 1530 cm−1) has shown the presence of Mn2O3 bond and (1250 cm−1 and 525 cm−1) has shown the presence of MnO2 and CO32- bond respectively. In the Mn-A catalyst at the transmittance conditions, there were six peaks we obtained. The IR band, (3590 cm−1) has shown the presence of -OH bond, (2350 cm−1) has shown the presence of COO bond, (1640 cm−1 and 1530 cm−1) has shown the presence of Mn2O3 bond, (525 cm−1) has
Table 2 The atomic and weight percentage of catalysts by EDX techniques. Catalyst
Elements atomic (%)
Elements weight (%)
Mn-A Mn-N
Mn(85.75) Mn (72.22)
Mn (84.80) Mn (71.85)
O (14.25) O (27.78)
O (15.20) O (28.15)
was increased in the following order: Mn-N > Mn-A and it matches with the SEM image analysis of the catalysts. The crystalline acetate phase was disappeared between 110 °C and 140 °C to leave a predominantly amorphous phase. The next observable phase change occurred at 240 °C with the broad peaks matching with the reflections of Mn2O3 becoming apparent at 35.7 and 36.72 at θ. The Mn2O3 phase was the most crystalline form, producing the narrow size high intensity diffraction lines (Dey et al., 2017). The experimental result was also confirmed that the lower particle size of Mn-A catalyst is
Fig. 4. XRD analysis of the catalysts (A) Mn-A and (B) Mn-N.
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Fig. 5. FTIR analysis of the catalysts (A) Mn-A and (B) Mn-N.
shown the presence of MnO2 bond and (1250 cm−1) has shown the presence of CO32- bond (Mirzaei et al., 2003). After the FTIR analysis, we have to observe that there will be some impurities like carbonate group and hydroxyl group presence in the catalyst sample. All the sample of catalysts; this originates from the stretching vibrations of the metal-oxygen bonds and confirm the presence of Mn2O3 and MnO2 peaks (Dey et al., 2017). This FTIR spectrum shows the presence of various functional groups on the surface of the as-synthesized MnOx catalyst. No organic groups were found to be adsorbed on the surface of the basis of FTIR spectra. The concentration of the nitrates or acetate on the reactive surface of MnOx catalyst with manganese content corresponding to a monolayer is considerably greater than that of the sample with higher manganese loading.
Table 4 Textural property of the catalysts.
Binding energy of Elements
Mn-A Mn-N
Mn2O3 MnO2
Mn (656.542) Mn (656.178)
Organic C=O Organic C-O
Pore Volume (cm3/g)
Ave. Pore Size (Å)
Mn-N Mn-A
35.90 37.84
0.430 0.480
48.36 46.05
Calcination Strategy
Reactive Calcination Flowing air Calcination Stagnant air Calcination
Mn-N catalyst
Mn-A Catalyst
Ti
T50
T100
Ti
T50
T100
25 °C 30 °C 30 °C
80 °C 100 °C 110 °C
160 °C 195 °C 220 °C
25 °C 30 °C 30 °C
70 °C 80 °C 100 °C
130 °C 150 °C 190 °C
and Mn2p1/2 transition at 644.3 and 656.24eV, respectively. The deconvolution of the Mn 2p3/2 transition exhibited the presence of Mn III (643.6eV) with a small shoulder in 645.6eV assigned to Mn II. From the Fig. 6, we have to observe that the Mn ions present in a Mn-N and Mn-A catalyst was MnO2 and Mn2O3 form respectively. The Mn2O3 has shown a broad peak at 645–657eV, which can be assigned to a mixed oxidation state of Mn2+ and Mn3+. From the table and figure, it was clear that the binding energy present was highest in Mn-A catalyst as compared to the other catalyst samples (Trivedi and Prasad, 2017). The XPS results suggest that the surface manganese oxide species were least partially reduced, which may lead to the formation of Mn4+/Mn3+/2+ redox couples on the surface of catalyst therefore promote the catalytic activity for CO oxidation. Although, it can be proposed that the highest binding energy was preferably for the CO oxidation (Dey et al., 2017). The increasing of Mn ions concentration in the Mn-oxide catalyst was helpful to the migration of oxygen from bulk to the catalyst surface, which can promote the activation and transportation of active oxygen species on the surface of catalyst. The binding energy of oxygen O (1s) spectra was illustrated in Fig. 7. In the XPS analysis, there were two diverse types of oxygen species present in the catalyst samples. First was known as chemisorbed oxygen (denoted as Oa, such as O22−, O−, OH−, CO32−, etc.) and second was known as lattice oxygen (denoted as Ol, such as O2−). In our case, the oxygen with the binding energy of (532.80eV-535.46eV) was the main form and could be assigned to the chemisorbed oxygen (Oa).
Table 3 The chemical state and binding energy of Mn-oxide Catalyst. Chemical state of Elements
Surface Area (m2/g)
Table 5 Activity test of Mn-oxide catalyst for CO oxidation.
3.1.5. XPS analysis With the help of XPS analysis, we have to calculate the surface valence state, binding energy and the chemical state of different elemental groups' present on the surface layer of catalysts. All the catalyst samples were prepared in RC conditions and their binding energy was preferably for the CO oxidation. In Table 3we have to discuss the binding energy and chemical state of different elemental groups present on the catalyst surfaces. The evaluation of Mn(2p) narrow scans allows the assignment of the oxidation state of Mn. By performing peak fitting deconvolution of the main Mnp3/2 in all three catalyst samples can be divided into the three components including Mn4+, Mn3+ and satellite. Since the differences between the binding energy values of Mn3+ and Mn4+ ions were small. The binding energy of Mn(2p) element present in a Mn-A and Mn-N catalyst was (645.946eV and 656.542eV) and (643.816eV and 656.178eV) respectively (see Tables 4 and 5). The observed binding energies 643.816eV, 645.946eV and 656.178eV were associated with the presence of Mn3+, Mn4+ and satellite respectively in all the three samples. The evolution of XPS, to ability the compensate for surface charging and accurately calibrate the binding energy scale and found +2 and +3 on the surface of MnOx catalyst. The pristine Mn2O3 spectrum was characterized by Mn2p3/2
Catalyst
Catalyst
O (532.113) O (532.274)
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Fig. 6. XPS analysis of the Mn(2p) in MnOx catalysts (A) Mn-N and (B) Mn-A.
structural varieties of Mn-oxide catalyst. It was possible to determine the FTIR spectra of MnOx phases and the XPS analysis at different precursor prepared catalyst to evidence a continuous evolution leading to the simultaneous presence of several MnOx phases.
The O(1s) peak in MnOx, Mn2O3, and MnO2 displayed the presence of Mn−O−Mn typical for the manganese oxides. The oxygen peaks spectra present in a Mn-A catalyst was much broader and more intensive than the other catalyst samples. Therefore, it was suggested that the highest binding energy of Mn-A catalyst was more preferable for the selective catalytic oxidation reaction (Njagi et al., 2010). The formation of adsorbed reactive oxygen species, such as superoxide ions (O2–), might be correlated to the presence of surface oxygen vacancies on the metal oxide support or at the metal–support interface. The oxygen vacancy present in the MnOx catalyst was responsible for the decreased lattice constant in Mn2O3 and MnO2 samples. The chemical state of C(1s) present in the Mn-oxide catalyst was C-O-C form and the binding energy of C(1s) was 286eV. The inelastic scatter of electrons through the adventitious carbon layer may cause a large change in background shape when compared with the spectrum from a clean sample. In summary, differences in the coordination environment of Mn (II) in MnO2 and Mn2O3 only slightly perturb the shape of the Mn(3p) lines, but appreciably affect on the Mn2p3/2. FTIR and XPS analysis provide information about the various oxidation states and compared the calcination of various manganese precursors. Our main work was to evaluate the use of FTIR spectroscopy and XPS analysis to probe the
3.1.6. BET surface area Nitrogen adsorption/desorption isotherms were analyzed to determine the specific surface area, pore volume and pore size distribution of the manganese precursors and their oxides obtained after the heat treatment process. The surface area of differently prepared catalyst samples like Mn-A and Mn-N in RC conditions was 37.84 m2/g and 35.90 m2/g respectively. The pore volume and pore size of the Mn-A catalyst was slightly higher than the Mn-N catalyst. The textural properties like surface area, pore volume and pore size of the catalysts were more favored for the CO oxidation. The specific surface area calculated using the BET method was reported in the above Table (Trivedi and Prasad, 2016). A large number of more pores present in a catalyst surface mean more numbers of CO molecules were trapped and they have to shows the better catalytic activity at a low temperature (Mirzaei et al., 2003). The porosity of the Mn2O3 nano-particles can be explained by considering the voids present in the particles on the
Fig. 7. XPS analysis of the O(1s) in MnOx catalysts (A) Mn-N and (B) Mn-A.
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Fig. 8. Activity test of Mn-N and Mn-A catalyst in A) Stagnant air, B) Flowing air and C) Reactive calcination.
network as the “pores” detected by the N2 adsorption–desorption measurements. The specific surface area of the various manganese oxide catalysts was measured by the BET analysis and it was also matched with the SEM and XRD results. However, we believe that a larger pore size to be advantageous for application as electro-catalysts, as electrochemical processes often produce the solid products, which might be easily clog pores in the micro- and meso-porous range (Dey et al., 2017).
catalyst, which was lowered by 10 °C than that of Mn-N catalyst respectively. The total oxidation temperature of CO was 190 °C for Mn-A catalyst, which was less by 30 °C than that of Mn-N catalyst. In comparison, between the stagnant air and flowing air calcination conditions, we have to observe that the flowing air calcination produced catalysts has shown the best catalytic activity for CO oxidation at a lower temperature as compared to the stagnant air calcination conditions. Fig. 8(B) shows that the initial oxidation of CO by these catalysts in flowing air calcination was 30 °C and half conversion of CO has occurred at 80 °C over the Mn-A catalyst, which was less by 20 °C than that of Mn-N catalyst. The total oxidation temperature of CO was 150 °C for Mn-A catalyst, which was less by 45 °C than that of Mn-N catalyst. The order of activity of Mn-oxide catalyst in flowing air calcination conditions was as follows: Mn-A > Mn-N. The improved catalytic activity of Mn-A catalyst can be ascribed to the unique structural, textural characteristics and the smallest crystallite size. The rate of CO oxidation was increase with time and flattened at the end. This might be due to the synergistic effects of exothermic oxidation, decomposition and redox surface reaction of the catalyst. In Fig. 8(C), we have to observe that the comparative study of CO
3.2. Catalyst performance and activity measurement Activity test of the catalyst was carried out to evaluate the effectiveness of different manganese oxide catalysts (Mn-A and Mn-N) as a function of temperature. Their activity was measured in different calcination conditions like stagnant air, flowing air and reactive calcination conditions. Effectiveness of the manganese oxide catalysts was increased with the increasing of temperature from room temperature to a certain high temperature for full conversion of CO as shown in the Fig. 8. In the stagnant air calcination (SAC) conditions Fig. 8(A), shows that the oxidation of CO was initiated at 30 °C in both the catalyst and the half conversion of CO was occurred at 100 °C over the Mn-A 8
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oxidation over various Mn-oxide catalysts prepared by RC conditions. The extraordinary performance of the resulting catalysts was achieved for full conversion of CO at a lower temperature (Dey et al., 2017). The novelty of the catalysts produced by RC conditions matched with their characterization results. The oxidation of CO was just initiated in RC conditions at 25 °C over the catalysts and the half conversion of CO was achieved at 70 °C over the Mn-A catalyst, which was less by 10 °C than that of Mn-N catalyst respectively. The total oxidation temperature of CO was 130 °C for Mn-A catalyst, which was less by 30 °C than that of Mn-N catalyst respectively. The light-off temperature showed that the RC route prepared catalyst was more active for CO oxidation at a low temperature as compared to the traditional method of calcination (Njagi et al., 2011). The activity order of the catalysts for CO oxidation was in accordance with their characterization by XRD, SEM-EDX, FTIR, XPS and BET analysis was as follows: Mn-A > Mn-N. The presence of highly dispersed and more specific surface area, which will be more favorable for the CO oxidation.
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Kondrat, S.A., Davies, T.E., Zu, Z., Boldrin, P., Bartley, J.K., Carley, A.F., Taylor, S.H., Rosseinsky, M.J., Hutchings, G.J., 2011. The effect of heat treatment on phase formation of copper manganese oxide: influence on catalytic activity for ambient temperature carbon monoxide oxidation. J. Catal. 281 (2), 279–289. Lee, J., Kim, H., Lee, H., Jang, S., Chang, J.H., 2016. Highly efficient elimination of carbon monoxide with binary copper-manganese oxide contained ordered nanoporous silicas. Nanoscale Res. Lett. 11, 2–6. Lee, S., Gavriilidis, A., Pankhurst, Q.A., Kyek, A., Wagner, F.E., Wong, P.C.L., Yeung, K.L., 2001. Effect of drying conditions of Au-Mn co-precipitates for low temperature CO oxidation. J. Catal. 200, 298–308. Li, M., Wang, D., Shi, X., Zhang, Z., Dong, T., 2007. Kinetics of catalytic oxidation of CO over copper-manganese oxide catalyst. Separ. Purif. Technol. 57 (1), 147–151. Manceau, A., Marcus, M.A., Grangeon, S., 2012. Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy. Am. Mineral. 97 (5−6), 816–827. Mirzaei, A.A., Shaterian, R.H., Habibi, M., Hutchings, G.J., Taylor, S.H., 2003. Characterization of copper-manganese oxide catalysts: effect of precipitate ageing upon the structure and morphology of precursors and catalysts. Appl. Catal. A: Gen 253, 499–508. Najafpour, M.M., Rahimi, F., Amini, M., Nayeri, S., Bagherzadeh, M., 2012. A very simple method to synthesize nano-sized manganese oxide: an efficient catalyst for water oxidation and epoxidation of olefins. Dal. Trans. 41, 11026–11031. Njagi, E.C., Chen, C., Genuino, H., Galindo, H., Huang, H., Suib, S.L., 2010. Total oxidation of CO at ambient temperature using copper manganese oxide catalysts prepared by a redox method. Appl. Catal. B 99, 103–110. Njagi, E.C., Genuino, H.C., Kingondu, C.K., Chen, C., Horvath, D., Suib, S.L., 2011. Preferential oxidation of CO in H2 rich feeds over mesoporous copper manganese oxide synthesized by a redox method. Int. J. Hydrogen Energy 36, 6768–6779. Roy, M., Basak, S., Naskar, M.K., 2016. Bi-template assisted synthesis of mesoporous manganese oxide nanostructures: tuning properties for efficient CO oxidation. Phys. Chem. Chem. Phys. 18, 5253–5263. Solsona, B., Hutchings, G.J., Garcia, T., Taylor, S.H., 2004. Improvement of the catalytic performance of CuMnOx catalysts for CO oxidation by the addition of Au. New J. Chem. 6, 708–711. Tang, C., Kuo, C., Kuo, M., Wang, C., Chien, S., 2006. Influence of pretreatment conditions on low-temperature carbon monoxide oxidation over CeO2/Co3O4 catalysts. Appl. Catal., A. 309, 37–43. Trivedi, S., Prasad, R., 2017. Choice of precipitant and calcination temperature of precursor for synthesis of NiCo2O4 for control of CO–CH4 emissions from CNG vehicles. J. Environ. Sci. 1–10. Trivedi, S., Prasad, R., 2016. Reactive calcination route for synthesis of active Mn–Co3O4 spinel catalysts for abatement of CO–CH4 emissions from CNG vehicles. J. Env. Chem. Eng 4, 1017–1028. Villalobos, M., Toner, B., Bargar, J., Sposito, G., 2003. Characterization of the manganese oxide produced by Pseudomonas putida strain MnB1. Geo. Cosmo. Acta 67 (14), 2649–2662. Xie, X., Li, Y., Liu, Z., Haruta, M., Shen, W., 2009. Low-temperature oxidation of CO catalysed by Co3O4 nanorods. Nat. Lett. 458, 746–749. Zhanga, X., Mab, K., Zhanga, L., Yonga, G., Yadib, M., Liu, S., 2011. Effect of precipitation method and Ce doping on the catalytic activity of copper manganese oxide catalyst for CO oxidation. Chin. J. Chem. Phys. 24, 97–102.
4. Conclusions The preparation conditions and calcination strategies have a great influence on the activity of resulting MnOx catalysts for CO oxidation. From the results and discussions, we have to conclude that the RC route was the most appropriated calcination strategy for the production of highly active manganese oxide catalyst for the oxidation of CO. The calcination order with respect to the performance of catalyst for CO oxidation was as follows: Reactive calcination > Flowing air > Stagnant air. In the activity test, we have to observe that the MnA catalyst prepared by RC conditions showed the best catalytic activity for CO oxidation at a low temperature (130 °C). The order of activity for various catalyst samples in different calcination conditions was as follows: Mn-A > Mn-N. The characterization by various techniques (XPS and FTIR) of the catalysts produced by RC revealed that the presence of Mn2O3 phases with the minor MnO2 phases in Mn-A catalyst. Therefore, it was suggested that the smallest particle size of Mn-A catalyst was highly active for the CO oxidation at a low temperature as compared to the other catalyst samples. These results have allowed us to conclude some characteristic FTIR and XPS peaks that was, observed, for the various transition MnO2 and Mn2O3 phases. Using these FTIR and XPS characterization, it was possible to detect the presence of transition MnOx naturally grown on Mn2O3-former catalyst. References Balgooyen, S., Alaimo, P.J., Remucal, C.K., Ginder-Vogel, M., 2016. Structural transformation of MnO2 during the oxidation of bisphenol A. Environ. Sci. Technol. 51 (11), 6053–6062. Berger, R.J., Perez-Ramirez, J., Kapteijn, F., Moulijn, J.A., 2002. Catalyst performance testing: the influence of catalyst bed dilution on the conversion observed. Chem. Eng. J. 90 (1–2), 173–183. Boreskov, G.K., 1966. Forms of oxygen bonds on the surface of oxidation catalysts. 41, 263–276. Cai, L., Guo, Y., Lu, A., Branton, P., Li, W., 2012. The choice of precipitant and precursor in the co-precipitation synthesis of copper manganese oxide for maximizing carbon monoxide oxidation. J. Mol. Catal. A Chem. 360, 35–41. Cholakov, G.S., 2000. Control of exhaust emissions from internal combustion engine vehicles. Pollut. Control Technol. 3, 1–8. Clarke, T.J., Davies, T.E., Kondrat, S.A., Taylor, S.H., 2015. Mechano-chemical synthesis of copper manganese oxide for the ambient temperature oxidation of carbon monoxide. Appl. Catal. B 165, 222–231. Dey, S., Dhal, G.C., Mohan, D., Prasad, R., 2017. Study of Hopcalite (CuMnOx) catalysts prepared through a novel route for the oxidation of carbon monoxide at low temperature. Bull. Chem. React. Eng. Catal. 12 (3), 393–407. Didenko, O.Z., Kosmambetova, G.R., Strizhak, P.E., 2011. Size effect in CO oxidation over magnesia-supported ZnO nanoparticles. J. Mol. Catal A 335, 14–23.
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Low-temperature complete oxidation of CO over various manganese oxide catalysts Subhashish Deya,∗, Ganesh Chandra Dhala, Devendra Mohana, Ram Prasadb 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
Keywords: Mn-acetate Mn-nitrate MnOx Precursors Calcination and characterization
The low temperature catalytic oxidation of CO conceivably the most extensively studied reaction in the history of heterogeneous catalyst and it is important in the context of cleaning the air and lowering the automotive emissions. Among a variety of manganese oxides catalyst, the Mn2O3 is considered to be the most favorable with respect to the catalytic activity. A novel route of reactive calcination (RC) of Mn-salts (Mn-Acetate and MnNitrate) for the synthesis of highly active catalysts was studied. The calcination of the precursor produced manganese oxide species (Mn2O3 and MnO2) in thermodynamic equilibrium, with the major nano-size Mn2O3 phase having a large surface area, and lower crystallinity. The amazing activity of the novel MnOx catalysts over the conventional ones obtained by calcination of the precursors in stagnant air, flowing air and reactive calcination (4.5%CO in air) in CO oxidation was associated with the presence of Mn2O3 and its unusual morphology as evidenced by XRD, SEM-EDX, XPS and FTIR characterization. The catalysts obtained by calcination of various MnOx precursors showed activity for the CO oxidation in the following order: Mn-Acetate > Mn-Nitrate and all the traditional route of calcination was also followed the same activity order. Further, the activity order of the catalysts obtained by various calcination conditions was as follows: RC > flowing-air > stagnant-air.
1. Introduction The catalytic oxidation of carbon monoxide (CO) receives a considerable attention due to its applications in many different fields like automobile vehicles, industry and environmental fields, such as personal respiratory protective devices, CO2 laser gas generation, proton exchange membrane fuel cells and automobile emission controls etc (Njagi et al., 2010; Didenko et al., 2011; Cai et al., 2012). It is widely used in the automobile vehicles to reduce CO from the environment, therefore it has been attracted much attention (Lee et al., 2016). CO has been termed as the silent killer for the 21st century (Xie et al., 2009). It is also called the carbonous oxide is a colorless, odorless, tasteless and nonirritating gas, which makes it difficult for humans to detect (Cholakov, 2000). This is a poisonous and life-threatening gas to humans and all other forms of air-breathing life, as inhaling even relatively small concentration of it can lead to serious injury, neurological damage and possibly death (Clarke et al., 2015). When CO enters the bloodstream it combines with hemoglobin and forms carboxy-hemoglobin, which reduces the oxygen-carrying ability of the blood. In the past 35 years, the catalytic converters have been installed on more
than 1000 million vehicles around the world (Ghaffari et al., 2008). The choice of appropriate catalyst is an important step for reducing the atmospheric pollution. After research for decades, two main types of catalysts for CO oxidation near room temperatures have been developed, the first type includes high surface area precious metal (Pt, Pd, Rh, Au, etc.) based catalysts and the second one transition metal (Cu, Mn, Cr, Co, Ni, Fe etc) based catalysts (Kanungo, 1979). The noble metals are widely used as catalysts for a long duration but due to its high price and sulfur poisoning, we have to search other substitute catalysts like mixed metal oxides and transition metal oxides for CO oxidation (Tang et al., 2006). The manganese oxide catalyst is also able for the oxidation of CO at a low temperature due to the presence of lattice oxygen on their surfaces (Dong et al., 2015). The size of catalysts has also been several advantages for CO oxidation purposes, because it's small size particles have a high surface area it causes the number of active sites presence per unit mass of the catalyst is increase (Halim et al., 2007; Hu et al., 2007). (see Table 5) The present work discussed about the experimental procedures to understand the characterization techniques applied to gain information about the manganese oxide catalyst structure and their activity for CO
Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. ∗ Corresponding author. E-mail address: [email protected] (S. Dey). https://doi.org/10.1016/j.apr.2018.01.020 Received 31 August 2017; Received in revised form 31 January 2018; Accepted 31 January 2018 1309-1042/ © 2018 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V.
Please cite this article as: Dey, S., Atmospheric Pollution Research (2018), https://doi.org/10.1016/j.apr.2018.01.020
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and Prasad, 2017). The RC process minimized a process step by converting two steps processes into single step process in a reactive CO-air mixture at a 300 °C (Trivedi and Prasad, 2016). Such single step thermal treatment of the precursor was called “RC method” by the authors (Dey et al., 2017). It is also produced MnOx catalysts with improved performance for CO oxidation as compared to the traditional method of calcination. It was also postulated that the RC method shows the concurrent multifarious phenomena for CO oxidation and precursor decomposition, cause a synergistic effect in the formation of the oxygendeficient catalyst surface at a low temperature.
oxidation. However, the relatively few comprehensive characterization studies of supported manganese oxide catalysts have appeared in comparison to those who devoted to the other transition metal oxides (Lee et al., 2001). This may in part, due to the fact that manganese forms very complex species with oxygen, especially in the presence of other cations and water, make a characterization of supported manganese oxide catalysts difficult (Roy et al., 2016). In addition, several manganese oxide catalysts can exist in different crystalline or pseudocrystalline forms, and a number of non-stoichiometric mixed-valence compounds are known, further complicating their characterization (Kapteijn et al., 1994). Emphasis has been put on the textural properties of the catalysts, there nature and distribution on the supported manganese oxide species (Jiao and Frei, 2010). The MnOx catalyst is preparing from the manganese nitrate precursor have obtained the high CO conversion efficiency by the addition of gold (Au/MnOx) catalyst (Jiao and Frei, 2010; Kapteijn et al., 1994). Among all the transition metal oxides, the Mn2O3 exhibits stable performance for CO oxidation. The reaction of Mn2O3 and O2, forming MnO2, is influenced by the oxygen concentration and temperature. Higher the oxygen concentration and lower the temperature are, the more conducive to the formation of MnO2 (Li et al., 2007). On the contrary, lower the oxygen concentration and higher the temperatures are, the slower the generation rate of MnO2 and Mn2O3 gradually forms as the reaction proceeds (Kondrat et al., 2011). Manganese oxide catalysts analysis by the XRD and FTIR is consistent with the presence of (Mn2O3, MnO2, and Mn3O4) phases, with the composition of strongly dependent on the calcination temperature between 200 and 300 °C (Jiao and Frei, 2010). When the catalyst is mainly MnO2, the hydroxyl radical oxidation mechanism is dominant and the content of Mn2O3 is high in the equilibrium phase of MnO2–Mn2O3 and the oxygen anion in oxidation mechanism is dominant (Najafpour et al., 2012). The oxidation state of manganese is highly influential when the using of manganese oxide as a support. The MnOx compounds have a typical benthollide structure containing the labile lattice oxygen (Balgooyen et al., 2016). The labile oxidation state facilitates Mn to act either as a reducing agent or an oxidizing agent. The oxygen storage capacity in the crystalline lattice and the capability of manganese to form the oxides with variable oxidation states determines its catalytic property (Ilton et al., 2016). The Au and MnOx interaction synergistically, exhibiting long-term CO oxidation at a low temperature with negligible activity decays (Solsona et al., 2004). The MnOx has high oxygen storage along with faster oxygen adsorption and oxide reduction rates, which can be used to reduce the amount of CO in the exhaust gas or natural gas vehicle (Manceau et al., 2012; Villalobos et al., 2003). Among the transition metals supported on manganese oxides are good substitute catalysts because of their low price and widespread use. The manganese or manganese oxide can exhibit higher activities per unit surface area than those of noble metal catalysts. The CuMnOx catalyst being produced under the oxygen-rich atmosphere and its confirmation of the retardation of copper oxide and reduction of manganese phases under the oxygen-deficient conditions, to produce residual Cu2O and Mn2+/3+ oxide phases (Jones et al., 2008). Several studies have shown that the preparation conditions and calcination strategy highly influenced on the activity of resulting catalyst. The success of MnOx catalysts has prompted a big deal of fundamental work devoted to the instructive role played by the each element and the nature of active sites (Zhanga et al., 2011; Solsona et al., 2004). The effect of surface area of the catalyst is the main reason for their high catalytic activity is also discussed in this research work. From the experimental results, we can get that the MnOx catalyst prepared by reactive calcination conditions is more active for the complete oxidation of CO as compared to the stagnant air and flowing air calcination conditions. The reactive calcination (RC) of the precursor is carrying out by the introduction of a low concentration of chemically reactive CO–Air mixture (4.5% CO) at a total flow rate of 32.5 ml min−1 over the hot precursor in a down flow bench-scale tubular reactor (Trivedi
2. Experimental 2.1. Catalyst preparation The manganese oxide catalysts were prepared from the precursors of manganese nitrate or manganese acetate separately. All the chemicals used for the preparation of catalyst they were of analytical grade. Ammonia (Verbièse, 25%) was added drop-wise to an aqueous solution of the Mn nitrate precursor (Mn(NO3)3H2O, 99.98%,Aldrich) or (Mn (CH3COO)2·4H2O, 99.98%, Aldrich) under constant stirring until the pH reached to a value of 10.0. The resulting solid was kept under stirring for 1 h. After filtration and washing, the solid was dried overnight at 110 °C and subsequently calcined in a similar procedure as above to yield the MnOx catalyst. The molecular weight of manganese nitrate [Mn(NO3)2·3H2O] and manganese acetate [Mn(CH3COO)2·4H2O] precursor was 178.95 g and 245.09 g respectively and converted into Mnoxide with the molecular weight (MnO2 = 87 g and Mn2O3 = 158 g). The 1 g MnO2 was equivalent to (87/178.95) for Mn(NO3)2·3H2O and 1 g Mn2O3 was equivalent to (158/245.09) for Mn(CH3COO)2·4H2O respectively (Dinh et al., 2015). The calcination of the precursor was done just before the activity measurement of the catalyst. It was carried out under the following three different conditions: (I) Stagnant air calcination: The calcination of the precursor was carried out in a furnace in the presence of stagnant air at 300 °C for 2 h to produce the catalyst. The calcined catalyst was stored in an air tight glass bottle. (II) Flowing air calcination: The calcination of the precursor was performed in situ under flowing air in the reactor at 300 °C for 2 h, just before the activity measurement experiment. (III) Reactive calcination: The calcination of the precursor was performed in situ under a flowing reactive mixture of 4.5% CO-Air at 300 °C for 2 h in a compact bench scale of fixed bed tubular reactor, just before the activity measurement experiment. The reactive calcination of differently prepared catalyst samples was carried out by passing a CO-Air mixture over the precursors at 160 °C for 30min and 300 °C for 1hr for the total decomposition of the catalyst. The RC process minimized a process step by converting two steps processes into single step process. It was also produced catalysts with improved performance for CO oxidation (Dey et al., 2017). 2.2. Characterization The X-ray diffraction (XRD) measurement of the catalyst was carried out by using Rigaku D/MAX-2400 diffractometer with Cu-Kα radiation at 40 kV and 40 mA. The mean crystallite size (d) of the catalyst was calculated from the line broadening of the most intense reflection using the Scherrer Equation. It provides information about the structure, phase, crystal orientation, lattice parameters, crystallite size, strain and crystal defects etc. d = 0.89λ/βcosθ
(1)
Where d is the mean crystallite diameter, 0.89 is the Scherrer 2
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min. To monitor the flow rate of CO and air passing through the catalyst presence in the reactor was done by the digital gas flow meters. The CO oxidation was analysis by the gas chromatogram to measure the activity of the resulting catalyst. Pure α–alumina spheres were used in the preheating section and the section after the catalyst bed. Eq. (2) can be representing the air oxidation of CO over the catalyst. For controlling the heating temperature of the catalyst present in a reactor was done by a microprocessor based temperature controller (Berger et al., 2002).
constant, l is the X-ray wave length (1.54056 Å), and b is the effective line width of the observed X-ray reflection, calculated by the expression β2 = B2-b2 (where B is the full width at half maximum (FWHM), b is the instrumental broadening) determined through the FWHM of the X-ray reflection at 2θ of crystalline SiO2. The Fourier transform infrared spectroscopy (FTIR) analysis was done by the Shimadzu 8400 FTIR spectrometer in the range of 400–4000 cm−1 and the wavelength range of Shimadzu 8400 FTIR was (700 nm - 25 μm). To calibrate the optical path length of a white cell of FTIR L = 21 m by means of a reference gas cell of an optical path length of Lref = (0.2003815 ± 0.000062) m. For the mid-IR region, 2−25 μm (5000–400 cm−1), the most common source is a silicon carbide element heated to about 1200 K. It provides information about the kind of materials present in a catalyst sample by their peak values. The scanning electron microscope (SEM) used a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. It was produced the topographical image of a catalyst by an electron beam and the image of catalyst was recorded on Zeiss EVO 18 (SEM) instrument. The accelerating voltage was used 15 kV and magnification of the image 5000X was applied. Areas ranging from approximately 1 cm to 5 μm in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50–100 nm). It provides information about the average aggregate size, crystallite degree and the microstructures of the catalyst. The best resolution is about 2–5 nm but many routine studies are satisfied with a lower value and exploit the case of image interpretation and the extraordinary depth of field to obtain a comprehensive view of the specimen. The SEM has more than 300 times the depth of field of the light microscope. The resolution we choose to image will obviously affect the number of pixels per row as well as the number of rows that constitute the scanned area. Magnification will increase if we reduce the size of the area scanned on the specimen. Magnification area scanned on the monitor/area scanned on the specimen. The X-ray photoelectron spectroscopy (XPS) analysis of the catalyst was measured with Amicus spectrometer equipped with Al Kα X-ray radiation at a voltage of 15 kV and current of 12 mA to measure the binding energy used for the calibration of adventitious carbon C(1s) present in the catalyst. The C(1s) peak is often used as an internal standard for the calibration of the binding energy scale. When X-ray photons strike a surface, they cause the emission of electrons. It provides information about the surface compositions and chemical states of the different constituent elements present in a catalyst. The features of charge compensation for the amicus spectrometer are patented ‘in the lens’ multi-mode electrostatic flood source, 0–5eV (mode 1) charge compensation, 0–1000eV (mode 2) reels/imaging/alignment, up to 500 μA electron emission, additional dual beam (ion electron) flood source. The evolution of XPS, the ability to compensate for surface charging and accurately determine the binding energies, particularly with electrically in homogenous samples. The Brunauer Emmett Teller Analysis (BET) provides information about the specific surface area, pore volume and pore size of the catalyst. The isotherm was recorded by micromeritics ASAP 2020 analyzer and the physical adsorption of N2 at the temperature of liquid nitrogen (−196 °C) with a standard pressure range of 0.05-0.30 P/Po.
2CO + O2→2CO2
(2)
The gaseous products were produced after the oxidation reaction in a reactor was analysis by an online gas chromatogram (Nucon series 5765) equipped with a porapack q-column, FID detector and a methaniser for measuring the concentration of CO and CO2. Where, the concentration of CO was proportional to the area of chromatogram ACO. The overall concentration of CO in the inlet stream was proportional to the area of CO2 chromatogram. (XCO) = [(CCO)in - (CCO)out] / [CCO]in = [(ACO)in - (ACO)out] / [ACO]in (3) The conversion of CO at any instant was calculate 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. (3). 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 the chromatogram of CO2 formed (ACO )out by the oxidation of CO. 3. Results and discussions The characterization of differently prepared catalyst samples in RC conditions was done by the following techniques and their activity for CO oxidation was discussed below. 3.1. Catalyst characterization The characterization of catalyst was carried out information about the morphology, surface area, binding energy, pore volume, pore size, chemical state, material composition and the percentages of different materials present on the surface of catalyst (see Figs. 1, 3 and 5). 3.1.1. Scanning electron microscopy analysis Morphology of the prepared catalyst samples in RC conditions was done by the scanning electron microscopy analysis. It was shown (Fig. 2) that the large differences in the surface texture and the other property of various MnOx catalysts. The SEM image clearly shows that the using of different precursors in the preparation of various catalysts makes a large difference in their morphology. In addition, the smaller size particles and the good distribution of active phase presence on the surface layer of catalyst, which causes a significant increasing of the effective surface area of the catalyst (Njagi et al., 2011). As seen in the SEM micrograph, the particles were comprised of coarse and fine by RC of Mn-N and Mn-A catalyst respectively. The particle size of Mn-N and Mn-A catalyst was 1.245 μm and 0.345 μm respectively. The particles present in the Mn-A catalyst was smaller size, less agglomerated and homogeneous as compared to the other catalyst sample (Jones et al., 2008). The particle size of the catalyst was also confirmed by the SEM image analysis and it was also observed that the particle size of the catalyst was increased in the following order: MnN > Mn-A. With the decreasing of particle size of the catalyst, therefore, more and more CO dispersed on their surfaces, it causes activity of the catalyst has been increased. The surface reconstruction behavior of
2.3. Catalytic activity measurement The oxidation of CO was carried out under the following reaction conditions: 100 mg of catalyst was diluted to α-alumina with feed gas consisting of a lean mixture of (2.5 vol% CO in air) and the total flow rate was maintained at 60 mL min−1. The. The air feed into the reactor was made free from moisture and CO2 by passing through it CaO and KOH pellet drying towers. The catalytic experiment was carried out under the steady state conditions and the reaction temperature was increased from room temperature to 300 °C with a heating rate of 2 °C/ 3
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Fig. 1. Schematic diagram of Experimental Set up.
Prasad, 2016). The decreasing of oxygen concentration in the surface layer of catalyst, therefore, the activity of the catalyst has been increased. Due to the presence of high oxygen deficiency in the Mn-A catalyst, therefore the activity of the catalyst has been increased (Dey et al., 2017). The high level of oxygen deficiency was created the high density of active sites present on the catalyst surfaces. It was also confirmed that the presence of pure oxides phases in the surface of catalyst was a good harmony with the XRD and FTIR results also.
the different size of particles presents in a catalyst surfaces during the prolonged exposure of CO gas (Mirzaei et al., 2003) (see Table 1).
3.1.2. Elemental analysis It was very clear from the SEM-EDX analysis that all the samples of catalysts were pure due to the presence of their respective element peaks only. After the SEM micrography was taken, the elemental mapping of different catalysts sample was analyzed to determine the elemental concentration distribution of the catalyst surface. The SEMEDX was performed on the different spots of cross-sectioned of the catalyst granules to determine the concentration of different elemental groups present at different locations on the catalyst surfaces. It was very clear from the SEM-EDX analysis that the entire catalyst sample was pure and there was not present any types of impurities in the catalyst surfaces (Mirzaei et al., 2003). In Table 2 we have to discuss about the relative atomic percentage and weight percentage of different elemental groups' present on the catalyst surfaces (Dey et al., 2017). The atomic and weight percentage of oxygen present in the surface layer of MnOx catalysts were decreased in the following order: Mn-N > Mn-A. The presence of high concentration of oxygen in the surface layer of catalyst, it causes activity of the catalyst has been decreased (Dey et al., 2017; Boreskov, 1966). This is the reason for Mn-N catalyst has shows a poor performance for the CO oxidation. It was very clear from the table and figure that the atomic and weight percentage of manganese present in a Mn-N and Mn-A catalyst was higher than oxygen (Trivedi and
3.1.3. X-ray diffractogram XRD analysis of the catalysts (Fig. 4) was used to determine the final phases after heat treatment at 300 °C in RC conditions. It was carried out to identify the crystallite size and coordinate dimensions present on the surface layer of the catalysts. The phase analysis of the differently prepared catalyst samples was done by the XRD studies (Villalobos et al., 2003). In the Mn-N catalyst, their diffraction peak at 2-Theta (2θ) was 37.25 and their corresponding lattice plane (h k l) value of largest peak was (1 2 1). The structure was end centered; monoclinic MnO2 phase at the JCPDS reference no. (89–2545) and the crystallite size of the catalyst was 7.19 nm. In the Mn-A catalyst, their diffraction peak at 2-Theta (2θ) was 36.95 and their corresponding lattice plane (h k l) value of largest peak was (1 1 1). Structure was end centered; monoclinic Mn2O3 phase at the JCPDS reference no. (89–2530) and crystallite size of the catalyst was 6.89 nm. The crystallite size of the catalyst Fig. 2. SEM image of the catalysts (A) Mn-N and (B) Mn-A.
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Fig. 3. The SEM-EDX image of the catalysts (A) Mn-N and (B) Mn-A.
highly active for the complete oxidation of CO at a low temperature. The activation or deactivation periods on the catalytic reaction and the loss or gain of catalytic activity on the reaction may be related to the appearance or loss of specific bulk phases. After the SEM and XRD analysis, we have to get that the crystallite size and particle size of the catalyst was followed the same order.
Table 1 Nomenclature of the catalyst samples in this study was as follows. Catalyst Name
Nomenclature
Mn(NO3)2·3H2O Mn(CH3COO)2·4H2O
Mn-N Mn-A
3.1.4. Fourier transforms infrared spectroscopy The identification of the metal-oxygen bonds present in the catalyst surfaces was made by the Fourier transform infrared spectroscopy (FTIR) analysis. In the invested region (4000-400 cm−1) to obtain the entire absorption spectra of different peaks to indicates the presence of different elemental groups (bond) in the catalyst samples. All the catalysts were prepared in RC conditions before applying in different characterization work. In the Mn-N catalyst at the transmittance conditions, there were seven peaks we obtained. The IR band (3590 cm−1) has shown the presence of -OH bond, (2350 cm−1 and 679 cm−1) has shown the presence of COO bond, (1640 cm−1 and 1530 cm−1) has shown the presence of Mn2O3 bond and (1250 cm−1 and 525 cm−1) has shown the presence of MnO2 and CO32- bond respectively. In the Mn-A catalyst at the transmittance conditions, there were six peaks we obtained. The IR band, (3590 cm−1) has shown the presence of -OH bond, (2350 cm−1) has shown the presence of COO bond, (1640 cm−1 and 1530 cm−1) has shown the presence of Mn2O3 bond, (525 cm−1) has
Table 2 The atomic and weight percentage of catalysts by EDX techniques. Catalyst
Elements atomic (%)
Elements weight (%)
Mn-A Mn-N
Mn(85.75) Mn (72.22)
Mn (84.80) Mn (71.85)
O (14.25) O (27.78)
O (15.20) O (28.15)
was increased in the following order: Mn-N > Mn-A and it matches with the SEM image analysis of the catalysts. The crystalline acetate phase was disappeared between 110 °C and 140 °C to leave a predominantly amorphous phase. The next observable phase change occurred at 240 °C with the broad peaks matching with the reflections of Mn2O3 becoming apparent at 35.7 and 36.72 at θ. The Mn2O3 phase was the most crystalline form, producing the narrow size high intensity diffraction lines (Dey et al., 2017). The experimental result was also confirmed that the lower particle size of Mn-A catalyst is
Fig. 4. XRD analysis of the catalysts (A) Mn-A and (B) Mn-N.
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Fig. 5. FTIR analysis of the catalysts (A) Mn-A and (B) Mn-N.
shown the presence of MnO2 bond and (1250 cm−1) has shown the presence of CO32- bond (Mirzaei et al., 2003). After the FTIR analysis, we have to observe that there will be some impurities like carbonate group and hydroxyl group presence in the catalyst sample. All the sample of catalysts; this originates from the stretching vibrations of the metal-oxygen bonds and confirm the presence of Mn2O3 and MnO2 peaks (Dey et al., 2017). This FTIR spectrum shows the presence of various functional groups on the surface of the as-synthesized MnOx catalyst. No organic groups were found to be adsorbed on the surface of the basis of FTIR spectra. The concentration of the nitrates or acetate on the reactive surface of MnOx catalyst with manganese content corresponding to a monolayer is considerably greater than that of the sample with higher manganese loading.
Table 4 Textural property of the catalysts.
Binding energy of Elements
Mn-A Mn-N
Mn2O3 MnO2
Mn (656.542) Mn (656.178)
Organic C=O Organic C-O
Pore Volume (cm3/g)
Ave. Pore Size (Å)
Mn-N Mn-A
35.90 37.84
0.430 0.480
48.36 46.05
Calcination Strategy
Reactive Calcination Flowing air Calcination Stagnant air Calcination
Mn-N catalyst
Mn-A Catalyst
Ti
T50
T100
Ti
T50
T100
25 °C 30 °C 30 °C
80 °C 100 °C 110 °C
160 °C 195 °C 220 °C
25 °C 30 °C 30 °C
70 °C 80 °C 100 °C
130 °C 150 °C 190 °C
and Mn2p1/2 transition at 644.3 and 656.24eV, respectively. The deconvolution of the Mn 2p3/2 transition exhibited the presence of Mn III (643.6eV) with a small shoulder in 645.6eV assigned to Mn II. From the Fig. 6, we have to observe that the Mn ions present in a Mn-N and Mn-A catalyst was MnO2 and Mn2O3 form respectively. The Mn2O3 has shown a broad peak at 645–657eV, which can be assigned to a mixed oxidation state of Mn2+ and Mn3+. From the table and figure, it was clear that the binding energy present was highest in Mn-A catalyst as compared to the other catalyst samples (Trivedi and Prasad, 2017). The XPS results suggest that the surface manganese oxide species were least partially reduced, which may lead to the formation of Mn4+/Mn3+/2+ redox couples on the surface of catalyst therefore promote the catalytic activity for CO oxidation. Although, it can be proposed that the highest binding energy was preferably for the CO oxidation (Dey et al., 2017). The increasing of Mn ions concentration in the Mn-oxide catalyst was helpful to the migration of oxygen from bulk to the catalyst surface, which can promote the activation and transportation of active oxygen species on the surface of catalyst. The binding energy of oxygen O (1s) spectra was illustrated in Fig. 7. In the XPS analysis, there were two diverse types of oxygen species present in the catalyst samples. First was known as chemisorbed oxygen (denoted as Oa, such as O22−, O−, OH−, CO32−, etc.) and second was known as lattice oxygen (denoted as Ol, such as O2−). In our case, the oxygen with the binding energy of (532.80eV-535.46eV) was the main form and could be assigned to the chemisorbed oxygen (Oa).
Table 3 The chemical state and binding energy of Mn-oxide Catalyst. Chemical state of Elements
Surface Area (m2/g)
Table 5 Activity test of Mn-oxide catalyst for CO oxidation.
3.1.5. XPS analysis With the help of XPS analysis, we have to calculate the surface valence state, binding energy and the chemical state of different elemental groups' present on the surface layer of catalysts. All the catalyst samples were prepared in RC conditions and their binding energy was preferably for the CO oxidation. In Table 3we have to discuss the binding energy and chemical state of different elemental groups present on the catalyst surfaces. The evaluation of Mn(2p) narrow scans allows the assignment of the oxidation state of Mn. By performing peak fitting deconvolution of the main Mnp3/2 in all three catalyst samples can be divided into the three components including Mn4+, Mn3+ and satellite. Since the differences between the binding energy values of Mn3+ and Mn4+ ions were small. The binding energy of Mn(2p) element present in a Mn-A and Mn-N catalyst was (645.946eV and 656.542eV) and (643.816eV and 656.178eV) respectively (see Tables 4 and 5). The observed binding energies 643.816eV, 645.946eV and 656.178eV were associated with the presence of Mn3+, Mn4+ and satellite respectively in all the three samples. The evolution of XPS, to ability the compensate for surface charging and accurately calibrate the binding energy scale and found +2 and +3 on the surface of MnOx catalyst. The pristine Mn2O3 spectrum was characterized by Mn2p3/2
Catalyst
Catalyst
O (532.113) O (532.274)
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Fig. 6. XPS analysis of the Mn(2p) in MnOx catalysts (A) Mn-N and (B) Mn-A.
structural varieties of Mn-oxide catalyst. It was possible to determine the FTIR spectra of MnOx phases and the XPS analysis at different precursor prepared catalyst to evidence a continuous evolution leading to the simultaneous presence of several MnOx phases.
The O(1s) peak in MnOx, Mn2O3, and MnO2 displayed the presence of Mn−O−Mn typical for the manganese oxides. The oxygen peaks spectra present in a Mn-A catalyst was much broader and more intensive than the other catalyst samples. Therefore, it was suggested that the highest binding energy of Mn-A catalyst was more preferable for the selective catalytic oxidation reaction (Njagi et al., 2010). The formation of adsorbed reactive oxygen species, such as superoxide ions (O2–), might be correlated to the presence of surface oxygen vacancies on the metal oxide support or at the metal–support interface. The oxygen vacancy present in the MnOx catalyst was responsible for the decreased lattice constant in Mn2O3 and MnO2 samples. The chemical state of C(1s) present in the Mn-oxide catalyst was C-O-C form and the binding energy of C(1s) was 286eV. The inelastic scatter of electrons through the adventitious carbon layer may cause a large change in background shape when compared with the spectrum from a clean sample. In summary, differences in the coordination environment of Mn (II) in MnO2 and Mn2O3 only slightly perturb the shape of the Mn(3p) lines, but appreciably affect on the Mn2p3/2. FTIR and XPS analysis provide information about the various oxidation states and compared the calcination of various manganese precursors. Our main work was to evaluate the use of FTIR spectroscopy and XPS analysis to probe the
3.1.6. BET surface area Nitrogen adsorption/desorption isotherms were analyzed to determine the specific surface area, pore volume and pore size distribution of the manganese precursors and their oxides obtained after the heat treatment process. The surface area of differently prepared catalyst samples like Mn-A and Mn-N in RC conditions was 37.84 m2/g and 35.90 m2/g respectively. The pore volume and pore size of the Mn-A catalyst was slightly higher than the Mn-N catalyst. The textural properties like surface area, pore volume and pore size of the catalysts were more favored for the CO oxidation. The specific surface area calculated using the BET method was reported in the above Table (Trivedi and Prasad, 2016). A large number of more pores present in a catalyst surface mean more numbers of CO molecules were trapped and they have to shows the better catalytic activity at a low temperature (Mirzaei et al., 2003). The porosity of the Mn2O3 nano-particles can be explained by considering the voids present in the particles on the
Fig. 7. XPS analysis of the O(1s) in MnOx catalysts (A) Mn-N and (B) Mn-A.
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Fig. 8. Activity test of Mn-N and Mn-A catalyst in A) Stagnant air, B) Flowing air and C) Reactive calcination.
network as the “pores” detected by the N2 adsorption–desorption measurements. The specific surface area of the various manganese oxide catalysts was measured by the BET analysis and it was also matched with the SEM and XRD results. However, we believe that a larger pore size to be advantageous for application as electro-catalysts, as electrochemical processes often produce the solid products, which might be easily clog pores in the micro- and meso-porous range (Dey et al., 2017).
catalyst, which was lowered by 10 °C than that of Mn-N catalyst respectively. The total oxidation temperature of CO was 190 °C for Mn-A catalyst, which was less by 30 °C than that of Mn-N catalyst. In comparison, between the stagnant air and flowing air calcination conditions, we have to observe that the flowing air calcination produced catalysts has shown the best catalytic activity for CO oxidation at a lower temperature as compared to the stagnant air calcination conditions. Fig. 8(B) shows that the initial oxidation of CO by these catalysts in flowing air calcination was 30 °C and half conversion of CO has occurred at 80 °C over the Mn-A catalyst, which was less by 20 °C than that of Mn-N catalyst. The total oxidation temperature of CO was 150 °C for Mn-A catalyst, which was less by 45 °C than that of Mn-N catalyst. The order of activity of Mn-oxide catalyst in flowing air calcination conditions was as follows: Mn-A > Mn-N. The improved catalytic activity of Mn-A catalyst can be ascribed to the unique structural, textural characteristics and the smallest crystallite size. The rate of CO oxidation was increase with time and flattened at the end. This might be due to the synergistic effects of exothermic oxidation, decomposition and redox surface reaction of the catalyst. In Fig. 8(C), we have to observe that the comparative study of CO
3.2. Catalyst performance and activity measurement Activity test of the catalyst was carried out to evaluate the effectiveness of different manganese oxide catalysts (Mn-A and Mn-N) as a function of temperature. Their activity was measured in different calcination conditions like stagnant air, flowing air and reactive calcination conditions. Effectiveness of the manganese oxide catalysts was increased with the increasing of temperature from room temperature to a certain high temperature for full conversion of CO as shown in the Fig. 8. In the stagnant air calcination (SAC) conditions Fig. 8(A), shows that the oxidation of CO was initiated at 30 °C in both the catalyst and the half conversion of CO was occurred at 100 °C over the Mn-A 8
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oxidation over various Mn-oxide catalysts prepared by RC conditions. The extraordinary performance of the resulting catalysts was achieved for full conversion of CO at a lower temperature (Dey et al., 2017). The novelty of the catalysts produced by RC conditions matched with their characterization results. The oxidation of CO was just initiated in RC conditions at 25 °C over the catalysts and the half conversion of CO was achieved at 70 °C over the Mn-A catalyst, which was less by 10 °C than that of Mn-N catalyst respectively. The total oxidation temperature of CO was 130 °C for Mn-A catalyst, which was less by 30 °C than that of Mn-N catalyst respectively. The light-off temperature showed that the RC route prepared catalyst was more active for CO oxidation at a low temperature as compared to the traditional method of calcination (Njagi et al., 2011). The activity order of the catalysts for CO oxidation was in accordance with their characterization by XRD, SEM-EDX, FTIR, XPS and BET analysis was as follows: Mn-A > Mn-N. The presence of highly dispersed and more specific surface area, which will be more favorable for the CO oxidation.
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4. Conclusions The preparation conditions and calcination strategies have a great influence on the activity of resulting MnOx catalysts for CO oxidation. From the results and discussions, we have to conclude that the RC route was the most appropriated calcination strategy for the production of highly active manganese oxide catalyst for the oxidation of CO. The calcination order with respect to the performance of catalyst for CO oxidation was as follows: Reactive calcination > Flowing air > Stagnant air. In the activity test, we have to observe that the MnA catalyst prepared by RC conditions showed the best catalytic activity for CO oxidation at a low temperature (130 °C). The order of activity for various catalyst samples in different calcination conditions was as follows: Mn-A > Mn-N. The characterization by various techniques (XPS and FTIR) of the catalysts produced by RC revealed that the presence of Mn2O3 phases with the minor MnO2 phases in Mn-A catalyst. Therefore, it was suggested that the smallest particle size of Mn-A catalyst was highly active for the CO oxidation at a low temperature as compared to the other catalyst samples. These results have allowed us to conclude some characteristic FTIR and XPS peaks that was, observed, for the various transition MnO2 and Mn2O3 phases. Using these FTIR and XPS characterization, it was possible to detect the presence of transition MnOx naturally grown on Mn2O3-former catalyst. References Balgooyen, S., Alaimo, P.J., Remucal, C.K., Ginder-Vogel, M., 2016. Structural transformation of MnO2 during the oxidation of bisphenol A. Environ. Sci. Technol. 51 (11), 6053–6062. Berger, R.J., Perez-Ramirez, J., Kapteijn, F., Moulijn, J.A., 2002. Catalyst performance testing: the influence of catalyst bed dilution on the conversion observed. Chem. Eng. J. 90 (1–2), 173–183. Boreskov, G.K., 1966. Forms of oxygen bonds on the surface of oxidation catalysts. 41, 263–276. Cai, L., Guo, Y., Lu, A., Branton, P., Li, W., 2012. The choice of precipitant and precursor in the co-precipitation synthesis of copper manganese oxide for maximizing carbon monoxide oxidation. J. Mol. Catal. A Chem. 360, 35–41. Cholakov, G.S., 2000. Control of exhaust emissions from internal combustion engine vehicles. Pollut. Control Technol. 3, 1–8. Clarke, T.J., Davies, T.E., Kondrat, S.A., Taylor, S.H., 2015. Mechano-chemical synthesis of copper manganese oxide for the ambient temperature oxidation of carbon monoxide. Appl. Catal. B 165, 222–231. Dey, S., Dhal, G.C., Mohan, D., Prasad, R., 2017. Study of Hopcalite (CuMnOx) catalysts prepared through a novel route for the oxidation of carbon monoxide at low temperature. Bull. Chem. React. Eng. Catal. 12 (3), 393–407. Didenko, O.Z., Kosmambetova, G.R., Strizhak, P.E., 2011. Size effect in CO oxidation over magnesia-supported ZnO nanoparticles. J. Mol. Catal A 335, 14–23.
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