Control of Carbon Monoxide (CO) from Automobile Exhaust by a

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Control of Carbon Monoxide (CO) from Automobile Exhaust by a Dealuminated Zeolite Supported Regenerative MnCo2O4 Catalyst P. S. Arun,† B.P. Ranjith,‡ and S. M. A. Shibli*,† †

Department of Chemistry, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala, 695 581 India Department of Mechanical Engineering, College of Engineering, College of Engineering, Trivandrum, Kerala, 695 516, India

‡

S Supporting Information *

ABSTRACT: We synthesized MnCo2O4 catalyst with very high porosity on the surface of dealuminated zeolite molecular sieves (DAZMS) for CO oxidation under actual automobile conditions. The MnCo2O4 catalyst was selected on the basis of preliminary DFT study using the software ADF BAND. The MnCo2O4 catalyst had comparatively higher CO adsorption energy and very low oxygen vacancy formation energy. The synthesized MnCo2O4/ DAZMS catalyst was characterized by XRD, XRF, BET, SEM, and Confocal Microscopy. The Confocal microscopic analysis revealed that porosity of the dealuminated zeolite surface was significantly enhanced after the catalyst loading process. The completely precious metal free and DAZMS-supported catalyst exhibited excellent CO oxidation ability with renewed activity for seven months under actual automobile conditions with reference to normal and cold start conditions. The synthesized MnCo2O4/ DAZMS not only exhibited surprisingly high catalytic activity for CO oxidation at a temperature resembling a cold start period but was also sufficiently stable/active under actual automobile conditions and ambient conditions containing large amounts of CO,H2O,CO2, and NOx at 155−715 °C. These significant results revealed the flexible use of the present catalyst system for a wide variety of automobiles from a small gasoline-fuelled vehicle to a large diesel-fuelled vehicle that may produce high COcontent exhaust.

1. INTRODUCTION Automobiles, due to incomplete combustion of hydrocarbon fuels in internal combustion engines, produce carbon monoxide (CO), a hemo-toxic chemical. There are extensive recent reports from newspapers and magazines highlighting numerous deaths caused around the world due to the automobile carbon monoxide. The defense offered by the scientific community against such CO pollution began in the early forties with the exploration of a hopcalite based catalytic converter, a mixture of manganese and copper oxides.1 Subsequently more effective noble metals, such as Pt,2−5 Pd,4,6−8 Au,9−13 Ru,4,14,15 and Rh16−18 based catalysts were explored for efficient conversion of CO. These catalysts, under actual automobile conditions, get exposed to heat and hence they become agglomerated, resulting in loss of active surface area.19Owing to this problem, the conventional catalysts are incorporated with excess of precious metals to ensure continuous catalytic activity and constant efficiency.19The very high consumption of precious metal catalysts led to exploration of self-regenerative catalysts which can vehemently suppress self-agglomeration during or after the use.20 Most of the self-regenerating catalysts are composed of precious metals at the core.19,20 Critical requirement of these precious metals are predicted in the coming days due to large production of automobiles especially in Asian countries and also due to stringent emission control norms.20 Many attempts © 2013 American Chemical Society

to develop, the so-claimed transition metal based catalysts did not get due recognition due to their disparity in their performance against their proposed extent of activity.21 In this context, development of such regenerative transition metal based catalysts with constant and efficient performance under actual automobile conditions is the need of the hour and it became the objective of the present study. Extensive reports are available on the oxides of Co and Mn, which can act as effective catalysts for complete oxidation of CO.1,22−29 Co3O4 and Co3O4-supported systems have been well proven to be highly active for CO oxidation.1,30−37 Different mechanisms of CO oxidation on Co 3 O 4 catalyst have been introduced.32,38−41Co3O4 catalyst is also known for nitrous oxide abatement.42 These catalysts are severely deactivated by trace amounts of moisture that are normally present in the feed gas/ automobile exhaust.1,43 Molecular sieve-traps can be used to provide dry condition.1 Clusters of Cobalt−manganese oxide/ layered manganese oxide can be used as efficient catalysts for water oxidation.44,45 Manganese oxides possess a wide range of simple and mixed compositions with Mn atoms at different Received: Revised: Accepted: Published: 2746

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to eq 1, then a steady adsorption occurs at the surface of the catalyst as described elsewhere.41

oxidation states such as MnO2, MnO3, Mn5O8, Mn2O3, and Mn3O4. According to Zener,46 these oxides establish the necessary electron mobile environment for an optimal surface redox catalyst. Recently, mesoporous cobalt oxide and manganese oxide have been explored for efficient removal of CO.47 (Mn,Co)3O4 spinel systems are used as effective catalytic materials for a variety of applications.48 On the basis of these available literature reports we first computationally studied the activity of MnCo2O4 spinel catalyst using density functional theory (DFT).41,49−54 We synthesized highly porous MnCo2O4 catalyst for the complete oxidation of CO from automobile exhaust. We proposed a modified zeolite as catalyst support after observing dissatisfactory results with other catalyst supports. The zeolite support was further significantly modified on sensing three major problems, namely: (a) there is a possibility of exchange of Na ion from zeolite by Co/Mn of the catalyst during its preparation; (b) the probability of dealumination that may happen in zeolite causing suppression of the catalytic activity during the application; and (c) zeolite easily adsorbs CO at normal and ambient conditions55 and may affect the activity of the catalyst if the zeolite molecular sieves are used as such as the support. In this context, a DAZMS supported MnCo2O4 catalyst was synthesized and explored for steady oxidation of CO from actual automobile exhaust.

Eads = Eadsorbate + Esubstrate − Eadsorbate/substrate

(1)

41,58

According to Mars-van Krevelen mechanism the surface oxygen oxidizes CO molecule and removes it from the surface leaving behind an oxygen vacancy. Oxygen vacancy formation energy must be calculated to understand about the extent of CO oxidation efficiency.50 Actually, the oxygen vacancy formation energy must be calculated to understand regarding what extent the oxide becomes a better oxidant. According to Ganduglia-Pirovano et al., the average oxygen vacancy formation energy can be described by the following equation.59 Ef (O) = 1/Ndef (Edef − Efree + Ndef Eo)

(2)

where Edef, Efree are the total energy of the system with and without oxygen vacancy respectively. Eo and Ndef correspond to the energy of free oxygen atom and the number of oxygen vacancies respectively. Significant results favoring the selection of the proposed MnCo2O4 catalyst were obtained from the computational study and hence further experiments were proceeded accordingly, leaving all other Mn or Co based catalyst systems initially considered.

3. EXPERIMENTAL SECTION Materials. Sodium zeolite molecular sieves pellets of porosity: 5 Å, Si/Al ratio: 1.78 and surface area: 570m2/g, were purchased from Zeolites & Allied Products Pvt. Ltd., India. CoCl2·6H2O, MnCl2·4H2O, isopropanol and Nile Blue A were purchased from Sigma Aldrich. Synthesis of DAZMS. Sodium zeolites molecular sieves were calcined at 800 °C for 5 h and then cooled down to room temperature rapidly. The samples were carefully washed with water and dried at 115 °C for 1h followed by steaming at 600 °C for 5h in pure steam of 1 bar pressure followed by 2 M HCl reflux at 100 °C. The content was carefully washed and dried at 250 °C in order to obtain DAZMS. All of the conditions were fixed based on reproducibility of the characteristics of the products, obtained after physico chemical analysis. Synthesis of MnCo2O4/DAZMS. A small amount of aqueous solution containing MnCl2·4H2O and CoCl2·6H2O with a mole ratio 1:2 was heated in a Pyrex glass beaker on a hot plate kept at 100 °C, and evaporated to dryness. Excess water isopropanol (1:1) mixture along with an appropriate amount of DAZMS were added, then heated at 350 °C with stirring, and then evaporated to dryness. The mixture in a silica crucible was placed in a furnace at 470 °C for 12 h. All of the conditions were fixed based on reproducibility of the characteristics of the products, obtained after physico chemical analysis. Characterization. XRD patterns of the samples were recorded from a Bruker AXS D8 Advance Diffractometer using Cu Kα radiation. XRF analysis was carried out using a Bruker Model S4 Pioneer Sequential Wavelength Dispersive Xray Spectrometer. The morphology of the samples was analyzed by SEM, using a Jeol Model JSM-6390LV microscope. The porosity was analyzed by Confocal Microscopy, using an Olympus Confocal Scanning Microscope. Samples for confocal fluorescence microscopy were prepared by following the method of Buurmans et.al.60 Surface area was analyzed by BET method using TriStar 3000 V6.05A surface area analyzer from NIIST Trivandrum, Kerala, India. A reference sample was

2. COMPUTATIONAL STUDY OF MnCo2O4(110) All of the DFT calculations were performed by using ADF (Amsterdam density functional) BAND (G.te Velde and E.J. Baerends, Phy. Rev. B 1991, 44, 7888) software and accordingly the MnCo2O4(110) surface was modeled as a two layered slab. Spin-restricted calculations were performed by generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE)56,57 and all of the atoms in the two-layered (110) surface were relaxed. The DZ basis set was used for all the atoms including Co and Mn. The ionic cores were described by scalar relativistic ultra pseudo potentials. It is clear from available literature,1 that the (110) plane of Co3O4 on its surface actually catalyzes CO oxidation efficiently. For that reason the (110) plane was focused for the present computational study1 as well. All of the results of DFT calculations of MnCo2O4(110) were compared with Co3O4(110), a wellknown catalyst for CO oxidation.1,41 The optimized slab structure of Co3O4 (110) and MnCo2O4 (110) are shown in Figure 1. The adsorption energy of CO and O2 on the catalyst surface has a crucial role for CO oxidation.41 If the CO adsorption energy of the catalyst has a positive value according

Figure 1. The optimized two layer slab structure of (A) Co3O4 (110) and (B) MnCo2O4 (110). 2747

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analyzed and the accuracy was confirmed prior to the analysis of the actual sample, each time. Catalytic Activity. Performance of the catalyst was analyzed using a four stroke diesel engine of ATUL Group of industries, India. A CO gas analyzer manufactured by Nevco Engineers Pvt.Ltd., model KM900PLUS was also used. The DAZMS supported MnCo2O4 catalyst was filled in an iron pipe and then fitted the catalyst chamber along with the exhaust of the actual engine. The performance was monitored continuously and intermittently under actual and under simulated conditions. Different batches of the experiments were conducted using particular sets of the catalyst chambers fitted with the exhaust system and the average results are reported here. The details of the engine and the operational conditions during the analysis of catalytic activity are shown in S3 of the Supporting Information, SI.

catalyst possesses very high CO adsorption energy and lower oxygen vacancy formation energy compared to Co3O4, a well established CO oxidant. Hence MnCo2O4 alone was focused for further experimental evaluation. 4.2. The Support for MnCo2O4 Catalyst. Various catalyst supports including activated carbon, silica, and mild steel were attempted to hold the catalyst, during the preliminary studies. However, none other than zeolite yielded satisfactory results in terms of long-term stability and activity under actual automobile conditions. Metals present in the zeolite framework have been identified as they behave as highly catalytically active species for CO and benzene oxidation.61−63Accordingly, zeolite was considered initially to develop a support for the proposed MnCo2O4 catalyst. Initially, zeolite molecular sieves as such were used as the catalyst support and their characteristics were analyzed. Zeolite exhibited satisfactory results in terms of stability. But zeolite could not be considered as an ideal support since zeolite can adsorb CO and other molecules from the automobile exhaust. The activity of the catalyst material loaded on the zeolite support will also be affected in such case. This necessitated the requirement of an alternate catalyst support or modification of the zeolite with eliminating such adsorption character. Hence deactivation of the zeolite support was opted first. The surface area and porosity was also minimized in order to suppress its inherent CO adsorption character. Accordingly zeolite molecular sieves were deactivated by a simple heat treatment to make it suitable as an efficient catalyst support for MnCo2O4 catalyst. 4.3. Effect of Deactivation of Zeolite Molecular Sieves by a Heat Treatment. Zeolite molecular sieves of porosity 5 Å, surface area 570m2/g and Si/Al ratio 1.78 was used for the synthesis of DAZMS. As per literature, zeolite 5 Å can easily adsorb CO at normal and ambient conditions.55 In the present case, it will adsorb CO gas and alter the activity of the catalyst if the zeolite molecular sieves are used as such as the support. Hence it was deactivated, i.e., surface area was reduced by a heat treatment. Zeolite was first heated to 800 ± 5 °C, kept at this temperature for two hours and then cooled down to room temperature rapidly. The temperature and the duration were optimized based on the analysis of the product in terms of surface area and porosity during preliminary studies. As a result of the process, the surface area of the catalyst was significantly reduced eliminating the possible interference on the catalytic activity by the support. Again one more possible setback was sensed in the case of zeolite based catalysts: there is a possibility of deactivation by dealumination of zeolite in the automobile exhaust leading to the misplacement of the catalyst from its actual position during long run,64 The results of the preliminary computational studies also evidenced that dealumination reduces CO adsorption ability of the zeolite. The details are provided in S1 of the SI. Hence zeolite was made dealuminated before its further use as the support for MnCo2O4 catalyst. 4.4. Effect of Dealumination. The purchased zeolite molecular sieves had a surface area of 570m2/g. The surface area was reduced by deactivation which was again reduced further by the dealumination process. In the course of these processes, the surface area was finally reduced to 10.12 m2/g after the processes of heat treatment followed by dealumination. The surface area was noted from the BET isotherm, shown in Figure S2-1 of the SI. There is a possibility of exchange between sodium ions of zeolite with Co or Mn ions from the precursors when zeolite molecular sieves (having low Si/Al ratio) are used as such for the synthesis of MnCo2O4 over

4. RESULTS AND DISCUSSION 4.1. Target of High Adsorption Energy and Low Oxygen Vacancy Formation Energy of the Catalyst. The adsorption energy of CO and O2 is crucial for the process of CO oxidation. The adsorption energy of CO and O2 on MnCo2O4(110) was determined by eq 1 and compared with Co3O4(110). The adsorption energies of CO and O2 for different sites are compared in Table 1. It is clear that Table 1. Calculated Adsorption Energy of CO (E.V.) and The Oxygen Vacancy Formation Energy (E.V.) on Different Sites (Cooct, Cotd, Mnoct) in Co3O4(110) and MnCo2O4 (110) catalysts Co3O4 MnCo2O4

adsorption energy of CO on different sites (E.V.) Cooct

Cotd

0.4219 0.4806

0.3884 0.4362

Mnoct

O2f

0.5626

0.2658 0.3198

oxygen vacancy formation energy (E.V.) 0.5854 0.5293

MnCo2O4 possesses very high CO adsorption energy in all of the sites compared to Co3O4 as evident from Table 1. The adsorption energy of CO on Mn at the octahedral site (Mnoct) is higher compared Co at octahedral (Cooct) or tetrahedral site (Cotd). Hence CO would preferentially adsorb on Mnoct first, then on Cooct and then on Cotd, respectively. The CO adsorption energy on the 2-fold oxygen (O2f) is found very small compared to other sites. These results reveal that adsorption of CO on the O2f site can occur only after Mnoct, Cooct, and Cotd sites got completely covered. According to Mars-van Krevelen mechanism41,58 the product CO2 then leaves the surface, forming an oxygen vacancy. Hence the extent of oxygen vacancy formation plays a crucial role in determining the catalytic activity.50 Following the approach of GandugliaPirovano et al., the average oxygen vacancy formation energy can be calculated by eq 2.59 The oxygen vacancy formation energy of MnCo2O4 is compared with Co3O4 in Table 1. The oxygen vacancy formation energy of MnCo2O4 is found to be significantly low. The low oxygen vacancy formation energy could facilitate easy removal of CO2 from the catalyst surface. According to Xu et al41 the higher adsorption energy of CO on the adjacent sites of O2f will also favor this phenomenon. The preferential adsorption of CO over oxygen molecule was analyzed based on the respective adsorption energy values. All of these computational results suggested that the MnCo2O4 2748

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zeolite. Hence this ratio should be increased in order to avoid such exchange reactions. Generally, high sodium content (low Si/Al ratio) in the zeolite causes easy exchange between sodium and Co or Mn ions.65 It is the reason that we initiated the process after procuring zeolite with low sodium content (Si/Al ratio: >1.5). The Si/Al ratio was further significantly increased to 4.06 by dealumination resulting in extremely low sodium content. This was proposed in order to avoid all probable exchange reactions and for easy formation of MnCo2O4 on the zeolite support. Hence, dealuminated zeolite was used for the synthesis of MnCo2O4 catalyst system to overcome this problem. The results of XRF analysis revealed that Si/Al ratio of the purchased zeolite molecular sieves was 1.78 and the DAZMS/DAZMS supported MnCo2O4 possessed a Si/Al ratio of 4.06. The results of XRF analysis are shown in Figure 2. As

Figure 2. XRF analysis of (A) zeolite molecular sieves alone and (B) MnCo2O4/DAZMS.

evidenced from the literature, the XRD lines of Co3O4 would hardly be visible even if sintering of a large amount of Co3O4 occurs in zeolite and hence the XRD diffraction pattern will be typically dominated by the zeolite diffraction peaks.65 Moreover, a higher surface area and higher porosity of the catalyst system was achieved when it was synthesized with dealuminated zeolite than with nondealuminated zeolite. Hence, DAZMS was used further for the synthesis and as a support material for the MnCo2O4 catalyst. 4.5. Standardization of the Synthesis Parameters. As discussed above, the crucial part of the synthesis process was tuning and optimization of the experimental parameters after finalizing the exact catalyst and its support. The influence of variation in all experimental parameters including temperature, duration, and the nature of the support on the synthesis product was evaluated with respect to their final catalytic activity including stability and porosity. The physico chemical characterization of the catalyst system and extended evaluation of its catalytic activity were carried out to fix the parameters based on reproducibility that was obtained with different batches of the products. 4.6. Identification of Spinel MnCo2O4 on the Surface. The Mn:Co ratio in the MnCo2O4/DAZMS system was found to be 1:2, as evidenced from XRF analysis (Figure 2). The nature of formation of spinel MnCo2O4 when it was synthesized with and without the presence of DAZMS was analyzed by XRD. The formation of MnCo2O4 with and without the presence of DAZMS was confirmed by comparing it with available literature reports.66 The XRD patterns revealed that no other phases except MnCo2O4 spinel peaks were formed without the presence of DAZMS (Figure 3C). The peaks were found to be sharp as Karppinen et al. have also observed similar MnCo2O4 spinel phases at 900 °C.48 In the case of MnCo2O4/DAZMS, a most intense peak was observed

Figure 3. X-ray diffraction data of (A) DAZMS, (B) spinel MnCo2O4 synthesized in presence of DAZMS (o: peaks of DAZMS; x: peaks of spinel MnCo2O4), and (C) MnCo2O4 alone synthesized in the absence of DAZMS.

for MnCo2O4 (Figure 3C) while other peaks were found merged.65 A peak broadening was also observed in the XRD patterns of MnCo2O4/DAZMS. This may be due to the size reduction of the particles as well as due to the variation in oxygen content or non uniform strain.66 Accordingly the surface area of the catalyst would have got increased. The data of BET analysis revealed that MnCo2O4/DAZMS possessed higher surface area compared to MnCo2O4 that was synthesized in the absence of zeolite. The XRD patterns of DAZMS, MnCo2O4/DAZMS, and that of MnCo2O4 synthesized alone was also compared as shown in Figure 3. The XRD patterns revealed the nature of MnCo2O4 formed with and without the presence of DAZMS. The sharp peak at 2θ 45.5 with a d spacing of 1.99 Å appeared only in Figure 3B was due to the presence of Al2O3 according to jcpds reference code 00−004− 0877. The pattern of the MnCo2O4 catalyst and their orientation in the DAZMS should be regular and uniform otherwise the resultant MnCo2O4/DAZMS would not have exhibited a smooth morphology. 4.7. Morphological Characteristics of the MnCo2O4/ DAZMS. The morphology of the pretreated sodium zeolite support and the MnCo2O4/ DAZMS catalyst, as compared in Figure 4, revealed that the MnCo2O4 catalyst has homogeneously grown and cured all over the catalyst DAZMS support. There was no significant irregular orientation, segregation, 2749

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Figure 4. The SEM images of (A) DAZMS and (B) MnCo2O4/ DAZMS at different magnifications. Figure 5. The confocal microscopic images and the corresponding surface topography of (A) DAZMS and (B) MnCo2O4/DAZMS using Nile Blue A as the fluorescent probe molecule.

voids, or any other irregular patterns of growth on the support. The visible macro spots could be attributed to the irregular orientation of the catalyst support and not the catalyst on it. Such a slight dissimilarity was also observed on the DAZMS support. In this context, confocal analysis was carried out in order to visualize the nature and extent of the pores by the interior morphological changes of the catalyst prepared at different stages. 4.8. Visualization of Enhanced Porosity in the Catalyst Loaded DAZMS by Confocal Analysis. BET analysis revealed that the surface area of MnCo2O4/DAZMS catalyst has very high surface area (137.65 m2/g) compared to the MnCo2O4 (5.48 m2/g) prepared without the presence of DAZMS and that of DAZMS alone (10.12 m2/g). The details are shown in S2 of the SI. It may be due to lesser particle size of the synthesized MnCo2O4 over DAZMS, as revealed from the XRD patterns. The results of preliminary UV absorption study using Nile Blue A revealed that the absorption of Nile Blue A was higher in MnCo2O4/DAZMS catalyst compared to DAZMS. It may be due to the enhanced porosity of the catalyst. Further, confocal microscopic analysis was carried out, by following the approach of Buurmans et al.,60 in order to confirm this inference. A fluorescent probe of Nile Blue A was used to visualize porosity and the corresponding surface topography. Due to the bigger size of Nile Blue A, it would be hard for it to get into the pores of DAZMS and its red florescence lights up the porous matrix. It is clear from Figure 5 that the DAZMS supported MnCo2O4 catalyst exhibited lesser red florescence compared to DAZMS. It was due to the absence of Nile blue A at the surface. Increased porosity of the catalyst loaded zeolite facilitated the passage of zeolite in to the pores of the catalyst loaded DAZMS due to which the intensity of red florescence observed on the surface got decreased. These confocal analysis results and the corresponding surface topography confirmed that the process of catalyst loading on DAZMS eventually yielded a surface of enhanced porosity. This was also confirmed by BET analysis. 4.9. The Efficiency and Practical Feasibility of the MnCo2O4/DAZMS Catalyst for CO Oxidation. The performance of the MnCo2O4/DAZMS catalyst was evaluated using a special catalyst-carrying-pipe system filled with the catalyst. The system as a whole was introduced at the exhaust

end of an actual automobile engine. A four stroke diesel engine of ATUL Group of industries, India, was fitted with thermo couples. Under such conditions, the processes of CO oxidation along with the MnCo2O4/DAZMS catalyst was continuously as well as intermittently monitored, depending upon the parameters to be evaluated. Under such actual/practical conditions, all of the parameters including percentage of CO conversion, temperature variation, and catalyst poisoning were instantly monitored and evaluated. The influence of excess CO in the exhaust stream was also monitored. A secondary lengthier exhaust line containing the catalyst components was also provided in order to avoid the catalyst components from getting heated due to the heat of the original exhaust. All of these parameters of the catalyst in terms of CO conversion efficiency were correlated with different functions such as time, temperature and CO concentration. The essential features are shown in Figure 6. The catalyst exhibited excellent CO conversion efficiency under normal and ambient conditions. Figure 6A reveals the catalytic activity of the MnCo2O4/DAZMS as a function of time. The catalyst oxidized 99.2% of CO at the beginning when the temperature at the exhaust line was 155 °C. At that temperature itself, it could facilitate maximum CO oxidation up to 99.4% continuously for 15 h. However, the actual temperature then exceeded even 300 °C after a continuous oxidation of 15 h. It is well-known that maximum conversion efficiency could be achieved only when the transition metal catalyst is maintained at a temperature above 200 °C.21 However, the present catalyst performed extremely well for CO conversion even below 200 °C.21 This is the maximum CO producing stage when a normal automobile exhaust process is considered i.e. cold start period. The engine was also worked for an average of 6 h per day, continuously for 7 months, resembling an actual automobile engine which works for a few hours and stops for a few hours, alternatively. The CO conversion efficiency was monitored periodically after every month by analyzing the data after every hour. An overall efficiency of 98.2% even after seven months of application was recorded, as shown in Figure 6B. A slight decrease in the efficiency after four months could be attributed 2750

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of 99.4 to 82.8% even after experiencing a working temperature as high as 700 °C. The data are presented in Figure 6D. Moreover, the catalyst showed more selectivity for CO oxidation in presence of NOx and HC. The details are provided in S4 of the SI. All of these studies confirmed that the synthesized MnCo2O4/DAZMS not only exhibited surprisingly high catalytic activity for CO oxidation at temperature resembling cold start period but also it was sufficiently stable/active in actual automobile condition and ambient condition containing large amount of CO, H2O, CO2, and NOx at 155−715 °C. The present catalyst system will be efficient for the development of next generation CO oxidation catalysts for a wide variety of automobiles from a small petrol fuelled vehicle to a large diesel fuelled vehicle which may produce high CO contented exhaust.

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ASSOCIATED CONTENT

S Supporting Information *

Computational evidence for low adsorption energy of DAZMS compared to zeolite, BET isotherms of DAZMS, MnCo2O4 synthesized without the presence of DAZMS, MnCo2O4/ DAZMS, engine operational conditions at catalytic activity analysis, and the CO oxidation selectivity of the catalyst in presence of NOx and HC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Figure 6. The variation in CO oxidation efficiency of MnCo2O4/ DAZMS catalyst under different experimental conditions: (A) The efficiency of MnCo2O4/DAZMS as a function of time (average inlet CO concentration: 240 ppm, average inlet NOx concentration: 98 ppm, average inlet HC concentration: 36 ppm, average inlet temperature: 155 °C and engine speed: 1500 rpm); (B) the efficiency of MnCo2O4/DAZMS as a function of time in month (average inlet CO concentration: 243 ppm, average inlet NOx concentration: 93 ppm, average inlet HC concentration: 36 ppm, average inlet temperature: 155 °C and engine speed: 1500 rpm); (C) the efficiency of MnCo2O4/DAZMS along with excess concentration of CO in ppm (engine speed: 1500 rpm and average inlet temperature: 165 °C); and (D) the efficiency of MnCo2O4/DAZMS as a function of temperature (average inlet CO concentration: 24o ppm, average inlet NOx concentration: 98 ppm, average inlet HC concentration: 36 ppm, and engine speed: 1500 rpm).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS STIC Cochin for XRD, SEM, and EDS. CESS, Trivandrum for providing XRF facility, NIIST, Trivandrum for BET analysis. Dr. T. V. Anil Kumar, Experimental Pathology Lab, SCTIMST, Trivandrum for Confocal microscopic analysis. Mechanical Engineering Department, College of Engineering Trivandrum for providing automobile engine and CO analyzer, and the University of Kerala for providing financial support for doing this work.

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ABBREVIATIONS DAZMS, dealuminated zeolite molecular sieves; Mnoct, Manganese at the octahedral site; Cotd, cobalt atom at the tetrahedral site; Cooct, cobalt atom at the octahedral site and; O2f, is the 2-fold oxygen; DFT, density functional theory

to slow poisoning of the catalyst by dust or smoke particles. The poisoning due to physical matters may be regenerated, but a new process should be explored to recover the catalyst undergoing chemical poisoning. To verify the effect of excess CO concentration in the exhaust stream on the activity of the catalyst, an excess concentration of CO was introduced by applying different loads in the engine system. The effect of CO concentration on the break-through behavior with the MnCo2O4/DAZMS catalyst is shown in Figure 6C. Even when the concentration of CO was largely increased to a maximum of 350 ppm, only a slight decrease in its conversion efficiency, i.e., 99.4 to 98.2% was observed. This is a significant parameter that revealed the flexible use of the present catalyst system for a wide variety of automobiles from a small petrol fuelled vehicle to a large diesel fuelled vehicle which may produce high CO contented exhaust. The catalyst exhibited almost a steady performance of yielding efficiency in the range

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REFERENCES

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