Catalytic activity and mechanistic approach of NO reduction by CO over M0.05Co2.95O4 (M = Rh, Pd & Ru) spinel system

Applied Surface Science 389 (2016) 344–353

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Catalytic activity and mechanistic approach of NO reduction by CO over M0.05 Co2.95 O4 (M = Rh, Pd & Ru) spinel system A.V. Salker ∗ , M.S. Fal Desai Department of Chemistry, Goa University, Goa, 403206, India

a r t i c l e

i n f o

Article history: Received 13 May 2016 Received in revised form 17 June 2016 Accepted 19 July 2016 Available online 22 July 2016 Keywords: Nitric oxide Carbon monoxide Cobalt oxide spinel Metal doped cobalt oxide CO-TPD NO-TPD

a b s t r a c t Pd, Rh and Ru doped cobalt oxide spinels have been prepared by citric acid assisted sol-gel method to yield nano-catalysts. Compositions have been characterized by XRD, FTIR, TG/DTA, BET, SEM, TEM, TPD and XPS techniques. Simultaneous catalytic detoxification of nitric oxide and carbon monoxide is investigated over the doped and pristine catalysts. Metal doping is found to enhance the activity of the catalysts due to their better adsorption capacity. Moisture and oxygen tolerance is investigated for Rh and Pd substituted catalysts. A probable reaction mechanism on the catalyst surface has been proposed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Detoxification of nitric oxide (NO) and carbon monoxide (CO) from exhaust gas treatment system is an interesting topic for many research groups due to its wide implications in air pollution control [1–4]. Stringent regulations have been imposed on NOx emission in developed and developing countries to reduce its emission in the environment. Heterogeneous catalysts are proven to be convenient and efficient solution for the detoxification of pollutant gases from the exhaust emission systems. Although, there are many other methods developed to reduce the discharge of NO from stationary and mobile exhaust systems, simultaneous catalytic redox reaction of NO and CO is considered to be advantageous over selective catalytic reduction (SCR) using NH3 or Urea, since CO is produced by the same automobile exhaust source and thus can be utilized as a reducing agent without adding it externally [5–11]. It is also a cost effective method as no sophisticated modification of exhaust system is needed. Variety of catalysts has been studied for de-NOx activity. Among them, precious metal catalysts have been proved their use advantageous. In growing market, availability and high cost of the precious metals make them difficult for the practical application. Therefore, highly dispersed precious metals on active support can be effective solution for the problem. Precious met-

∗ Corresponding author. E-mail addresses: sal [email protected], [email protected] (A.V. Salker). http://dx.doi.org/10.1016/j.apsusc.2016.07.121 0169-4332/© 2016 Elsevier B.V. All rights reserved.

als like Pt and Pd dispersed on inert support like Al2 O3 have been studied [12,13]. Pd supported on active supports like CeO2 and TiO2 is found to be very active de-NOx catalysts [14]. Hexa-ruthenium cluster and Pd cluster for NO-CO redox reaction are reported in the literature [15,16]. H. Muraki and Y. Fujltanl performed comparative NO–CO activity studies over precious metals, where activity in the order Rh > Ru > Ir > Pd > Pt was observed [17]. Cobalt oxides have promising activity in many catalytic reactions such as selective oxidation of hydrocarbons, steam reforming, hydrogen production, Fischer Tropsch synthesis etc, due to their high surface oxygen mobility [18–21]. Cobalt oxide spinel has been studied for CO oxidation and has shown excellent activity [22–24]. The doped and pristine cobalt oxide spinels have been prepared by various synthetic methods such as precursor, co-precipitation, combustion and sol-gel [25–29]. Among them sol-gel method is found to be effective in synthesis of meso-porous nano particles [19,20]. Precious metal supported on cobalt spinels are also reported in the literature [2,8]. Cobalt oxide is found to be active in N2 O decomposition [30]. Since precious metals can be readily doped in low concentrations, their promotional effect is investigated for NO and CO redox reaction leading to nontoxic products. Effect of moisture and oxygen on the catalyst activity is also studied. Researchers have supported the formation of NO2 species that facilitate the reduction of NO in Lean NOx Trap type of catalysts [31]. Sol-gel assisted synthesis is successfully employed to prepare highly active nano catalysts. In the present work, emphasis is laid on the enhancement

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of the activity of cobalt oxide spinels by doping active metals. Many working groups have identified the isocynate type of intermediates in NO-CO reaction. Present study on NO-CO reaction under various conditions does not indicate such intermediate adsorbed on the surface from infrared studies, as no infra-red absorption is observed around 2290 cm−1 [32,33]. Also in presence of water, NH3 formation was not observed in current investigation as reported by M. Unland [34]. 2. Experimental 2.1. Catalyst preparation Doped Mx Co3-x O4 (M = Rh, Pd, Ru and x = 0.05) and pristine Co3 O4 were prepared from Co(NO3 )2 ·6H2 O (Sigma Aldrich), RhCl3, PdCl2 and RuCl2 (Thomas Baker). To the calculated amount of cobalt nitrate and metal chlorides, citric acid monohydrate (Thomas Baker) is added in molar ratio 1: 1 (Co/Citric acid). Solution was stirred for 1 h and then evaporated to get a viscous gel. This gel was dried in oven at 150 ◦ C for 3 h which resulted in a foamy metal citrate complex. This foamy complex was ground and then sintered at 600 ◦ C for 7 h in air to get the final metal substituted oxide nano catalyst.

Fig. 1. XRD patterns of a) Co3 O4 , b) Pd0.05 Co2.95 O4 , c) Rh0.05 Co2.95 O4 and d) Ru0.05 Co2.95 O4 .

2.2. Characterizations Thermo gravimetric analysis (TG) and Differential thermal analysis (DTA) were carried out in dry air at the flow rate of 60 mL min−1 using NETZSCH–TG STA 409PC thermal analyzer to get the decomposition pattern and weight loss of metal citrate complexes. TG/DTA was also performed in inert atmosphere of N2 at the flow rate of 60 mL min−1 to check the thermal stability of the oxides. The spinel phase of pristine Co3 O4 was identified using X-ray powder diffractometer (Regaku Ultima IV) with Cu-K␣ source. FTIR analysis of powder catalysts was done using Shimadzu IR-Prestige-21 spectrometer. Morphology of the particles was analyzed with the help of Zeiss Evo 18 Scanning electron microscope (SEM). HR-TEM images were obtained from JEOL 3010 electron microscope. BET surface area was measured at liquid nitrogen temperature using QUANTACHROME AUTOSORB IQ-MP-C surface area analyzer. XPS studies of the catalysts were carried out using PHOIBOS HSA3500 DLSEGD 150 R7 with Al K␣ excitation source (E = 1486.74 eV). TPD experiments were performed on QUANTACHROME AUTOSORB IQMP-C equipped with TCD using 150 mg of catalyst. Prior to the TPD studies, the catalysts were pre-treated at 120 ◦ C for 20 min in N2 (20 mL min−1 ) stream. The furnace temperature was lowered to 100 ◦ C, and the samples were then saturated with 5% CO in N2 at the flow rate of 20 mL per min for 1 h. The physisorbed CO was removed by flushing the catalyst with N2 at 40 ◦ C for 20 min before starting the TPD experiments. The heating rate of 10 ◦ C min−1 was used for desorption studies. Similarly for NO chemisorption studies were done at 100 ◦ C and 5% NO in N2 was passed at the flow rate of 20 mL min−1 . To study the effect of reduction of catalyst on adsorption of NO, catalyst samples were reduced by passing CO at 250 ◦ C. Reduction temperature was evident from CO TPR profile (supplementary Fig. S1). Samples were then cooled at 40 ◦ C and N2 was passed for 20 min. These reduced samples were subjected to NO chemisorption at 100 ◦ C under same flow rate and desorption is studied at heating rate of 10 ◦ C min−1 . 2.3. Catalytic studies of NO–CO redox reaction Simultaneous redox reaction of NO and CO was carried out in a continuous flow, fixed bed glass reactor. NO and CO gases were prepared by known standard laboratory procedure in the lab and purified by passing over appropriate traps. Every time about 0.8 g

of the catalyst was loaded in the glass reactor supported in between glass wool. Catalysts were heated prior to the reaction by passing N2 at 100 ◦ C for 30 min to make the surface free, from adsorbed moisture and oxygen. The catalytic activity was measured using feed gas composition of 5% CO and 5% NO in Argon at the rate of 5000 mL h−1 . Individual gases flow rates were controlled using gas flow meters and precision needle valves. Activity of catalysts was measured with respect to temperature. The feed gases and products were analyzed using on line Gas Chromatograph employing Molecular sieve 13x and Porapak Q columns with TCD detector. Details about the analysis are given in Supplementary information. To investigate the interaction of CO and NO over all the catalyst, FTIR spectrum after the catalytic test was recorded which showed some characteristic absorbed species. Stability of catalyst was also analyzed in feed gas composition of 5% CO and 5% NO in Ar at the rate of 5000 mL h−1 in high and low moisture presence, i.e. 2% and 9% respectively. The effect of two different partial pressure of oxygen on catalytic activity was investigated with the following feed gas compositions (5% CO, 5% NO and 2.5% O2 ) and (5% CO, 5% NO and 5% O2 ) in Ar. Simultaneous effect of moisture and O2 was also investigated using 5% CO, 5% NO and 2.5% O2 with 2% of water in Ar. All the stability tests were carried out at 250 ◦ C. 3. Result and discussion 3.1. X-Ray diffraction analysis Crystal phase of the prepared Co3 O4 catalyst was analysed and confirmed by XRD patterns. The XRD pattern indicated the formation of cubic phase and 2 theta values are matched well with JCPDS card no. 42-1467 [29]. In metal substituted catalyst samples, no additional peaks were observed which confirmed monophasic nature of the catalyst (Fig. 1). Broadening of the peaks indicate poor crystalline and nano nature of the particles. An increase in lattice parameter was observed with the substitution (Table 1). Increase in lattice parameter was observed after precious metal incorporation in the cubic lattice of cobalt oxide spinel. Substitution of bigger 4d transition metals has increased the size of the Co3 O4 unit cell. Thus, XRD pattern confirmed that sol-gel assisted synthesis of metal doped cobalt oxide spinel yielded monophasic compound. It is dif-

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Fig. 2. XPS spectra of Co3 O4 and Rh0.05 Co2.95 O4, a) Full scan, b) Co 2p, c) Rh 3d of Rh0.05 Co2.95 O4 and d) O1s. Table 1 Lattice parameter and BET surface area. Catalyst

Lattice parameter (Å)

Surface area (m2 g−1 )

Co3 O4 Pd0.05 Co2.95 O4 Rh0.05 Co2.95 O4 Ru0.05 Co2.95 O4

8.0642 8.0895 8.1059 8.0855

55.7 44.2 55.6 51.5

ficult to obtain information about the dispersion state of doped metals only on the basis of XRD, therefore XPS studies were done to find the oxidation states of metals. 3.2. XPS studies Surface oxidation state of doped cobalt oxide was investigated employing XPS technique as seen in Fig. 2. It is well known that the binding energies (B. E.), satellite intensity to main peak intensity ratio (S/M) and spin orbit coupling (E = Co 2p1/2 –Co 2p3/2 ) are important parameters for analysing the chemical state of cobalt as seen in Fig. 2 (b). Both analysed catalysts showed S/M ratios 0.53 and 0.56 for pristine and Rh doped spinels, respectively [35]. E for Rh doped and un-doped spinel was calculated to be 15.03 eV and 15.09 eV. The B. E., S/M ratio and spin orbit coupling energies are characteristic of Co3 O4 which conclude precious metals are highly disperse in Co3 O4 spinel. Since carbon 1 s peak at B. E. 285 eV is considered to be standard which was observed at 285.48 eV in case of parent cobalt oxide spinel and in case of Rh doped at 285.26 eV, XPS peaks are subtracted with values 0.48 eV for cobalt spinel and 0.26 eV for Rh doped catalyst. Thus B. E. for Co 2p3/2 and Co 2p1/2 were observed at 779.4 eV and 794.52 eV respectively for Co3 O4 , whereas for Rh0.05 Co2.95 O4 these were observed at 779.5 eV and 794.6 eV. The Photoemission spectra of pristine and doped com-

pound shows a weak shoulder at 790.2 eV and 789.3 eV respectively and absence of the 787 eV Co 2p3/2 satellite peak, which are identical to Co 2p photoemission feature of Co3 O4 indicates the absence of Co2+ from octahedral site as suggested from the literature [8]. B. E. of the O1 s peak observed in Fig. 2(d) at 530.47 and 530.26 in un-doped and doped compound respectively, actually correspond to 529.9 eV and 530 eV which is the contribution from lattice oxygen (O␤). While O␣ peak observed at higher B. E. which is due to non-stoichiometric oxygen vacancies are also seen [36]. Percentage of O␣ species is observed higher in Rh substituted catalyst. This indicates oxide defects are enhanced by Rh substitution. Oxidation state of Rh in doped catalyst was investigated from photoemission spectra as shown in Fig. 2(c). B. E. at 308.9 eV of 3d5/2 clearly indicate +3 oxidation state of Rh atom instead of metallic Rh. Overall XPS result suggests that Rh3+ is incorporated in Co3 O4 cubic lattice through the bonding with oxygen atoms. XPS studies of Pd and Ru substituted reveal that Pd is in +2 and Ru is in multiple oxidation state (supplementary Fig. S2).

3.3. FTIR studies Formation of cobalt spinel was also evident from IR spectroscopy as seen in Fig. 3. No characteristic peaks due to citric acid or nitrate were observed which shows that sintering at 600 ◦ C for 8 h is sufficient for complete decomposition of precursor gel to form desired oxide. The two distinct Co O stretching frequencies indicate the presence of two different oxygen environments. Lower energy bands in the range 664–673 cm−1 are assigned to Co O vibrations from Co+2 in tetrahedral sites, whereas peak from 579 to 556 cm−1 are attributed to the OB3 vibration in a spinel lattice, where B denotes the Co3+ in octahedral sites [37]. Therefore, IR

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Fig. 5. TG/DTA plots of Co3 O4 and Rh0.05 Co2.95 O4 showing weight loss due to loss of lattice oxygen and phase transition around 930 ◦ C. Fig. 3. FTIR spectra of a) Rh0.05 Co2.95 O4 , b) Pd0.05 Co2.95 O4 , c) Ru0.05 Co2.95 O4 and d) Co3 O4 , recorded at room temperature.

3.5. Surface morphology by SEM and HR-TEM Powder catalyst prepared by sol-gel method and sintered at 600 ◦ C were suspended in methanol and deposited on carbon film and morphological properties were investigated by recording SEM and TEM as observed in Fig. 6. Images revealed nano size of cobalt catalyst. Particles were found to be in the range 15–20 nm. The Figs. 6(a) and (b) show representative images of doped cobalt oxide spinels. From this it can be concluded that the method adopted for preparation yielded nano particles. 3.6. Surface area measurement

Fig. 4. TG/DTA curves showing thermal behaviour of pristine cobalt citrate precursor.

spectra also indicate the formation of cubic cobalt spinel structure. XPS and IR studies showed the presence of Co3+ and Co2+ .

Surface area is one of the important parameter in catalytic reaction studies. BET surface area of the catalysts has been tabulated in Table 1. The sol-gel method was helpful in synthesis of catalysts with high surface areas. Surface areas of the catalysts were found to be in the range 44–57 m2 g−1 . High surface area and fine nanoparticles are one of the essential criteria for the activity catalysts and these depends entirely on the preparative method although converse may be also true in some cases. Sol-gel method employed to prepare the cobalt spinel oxides showed better surface area compared to ceramic method employed by R. Sundararajan and V. Srinivasan [30]. Surface area was inconsistent with cobalt spinel oxides prepared by sol-gel method [23]. 3.7. Catalytic activity

3.4. Thermal analysis The metal citrate precursor was subjected to TG/DTA analysis in air. TG/DTA curve showed sharp decrease in weight at 250–300 ◦ C due to the decomposition of organic matter as shown in Fig. 4. Although it was evident from TG/DTA that cobalt oxide spinel phase could be obtained at 350 ◦ C, in order to incorporate precious metal in cubic lattice of Co3 O4 , all catalysts were sintered at 600 ◦ C. To check the thermal stability of catalysts, TG/DTA analysis was carried out in N2 for pristine Co3 O4 and Rh substituted Co3 O4 represented in Fig. 5. The gradual decrease in weight and broad collective exothermic peak was observed in both the thermo-grams with increase in temperature. Above 900 ◦ C complete change of spinel phase was observed indicating phase transition of Co3 O4 to CoO phase which is indicated by a sharp endothermic peak in DTA. Similar decomposition pattern is shown by Ru and Pd substituted catalysts (supplementary Fig. S3).

3.7.1. NO–CO reaction Redox reaction of NO by CO over pristine Co3 O4 and promotional effect of precious metal on cobalt spinel was studied and is presented in Fig. 7. The pristine Co3 O4 showed around 40% NO conversion in temperature range 300–400 ◦ C. There was a decrease in the activity of catalyst for NO reduction above 400 ◦ C because of lattice oxygen desorption above this temperature and also the stability of NO increases at higher temperatures. There is decrease in NO conversion since high valence state cobalt spinel Co3 O4 is getting reduced as reported in literature [28,38]. The precious metal substitution enhanced activity of the catalyst in the temperature range 200–300 ◦ C for Rh and Pd substituted catalysts. Metal doped compound have increased the adsorption capacity of CO and NO which is also evident from CO chemisorption studies (supplementary Fig. S4). The dissociative desorption of NO is facilitated, which in turn give higher activity for NO reduction by CO to form N2 and N2 O. Prepared substituted catalysts have been found more active

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Fig. 8. Catalyst selectivity for N2 O (solid lines) and N2 (dotted lines) for all the catalysts.

Fig. 6. SEM and TEM images of a) Pd0.05 Co2.95 O4 and b) Rh0.05 Co2.95 O4 (Note: Marked area in TEM image is 50 nm).

Fig. 7. Simultaneous NO–CO redox reaction NO conversion (5% NO, 5% CO in argon at 5000 mL h−1 ).

even though the concentration of precious metal is low as compared to some literature reports [39,40]. The substituted catalysts have helped in stability of the compositions and increased the number of active sites on the surface, evidenced from activity and CO chemisorption studies. Rh doped catalyst showed better activity among other catalysts for NO–CO reaction. Pd and Rh metals show high sensitivity towards CO oxidation. Since Rh is substituted in octahedral site and Pd in tetrahedral site of the spinel, Rh incorporation has enhanced the catalyst activity better than Pd. Although Ru is also found to be active for CO oxidation, but its incorporation in the lattice has not significantly increased the activity. CO oxidation was observed in line with NO conversion (supplementary Fig. S5A). Higher concentration of substitution was also analysed for NO–CO reaction which showed improved activity (supplementary Fig. S5B). Selectivity for N2 O and N2 formation is presented in Fig. 8. All the spinel compounds showed higher selectivity for N2 . The 100% selectivity for N2 was observed for Rh and Pd substituted catalysts at around 200 ◦ C and 250 ◦ C respectively. Thus the stability of Rh and Pd doped catalysts was tested for ten hours at 250 ◦ C respectively. The compounds showed good stability and there was enhancement in the activity with increase in time as shown in Fig. 9, this may possibly be due to the equilibrium condition that is attained progressively. Catalyst activity was repeated without any regeneration procedures and NO conversion was found reproducible (supplementary Fig. S6). It is very important to reduce NO 100% below 350 ◦ C, as NO attains more stability above this temperature and hence difficult to reduce at this temperatures. The pristine catalyst has shown decrease in NO conversion above 350 ◦ C. Therefore, cent percentage NO reduction below 350 ◦ C has great significance. Rh, Pd doped catalysts have proved to be efficient well below 350 ◦ C. 3.7.2. Effect of moisture and oxygen in NO–CO reaction Moisture tolerance for Rh and Pd substituted catalyst was studied at low and high moisture levels, 2% and 9% respectively and the profiles are depicted in Fig. 10. It was observed that the low moisture level does not affect the catalytic activity to considerable extent in both the catalysts. However decrease in activity was observed at high moisture level in catalytic systems studied. Presence of moisture in the feed gas is expected to decrease the NO conversion due to competitive adsorption of moisture which was also observed by Mn substituted cobalt spinel oxide (Mnx Co2-x O4 ) [41,42]. Effect of oxygen partial pressure was analyzed over the

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Fig. 9. Stability study of the catalysts at 10 h for NO–CO reaction (5% NO, 5% CO in argon at 5000 mL h−1 ).

Fig. 10. Effect of moisture low (2%) and high (9%) for NO–CO reaction at 250 ◦ C (5% NO, 5% CO in argon at 5000 mL h−1 ).

same catalysts at 250 ◦ C (Fig. 11). Presence of high oxygen levels also reduced the catalytic activity to a considerable level, F. J. Williams et al. supports this observation, as O2 oxidizes NO to NO2 making it difficult for NOx reduction [43]. Although moisture has not affected the selectivity of the catalysts, presence of oxygen has decreased the selectivity of catalysts (supplementary Fig. S7). Low moisture and low oxygen partial pressure have marginally decreased the activity, thus catalytic activity in 2.5% O2 in feed gas and low moisture (2%) was studied at 250 ◦ C which is presented in Fig. 12. Simultaneous effect of low moisture and oxygen has affected the catalytic activity of Rh and Pd substituted cobalt spinels. Theoretically, bond order of NO is higher than the bond order of oxygen. Thus oxygen is expected to re-oxidize the catalyst surface in competitive environment as suggested by S. Stegenga et al. [44]. Oxygen gas will also help NO to get oxidized to NO2 , decreasing the reduction ability further. Moisture may be adsorbing on the surface that reduces the active sites for NO and CO adsorption. The resultant effect will be fall in activity to some extent. The studies by C. A. Pereira and Ernesto A. U. Gonzalez [45] also point out the reduction of active sites as there is competition for NO by water.

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Fig. 11. Effect of partial pressure of O2 for NO–CO reaction at 250 ◦ C (2.5% and 5% O2 , 5% NO, 5% CO in argon at 5000 mL h−1 ).

Fig. 12. Simultaneous effect of moisture and O2 on NO–CO reaction at 250 ◦ C (2% H2 O, 2% O2 , 5% NO, 5% CO in argon at 5000 mL h−1 ).

S. Stegenga et al. [44] observed that moisture inhibits the reaction by reversible competitive adsorption. There was no noticeable decrease in NO conversion after six hours. 3.7.3. NO and CO temperature programed desorption CO adsorption was performed at 100 ◦ C on all the samples. Desorption profile of CO with increase in temperature is represented in Fig. 13. All the catalysts showed desorption of CO which indicate their affinity towards CO chemisorption. The quantum of CO adsorbed on the pristine and Ru doped sample were found to be less. Comparatively, weaker adsorption was observed at lower temperature as indicated by desorption maxima, whereas Pd and Rh substitution showed prominent and stronger chemisorption of CO. NO chemisorption was also performed at 100 ◦ C. Desorption of NO on the catalysts which are not pre-treated with CO was observed at rather lower temperatures (Fig. 14A). This indicates weak chemisorption of NO on the surface. Rh, Pd, and Ru shows higher quantum of NO adsorption then the pristine compound. In

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Fig. 13. CO TPD profiles of a) Co3 O4 , b) Ru0.05 Co2.95 O4 , c) Pd0.05 Co2.95 O4 and d) Rh0.05 Co2.95 O4 .

Fig. 14. A) NO TPD profiles of a) Co3 O4 , b) Ru0.05 Co2.95 O4 , c) Pd0.05 Co2.95 O4 and d) Rh0.05 Co2.95 O4 and B) NO TPD profiles of a) Co3 O4 , b) Ru0.05 Co2.95 O4 , c) Pd0.05 Co2.95 O4 and d) Rh0.05 Co2.95 O4 . CO reduced catalyst.

Fig. 15. Infra-red spectrum of a) Co3 O4 , b) Ru0.05 Co2.95 O4 , c) Pd0.05 Co2.95 O4 and d) Rh0.05 Co2.95 O4 treated with 5% NO, 5% CO in Argon at flow rate of 5000 mL h−1 for 3 h at 400 ◦ C.

order to examine the possible cause of activity, NO desorption on reduced catalyst was performed (Fig. 14B). CO gas at flow rate of 20 mL min−1 in N2 was passed at 250 ◦ C. Samples were then cooled at room temperature and NO gas at a flow rate of 20 mL min−1 in N2 was passed at 100 ◦ C for 20 min. All catalyst showed remarkable change in adsorption of NO. Weaker chemisorption of NO was significantly reduced in un-doped sample and only small desorption peak maxima at 340 ◦ C was observed due to strong chemisorption. However in Rh, Pd and Ru weak chemisorption was reduced and broad desorption peak at higher temperature, due to stronger chemisorption was observed. Systematic studies performed by H. Hu and co-worker’s over Mn substituted cobalt oxide and CoMn/TiO2 , attributed these desorption peaks of NOx to monodentate nitrites, bridged nitrates, and bidentate nitrates [46,47]. NO-TPD over Ni substituted cobalt spinel oxide showed strongly coordinated species, such as bidentate or bridging nitrates [48]. Thus, we can conclude from the literature and TPD studies that the electron rich transition metals like Pd, Rh and Ru have facilitated stronger NO adsorption. NO adsorption on the surface can lead to various kind of interaction such as M-NO, M-ONO, M2 NO etc. NO adsorption on the untreated catalyst is poor whereas it enhanced when catalyst was treated with CO. M. A. Henderson assigned lower and higher temperature peaks to desorption of NO molecules from Fe3+ and Fe2+ respectively in Fe3 O4 , which is in agreement to the desorption peaks observed around 100 ◦ C due to the Co3+ sites [49]. After reduction, there is increased number of reduced metal centres and this facilitated the stronger chemisorption which requires higher temperature for desorption. IR spectrum of the catalysts after catalytic activity test shows the formations of some intermediates as evident from their characteristic bands. IR data after chemisorption reveal one of the possible binding mode i.e. M-ONO. IR spectra as seen in Fig. 15 shows a distinct peak at 1629 cm−1 for the pristine compound due to the formation of bi-dentate carbonate species after adsorption [40–52]. Peaks resembling adsorbed nitrate species are evident at around 1260 cm−1 [3,32,52]. Reduction in the intensities of Co O (td) and Co O (oh) can be attributed to partial reduction of Co3+ to Co2+ . The same catalyst when heated in N2 at 100 ◦ C for 45 min showed no absorption in IR corresponding to carbonates and nitrates species, which rules out the possibility

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Scheme 1. Mechanistic approach of the reaction path ways on the surface.

of catalyst denaturation due to the formation of stable carbonates and nitrates. The Co O bond intensities were also restored. 3.7.4. Mechanism Many reaction mechanisms are put forward to explain the catalytic cycle of simultaneous NO − CO reaction over different catalysts. H. Yao and M. Shelef have proposed formation of surface carbonate species with CO2 desorption taking place by the reduction of Co3+ sites [27]. This reduced site is suitable for NO adsorption. Rh incorporated in octahedral sites of cobalt oxide spinel has high affinity for NO adsorption over cobalt [8]. The adsorbed nitric oxides have oxidizing effect on reduced catalyst sites. Further desorption of CO2 reduces the surface site creating oxygen vacant sites which are active for NO adsorption. N2 O is expected from partial dissociation of NO molecules. Complete dissociative desorption of NO forms N2 which results in re-oxidation of catalyst site and likewise adsorption of CO and NO continue in cyclic manner.

One of the probable mechanistic approaches has been presented in Scheme 1. Co-O surface is found to be active for CO adsorption (equation 1). Since CO is a reducing gas, it takes up lattice oxygen from the cobalt spinel oxide forming CO2 (equations 2 and 3). This creates reduced sites on the surface where NO gets adsorbs, as indicated in equation 4. At higher temperature (above 350 ◦ C), adsorbed NO molecules will desorb dissociatively to form N2 and thus regenerating the surface active sites for further CO and NO adsorption as seen from (equations 5 and 6). At temperatures below 250 ◦ C, there is partial reduction of NO which leads to the formation of N2 O (equations 7 and 9). The fact that oxygen decreases the activity for NO conversion is explained by equations (10 and 11). Oxygen oxidizes the reduced catalyst and decreases the availablity of reduced sites for NO adsorption, decreasing NO conversion to N2 . Xie et al. [24] considered Co3+ at an octahedral site as an active site for CO oxidation. Since Rh3+ can occupy octahedral site in Co3 O4 spinel, its substitution enhances the reduction process favorably by CO2 desorption from the surface and increasing stronger NO

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chemisorption [53] Pd+2 and Ru4+ cations occupy tetrahedral sites in cobalt spinel oxide thus it does not have significant effect on NO conversion compared to Rh3+ . 4. Conclusions Citric acid assisted sol-gel method was successfully employed to prepare metal substituted cobalt oxide spinel nano catalysts. The catalysts prepared were found to be monophasic as detected from XRD patterns. Precious metal substitution has resulted in increase in lattice parameter, which reflects their incorporation in the cubic lattice of Co3 O4 . XPS spectra revealed the presence of Rh3+ , Pd2+ , Ru4+ , Co2+ and Co3+ in the compounds. There was no evidence of precious metals in the metallic (M0 ) form. TEM images shown the formation of nano sized particles with uniform distribution in the range 10–20 nm. Catalysts were found to be active for NO reduction by CO in the order Rh0.05 Co2.95 O4 > Pd0.05 Co2.95 O4 > Ru0.05 Co2.95 O4 » Co3 O4 . The metal substitution has enhanced the activity and stability of the catalysts. Pd and Rh doped spinel compounds showed high N2 selectivity, good stability and 100% reduction well below 300 ◦ C, as stability of NO increases above 350 ◦ C. However in the presence of higher concentrations of moisture and oxygen, conversion of NO to N2 decreased marginally. The doped metals facilitate the adsorption of CO and NO, enhancing the activity of the catalysts. A probable reaction mechanism is proposed based on activity and TPD measurements. Electron rich transition elements like Rh, Pd and Ru increased the stronger chemisorption probably because of their good back bonding ability. This lowers the bond order of NO and facilitates its reduction forming N2 .

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Acknowledgments [21]

Authors would like to thank “University Grant Commission”, New Delhi, for the financial support. Authors also thank the UCGDAE Consortium for Scientific Research, Kalpakkam, for XPS facility.

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Appendix A. Supplementary data

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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.07. 121.

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