Text Material Preview
Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=ycsr20 Catalysis, Structure & Reactivity ISSN: 2055-074X (Print) 2055-0758 (Online) Journal homepage: www.tandfonline.com/journals/ycsr20 Investigation of support effect on CO adsorption and CO + O2 reaction over Ce1 − x − yMxCuyO2 − δ (M = Zr, Hf and Th) catalysts by in situ DRIFTS Tinku Baidya & Parthasarathi Bera To cite this article: Tinku Baidya & Parthasarathi Bera (2015) Investigation of support effect on CO adsorption and CO + O2 reaction over Ce1 − x − yMxCuyO2 − δ (M = Zr, Hf and Th) catalysts by in situ DRIFTS, Catalysis, Structure & Reactivity, 1:3, 110-119, DOI: 10.1179/2055075815Y.0000000004 To link to this article: https://doi.org/10.1179/2055075815Y.0000000004 © 2015 The Author(s). Published by Taylor & Francis Published online: 05 May 2015. Submit your article to this journal Article views: 1344 View related articles View Crossmark data Citing articles: 1 View citing articles https://www.tandfonline.com/action/journalInformation?journalCode=ycsr20 https://www.tandfonline.com/journals/ycsr20?src=pdf https://www.tandfonline.com/action/showCitFormats?doi=10.1179/2055075815Y.0000000004 https://doi.org/10.1179/2055075815Y.0000000004 https://www.tandfonline.com/action/authorSubmission?journalCode=ycsr20&show=instructions&src=pdf https://www.tandfonline.com/action/authorSubmission?journalCode=ycsr20&show=instructions&src=pdf https://www.tandfonline.com/doi/mlt/10.1179/2055075815Y.0000000004?src=pdf https://www.tandfonline.com/doi/mlt/10.1179/2055075815Y.0000000004?src=pdf http://crossmark.crossref.org/dialog/?doi=10.1179/2055075815Y.0000000004&domain=pdf&date_stamp=05 May 2015 http://crossmark.crossref.org/dialog/?doi=10.1179/2055075815Y.0000000004&domain=pdf&date_stamp=05 May 2015 https://www.tandfonline.com/doi/citedby/10.1179/2055075815Y.0000000004?src=pdf https://www.tandfonline.com/doi/citedby/10.1179/2055075815Y.0000000004?src=pdf Investigation of support effect on adsorption and þ O2 reaction over Ce12x2yMxCuyO22d (M ¼ Zr, Hf and Th) catalysts by in situ DRIFTS Tinku Baidya1 and Parthasarathi Bera*2 1Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India 2Surface Engineering Division, CSIR–National Aerospace Laboratories, Bangalore 560017, India Abstract Adsorption of CO as well as CO þ O2 reaction over Cu2þ ion substituted Ce12xMxO2 (M ¼ Zr, Hf and Th) supports have been studied by DRIFTS. Linear CuþZCO bands are observed over all catalysts upon introduction of CO. But, CuþZCO band positions are shifted to little higher frequencies in Ce0.68M0.25Cu0.07O22d compared to Ce0.93Cu0.07O22d. However, Cu þZCO bands are in same positions when CO and O2 are adsorbed simultaneously over all the catalysts. Ramping the temperature in the DRIFTS cell after simultaneous CO and O2 adsorption shows the formation of CO2 as well as decrease of CO. Comparison of intensities of CO2 bands of different catalysts as a function of temperature indicates that Ce0.68Th0.25Cu0.07O22d shows lowest temperature CO oxidation among all the catalysts that is because of its more electron withdrawing power. Keywords Adsorption, CO, Oxidation, DRIFTS, Ce0.93Cu0.07O22d, Ce0.68M0.25Cu0.07O22d Cite this article Tinku Baidya and Parthasarathi Bera. Catal. Struct. React., Introduction CeO2 is well known as an oxygen storage material and redox support for various noble and transition metals.1 It finds extensive applications in NOx reduction, three-way catalysis and many other oxidation reactions.2–5 The unique oxygen storage property of CeO2 based oxides act as a buffer during oxidation/reduction cycle in a reaction. The oxygen storage capacity (OSC) has been enhanced by doping with tetravalent cations like Zr4þ, Hf4þ, Sn4þ and Ti4þ into CeO2matrix, but their effect is observed to be different on the extent of reducibility of themixed oxides.6–9 However, activemetal dispersed on these doped cerium oxide catalysts acts as an adsorption site for reactants that remains in close proximity of lattice oxygen. Therefore, reactant can easily utilize the lattice oxygen to form oxidized product. Thus, created oxygen vacancies act as adsorption sites for the dissociation of feed oxygen. This was reported as bi-functional mechanism in the literature.4,10–12 In recent years, CeO2 and TiO2 based noble metal ionic catalysts have been found to show significant enhancement of CO oxidation at a lower temperature.3 5 It has been demonstrated that substituted noble metal ions on CeO2 matrix are catalytically more active than fine metal particles dispersed on Al2O3. Cu 2þ, Agþ, Au3þ, Pd2þ and Pt2þ are the active sites for the catalytic reactions. Structural studies show that metal ions are incorporated into CeO2 substrate to a certain limit in the solid solution form of Ce12xMxO22d (x # catalysts have been found to be very active for CO oxidation, NO reduction and CO-PROX reaction.13–17 However, in our previous study, it has been found that Pd2þ ion substituted in different CeO2 based supports show different activities.18 It has been observed that CO oxidation activation energy decreases with increasing ionic character of Pd2þ in CeO2 based oxides. Based on this finding, it would be worthwhile to extend this idea of ionic character in other metal ions such as Cu2þ that are very much active for several catalytic reactions when they are supported on different matrices. This can provide a more general outlook into how active metal ion can be more activated by choosing suitable support. In this regard, it would be interesting to study CO adsorption and variation of catalytic activities of Cu2þ ion in different Cu substituted CeO2 supports toward CO oxidation. Diffuse reflectance infrared Fourier transform spec- troscopy (DRIFTS) is a versatile tool to get information about the nature of adsorbed species on the surface of catalyst*Corresponding author, Email: partho@nal.res.in Original Research Paper Received 02 February 2015; accepted 11 March 2015 DOI 10.1179/2055075815Y.0000000004 1 CO CO 0.05). Among the transition metals, Cu based oxide 110 2015, 1, 1 -110 19 – – © 2015 The Author(s) . Published by Taylor & Francis This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. during reaction.19–21 It can also provide the trends of reaction with respect to the temperature. The present work focus on DRIFTS study to understand CO adsorption at room temperature as well as CO oxidation by O2 at different temperatures over different Cu substituted Ce12xMxO2 (M ¼ Zr, Hf and Th) oxides. The effect of support on the substituted Cu2þ ions toward CO adsorption/oxidation behavior has been investigated. Experimental methods Preparation Cu substituted Ce12xMxO2 (M ¼ Zr, Hf and Th) were prepared by solution combustion method. The preparation of the above catalysts was slightly different because of preferring suitable fuel for the combustion reaction. For the preparation of Ce0.93Cu0.07O22d, 30mL aqueous solution of 5 g (NH4)2Ce(NO3)6.6H2O, 1.255 g Cu(NO3)2.5H2O and 2.33 g of C2H6N4O2 (oxalyl dihydrazide) was taken in a 300mL borosilicate dish. The dish was kept in the furnace preheated at 400 C. Initially, the solution boiled with frothing and foaming and underwent dehydration. At the point of com- plete dehydration, the combustion started with a burning flame on the surface leading to a voluminous solid product within 5min. Ce0.68Zr0.25Cu0.07O22d was prepared from (NH4)2Ce(NO3)6.6H2O, Zr(NO3)4, Cu(NO3)2.5H2O and C2H5NO2 (glycene) in 0.73:0.25:0.02:2.5 molar ratio. Similarly, Ce0.68 Hf0.25Cu0.07O22d was prepared from (NH4)2- Ce(NO3)6.6H2O, Hf(NO3)4, Cu(NO3)2.5H2O and C2H5NO2 (gly- cene) in 0.73: 0.25: 0.02:2.5 molar ratio.However, Hf(NO3)4 was prepared in situ using HfCl4 as a precursor. Initially, the required amount of HfCl4 was hydrolyzed in water giving a white precipitate of Hf(OH)4 which was dissolved in a minimum volume of concentrated HNO3. Ce0.68Th0.25Cu0.07O22d was prepared from the solution of 5 g of (NH4)2Ce(NO3)6.6H2O, 1.61 g of Th(NO3)4, 0.272 g of Cu(NO3)2.5H2O and 2.46 g of C2H5NO2 (glycene). The solution of ,30mL taken in a 300mL borosilicate dish was placed in the furnace at 400 8C to carry out the combustion and similar procedures were followed as above to obtain the solid product. All as-prepared catalysts were heated at 500 C for 1 h to remove all organic residues present in the samples. Characterization BET surface area of Cu substituted Ce12xMxO2 (M ¼ Zr, Hf and Th) catalysts were measured using Quantachrome Autosorb Automated Gas Sorption System (Quantachrome Instruments). X-ray diffraction (XRD) patterns of all catalysts were recorded on a Philips PANalytical X'Pert PRO diffractometer at a scan rate of 0.128min with 0.028 step size in the 2h range between 208 and 658. X-ray photo- electron spectra (XPS) of Cu/CeO2 based materials were recorded in a Thermo Fisher Scientific Multilab 2000 with AlKa radiation (1486.6 eV) operated at 15 kV and 10mA (150W). Binding energies reported here are with reference to graphite at 284.5 eV and they are accurate within ^ 0.1 eV. For XPS analysis, the powder samples were pressed to 0.5mm thick pellets and placed into a preparation chamber with ultrahigh vacuum (UHV) at 1029 Torr for 5 h in order desorb any adsorbed species present on the sample surface and then it was transferred to analyzer chamber with UHV at 1029Torr. All the spectra were obtained with apass energy of 30 eV and step increment of 0.05 eV. DRIFTS measurements CO. adsorption at room temperature and in situ DRIFT spectra of the CO. þ O2 reaction as a function of tempera- ture over Cu substituted Ce12xMxO2 (M ¼ Zr, Hf and Th) catalysts were recorded with an accumulation of 20 scans at a resolution of 4 cm using a FTIR spectrometer from Thermo Scientific Nicolet 380 FTIR with a liquid N2 cooled high sensitivity MCT detector and a DRIFTS cell. Aliquots of ca. 100mg catalyst powder were placed inside the DRIFTS cell with ZnSe windows and a heating cartridge that allowed samples to be heated up to 300 8C. DRIFTS cell was connected with a gas handling system in order to measure in situ spectra under controlled gas environments at atmospheric pressure. Catalysts were activated in the DRIFTS cell by calcination for 2 h in a diluted O2/He stream at 300 8C before introducing the reaction mixture. Subsequently, the system was cooled down to room temperature and the background spectra were recorded. Adsorption studies over the catalysts were carried out by passing CO (0.5% in He). The reaction mixture (0.5% CO. þ0.5% O2 in He) was passed over the catalysts at a total flow rate of 100cm3min at atmospheric pressure for in situ studies. DRIFTS cell was heated to different temperatures and the spectra were collected after getting 10min of steady state. Temperature of the DRIFTS cell was controlled with a thermocouple in direct contact with the sample. Circulating water was used to cool the body of the reaction chamber. All spectra obtained were transformed into absorption spectra by using Kubelka- Munk (K-M) function which is linearly related to the absorber concentration in the DRIFT spectra. CO, O2 and He gases used in this study were supplied by Bhoruka Gas (purity higher than 99.95%). Results and discussion Characterization studies BET surface areas of 7 at.% Cu substituted CeO2, Ce0.75Zr0.25O2, Ce0.75Hf0.25O2 and Ce0.75Th0.25O2 catalysts are 15, 22, 38 and 26m2 g , respectively. At P/Po ¼ 0.3, pore volumes of these catalysts are 4.68, 7.22, 9.25 and 8.37 cm3 g , respectively and pore diameter is 5 Å. XRD patterns of the Cu substituted Ce0.75M0.25O2 (M ¼ Zr, Hf and Th) catalysts are presented in Fig. 1. Observed dif- fraction lines in the patterns are indexed to fluorite structure. The diffraction peak shifts indicate the substitution of Zr, Hf and Th into CeO2 lattice. As Zr4þ(0.84 Å) and Hf4þ(0.83 Å) ions are smaller than Ce4þ(0.97 Å),22 the peak shift toward higher angle compared to CeO2 is observed that is an indi- cation of Ce0.75M0.25O2 solid solution formation. Similarly, peak is shifted toward lower angle in Th doped CeO2 as Th4þ(1.05 Å) ion is larger than Ce4þ. In Cu substituted Ce0.75M0.25O2 oxides, Cu metal or CuO related peaks are not detected even after magnification, indicating incorporation of ionic Cu2þ into CeO2 or Ce0.75M0.25O2. However, finely dispersed CuO species can also be present over CeO2 or Ce0.75M0.25O2 supports which is likely to be XRD silent 2 8 8 -1 -1 -1 -1 -1 111Catalysis, Structure & Reactivity 2015 VOL NO1 3 Catal. Struct. React., 2015, 1, 110-11 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reaction9 because of their low crystal size or amorphous nature. Crystallite size of Cu/Ce0.75Zr0.25O2 and Cu/Ce0.75Th0.25O2 catalysts obtained by using Scherrer formula is ,25 nm. However, the average crystallite size of Cu/Ce0.75Hf0.25O2 XPS has been carried out to know the oxidation states of elements present in these catalysts. Cu2p core level spectra in various Cu substituted Ce0.75M0.25O2 (M ¼ Zr, Hf and Th) catalysts are shown in Fig. 2. Cu2p spectrum of Ce0.93Cu0.07O22d is also included in the figure for compari- son. Cu2p3/2,1/2 peaks are resolved into spin-orbit doublets. Accordingly, Cu2p3/2,1/2 core level peaks around 933.9 and 953.6 eV with corresponding satellite peaks and spin-orbit separation of 19.7 eV are assigned to Cu2þ in these type of catalysts.13,14,23 Satellite peaks are characteristics of oxidized transition metals especially, Fe, Co, Ni and Cu.24 It is well documented that an additional excitation of a second elec- tron occurs during emission of a photoelectron of a core level creating a hole in it. Sudden creation of a hole in Cu2p6 filled orbital from Cu2þ ion present in the catalyst makes Cu3þ ion and it becomes unstable. Therefore, an electron transfer from O2p level to Cu3d level occurs that leads to satellite peaks in the Cu2p core level spectrum as seen in the figure. It is to be noted that satellite peak (S) to main Cu2p3/2 core level peak (M) intensity ratio (IS/IM) in CuO is found to be 0.55.25–27 The intensity ratio varies from 0.39 to 0.48 in the case of Cu2þ ion in the square pyramidal position in Bi2- Ca12xRxSr2Cu2O8þd (R ¼ Y, Yb).28 Further, IS/IM values are 0.55 and 0.85 for Cu2þ in the octahedral and tetrahedral sites of a cubic spinel.29 In the present study, intensity ratios obtained from the areas under the satellite and main peaks after background subtraction are calculated to be in the range of 0.35 to 0.5 that is lower than the value of CuO. Therefore, Cu2þ ions in these catalysts are not in tetrahedral coordination and amount of finely dispersed CuO species is less in these catalysts. Figure 3 displays Ce3d core level spectra of Ce0.93Cu0.07O22d and Ce0.68M0.25Cu0.07O22d (M ¼ Zr, Hf and Th) catalysts. Ce3d5/2,3/2 peaks at 882.5 and 900.7 eV with characteristic satellites are attributed to CeO2 whereCe is in 4 13,14,23,30 Satellitepeak at 916.5eV is relatively well separated from the rest of the spectrum and is the characteristic of the presence of tetravalent Ce (Ce4þ) in Ce compounds. However, possibility of the presence of Figure 3 X-ray photoelectron spectra of Ce3d core levels of a Ce0.93Cu0.07O22d,; b Ce0.68Zr0.25Cu0.07O22d,; c Ce0.68- Hf0.25Cu0.07O22d and d Ce0.68Th0.25Cu0.07O22d catalysts Figure 1 X-ray diffraction patterns of a Ce0.93Cu0.07O22d,; b Ce0.68Zr0.25Cu0.07O22d,; c Ce0.68Hf0.25Cu0.07O22d and d Ce0.68Th0.25Cu0.07O22d catalysts Figure 2 X-ray photoelectron spectra of Cu2p core levels of a Ce0.93Cu0.07O22d,; b Ce0.68Zr0.25Cu0.07O22d,; c Ce0.68- Hf0.25Cu0.07O22d and d Ce0.68Th0.25Cu0.07O22d catalysts 3 is 10 nm. oxidation state.þ 112 Catalysis,Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reaction Catal. Struct. React., 2015, 1, 110-119 a small amount of Ce3þ on the surface level cannot be ruled out. In this regard, Ce3d spectra of all catalysts can be curve- fitted into several Ce3d5/2,3/2 spin-orbit doublet peaks related to Ce4þ and Ce3þ species along with satellites. Peak areas of Ce4þ and Ce3þ components are commonly used to estimate their relative concentrations in the catalysts.31 Concen- trations of Ce3þ in all catalysts are evaluated to be in the range of 20–23 at.% with respect to the total amount of Ce species. Zr3d, Hf4f and Th4f core level spectra of respective catalysts are displayed in Fig. 4. Zr3d5/3,3/2 peaks observed at 182.6 and 184.9 eV in Ce0.68Zr0.25Cu0.07O22d are associated with Zr4þ.32 In Ce0.68Hf0.25Cu0.07O22d, Hf4f7/2,5/2 core level peaks at 16.9 and 18.2 eV correspond to Hf4þ.33 Th4f7/2,5/2 peaks at 334.4 and 343.6 eV in Ce0.68Th0.25Cu0.07O22d are related to Th4þ.32,34 Thus, XPS studies demonstrate that Cu, Ce, Zr, Hf and arepresent inþ2, þ4,þ4,þ4 andþ4 oxidati- on states, respectively. From XPS studies, relative surface concen- trations of Cu, Ce, Zr, Hf and Th in respective catalysts have been estimated by the relation given below35 CM ¼ IM sMlMDM S IM sMlMDM � � (1) where CM, IM, sM, lM and DM are relative surface concen- tration of a particular metal (M), integrated intensity of the metal core level peak, photoionization cross-section of metal core level photoelectron, mean escape depth of the re- spective metal core level photoelectron and geometric factor, respectively Integrated intensitiesof Cu2p, Ce3d, Zr3d, Hf4f and Th4f core level peaks have been taken into account to estimate their concentrations, whereas photoionization cross-sections and mean escape depths have been obtained from the literature.36,37 The geometric factor has been taken as one, since the maximum intensity in this spectrometer is obtained at 908. Accordingly, surface concentrations of Cu are evaluated to be in the range of 20 to 26 at .% in these cat- alysts which is around three times more than what is taken in the preparation indicating surface segregation of Cu2þ ions on CeO2 and Ce12xMxO2 (M ¼ Zr, Hf and Th) surfaces. Therefore, some of the Cu2þ species present in the catalysts can be related to finely dispersed CuO entities typically observed over these types of catalysts. DRIFTS studies Adsorption of CO has been carried out over Ce0.93Cu0.07O22d and Ce0.68M0.25Cu0.07O22d (M ¼ Zr, Hf and Th) catalysts to know the differences in the nature of their surfaces. Detailed in situ DRIFTS studies of CO þ O2 reaction have also been carried out over these catalysts. Figure 4 X-ray photoelectron spectra of Zr3d, Hf4f and Th4f core levels of Ce0.68Zr0.25Cu0.07O22d, Ce0.68Hf0.25Cu0.07O22d and Ce0.68Th0.25Cu0.07O22d catalysts Th 113Catalysis, Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reactionCatal. Struct. React., 2015, 1, 110-119 CO adsorption over Ce0.93Cu0.07O22d and Ce0.68M0.25- Cu0.07O22d catalysts The interaction of CO over Ce0.93Cu0.07O22d catalyst is shown in Fig. 5. In general, CO interaction with catalyst is associated with various chemical transformations of CO molecule. Usual CO derived products during adsorption are carbonates (CO22 3 ), bicarbonates (HCO2 3 ), formates (HCOO2), carboxylates (COO2) and carbonites (CO22 2 ). Carboxylates and bicarbonates are formed when cations can be easily reduced such as Cu2þ or when non-stoichiometric oxygen exists on the surface. On the other hand, carbonites are observed on strongly basic surfaces like MgO, CaO. Surface OH2 species are necessary to form bicarbonates and formates. However, all these species are characterized in the stretching frequencies below 1800 cm . In the present study, several peaks are occurred in the carbonyl (1900 2 2300 cm ) and carbonate (900 2 1600 cm ) regions when CO is introduced over Ce0.93Cu0.07O22d cata- lyst. Carbonyl and carbonate type species are formed upon the first contact of the catalyst with CO at room tempera- ture. A band at 2100 cm which is gradually developed on the catalyst surface is because of CO adsorption over Cuþ sites19,38,39 and it is shifted to 2102 cm over time with higher intensity. In general, bands in the region of 2123–2114 cm are attributed to Cuþ carbonyl species, where as bands in 2110–2080 cm regime correspond to Cu metal carbonyl.39 In this sense, observed band at 2102 cm over Ce0.93Cu0.07O22d may create confusion about its nature as it is in the borderline. Its low frequency suggests that they may be related to metallic copper carbonyls. No significant change in the intensities of this band is observed even after purging the DRIFTS cell with He for 5min. Again, Cu0 CO species is unstable at room temperature.40 Therefore, in this study, adsorption center can correspond to partially oxidized copper species and observed peak at 2102 cm is related to Cuþ CO species. Peaks related to mono, bi and polydentate carbonates, bicarbonate and formate species are found in the 900 2 1600 cm region. It is important to note that carbonate and hydroxyl species are difficult to remove by oxidizing thermal treatment requiring temperatures above 750 8C which can induce cat- alyst sintering and loss of surface area.19,38 Carbonate related species are formed over CeO2 region of the catalysts. According to the literature, peaks observed at 1575, 1300, 1014 and 940 cm are attributed to bidentate carbonate species, whereas the peaks at 1470 and 1040 cm corre- spond to mono or polydentate carbonates formed on the surface.19,20,38,39,41 Carbonyl peaks at 1396 and 1190 cm are assigned to hydrogen carbonate species. Observed peak at 1550 cm21 is related to formate species generated by interacting with hydroxyl species present on the surface (CO þ OH2 ! HCOO2).38 In XPS studies, it has been demonstrated that Cu is in the þ2 oxidation state. When CO is introduced over the catalyst, surface Cu species in þ2 oxidation state gets partially reduced by CO and consequently, CO derived species such as carbonates are formed over the CeO2 support (2Cu2þ þ 2O22 þ CO! 2Cuþ þ CO22 3 ) that is reflected in the DRIFT spectra. Complete reduction of Cu2þ to Cu0 cannot be possible as CeO2 is a reducible support. Therefore, Cu may remain as Cuþ form in the catalyst. In this sense, frequencies related to Cu and CO interaction in Ce0.93Cu0.07O22d catalyst are ascribed for CuþZCO species. A similar kind of reduction process at room temperature and formation of CuþZCO species has also been observed over Cu/CeO2 based catalysts in earlier studies. Interactions of CO with Ce0.68Zr0.25Cu0.07O22d and Ce0.68- Hf0.25Cu0.07O22d are displayed in Fig. 6. Similar to the Figure 5 Diffuse reflectance infrared Fourier transform spectra of CO adsorption on Ce0.93Cu0.07O22d catalyst -1 -1-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 19,38,39 114 Catalysis, Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reaction Z Z Catal. Struct. React., 2015, 1, 110-119 Ce0.93Cu0.07O22d catalyst, an intense peak at 2108 cm observed over Ce0.68Zr0.25Cu0.07O22d in carbonyl region is ascribed to CO adsorption over reduced Cu (CuþZCO) species. CuþZCO peak positions are found to be at 2110 and 2109 cm in cases of Ce0.68Hf0.25Cu0.07O22d and Ce0.68Th0.25Cu0.07O22d catalysts. It is important to note that CuþZCO bands have been shifted to higher frequencies (blue shift) because of presence of different environment of Cu2þ in Ce0.68M0.25Cu0.07O22d (M ¼ Zr, Hf and Th) catalysts compared to Ce0.93Cu0.07O22d catalyst. This shift is related to the electron withdrawing ability of the substituent Zr4þ, Hf4þ and Th4þ ions. With more electron withdrawing character of the metals around Cu2þ in Zr, Hf and Th substituted CeO2 catalysts, CZO bond becomes stronger because of lowerp back donation. Peaks associated with mono, bi and poly dentate carbonate, bicarbonate and formate species are discernible in the 900 2 1600 cm21 region. These species are already produced upon the first contact of the samples with CO at room temperature. Peaks observed at 1520, 1430 and 1290 cm21 on the surface of Ce0.68Zr0.25Cu0.07O22d Figure 6 Diffuse reflectance infrared Fourier transform spectra of CO adsorption on (top) Ce0.68Zr0.25Cu0.07O22d and (bottom) Ce0.68Hf0.25Cu0.07O22d catalysts -1 -1 115Catalysis, Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reactionCatal. Struct. React., 2015, 1, 110-119 are assigned for formate, hydrogen carbonate and bidentate carbonate. In Ce0.68Hf0.25Cu0.07O22d, peaks at 1475, 1310 and 1075 cm correspond to monodentate carbonate species, whereas 1010 cm is related to bidentate species.41 Peaks at 1400, 1270 and 1200 cm are attributed to hydrogen car- bonate species. However, intensities of these carbonate species are less in Ce0.68M0.25Cu0.07O22d (M ¼ Zr, Hf and Th) catalysts compared to Ce0.93Cu0.07O22d. In the present study, it has been confirmed with XPS technique that Cu is in þ2 oxidation state in Ce0.68M0.25Cu0.07O22d catalysts. Similar to Ce0.93Cu0.07O22d, introduction of CO over Ce0.68M0.25Cu0.07- O22d partially reduces the surface Cu species which is in þ2 oxidation state and consequently, CO derived species such as carbonates are formed over the catalysts which is confirmed from DRIFT spectra. There is no significant change in the intensities of all bands even after purging the DRIFTS cell with He for 5min after CO exposure. CO þ O2 reaction over Ce0.93Cu0.07O22d and Ce0.68M0.25- Cu0.07O22d catalysts DRIFTS studies of CO þ O2 reaction have been carried out over Ce0.93Cu0.07O22d and Ce0.68M0.25Cu0.07O22d catalysts to monitor the reaction sequences during the reaction. After simultaneous CO and O2 adsorption, catalysts have been heated up to 300 8C and every 20 8C DRIFT spectra are recorded. Spectra of carbonyl and carbonate regions of Ce0.93Cu0.07O22d catalyst at different temperatures are shown in Fig. 7. CuþZCO peak in Ce0.93Cu0.07O22d is shifted to higher frequency (2114 cm ) in presence of O2. Intensity of carbonyl peak at 2114 cm slowly comes down above 100 8C and peaks at 2362 and 2335 cm associated with gas phase CO2 are started to be observed. As temperature increases CO oxidation is observed to accelerate that can be seen from the increase in the gas phase CO2 and corre- sponding carbonate bands because of CO2 adsorption on the catalyst. CuþZCO peak is observed to disappear at higher temperatures over the catalyst. However, strong CO2 bands are found even after disappearance of CO bands at higher temperatures during reaction indicating CO conver- sion. DRIFT spectra of Ce0.68M0.25Cu0.07O22d catalysts are similar to those of Ce0.93Cu0.07O22d, except the differences in onset temperatures and changes in the nature of carbonate species in presence of O2. Figure 8 displays DRIFT spectra of Ce0.68Zr0.25Cu0.07O22d and Ce0.68Hf0.25Cu0.07O22d catalysts during CO þ O2 reaction. It has been observed that CO oxidation starts at higher temperature over Ce0.68Zr0.25- Cu0.07O22d in comparison with Ce0.68Hf0.25Cu0.07O22d. During CO þ O2 reaction, intensities of bands at 1335 and 1080 cm related to monodentate carbonate species and bands at 1290 and 990 cm corresponding to bidentate carbonate species increase over Ce0.93Cu0.07O22d catalyst. Increase in the intensities of bands at 1285 and 1038 cm associated with bidentate carbonate species along with hydrogen carbonate species (1480 and 1395 cm ) is observed in Ce0.68Zr0.25Cu0.07O22d catalyst. Intensities of monodentate carbonate (1340 and 1070 cm ) and biden- tate carbonate (1300 and 990 cm ) species are found to increase over Ce0.68Hf0.25Cu0.07O22d catalyst. Increase in intensities of monodentate and bidentate carbonate species and decrease in hydrogen carbonate species is noticed over Ce0.68Th0.25Cu0.07O22d catalyst. Ionic substitution of Cu in CeO2 based oxides forming ZCe4þ 2þZ type of linkages on their surfaces is established in the literature where metal ions and oxide ion vacancies are two distinct sites for adsorption of reducing and oxidizing molecules and subsequent catalysis.3,4,13,14 Figure 7 Diffuse reflectance infrared Fourier transform spectra during temperature ramping after simultaneous CO and O2 adsorption on Ce0.93Cu0.07O22d catalyst -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Cu 116 Catalysis, Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reaction Catal. Struct. React., 2015, 1, 110-119 � More than 10 at.% Cu substitution into CeO2 matrix has been reported in the literature.42 In this study, 7 at.% Cu has been substituted in the CeO2 matrix so that effect of surrounding environment in different CeO2 based matrices can be understood. Therefore, Zr, Hf and Th have been chosen because their oxides are non-reducible and having stable tetravalent oxidation state with similar structure of substituted CeO2 based oxides. Surface areas of the catalysts are not significantly different in these catalysts except in Ce0.68Hf0.25Cu0.07O22d. Preparation procedure of Ce0.68Hf0.25Cu0.07O22d can be responsible for its higher surface area, because its preparation was started from hy- droxide precipitation of HfCl4, followed by washing of chloride and then dissolving in excess HNO3. In the present work, change of nature of ionic Cu species with different substituents in CeO2 based oxides has been investigated as substitution of Cu in CeO2 based oxides occur up to high concentration. The effect of support can be observed when CO adsorption shows variation in the intensity of CuþZCO in different supports in similar Figure 8 Diffuse reflectance infrared Fourier transform spectra during temperature ramping after simultaneous CO and O2 adsorption on (top) Ce0.68Zr0.25Cu0.07O22d and (bottom) Ce0.68Hf0.25Cu0.07O22d catalysts 117Catalysis, Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reactionCatal. Struct. React., 2015, 1, 110-119 conditions. In this study, variation of CuþZCO peak inten- sities during CO oxidation have been accepted as a probe to find the differences in supports and it is shown in Fig. 9. The observed intensities of CuþZCO peaks increase with the following substitutions: Zr , Hf , Th. This trend can also be explained based on the higher effective nuclear charge of Hf4þ and Th4þ as compared to Zr4þ. As all the substituents are non-reducible, only electron-withdrawing power as determined by effective nuclear charge can influence the activity of these catalysts. Effective nuclear charge of Hf4þ or Th4þ is higher because of presence of f electrons as compared to Zr, indicating that lattice oxygen is more polarized toward Hf4þ and Th4þ. This means charge on Cu2þ ion is more localized and able to withdraw more electron toward itself or more reducing nature in the Hf/Th contain- ing oxides. Therefore, transformation of more Cu2þ ion to Cuþ occur significantly in the case of Hf and Th doped cat- alysts giving higher CuþZCO intensities. This also influences the CO oxidation activities of these catalysts, because loca- lized charge on Cu2þ induces more þ ve charge on `C' atom end of the adsorbed CO molecule while reacting with lattice oxygen. This explains lowest activity of Zr substituted cata- lyst that is shown in Fig. 10. Conclusion CO adsorption and oxidation are observed to be lowest over þZ CO vibrational intensity increases as Zr , Ce , Th , Hf. With increasing effective nuclear charge of the substituent metal (Zr, Hf and Th) in CeO2 matrix, ionic charge on the Cu2þ is more localized as negative charge on oxygen gets depleted because of more polarization in the MZO bond around Cu2þ ion. This effect may be alleviated by converting more Cu2þ to Cuþand thereby, increasing the intensity of CuþZCO peak. Therefore, CO oxidation activity increases in the following substitutions: Zr , Ce , Hf , Th. Acknowledgements The authors are grateful to Prof. M. S. Hegde, Indian Institute of Science, Bangalore, for providing DRIFTS and XPS facilities. References 1. A. Trovarelli: Catal. Rev. Sci. Eng., 1996, 38, 439. 2. A. Trovarelli: ‘Catalysis by ceria and related materials’; 2002, London, Imperial College Press. 3. M. S. Hegde, K. C. Patil and G. Madras: Acc. Chem. Res., 2009, 42, 704. 4. P. Bera and M. S. Hegde: Catal. Surv. Asia, 2011, 15, 181. 5. P. Bera and M. S. Hegde: J. Indian Inst. Sci., 2010, 90, 299. 6. G. Dutta, U. V. Waghmare, T. Baidya, M. S. Hegde, K. R. Priolkar and P. R. Sarode: Catal. Lett., 2006, 108, 165. 7. G. Dutta, U. V. Waghmare, T. Baidya, M. S. Hegde, K. R. Priolkar and P. R. Sarode: Chem. Mater., 2006, 18, 3249. 8. T. Baidya, M. S. Hegde and J. Gopalakrishnan: J. Phys. Chem. B, 2007, 111, 5149. 9. T. Baidya, A. Gupta, P. A. Deshpandey, G. Madras and M. S. Hegde: J. Phys. Chem. C, 2009, 113, 4059. 10. T. Baidya, A. Marimuthu, M. S. Hegde, N. Ravishankar and G. Madras: J. Phys. Chem. C, 2007, 111, 830. 11. S. Roy, A. Marimuthu, M. S. Hegde and G. Madras: Appl. Catal. B, 2007, 71, 23. 12. S. Roy, A. Marimuthu, M. S. Hegde and G. Madras: Catal. Commun., 2008, 9, 101. 13. P. Bera, S. T. Aruna, K. C. Patil and M. S. Hegde: J. Catal., 1999, 186, 36. 14. P. Bera, K. R. Priolkar, P. R. Sarode, M. S. Hegde, S. Emura, R. Kumashiro and N. P. Lalla: Chem. Mater., 2002, 14, 3591. 15. D. Gamarra, G. Munuera, A. B. Hungría, M. Fernández-García, J. C. Conesa, P. A. Midgley, X. Q. Wang, J. C. Hanson, J. A. Rodríguez and A. Martínez-Arias: J. Phys. Chem. C, 2007, 111, 11026. 16. A. Martínez-Arias, D. Gamarra, M. Fernández-García, A. Hornés, P. Bera, Zs Koppány and Z. Schay: Catal. Today, 2009, 143, 211. 17. A. Martínez-Arias, D. Gamarra, A. B. Hungría, M. Fernández-García, G. Munuera, A. Hornés, P. Bera, J. C. Conesa and A. L pez Cámara: Catalysts, 2013, 3, 378. 18. T. Baidya, G. Dutta, M. S. Hegde and U. V. Waghmare: Dalton Trans., 2009, 3 , 455. 19. A. Hornés, P. Bera, A. L pez Cámara, D. Gamarra, G. Munuera and A. Martínez-Arias: J. Catal., 2009, 268, 367. 20. T. Baidya, P. Bera, B. D. Mukri, S. K. Parida, O. Kröcher, M. Elsener and M. S. Hegde: J. Catal., 2013, 303, 117. 21. D. D. Miller and S. S. C. Chuang: Catal. Commun., 2009, 10, 1313. 22. R. D. Shanon: Acta Crystallogr. A, 1976, 32, 751. 23. J. Huang, Y. Kang, T. Yang, Y. Wang and S. Wang: React. Kinet. Mech. Catal., 2011, 104, 149. 24. S. Hüfner: ‘Photoelectron spectroscopy: principles and applications’. 3rd edn; 2003, Berlin, Springer. Figure 9 Cu1ZCO intensity versus temperature (8C) in Ce0.93Cu0.07O22d and Ce0.68M0.25Cu0.07O22d (M 5 Zr, Hf, and Th) catalysts Figure 10 CO2 formation from IR intensity as a function of temperature (8C) over Ce0.93Cu0.07O22d and Ce0.68M0.25Cu0.07- O22d (M 5 Zr, Hf and Th) catalysts 4þ Ce0.68 0.25Zr 0.07Cu O2-d among Ce0.68 0.25M 0.07Cu O2-d (M 88 ¼Zr Hf and Th) catalysts. It has been found that Cu 118 Catalysis, Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reaction ó ó Catal. Struct. React., 2015, 1, 110-119 25. D. C. Frost, A. Ishitani and C. A. McDowell: Mol. Phys., 1974, 24, 861. 26. G. Moretti, G. Fierro, M. Lo Jacono and P. Porta: Surf. Interface Anal., 1989, 14, 325. 27. Y. Okamoto, K. Fukino, T. Imanaka and S. Teranishi: J. Phys. Chem., 1983, 87, 3740. 28. C. N. R. Rao, G. R. Rao, M. K. Rajumon and D. D. Sarma: Phys. Rev. B, 1990, 42, 1026. 29. R. Bechara, A. Aboukaïs and J. -P. Bonnelle: J. Chem. Soc. Faraday Trans., 1993, 89, 1257. 30. D. D. Sarma, M. S. Hegde and C. N. R. Rao: J. Chem. Soc., Faraday Trans., 1981, 77, 1509. 31. C. Anandan and P. Bera: Appl. Surf. Sci., 2013, 283, 297. 32. J. F. Moulder, W. F. Stickle, P. E. Sobol and B. D. Bomben: ‘Handbook of X-ray photoelectron spectroscopy’, (ed. J. Chastain); 1992, Eden Prairie, PerkinElmer Corporation. 33. W. -E. Fu and Y. -O. Chang: Appl. Surf. Sci., 2011, 257, 7436. 34. Z. -W. Lin, Q. Kuang, W. Lian, Z. -Y. Jiang, Z. -X. Xie, R. -B. Huang and L. -S. Zheng: J. Phys. Chem. B, 2006, 110, 23007. 35. C. J. Powell and P. E. Larson: Appl. Surf. Sci., 1978, 1, 186. 36. J. H. Scofield: J. Electron Spectrosc. Relat. Phenom., 1976, 8, 129. 37. D. R. Penn: J. Electron Spectrosc. Relat. Phenom., 1976, 9, 29. 38. P. Bera, A. L pez Cámara, A. Hornés and A. Martínez-Arias: J. Phys. Chem. C, 2009, 113, 10689. 39. P. Bera, A. L pez Cámara, A. Hornés and A. Martínez-Arias: Catal. Today, 2010, 155, 184. 40. M. B. Padley, C. H. Rochester, G. J. Hutchings and F. King: J. Catal., 1994, 148, 438. 41. K. I. Hadjiivanov and G. N. Vayssilov: Adv. Catal., 2002, 47, 307. 42. A. Gayen, T. Baidya, A. S. Prakash, N. Ravishankar and M. S. Hegde: Indian J. Chem., 2005, 44A, 34. 119Catalysis, Structure & Reactivity 2015 VOL NO1 3 Baidya and Bera Investigation of support effect on CO adsorption and CO þ O2 reaction ó ó Catal. Struct. React., 2015, 1, 110-119
