A comparative study of CuOTiO2SnO2, CuOTiO2 and CuSnO2 catalysts for lowtemperature CO oxidation

Journal of Natural Gas Chemistry 18(2009) – A comparative study of CuO/TiO2-SnO2, CuO/TiO2 and CuO/SnO2 catalysts for low-temperature CO oxidation Kairong Li, Yaojie Wang, Shurong Wang∗ , Baolin Zhu, Shoumin Zhang, Weiping Huang, Shihua Wu Department of Chemistry, Nankai University, Tianjin 300071, China [ Received April 22, 2009; Revised May 11, 2009; Available online November 22, 2009 ] Abstract Nanometer SnO2 particles were synthesized by sol-gel dialytic processes and used as a support to prepare CuO supported catalysts via a deposition-precipitation method The samples were characterized by means of TG-DTA, XRD, H2 -TPR and XPS The loading of CuO in the CuO/TiO2 -SnO2 catalysts markedly influenced the catalytic activity, and the optimum CuO loading was wt.% (T100 = 80 ◦ C) The CuO/TiO2 -SnO2 catalysts exhibited much higher catalytic activity than the CuO/TiO2 and CuO/SnO2 catalysts H2 -TPR result indicated that a large amount of CuO formed the active site for CO oxidation in wt.% CuO/TiO2 -SnO2 catalyst Key words sol-gel dialytic processes; CuO/TiO2 -SnO2 catalyst; low-temperature CO oxidation Introduction Carbon monoxide, emitted from automated vehicles, aircraft, natural gas emission, industrial wastage, sewage leaking, mines, etc, is one of the most common and dangerous pollutants present in the environment Catalytic oxidation is an efficient way to convert CO to CO2 at low-temperature Many precious metal supported catalysts, such as Au/TiO2 , Au/Fe2 O3 , Au/CeO2 , Pt/SnO2 , Pd/CeO2 , Ir/TiO2 , have been demonstrated to be very efficient for low-temperature CO oxidation [1−5] However, due to the high cost and the scarcity of precious metal, attention has been given to search alternative catalysts, especially for copper oxide [6] Tin dioxide is one of the most widely applied materials in solid-state gas sensor devices detecting toxic and combustible gases in air [7] Moreover, tin dioxide as CO oxidation catalyst has attracted particular attention in the past [8] The preparation of nanometer SnO2 powders has been widely studied, and different methods have been proposed to synthesize pure or doped SnO2 At present, the main methods used for preparing nanometer SnO2 powders are the sol-gel method, the alkoxide method, and the hydrothermal method The sol-gel method is one of the best methods used to prepare nanometer powders However, this method has two restric- tions, one is time-consuming, generally 15−30 days, the other is the difficulty in removing Cl− In this paper, tin (IV) chloride and alcohol were used as the start materials Ammonia gas as catalyst for forming a colloidal solution and as agent for removing Cl− , was introduced to the dialytic processes to improve and accelerate the formation of gels It took about 18 h to form SnO2 wet-gels, which did not need washing CuO/TiO2 -SnO2 catalysts were prepared via a deposition-precipitation method and their catalytic activities in CO oxidation were studied The catalytic activity of CuO supported on TiO2 -SnO2 was compared with that of CuO supported on TiO2 or SnO2 The samples were characterized by means of TG-DTA, XRD, H2 -TPR and XPS Experimental 2.1 Catalyst preparation Nanometer tin dioxide particles were prepared by sol-gel dialytic processes SnCl4 (A R.) was mixed with anhydrous alcohol (A R.) to obtain SnCl4 alcohol solution The precursor Sn(OC2 H5 )4 colloidal solution was prepared by adding ammonia gas to SnCl4 alcohol solution at ◦ C under vigorous ∗ Corresponding author Tel: +86-22-23505896; Fax: +86-22-23502458; E-mail: shrwang@nankai.edu.cn (S R Wang) This work was supported by the National Natural Science Foundation of China (20771061 and 20871071), the 973 Program (2005CB623607) and Science and Technology Commission Foundation of Tianjin (08JCYBJC00100 and 09JCYBJC03600) Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences All rights reserved doi:10.1016/S1003-9953(08)60144-9 Kairong Li et al./ Journal of Natural Gas Chemistry Vol 18 No 2009 stirring After white precipitation filtered, colloidal solution was inputted a dialytic membrane, and then put in the distilled water When no Cl− was examined by 0.1 mol/L AgNO3 aqueous solution, and the pH of dialysate was 7, the wet-gels formed in the process of dialysis were dried at 80 ◦ C for 24 h Based on the result of TG-DTA analyses, the dried samples were calcined at 400 ◦ C in air for h TiO2 /SnO2 nano-composite was prepared by depositionprecipitation method with nominal 50 wt.% TiO2 contents A suitable amount of SnO2 powders were dispersed in alcohol solution, and the slurry was constantly stirred for h Then the Ti(OBu)4 alcohol solution was added drop-wise to the above resulting slurry The mixture was stirred for another 30 The samples were dried at 80 ◦ C in water bath, and then dried in oven at 80 ◦ C for h The as-made material was calcined at 400 ◦ C in air for h TiO2 nanopowders were prepared by hydrolyzation-precipitation from Ti(OBu)4 alcohol solution, and the preparation procedure was similar to that of the preparation of the TiO2 /SnO2 samples CuO/SnO2 , CuO/TiO2 and CuO/TiO2 -SnO2 catalysts were prepared by a deposition-precipitation method Under vigorous stirring, a suitable amount of SnO2 , TiO2 or TiO2 SnO2 powders were dispersed in Cu(NO3 )2 solution, and the slurry was constantly stirred for h 0.25 mol/L Na2 CO3 solution was added drop-wise to the above slurry under vigorous stirring until the pH was 10 The mixture was stirred for another an hour, then filtered and washed with distilled water The samples were dried at 80 ◦ C in air in an oven, followed by calcination at 400 ◦ C in air for h 2.2 Measurement of catalytic activity Catalytic activity tests were performed in a continuous flow fixed bed microreactor, using 100 mg catalyst powder A stainless steel tube with an inner diameter of mm was chosen as the reactor tube The reaction gas mixture consisting of vol.% CO balanced by air passed through the catalyst bed at a total flow rate of 33.4 ml/min A typical weight hourly space velocity (WHSV) was 20040 ml·h−1 ·g−1 After 30 under reaction conditions, the effluent gases were analyzed online by GC-508A gas chromatography equipped with a thermal conductivity detector (TCD) and a TDX 01 column The activity was expressed by the degree of CO conversion ment with the literature, at a binding energy of 284.6 eV The reduction properties of CuO supported catalysts were measured by means of temperature-programmed reduction (TPR) techniques under a mixture of 5% H2 in N2 100 mg catalyst was placed in a quartz reactor which was connected to a conventional TPR apparatus, and the heating rate is 10 ◦ C/min Results and discussion 3.1 Catalyst characterization Figure shows thermogravimetry and differential thermal analyses (TG-DTA) curves of as-prepared SnO2 dried at 80 ◦ C The TG curve can be divided into three stages The first stage is from room temperature to 97 ◦ C The weight loss is about 3%, which is caused by dehydration and evaporation of alcohol existing in the gel The second stage is from 97 to 257 ◦ C The weight loss is about 3.1%, which is attributed to the removal of the strongly bound water or the surface hydroxyl groups in the gel The third stage is 257 to 377 ◦ C The weight loss is about 3.5%, which results from the decomposition of the remained ammonium chlorate The existence of a small amount of water, alcohol and ammonium chlorate in samples is a normal phenomenon, and they can be removed by heating to above 377 ◦ C In the DTA curve, the broad weak endothermic peak at 97 ◦ C comes from desorption of water and alcohol Between 257 and 377 ◦ C, a small endothermic peak is observed due to the removal of the strongly bound water or the surface hydroxyl groups The endothermic peak centered at 340 ◦ C is assigned to the decomposition of the remained ammonium chlorate No any exothermal peak exists in the range of room temperature to 800 ◦ C, which indicates that there is no phase transformation in this temperature range From the result of the above TG-DTA, therefore, the as-prepared SnO2 was calcined at 400 ◦ C in air for h for the subsequent use 2.3 Catalyst characterization Thermogravimetry and differential thermal analyses (TGDTA) were performed by ZRY-2P thermal analyzer at a linear heating rate of 10 ◦ C/min α-Al2 O3 was used as reference Xray diffraction (XRD) analyses were performed on D/MAXRAX diffractometer operating at 40 kV and 100 mA, using Cu Kα radiation (scanning range 2θ: 20o – 75o ) X-ray photoelectron spectroscopy (XPS) measurements were performed with a Perkin-Elmer PHI 5600 spectrophotometer with the Mg Kα radiation The operating conditions were kept constant at 187.85 eV and 250.0 W In order to subtract the surface charging effect, the C 1s peak has been fixed, in agree- Figure TG-DTA curves of the as-prepared SnO2 dried at 80 ◦ C Figure shows the X-ray patterns of nanosized SnO2 , TiO2 and TiO2 -SnO2 calcined at 400 ◦ C Compared to JCPDS standard pattern, these XRD results indicate that SnO2 in both Journal of Natural Gas Chemistry Vol 18 No 2009 pure SnO2 and TiO2 -SnO2 samples is tetragonal rutile structure, and TiO2 in pure TiO2 and TiO2 -SnO2 samples is all tetragonal anatase structure Average particle size of SnO2 powders in pure SnO2 and TiO2 /SnO2 is 6.7 and 6.3 nm, respectively, obtained by Scherrer’s equation from SnO2 (101) Average particle size of TiO2 in pure TiO2 and TiO2 /SnO2 , calculated by Scherrer’s equation from TiO2 (110), is 12.7 nm and 12.4 nm, respectively Figure presents the XPS spectra in the Sn 3d, Ti 2p and Cu 2p region for wt.% CuO/TiO2 -SnO2 catalyst Two distinguished Sn 3d peaks, centered at 486.5eV and 495.0 eV, respectively, can be observed and were attributed to Sn4+ [9] In Ti 2p spectrum, two shoulder peaks at 458.4 and 464.0 eV indicated the presence of Ti4+ [10] The Cu 2p3/2 binding energy was 934.5 eV for the CuO/TiO2 -SnO2 catalyst, which is similar to that of pure CuO [11] 3.2 Catalytic performance Figure shows the activity of CuO/TiO2 -SnO2 catalysts with various CuO loadings in CO oxidation The loading of CuO in the CuO/TiO2 -SnO2 catalysts markedly influences the catalytic activity The optimum CuO loading is wt.%, which has an appreciably high catalytic activity [the temperature of 100% CO conversation (T 100) is 80 ◦ C] When the CuO loading is below wt.%, T 100 decreases with the increase of the CuO loading When the CuO loading is above wt.%, T 100 increases with the increased CuO loading This indicates that a proper CuO loading is necessary to gain high activity for CuO catalysts Figure X-ray diffraction patterns of nanosized SnO2 (1), TiO2 (2), and TiO2 -SnO2 (3) calcined at 400 ◦ C for h Figure Influence of CuO loading on the activity of CuO/TiO2 -SnO2 catalysts for CO oxidation Figure XPS spectra of Sn3d (a), Ti2p (b) and Cu2p (c) region for wt.% CuO/TiO2 -SnO2 catalyst calcined at 400 ◦ C Figure shows the CO oxidation activity of wt.% CuO/SnO2 , wt.% CuO/TiO2 and wt.% CuO/TiO2 -SnO2 catalysts It can be seen that the 100% CO conversion over CuO/SnO2 , CuO/TiO2 and CuO/TiO2 -SnO2 catalysts takes place at 130, 110, and 80 ◦ C, respectively This shows that the CuO/TiO2 -SnO2 catalyst exhibits much higher catalytic activity in CO oxidation than CuO/SnO2 and CuO/TiO2 catalysts We also measured the activity of the supports in CO oxidation TiO2 support displayed no catalytic activity below 310 ◦ C T100 on SnO2 support was 300 ◦ C However, the TiO2 /SnO2 support exhibited the higher catalytic activity than SnO2 support, and T 100 decreased to 270 ◦ C Compared to the single oxides, the particle sizes of SnO2 and TiO2 in the TiO2 /SnO2 composite have no obvious difference It was suggested that the high catalytic activity of CuO/TiO2 -SnO2 catalyst was related to the synergistic effect between Ti and Sn in the TiO2 -SnO2 support 4 Kairong Li et al./ Journal of Natural Gas Chemistry Vol 18 No 2009 Figure Activity of CuO/SnO2 , CuO/TiO2 and CuO/TiO2 -SnO2 catalysts for CO oxidation 3.3 H -TPR of CuO supported on dif ferent supports H2 -TPR has been extensively used to characterize the reducibility of oxygen species in metal oxide containing materials To study the reduction ability of CuO supported on different supports, we compare the H2 -TPR profiles of CuO/SnO2 , CuO/TiO2 , and CuO/TiO2 -SnO2 catalysts (Figure 6) H2 -TPR profile of the CuO/SnO2 catalyst shows one broad reduction peak centered at 227 ◦ C (β) with two shoulder peaks at 217 and 258 ◦ C (α and γ) There are two reduction peaks (α and γ) at around 209 and 258 ◦ C, respectively, in the H2 -TPR profile of CuO/TiO2 catalyst H2 -TPR profile of CuO/TiO2 -SnO2 catalyst exhibits one sharp reduction peak centered at 193 ◦ C (α) and a strong broad shoulder peak at 227 ◦ C followed a very weak peak at around 258 ◦ C It is likely that the α peak results from the reduction of welldispersed CuO species on support [12], and the β peak is assigned to the reduction peak of the surface-capping oxygen of SnO2 , and the γ peak is ascribed to the reduction of larger CuO species on the surface Due to the TiO2 reduction is very difficult at low temperature, there are no peaks related to the reduction of TiO2 are observed during the TPR from 24 to 400 ◦ C The strong broad β peaks in the H2 -TPR profile of the CuO/SnO2 and CuO/TiO2 -SnO2 catalysts suggested that a Figure H2 -TPR profiles of wt.% CuO/SnO2 (1), wt.% CuO/TiO2 (2), and wt.% CuO/TiO2 -SnO2 (3) significant amount of SnOx can be reduced even at low temperature In the CuO/TiO2 -SnO2 catalyst, the intensity of α peak is stronger than that in the CuO/SnO2 and CuO/TiO2 catalysts, and α peak shifted to lower temperature, while the γ peak becomes pretty weak The result indicates the amount of well-dispersed CuO is large and the reduction temperature of well-dispersed CuO is low in the CuO/TiO2 -SnO2 catalyst, compared with those in the CuO/SnO2 and CuO/TiO2 catalysts The reduction of CuO depends strongly on the sort of support, and the interaction of SnO2 and TiO2 might largely promote CuO well-dispersion on TiO2 -SnO2 support and make CuO more easily to be reduced Therefore, it is suggested that CuO species with high dispersion is the active site of CO oxidation, and the improvement in catalytic activities is probably related to highly dispersion of CuO species [13,14] Conclusions The loading of CuO in the CuO/TiO2 -SnO2 catalysts markedly influences the catalytic activity, and the optimum CuO loading is wt.% CuO/TiO2 -SnO2 catalyst exhibits much higher catalytic activity for CO oxidation than CuO/SnO2 and CuO/TiO2 catalysts H2 -TPR result shows that there is a large amount of CuO to form the active site for CO oxidation in CuO/TiO2 -SnO2 catalyst Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (20771061 and 20871071), the 973 Program (2005CB623607) and Science and Technology Commission Foundation of Tianjin 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