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Langmuir 2006, 22, 7072-7077
Sonochemically Prepared Pt/CeO2 and Its Application as a Catalyst in Ethyl Acetate Combustion Nina Perkas,† Hadar Rotter,‡ Leonid Vradman,§ Miron V. Landau,‡ and Aharon Gedanken*,† Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan, 52900, Israel, Chemical Engineering Department, Ben-Gurion UniVersity of the NegeV, Beer-SheVa, 84105, Israel, and Department of Chemical Engineering, Sami Shamoon College of Engineering, Beer-SheVa 84100, Israel ReceiVed January 10, 2006. In Final Form: May 23, 2006 Highly dispersed Pt nanoparticles were incorporated in CeO2 nanopowders by an ultrasound-assisted reduction procedure. The activity of the Pt/CeO2 catalysts was studied in the reaction of the ethyl acetate combustion, and complete conversion was achieved at low temperature. It was demonstrated that the higher dispersion of the CeO2 support, the better the performance of the Pt/CeO2 catalysts. The catalysts were characterized by XRD, TEM, HRTEM, EDX, BET, and XPS. The homogeneous incorporation of 2-4 nm Pt nanoparticles into the interparticle distance of the CeO2 nanopowders was demonstrated. The advantage of the sonochemical method for catalyst preparation, in comparison with the traditional incipient wetness impregnation, was explained as the result of the homogeneity and better dispersion of the active metal phase obtained by ultrasound irradiation.
Introduction Cerium oxide is widely used as a support for many oxidation catalysts. These catalysts are active in oxidation of CO,1 elimination of CO and NOx contaminants from automotive exhaust gases,2 water-gas shift reactions,3 combustion of hydrocarbons,4 etc. The industrial applications of CeO2 as a catalyst support are due to its high oxygen transport and storage capacities.5 The properties of cerium oxide powder are influenced by its structure, morphology, and preparation method. Many techniques were applied to obtain homogeneously dispersed ceria nanoparticles, providing a significant improvement in their redox and thermal stability with respect to microsized or bulk materials.6-8 Recently, it was demonstrated that an ultrasound-assisted method enables the production of cerium oxide nanoparticles homogeneously distributed in a 3.5-4 nm size range by a one-step procedure.9-11 These monodispersed nanoparticles were found to form mesoporous (MSP) structures, which can be used as a suitable support for the synthesis of many effective catalysts. * To whom correspondence should be addressed. E-mail: gedanken@ mail.biu.ac.il. † Bar-Ilan University. ‡ Ben-Gurion University of the Negev. § Sami Shamoon College of Engineering. (1) Martinez-Arias, A.; Fernandez-Garcia, M.; Galvez, O.; Coronado, J. M.; Anderson, J. A.; Conesa, J. C.; Soria, J.; Munuera, J. J. Catal. 2000, 195, 207. (2) Manuel, I.; Thomas, C.; Colas, H.; Matthess, N.; Diega-Mariadassou, G. Top. Catal. 2004, 30-1, 311. (3) Goquet, A.; Meunier, F. C.; Tibiletti, D.; Breen, J. P.; Burch, R. J. Phys. Chem. B 2004, 108, 20240. (4) Bera, P.; Patil, K. C.; Jayaram, V.; Subbana, G. N.; Hegde, M. S. J. Catal. 2000, 196, 293. (5) In Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; Imperial College Press: London, 2002. (6) Zhang, F.; Chan, S.-W.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson, R. D.; Herman, P. Appl. Phys. Lett. 2002, 80, 127. (7) Hernandez-Alonso, M. D.; Hungria, A. B.; Martinez-Arias, A.; Coronado, J. M.; Conesa, J. C.; Soria, J.; Fernandez-Garcia, M. Phys. Chem. Chem. Phys. 2004, 6, 3524. (8) Chen, H.-I.; Chang, H.-Y. Colloids Surf. Sci. A: Physicochem. Eng. Aspects 2004, 242, 61. (9) Yin, L.; Wang, Y.; Pang, G.; Koltypin, Y.; Gedanken, A. J. Colloid Interface Sci. 2002, 246, 78. (10) Wang, Y.; Yin, L.; Gedanken, A. Ultrason. Sonochem. 2002, 9, 285. (11) Yu, J. C.; Zhang, L.; Lin, J. J. Colloid Interface Sci. 2003, 260, 240.
The properties of Pt/CeO2 catalysts prepared by combustion and modified reduction-deposition methods were studied in CO and hydrocarbons oxidation.4,12,13 The promoting effect of CeO2 was attributed by these authors to the strong Pt2+-CeO2 interaction creating the Ce1-xPtxO2-δ species responsible for the higher catalytic activity. The insertion of Pt into the mesoporous CeO2 oxides was not reported in the literature. Ultrasound-assisted preparation of catalysts was previously studied by Suslick et al.14-22 In our previous publications we demonstrated the advantages of the sonochemical method for the deposition/insertion of an active metal phase on/into different ceramic supports as well as for the synthesis of mesoporous supports.23-26 Ultrasound irradiation results in the production of highly dispersed nanoparticles having a narrow size distribution. The ultrasonic waves form bubbles, which tend to collapse preferentially near solid surfaces. The after effects of the collapse are microjets and shock waves directed toward the solid. The nanoparticles formed during the collapse of the ultrasonic bubbles (12) Bera, P.; Gayen, A.; Hegde, M. S.; Lalla, N. P.; Sparado, L.; Frusteri, F.; Arena, F. J. Phys. Chem. B 2003, 107, 6122. (13) Tang, X.; Zhang, B.; Li, Y.; Xu, Y.; Xin, Q.; Shen, W. Catal. Lett. 2004, 97, 163. (14) Suslick, K. S. Handbook of Heterogeneous Catalysis; Wiley-VSH: Weinheim, 1997; Vol. 3, pp 1350-1357. (15) Mdleleni, M. M.; Hyeon, T.; Suslick, K. S. J. Am. Chem. Soc. 1998, 120, 6189. (16) Cain, P. W.; McCausland, J.; Bates, D. M.; Mason, T. J. Ultrason. Sonochem. 1994, 1, S45. (17) Tai, A.; Kikukawa, T.; Sugimura, T.; Inoue, Y.; Osawa, T.; Fujii, S. J. Chem. Soc., Chem. Commun. 1991, 795. (18) Torok, B.; Felfoldi, K.;Szakonyi, G.; Balazsik, K.; Bartok, M. Catal. Lett. 1998, 52, 81. (19) Kun, I.; Torok, B.; Felfoldi, K.; Bartok, M. Appl. Catal. A 2000, 203, 71. (20) Mhadgut, S. C.; Busci, I.; Torok, M.; Torok, B. Chem. Commun. 2004, 984. (21) Disselkamp, R. S.; Judd, K. M.; Hart, T. R.; Peden, C. H. F.; Posakony, G. J.; Bond, L. J. J. Catal. 2004, 221, 347. (22) Disselkamp, R. S.; Chin, J. H.; Peden, C. H. F. J. Catal. 2004, 227, 552. (23) Perkas, N.; Wang, Y.; Koltypin, Y.; Gedanken, A.; Chandrasekaran, S. Chem. Commun. 2001, 988. (24) Landau, M. V.; Vradman, L.; Herskowitz, M.; Koltypin, Y.; Gedanken, A. J. Catal. 2001, 201, 22. (25) Perkas, N.; PhamMinh, D.; Gallezot, P.; Gedanken, A.; Besson, M. Appl. Catal. B 2005, 59, 121. (26) Perkas, N.; Zhong, Z.; Chen, L.; Besson, M.; Gedanken, A. Catal. Lett. 2005, 103, 9.
10.1021/la0600907 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/30/2006
Sonochemically Prepared Pt/CeO2
near the solids are thrown by these microjets at their surfaces and into the mesopores of the support. As a result, the metal nanoparticles are anchored to the surface of the support and on the inner walls of the mesoporous material, without blocking the mesopores. In the present work we continue these studies with another sonochemically prepared mesoporous oxide, namely, CeO2. A one-step sonochemical process created the Pt nanoparticles and immediately deposited them on the surface of the CeO2 support. Two ceria supports were used for comparison. The first was commercial CeO2 nanopowder, and the second was mesoporous CeO2 prepared by ultrasound radiation. In addition to sonochemical synthesis, the Pt/CeO2 catalysts were also prepared by the classic incipient wetness-impregnation method on both mesoporous and commercial CeO2 nanopowder. The catalytic performance of Pt/CeO2 was tested in ethyl acetate (EA) combustion. This is an important model reaction in the environmental protection technology of volatile organic compound (VOC) combustion.27 Pt catalysts on different ceramic supports such as Al2O3, TiO2, and silica SBA-15 were previously studied by this process.28-30 Different modifications of the supports were used to enhance the catalytic activity. In the present work, high activity of the novel Pt/CeO2 catalysts at low temperature was achieved. The effect of the support structure and preparation method on the catalytic properties was demonstrated. Experimental Section Catalyst Preparation. All reagents (chemically pure) were purchased from Aldrich and used as received. Mesoporous CeO2 (MSP) was prepared by the sonochemical method following a previously described procedure.10 In short, the reaction involved an aqueous solution of Ce(NO3)3‚9H2O as the Ce source, sodium dodecyl sulfate (SDS) as a surfactant template, and urea as a precipitating agent. Removal of the surfactant was done by extraction with sodium acetate following Yada’s method.31 Insertion of Pt nanoparticles into CeO2(MSP) was done by an ultrasound-assisted reduction method. A typical sonochemical reaction was performed as follows: to a solution of 0.01 g of H2PtCl6 in 100 mL of ethanol, the corresponding amount of CeO2 (MSP) was added to get a loading of 0.5 wt % Pt on the support. The pH was adjusted to 6 by addition of NaOH to the solution. The reaction mixture was purged with a gas mixture of 95:5 Ar:H2 for 1 h to remove the traces of oxygen, and the sonication was carried out under a slow flow (bubbling) of this gas using a high-intensity direct immersion titanium horn (20 kHz, 100W/cm2) at 30 °C for 1 h. The solid product was separated by centrifugation, thoroughly washed with ethanol, and dried under vacuum overnight. The 0.5%Pt/ CeO2 catalysts were prepared by the same ultrasound-assisted method on the commercial ceria nanopowder (CCN) purchased from Aldrich. The catalysts are marked as 0.5%Pt/CeO2(MSP)US and 0.5%Pt/ CeO2(CN)US, respectively. For comparison, the Pt catalysts were prepared by the classic incipient-wetness impregnation (IWI) of the mesoporous ceria and CCN with an aqueous solution of H2PtCl6 to get a 0.5% loading of Pt on the support. The dried mixture was reduced by H2 at 300 °C for 2 h, as previously reported.30 These catalysts are marked as 0.5%Pt/CeO2(MSP)IWI and 0.5%Pt/CeO2(CN)IWI, respectively. In addition, 0.5 wt % Pt was deposited on the commercial reference support γ-Al2O3 (Norton SA 6175, 270 m2/g) via the IWI method. (27) Hodnett, B. K. Heterogeneous Catalytic Oxidation, Fundamental and Technological Aspects of the SelectiVe and Total Oxidation of Organic Compounds; Wiley: Chichester, 2000; p 191. (28) Papaefthimiou, P.; Ioannides, T.; Verykios, X. E. Appl. Catal. B 1998, 15, 75. (29) Rotter, H.; Landau, M. V.; Carrera, M.; Goldfarb, D.; Herskowitz, M. Appl. Catal. B 2004, 47, 111. (30) Wang, X.; Landau, M. V.; Rotter, H.; Vradman, L.; Wolfson, A.; Erenburg, A. J. Catal. 2004, 222, 565. (31) Yada, M.; Ohya, M.; Machida, M.; Kijima, T. Langmuir 2000, 16, 1536.
Langmuir, Vol. 22, No. 16, 2006 7073 The comparative Pt catalysts prepared by impregnation of ceria and alumina supports with an aqueous solution of H2PtCl6 required regular reduction of platinum with hydrogen to convert it to the metallic state. Hydrogenative pretreatment of Pt-containing catalysts before the reaction is a common practice in combustion.27,29,30 Therefore, a reductive treatment under the same conditions was conducted prior to the catalytic tests for samples prepared by IWI and US to compare the catalytic performance. Catalyst Characterization. The solid products were identified by XRD analysis using a Bruker D8 diffractometer with Cu KR radiation. The CeO2 particle size was determined using the DebyeScherrer equation. Transmission electron microscopy (TEM) studies were carried out on a JEOL-JEM 100 electron microscope. Highresolution TEM (HRTEM) images were obtained by employing a JEOL-3010 device with 300 kV accelerating voltage. Elemental analysis was carried out by energy-dispersed X-ray analysis (EDX) on a JEOL-JSM 840 scanning microscope. Nitrogen isotherms were measured at -196 °C using a Micromeritics (Gemini 2375) analyzer after outgassing of the samples at 120 °C for 1 h. The surface area was calculated from the linear part of the BET plot. The pore size distribution was estimated using the Barret-Joyner-Halenda (BJH) model with the Halsey equation.32 The oxidation state of the components was determined by XPS on a KRATOS AXIS HS spectrometer using Al KR radiation. The C 1s (Eb ) 285.0 eV) peak was chosen as a reference line for calibration of the energy scale. Catalyst Testing. Oxidation of ethyl acetate (EA) was studied under atmospheric pressure in the temperature range of 200-350 °C. The concentration of EA in air was 0.5 vol % and EA WHSVEA ) 2.5-5 h-1, as described elsewhere.30 The experiments were carried out in a stainless steel tubular reactor of 20 mm i.d. and 30 cm long. Catalyst activation was done under a hydrogen flow at 300 °C for 2 h. The catalyst pellets, 0.3 g, 0.1-0.2 mm, were diluted with quartz particles of the same size at a 1:5 vol. ratio. The concentrations of EA and CO2 in the effluent gas were analyzed on a Varian 3300 GC equipped with a TCD detector and a Poropack Q packed column (6 ft, 1/8 in. i.d., 3 mm). At low EA conversions (
