Activation of Gold Nanoparticles on Titania: A Novel DeSOx Catalyst

Chapter 27

Activation of Gold Nanoparticles on Titania: A Novel DeSOx Catalyst José A. Rodriguez

Downloaded by UNIV OF PITTSBURGH on March 17, 2016 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch027

Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973 ([email protected])

Abstract The DeSO activity of Au nanoparticles supported on titania and magnesium oxide is examined and compared. The Au/TiO2 system exhibits a remarkable activity for the destruction of SO . Extensive dissociation of SO is observed on Au/TiO (110), with negligible dissociation on Au/MgO(100). Similar trends are found in Au/TiO and Au/MgO high surface area catalysts. These results illustrate the importance of the oxide support for the activation of gold nanoparticles. x

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2005 American Chemical Society

Karn et al.; Nanotechnology and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

206 The destruction of S 0 (DeSO ) is a very important problem in environmental chemistry (1,2). S 0 isfrequentlyformed during the combustion of fossil-derived fuels in factories, power plants, houses, and automobiles (1). Every year the negative effects of acid rain (main product of the oxidation of S 0 in the atmosphere) on the ecology and corrosion of monuments or buildings are tremendous. Thus, new environmental regulations emphasize the need for more efficient technologies to destroy the S 0 formed in combustion processes (2). Titania is the most common catalyst used in the chemical industry and oil refineries for the removal of S 0 through the Claus reaction: S 0 -i- 2H S -> 2H 0 + 3S„u4 (2). Different approaches are being tested for improving the performance of titania in DeSOx operations. We have found that the addition of gold to T i 0 produces desulfurization catalysts with a high efficiency for the cleavage of S-0 bonds. In this respect, the Au/Ti0 system is much more chemically active than either pure titania or gold (3). 2

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Bulk metallic gold typically exhibits a very low chemical and catalytic activity (4). Among the transition metals, gold is by far the least reactive and is often referred as the "coinage metal". Recently, gold has become the subject of a lot of attention due to its unusual catalytic properties when dispersed on some oxide supports (Ti0 , CrO*, MnO„ Fe 0 , A1 0 , MgO) (3,5-9). Several models have been proposed for explaining the activation of supported gold: from special electronic properties resulting from the limited size of the active gold particles (usually less than 10 nm), to the effects of metal-support interactions (i.e. charge transfer between the oxide and gold). In principle, the active sites for the catalytic reactions could be located only on the supported Au particles or on the perimeter of the gold-oxide interface. To address some of these issues, the reactivity of Au/TiO (110) and Au/MgO(100) towards S 0 was compared (3,10). 2

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Experimental Methods Photoemission was used to study the chemistry of S 0 on the supported nanoparticles. The experiments were performed in a standard ultra-high vacuum chamber (base pressure - 6 χ 10" Torr) that is part of the U7A beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (3). The overall instrumental resolution in the photoemission experiments was approximately 0.35 eV. The Au/TiO (110) and Au/MgO(100) surfaces were prepared as described in refs 3 and 10. S 0 was dosed through a glass-capillary array doser positioned to face the sample at a distance of - 2 mm. The S 0 exposures are based on the ion gauge reading and were not corrected for the capillary array enhancement (> 5 enhancement factor with respect to background dosing). 2

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Au coverage (ML) Fig 1. Amount of atomic sulfur generated after dosing SO2 to a series of Au/TiO (U0) andAu/MgO(100) surfaces at 300 K. The coverage of sulfur is assumed to be proportional to the area under the S 2p features in photoemission 2

RESULTS Figure 1 compares S 2p areas measured for atomic S after dosing S 0 (5 langmuir) to Au/MgO(100) and Au/TiO (110) surfaces at 300 K . Neither MgO(lOO) nor TiO (110) are able to dissociate Sty on their own. On both oxide supports, the largest activity for the full dissociation of S 0 is found in systems that contain Au coverages smaller than 1 ML when the size of the Au nanoparticles is below 5 nm (3,6,10). Clearly the Au/TiO (110) system is much more chemically active than the Au/MgO(100) system. These data indicate that titania either plays a direct active role in the dissociation of S 0 or modifies the chemical properties of the supported Au nanoparticles (3,10). 2

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208 Photoemission results and TPD data indicate that Au particles supported on MgO(lOO) bond S 0 substantially stronger than extended surfaces of gold (10) . The heat of adsorption of the molecule on the Au nanoparticles is - 15 kcal/mol compared to 8 kcal/mol for S 0 on A u ( l l l ) . Density functional calculations indicate that the enhancement in the S 0 adsorption energy is simply due to the presence of corner sites (i.e. Au atoms with a low coordination number) in the nanoparticles (10). The dissociation of S 0 on Au/MgO(100) is very limited due to weak Au/MgO(100) interactions. 2

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Recent catalytic tests for the Claus reaction (S0 + 2H S -> 2H 0 + and the reduction of S 0 by carbon monoxide (S0 + 2CO -> 2C0 + Ssoiid) show that the Au/Ti0 system is 5-10 times more active than pure T i 0 (11) . Interestingly, polycrystalline foils of gold are inactive as catalysts for these DeSOx processes. In the case of Au/MgO, a limited DeSO activity is observed


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Acknowledgements The studies described here were done in collaboration with Z. Chang, J. Dvorak, J. Evans, L, Gonzalez, J. Hrbek, T. Jirsak, G. Liu, A . Maiti, and M . Perez. This research was carried out at Brookhaven National Laboratory and supported by the US Department of Energy.

References 1.

Slack, Α.V.; Holliden, G.A. Sulfur Dioxide Removal from Waste Gases, 2nd ed ; Noyes Data Corporation: Park Ridge, NJ 1975. 2. Pieplu, Α.; Saur, Ο.; Lavalley, J.-C.; Legendre, O.; Nedez, C. Catal. Rev.Sci. Eng. 1998, 40, 409. 3. Rodriguez, J.A.; Liu, G.; Jirsak, T.; Hrbek, J.; Chang, Z.; Dvorak, J.; Maiti, A . J. Am. Chem. Soc. 2002, 124, 5242. 4a. Thomas, J. M . ; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH: New York, 1997 b. Somorjai, G.A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. 5. Haruta, M . Catal. Today, 1997, 36, 153. 6. Valden, M . ; Lai, X.; Goodman, D.W. Science, 1998, 281, 1647. 7. Rodriguez, J.A.; Chaturvedi, S.; Kuhn, M . ; van Ek, J.; Diebold, U.; Robbert, P.S.; Geisler, H.; Ventrice, C.A. J. Chem. Phys. 1997, 107, 9146.


209 Bondzie, V.A.; Parker, S.C.; Campbell, C.T. J. Vac. Sci. Technol. A , 1999, 17, 1717 9. Campbell, C.T. Curr. Opin. Solid State Mater. Sci. 1998, 3, 439. 10. Rodriguez, J.A.; Perez, M . ; Jirsak, T.; Evans, J.; Hrbek, J.; Gonzalez, L. Chem. Phys. Lett., 2003, in press. 11. Evans, J.; Lee, H; Fischer, I. research in progress.


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Activation of Gold Nanoparticles on Titania: A Novel DeSOx Catalyst

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