We have studied the structure of mononuclear gold supported on acidic form of faujasite zeolite in two oxidation states, namely, 0 and +1, using density functional theory. The binding of the gold monomer to the zeolite support is stronger in the oxidation state +1 than in the oxidation state 0. For the oxidation state 0, the hydrogenated clusters AuH/(2H)-FAU, AuH2/H-FAU generated by stepwise reverse hydrogen spillover from bridging OH groups of zeolite are energetically preferred over the Au/(3H)-FAU structure. Reverse hydrogen spillover of all the three acidic protons from the zeolite to the Au monomer did not lead to a stable structure. The calculated reverse hydrogen spillover energy per hydrogen atom for zeolite supported AuH and AuH2 clusters are ？10.2 and ？5.1？kJ/mol, respectively, in the oxidation state 0, while in the oxidation state +1 it is 20.9？kJ/mol for zeolite supported Au+H cluster. 1. Introduction Oxide-supported transition metal clusters form an important class of system both for theoretical and experimental investigations mainly due to their very important applications as catalysts. The activity of supported metal clusters is found to be higher than bare clusters and these metal-support interfaces are believed to act as active sites for catalysis. The factors influencing the reactivity of supported clusters are the size, structure and oxidation state of the cluster, the nature of the support, and cluster support interaction. Zeolites form an important class of support material for nanoclusters because their pores and cavities facilitate the formation of size-selective clusters of nanometer and sub-nanometer dimensions. Also, the zeolite support enables the tuning of the charge state of the cluster, as it depends upon the concentration of the acidic centres on the support, which can be modified. Among the transition metals gold has been highly investigated due to its ability to catalyse a number of reactions like CO oxidation [1], water gas shift reaction [2], epoxidation of propylene, [3] vinyl chloride synthesis [4], and so forth. Spurt in research activities involving supported gold clusters began after the pioneering discoveries of Haruta et al. [5] and Hashmi and Hutchings [6] demonstrating strong catalytic activities of highly dispersed gold. The common metal oxide supports used for gold cluster catalysis are MgO, Al2O3, SiO2, TiO2, and so forth. It has been found by Vayssilov and R？sch [7] that transition metal clusters M6 with hydrogen impurity adsorbed on a zeolite support have more nearest neighbour M-O contacts than the
References
[1]
M. Haruta, “Size- and support-dependency in the catalysis of gold,” Catalysis Today, vol. 36, no. 1, pp. 153–166, 1997.
[2]
Q. Fu, H. Saltsburg, and M. Flytzani-Stephanopoulos, “Active nonmetallic Au and Pt species on ceria-based water-gas shift catalysts,” Science, vol. 301, no. 5635, pp. 935–938, 2003.
[3]
E. E. Stangland, K. B. Stavens, R. P. Andres, and W. N. Delgass, “Characterization of gold-titania catalysts via oxidation of propylene to propylene oxide,” Journal of Catalysis, vol. 191, no. 2, pp. 332–347, 2000.
[4]
B. Nkosi, M. D. Adams, N. J. Coville, and G. J. Hutchings, “Hydrochlorination of acetylene using carbon-supported gold catalysts: a study of catalyst reactivation,” Journal of Catalysis, vol. 128, no. 2, pp. 378–386, 1991.
[5]
M. Haruta, N. Yamada, T. Kobayashi, and S. Iijima, “Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide,” Journal of Catalysis, vol. 115, no. 2, pp. 301–309, 1989.
[6]
A. S. K. Hashmi and G. J. Hutchings, “Gold catalysis,” Angewandte Chemie International Edition, vol. 45, no. 47, pp. 7896–7936, 2006.
[7]
G. N. Vayssilov and N. R？sch, “Reverse hydrogen spillover in supported subnanosize clusters of the metals of groups 8 to 11. A computational model study,” Physical Chemistry Chemical Physics, vol. 7, no. 23, pp. 4019–4026, 2005.
[8]
W. A. Weber and B. C. Gates, “Hexarhodium clusters in NaY zeolite: characterization by infrared and extended X-ray absorption fine structure spectroscopies,” The Journal of Physical Chemistry B, vol. 101, no. 49, pp. 10423–10434, 1997.
[9]
G. N. Vayssilov and N. R？sch, “Free and zeolite-supported hexarhodium clusters with light impurity atoms,” Journal of Physical Chemistry B, vol. 108, no. 1, pp. 180–192, 2004.
[10]
M. N. Mikhailov, I. V. Mishin, and L. M. Kustov, “A possible mechanism of hydrogen reverse spillover in platinum-zeolite catalysts,” Catalysis Letters, vol. 128, pp. 313–317, 2008.
[11]
R. C. Deka, A. Deka, and A. Miyamoto, “Density functional studies on the structure and reverse hydrogen spillover in Au6 cluster supported on zeolite,” Catalysis Letters, vol. 131, no. 1-2, pp. 155–159, 2009.
[12]
J. C. Fierro-Gonzalez and B. C. Gates, “Mononuclear AuIII and AuI complexes bonded to zeolite NaY: catalysts for CO oxidation at 298 K,” Journal of Physical Chemistry B, vol. 108, no. 44, pp. 16999–17002, 2004.
[13]
W. L？wenstein, “The distribution of aluminium in the tetrahedra of silicates and aluminates,” American Mineralogist, vol. 39, no. 1-2, pp. 92–96, 1954.
[14]
D. H. Olson, “The crystal structure of dehydrated NaX,” Zeolites, vol. 15, no. 5, pp. 439–443, 1995.
[15]
B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” The Journal of Chemical Physics, vol. 92, no. 1, pp. 508–517, 1990.
[16]
B. Delley and D. E. Ellis, “Efficient and accurate expansion methods for molecules in local density models,” The Journal of Chemical Physics, vol. 76, no. 4, pp. 1949–1960, 1982.
[17]
S. J. Vosko, L. Wilk, and M. Nusair, “Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis,” Canadian Journal of Physics, vol. 58, no. 8, pp. 1200–1211, 1980.
[18]
A. D. Becke, “Density-functional exchange-energy approximation with correct asymptotic behavior,” Physical Review A, vol. 38, no. 6, pp. 3098–3100, 1988.
[19]
C. Lee, W. Yang, and R. G. Parr, “Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density,” Physical Review B, vol. 37, no. 2, pp. 785–789, 1988.
[20]
J. C. Fierro-Gonzalez, Y. Hao, and B. C. Gates, “Gold xanoclusters entrapped in the α-cages of y zeolites: structural characterization by X-ray absorption spectroscopy,” Journal of Physical Chemistry C, vol. 111, no. 18, pp. 6645–6651, 2007.
[21]
R. C. Deka and S. Baishya, “Density functional investigation of reverse hydrogen spillover on zeolite supported Pd6 and Au6 clusters,” Catalysis Today, vol. 198, no. 1, pp. 110–115, 2012.
