JOURNAL OF CATALYSIS 108, 161-174 (1987) The Oxidation of Ethylene over Silver-Based Alloy Catalysts 3. Silver-Gold Alloys NOREDDINE TOREIS’ AND XENOPHON E. VERYKIOS’ Department of Chemical Engineering, Drexel University, Philadelphia, Pennsylvania 19104 Received December 1, 1986; revised May 19, 1987 Supported silver-gold alloy catalysts were prepared by impregnation of low surface area a-A1203 with mixed silver cyanide and gold cyanide solutions. Alloying was achieved by calcination at 250°C.X-ray diffraction analysis indicates that complete alloying was achieved. Surface composi- tion of alloy particles was determined as a function of bulk composition by high resolution ESCA analysis. Surfaces were found to be significantly enriched in silver. Turnover numbers of ethylene epoxidation and combustion were found to exhibit maxima on surfaces containing lo-15% Au and drop to zero on surfaces containing 30% Au or more. Temperature-programmed desorption of oxygen from Ag-Au ahoy surfaces revealed that oxygen desorbs in two modes from surfaces containing less than 30% Au. The high-temperature desorption peak was assigned to multicoor- dinated adsorbed atomic oxygen and the low-temperature peak to monocoordinated atomic oxygen. Based on these results a mechanism of ethylene epoxidation and combustion is formulated. 0 1987 Academic Press. Inc. INTRODUCTION Geometric and electronic factors in ca- talysis by alloys are investigated using eth- ylene oxidation as a probe reaction. In previous publications (l-3), the effects of alloying silver with Pd, Cd, and Zn on its performance in ethylene oxidation catalysis were described. The final metal which was selected for this study is gold, on the basis of its electronegativity, which is similar to that of Pd relative to silver. A major advan- tage that gold presents as an alloying metal in this catalytic system is the fact that it is totally inert toward most chemisorptive and catalytic processes. In addition, silver and gold form random solid solutions over the entire composition range. As a result, alloy catalysts with a large variation in surface composition can be formulated and studied. ’ Present address: Ecole Mohammadia d’ Inge- nieurs, BP 765 Agdal, Rabat, Morocco. ’ Present address: Institute of Chemical Engineering and High Temperature Chemical Processes, Depart- ment of Chemical Engineering, University of Patras, GR-26110 Patras, Greece. Finally, the formation of Ag-Au alloys is moderately exothermic, which ensures fairly rapid equilibration of the alloys. It is probably for these reasons that gold is one of the few metals whose role as an additive to silver has been investigated in ethylene oxidation (4-6) and other catalytic systems (7, 8). Contradictory results con- cerning the effects of Au on kinetic pa- rameters in ethylene epoxidation and com- bustion have been reported. Flank and Beachell (4) have concluded that activity for overall ethylene oxidation is dependent on lattice spacing of the alloy particles and that gold at low concentrations is a pro- moter of ethylene oxide formation, markedly improving selectivity. However, Geenen et al. (5) observed selectivity to decrease with increasing gold content and ultimately drop to zero over gold-rich al- loys. Such discrepancies can probably be traced to the method of preparation of the catalysts and to different precursors of the metal components. In the present communication, the ef- fects of alloying silver with gold on kinetic 161 0021-9517187 $3.00 Copyright 8 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.