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4711791 
Journal Article 
Phase Coexistence of Multiple Copper Oxides on AgCu Catalysts during Ethylene Epoxidation 
Greiner, MT; Cao, J; Jones, TE; Beeg, S; Skorupska, K; Carbonio, EA; Sezen, H; Amati, M; Gregoratti, L; Willinger, MG; Knop-Gericke, A; Schloegl, R 
2018 
ACS Catalysis
ISSN: 2155-5435 
AMER CHEMICAL SOC 
WASHINGTON 
8 
3 
2286-2295 
WOS:000426804100074 
Alloy catalysts under reaction conditions are complex entities. In oxidizing atmospheres, multiple phases can coexist on a catalyst's surface as a result of phase segregation and preferential oxidation. Such a scenario can result in unusual substoichiometric and metastable phases that could play important roles in catalytic processes. For instance, AgCu alloy-known to exhibit enhanced epoxide selectivity in partial oxidation of ethylene-form an oxide-like surface structure under reaction conditions. Under these conditions, copper oxides are stable, while silver oxides are not. Consequently, copper segregates to the alloy's surface and forms an oxide overlayer. Little is known about the structure or function of such overlayers, and it is unknown whether they play an active role in the catalyst's enhanced selectivity. In order to develop a clearer picture of such catalysts, the current work utilizes several in situ spectroscopic and microscopic techniques to examine the copper oxide phases that form when AgCu is exposed to epoxidation conditions. It is found that several forms of oxidic Cu coexist simultaneously on the active catalyst's surface, namely, CuO, Cu2O, and some previously unreported form of oxidized Cu, referred to here as CuxOy. Online product analysis, performed during the in situ spectroscopic measurements, shows that increased epoxide selectivity is correlated with the presence of mixed copper oxidation states and the presence of the CuxOy species. These results support previous theoretical predictions that oxidic copper overlayers on silver play an active role in epoxidation. These results furthermore emphasize the need for in situ spectromicroscopic methods to understand the complexity of alloy catalysts. 
alloy catalyst; ethylene epoxidation; in situ XPS; in situ SEM; oxidation; phase segregation