Bimetallic AgCu/SBA-15 System: The Effect of Metal Loading and Treatment of Catalyst on Surface Properties

Monometallic (Ag, Cu) and bimetallic (Ag + Cu) catalysts were prepared by metal loading (Ag:Cu = 0.3 and 1.8) on 3-aminopropyl-trimethoxysilane-grafted SBA-15 and calcination at 773 K and either reduction by NaBH4 before calcination or activation in inert gas after calcination. The catalysts treated in this way were fully characterized. Cu! SBA-15 samples contained CuO and oligonuclear [Cu delta+center dot center dot center dot Cu delta-center dot center dot center dot Cu delta+](n) clusters irrespective of the catalyst treatment. Silver-SBA-15 contained cationic silver in the form of Ag2O which was transformed to metallic Au by the reduction with NaBH4 and was not reoxidized during calcination. In bimetallic catalysts, different species were identified depending mainly on the Ag:Cu atomic ratio and the post modification treatment. When the excess of copper was applied the core (Ag2O)-shell (CuO) structure of the bimetallic phase was formed. If the reduction with NaBH4 was used prior to calcination, the same core-shell structure was present but with higher dispersion of CuO, manifested as a higher basicity of the catalysts revealed as a higher selectivity to acetone in 2-propanol dehydrogenation and to CO2 in methanol oxidation. The use of silver in excess led to the presence of both cationic silver and copper species in calcined AgCu(1)/S(C) material. In the sample reduced with NaBH4 and then calcined (AgCu(1)/S(RC)), metallic copper was partially surrounded by metallic silver. In bimetallic samples Cu-Ag interaction led to the electron transfer from copper to silver species enhancing their redox properties and causing the superior activity in the low-temperature total oxidation of methanol to CO2.