The effect of AuAg–MCF and AuAg–NbMCF catalysts pretreatment on the gold–silver alloy formation and the catalytic behavior in selective methanol oxidation with oxygen

Abstract

MCF (mesostructured cellular foams) and NbMCF were used as supports for gold and silver. Bimetallic (Au–Ag) catalysts were prepared by metals loading (Au/Ag atomic ratio=0.5 and 2) on 3-aminopropyl-trimethoxysilane grafted supports and reduction by NaBH4 before calcination at 773K. The prepared samples were then activated in inert gas at 623K or reduced with hydrogen at 723K. The catalysts treated in these ways were fully characterized by XRD, TEM, UV–vis and XPS techniques. Au–Ag alloy was identified on the surface of AuAg–MCF and AuAg–NbMCF catalysts. The composition of Au–Ag alloy was found to be sensitive to the catalyst pretreatment conditions. The reduction of Au–Ag containing materials with NaBH4 led to the formation of alloy with low silver content. The Au–Ag alloy was enriched in silver partially by the activation in argon flow and to a substantial degree by the reduction with hydrogen. The conditions of catalysts treatment also determined the alloy particle size. Catalytic activity of Au–Ag catalysts was tested in the oxidation of methanol in the gas phase. The results showed that the presence of gold–silver alloy on MCF surface was responsible for high activity of the catalysts as a result of synergetic interactions between Au and Ag species. Niobium in MCF support influenced both the activity of the catalysts and the selectivity to formaldehyde.