Methanol oxidation on AuAg-Zn/MCM-36 – The effect of catalyst components and pretreatment

Abstract

• ZnO loaded on MCM-36 played a role of structural and electronic promoter. • Alloy was identified after thermal treatment of AuAg-zeolites in Ar and H 2 flows. • Zinc favored the deeper alloying of noble metals in hydrogen reduction conditions. • Zinc on MCM-36 directed the selectivity of Au- and AuAg-catalysts to formaldehyde. • Alloy rich in silver favored the total oxidation of methanol. MCM-36 and Zn/MCM-36 layered zeolites were used as supports for gold and silver. The calcined samples prepared were treated in inert gas or hydrogen flow at 673 K. The effect of gold, silver, zinc and Au-Ag alloy on the surface properties, activity and selectivity of the catalysts in methanol oxidation was studied. The catalysts were fully characterized by XRD, TEM, UV–vis and XPS techniques. The surface acidity of catalysts was measured by pyridine adsorption combined with FTIR studies. The results showed that metallic gold dominated on the surface of calcined catalysts and the amount of negative charge on metal particles was dependent on the chemical composition of the matrix and the active phase. Au-Ag alloy was identified on the surface of bimetallic catalysts after their treatment in argon and hydrogen flow. It was proved that zinc on the surface of MCM-36 promoted the deep alloying of noble metals in strong reducing (hydrogen flow) conditions with the formation of gold-silver alloy rich in silver. The species on the surface of the catalysts, dependent on the activation treatments, determined their activity and selectivity in methanol oxidation. The presence of zinc in the support (Zn/MCM-36) significantly improved the activity of Au-Zn/MCM-36 due to the effect of zinc on the electronic and structural properties of gold. In bimetallic AuAg-samples, the influence of gold-silver alloy on the increase in their activity was dependent on the content of silver in the alloy. Zinc on MCM-36 support increased the selectivity to formaldehyde, whereas the alloy rich in silver favoured the total oxidation of methanol because of the segregation of alloy during the reaction.