Understanding the Promotion Effect of Lanthanum Oxide on Gold-Based Catalysts in the Partial Oxidation of Methanol by in Situ XAFS and DSC Studies

Abstract

Promoting supported gold nanoparticles with lanthanum oxide largely increases the hydrogen selectivity in the partial oxidation of methanol. In this study, the origin of the promotion effect of lanthanum oxide on supported gold catalysts was investigated. The formation of small gold nanoparticles on both the high surface area alumina and the low surface area lanthanum oxide support materials was confirmed by Transmission Electron Microscopy (TEM) and Extended X-ray Absorption Fine Structure (EXAFS). In situ X-ray absorption spectroscopy during partial methanol oxidation revealed the formation of oxidized gold species on the reduced Au/Al2O3 catalyst material, whereas La2O3 was found to facilitate the reduction of initially present oxidic gold species. This was confirmed by a larger measured heat of reaction for the exothermic decomposition of the oxidic gold species supported on lanthanum oxide as was found by differential scanning calorimetry. The catalysts under study did not show significant differences inmethanol oxidation activity; however, the Au/Al2O3 catalyst exhibited much higher activity in CO and H2 oxidation. These observed differences in catalytic activity and selectivity of the Al2O3- andLa2O3-supported Au catalysts are explained by the differences in redox behavior of the gold nanoparticles. It is proposed that zerovalent gold species limit dissociative H2 adsorption during the partial oxidation of methanol and thus improve the H2 selectivity by reducing CH4 and H2O formation.