Surface nature of nanoparticle gold/iron oxide aerogel catalysts

Abstract

The aerogel catalysts investigated are constituted by two chemically different nanoparticle systems consisting of the gold phase and the iron oxide support. High-resolution transmission electronic microscopy (HRTEM) showed an increased surface roughness for aerogel particles with higher gold loading. X-ray photoelectron spectroscopy (XPS) revealed increases in the surface coverage of hydroxyl groups and the Fe 2+/Fe 3+ ratio due to the addition of gold, and showed the transition of gold from oxidized to metallic states due to calcination. In the presence of gold species, the Fe 3+ satellite structure in XPS was not produced. The crystallinity of maghemite as the support was found quite stable with respect to gold addition and thermal treatment. The aerogels were evaluated for methanol oxidation carried out in an ambient flow reactor. The oxidation activity enhanced with decreasing catalyst pretreatment temperatures and with increasing gold loadings up to 5 wt%. A wide selectivity pattern formed between dimethyl ether and carbon dioxide products. The size of gold particles and the status of surface gold species played a crucial role in the catalytic conversion of methanol. The oxidized gold was more active than the metallic gold towards the total combustion to carbon dioxide. The surface nature has been proven to transform from strong Lewis acidic to high basic characters due to the formation of reactive hydroxyl groups near by the gold sites.