Effect of FeOx Modification of Al2O3 on Its Supported Au Catalyst for Hydrogenation of 5-Hydroxymethylfurfural

To enhance the hydrogenation activity of alumina supported Au (Au/Al 2 O 3 ) catalyst for selective hydrogenation of 5-hydroxymethylfurfural (HMF), an important biomass-derived aldehyde, Al 2 O 3 support was modified with iron oxide (FeO x ). The apparent catalytic activity of the FeO x /Al 2 O 3 supported Au (Au/FeO x /Al 2 O 3 ) catalysts increased as an increase of Fe loading up to 10 wt % (3–4 times higher than Au/Al 2 O 3 ). When the Fe loading was more than 10 wt %, the apparent catalytic activity decreased, and Au/α-Fe 2 O 3 showed much lower activity than Au/Al 2 O 3 . The Fe K-edge X-ray absorption fine structure (XAFS) spectra and the atomic scale observation using aberration corrected scanning transmission electron microscopy (Cs-STEM) suggested positive and negative effects of FeO x support. As the positive effect, FeO x promotes formation of Au clusters by reduction of Au single atoms having low or no catalytic activity for the hydrogenation. On the other hand, as the negative effect, FeO x, more specifically, large α-Fe 2 O 3, can form buried structure of Au clusters in FeO x due to reduction of α-Fe 2 O 3 to Fe metal by H 2 pretreatment. The buried structure of Au clusters is responsible for the decrease of hydrogenation activity at high Fe loading. In contrast, dispersed Fe 2 O 3 species of Au/FeO x /Al 2 O 3 with less than 20 wt % of Fe loading was not subjected to the reduction of Fe 2 O 3 species to Fe metal, and the Au clusters were exposed on the support surface. Therefore, the dispersed Fe 2 O 3 on Al 2 O 3 is effective to enhance the activity of Au catalysts for the hydrogenation because of the formation of exposed small Au clusters.