Gold is active in various hydrogenation reactions and often shows exceptional high chemoselectivity when multiple functional groups are present. All hydrogenation reactions have in common, that the hydrogen molecules have to adsorb and dissociate before they react. Recent insights in the interaction of gold with hydrogen and simple reactions involving hydrogen are described.
Controlling the particle size of supported metal catalysts is essential to achieving active, selective, and stable catalysts. High-resolution electron microscopy has revealed that large Au particles (∼8 nm), supported on titania and if multiple-twinned, broke up into small particles (2−3 nm) during liquid-phase hydrogenation. Multiple-twinned Au particles, obtained after gas-phase reduction of as-prepared Au/TiO 2 at elevated temperature, break up into smaller particles. EXAFS confirmed that the multiple-twinned particles were agglomerates of smaller particles. In contrast, calcination caused a strong interaction between Au and titania and the formation of mainly single crystalline Au particles (4 to 5 nm), which were relatively stable during liquid-phase hydrogenation. Reduction and calcination lead to gold particles that are fundamentally different.
An autoclave reactor was modified to perform simultaneously high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy measurements without altering the reactor design. This operando cell allows one to follow changes in the electronic and geometric structure of the catalyst with HERFD XAS and relate them to the simultaneously detected activity and reaction species with ATR-FTIR formed during the reaction in the liquid phase. The capability of the cell is demonstrated by two studies. The first shows the reduction of Au/CeO(2) precursors in different solvents. The second shows that mainly Au(0) is present in the hydrogenation of nitrobenzene over Au/CeO(2).
With regard to heterogeneous catalysis by gold and, in particular, hydrogenation reactions, there is no consensus on the oxidation state of the catalytically active species. By means of high‐resolution X‐ray absorption spectroscopy, we determined in situ the oxidation state of gold in the functioning catalyst Au/CeO2 in the slurry phase of the hydrogenation of nitrobenzene at high pressure. The conversion of nitrobenzene was monitored simultaneously by means of online attenuated total reflectance–Fourier transform infrared spectroscopy. We found that catalysts without measurable amounts of cationic gold were more active than catalysts with cationic gold. Any cationic gold that remained after pretreatment was reduced under reaction conditions, without any loss of activity. No evidence was found that the cationic gold contributed to the catalytic activity.
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