Effect of support redox character on catalytic performance in the gas phase hydrogenation of benzaldehyde and nitrobenzene over supported gold

A range of oxides (γ-Al 2 O 3, TiO 2, ZrO 2, CeO 2, α-Fe 2 O 3 and Fe 3 O 4 ) with different redox properties were used to support nano-scale (mean = 2–8 nm) Au and employed in the gas phase hydrogenation of benzaldehyde and nitrobenzene. The catalysts were subjected to TPR, H 2 /O 2 titration, H 2 TPD, XRD, TEM/STEM and XPS analysis. The supported Au phase promoted partial reduction of the reducible supports through the action of spillover hydrogen (based on TPD), which generated surface oxygen vacancies (demonstrated by O 2 titration) that inhibit Au particle sintering during catalyst activation. Electron transfer to generate charged Au species (determined by XPS) correlates with support ionisation potential. Higher nitrobenzene hydrogenation (to aniline) TOF s were recorded relative to benzaldehyde where rate increased with decreasing Au size (from 8 to 4 nm) with measurably lower TOF over Au <3 nm. Strong binding of CH O and NO 2 functions to oxygen vacancies resulted in lower hydrogenation rates. Higher temperatures (>413 K) promoted benzaldehyde hydrogenolysis to toluene and benzene. The formation of Au δ− on non-reducible Al 2 O 3 favoured selective reduction of CH O with full selectivity to benzyl alcohol at 413 K.