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-AlR2ROR3R, TiOR2R, ZrOR2R, CeOR2R, α-FeR2ROR3R and FeR3ROR4R) with differentredox properties were used to support nano-scale (mean = 2-8 nm) Au and employed in thegas phase hydrogenation of benzaldehyde and nitrobenzene. The catalysts were subjected toTPR, HR2R/OR2R titration, HR2R TPD, XRD, TEM/STEM and XPS analysis. The supported Auphase promoted partial reduction of the reducible supports through the action of spilloverhydrogen (based on TPD), which generated surface oxygen vacancies (demonstrated by OR2 Rtitration) that inhibit Au particle sintering during catalyst activation. Electron transfer togenerate charged Au species (determined by XPS) correlates with support ionisationpotential. Higher nitrobenzene hydrogenation (to aniline) TOFs were recorded relative tobenzaldehyde where rate increased with decreasing Au size (from 8 to 4 nm) with measurablylower TOF over Au <3 nm. Strong binding of –CH=O and –NOR2R functions to oxygenvacancies resulted in lower hydrogenation rates. Higher temperatures (>413 K) promotedbenzaldehyde hydrogenolysis to toluene and benzene. The formation of AuPδ-P on nonreducibleAlR2ROR3R favoured selective reduction of –CH=O with full selectivity to benzylalcohol at 413 K.