Palladium-Rhenium Catalysts for Selective Hydrogenation of Furfural: Evidence for an Optimum Surface Composition

Bimetallic catalysts comprising a platinum group metal (e.g., Pt, Pd, Ru) and rhenium (Re) have important applications in petroleum refining, industrial chemicals production, and biomass conversion. In this work, a series of PdRe/Al2O3 catalysts was investigated for selective hydrogenation of furfural to furfuryl alcohol (FAL) at 150 degrees C and 1 atm in a differential reactor. The results demonstrate that PdRe/Al2O3 catalysts have greater FAL selectivity and activity than Pd/Al2O3 catalysts. Over the bimetallic catalysts, decreased furan production is accompanied by a marked increase in hydrogenation activity. PdRe 1:1 catalysts prepared using [Pd(NH3)(4)](NO3)(2) are significantly more active than catalysts prepared using Pd(NO3)(2). PdRe 1:2 catalysts are more selective to FAL but less active than 1:1 catalysts prepared using the same precursors. The superior activity of PdRe/Al2O3 catalysts for selective hydrogenation of furfural is inferred to result from Re surface modification of Pd nanoparticles disrupting Pd ensembles and creating new highly active Pd-Re sites. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicates an enhanced linear to bridging ratio for CO chemisorbed on surface Pd atoms, supporting this hypothesis. The H/CO chemisorption ratio at 35 degrees C varies inversely with Re surface coverage and correlates with furfural turnover frequency (TOF) and FAL selectivity. Thus, the observed TOP maximum at H/CO approximate to 0.25 suggests an optimum surface composition of approximately 75% Re and 25% Pd. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) with energy-dispersive X-ray (EDX) analysis shows intimate contact of Re clusters with Pd nanoparticles in the most active catalysts.