Phosphate modification of Pd/Al2O3 enhances activity and stability in aromatic hydrogenation under CO-contaminated hydrogen

Abstract

Hydrogenation reactions are essential to synthesize platform and fine chemicals today and to establish chemical hydrogen storage in the future. However, hydrogen from fossil or biogenic sources contains CO, a potent poison for noble metal hydrogenation catalysts, necessitating costly purification steps. In this work, we demonstrate phosphate modification as an effective strategy to enhance activity and CO tolerance of Pd/Al₂O₃ in benzyltoluene hydrogenation using pure and impure H₂ streams. Under 1.6 vol% CO in H₂, phosphate modified catalysts achieve a 230% increase in productivity over unmodified Pd/Al₂O₃. Characterization reveals that highly dispersed monomeric phosphate species on Al₂O₃ enhance metal–support interaction and induce Pd redispersion, forming smaller, more stable Pd nanoparticles with enhanced resistance against sintering. Notably, the local electronic environment of Pd remains unchanged by phosphate species. We further show that under CO-rich conditions, benzyltoluene is preferentially hydrogenated at Pd edge sites rather than terrace sites, which explains the pronounced activity increase of the smaller Pd nanoparticles. Phosphate-induced acidity provides additional sites for aromatic hydrogenation with spilled-over hydrogen that remain active in the presence of CO.