Unraveling hydrogen-induced reconstruction of Pd catalysts and their impact on the anthraquinone hydrogenation mechanism: a combined PSO–DFT study

Abstract

Anthraquinone (AQ) hydrogenation, a critical industrial step for hydrogen peroxide production, is catalyzed by γ-Al₂O₃-supported Pd catalysts. However, the reaction mechanism remains poorly understood due to unresolved structural evolution of the active Pd phase under hydrogenation conditions. Thus, hydrogenated Pd surface/cluster structures and their catalytic impact on AQ hydrogenation are elucidated by particle swarm optimization (PSO) and density functional theory (DFT) in this study. Under industrial conditions, β-PdH_0.5 and Pd₉H₄ are identified as thermodynamically stable phases for Pd(111) surfaces and γ-Al₂O₃-supported clusters, respectively. Hydrogenation induces subsurface H penetration and lattice distortion at high coverage. Electronic structure analysis reveals d-band center downshifting on hydrogenated Pd(111) weakens adsorbate bonding, while supported Pd₉H₄ enhances AQ adsorption. Reaction pathway studies demonstrate that clean Pd(111) favors aromatic ring hydrogenation, yielding dihydroanthraquinone (H₂AQ). In contrast, hydrogenated Pd(111) achieves a lower energy barrier at the rate-determining step and higher selectivity for target anthrahydroquinone (AH₂Q) via carbonyl oxygen hydrogenation. This high selectivity is attributed to steric effects that suppress side reactions. Pd₉H₄ clusters promote undesired OAN formation due to restricted H diffusion. Large particles (>2 nm), represented by hydrogenated Pd(111), enable efficient AH₂Q production but exhibit low atom utilization, while small clusters (<1 nm), represented by the Pd₉H₄ cluster, suffer from low activity and poor selectivity. Particle size and hydrogenated surface structure are identified as critical factors for optimizing Pd-based catalysts, enhancing activity and selectivity in industrial anthraquinone hydrogenation processes.