Identifying the activity origin of silver catalysts induced by interfacial electron localization for regioselective C[sbnd]O bond hydrogenation

Abstract

Deciphering intrinsic reactive sites of metal–oxide catalysts, where the outer surface and/or interfaces coupled with oxygen species in the vicinity of nanoparticles concurrently work, remains a challenge. Given the extreme complexity of heterogeneous catalysis (e.g., metal catalysts for preliminary hydrogenation process), catalytically active sites, the activity origin, and reaction mechanism generally present a so-called 'black box'. Herein, Ag/ZrO2−x achieves a higher intrinsic hydrogenation rate for selective scission of CO bond per mass Ag than the reference Ag/SiO2 catalyst, as illustrated by the preliminary hydrogenation of dimethyl oxalate to methyl glycolate. The in-depth characterization revealed that the Agδ+–Ov–Zr3+ (Agδ+–O–Zr4+, Ov refers to as oxygen vacancy) structure could be engineered owing to the higher affinity of Ag for the partially reduced ZrO2 surface and heteroatom junctions (in preference to the inert support, such as SiO2 and Al2O3). Furthermore, the local environment of different crystallite phases (tetragonal or monoclinic ZrO2) induced tunable (electronic) metal–support interactions (EMSIs), resulting in enhanced catalytic activity and ultra-stability for CO bond hydrogenation. XPS, methyl acetate-TPD, and in situ DRIFTS results unveiled the activity origin of silver–oxide systems, where the location of active metal sites for distinct functionalities over superficial and interfacial phases was validated. The mediated Ov was verified to stabilize the thermodynamically unstable metallic (Ag0) and charged silver species (Ag+ or Agnδ+). More importantly, time-resolved DRIFTS confirmed the crucial interfacial sites of Ag–ZrO2−x for regioselective CO bond adsorption and activation. Further, the distinct active sites over Ag–ZrO2−x may release the altered reaction mechanisms, i.e., conventional Langmuir–Hinshelwood (LH) on Ag/SiO2 versus quasi Mars–Van Krevelen process (Ag–ZrO2−x), which presents the new pathway for regioselective CO bond hydrogenation. The coordinated EMSIs were proposed as the origin of the catalytic activity caused by the synergistic catalysis of the interfacial/superficial sites. These findings rationalize the underlying understanding of the EMSI and highlight the instructive role for perceiving the structure-performance relationships and the activity origin in heterogeneous catalysts, thus moving away from the 'black-box' cognition.