Significant Roles of Carbon Pore and Surface Structure in AuPd/C Catalyst for Achieving High Chemoselectivity in Direct Hydrogen Peroxide Synthesis

Direct synthesis of hydrogen peroxide (H₂O₂) from hydrogen (H₂) and oxygen (O₂) has been widely investigated as an attractive way for small-scale/on-site H₂O₂ production. Among various catalysts, carbon-supported AuPd catalysts have been reported to exhibit the most promising H₂O₂ productivity and selectivity. In this work, to better understand the catalytic role of the surface properties and porous structures of the carbon supports, we systematically investigated AuPd catalysts supported on various nanostructured carbons including activated carbon, carbon nanotube, carbon black, and ordered mesoporous carbons. The results showed that a high density of oxygen functional groups on the carbon surface was essential for synthesizing highly dispersed bimetallic catalysts with effective AuPd alloying, which is a prerequisite for achieving high H₂O₂ selectivity. Regarding porous structure, a solely mesoporous carbon support was superior to microporous ones. Microporous carbons such as activated carbon suffered from diffusion limitation, leading to significantly slower H₂ conversion than mesoporous catalysts. Furthermore, H₂O₂ produced from AuPd catalyst in the micropores was more prone to subsequent disproportionation/hydrogenation into H₂O due to retarded diffusion of the H₂O₂ out of the microporous structure, which led to decreased H₂O₂ selectivity. The present study showed that solely mesoporous carbons with high surface oxygen content are most desirable as a support for AuPd catalyst in order to achieve high H₂O₂ productivity and selectivity

Cross-Linked “Poisonous” Polymer: Thermochemically Stable Catalyst Support for Tuning Chemoselectivity

Designed catalyst poisons can be deliberately added in various reactions for tuning chemoselectivity. In general, the poisons are “transient” selectivity modifiers that are readily leached out during reactions and thus should be continuously fed to maintain the selectivity. In this work, we supported Pd catalysts on a thermochemically stable cross-linked polymer containing diphenyl sulfide linkages, which can simultaneously act as a catalyst support and a “permanent” selectivity modifier. The entire surfaces of the Pd clusters were ligated (or poisoned) by sulfide groups of the polymer support. The sulfide groups capping the Pd surface behaved like a “molecular gate” that enabled exceptionally discriminative adsorption of alkynes over alkenes. H₂/D₂ isotope exchange revealed that the capped Pd surface alone is inactive for H₂ (or D₂) dissociation, but in the presence of coflowing acetylene (alkyne), it becomes active for H₂ dissociation as well as acetylene hydrogenation. The results indicated that acetylene adsorbs on the Pd surface and enables cooperative adsorption of H₂. In contrast, ethylene (alkene) did not facilitate H₂–D₂ exchange, and hydrogenation of ethylene was not observed. The results indicated that alkynes can induce decapping of the sulfide groups from the Pd surface, while alkenes with weaker adsorption strength cannot. The discriminative adsorption of alkynes over alkenes led to highly chemoselective hydrogenation of various alkynes to alkenes with minimal overhydrogenation and the conversion of side functional groups. The catalytic functions can be retained over a long reaction period due to the high thermochemical stability of the polymer

Hydrogen Peroxide Synthesis via Enhanced Two-Electron Oxygen Reduction Pathway on Carbon-Coated Pt Surface

Continuous on-site electrochemical production of hydrogen peroxide (H₂O₂) can provide an attractive alternative to the present anthraquinone-based H₂O₂ production technology. A major challenge in the electrocatalyst design for H₂O₂ production is that O₂ adsorption on the Pt surface thermodynamically favors “side-on” configuration over “end-on” configuration, which leads to a dissociation of O–O bond via dominant 4-electron pathway. This prefers H₂O production rather than H₂O₂ production during the electrochemical oxygen reduction reaction (ORR). In the present work, we demonstrate that controlled coating of Pt catalysts with amorphous carbon layers can induce selective end-on adsorption of O₂ on the Pt surface by eliminating accessible Pt ensemble sites, which allows significantly enhanced H₂O₂ production selectivity in the ORR. Experimental results and theoretical modeling reveal that 4-electron pathway is strongly suppressed in the course of ORR due to a thermodynamically unfavored end-on adsorption of O₂ (the first electron transfer step) with 0.54 V overpotential. As a result, the carbon-coated Pt catalysts show an onset potential of ∼0.7 V for ORR and remarkably enhanced H₂O₂ selectivity up to 41%. Notably, the produced H₂O₂ cannot access the Pt surface due to the steric hindrance of the coated carbon layers, and thus no significant H₂O₂ decomposition via disproportionation/reduction reactions is observed. Furthermore, the catalyst shows superior stability without considerable performance degradation because the amorphous carbon layers protect Pt catalysts against the leaching and ripening in acidic operating conditions