Mechanism for the Direct Synthesis of H₂O₂ on Pd Clusters: Heterolytic Reaction Pathways at the Liquid–Solid Interface

Abstract

Direct synthesis (H2 + O2 → H2O2) is a promising reaction for producing H2O2, which can replace chlorinated oxidants in industrial processes. The mechanism of this reaction and the reasons for the importance of seemingly unrelated factors (e.g., Pd cluster size and solvent pH) remain unclear despite significant research. We propose a mechanism for H2O2 formation on Pd clusters consistent with steady-state H2O2 and H2O formation rates measured as functions of reactant pressures and temperature and the interpretations of proton concentration effects. H2O2 forms by sequential proton-electron transfer to O2 and OOH surface intermediates, whereas H2O forms by O-O bond rupture within OOH surface species. Direct synthesis, therefore, does not proceed by the Langmuir-Hinshelwood mechanism often invoked. Rather, H2O2 forms by heterolytic reaction pathways resembling the two-electron oxygen reduction reaction (ORR); however, the chemical potential of H2 replaces an external electrical potential as the thermodynamic driving force. Activation enthalpies (ΔH(⧧)) for H2O formation increase by 14 kJ mol(-1) when Pd cluster diameters increase from 0.7 to 7 nm because changes in the electronic structure of Pd surface atoms decrease their propensity to cleave O-O bonds. ΔH(⧧) values for H2O2 remain nearly constant because barriers for proton-electron transfer depend weakly on the coordinative saturation of Pd surface atoms. Collectively, these results provide a self-consistent mechanism, which clarifies many studies in which H2O2 rates and selectivities were shown to depend on the concentration of acid/halide additives and Pd cluster size. These findings will guide the rational design of selective catalysts for direct synthesis.

Direct Synthesis of H2O2 on Pd Based Catalysts: Modelling the Particle Size Effects and the Promoting Role of Polyvinyl Alcohol

“…The differences, however, are relatively minor with respect to NR reference catalyst, while nearly twice of that of ID catalysts. The interpretation that PVA could assist the formation of H 2 O 2 , for example by increasing the local concertation of protons at the Pd surface and consequently favors the selective transformation with −OOH* or O 2 * surface species,, is consistent with the experimental observation that the selectivity and productivity initially increases after the removal of PVA layer and the fact that catalysts prepared by ID are worst (in terms of rate constant of direct synthesis) with respect to those prepared by NR method, even considering the different Pd particle size. Thus likely the presence of Br − ions in the tests of the ID catalyst series induces a partial blocking of some of the sites for H 2 O 2 synthesis and thus not only inhibit side reactions as reported in literature .…”

Recent Advances in the Direct Synthesis of H2O2

“…Recently Wilson and Flaherty have completed a comprehensive mechanistic study of the direct synthesis of H 2 O 2 over a supported Pd and have demostrated that the presence of protons are key for H 2 O 2 synthesis, with H 2 O 2 yields in protic solvents such as H 2 O and CH 3 OH much greater than in aprotic solvents, such as acetonitrile (Figure ). In the same work they report that the presence of protons, from mineral aicds can aid in the the reduction of molecular O 2 , a key step in H 2 O 2 synthesis and make compelling arguments for the importance of the counterions, such as SO 4 2− and Cl − in determining selectivitiy towards H 2 O 2 .…”

Influence of Metal Precursors and Reduction Protocols on the Chloride‐Free Preparation of Catalysts for the Direct Synthesis of Hydrogen Peroxide without Selectivity Enhancers

“…The fundamental aspects of the CDS process have been reviewed recently, but many issues that concern the reaction mechanism still remain unclear because of the complexity of the three‐phase system and the reaction network (Scheme ). Metallic Pd is the most active catalyst for this reaction and is applied generally in a supported form.…”