H2O2 is less environmentally impactful than many industrial oxidants such as Cl2. The auto-oxidation of anthraquinones is the current standard for industrial H2O2 production, however, this process requires significant energy input due to the extensive purification and concentration processes involved, making H2O2 cost-prohibitive for many oxidation processes. As a result, there is clear motivation for less expensive and energy-demanding chemistries for H2O2 production, such as the direct synthesis of H2O2 (H2 + O2 → H2O2), the most promising alternative to using anthraquinones. Unfortunately, the combustion of H2 is thermodynamically favored over direct synthesis to H2O2 on most transition metal catalysts, and so significant research has been directed towards improving catalyst selectivity towards H2O2 through various methods. Despite significant research, the mechanism of this reaction, and the reasons for the importance of seemingly unrelated factors (e.g., metal cluster size, solvent pH, alloying), have remained unclear. The aim of this work is to provide a fundamental understanding of these factors through rigorous experimental procedures and analysis. Here, 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. Similar experimental procedures have shown that this proton-electron transfer mechanism is the same also on AuPd and PdZn catalysts.Among AuPd and PdZn catalysts, increases in the Au:Pd or Zn:Pd ratio leads to simultaneous but unequal increases in the activation enthalpies (∆H‡) for both H2O2 and H2O formation, which must result from significant electronic changes to Pd by Au or Zn. Detailed comparisons of these changes in ∆H‡ for H2O2 and H2O production to H2O2 selectivities provide compelling evidence that these electronic effects are primarily responsible for the high H2O2 selectivities commonly reported on bimetallic catalysts. Additionally, these results lack trends that suggest ensemble effects contribute significantly to the increase in H2O2 selectivity observed on bimetallic catalysts within the ranges of metal compositions tested here.AgPt octahedra with dilute concentrations of surface Pt atoms were synthesized to test for the presence of ensemble effects which presumably do not manifest until the active sites (i.e., Pt) are sufficiently diluted with a metal which weakly binds O2 (i.e., Ag). Combining Pt with Ag increases H2O2 selectivity and significantly modifies the electronic structure of Pt active sites, which is reflected by a red shift in the ν(C=O) singleton frequency in 13CO, which is accompanied by a significant decrease in ∆H‡ values for H2O2 formation and a smaller decrease for H2O formation. These combined results show that adding Ag to Pt increases H2O2 selectivity by a combination of electronic modification of Pt atoms (likely due to tensile strain effects) and a commensurate increase in the number of isolated Pt atoms that lack sufficiently large numbers of Pt atoms to cleave O-O bonds to form H2O. Collectively, the results of these studies clarify many previously reported phenomena and help to guide the rational design of selective catalysts for the direct synthesis of H2O2.
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Mechanistic insights into the direct synthesis of H2O2 on transition metal catalysts
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