Direct Synthesis of H₂O₂ from H₂ and O₂ on Pd Catalysts: Current Understanding, Outstanding Questions, and Research Needs

Successful implementation of the direct synthesis of H₂O₂ (and the resultant inexpensive H₂O₂) would decrease the environmental impact of industrial oxidation reactions and could enable a number of ground-breaking processes (e.g., selective oxidations of alkanes to alcohols). The greatest barriers that stand in the way of implementation stem from outstanding scientific challenges. Namely, the combinations of solvents and promoters that provide high selectivities to H₂O₂ on Pd catalysts (and Pd-based bimetallic catalysts) also decrease the stability of those catalysts by increasing rates of Pd dissolution and leaching. Current understanding of this presumably simple reaction between small molecules (H₂ and O₂) on nanoparticle surfaces is surprisingly limited. Consequently, the pathway to develop new combinations of solvents and promoters that may provide both selective and stable catalysts is uncertain. Further progress in understanding this deceptively intricate chemistry will require rigorous studies that combine kinetic, spectroscopic, and computational methods and which develop quantitative descriptions of how the many components of the system influence intrinsic kinetic parameters and reaction mechanisms. To do so, researchers will need to embrace the complexity of this reaction and consider that Pd nanoparticles are only one part of the complete catalytic system that produces H₂O₂ selectively.