Active sites and mechanisms for H2O2 decomposition over Pd catalysts
Significance The use of hydrogen peroxide (H2O2) for catalytic oxidations is limited by the energy intensive and wasteful process by which H2O2 is currently produced the anthraquinone process. The direct synthesis of H2O2 (DSHP) is a promising alternative process, yet catalysts active for this reaction (Pd being the most widely studied) are generally hindered by subsequent H2O2 decomposition. Through a combined theoretical and experimental approach, our work (i) provides an understanding of the nature of Pd active sites responsible for H2O2 decomposition and (ii) identifies a single type of elementary step that controls the rate. These structural and mechanistic insights are important for designing improved DSHP catalysts and for developing transition metal catalyzed oxidations that efficiently use H2O2. A combination of periodic, self consistent density functional theory (DFT GGA PW91) calculations, reaction kinetics experiments on a SiO2 supported Pd catalyst, and mean field microkinetic modeling are used to probe key aspects of H2O2 decomposition on Pd in the absence of cofeeding H2. We conclude that both Pd(111) and OH partially covered Pd(100) surfaces represent the nature of the active site for H2O2 decomposition on the supported Pd catalyst reasonably well. Furthermore, all reaction flux in the closed catalytic cycle is predicted to flow through an O O bond scission step in either H2O2 or OOH, followed by rapid H transfer steps to produce the H2O and O2 products. The barrier for O O bond scission is sensitive to Pd surface structure and is concluded to be the central parameter governing H2O2 decomposition activity.
Proceedings of the National Academy of Sciences