(90c) Direct Synthesis of H2O2 from H2/O2 Mixtures and Its Decomposition over Intermetallic Pd-Zn Catalysts

Understanding the mechanism of H₂O₂ decomposition provides insight into the design of catalysts for the direct synthesis of H₂O₂ from H₂ and O₂, a promising and environmental-friendly reaction to replace the current the anthraquinone autoxidation (Riedl-Pfleiderer) process. Bimetallic catalysts developed by Hutchings and co-workers appear to be the state-of-the-art catalysts for direct synthesis, but these materials still demonstrate low apparent selectivity. The observed apparent selectivity are most likely influenced by decomposition reactions of hydrogen peroxide. Alkanol solvents or additives (mineral acids) increase H₂O₂ selectivity, but the impact of solvent, additives, and catalyst modification have been predominantly examined for apparent H₂O₂ selectivity. In this talk, we will focus on the impact of these variables on H₂O₂ decomposition and to a lesser extent on direct synthesis over phase-pure beta- and gamma-phase Pd-Zn nanoparticles supported on SiO₂. The addition of Zn to Pd leads to increased rates of H₂O₂ decomposition even though supported Zn (or ZnSiO₄) is completely inactive for this reaction. The turnover frequency based on the number of exposed Pd atoms was 2-10 times greater on the gamma-brass Pd-Zn/SiO₂ at low temperature. Both monometallic Pd and Pd-Zn demonstrated Langmuir-Hinshelwood saturation kinetics, but the addition of Zn to Pd led to a reduction in the apparent activation energy for H₂O₂ decomposition. An interesting observation was the kinetically relevant role that solvent (water) played during decomposition of H₂O₂. The k_H/k_D ratio was far greater than a primary effect and differed between the monometallic and bimetallic catalysts. The impact of intermetallic stoichiometry and solvent was additionally examined for the direct synthesis of H₂O₂.