The direct synthesis of H2O2 (DSHP) from H2 and O2 offers a “greener”, single-pot alternative to the current industrial-scale anthraquinone process for H2O2 production. An optimal DSHP catalyst must maintain high rates and selectivity at industrially relevant H2O2 concentrations. However, catalysts active for the DSHP generally also show activity for H2O2 decomposition reactions, limiting H2O2 yields.1 This problem is avoided in the anthraquinone process, as the hydrogenation and oxidation steps are separated – avoiding contact of H2O2 produced with the hydrogenation catalyst.
Pd is the most widely studied catalyst for the DSHP, and one strategy to quench the undesired H2O2 decomposition reactions on Pd is to add acids and halides to the reaction medium.2 Some proposed actions of the halide include blockage of coordinatively unsaturated Pd sites3 and/or prevention of O-O bond cleavage.1 We utilize density functional theory (DFT) calculations to further probe the role of halides on the H2O2 decomposition reaction mechanism on Pd. This presentation will focus on the stability of reaction intermediates and energetics of select elementary steps in the presence of adsorbed halides. The results are discussed in the context of the experimentally observed trends for halide inhibition of H2O2 decomposition on Pd.4
1. Samanta C. Direct synthesis of hydrogen peroxide from hydrogen and oxygen: An overview of recent developments in the process. Applied Catalysis a-General. Nov 30 2008;350(2):133-149.
2. Edwards JK, Freakley SJ, Lewis RJ, Pritchard JC, Hutchings GJ. Advances in the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Catalysis Today. 6/15/ 2015;248(0):3-9.
3. Deguchi T, Iwamoto M. Catalytic Properties of Surface Sites on Pd Clusters for Direct H2O2 Synthesis from H2 and O2: A DFT Study. Journal of Physical Chemistry C. Sep 12 2013;117(36):18540-18548.
4. Choudhary VR, Samanta C. Role of chloride or bromide anions and protons for promoting the selective oxidation of H2 by O2 to H2O2 over supported Pd catalysts in an aqueous medium. Journal of Catalysis. Feb 15 2006;238(1):28-38.