Hydrogenation and decomposition kinetic study of H2O2 over Pd/C catalyst in an aqueous medium at high CO2 pressure

Hydrogenation and decomposition of H2O2 cause an undesired decrease in selectivity when producing H2O2 over Pd/C catalysts via direct synthesis. Hydrogenation accounts for more than 70–85% of the decrease in selectivity itself, which means that the rate of hydrogenation is 3–4-fold the decomposition rate. In this study we have studied the operational interval for H2O2 concentration (1–10%, w/v), temperature (23–50 °C), pH (2–3.8), halide to active metal ratio (1.5–8.0), catalyst amount and palladium loading (1, 3 and 5 wt% Pd/C) at 80 bar using CO2 as inert gas. A kinetic model coupling both decomposition and hydrogenation was fit to the results of a semicontinuous reactor achieving average deviations lower than 5% in most cases. Using the right proportion of promoters the activation energies obtained were Ead=18803.6   J   mol−1Ead=18803.6   J   mol−1 (decomposition) and Eah=7746.2   J   mol−1Eah=7746.2   J   mol−1 (hydrogenation). The order of reaction for decomposition respect to H2O2 was 1.031 and with hydrogenation −0.161. Turn over frequencies of decomposition and hydrogenation rates between 0.31 and 9.62 mol H2O2 h−1 mol Pd−1 and 1.39 to 23.01 mol H2O2 h−1 mol Pd−1 respectively have been observed in this study.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Kinetics of hydrogenation + decomposition of H2O2 in water at high pressures with CO2. ► Model explaining the effect of H2O2 concentration, acid, halide, temperature, pressure and catalyst loading. ► Hydrogenation is important at low H2O2 concentration (under 3 wt%) and decomposition at high H2O2 concentrations (over 3 wt%). ► Base for optimization of the direct synthesis of H2O2 using supercritical CO2 in water.