Quantitative Insights into the Reaction Mechanism for the Direct Synthesis of H2O2over Transition Metals: Coverage-Dependent Microkinetic Modeling

The direct synthesis is the most promising alternative method for the production of hydrogen peroxide, and the bottleneck is still unsolved. The breakthrough lies in elusive reaction mechanism issues. In this work, advanced coverage-dependent kinetic modeling is combined with the energetics from first-principles calculations to investigate the formation of H2O2 over transition metals. We show that the adsorbate–adsorbate interactions considerably affect the reaction mechanism of synthesis of hydrogen peroxide on Pd(111). Without the coverage effect, O2 is likely to go through the direct dissociation mechanism, and water is the major product. When the coverage effects are included, the dissociations of O–O and O–OH bonds are significantly inhibited, and on the contrary, the hydrogenations of O2 and OOH are promoted, leading to the production of H2O2. We demonstrate that the reaction temperature induces strong variations in the coverage of intermediates, which in turn causes changes in product selectivity. Being consistent with the operando experiment, our kinetic simulations indicate that the H2/O2 partial pressure ratio has great effects on H2O2 selectivity and the reaction rate of H2O2 is lower under hydrogen-rich (oxygen-lean) and oxygen-rich (hydrogen-lean) conditions, which is highly related to the intermediate coverage. The same approach is also applied to other important relevant metals, i.e., Cu(111), Au(111), PdAu, and PdHg alloys, and the trends of activity and selectivity have been obtained.