Direct H2O2 synthesis over dilute PdAu alloys

Licentiate thesis, 2024

Heterogeneous catalysis is crucial in a range of technological and industrial processes. Depending on the composition of the catalyst and the reaction conditions, it is possible to steer the activity and the selectivity towards the desired products. However, finding a proper catalyst for a specific reaction is difficult, and several reactions still lack efficient catalysts. To aid the search for new catalysts, atomic scale understanding is desirable. In this thesis, kinetic Monte Carlo simulations are used for atomistic simulations of reaction kinetics.

The direct formation of H2O2 from H2 and O2 over single atom alloy (Pd@Au) nanoparticles, in an aqueous solution, is investigated and compared to the reaction on extended surfaces. The influence of the metal-water interface is studied using ab initio molecular dynamics simulations. To investigate the durability of the dilute alloy particles, kinetic Monte Carlo simulations are employed to simulate the activation and deactivation of the catalyst.

Pd monomers are found to act as active centers for H2 dissociation, whereas the formation of H2O2 occurs on Au atoms located at the edges and corners of the nanoparticle. The kinetic coupling between Pd and Au sites is crucial to maintain a high selectivity towards H2O2. Hydrogen adsorbed on the surface is found to undergo a charge separation, where a proton desorbs to the water solution, whereas the electron is trapped in the metal. The simulations reveal that the process is facile at room temperature over a range of metal (111) surfaces, thus, providing reaction pathways that drastically differ from conventional surface reactions. Kinetic Monte Carlo simulations show that the dilute alloy particles are deactivated at elevated temperatures, in the absence of adsorbates. The deactivation depends both on the positions of the Pd monomers, and the global structure of the system.