Impacts of Pd–PdO phase transformation and Pd/PdO nanoparticle sizes on H2O2 synthesis and decomposition pathways

Pd is used for the direct synthesis of H₂O₂ from H₂ and O₂, but easily decomposes H₂O₂ (to H₂O and O₂) resulting in low H₂O₂ yields. In this work, we implement molecular dynamic simulations and density functional theory (DFT) calculations to assess the entangled effects of Pd-O coordination and Pd/PdO nanoparticle sizes on primary H₂O₂ selectivities and H₂O₂ decomposition reactivities. Primary H₂O₂ selectivities are estimated by assessing the formation of H₂O₂ via O-H formation from OOH* (k_O-H) or the decomposition of OOH* via O-O cleavage (k_O-O). The k_O-H/k_O-O ratio is estimated from DFT-derived free energy barriers on metallic Pd, surface oxide, and bulk oxide models of varying particle sizes and exposed facets. These results show that k_O-H/k_O-O ratios are smaller than unity for all metallic Pd models, indicating poor primary H₂O₂ selectivities; these ratios are much smaller for Pd₁₃ than for Pd(111), suggesting that larger Pd particles are more selective for H₂O₂. As the oxygen chemical potential increases and Pd-Pd ensemble sites are perturbed by O atoms, their selectivities dramatically change. For example, the k_O-H/k_O-O ratio increases from 10^-4 to 10⁹ to 10¹⁶ as Pd(111) oxidizes to Pd₅O₄/Pd(111) and PdO(100). This selectivity improvement is minimal for surfaces that persistently contain Pd-Pd ensembles, such as PdO(101)/Pd(100) and PdO(101). Smaller PdO nanoparticles, fully saturated with O atoms, resulted in the highest primary H₂O₂ selectivity. DFT-derived barriers also show that the catalysts with higher primary H₂O₂ selectivity are less prone to H₂O₂ decomposition, leading to high H₂O₂ yields. Ab initio thermodynamic calculations reveal that smaller Pd nanoparticles are more likely to transform to PdO during H₂/O₂ reactions and H₂O₂ decomposition than their larger counterparts. This study highlights how Pd transformation to PdO impacts H₂O₂ synthesis and decomposition rates and selectivities, which are highly sensitive to Pd-O coordination environments and particle sizes.