Theoretical assessments of Pd–PdO phase transformation and its impacts on H2O2 synthesis and decomposition pathways

Abstract

The direct synthesis of H₂O₂ from O₂ and H₂ provides a green pathway to produce H₂O₂, a popular industrial oxidant. Here, we theoretically investigate the effects of Pd oxidation states, coordination environments, and particle sizes on primary H₂O₂ selectivities, assessed by calculating the ratio of rate constants for the formation of H₂O₂ (via OOH* reduction; k_O–H) and the decomposition of OOH* (via O–O cleavage; k_O–O). For Pd metals, the k_O–H/k_O–O ratio decreased from 10⁻⁴ for Pd(111) to 10⁻¹⁰ for the Pd₁₃ cluster at 300 K, indicating poorer H₂O₂ selectivity as Pd particle size decreases and low primary selectivities for H₂O₂ overall. As the oxygen chemical potential increases and metals form surface and bulk oxides, the perturbation of Pd–Pd ensemble sites by lattice O atoms results in selectivities that become dramatically higher than unity. For instance, at 300 K, the k_O–H/k_O–O ratio increases significantly from 10⁻⁴ to 10⁹ to 10¹⁶ as Pd(111) oxidizes to Pd₅O₄/Pd(111) and to PdO(100), respectively. In contrast, such selectivity enhancements are not observed for surface and bulk oxides that persistently contain rows of more metallic, undercoordinated Pd–Pd ensemble sites, such as PdO(101)/Pd(100) and PdO(101). These Pd–Pd ensembles are also absent when smaller Pd nanoparticles fully oxidize, indicating that smaller PdO clusters can be more selective for H₂O₂ synthesis. These trends for primary H₂O₂ selectivities were found to inversely correlate with trends for H₂O₂ decomposition rates via O–O bond cleavage, demonstrating that catalysts with high primary H₂O₂ selectivity can also hinder H₂O₂ decomposition. Ab initio thermodynamic calculations are used to estimate the thermodynamically favored phase among Pd, PdO/Pd and PdO in O₂, H₂O₂/H₂O, and O₂/H₂ environments. These results are combined to show that smaller Pd nanoparticles are more prone to be oxidized at lower oxygen chemical potentials, upon which they become more selective than larger Pd particles for H₂O₂ synthesis.