(679e) Theoretical Assessments of Pd-Pdo Phase Transformation and Its Impacts on H2O2 Synthesis and Decomposition Pathways

Direct H₂O₂ synthesis from H₂ and O₂ provides a green route to produce H₂O₂. Pd has been widely used for this process, although its commercialization is limited due to low H₂O₂ yields.¹ This study aims to systemically assess the impacts of Pd-O coordination and particle sizes on primary H₂O₂ selectivities and H₂O₂ decomposition reactivities. Density functional theory (DFT) methods are implemented to calculate the rate constant ratios for H₂O₂ formation (via OOH* reduction; k_O-H) and OOH* decomposition (via O-O cleavage; k_O-O) for metallic Pd, surface oxide, and bulk oxide models. The k_O-H/k_O-O ratio is much smaller for Pd₁₃ than for Pd(111) (10^-10 vs. 10^-4 at 300 K), indicating poorer primary H₂O₂ selectivities for smaller particles. Yet, this ratio remained much smaller than unity for all Pd models. As the oxygen chemical potential increases, the Pd-Pd ensemble sites are perturbed by O atoms, which dramatically change their selectivities. The k_O-H/k_O-O ratio increased from 10^-4, 10⁹ to 10¹⁶ as Pd(111) oxidizes to Pd₅O₄/Pd(111) and PdO(100). This selectivity improvement, however, is very structure sensitive and the improvement remained minimal for surfaces that persistently contain Pd-Pd ensembles, such as PdO(101)/Pd(100) and PdO(101). Smaller PdO nanoparticles do not contain these facets and thus fully benefit from the selective enhancement. DFT-derived energy trends show that the catalysts with higher primary H₂O₂ selectivity are also less prone to H₂O₂ decomposition, leading to high H₂O₂ yields. Furthermore, ab initio thermodynamics calculations are used to assess the relevant Pd phase in O₂ and H₂O₂/H₂O environments, revealing that smaller Pd nanoparticles are likely to present as PdO during H₂O₂ decomposition experiments. This study highlights how Pd transformation to PdO impacts H₂O₂ synthesis and decomposition rates and selectivities, which are sensitive to the Pd-O coordination environments and particle sizes.

References

• Flaherty, D.W. ACS Catal 2018, 8(2),1520–1527.