Issue 13, 2023

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; kO–H) and the decomposition of OOH* (via O–O cleavage; kO–O). For Pd metals, the kO–H/kO–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 kO–H/kO–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.

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

Supplementary files

Article information

Article type
Paper
Submitted
24 Mar 2023
Accepted
21 May 2023
First published
31 May 2023

Catal. Sci. Technol., 2023,13, 3828-3848

Author version available

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

M. Vyas, F. Fajardo-Rojas, D. A. Gómez-Gualdrón and S. Kwon, Catal. Sci. Technol., 2023, 13, 3828 DOI: 10.1039/D3CY00404J

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