Direct synthesis of H₂O₂ on PdZn nanoparticles: The impact of electronic modifications and heterogeneity of active sites

The direct synthesis of H₂O₂ (H₂ + O₂ → H₂O₂) would reduce the cost of H₂O₂, but few catalytic materials demonstrably provide the necessary combination of high H₂O₂ selectivity and stability over long periods of continuous reaction. Here, we show that silica supported PdZn_x nanoparticles (where x indicates the bulk composition, 0 ≤ x ≤ 30) are stable and selective catalysts for the continuous production of H₂O₂ in the absence of liquid-phase promoters. Increasing the ratio of Zn to Pd precursors in the synthesis lead to distributions of nanoparticles that contain greater fractions of β₁-Pd₁Zn₁ and lower fractions of face-centered cubic (fcc) Pd nanoparticles. A combination of the measured functional dependence of steady-state H₂O₂ and H₂O formation rates on H₂ and O₂ pressures and the need for protic solvents indicate that H₂O₂ forms on all members of the PdZn_x series by proton-electron transfer steps to dioxygen and hydroperoxy surface intermediates that saturate active sites during catalysis. Primary H₂O₂ selectivities increase systematically with the Zn to Pd ratio and range from 26% on fcc Pd nanoparticles to 69% on PdZn₃₀ catalysts, which contains β₁-Pd₁Zn₁ nanoparticles and lacks detectable amounts of fcc Pd. The differences in H₂O₂ selectivities among the PdZn_x catalysts reflect changes in activation enthalpies (ΔH^‡) for H₂O₂ (from -3 to 14 kJ mol⁻¹) and H₂O (11 to 44 kJ mol⁻¹) formation. These results demonstrate that pathways for H₂O₂ formation are less sensitive than H₂O formation to electronic differences between active sites on fcc Pd and intermetallic β₁-Pd₁Zn₁ nanoparticles, whose densities of states differ significantly near the Fermi level. Comparisons of the ratios of H₂O₂ and H₂O formation rates to ΔH^‡ values among Pd and PdZn_x catalysts show that the increases in H₂O₂ selectivities are less than predicted from the differences in ΔH^‡ values alone. This analysis shows that the current synthesis method introduces at least two distinguishable catalytically active sites, and a portion of the sites formed predominantly cleave O-O bonds in primary and secondary reaction pathways that form H₂O and lead to lower H₂O₂ selectivities than would be achieved in their absence. These findings demonstrate that differences in electronic structure of active sites cause β-Pd₁Zn₁ nanoparticles to give greater H₂O₂ selectivities than Pd catalysts and suggest that a synthesis procedure that uniformly creates β-Pd₁Zn₁ and alloys all Pd on the support may lead to increasingly selective conversion of O₂ and H₂ to H₂O₂.