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

The direct synthesis of H2O2 (H-2 + O-2 -> 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 PdZnx 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 beta(1)-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 H-2 and O-2 pressures and the need for protic solvents indicate that H2O2 forms on all members of the PdZnx 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 beta(1)-Pd1Zn1 nanoparticles and lacks detectable amounts of fcc Pd. The differences in H2O2 selectivities among the PdZn catalysts reflect changes in activation enthalpies (Delta H double dagger) for H2O2 (from -3 to 14 kJ mol(-1)) and H2O (11 to 44 kJ mol(-1)) formation. These results demonstrate that pathways for H202 formation are less sensitive than H2O formation to electronic differences between active sites on fcc Pd and intermetallic beta(1)-Pd1Zn1 nanoparticles, whose densities of states differ significantly near the Fermi level. Comparisons of the ratios of H2O2 and H2O formation rates to AHt values among Pd and PdZn catalysts show that the increases in H2O2 selectivities are less than predicted from the differences in Delta H double dagger 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 beta-Pd1Zn1 nanoparticles to give greater H2O2 selectivities than Pd catalysts and suggest that a synthesis procedure that uniformly creates beta-Pd1Zn1 and alloys all Pd on the support may lead to increasingly selective conversion of O-2 and H-2 to H2O2. (C) 2018 Elsevier Inc. All rights reserved.