Direct synthesis of H2O2, a selective and environmentally benign oxidant, remains out of reach due to lack of catalysts with sufficient stability and selectivity to H2O2. Alloyed PdAu nanoparticle catalysts show higher selectivities than monometallic Pd, but the reasons for the increased selectivities remain controversial and conflicting hypotheses have been proposed. Here, we analyze the effect of alloying and diluting Pd with Au on H2O2 formation by experimental rate measurements and in situ structural characterization of a series of bimetallic PdAux catalysts of varying composition (0 ≤ x ≤ 220) and ab initio quantum mechanical calculations of reaction barriers on representative surfaces. Steady-state H2O2 selectivities on PdAux nanoparticles only become large (&gt; 50%) in comparison to Pd (18%) when the Au to Pd ratio is also large (&gt; 37). In general, greater Au to Pd ratios lead to higher selectivities, which approach ~100% for materials with the most dilute Pd atoms. In contrast, monometallic Au nanoparticles show negligibly low H2O2 formation rates. We elucidate the pathways that form H2O2 and H2O and identify the structural motifs responsible for high selectivities. Both kinetic measurements and ab initio quantum mechanical calculations show that in pure water proton-electron transfer (not homolytic hydrogenation) processes form H2O2 and H2O even as the structure of the active site changes by increasing Pd-Au coordination determined by in situ EXAFS and by measurements of vibrational Stark effects of mixed adlayers of 13CO and 12CO. Moreover, the identities of the rate and selectivity determining steps remain constant. Instead, structural characterization shows that a lack of direct Pd-Pd coordination (i.e., formation of a PdAu single-atom alloy) coincides with high H2O2 selectivity. However, selectivities differ among multiple materials that lack Pd-Pd coordination, which implies other factors also matter. Moreover, these observations do not exclude either electronic or geometric effects. Comparisons between the densities of states on Pd atoms indicate that the electronic structure of Pd changes slightly as the Pd – Au coordination number increases. Geometric structures, the number of nearest and next-nearest Pd neighbors to Pd, correlate strongly with experimentally measured and computationally predicted barriers and selectivities. These findings underscore the importance of next-nearest-neighbor atoms particularly for single-atom alloy catalysts.

Effect of Pd Coordination and Isolation on the catalytic reduction of O2 to H2O2 over PdAu bimetallic nanoparticles.

ACS Spring Meeting 2021

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