Direct synthesis of H2O2 (H2 + O2 → H2O2) could enable on-site, and even in situ, H2O2 production, which motivates searches for highly selective catalysts and process conditions. H2O2 formation rates and selectivities depend sensitively on the addition of other transition metal atoms (e.g., Au, Zn, Sn) or the adsorption of halides to Pd nanoparticles. Moreover, apparent kinetics for the formation of H2O2 and H2O differ between systems operating within alcohol (or aqueous alcohol solutions) solvents or within pure water. The reasons for these changes are not completely understood and are difficult to explain mechanistically. Here, we combine rate measurements on Pd and Pd-based bimetallic clusters to determine the mechanism of this reaction and to understand the reasons why alloying Pd often increases H2O2 selectivities. In aqueous alcohols, the change in H2O2 and H2O formation rates with H2 and O2 pressures are not consistent with a Langmuirian mechanism, but instead suggest O2* species react in steps mediated by the solvent. In addition, H2O2 formation rates in protic solvents are 103 larger than those measured in aprotic liquids and large kinetic isotope effects (kH/kD &gt; 7) strongly suggest that alcohols serve as reactants in the kinetically relevant step for H2O2 formation. In parallel, O-O bonds within chemisorbed intermediates cleave to form H2O with rates that are less sensitive to the solvent identity. In pure water (i.e., without alcohol co-solvents), H2O2 forms via distinct steps that involve H*-atom transfer, shows that long-standing observations that H2O2 forms in greater yields within alcoholic solvent are not explained by simple differences in the solubility of H2 in the liquid-phase. Similar rate laws and solvent requirements indicate that these reactions proceed by the same pathways on Pd-based bimetallic clusters and also in the presence of strongly binding halide adsorbates and acids. These modifications change barriers for the formation of H2O (significantly) with lesser effects on barriers for steps that lead to H2O2, and are consistent with electronic modifications of Pd active sites by intra-atomic orbital rehybridization or by charge transfer from Pd atoms, respectively. Overall, this work presents evidence for the mechanism for H2O2 formation and explains the roles of solvent identity, cluster composition, and surface modification strategies on H2O2 selectivities.

Modifying metal nanoparticle surfaces to influence hydrogen transfer and oxygen dissociation for the direct synthesis of H2O2

255th National Meeting (2018)

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