Effects of bromide adsorption on the direct synthesis of H₂O₂ on Pd nanoparticles: Formation rates, selectivities, and apparent barriers at steady-state

The direct synthesis reaction (H₂ + O₂ → H₂O₂) may provide a low-cost method to form H₂O₂ if catalytic systems with sufficient selectivities and stabilities are developed. Here, we examine how sodium bromide, a ubiquitous promoter for direct synthesis, impacts steady-state rates and apparent barriers for H₂O₂ and H₂O formation in water and relate changes to the effect of NaBr on surface and bulk properties of Pd nanoparticle catalysts. Comparisons of turnover rates and selectivities measured over an extended period (>80 h) demonstrate that these systems require more than 10 h to reach steady-state, whereupon, H₂O₂ selectivities depend strongly upon [NaBr] and increase from 17% in pure water to ~65% at 10⁻⁴ M NaBr (55 kPa H₂, 200 kPa O₂). Contact with these NaBr solutions irreversibly modifies Pd nanoparticles, such that H₂O₂ selectivities remain at 40% in pure water, due to irreversible uptake of Br*-atoms. Bromide adsorption isotherms measured show that reduced Pd nanoparticles adsorb several monolayers of Br*-atoms, which suggests Br saturates surfaces but also resides within the near-surface region of Pd nanoparticles. Ex situ X-ray photoelectron spectroscopy indicates that Br*-atoms withdraw charge from Pd atoms and yield greater fractions of Pd²⁺. Comparisons among infrared spectra of adsorbed CO imply that persistent Br atoms preferentially bind to undercoordinated sites. H₂O₂ and H₂O formation rates, both in the presence and absence of Br*-atoms, change with H₂ and O₂ pressures in ways consistent with elementary steps that involve H₂O-mediated proton-electron transfer (PET). Therefore, increased selectivities on Br*-modified surfaces reflect differences in apparent activation enthalpies for H₂O₂ (ΔH^‡_H2O2)and H₂O (ΔH^‡_H2O) formation. ΔH^‡_H2O2and ΔH^‡_H2O increase systematically with [NaBr], although with different sensitivities. Comparisons of activation enthalpies alongside ex situ characterization provide compelling evidence that Br atoms adsorb onto and intercalate beneath the surfaces of Pd nanoparticles and increase steady-state H₂O₂ selectivities, which persist without further addition of liquid-phase bromide for at least 15 h. These findings indicate H₂O₂ selectivities for a continuous catalytic process may be increased by a one-time (or infrequent) addition of halide promoters, which can reduce the complexity of subsequent product purification for many applications of H₂O₂.