(597e) Understanding the Effects of Bromide Adsorption on the Direct Synthesis of H2O2 on Pd Nanoparticles

Direct synthesis (H₂+O₂→H₂O₂) can replace the cost- and energy- intensive, Riedl-Pfleiderer process as the primary method to produce H₂O₂ (replacement for harmful chlorinated oxidants). However, direct synthesis exhibits low H₂O₂ selectivities (< 60%) on benchmark Pd catalysts. Adding halides, especially bromide (as inorganic acids and salts) in the reaction mixture improves H₂O₂ selectivities. However, under acidic conditions, metal dissolution occurs which adversely affects the reaction rates. Additionally, the role of bromide-based promoters in influencing catalysis is unclear because of the convoluting presence of multiple promoters (acids, organic additives). We examine how sodium bromide modifies the properties of Pd nanoparticles and impacts the rates, selectivities and apparent activation barriers for product formation in water in the absence of any other promoter.

H₂O₂ selectivities increase from 17% in pure water to ~65% at 10^-4 M NaBr (55 kPa H₂, 200 kPa O₂) and decrease beyond 10^-4 M NaBr on Pd-SiO₂ catalyst. Br*-atoms adsorb onto and irreversibly modify Pd nanoparticles, increasing the H₂O₂ selectivities in pure water (~40%). Infrared spectra of adsorbed CO indicate that Br atoms preferentially bind onto undercoordinated Pd sites. Bromide adsorption isotherms suggest that Br saturates the surface of reduced Pd nanoparticles and intercalates to the subsurface of Pd nanoparticles. H₂O₂ and H₂O form by H₂O-mediated proton-electron transfer (PET) steps in the presence and absence of Br*-atoms. Increase in [NaBr] raises the apparent activation enthalpies for H₂O₂ and H₂O formation, albeit with different sensitivities leading to the observed increases in H₂O₂ selectivities. The adsorption and intercalation of Br in Pd nanoparticles increase the steady-state H₂O₂ selectivities in pure water, requiring no further addition of liquid-phase bromide for at least 15 h showing that infrequent additions of benign halide promoters can reduce the complexities associated with product purification for many applications of H₂O₂ without compromising rates and selectivities.