(535d) Mechanism and Factors That Determine Selectivity for H2O2 Formation on Pd and Aupd Clusters Via Direct Synthesis

Hydrogen peroxide (H₂O₂) is an environmentally benign oxidant with applications in paper bleaching, wastewater treatment, and epoxidation of organic molecules.¹ H₂O₂ is currently produced by the auto-oxidation of anthraquinones (AO) yet direct synthesis (DS) is a greener alternative with the potential to make H₂O₂ more efficiently than AO. However, DS currently suffers from low H₂O₂ selectivity in the absence of acids and halides (< 60% H₂O_2­).² The key to economically viable DS (> 90% H₂O₂)¹is to: (1) prevent scission of the O-O bond in chemisorbed dioxygen (O₂*) while, (2) increasing the rates of H-addition to O₂*. We intend to test these hypotheses by elucidating the mechanisms for H₂O₂ and H­₂O formation by DS.

Steady-state H₂O₂ and H₂O formation rates were measured as functions of H₂ pressure (10-400 kPa), O₂ pressure (10-400 kPa), and temperature (263-303 K) on silica-supported Pd and AuPd clusters in methanol solutions within a fixed bed, plug flow reactor. These data are inconsistent with Langmuirian mechanisms for H₂O₂ formation, which suggests an Eley-Rideal mechanism creates H₂O₂. H₂O₂ formation was found to occur on primarily O₂* or OOH* covered surfaces and increases in proportion to the liquid-phase concentration of protons, which was varied independent of H₂ using a variety of proton donors. Lowering the pH increases H₂O₂ formation rates over those for H₂O and, thus, improves selectivity. High H₂O₂ selectivities occur when the rate of protonating O₂* and OOH* is much greater than that for irreversibly breaking O-O bonds in these intermediates. Increasing the size of the Pd clusters was found to dramatically increase selectivity by reducing the amount of under-coordinated Pd sites that promote O-O bond scission. It was found that the activation enthalpies for H₂O formation increased with Pd cluster size while enthalpies for H₂O₂ formation were primarily unaffected, indicating that electronic effects primarily contribute to O-O bond scission on under-coordinated Pd sites. Similar increases in activation enthalpies for H₂O formation over H₂O₂ are seen when alloying Au with Pd, which suggests that changes in the electronic structure of cluster surfaces are primarily responsible for increased selectivities observed on AuPd clusters. This improved understanding of the mechanisms behind DS will help to guide the future synthesis of selective catalysts so that H₂O₂ can be made more efficiently using DS.

(1) Goor, G.; Glenneberg, J.; Jacobi, S. Ullmann's Encyclopedia of Industrial Chemistry 2012, 18, 393-427.       

(2) Samanta, C. App. Catal. A 2008, 350, 133-149.