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% H2O2).2 The key to economically viable DS (> 90% H2O2)1 is 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.