Selective Water Oxidation to H₂O₂ on TiO₂ Surfaces with Sub-Surface Redox-Active Allosteric Sites

Hydrogen peroxide H₂O₂, is a versatile chemical with applications ranging from industrial to household use such as pulp bleaching, wastewater treatment, and sterilization. In addition to these established uses, H₂O₂is emerging as a green energy carrier which can release 96 kJ *mol^-1 of energy via exothermic chemical decomposition with only water and oxygen as by-products. Currently, more than 95% of H₂O₂ produced globally originates from anthraquinone autoxidation. This process requires precious metal catalysts and expensive liquid-liquid extraction processes that are only feasible for large-scale chemical plants.<br/>Electrochemical and photochemical syntheses are promising alternatives for distributed H₂O₂production. H₂O₂can be produced by 2e^- processes both via oxidatively, via water oxidation reaction (WOR) or reductively via oxygen reduction reaction (ORR). These approaches take advantage of abundant energy sources, i.e., renewable electrical or solar energy, and enables the storage of intermittent energy as the free energy of chemical bonds for distributed energy storage. In this work we focus on the water oxidation pathway.<br/><b>Electrochemical Charge Transfer:</b><br/>H₂O₂can be (photo-)electrochemically synthesized via a two-electron (2e−) water oxidation pathway, as shown in equation 1.<br/>H₂O₂ → H₂O + 2H⁺ + 2e^-, E = +1.77 V (1)<br/>However, existing catalysts often exhibit low faradaic efficiency (FE) due to the competitive four-electron (4e ) pathway of O₂ evolution reaction (OER) equation 2.<br/>H₂O₂→ O₂ + 4H⁺ + 4e^- , E = +1.23 V (2)<br/>The thermodynamic driving force for OER is always at least +0.54 V greater than that of the 2e H₂O₂pathway which can allow the undesired O₂ evolution pathway to overcome any kinetic barriers and dominate. In addition, the produced H₂O₂ can be over-oxidized to O₂ at an excess potential of at least +1.09 V (equation 3).<br/>H₂O₂→ O₂+ 2H⁺ + 2e^- , E = +0.68 V (3)<br/>Thus, the partial oxidation of water to hydrogen peroxide presents a model system of a common challenge in catalysis where the desired product is kinetically feasible but thermodynamically unfavorable.<br/><b>Conclusion:</b><br/>In this work, we utilize X-ray Photoelectron Spectroscopy to quantify the presence of Mn(III) defect states within the TiO₂ band gap which are energetically aligned to peroxide generating reactive intermediates. During operation, the energy levels of CB and VB of TiO₂ are fixed, or “pinned”, at −0.05 and 3.29 VRHE, respectively, yet the empty states of the Mn defect states can precisely align with the energetics of H₂O₂ producing intermediates. Under a higher applied potential, i.e., +2.3 V , there would exist a moderate overpotential for the 2e water oxidation, but such a potential also enables the other pathways including 1e water oxidation to produce radicals, leading to an overall reduction in H₂O₂production rates due to the reduced selectivity. Thus, this strategy of defect state tuning is best suited to low overpotentials, 90% at &lt; 150 mV overpotentials was achieved for H₂O₂production, accumulating 2.97 mM H₂O₂ after 8 hours. Nanoscale mixing of MnOx and TiOx resulted in a partially filled, highly conductive Mn intermediate band (IB) within the TiO₂ mid-gap to transport charge across the TiMnOx coating. This IB energetically matched that of H O -producing surface intermediates, turning a wide bandgap oxide into a selective electrocatalytic material that can operate in the dark.