Microstructural Origin of Selective Water Oxidation to Hydrogen Peroxide at Low Overpotentials: A Study on Mn-Alloyed TiO2
Authors: Solanki Devan , Hu Shu 
Published: September 2023

Introduction:

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, H2O2 is 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.

Electrochemical and photochemical syntheses are promising alternatives for distributed H2O2 production. H2O2 can 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.

Charge Transfer:

H2O2 can be (photo-)electrochemically synthesized via a two-electron (2e−) water oxidation pathway, as shown in equation 1.

2H2O → H2O2 + 2H+ + 2e−, E = +1.77 VRHE (1)

However, existing catalysts often exhibit low faradaic efficiency (FE) due to the competitive four-electron (4e−) pathway of O2 evolution reaction (OER) equation 2.

2H2O → O2 + 4H+ + 4e−, E = +1.23 VRHE (2)

The thermodynamic driving force for OER is always at least +0.54 V greater than that of the 2e− H2O2 pathway 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.

H2O2 → O2 + 2H+ + 2e−, E = +0.68 VRHE (3)

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.

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 VRHE, 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 H2O2 production rates due to the reduced selectivity. Thus, this strategy of defect state tuning is best suited to low overpotentials, <100mV, catalysis.

Conclusion:

This study presents a new strategy of introducing charge-transport defect states and demonstrates the design of H2O2-producing surfaces that oxidize water to H2O2 with high selectivity at very low overpotentials. Given the growing literature related to H2O2 selective catalysis, this approach is expandable to a range of host oxides that prevent H2O2 disproportionation to favor 2e− H2O2 electrochemistry.

Faradaic efficiency of >90% at < 150 mV overpotentials was achieved for H2O2 production, accumulating 2.97 mM H2O2 after 8 hours. Nanoscale mixing of Mn2O3 and TiO2 resulted in a partially filled, highly conductive Mn3+ intermediate band (IB) within the TiO2 mid-gap to transport charge across the (Ti,Mn)Ox coating. This IB energetically matched that of H2O2-producing surface intermediates, turning a wide bandgap oxide into a selective electrocatalyst capable of operating in the dark.

Figure 1