In-situ Oxidant Production from Oxygen Reduction Reaction (ORR) for Water Treatment

Abstract

Advanced oxidation processes (AOPs) have been an attractive method to remove toxic, non-biodegradable, and recalcitrant contaminants which easily permeate the membrane. When considering the cost-prohibitive complete removal process, AOPs have the advantage to achieve mineralization of organics or partial oxidation to inert or less toxic products. The basic steps for the technologies are efficient production of reactive oxidative species and reaction of oxidants with target contaminants. Common homogeneous AOPs yield highly reactive radicals such as hydroxyl radicals (•OH) from activating weaker oxidants such as O3 or H2O2 combined with ferric ion, electricity, or UV light. However, these systems require often substantial chemical and external energy input. Using a heterogeneous catalytic process based on engineered nanomaterials have been studied for oxidant production with lower energy and with in-situ production of the oxidants. Especially, photocatalytic redox reactions caused by photoinduced electrons (e-) and holes (h+) on the heterogeneous solid surfaces generate several reactive species from oxygen reduction reaction and water oxidation. For example, •OH production as a strong oxidant can be produced from two pathways which are one-hole oxidation of water and three-electron reduction of O2. However, only a few photocatalysts (e.g. TiO2, ZnO, and WO3) can produce •OH from water oxidation because the more positive valence band potential should be needed compared with water oxidation potential for •OH production (E° = 2.73 V). In comparison, there are many candidates which have appropriate conduction band potential for oxidant production (e.g. O2-, H2O2, and •OH) from stepwise oxygen reduction reaction. Among them, hydrogen peroxide (H2O2) has the potential to combine with other AOPs for the production of •OH or consume itself as a clean oxidant. The aim of this study is the development of the system for chemical and photochemical production of H2O2 from the oxygen reduction reaction using a heterogeneous catalytic process based on engineered nanomaterials. In the first study, the carbon nitride (CN), which has a high negative conduction band for the reduction of dioxygen, used for the H2O2 production and the oxidation of organic pollutants. For a better understood of environmental application, the behaviors of CN were systematically studied in comparison with a well-known photocatalyst (TiO2). The two photocatalysts exhibit different photocatalytic oxidation (PCO) behaviors and dependencies on the experimental conditions (e.g., pH, Pt loading, and the kind of organic substrate and scavenger). The negative effect of Pt on CN is ascribed to that Pt catalytically decomposes in-situ generated H2O2 (the main precursor of OH radical) with hindering •OH production. Instead of using metals to separate charge pairs, we introduce a new strategy for enhancing its visible-light photocatalytic activity by designing in which the nitrogen of tertiary amine is substituted with a benzene molecule connected by three heptazine rings. The intramolecular benzene doping induced the structural changes from planar symmetric structure to distorted geometry. This structural distortion facilitated the spatial separation of photogenerated charge pairs and retarded charge recombination via exciton dissociation. For overcoming pH, co-catalyst, and hole scavenger dependent H2O2 production, we choose C, N, O, P-containing organic polymer to H2O2 production by dioxygen reduction with a water oxidation reaction. CNOP are able to produce H2O2 concurrent with the generation of dioxygen from water oxidation. The effects of phosphorus and oxygen incorporated in organic polymers were evaluated for H2O2 production concurrent with water oxidation in a systematic way. In addition, to realize the H2O2 production system under even in a lightless environment, copper phosphide (CuxP) was synthesized and tested for its reactivity for generating H2O2 through spontaneous reduction of dioxygen under ambient aqueous condition. The in-situ generated H2O2 was subsequently decomposed to generate OH radicals, which enabled the degradation of organic compounds in water. Finally, in the case of industrial wastewater containing a large number of specific pollutants, a water treatment process by fit-for-purpose is required. For one example, free cyanide (CN-) removal using the photoelectrochemical (PEC) system which can produce reactive chlorine species and H2O2 was introduced in the appendix.