Electrochemical Hydrogen Peroxide Generation with Anthraquinone Chemistry for Advanced Oxidation Process

Hydrogen peroxide plays a critical role in many industrial applications, including chemical synthesis and environmental remediation. For example, in advanced oxidation process of water treatment, hydrogen peroxide is activated to hydroxyl radical by catalysts or UV light for non-selective and rapid degradation of non-biodegradable, toxic, recalcitrant organic micropollutants. Industrial anthraquinone process, which synthesizes hydrogen peroxide from hydrogen and oxygen gas in organic phase, requires intensive capital investment, expensive reactants, high energy inputs, and raises many environmental concerns. Electrochemical production of hydrogen peroxide with air, water and electricity as inputs has emerged as a promising alternative method because it is low-cost, easy-to-operate, and environmentally-friendly. It would also greatly reduce the costs and dangers related to handling and transportation of commercial concentrated hydrogen peroxide solution and it can be potentially used for in-situ advanced oxidation process in decentralized water treatment applications. However, electrocatalyst synthesis, electrochemical reactor design, and H2O2 activation method all require significant improvement for real world usage. For electrocatalyst development, we propose to transfer anthraquinone’s homogeneous chemistry to heterogeneous interfaces. Most of the electrocatalysts reported have either low selectivity or low overall production and accumulation, as their environmental applications are hindered by the unclear reaction mechanism and uncontrolled material structure. We expect to produce hydrogen peroxide at the well-defined reactive sites selectively and stably. Anthraquinone molecular catalyst is attached to conductive substrates to construct immobilized heterogeneous electrocatalysts, and their performance under various electrochemical conditions are measured and optimized. The electro-driven anthraquinone is hydrogenated and oxidized sequentially to produce hydrogen peroxide, as in industrial processes. Various conductive supports with high electron mobility, stability with reduction potential, and compatibility with peroxide are investigated, including polymeric carbon nitride and conductive polymers. The material characterizations prove that anthraquinone can be chemically attached to conductive substrates with a facile, one-step synthesis method, and both catalytic composites show capacities of producing H2O2 with high activity and selectivity. For device-level optimization, two different electrode configurations, immersed electrode and gas diffusion electrode, are tested for their electrochemical performance respectively. Due to the oxygen mass transport limitation in the liquid phase, GDE electrode architecture is expected to boost the electrocatalytic activity and selectivity due to the formation of gas-liquid-solid triple phase boundary layer. We here present an electrochemical H2O2 generation cell that produces 1.8 mol gcatalyst–1 hr–1 at 100 mA with a Faradaic efficiency of 96%. Our calculation indicates that H2O2 production consumes only 0.2 to 20% of the total electricity consumption of AOPs in various AOP application scenarios employing UV activation. We also demonstrate the H2O2 production capability of the device with simulated drinking water and wastewater as feed electrolytes to demonstrate its potential for real-world operation scenarios. For H2O2 activation in an AOP system, we further explore the possibilities of activating H2O2 with no chemical input and with no or little energy input by utilizing a heterogeneous Fenton catalyst, iron oxychloride (FeOCl) in two configurations: packed bed reactor and electro-Fenton reactor. In packed bed reactor, FeOCl is loaded to molecular sieve substrate as heterogeneous catalyst for Fenton reaction, where H2O2 is activated to hydroxyl radical; in electro-Fenton reactor, FeOCl is loaded to conductive carbon substrate as electrocatalyst for electrochemically-driven Fenton-like reaction. Both reactors exhibit capacities to degrade the model contaminant compound. Based on these results, future research directions are outlined to fully realize the research goal of modular, electrified, and decentralized water treatment with in-situ H2O2 generation and in-situ H2O2 activation