Reactivity Mapping of Photocatalysts Using Dynamic Environmental Electron Microscopy

The aim of this proposal is to advance the development of technology to reduce indoor air pollution that is harmful to human health. Outdoor urban pollution is clearly detrimental to health and is of widespread public concern, however it is estimated that in the UK approximately 90% of our time is spent indoors. Therefore, clean indoor air is critical, and long-term exposure to low concentrations of harmful airborne particles and molecules becomes important. Recent reports including that of the Chief Medical Officer (2022) have identified carbonaceous particulate matter (PM) and volatile organic compounds (VOCs) as of significant concern. Reducing exposure requires both changing human behaviour and developing technological solutions. Current indoor air purification systems typically use disposable filters requiring regular replacement and disposal. They are also much less effective in humid air. A promising alternative technology is heterogeneous photocatalysis using titanium dioxide (TiO2). Here, light drives photooxidation of pollutants, and can operate more efficiently in humid conditions, potentially for many years. However, there remain long-standing challenges of photocatalyst efficiency, selectivity, and deactivation. Overcoming these challenges requires fundamental understanding of the chemical dynamics of TiO2 particles that mediate photooxidation. Pollutant photooxidation can occur directly at the pollutant-photocatalyst interface or via reactive oxygen species (ROS) which are generated from photoreactions of water and oxygen mediated by the photocatalyst. However, little is known of where and how these processes occur in practical nanoparticulate systems and their role in photocatalyst efficiency, selectivity, and deactivation. Here we propose to use state-of-the-art aberration-corrected environmental scanning transmission electron microscopy (ESTEM) for unprecedented in-operando atomistic understanding of practical photocatalytic nanocrystalline systems. New ‘reporter’ methodology based on carbon nanoparticles will provide the spatial resolution to identify the active location(s) of photooxidation. Data will also be acquired at millisecond timescale giving the temporal resolution to measure the dynamics of structure and oxidation state changes at these locations. These experiments will be performed under variable air and water atmospheres to ultimately map the chemical dynamics and mechanism of photooxidation onto nanoparticle photocatalyst structure. This data will guide catalyst development in the second phase of this project to promote complete oxidation of pollutants to carbon dioxide avoiding noxious byproducts and prevent catalyst deactivation. Photocatalytic testing will include indoor air cleansing at room-scale with full byproduct analysis and explore pathways to commercialisation in collaboration with our industrial partner. Furthermore, the new ESTEM carbon nanoparticle reporter methodology developed here can also have wider application to the study of other photooxidative and thermal oxidation processes relevant to catalysis, corrosion, and healthcare. Commercial examples include established TiO2 industries ($20 billion in 2023), such as white pigments, coatings, and sunscreens where suppression of ROS production is required. Other emerging TiO2 technologies include antimicrobial composite materials where (photo)oxidation of pathogens reduces the probability of infection. Understanding the chemical dynamics of TiO2 (photo)oxidation and its composites will guide the targeting of bespoke structures and compositions for specific application.