Modelling the photocatalytic oxidation of methane and other air pollutants for applications in ventilation systems

Abstract

Photocatalytic oxidation (PCO) is a promising strategy for indoor air purification and outdoor pollutant abatement, potentially offering treatment for climate- and health-relevant pollutants such as methane (CH$_4$), nitrogen oxides (NO$_\text{x}$) and volatile organic compounds (VOCs). In this work, we present experiments evaluating the PCO of CH$_4$ (2 to 10 ppm) under varying UV-C light intensities (4 to 59 W/m$^2$), using titanium dioxide (TiO$_2$) as the photocatalyst. At 2 ppm CH$_4$, TiO$_2$ achieves a maximum conversion efficiency of 24.4% and a maximum apparent quantum yield of $0.013$\% over the tested UV-C light intensities, demonstrating activity at environmentally relevant concentrations. We develop a model to interpret the experimental results and assess the potential of PCO for ventilation applications. The model is validated against our CH$_4$ data and literature results for formaldehyde (HCHO) and NO$_\text{x}$. While laboratory-scale configurations achieve high conversions (e.g., 24.4% for CH$_4$), ventilation-scale performance is predicted to be limited by thin concentration boundary layers and short residence times, with conversion efficiencies dropping to around $0.017$%. Finally, we estimate the climate impact of CH$_4$ removal in terms of CO$_2$e emission rates, demonstrating that TiO$_2$-based PCO in ventilation applications can yield a net climate benefit (i.e., a net-negative CO$_2$e emissions rate) when the modelled CO$_2$e removal rate exceeds the emissions from catalyst material production and UV operation, particularly when pre-existing UV-C irradiation is leveraged.