Photocatalytic paint is a proposed approach to oxidize atmospheric methane with light-activated paint on surfaces such as buildings, vehicles, solar panels, or wind turbines. This is a passive, non-energy-intensive approach to oxidizing methane. However, more research is needed into potential byproducts and the catalytic efficiency of this method. Laboratory research is underway to improve the performance of certain photocatalysts, and to study the potential effects and side effects of painted photocatalysts.
For photocatalytic paint to be a feasible approach there would have to be significant increases in catalytic efficiency, drastic reductions in the cost per area of painting a surface, and resolution on land use considerations for scalability and potential environmental concerns. However, even then, forthcoming modeling suggests that even with arbitrarily high catalytic efficiency this approach is unlikely to become cost-effective due to slow convective mass transfer of methane in the atmosphere.
More research, including a full lifecycle analysis of the process emissions embedded in the painting process itself, is required to determine whether photocatalytic paints are a feasible approach to atmospheric methane removal since there is currently no peer-reviewed literature on this topic.
Apparent Quantum Yield (AQY), the ratio of incident photons to oxidized methane molecules, is a key metric of catalytic efficiency for determining the costs and climate impacts of photocatalysis. It helps to determine the surface area required for photocatalytic paints to oxidize a certain amount of methane.
The potential costs of this method on a per ton of methane basis have not been established in the literature, but forthcoming research offers a few estimates. Modeling suggests that even with arbitrarily high AQY this approach is unlikely to become cost-effective due to slow convective mass transfer of methane in the atmosphere. Reaching the cost-plausible threshold with painting rooftops (costing ~$10/m2 to paint) would require at least a ~30x increase in AQY to 1% from its current state-of-the-art of only 0.03% at 2 ppm methane. Photocatalytic paints will in most cases increase the albedo, cooling the air locally. This additional effect should be factored into climate impact calculations. Advances in photocatalyst durability and self-cleaning behavior would also likely be needed.
Photocatalytic paints appear unlikely to be meaningfully scalable as an atmospheric methane removal method due to fundamental limits on potential cost-effectiveness driven by slow convective mass transfer.
Preliminary evaluations of the surface area required to oxidize 10 million metric tons of methane per year gives an estimate of roughly > 200,000 km2 (equivalent to the estimated total rooftop surface area on Earth) if the current best AQY (~0.03%, forthcoming research) could be maintained over such an area, though this would remain cost-implausible. Should the required 30x AQY improvements to achieve cost-plausibility be achieved, this would go down to 6,000 km2. However, if the area that was painted was in close proximity resulting in substantial local oxidation of methane, the local concentration would be much lower, meaning that the AQY would decrease as well; modeling is required to determine the magnitude of this effect. The scale required would therefore likely be much larger, but modeling work is required to determine this value. Furthermore, any surface area used for photocatalytic paint will also be competing with rooftop solar installations and other uses of that surface area, so tradeoffs will likely be inherent.
Given the very early state of understanding this potential pathway, health and environmental co-benefits and concerns are not yet well understood. It would be critical to study them before considering any future testing or deployment.
In addition to methane, photocatalysts also oxidize VOCs and ozone, pollutants with negative human health and environmental impacts near ground level. Increasing the oxidation of these species could have co-beneficial effects. At the same time, photocatalysts are also known to generate undesirable end products (nanoparticles, secondary VOCs, etc.). More research is needed to better understand the tradeoffs between these potential positive vs. negative impacts.