Theoretical

Atmospheric Methane Removal Approaches

Photocatalytic paints and coatings

Rising temperatures are increasing the risk of natural systems releasing methane, which would drive further warming. Existing efforts towards reducing anthropogenic greenhouse gas emissions and removing atmospheric carbon dioxide are crucial, but may be insufficient to maximally decrease the chance of, and then possible impact of, these risks. Atmospheric methane removal approaches are being researched to determine how to remove methane from the atmosphere faster than natural systems alone, in order to help lower peak temperatures and counteract some of the impacts of large-scale natural systems methane releases.

Atmospheric methane removal, should any approaches prove highly scalable, effective, and safe, could help address some of the current 0.5°C—and rising—of methane-driven warming. All proposed atmospheric methane removal approaches are at a very early stage today: some ideas have been proposed, some are being researched in laboratories, but none are yet ready for deployment. Spark believes that accelerating research to develop and assess which, if any, of these approaches might be possible and desirable is an important additional risk mitigation strategy.

A number of atmospheric methane removal approach ideas have been raised—including
photocatalytic paint
, which is currently
Theoretical
, with major breakthrough innovations required to change this
.
This approach, based on early analysis, will likely require multiple breakthroughs in order to feasibly address atmospheric methane levels. It may hold the most promise if it also delivers separate benefits (e.g. for climate or pollution), as part of systems deployed for other primary reasons, or to address low-concentration methane sources.

Photocatalytic paints and coatings

Overview

Photocatalytic paints (and coatings) cover surfaces such as buildings, vehicles, solar panels, or wind turbines to oxidize methane through light activation. 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 photocatalytic performance and to study the potential side effects of painted photocatalysts.

Photocatalytic paint is potentially promising for breaking down methane at its emission sources where its concentration is elevated. For photocatalytic paint to be a feasible standalone approach at atmospheric methane concentrations 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, adding photocatalysts to paint or coatings being applied for other purposes (such as cooling buildings and increasing their albedo) may be a cost-plausible methane removal approach.

Feasibility

Learn more about how we evaluate cost plausibility and climate impacts

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 photocatalytic paint is unlikely to become cost-effective as a standalone approach 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. However, photocatalyst added to paint being applied for other purposes may be cost-plausible. Further research is required to determine and compare the rates of methane breakdown and the reduction in albedo that photocatalysts may cause. Advances in photocatalyst durability and self-cleaning behavior would also likely be needed.

Scalability

Learn more about how we evaluate scalability

The potential scale photocatalytic paints could reach is primarily limited by the surface area available to be painted. Preliminary evaluations of the surface area required to oxidize a benchmark annual scale of 10 million metric tons of methane (830 Mt CO2e using GWP20) 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% at 2 ppm CH4, forthcoming research) could be maintained over such an area. 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 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, unless the photocatalyst could be added to paint already being used for other purposes.

Health & Environmental Considerations

Given the very early state of understanding this potential approach, health and environmental co-benefits and concerns are not yet well understood. It is 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 products (nanoparticles, secondary VOCs, etc.). More research is needed to better understand the tradeoffs between these potential positive and negative impacts.

Learn More

State of Research
References and Resources
Thank you to
Richard Randall (Stanford)
for their contributions to, and review of, this content.
This is live and evolving content, we are always open to well-referenced updates and suggestions, which can be shared here.

Explore Other Potential Approaches

Approaches Overview

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