Photocatalytic reactors use artificial ultraviolet light to oxidize methane. Air must be moved through the localized reactor, either through active methods such as fans, or passive methods such as solar updraft chimneys that use convective heat transfer to move air.
Photocatalytic reactors are promising for breaking down methane at its emission sources where its concentration is elevated. For photocatalytic reactors to be a feasible atmospheric methane removal approach there would have to be a >200x improvement in catalytic efficiency and extremely low energy airflow. The required airflow to achieve megaton scale atmospheric methane removal would be far beyond planned carbon dioxide direct air capture capacity in the upcoming few decades.
Significant improvements in catalytic efficiency would be necessary to make photocatalytic reactors viable to operate atmospheric methane concentrations. For this application, they are currently climate detrimental and cost-implausible.
A key metric for determining cost and climate impacts for photocatalysis is Apparent Quantum Yield (AQY), the ratio of incident photons to oxidized methane molecules. This determines the energy input required to produce artificial light to oxidize methane. Cost and climate impacts are also driven by the energy requirements to move air through the reactor. This could be negligible for passive air flow, but significant for active air flow methods. Where artificial light is required, the energy requirements to generate it add to the cost. Sunlight does not provide a high enough photon flux to be a viable alternative.
Assuming best-case 2030 energy cost and carbon footprint projections are met, current photocatalytic reactors are not climate beneficial or cost-effective. Before taking into account airflow requirements, the minimum AQY thresholds are ~0.1% for climate beneficial, ~1.4% for cost-plausible, and ~7% for cost-effective. The best AQY measured so far is ~0.03% for 2 ppm methane, so at least a 200x increase in AQY is required to be climate beneficial and cost-effective.
The potential scale photocatalytic reactors could reach is limited by the energy requirements of air movement and potentially resource limitations of raw materials. Given the low atmospheric concentration of methane, any flow through system would have to process massive amounts of air to oxidize a benchmark annual scale of 10 million metric tons of methane (830 Mt CO2e using GWP20). For example, if you add methane breakdown reactors to every carbon dioxide direct air capture system that is projected to be installed by 2030 (~60 Mt CO₂/yr or around 5 billion cubic feet per minute), only around 0.1 Mt/yr of methane could be removed.
Even if photocatalytic reactors become feasible, the amount of methane addressed will be directly related to the number and scale of the reactors built, and resource intensive to reach meaningful scale. Therefore we estimate an approach of this sort would take decades to scale after feasibility was established.
In a closed reactor system, byproducts could be measured in situ and potentially be selectively removed, reducing the uncertainty as to what gasses would be produced and emitted. This drastically lowers the health and environmental risks compared to open system interventions.
In addition to methane, photocatalysts are expected to react with other atmospheric pollutants, including sulfur dioxide (SO₂), carbon monoxide (CO), ozone (O3) and non-methane volatile organic compounds (NMVOC). These compounds also contribute to climate change and the positive and negative climate impacts of their breakdown would need to be accounted for. Increasing oxidation of these species could have additional air quality benefits near ground level. However, photocatalysts are also known to generate undesirable end products (nanoparticles, secondary volatile organic compounds, etc.). More research is needed to better understand the tradeoffs between these potential positive vs. negative impacts.
The photocatalytic mechanism is not well understood, especially the extent to which radicals like hydroxyl or chlorine participate in the breaking of the carbon-hydrogen bonds, as opposed to electron/hole charge pairs causing the break themselves.
Most photocatalysts are excited only by very small segments of the light spectrum, usually around 365 nm (UV radiation). As a result, lab tests have almost exclusively used artificial UV lights to cause excitation. Photocatalysts have so far only been tested in the lab, usually using well controlled artificial air mixtures from pure gases, and often in batch reactors. More realistic conditions, long-term stability testing, and flow-through reactor experiments will be necessary to fully understand the performance of photocatalyst candidates.
Titanium dioxide and zinc oxide compounds are the most studied photocatalysts. Performance is often measured in terms of Apparent Quantum Yield (AQY), which is the ratio of incident photons to oxidized methane molecules. It has been hypothesized that the upper limit for AQY is 12.5% given that 8 electrons are required to oxidize one molecule of methane.
Actual AQY is a function of the concentration of methane present in the air (lower concentrations produce lower AQY) and the wavelength of light being applied (visible light tends to produce lower AQY than UV light, due to its lower energy).
An Apparent Quantum Yield of at least 7% will be required to make radical reactor energy costs approach the social cost of methane. State-of-the-art AQY is 1% at 5000 ppm CH4 using Ag-ZnO as the photocatalyst. At 2 ppm methane, both Ag-ZnO and TiO2 have AQYs of ~0.03% (Zhang et al 2022, Wei Li, personal communication).
Performance can potentially be improved by adding co-catalysts or dopants, increasing surface area, and otherwise altering the preparation process and composition of the material to produce higher AQYs, access lower concentrations, and unlock additional bandwidth of usable light spectrum.
Key questions that need to be answered regarding the photocatalytic reactor approach include: