Improving Quantum Yield for Photolytic Reactors

Reactors which use photolysis to convert chlorine gas into chlorine radicals to oxidize methane in an enclosed chamber have been proposed as a potential approach to remove methane from the atmosphere (1). They have relatively straightforward verifiability but are currently estimated to be too costly and difficult to scale to significantly impact the atmospheric methane concentration on decadal timelines (2). 

The cost is driven by the energy required to generate the UV light to break the methane bond and the energy required to move the air through the reactor. The energy cost for UV light depends directly on the apparent quantum yield (AQY; the ratio of incident photons to oxidized methane molecules) for which the current state-of-the-art is 0.83% (3). The current estimate for the minimum AQY that could achieve cost-effectiveness when oxidizing 2 ppm methane is 9% (4). Therefore, an order of magnitude improvement in AQY is required for cost-effectiveness for this technology.

The AQY of photolysis of chlorine gas into chlorine radicals limits the feasibility of reactors relying on this mechanism. The dependence of AQY and methane oxidation rates on a number of design variables is poorly understood; these include the wavelength of UV light, airflow rates, reflection of light within the reactor, humidity, and the gas composition. 

The primary goal, by which success will be measured, is improvement of AQY. To do so, design choices will be made based on the understanding of AQY’s dependence on the variables above, so another indicator of success will be the determination of quantitative relationships between AQY and variables that influence it.

Furthermore, the production of unintended oxidation products (such as CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄) should be assessed in different experimental conditions. Understanding the factors that drive the production of these gases will be critical to the future acceptability of such a technology. Measuring these unintended products and making reactor design choices to minimize their production will be a secondary key goal.

If successful, the understanding of atmospheric methane oxidation would be improved in the following ways:

• Improved understanding of gas-phase chemistry reactions (and rates) relevant for chlorine radical generation and the byproducts produced.
• A rigorous quantification of the AQY that is required for photolytic methane oxidizing reactors to be climate beneficial and cost-effective for methane removal at atmospheric concentration (2 ppm).
• Comparisons between gas phase AQY determined for photolytic reactors with other methane oxidizing reactors, such as hydroxyl radical-based reactors and solid photocatalyst-based reactors that require mass transfer at their surfaces.