The Efficiencies and Products of Dilute Methane Oxidation in a Chlorine Radical Photoreactor.

A potential approach for mitigating the climate impact of hard-to-abate, dilute (less than 1000 ppm; ppm) methane (CH4) is gas-phase advanced oxidation (GPAO). Here, we present experimental results from a benchtop GPAO reactor oxidizing CH4 with chlorine radicals (Cl·) produced from the photolysis of chlorine gas (Cl2), a process we term "Cl2-GPAO." We find that at CH4 concentrations from 2 to 90 ppm, supplying Cl2 in a ratio of 1:1 with CH4 uses photogenerated Cl· efficiently, with 3 Cl· photogenerated for each CH4 oxidized with only 20% of Cl· recombining to Cl2. When more Cl2 is added, the product balance shifts from mainly carbon monoxide to mainly carbon dioxide. Cl2-GPAO performs similarly under relative humidities between 5 and 60%. In the presence of ppm-level ammonia, toluene, nitric oxide, and hydrogen sulfide, CH4 conversion decreases, suggesting that gaseous contaminants compete with CH4 for Cl·. Gas chromatography-mass spectrometry revealed that Cl2-GPAO with 30 ppm CH4 and Cl2 produces 1.05 ± 0.32 part per billion (ppb) methyl chloride and 1.25 ± 0.38 ppb chloroform. Methylene chloride and carbon tetrachloride were not detected. We discuss the implications for cost-effective and climate-beneficial Cl2-GPAO at scale.