Reliable quantification of liquid oxygenated products in CO2/CH4 plasma-catalytic conversion

• Oxygenate quantification methods strongly affect conclusions on catalytic performance. • Indirect subtraction method overestimates oxygenate selectivity by up to 10 × . • Cold bath recovers only 50% alcohols and 10% ketones, underestimating selectivity. • Direct oxygenate quantification with a heated transfer line is more reliable. • Oxygenate selectivity remains low (< 3 %) and barely depends on the catalyst. The direct conversion of CO 2 and CH 4 into oxygenated products via plasma catalysis offers a promising route for biogas valorization, generating molecules usable as fuels or fuel additives, or convertible into energy carriers. However, many studies lack rigor in the quantification of oxygenate selectivity, often leading to overestimations and misleading conclusions regarding catalyst performance and energy efficiency. Typically, gaseous products are analyzed online, while liquid products are collected and analyzed offline. Oxygenate selectivity is often estimated indirectly by subtracting selectivities of other carbon products from 100%, a method that lacks accuracy and tends to overestimate performance. This work highlights the need for accurate and reliable analytical approaches to provide a realistic assessment of plasma-assisted CO 2 and CH 4 conversion and guide future research. An improved experimental setup is proposed, keeping all products in the gas phase using a heated transfer line and enabling direct online quantification by gas chromatography directly after the reactor. This approach minimizes errors from incomplete condensation and indirect calculations, achieving a carbon balance close to 95%. Beyond analytical considerations, the conversion of CO 2 and CH 4 into oxygenated products by plasma catalysis remains a challenge. Results indicate that while catalysts enhance overall conversion, their effect on oxygenate selectivity is limited. These findings suggest that future work should focus on tailoring plasma conditions rather than solely on catalyst optimization. Overall, this study emphasizes the importance of robust analytical methods as a foundation for better understanding reaction mechanisms and for guiding future research on energy conversion by plasma catalysis.