Designing reliable and accurate isotope-tracer experiments for CO2 photoreduction

The photoreduction of carbon dioxide (CO 2 ) into renewable synthetic fuels is an attractive approach for generating alternative energy feedstocks that may compete with and eventually displace fossil fuels. However, it is challenging to accurately trace the products of CO 2 photoreduction on account of the poor conversion efficiency of these reactions and the imperceptible introduced carbon contamination. Isotope-tracing experiments have been used to solve this problem, but they frequently yield false-positive results because of improper experimental execution and, in some cases, insufficient rigor. Thus, it is imperative that accurate and effective strategies for evaluating various potential products of CO 2 photoreduction are developed for the field. Herein, we experimentally demonstrate that the contemporary approach toward isotope-tracing experiments in CO 2 photoreduction is not necessarily rigorous. Several examples of where pitfalls and misunderstandings arise, consequently making isotope product traceability difficult, are demonstrated. Further, we develop and describe standard guidelines for isotope-tracing experiments in CO 2 photoreduction reactions and then verify the procedure using some reported photoreduction systems. Current verification of CO 2 photoreduction products using isotope-tracer methods is insufficient and can provide false-positive results. Here, rigorous protocols and case studies are presented to avoid pitfalls in isotope-tracer experiments.