Photo-thermochemical decomposition of carbon-dioxide in a direct solar receiver-reactor

The synergistic combination of the spectral and intensity features of solar energy can potentially lead to chemical synthesis processes under milder temperature conditions than those in solar thermochemical processes. A direct solar receiver-reactor is built and used for the catalytic photo-thermochemical processing of carbon dioxide. The reactor is designed to be representative of field application solar reactors by depicting fluid flow, heat transfer, mass transport, and radiation transport effects, often suppressed in reactors aimed to fundamental chemical kinetics studies. Two different types of catalytic monoliths, tubular quartz and zirconia foam, are tested with coatings of 1 and 2 wt% Cu-doped TiO₂. The zirconia foam monolith is optically thick and has more than four times higher coating mass compared to the tubular quartz monolith; nevertheless, the latter, while being optically thin, leads to four times higher CO production, with the 1 wt% coated monolith producing the highest CO yield. Moreover, the experimental results show that the CO production rate increases almost exponentially with increasing incident solar radiation flux. A computational fluid dynamics (CFD) model, describing fluid flow, radiation transport, and heterogeneous photo- and thermo-chemistry, is used to analyze the CO₂ decomposition process in the 1 wt% tubular quartz reactor. The modeling results provide assessment of the photochemical and thermochemical effects, and suggest the synergistic enhancement due to the photolytic mechanism on the thermochemical process. Solar photo-thermochemical processing of CO₂ using an optically and catalytically tuned catalytic monolith can lead to higher decomposition yields than photochemical processes, potentially comparable to those of solar thermochemical approaches while operating at lower temperatures.