Performance analysis of monolith photoreactor for CO2 reduction with H2

In this study a monolith photoreactor was compared with a cell type for testing photocatalytic CO2 reduction with H2 as a reducing agent. The monolith channels were dip-coated with TiO2 nanoparticles and were characterized using XRD, SEM, BET and UV–Vis spectroscopy. The performance of monolith photoreactor for CO2 photoreduction was much higher in the presence of H2 as a reducing agent than H2O with cell density of 200 CPSI. The CO2/H2 molar ratio of 1.5 was optimum at which higher CO evolution was observed. The efficiency of monolith photoreactor in batch process for CH4 production was 6 times higher than TiO2 dispersed in a cell reactor. The production rates were in the following order: monolith-CH4 (69 μmole g−1 h−1) > monolith-CO (59 μmole g−1 h−1) > Cell-CH4 (12 μmole g−1 h−1) > Cell-CO (6 μmole g−1 h−1). The higher yield rates in monolith photoreactor were due to the larger illuminated surface area of its multiple microchannels and efficient light utilization compared to the cell type reactor. More importantly, the quantum efficiency for CH4 production over TiO2 supported monolith was much higher (0.042%) than the cell type reactor (0.0038%). The significantly improved quantum efficiency indicated higher photonic efficiency in the microchannel monolith photoreactor. On the other hand, CO was the main product in the continuous monolith photoreactor with lesser yield rate compared to a batch process. The higher yield rate in batch process was obviously due to accumulation of products over the entire reaction period. The reaction mechanism revealed that CO2 was initially converted to CO before transforming to hydrocarbons at elongated time and in the presence of multi-electron processes.