A novel gas flow-through photocatalytic reactor based on copper-functionalized nanomembranes for the photoreduction of CO2 to C1-C2 carboxylic acids and C1-C3 alcohols

A novel concept of gas flow-through photo(electro)catalytic reactor based on copper-functionalized nanomembranes for the conversion of CO2 is presented. The nanomembranes are based on aligned TiO2 nanotube arrays grown over a microperforated metallic substrate, acting as an electron collector and to provide the necessary robustness, which are then functionalized by copper oxide (Cu2O or CuO) electrodeposition. For comparison, the behavior of copper oxide electrodeposited over the bare microperforated metallic substrate is also given. The concept is quite different from conventional photocatalytic approaches. Due to the peculiar characteristics and conditions in the photoreactor (working under a cross-flow of gaseous CO2 saturated with water passing through the nanomembrane photocatalyst layer), it is possible to evidence for the first time the highly selective CO2 conversion to C1-C2 carboxylic acids (formic, acetic and oxalic acids) without formation of H2, CO, CH4 or other hydrocarbons. Copper-oxide introduces an additional reaction pathway to C1-C3 alcohols (methanol, ethanol and isopropanol) or derived products (methyl formate). The best performances are obtained with p-type Cu2O over n-type TiO2 nanotubes (Faradaic Efficiency -FE- = 42% to methanol and 44% to acetic acid) due to the creation of a p-n-type heterojunction that improves visible light harvesting, giving an apparent quantum yield (ratio between electrons reacted and photons absorbed) with solar illumination of about 21%. FE up to 47% to methanol or 73% to acetic acid are observed among the other tested samples. The relevance of these results to the mechanism of CO2 conversion is also discussed.