CO2 photoreduction with H2O to C1 and C2 products over perovskite films of alkaline niobates ANbO3 (A = Li, Na, K)

Abstract

CO2 photoreduction represents one promising solution for environmental sustainability to produce renewable fuels with photocatalysts, H2O and solar light. For this reaction, photocatalytic materials with perovskite structure showed adequate properties, such as enough negative conduction band potentials and efficient charge transfer due to its internal electric field. Thus, here is proposed the use of alkaline niobates ANbO3 (A = Li, Na, K) with perovskite structure as catalysts in the CO2 photoreduction. The materials were prepared as thin films by ink-jet printing, which is versatile and rapid technology for film manufacturing. Highly crystalline ANbO3 (A = Li, Na, K) films were grown on glass substrates that exhibited a variety of morphologies and absorption edges in the UV region. All the films showed activity for CO2 photoreduction favoring the formation of different C1 and C2 products such as HCOOH, HCOH, CH3OH, and CH3CH2OH. LiNbO3 film was selective for CH3OH formation (35 μmol/h), while NaNbO3 selective produced HCOOH (54 μmol/h). These results were related to the potentials of their conduction bands and the photocarrier density of each semiconductor. Particularly, a higher carrier density in LiNbO3 favored CH3OH generation since it requires more electrons than HCOOH to generate it; however, the formic acid production was higher than the methanol. Thus, since the NaNbO3 perovskite exhibited the highest production of solar fuels, it was chosen to be deposited on stainless steel and alkali-activated material substrates. This strategy allowed the increase of HCOOH production up to two times compared with the reference, which was associated with enhanced light absorption, more efficient charge transfer, and more active sites for CO2 adsorption. As a result, the best energy conversion efficiency (6.85%) and the highest apparent quantum yield (AQYHCOOH = 25.41%) obtained are higher than recent reports of CO2 photoreduction to renewable solar fuels. Furthermore, the stability and reuse of the best system (NaNbO3 deposited on the alkali-activated material) were demonstrated after five consecutive cycles of photocatalytic evaluation.