Carbon nitride-based catalysts for high pressure CO2 photoreduction

INTRODUCTION The metal-free polymeric catalyst, graphitic carbon nitride (g-C3N4) is a relatively novel catalyst compared to TiO2. The g-C3N4 is characterized by a graphitic 2D type structure and a narrow band gap (on average 2.7 eV) so, in principle it is suitable to be activated by visible light. Unfortunately, this photocatalyst often shows a moderate activity due to the high electron/hole recombination rate, which may be lowered through the deposition of noble metal particles on its surface. g-C3N4 is commonly prepared via direct thermal condensation of an organic precursor containing carbon and nitrogen, e.g. urea, thiourea, melamine, dicyanamide, etc. however the surface area of the produced material resulted low for most of the applications, usually less than 10 m2g-1. In order to increase the surface area several strategies have been developed, as chemical, mechanical or thermal exfoliation or the use of templates. The aim of this work is to prepare solar sensitive nanophotocatalysts based on g-C3N4, as such or loaded over TiO2 or WO3, to accomplish the photoreduction of CO2 at high pressure (up to 20 bar). EXPERIMENTAL/THEORETICAL STUDY Bulk graphitic carbon nitride (g-C3N4) was prepared by thermal condensation of melamine. Melamine was placed in a covered ceramic crucible and heated at 2 °C min-1 up to 500-600 °C. The resulting yellow powder underwent a successive heating or sonication treatment, at different temperature or ultrasound intensity, in order to obtain an exfoliated material showing an increased specific surface area. Binary materials composed of C3N4 and TiO2 P25 or WO3 were prepared by mechanically mixing both components in different proportions. All the tests for the photoreduction of CO2 were performed using an innovative pressurized batch photo-reactor, designed to work under pressure up to 20 bar and temperatures not higher than 90 °C. Irradiation is accomplished by means of a 125 W medium pressure Hg vapour lamp made of two bulbs which emits in the range of 254-364 nm (average irradiance 152.63 W/m2). The optimal catalyst and HS concentration were 31 mg·L-1 of photocatalyst and 1.67 g·L-1 of hole scavenger (Na2SO3). Each test lasted for 24 h if not specified differently. The products in liquid phase were analysed via HPLC (LC-4000 series, Jasco) and the gas phase products by GC (Agilent 7890). RESULTS AND DISCUSSION An increased pressure showed an increase of the productivity, with a greater effect on the liquid product, i.e. formic acid production. Graphitic carbon nitride showed a performance comparable with bare P25. On the other hand, it was observed that when TE was combined with TiO2 the overall CO2 conversion to regenerated fuels improved. The composite material showed a much greater productivity of formic acid with respect to both its constituents. The reason must be searched in the interaction between the phases involved, that is titania-carbon nitride-water and synergistic effects among them. The exfoliating ultrasound power directly affected the surface area and structure of carbon nitride, achieving better performance with increasing power. The production of adducts with WO3 also proved effective to achieve high formic acid productivity. CONCLUSION Visible responsive materials were prepared by mixing g-C3N4 with TiO2 or WO3. The coupling of visible responsive materials and high pressure photoreactor allowed unprecedented activity for the photoreduction of CO2. ACKNOWLEDGMENTS The authors are grateful to Fondazione Cariplo (Italy) for supporting this research through the grant 2021-0855 - “SCORE - Solar Energy for Circular CO2 Photoconversion and Chemicals Regeneration” within the Circular Economy call 2021.