Advanced materials for photocatalytic carbon dioxide reduction and ammonia production
Abstract
Among the most important megatrends for the life on Earth nowadays are climate changes, clean energy and sustainable environment. To prevent dramatic climate changes, the anthropogenic emissions of greenhouse gases have to be limited. Carbon dioxide is one of the greenhouse gases, responsible for climate changes. Its global emissions in 2022 reached 36.6 Gt. Despite many successful CCS (carbon capture and storage) technologies, CCU (carbon capture and utilization) processes should be developed, enabling a transformation of CO2 into useful products, as carbon monoxide, methanol, methane or other hydrocarbons. Our research in the field is focused on the application of photocatalysis to CO2 reduction, carried out on microporous carbon spheres decorated with semiconductor oxides (mainly TiO2 and ZnO). The role of the oxide is to enhance the photocatalytic reduction, while that of carbon spheres is to enhance the CO2 adsorption and inhibit charge recombination. The modification with semiconductor oxides was performed in »one pot« synthesis or after the synthesis of carbon spheres, through impregnation. The photocatalytic tests were performed using a gas-phase or liquid-phase photocatalytic reactor with the bed in the form of a UV transparent glass fiber cloth coated with a photocatalyst. The photocatalytic properties of the composites containing titania or zinc oxide were compared. Hydrogen (product of the CO2 mediated water splitting), carbon monoxide, and methane were produced in the photoreduction process [1]. It was found that TiO2/CS and ZnO/CS composites showed similar activity in carbon dioxide reduction process. However, the higher addition of carbon spheres was more beneficial for the zinc-containing material. We stated that a decrease of ZnO particle size 50 um to 26 nm was favorable for the carbon dioxide photoreduction process efficiency. Additionally, good results were obtained also in the case when zinc oxide was totally dissolved in the liquid phase. Such a homogeneous method of carbon dioxide reduction and hydrogen production was declared under the Polish patent [2]. Photocatalysts based on commercial titania (P25) decorated with various metal compounds were investigated as well and platinum, ruthenium or copper addition were the most efficient. Under specific conditions of the photocatalytic process, green ammonia formation [3] from nitrogen and hydrogen (generated in the photocatalytic process) can occur as well. The bed in the reactor is located just above the water surface and the produced ammonia is easily absorbed in water and continuously separated from the gas phase, shifting the ammonia synthesis reaction equilibrium towards the product. Combining carbon dioxide reduction and low-temperature nitrogen fixation to produce ammonia in one photocatalytic process would be an interesting challenge for the future.
