Modelling and experimental validation of the photocatalytic CO2 reduction in a TiO2 slurry

BALLARI, MARÍA DE LOS MILAGROS; EDELMANNOVÁ, MIROSLAVA; RICKA, RUDOLF; RELI, MARTIN; KOCÍ, KAMILA

Rostock

Conferencia; The 25th International Conference on Semiconductor Photocatalysis and Solar Energy Conversion (SPASEC-25); 2022

One of the most addressed problems of nowadays is energy production from non-renewable fossil fuels. Not only the fossil fuels are beginning to be scarce but their combustion produces, among others, carbon dioxide. This is a primary greenhouse gas, with approximately 31.5 billion tons generated worldwide per year from fossil fuels combustion. So, photocatalytic reduction of carbon dioxide seems to be a promising way how to address both society problems. This process does not require an extra input energy, except solar radiation, and it is considered as the most attractive route for the transformation of CO2 in the long term. Unfortunately, the photocatalytic conversion of carbon dioxide is still very low. This reaction is a very complicated combination of photophysical and photochemical processes. Formation of the desired products - methane or methanol - is more difficult than formation of other possible products like carbon monoxide, formaldehyde and formic acid, because more electrons are required for the former reactions. In addition, the water splitting reaction competes for the electrons in a reductive media producing hydrogen. In photocatalytic applications, CO2 conversion and yield rates can only be enhanced if equal importance is given to both efficient photocatalysts and optimal design of the photoreactors. In this work, a kinetic study of the photocatalytic CO2 reduction is proposed, which can be useful for the photoreactor optimization to carry out the process. The experimental work was carried out in a TiO2 slurry that was saturated with CO2 and irradiated by a UV-C lamp. The product yields, mainly hydrogen, methane, and carbon monoxide, were followed in the gas phase by gas chromatography. Simplified kinetic expressions were proposed based on the photocatalytic complex reaction mechanism of the photocatalytic CO2 reduction and including the dependence with the radiation field in the TiO2 suspension. The mass balance equations in the liquid and gas phase considering the interphase mass transfer were solved and the kinetic and mass transport parameters were estimated by employing the experimental information. The main studied operating conditions were the magnetic stirrer speed and the TiO2 photocatalyst mass concentration. A good agreement between the experimental data and mathematical model predictions was observed. The fastest stirring speed in the liquid phase guarantees quicker species transport to the gas phase. On the other hand, the best products yield was observed for the lowest tested photocatalyst concentration, which presents better irradiation distribution in the reaction media and less particles agglomeration providing higher available active surface for the reaction. With a rigorous modelling methodology for photocatalytic systems, and from a relatively simple kinetic model, the obtained results in this work are valuable for the scaling up, optimization and performance prediction of the photocatalytic CO2 reduction process.