Compared to electrocatalytic and thermocatalytic CO2 reduction, the reaction rate of photocatalytic CO2 reduction is relatively slow. A thorough analysis of the reaction kinetics and rate-determining steps is required to design rational photocatalysts or reaction systems to enhance the efficiency of photocatalytic CO2 reduction.
Currently, two models that fit well with experimental data are the Microkinetic model and Langmuir-Hinshelwood (L-H) model. Both models describe the reaction mechanisms based on the theory of adsorption sites, involving gas adsorption and reaction processes on the surface of photocatalysts. They also consider adsorption, dissociation, and association of reactant molecules on the surface, and can be used to analyze reaction pathways and determine reaction rates.
In terms of analysis of intermediate species, the L-H model is simpler and more direct, usually considering adsorption and generation of a single intermediate species. In contrast, the Microkinetic model can consider adsorption and generation of multiple intermediate species, providing a more detailed description of reaction pathways.
In addition, the L-H model usually assumes that the amount of adsorption does not change with changes in the surface coverage of the catalyst, i.e., adsorption and reaction time are independent. The Microkinetic model can consider the influence of surface coverage on the adsorption energy, more realistically describing the adsorption process.
Furthermore, the L-H model usually assumes that the reaction rate constant is the apparent rate constant, i.e., proportional to the concentration of adsorbed species. The Microkinetic model can describe the relationship between adsorbed and reactive states through kinetic parameters, more accurately representing reaction rates.
In general, the Microkinetic model is more comprehensive and detailed, suitable for complex catalytic reactions, providing more physical and chemical information. The L-H model is simpler and suitable for simple reaction systems. In practical applications, the choice of which model to use should be based on specific situations and needs.
The Microkinetic model is based on the physical and chemical reaction processes of individual molecules on the surface of the photocatalyst. To simplify calculations, this model typically considers only the major gaseous products in actual applications: CH4, CO, and H2, excluding other hydrocarbons. It also assumes that all reactions are first-order kinetics.
The adsorption rate of gas can be calculated using formula 1:
The desorption rate of gas can be calculated using formula 2:
After calculating the adsorption and desorption rates of the gas, the consumption rate of a single reaction gas or the production rate of a single gas product in the entire gas reaction process can be expressed using formula 3:
Below is a brief description of the usage of the Microkinetic model using CO2 as an example.
The CO2 photocatalytic reduction process on the surface of the photocatalyst mainly involves three processes: reversible adsorption and desorption of CO2, and the reaction of hydrogen atoms adsorbed by adsorption. The reaction products are CO or CH4[1].
Therefore, the reaction rate of CO2 can be calculated using the following formula 5-1:
The reaction rates of other substances such as CO, CH4, active sites, etc., can be calculated using formula 5 and the reactions involved; refer to the literature[1] for details.
The L-H model is based on the relationship between adsorption of reactants and reaction rates on the surface of the photocatalyst. It is the most commonly used model for explaining multiphase catalytic dynamic processes, and its empirically derived rate expression is shown in formula 6:
Where, θA and θB represent the surface coverage of reactants, PA and PB represent the partial pressure of reactants, with units in Pa, k is the rate constant, and KA and KB are adsorption equilibrium constants for reactants, which are temperature-dependent.
Here is a brief description of the usage of the L-H model using the example of photocatalytic CO2 reduction.
Under conditions where H2O is involved in the reaction, the main products of CO2 reduction are CH4 and CO[2].
Assuming that only reactants are adsorbed on the surface of the photocatalyst and all products are desorbed immediately after the chemical reaction, formula 7 can be further simplified to formula 8:
*The effect of light intensity on photocatalytic reactions is usually described as a reaction order. The reaction order represents the degree of dependence of the reaction rate on light intensity. The reaction order of light intensity can be determined based on experimental data or theoretical predictions.
The information provided above is from literature, and the editor has compiled it. If there are any errors, please point them out promptly!
Article Information
Som, I. and M. Roy*, Recent development on titania-based nanomaterial for photocatalytic CO2 reduction: A review [J]., Journal of Alloys and Compounds, 2022. 918: p. 165533.
Article Link:
https://www.sciencedirect.com/science/article/pii/S0925838822019247?via%3Dihub
