The photocatalytic reduction of CO2 has the characteristics of green, mild conditions and abundant raw materials, so it is considered as one of the effective ways to achieve "carbon peak" and "carbon neutralization". Limited by conversion and selectivity problems, the current research on photocatalytic CO2 reduction is still in the laboratory stage. In addition to the development and rational design of efficient catalysts, the efficient transformation of CO2 can also be realized by optimizing the reaction process and changing the reaction conditions.
In general, photocatalytic CO2 the reduction reaction mainly takes place in the gas or liquid phase。
The liquid phase reaction system takes place in a saturated solution of CO2, in which the catalyst is uniformly dispersed in the solution.
In the gas reaction system, the photocatalyst is fixed on the substrate support, and the mixture of CO2 and water vapor directly reacts with the photocatalyst, as shown in Figure 1。
Figure 1. Comparison of photocatalytic CO2 reduction models in gas and liquid phases.
In the liquid phase reaction system, the charge transfer efficiency and heat transfer efficiency are higher because the solid catalyst dispersed in the solution is always in the agitation state [3, 4]. However, in the liquid phase reaction system, the limited solubility and diffusion coefficient of CO2 in H2O limit the mass transfer efficiency of the photocatalytic CO2 reduction reaction。
At 25℃ and 101.325 kPa, the solubility of CO2 in H2O is less than 0.033 mol·L-1, which weakens the diffusion of CO2 molecules from gas to the surface of the photocatalyst。
Compared with neutral and acidic conditions, THE solubility of CO2 under alkaline conditions is higher , and the solubility of CO2 can be improved by increasing the pH value of the solution. Organic solvents such as acetonitrile (ACN)  and ethyl acetate (EAA)  can also be added to H2O to promote the dissolution of CO2.
In order to solve the above problems, researchers proposed a photocatalytic CO2 reduction reaction in the gas phase. Compared with liquid phase reaction, gas reaction is not affected by sacrificing agent, photosensitizer, solvent and other factors, and is a relatively simple reaction system.
The diffusion coefficient of CO2 in the gas phase is about 0.1 cm2·s-1, which is about four orders of magnitude higher than that in the liquid phase [9, 10]. Therefore, the mass transfer efficiency between CO2 and photocatalyst is higher in the gas phase reaction.
Another advantage of gas-phase photocatalytic CO2 reduction reaction is that it can effectively inhibit hydrogen evolution reaction [2, 11]. Since the reduction of H2O to H2 is more advantageous in terms of kinetics and dynamics, photocatalytic CO2 reduction in liquid phase reaction may induce hydrogen evolution reaction and reduce the conversion rate of CO2 [1, 6]. The photocatalytic CO2 reduction in gas-solid reaction can effectively solve this problem.
At present, the gas reaction in the photocatalytic CO2 reduction reaction is mainly divided into two ways. One is to coat the photocatalyst on the substrate to form a film, and CO2 with a certain humidity flows through the upper layer of the film, as shown in Figure 2 (a). The other is fixed-bed gas reaction, in which CO2 with certain humidity directly passes through the photocatalyst bed, as shown in FIG. 2 (b). Compared with the first method above, the mass transfer effect of the fixed bed method is more sufficient, which helps to improve the photocatalytic CO2 conversion rate.
Figure 2. Film gas reaction mode (a) and fixed bed gas reaction mode (b).
In order to meet the demand of gas-phase photocatalytic CO2 reduction reaction, Pofila Technology Co., Ltd. launched an online gas-solid photocatalytic reactor for temperature measurement, which is mainly suitable for our LabSOLAR-6A all-glass automatic online trace gas analysis system (hereinafter referred to as LabSOLAR-6A system), as shown in Figure 3. On-line temperature measurement gas-solid photocatalytic reactor is different from passive diffusion, gas-solid reactor adopts gas "penetration" scheme, combined with magnetic drive plunger pump in LabSOLAR-6A system, so that CO2 and catalyst fully contact, improve mass transfer efficiency, improve reaction conversion rate.
Figure 3. Actual scene of labSOLAR-6A gas-solid photocatalytic reactor equipped with online temperature measurement (Hunan University).
In addition to photocatalytic CO2 reduction, the gas-solid photocatalytic reactor is also suitable for photothermal CO2 reduction. The reactor is equipped with a special in situ infrared temperature measuring port, non-contact real-time measurement of catalyst surface temperature and record, at the same time equipped with a constant temperature jacket to minimize heat dissipation, as shown in FIG. 4.
Figure 4. Actual scene of labSOLAR-6A system equipped with on-line gas-solid photocatalytic reactor.
Basic parameters of gas-solid photocatalytic reactor for on-line temperature measurement:
The reactor material: the reactor is high borosilicate glass, the light window is quartz glass;
Powder catalyst placement: tiled on the surface of the quartz filter membrane of the reactor;
Reactor volume: total volume (with cantilever) : 118 mL; Column volume: 96 mL;
Reactor size: flange outer diameter of 60 mm, total height of about 200 mm;
Temperature measurement range: 0~600℃;
Measurement accuracy: 0.1℃.
Figure 5. Gas-solid photocatalytic reactor and its accessories.