Experimental Study on the Photocatalytic Reduction of CO2 by Fe2O3/BiOIO3 Composite Photocatalyst

Photocatalytic reduction of CO2 to produce renewable hydrocarbon fuels is of great significance for solving the greenhouse effect and energy crisis. However, due to some shortcomings of the photocatalyst itself, further research is needed to realize its industrial application. In this article, we prepared Fe2O3/BiOIO3 composite photocatalyst by a simple one-step hydrothermal method, and discussed the photocatalytic activity and reaction mechanism of Fe2O3/BiOIO3 composite photocatalyst through a series of characterization and photocatalytic reduction experiments. The band gap energy of the composite photocatalyst (2.32 ev) is smaller than that of pure BiOIO3 (2.98 ev), so the composite photocatalyst can absorb visible light. Since the edge of the conduction band of BiOIO3 is more positive than the edge of the Fe2O3 conduction band, the photogenerated electrons in the Fe2O3 conduction band can be transferred to the BiOIO3 conduction band, which makes BiOIO3 more reducible and can reduce CO2 to CO. This study can provide some reference for the field of photocatalytic reduction of carbon dioxide.


Introduction
With the rapid development of human society, the demand for energy is increasing, which makes the excessive use of fossil fuels, which causes a large amount of greenhouse gases to be discharged into the air [1,2] . Carbon dioxide accounts for about half of the total greenhouse effect. Therefore, effective and innovative means are needed to keep the global net CO 2 emissions within an acceptable range. Among the semiconductors that have been studied for photocatalytic reduction of CO 2 , BiOIO 3 has good product selectivity and photocatalytic activity [3] . Nevertheless, pure BiOIO 3 as a photocatalyst also has some limitations. For example, pure BiOIO 3 has a wide band gap width and a small specific surface area. In order to overcome these shortcomings, researchers often use the method of preparing composite photocatalysts to regulate and control BiOIO 3 [4,5] . For example, Zeng et al. prepared Fe 2 O 3 /BiOIO 3 composite photocatalyst. In this study, the band gap energy of pure BiOIO 3 is 3.0 ev, and the band gap energy of the composite photocatalyst is 1.81 ev, which indicates that the photocatalytic activity of the composite photocatalyst is stronger than that of BiOIO 3 [6] . Guan et al. successfully prepared BiOIO 3 /g-C 3 N 4 /MoS 2 ternary composite photocatalyst. In this study, due to the in-situ electronic channel, the internal electric field was formed through the corresponding band gap engineering to improve the photocatalytic reaction [7] . Huang Hedong reported a new type of dual nonlinear optical material BiOIO 3 , the research is that the layered structure of BiOIO 3 leads to the formation of an internal automatic electric field, which enhances the charge separation efficiency [8,9] . Due to the small size of Fe 2 O 3 particles, the surface area occupies a large volume percentage, the bonding and electronic states of the surface are different from the inside of the particles, and the coordination of surface atoms is different, which leads to an increase in active sites on the surface. And the band gap energy of Fe 2 O 3 (2.2 ev) is small, which can be activated under visible light. Therefore, in this paper, Fe 2 O 3 /BiOIO 3 composite photocatalyst was prepared by a simple one-step hydrothermal method. A series of characterizations were used to analyze the microscopic morphology, band structure and optical properties of the composite photocatalyst. And the composite photocatalyst was used in the experimental study of photocatalytic reduction of CO 2 , and the photocatalytic activity and reaction mechanism of the composite photocatalyst were discussed.

Preparation of sample
BiOIO 3 was prepared by hydrothermal method, and the hydrothermal condition is hydrothermal at 160°C for 10 h. A certain amount of iron oxide was added to deionized water and stirred for 30 minutes. After being fully dissolved, BiOIO 3 with different mass ratios was added, and stirring was continued for 30 minutes. Then the mixed solution was placed in a stainless steel high-pressure hydrothermal reactor lined with 100 Teflon and heated at 160℃ for 10 hours. Finally, the obtained sample was washed three times with deionized water and ethanol, and then the above sample was placed in a drying box at 80℃ for 12 h and ground into powder. The mass ratios of Fe 2 O 3 and BiOIO 3 in the prepared catalyst were 1, 0.1 and 0.05, respectively, named FB-1, FB-0.1 and FB-0.05, respectively.

Results and discussion
The detailed crystal structure of the pure BiOIO 3 and Fe 2 O 3 /BiOIO 3 composite photocatalyst (FB-X, X = 1, 0.1, 0.05) sample was studied by XRD technology. The results are shown in Figure. 1. The diffraction peaks of pure BiOIO 3 are consistent with the diffraction peak data of orthorhombic BiOIO 3 crystals (ICSD # 262019) [10] . All patterns of the samples show sharp diffraction peaks, indicating that all samples are highly crystalline [11] . With the increase of Fe 2 O 3 , the intensity of the (121) peak first decreases and then increases. When the sample is FB-0.05, the diffraction peak intensity is the weakest. The results show that an appropriate amount of iron oxide will affect the crystallinity.  Figure. 2b shows the lattice fringes of the sample FB-0.1. Two kinds of lattice fringes are analyzed in Figure. 2b. The lattice fringe spacing in the upper part of the figure is 0.368 nm, and the corresponding crystal plane is the (017) crystal plane of Fe 2 O 3 [12] . The lattice fringe spacing in the lower left corner of  Figure. 2b is 0.289 nm, and the corresponding crystal plane is the (002) crystal plane of BiOIO 3 [13] . Figure. 2c-f is the element map, showing the spatial distribution of atoms, corresponding to Figure.  (1) αhν A hν Eg / (2) In the equation, the value of n is 4, which depends on the characteristics of the indirect optical transition in BiOIO 3 [14] . Then, by intersecting the tangent to the inflection point of the curve with the photon energy axis, the Eg value of the sample can be determined from the graph of (αhν) 1/2 and hν. Then, by stretching the linear part of the curve, the Eg value of the sample can be fixed according to vs. As shown in Figure.

Photocatalytic reduction of CO 2 activity test
In order to study the photocatalytic reduction of CO 2 activity of different samples, the experiment was carried out in a photocatalytic online analysis system. The light source used in the experiment was a 300 W xenon lamp and equipped with a filter with λ>420 nm to simulate visible light. The product in the experiment was Gas chromatography (GC) for qualitative and quantitative analysis. At the same time, comparative experiments were carried out, 1) the photocatalysis experiment was carried out under the condition of no catalyst; 2) the photocatalysis experiment was carried out in the dark; 3) the photocatalysis experiment was carried out with nitrogen instead of CO 2 . No CO product was detected in the above control experiment, indicating that the light source and photocatalyst were not replaceable in the CO 2 photocatalytic reduction process, and the CO product produced was derived from the gaseous CO 2 in the experiment. Figure. 4a shows the product yield of the photocatalytic reduction of CO 2 by prepared samples without sacrificial agents. From the figure, we can see that the main products of composite photocatalytic reduction of CO 2 are CO. The composite material FB-0.1 showed the highest CO yield of 5.2 μmol g -1 h -1 , which was about 1.6 and 9.8 times higher than the CO yields of other materials BiOIO 3 and Fe 2 O 3 . It is consistent with the results of the analysis of the optical properties of the sample, which proves that the optical properties and electronic structure of the semiconductor affect its photocatalytic activity. In the cyclic test of the photocatalytic reduction of CO 2 with sample FB-0.1, as shown in Figure. 4b, the output of the composite material did not decrease significantly under the 6 cyclic tests in 24 hours, which indicates that the photocatalyst is highly stable Sex. Figure. 4 (a) Product yield of all samples, (b) Cycle test of sample FB-0.1 As for the influence of the selectivity of the photocatalytic product, it can be obtained from the above analysis of the optical properties of the composite material. It can be clearly observed that the conduction band of the composite material gradually decreases with the increase of the relative proportion of Fe 2 O 3 . All composite materials The conduction band potential energy of CO 2 is more negative than the reduction potential of CO reduction, so the conduction band potential is adjusted to an appropriate position, so that the composite material can selectively reduce CO 2 to CO.

Analysis of the mechanism of photocatalytic reduction of CO 2
Under visible light irradiation, Fe 2 O 3 can be excited to generate electrons and holes in the Fe 2 O 3 /BiOIO 3 composite material. A heterojunction is formed between Fe 2 O 3 and BiOIO 3 , which facilitates the separation of photo-generated electrons and holes. Since the edge of the conduction band of BiOIO 3 is more positive than that of Fe 2 O 3 , the photogenerated electrons in the conduction band of Fe 2 O 3 can be transferred to the conduction band of BiOIO 3 , which makes BiOIO 3 more reducible and can reduce CO 2 to CO. As shown in Figure. 5 Figure. 5 Mechanism diagram of photocatalytic reduction of CO 2 with composite photocatalyst

Conclusions
In this chapter, a one-step hydrothermal method is used to prepare Fe 2 O 3 /BiOIO 3 composite photocatalyst, and a series of characterizations are used to analyze its phase composition, microscopic morphology and optical properties. The analysis of the phase composition shows that with the increase of Fe 2 O 3 content, the crystallinity of the composite material changes, and the micro morphology of the composite material also changes, which means that the adsorption capacity of the composite material changes. The analysis showed that the Fe 2 O 3 /BiOIO 3 composite photocatalyst formed a heterojunction after hydrothermal treatment. In the experimental study of visible light photocatalytic reduction of CO 2 , the composite material FB-0.1 showed the highest carbon monoxide yield of 5.2 μmol g -1 h -1 , which was consistent with the results of the sample's optical characteristics analysis. The transfer of photogenerated electrons effectively enhances the charge separation. In this case, most electrons can be driven to the conduction band of BiOIO 3 , selectively reducing CO 2 light to CO.