(307c) Optimization of Synthesis of Cu2O/Cu Photocatalyst Using Batch and Flow Process to Improve Photocatalytic Activity

Cuprous oxide (Cu₂O) has been widely applied in photocatalysis. This oxide is abundant and non-toxic and enables the use of visible light in the photocatalytic process. These important characteristics make this oxide a promising material for organic photoreaction and carbon dioxide photoreduction (1). However, the Cu₂O photoactivity is still limited due to its low stability and high photo-oxidation during the photocatalytic process. Because of these limitations, coupling metallic copper (Cu⁰) to Cu₂O, has been studied recently to increase the photocatalytic activity of this material (2). However, this combination of Cu₂O/Cu⁰ has been mainly synthetized using batch process. The batch process presents not only scalability and reproducibility issues, but also demands the manipulation of Cu₂O/Cu, which may result in changes in the oxidation state of copper. Because of this, the synthesis of nanoparticles using continuous flow systems has received attention lately (3). The flow synthesis improves the heat and mass transfer control and additionally avoids changes in concentration due to the evaporated solvent in the headspace of the reactor. These advantages of the flow synthesis have a direct impact in reproducibility and product consistency (4).

The reduction of copper acetylacetonate (Cu(acac)₂) using benzyl alcohol has been proven feasible for the synthesis of Cu₂O and Cu (5). In this synthesis, Cu⁺² is first reduced to Cu⁺ and if the reaction is maintained, Cu⁰ is produced. However, the intensification of this synthesis by using a continuous process and the testing of the obtained material in photocatalytic reaction has not yet been investigated.

In this project, the synthesis of Cu₂O/Cu using batch process was first investigated. The obtained materials were characterized using different techniques. X-ray diffraction (XRD), Thermogravimetric analysis (TGA) and X-ray Photoelectron Spectrometer (XPS) were used to determine the composition of the obtained photocatalyst. Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were applied to investigate the particle morphology and confirm the crystalline structure. Finally, UV-visible diffuse reflection spectroscopy (Uv-Vis DRS) and photocurrent response were used to evaluate the light absorption and photoactivity of the obtained material.

Then, a synthesis in flow of Cu₂O/Cu was developed based on the batch procedure. To optimize this synthesis, a design of experiment (DOE) approach was applied. In the DOE, temperature, residence time and initial concentration were varied. The impact of these variables was investigated by measuring conversion of the reaction, particle size, and photocatalytic activity of the obtained material. The obtained material was characterized using scanning electronic microscopy, X-ray diffraction and photocurrent response.

The results of these characterization techniques confirmed that nanoparticles of Cu₂O-Cu were produced as sole solid product. The synthetized Cu₂O nanoparticles were squares. The introduction of small amount of Cu⁰ in the photocatalyst structure improved the light absorption in the visible range and increased the photocurrent response. The use of DOE in the flow synthesis suggested that temperature and residence time were the most relevant variables in the synthesis. Therefore, by controlling temperature and residence time, it was possible to control the proportion between Cu₂O and Cu⁰ in order to optimize the photoactivity of this material. The obtained models from the DOE for the conversion and the particle size were statistically significant. These results open the possibility to couple the synthesis in flow to a continuous photocatalytic process avoiding any change in the catalyst composition that may occur when manipulated.

References

1. Rej S, Bisetto M, Naldoni A, Fornasiero P. Well-defined Cu2O photocatalysts for solar fuels and chemicals. Journal of Materials Chemistry A. 2021;9(10):5915-51.
2. Zheng Y, Zhang L, Guan J, Qian S, Zhang Z, Ngaw CK, et al. Controlled Synthesis of Cu0/Cu2O for Efficient Photothermal Catalytic Conversion of CO2 and H2O. ACS Sustainable Chemistry & Engineering. 2021;9(4):1754-61.
3. Zardi P, Carofiglio T, Maggini M. Mild Microfluidic Approaches to Oxide Nanoparticles Synthesis. Chemistry – A European Journal. 2022;28(9):e202103132.
4. Kusada K, Kitagawa H. Continuous-flow syntheses of alloy nanoparticles. Materials Horizons. 2022;9(2):547-58.
5. Staniuk M, Zindel D, van Beek W, Hirsch O, Kränzlin N, Niederberger M, et al. Matching the organic and inorganic counterparts during nucleation and growth of copper-based nanoparticles – in situ spectroscopic studies. CrystEngComm. 2015;17(36):6962-71.