硅酸盐学报

光催化CO₂还原技术利用太阳能将CO₂转化为燃料(如CO、CH₄等)，为解决气候变化和能源短缺问题提供可能，但其实际应用仍受限于高成本、低产率等问题。本工作采用一步水热法成功制备了一种C₄₈H₃₀N_6O₃₂Zr₆(UiO-66-NH₂)/硅藻土复合材料，并采用X射线衍射、红外光谱、扫描电子显微镜、X射线光电子能谱、原位漫反射傅里叶变换等表征手段对复合材料理化特性进行分析，系统研究了复合材料光催化CO₂还原的性能与机理。结果表明：当UiO-66-NH₂的理论负载量为60%时，所制备的复合材料光催化CO₂还原性能相对最优；优化条件下制备的UND-60%复合材料表现出优异的光还原CO₂性能(6.69μmol·g^–1·h^–1)，为纯UiO-66-NH₂(4.57μmol·g^–1·h^–1)的1.46倍；复合材料光催化CO₂还原性能的提升主要归因于硅藻土作为载体有效增强了UiO-66-NH₂的分散性，为光还原反应提供了更多的活性位点；此外，Ui O-66-NH₂/硅藻土复合材料带隙能的降低、光生载流子的迁移和分离效率的提升同样有助于提升其光催化性能。

Introduction UiO-66-NH₂ is used as a photocatalyst for CO₂ reduction, but it still faces some problems such as a narrow light absorption range, easy agglomeration, and a high recombination rate of electron-hole pairs. Various modification strategies(i.e., metal doping, introducing cocatalysts, and constructing heterojunctions with semiconductors) are proposed to enhance their photocatalytic efficiency. Although these methods can effectively improve the performance of the catalysts, the high synthesis cost of MOFs still restricts their practical application. Using widely available and low-cost natural non-metallic minerals as supports to develop high-performance composite catalysts can significantly reduce the production and application costs of catalysts and effectively enhance their application performance. Compared to conventional three-dimensional porous materials, diatomite has abundant active hydroxyl groups on its surface, which are the main reaction sites for surface grafting and coordination, having a good affinity for organic ligands. In addition, the alkaline centers on the surface of diatomite also have a certain catalytic assistance effect. In this paper, a purified diatomite was used as a support, and UiO-66-NH₂ was anchored on the surface and in the pores of diatomite by an one-step hydrothermal method to prepare UiO-66-NH₂/diatomite composite, thereby providing an idea for the development of low-cost and high-performance photocatalytic CO₂ reduction materials. Methods In the synthesis of pure UiO-66-NH₂, ZrCl₄(0.170 g) and 2-aminoterephthalic acid(0.132 g) were dissolved in a mixed solvent of 24 m L of DMF(N,N-Dimethylformamide) and 6 mL of CH₃ COOH. After magnetic stirring for 2 h, the solution was transferred to a 100 mL autoclave and reacted at 120 ℃ for 24 h. After reaction completion, the sample was cooled naturally to room temperature. Subsequently, the product was centrifugally washed for three times with DMF and for three times with C₂ H₅ OH. Finally, the washed sample was dried in a vacuum oven at 60 ℃ for 8 h to obtain the UiO-66-NH₂ catalyst, designated as UN(UiO-66-NH₂). In the purification via acid leaching, the refined diatomite was employed as a catalyst support. The synthesis method of UND(UiO-66-NH₂/diatomite) composite was as follows, i.e., a similar method to UN synthesis was employed, with the sole difference at varying masses of diatomite during synthesis. The composites were named as UND-X(where X denotes the theoretical loading of UN). A 5 mg sample of the photocatalyst was dispersed in 10 mL deionized water. The mixture was vacuum-filtered, and the resulting catalyst-loaded mixed cellulose filter membrane was dried at 40 ℃ for overnight. The membrane was then positioned in a 115 mL photoreactor with 1 mL deionized water added as a sacrificial agent. High-purity CO₂(99.999%) served as a feedstock gas. In dark adsorption equilibrium, irradiation was performed using a 300 W xenon lamp(i.e., at 320 ≤ λ ≤ 780 nm). Gaseous products were quantified by gas chromatography(GC). The product identification was established through retention time matching, with quantification calibrated against standard curves generated with certified gas mixtures. The photocatalytic CO₂ reduction performance was evaluated via calculating the yield of reduction products per unit mass of catalyst per unit time. Results and discussion UiO-66-NH₂/diatomite composite catalysts with different theoretical loadings are prepared by a hydrothermal method. The results indicate that UND-60% composite demonstrates the maximum CO production rate of 6.69 μmol·g^–1·h^–1, which is 46% higher than that of UN(i.e., 4.57 μmol·g^–1·h^–1). In addition, the results of cyclic experiments also demonstrate that UND-60% composite has a superior stability. After five cycles, the production rate of CO remains > 6.30 μmol·g^–1·h^–1. From the images of scanning electron microscopy(SEM) and high resolution transmission electron microscope(HRTEM), octahedral UiO-66-NH₂ particles are uniformly dispersed on the surface and pores of diatomite. Based on the energy dispersive spectrometer(EDS) elemental mapping images, the presence of Si, O, and Zr elements in the composite indicates the effective preparation of UND-60% composite. According to the X-Ray photoelectron spectroscopy(XPS) analysis, the increased surface hydroxyl groups in the UND-60% composite, stemming from the abundant hydroxyl groups on diatomite, enhance its photocatalytic CO₂ reduction performance. The results of ultraviolet-visible spectroscopy(UV-Vis) spectra, fluorescence spectra, electrochemical impedance spectra, transient photocurrent response spectra and Mott-Schottky curves indicate that UND-60% composite broadens the absorption range of the solar spectrum, and diatomite introduces an promoted separation efficiency of photogenerated carriers, thus improving the photocatalytic performance. The results of In-situ DRIFTs reveal the photocatalytic CO₂ reduction mechanism in the UND-60% composite, i.e., CO₂ and H_2O adsorb onto the photocatalyst surface, where photo-generated electrons activate CO₂ to form bent anionic radicals(~*CO₂~–). Subsequently, ~*CO₂~– undergoes proton-coupled electron transfer to generate a carboxylate intermediate(~*COOH), follows by further protonation and reduction to yield ~*CO and H_2O. This ~*CO formation step is rate-limiting. Finally, *CO desorbs from the catalyst surface as gaseous CO, completing the reduction pathway. Conclusions The UiO-66-NH₂/diatomite composite was synthesized by a one-step hydrothermal method. UND-60% composite demonstrated a superior photocatalytic CO₂ reduction performance, achieving a CO generation rate of 6.69 umol·g^–1·h^–1. This represented a 1.46-fold enhancement over pristine UiO-66-NH₂(4.57 umol·g^–1·h^–1). The results of cyclic testing indicated a superior photostability of the composite. Compared to pure UiO-66-NH₂, diatomite incorporation imparted an optimized specific surface area and pore structure, while enhancing catalyst dispersion. This synergistic effect exposed additional active sites and facilitated an improved CO₂ photoreduction performance. Furthermore, the composite had an expanded light absorption range and enhanced photoinduced charge carrier separation efficiency. The results of In-situ DRIFTs analysis on the UND-60% composite revealed the dynamic evolution of surface functional groups during CO₂ photoreduction, establishing the reaction pathway: CO₂ → ~*CO₂ → CO₂~-→ COOH → ~*CO → CO. These findings could provide valuable insights for designing cost-effective photocatalytic systems for CO₂ reduction.