Preparation Of Metal-doped Cu₂O Using Low-valence Metal Ions As Reducing Agents And Its Photocatalytic Performance

Since industrialization,the use of a large amount of fossil fuels has caused a sharp increase in the content of greenhouse gases in the atmosphere.The resulting increase in global temperature and a series of environmental problems that follow have become more and more important to ignore.CO₂ is one of the most abundant greenhouse gases and contributes the most to global warming.Therefore,more and more researchers are trying to find effective ways to reduce the CO₂ content in the atmosphere.Using photocatalysis,electrocatalysis,thermal catalysis and other ways to directly convert CO₂ into low-carbon fuels and industrial raw materials is an ideal way.The use of ubiquitous and huge reserves of solar energy to directly reduce CO₂ into low-carbon products has broad application prospects.However,the current field of photocatalytic CO₂ reduction still has problems to be solved,such as low catalyst conversion efficiency,low catalytic stability,and unclear reaction mechanism.Cu₂O is an ancient semiconductor material.It has the advantages of simple synthesis method,low price,safety and non-toxicity.At the same time,the forbidden band width of Cu₂O is about 2.2 e V,which has a good absorption of visible light with the highest energy proportion in the solar spectrum,which makes Cu₂O have a good application prospect in the field of photocatalysis.However,Cu₂O itself also has disadvantages such as weak carrier separation efficiency and unsatisfactory catalytic stability.It is a simple and effective strategy to modify Cu₂O by doping other elements.The solution method is a classic method for the synthesis of Cu₂O.Generally,Cu₂O is prepared by reducing the Cu²⁺with reducing agents such as hydrazine hydrate and hydroxylamine hydrochloride in an aqueous solution;if different elements need to be doped,the element ions are usually dissolved in the aqueous solution,when the Cu₂O is generated,element ions enter the lattice to complete the doping.The purpose of this thesis is to use metal ions with reducibility in a low-valence state as a reducing agent,and at the same time dope metal elements into the Cu₂O crystals,and adjust the state of the final product by changing the concentration of the metal ions used to obtain a better photocatalysis performance catalyst.Using SnCl₂ as the reducing agent,a series of Sn-doped Cu₂O microspheres were synthesized.When the molar amount of Sn in the reactant is 30%to 100%of Cu,a series of different Sn-doped Cu₂O will be obtained.Through a variety of tests,it is found that the amount of SnCl₂ has a significant effect on the morphology,composition,crystallinity,and energy band structure of the product.When the content of Sn in the reactant is 50%of Cu,the catalytic performance of the product is the best,the yield of the product-CO reaches 15.8μmol·g^-1·h^-1,and the selectivity is almost 100%.The performance of this product is superior to other products mainly because its band structure is suitable for catalyzing the half reactions of reducing CO₂ to CO and oxidizing H₂O to O₂.At the same time,the carrier separation efficiency of this product is the highest among a series of products.Finally,by in-situ infrared absorption spectroscopy,it is determined that the main product is CO because the CO*intermediate has a weak adsorption capacity on the surface of the catalyst.The CO is desorbed from the surface of the catalyst and the next hydrogenation reaction cannot be carried out.Using MnCl₂ as a reducing agent and changing the amount of MnCl₂,a variety of Mn-doped Cu₂O cubes were synthesized.Only when the molar amount of MnCl₂ is 50%to150%of that of CuCl₂,a Cu₂O product with better crystallinity can be obtained.Experiments have revealed that the doped into the Cu₂O lattice is Mn²⁺,and the high-valent Mn ions involved in the reaction are separated from the Cu₂O in the form of oxides.The morphology,crystallinity and energy band structure of the final product are greatly affected by the amount of MnCl₂.When the ratio of MnCl₂ to CuCl₂ is 1:1,the crystallinity and carrier separation efficiency of the product are the highest.By comparison with the energy band structure of other products,it is found that the energy band structure of this product is also the most suitable for catalyzing the reduction of CO₂ without a sacrificial agent.The experimental results also confirmed this point.The CO yield of the product reached 5.192μmol·g^-1·h^-1,and the selectivity was 78.3%,which was the highest among the products.When FeCl₂ and TiCl₃ are used as reducing agents,the products have similarities.Both of the two reducing agents can generate pure phase Cu₂O crystals only within a small amount of change,too little will result in an amorphous product,and too much will produce a mixed phase of metallic copper and Cu₂O.The microscopic morphology of the two products is nanoparticle.And the band gap of the two products is almost the same,but the band structure is very different.The potential of the top of the valence band of Fe-Cu₂O is 0.5 e V smaller than that of Ti-Cu₂O;This makes the difference between the bottom potential of the conduction band of Fe-Cu₂O and the oxidation-reduction potential of the reduction of CO₂ to CO much larger than that of Ti-Cu₂O.However,the excessively small potential of the valence band limits the progress of the oxidation half-reaction,and ultimately is not conducive to the overall performance of the catalyst.However,the carrier separation of Ti-Cu₂O is lower than that of Fe-Cu₂O.This makes the CO yield of Fe-Cu₂O reaches 4.562μmol·g^-1·h^-1 with a selectivity of85.2%,while the yield of Ti-Cu₂O reaches 3.607μmol·g^-1·h^-1with a selectivity of91.4%.