Photocatalytic Reduction Of CO₂ On Modified Molecular Sieve Ti-MCM-41

Nowdays,the world's energy is still dominated by non-renewable fossil energy.These fossil energy sources produce a large amount of CO₂ after combustion,which leads to an increase in the concentration of greenhouse gas CO₂.How to effectively reduce the CO₂ content of greenhouse gases and further utilize it has already caused worldwide attention.From the perspective of sustainable development,solar energy is rich,clean and pollution-free,and it is a sustainable energy.Photocatalysis is considered as an environmentally friendly technology,which can convert CO₂ into hydrocarbon fuel to realize CO₂ recycling.The core of CO₂ efficient photocatalytic resource utilization lies in the development of high efficiency photocatalysts.In addition to the common semiconductor photocatalysts,porous materials have attracted much attention in the field of photocatalysis due to their large specific surface area and abundant pore structure.Ti-MCM-41 molecular sieves have large specific surface and the pore structure with regular order.And adjustable pore diameter is beneficial to the adsorption of CO₂.In addition,Ti atoms are embedded in the skeleton of MCM-41 molecular sieve,and the photocatalytic properties of quantum-limited TiOx clusters can be utilized.In this paper,Ti-MCM-41 was prepared by hydrothermal method,and then the molecular sieve was modified by alkaline earth metal oxide and organic amine,and the pore size of the molecular sieve was adjusted.The photocatalytic reduction of CO₂ of the prepared samples was studied.The main contents are as follows:?1?Molecular sieves with different Si/Ti molar ratios were prepared by hydrothermal method.It was found that when n_Si/n_Ti=10,the highest CH₄ yield was 93 ppm·g^-1·h^-1,which was 3.9 times than that of P25.This is because when n _Si/n_Ti=10,molecular sieves can maintain the skeleton structure and there are more photogenerated electron hole pairs in the presence of titanium dioxide.Modified zeolites with alkaline earth metal oxides can enhance the adsorption of CO₂.Among them,MgO had the strongest adsorption capacity for CO₂ at that time,and the CH₄ yield reached 157 ppm·g^-1·h^-1.This indicated that the alkaline sites on the catalyst surface can enhance the adsorption of CO₂ and make more CO₂ reduced.Ti-MCM-41 molecular sieves were modified with MgO and then loading by Pt and Pd.The results showed high selectivity for CH₄ as Pt loading.The selectivity for CH₄ reached 93%when Pt loading was 1%,and the highest yield of CH₄ was 8835 ppm·g^-1·h^-1,while Pd showed high selectivity for CO when Pd loading was 1%the selectivity was as high as 94%.?2?Inorganic oxide and CO2 molecules are combined by van der Waals force,in order to further enhance the adsorption of CO₂,Ti-MCM-41 zeolite was aminated by TEPA,a commonly used adsorbent in industry.After modification,it was found that TEPA has stronger adsorption capacity for CO₂ than MgO,because TEPA contains a large amount of amino,which can adsorb more CO₂.After evaluation of the photocatalytic reduction activity of CO₂ on the aminated molecular sieve,it was found that when the TEPA content was 1%,the CH₄ yield was up to 232 ppm·g^-1·h^-1,and when the TEPA content was 30%,the CO yield was up to 2573 ppm·g^-1·h^-1,the yield of CH₄ and CO decreased with the increase of TEPA content in the sample.?3?The pore size of the molecular sieve has an important influence on the adsorption amount of the supported cocatalyst and CO₂.We had adjusted the pore size of the Ti-MCM-41 molecular sieve by adding N,N-dimethylguanamine?DMDA? during the hydrothermal process.It was found that the pore size could be adjusted from 3.8 nm to about 12 nm.The sample after adjusting the pore size did not change the amount of CO₂ adsorbed.But the Pt particles loaded after adjusting the pore size were more uniformly dispersed than the Pt particles directly supported onTi-MCM-41.Besides,the CH₄ yield of the sample Ti-2.5DMDA-1.5% Pt after adjusting the pore diameter reached 20837 ppm·g^-1·h^-1.It was 3.4 times than the unadjusted pore size sample Ti-MCM-41-1.5% Pt,and the selectivity to CH₄ was increased from 80% to 90%,because after adjusting the pore size the molecular sieve can make the supported Pt particles more uniformly dispersed,so that more electrons were transferred to the Pt particles.