加热和光照条件下Ru/TiO2催化二氧化碳甲烷化研究

本研究利用浸渍法制备了Ru/TiO₂催化剂, 并在光照和加热两种条件下考察了其催化二氧化碳与氢的反应, 发现催化剂在两种条件下均可引发显著的甲烷化反应(CO₂ + 4H₂ → CH₄ + 2H₂O)。结果显示, 在光照和加热(150~350℃)条件下, CH₄为唯一含C产物。而在更高温度的加热条件下(>400℃)除了生成CH₄外, 还产生少量CO副产物, 表明反应温度对产物选择性有显著影响。随着反应温度由150℃升高到550℃, 对于不同负载量的担载Ru催化剂, CO₂转化率均先增加后降低, 其中在Ru担载量为1.5wt%Ru/TiO₂催化剂上CO₂转化率在350℃时达到最高, 为77.58%。而在温度>400℃条件下, CO的选择性也随反应温度的升高而逐渐增加。综合反应结果和XRD、XPS和N₂吸附-脱附等表征结果, 发现二氧化碳与氢在光照和加热条件下(150~550℃)反应机制不同。在光照条件下, 光激发电子首先被金属Ru捕获, 进而将吸附在金属Ru上的二氧化碳还原, 活性物种经由RuC中间体形成CH₄。而加热条件下(150~550℃), H₂先被Ru活化成氢原子, 氢原子还原吸附在催化剂表面的CO₂形成RuC中间体, 最后RuC中间体进一步加氢生成CH₄。虽然在两种反应条件下经历相同的中间体, 但是中间体的形成路径不同, 即反应物CO₂被活化的方式不同, 因而产物选择性不同。

CO2 Methanation over Ru/TiO2 Catalysts under UV Irradiation and Heating

CO₂ hydrogenation was carried out over Ru/TiO₂ catalysts prepared by wet impregnation method. It was found that CO₂ methanation (CO₂ + 4H₂ → CH₄ + 2H₂O) could be catalyzed by Ru/TiO₂ both under UV irradiation and heating. Results showed that CH₄ was the only C-contained product under UV irradiation in temperature range from 150-350℃, while both CH₄ and CO (minor product) were formed at temperature higher than 400℃ under heating conditions. This indicated that reaction temperature affected obviously on selectivities of products. For all the catalysts with different Ru loadings, CO₂ conversion increased firstly and then decreased from 150℃ to 550℃, and corresponding maximum value of CO₂ conversion of 77.58% was achieved at 350℃ over 1.5wt% Ru/TiO₂ catalysts. Further increasing of reaction temperature over 400℃ led to the increase of CO selectivity. Based on reaction data, XRD, XPS and N₂ adsorption-desorption characterization results, we found that CO₂ hydrogenation occurred in different ways under UV irradiation and heating conditions (150-550℃). Under UV irradiation, excited electrons were firstly trapped by Ru site and then these electrons reduced CO₂ species, which was adsorbed on Ru surface to form CH₄ via RuC intermediate. However, under heating condition (150-550℃), H₂ was first decomposed into H atoms over Ru site, then the adsorbed CO₂ species on Ru site was reduced by H atoms to form RuC intermediate. Finally, RuC reacted with H atoms to produce CH₄. Although CO₂ hydrogenation was taken place via same intermediate under UV irradiation and heating conditions, the RuC intermediate was formed in different ways, i.e., the activation of CO₂ was induced in the different routes, therefore the different selectivities of products were obtained under UV irradiation and heating at temperature higher than 400℃ consequently.