The Study On The Photo-assisted Catalytic Methanation Of CO₂ Over Ru/TiO_?2-x?N_x Or Ni/TiO₂

With energy sources are dwindling and the environmental problems increasing,the technology of energy conversion such as CO₂ methanation has been paid attentions wide-spreadly now.But traditional methanation technology exists lots of problems such as enhanced side effect,sintered and carbon deposited catalysts.So,there are some common goals for researchers about how to improve the methanation catalyst's activity,instability and the selectivity for methane at low reaction temperature.In this paper,the light has been introduced into the reaction system of CO₂ methanation over the supported Ru catalysts based on a modified TiO₂.It was expected that the reaction activity can be improved through the enhancement of the interaction between support and the active metals induced by the photo-response of nitrogen-doped TiO₂.The following research has been carried out in details:?1?Ru/TiO_?2-x?N_x?Ru/TiO₂ catalysts were prepared by a facile impregnation-reduction method respectively.The changes of catalytic performances of the catalysts were tested for CO₂ conversion as well as CH4 selectivity under visible light or not.After testing the chemisorption behavior to CO₂ and H₂ respectively over the catalysts under visible light or not,together with the properties of the catalysts,such as the crystal structure,surface area,morphology,light absorption and surface chemical state,the possible photo-assisted mechanism of CO₂ methanation over Ru/TiO_?2-x?N_x catalysts under visible light has been discussed.?2?According to the steps above,the effect of UV-Vis light on CO₂ methanation over Ni/TiO₂ has also been dissussed.The main results and conclusions are presented as follows:?I?The CO₂ methanation over Ru/TiO_?2-x?N_x catalyst under visible light.?1?Visible light could substantially improve the methanation of CO₂ over Ru/TiO_?2-x?N_x at the same reaction temperature in the atmosphere?CO₂:H₂=1:4?.?2?The photo-assisted effect was the most prominent over the catalyst with 1.5wt%metal loading and with 2.57wt%?x=0.15?nitrogen doping.?3?It is proposed that the photo-assisted role comes from two aspects:one is that there is interaction between nitrogen-doped TiO₂ support and the active metal sites,and the photo-generated electrons induced by the band-gap excitation of TiO_?2-x?N_x can transfer to the adjacent metal nanoparticles,leading to the increase in the surface electron density of the reactive metals.This will further promote the adsorption and activation of CO₂ species?CO₂ adsorbed in M-C form?.Another one is that the TiO_?2-x?N_x itself is very adapt to the adsorption and activation of CO₂,and can also translate CO₂ to CO species which acts as intermediate.Moreover,the production of oxygen vacancies over TiO_?2-x?N_x induced by visible light further promote the process.Meanwhile,the left photo-generated holes can also benefit to the adsorption of H₂ and its activation together with the dissociation of H₂ overflow from Ru to TiO_?2-x?N_x.?II?The CO₂ methanation over Ni/TiO₂ catalyst under UV-Vis light.?1?The methanation of CO₂ over Ni/TiO₂ could not be improved under UV-Vis light at the same reaction temperature.?2?The tests of TPD?TPSR and FT-IR showed that the Ni of Ni/TiO₂ catalyst?reduced at 270??was in the oxidation state.During the methanation process,CO₂ could react with the adsorbed water to produce the carbonate and bicarbonate first,then translate into CO₂ and finally hydrogenate to methane.?3?XPS results showed that it was different with the M-C adsorbed form which CO₂ adsorbed over the reduced metal sites,CO₂ may be adsorbed at Ni surface to form like M+-O=C=O adsorbed because of charges repel with the cation state of oxidation nickel.This adsorbed CO₂ need to be activated through O lone pairs donating electrons to the unoccupied Nin+orbits.Here,the increase in electron density of oxidation nickel,induced by the photo-generated electrons of TiO_?2-x?N_x under UV-Vis light transfering to the adjacent metal nanoparticles,could not help the polarization and activation of CO₂ Therefore,the photo-response of TiO₂ could not improve the progress of CO₂ methanation.The study indicates that the improvement of CO₂ methanation is mainly dependent on the CO₂ activation through the electron interaction between CO₂ with active adsorption sites.This electron interaction process will disrupts the internal electron state of CO₂ and then improve the polarization and activation of CO₂.However,the adsorption modes of CO₂ at metal sites are different due to the different catalysts' chemical state,which resulted in the different electron interaction behaviors between CO₂ and active metal sites,and then the different activation mode of CO₂.Therefore,the photo-generated electrons induced by the band-gap excitation of supports under light irradiation will exert the different effects on the activation of CO₂ over the active adsorption sites with a different chemical state.This study provides a new approach to improve the CO₂ methanation reaction by changing the surface electron density of metal active sites on Ru/TiO_?2-x?N_x and Ni/TiO₂ catalysts through the photo-response of TiO_?2-x?N_x or TiO₂ supports,which may be also extended to the study and application of improving other thermal catalytic reaction performance.