Study Of Catalytic System For CO₂ Based Dry Reforming Of Methane And Reverse Water-Gas Shift To Syngas

The overuse of fossil resources has caused a dramatic increase in CO₂concentration in atmospheric,which will lead to a series of serious environmental problems such as ocean acidification and warmer average temperatures.CO₂ is not only a greenhouse gas,but also a cheap and widely available carbon resource.In China,the"double carbon"strategic goal of carbon emission peak and carbon neutrality will promote the development of green technologies for the efficient preparation of high value-added chemicals from CO₂,thus promoting the reuse of waste carbon resources.Syngas is the collective name for a mixture of CO and H₂.It is an important platform,which can be used as raw material to further synthesize oil,methanol,mixed high-carbon alcohols,aromatics,olefin and other important basic chemical raw materials.CO₂ was converted into syngas and downstream products through efficient catalytic method,which can fix and transform waste carbon resources,and recycling of CO₂ was promoted.Herein,we mainly focused on two paths to achieve the CO₂ to syngas conversion process:Dry Reforming of Methane（DRM）and CO₂ hydrogenation to CO（Reverse Water Gas Shift,RWGS）.For the supported metal catalysts,the catalytic systems with high activity and stability were designed and developed,and the fine structure of the catalysts were characterized by static methods.The reaction mechanisms of the catalytic systems were analyzed and explored in detail by combining in situ characterization methods and DFT calculations.In the dry reforming of CO₂-CH₄（DRM）system,firstly,an equilibrium theory of carbon deposition and carbon elimination in the reaction process was conceived.The equilibrium ofν（CH₄）≈ν（CO₂）during the reaction process would have the potential to achieve no carbon deposition in DRM.Based on this,a highly dispersed Ni-Ir/Mg Al₂O₄alloy system was constructed.TPSR-MS showed that CH₄ was mainly activated on metal Ni,CO₂-TGA and CO₂-DRIFTS proved that Mg Al₂O₄ adsorbed CO₂ to form carbonate species,which was likely a carbon capture cell.It is presumed from the in situ-DRIFTS results that this carbon capture cell could be effectively activated by the metal Ir to eliminate the carbon species generated by CH₄ activation.DFT calculations demonstrated that the promotion of CO₂ adsorption and activation ability on Ir was due to its strong oxyphilicity.The Ni-Ir ratio is adjusted to match the activation rates of CH₄and CO₂,thus achieving high activity and zero carbon accumulation in the dry reforming of methane at low temperatures（650°C）and long term（600 h）.Secondly,a new active center for dry reforming of methane,RuO_0.25/Mg Al₂O₄ with lattice distortion on Ru surface,was designed and synthesized.In general,the active center of DRM was the zero-valent（M⁰）metal center,but the initial activity of this Ru O_0.25/Mg Al₂O₄ oxide catalyst with this special structure was about 6 times than that of the metallic Ru/Mg Al₂O₄ catalyst.At a high induction temperature（850°C）,CO₂dissociated on the Ru surface to produce O*,which combined with the Ru atoms on the surface to form a Ru O_x structure and formed a stable oxide layer with 12.44%coverage of O*after 40 min.Furthermore,the coverage of O*on Ru was linearly related to the DRM activity,and the partially oxidized Ru O_0.25/Mg Al₂O₄ structure could maintain high activity and stability in high-temperature and reductive atmospheres（H₂,CO）.The catalytic system was extended to the Bi-reforming of methane（CH₄ reformed with H₂O and CO₂）and the reforming of coke oven gas（a methane-rich industrial waste gas）,which exhibited excellent activity and stability.In the RWGS（CO₂ hydrogenation）system,the Ru-Sn/La₂O₂CO₃ bimetallic catalyst was designed and prepared,which was applied to the CO₂ hydrogenation process to achieve CO selectivity more than 99%at 400°C.XPS,CO-DRIFTS,and charge density difference indicated electron transfer from Sn to Ru.H-D exchange experiments and CO₂-TPD confirmed that this electron transfer reduced the H₂dissociation ability and changed the CO₂ adsorption mode on Ru.DFT calculations and in situ-DRIFTS demonstrated that the reduction of dissociation capacity of H₂ inhibited CH₄ generation,while the change of CO₂ adsorption mode reduced the C-O bond energy of CO₂ adsorbed at the Ru-Sn interface and formed Ru-CO*and Ru-O*intermediate species,which was beneficial to generate CO with high selectivity.The formation rate of CO on the ultra-low metal loading（0.01wt%）Ru Sn/La₂O₂CO₃catalyst could reach 3.6*10⁵ mmol _CO·g _Ru^-1·h^-1.Meanwhile,the catalyst had been successfully applied to the hydrogenation of blast furnace gas（a CO₂-rich industrial waste gas）to syngas.The CO generation rate was as high as 2.3*10⁶ mmol _CO·g _Ru^-1·h^-1and it was kept stable for 1000 h.