CO₂ Hydrogenation:Structured NiIn（Ga）-Alloy And In₂O₃ Catalysts And Their Perfromance For RWGS And Methanol Synthesis

The excessive emissions of CO2,a major greenhouse gas,contributes to global climate change and ocean acidification.CO2 as a feedstock combined with renewable H2 for producing chemicals,being a promising approach for the simultaneous chemical storage of excess intermittent renewable power and the reduction of atmospheric CO2concentration,has attracted increasing attention.CO2 hydrogenation to CH3OH and the reverse water-gas shift（RWGS）are attractive direct and indirect routes for CO2conversion and utilization.CH3OH is not only used as a valuable fuel substitute and additive,but also a key feedstock for the synthesis of other high-value chemicals.Syngas（CO+H2）obtained by RWGS reaction is an important chemical feedstock for the synthesis of a range of platform chemicals and synthetic fuels through existing processes such as Fischer-Tropsch process and methanol synthesis.Therefore,developing high-efficiency catalysts for RWGS and methanol synthesis is an important topic of both academic significance and application value,which represents a grand challenge.In this dissertation,we studied the preparation of Al-fiber thin-felt microfibrous-structured NiIn（Ga）alloys and In₂O₃ catalysts as well as their catalytic performance for CO₂ hydrogenation to syngas（RWGS）and methanol.Such microfibrous-structured catalyst’s design takes into account the unification of high efficiency catalysis and the enhanced heat/mass transfer.The main content and results of the research can be briefly summarized as follows.（1）Monolithic Ni₅Ga₃/SiO₂/Al₂O₃/Al-fiber catalyst for CO₂ hydrogenation to methanol at ambient pressureThin-felt microfibrous-structure Al-fiber（consisting of 10 vol%60-μm Al-fiber and90 vol%void volume）was used as the substrate for endogenous growth of free-standing boehmite（AlOOH）nanosheets with the aid of steam-only hydrothermal oxidation reaction between Al metal and H₂O（2Al+4H₂O→2AlOOH+3H₂）.After calcination and SiO₂-modification of the as-obtained AlOOH/Al-fiber,the SiO₂/Al₂O₃/Al-fiber supports were obtained.A series of Ni₅Ga₃/m-SiO₂/Al₂O₃/Al-fiber（m=0-5.0 wt%）catalysts were obtained by incipient wetness co-impregnation method followed by reduction in H₂ at 630 ℃.The promising Ni₅Ga₃/1-SiO₂/Al₂O₃/Al-fiber catalyst is capable of converting 2.3%CO₂ into CH₃OH with a high selectivity of 86.7%as well as 10.3%/3.0%selectivities to CO/CH₄ at 210 ℃,3000 mL g^-1_cat h^-1,and H₂/CO₂/N₂ molar ratio of 66/22/12.Such catalyst delivers a promising methanol productivity of 19.1 g_CH3OH kg^-1_cat h^-1 and is stable for at least 75 h without any sign of deactivation.Compared with the pure SiO₂ support,the SiO₂/Al₂O₃/Al-fiber support is more advantageous for the Ni₅Ga₃ alloying.In addition,suitable SiO₂-modification（1 wt%）is beneficial to the formation of small-sized Ni₅Ga₃ nanoparticles,thus increasing the CO₂ conversion.（2）Hexagonal-phase indium oxide in situ structured onto a thin-felt Al₂O₃/Al-fiber for hydrogenation of CO₂ to methanolIn₂O₃ is considered as a potential and excellent catalyst for CO2 hydrogenation to methanol.However,previous studies have mainly focused on the cubic-In₂O₃（C-In₂O₃）,while the hexagonal-phase In₂O₃（H-In₂O₃）has been rarely studied because of the great difficulty in its synthesis.In this work,morphology-controllable H-In₂O₃ in situ structured onto a thin-felt Al₂O₃/Al-fiber has been prepared by the mix-solvothermal method,and the preparation parameters have been optimized.The amount of H-In₂O₃and catalytic performance are strongly dependent on the urea/In molar ratio during the mix-solvothermal synthesis.H2-TPR,XPS and CO2-TPD results indicate that the optimal catalyst（urea/In ratio=4.5）possesses the highest amounts of oxygen vacancy and strong basic site.Such catalyst achieves a turnover frequency of 47.8 h^-1（the number of CO2 converted into methanol per oxygen vacancy site per hour）,being capable of converting 4.4%CO2 into methanol at a selectivity of 67.6%at 325 ℃ with a methanol space time yield of 0.20 g_CH3OH g^-1_cat h^-1.In contrast,the microfibrous-structured C-In₂O₃/Al₂O₃/Al-fiber catalyst（prepared by incipient wetness impregnation method）delivers a very low methanol selectivity 36.8%under identical reaction conditions.In-situ FTIR experiments reveal that the H-In₂O₃/Al₂O₃/Al-fiber catalyzes CO2-to-methanol through formate intermediate.（3）In-Ni alloy catalysts for the RWGS reactionThe reverse water gas shift（RWGS）reaction catalyzed by the In-Ni nano-intermetallic catalyst is preliminarily investigated using density functional theory（DFT）calculations.The calculation results display that the In-Ni alloys exhibit obvious advantages in activation CO2 compared with the typical Cu catalyst.Based on the above results,a series of pure alloys of InNi,InNi2,and InNi3 were successfully synthesized and were evaluated for the RWGS reaction.In-Ni alloys deliver exciting intrinsic RWGS performances,especially for InNi3 with a high CO selectivity（>96%）and formation rate of 1.96 mmol g^-1_cat min^-1 and a considerably low CH4 selectivity at 500℃ with GHSV of 30000 mL g^-1_cat h^-1.It is very intriguing to find that after reaction the In-Ni phases are in situ changed in association with a new phase formation of InNi3C0.5.InNi3C0.5 formation is thermodynamically favorable with large ordering energy,which portends that the InNi3C0.5 is stable under the RWGS conditions.Notably,only InNi3could be fully transformed into pure InNi3C0.5 and offers the highest RWGS performance,indicating that InNi3C0.5 should be responsible for the RWGS reaction.（4）The study of InNi₃C_0.5 alloy catalyst for CO₂ catalytic hydrogenationInNi3C0.5 alloy catalyst is discovered to be particularly active,selective,and stable for the RWGS reaction.DFT calculation found that the InNi3C0.5（111）surface is dominantly exposed and gifted with dual active sites（hollow3Ni-In and hollow3Ni-C）,which in synergy efficiently dissociate CO2 into CO*（on hollow3Ni-C）and O*（on hollow3Ni-In）.O*can facilely react with 3Ni-C-offered H*to form H2O.Interestingly,CO*is exclusively desorbed above 400 ℃ whereas alternatively hydrogenated to CH3OH highly selectively below 300 ℃.For example,the InNi3C0.5/Al₂O₃/Al-fiber catalyst is capable of converting 4.3%CO2 into CH3OH with 83.1%selectivity as well as 16.6%/0.3%selectivities to CO/CH4(corresponding to the CH₃OH space time yield of 242.6 g_CH3OH kg^-1_cat h^-1)at 275 ℃,4.0 MPa,and H2/CO2/N2 molar ratio of 66/22/12.The reaction mechanism of CO2 hydrogenation on the InNi3C0.5 is studied by theoretical-calculation and infrared-analysis.RWGS reaction follows the redox mechanism and CH3OH can be formed through the CO*-to-HCO*-to-CH2O*-to-CH2OH*-to-CH3OH*pathway.Moreover,the InNi3C0.5 also presents the general and efficient ability to activate C=O bond in carbonyl-to-hydroxyl processes.In the reaction of gas-phase hydrogenation of dimethyl oxalate to ethylene glycol（DMO-to-EG）,the InNi3C0.5/Ni-foam structured catalyst is capable of completely converting DMO at a high EG selectivity of 96%and is stable for at least 500 hours without any sign of deactivation at 210 ℃,2.5 MPa,and H2/DMO molar ratio of 90.