Basic Research On The Mechanism Of Low-carbon Alkane Conversion Over Transition Metal-modified Cerium-based Catalysts

Climate change is a severe challenge for mankind in the 21 st century,and CO₂ is the culprit of climate change.Therefore,the research on CO₂ emission reduction and utilization is particularly important.Using fossil fuel combustion and metallurgical industry to produce CO₂ emitted into the air and low-carbon alkanes(mainly CH₄ and C₂H₆)in abundant natural gas and unconventional natural gas through co-transformation reaction can not only reduce the CO₂ content in the atmosphere but also produce important chemical production raw materials(syngas and ethylene),which has become the focus of scholars in various countries.At present,the understanding of the microscopic reaction mechanism and reaction path for the synthesis of syngas and ethylene from low-carbon alkanes through co-transformation reaction,and the microscopic mechanism of catalyst regulation on reaction performance are still unclear.These microscopic understandings are very important for the design and development of high stability,high selectivity and high activity catalysts.CeO₂ is widely used in various catalytic redox reactions due to its unique physical and chemical properties,ultra-high oxygen storage and release capacity and cerium ion coupling valence characteristics.Therefore,the Ce-based catalysts modified by transition metals were used as the research object in this paper.Based on the density functional theory(DFT)calculation,the effects of transition metals(Fe,Co and Ni)modification(adsorption,insertion and substitution)on the structure,electronic properties,oxygen vacancy formation and methane activation of CeO₂ were systematically investigated.Insitu characterization techniques(such as in-situ X-ray absorption spectroscopy(XAFS)and in-situ XRD analysis)combined with DFT calculations were used to systematically investigate the effects of CeO₂-supported Pd monometallic and Pd Fe bimetallic catalysts on the catalytic conversion of C₂H₆-CO₂ to ethylene.The main conclusions are as follows:(1)Methane activation and oxygen vacancy formation over transition metal Fe adsorption on CeO₂(110)were investigated by using DFT+U method.A set of model configurations are generated by placing Fe at five surface sites,viz.,O-top site,O-bridge site,Ce-bridge site,Ce-top and Double oxygen-bridge sites.The investigation shows that the energetically most favorable configuration is Fe adsorption at the Double oxygen-bridge site.Based on the calculated surface,subsurface and the second oxygen vacancies formation energy with(or without)Fe adsorption,it shows that the Fe adsorption is in favor of the surface,subsurface and second oxygen vacancies formation.For the surface and subsurface oxygen vacancy on the Fe/CeO₂(110)surface,the main factor responsible for lowering of Evac is that the adsorption induces structural distortions,whereas,for the second oxygen vacancy,half can be attributed to the large structural relaxation,half can be attributed to the electronic effects.After calculating and discussing about the CH₄ activation on CeO₂(110)and Fe/CeO₂(110)surface with(or without)the surface or subsurface oxygen vacancies at the possible adsorption sites,the results show that when the CH₄ adsorbed on the Fe/CeO₂(110)with the surface oxygen vacancy at the Ce₁ and Ce₂ sites,the CH₄ decomposed into the CH(ads)and H(ads),its belongs to the chemical absorption,whereas,when the CH₄ adsorbed on the other possible sites,the mentioned phenomenon is not occurred,its belongs to the physical absorption.(2)The effects of transition metal(Fe,Co and Ni)modification(adsorption,insertion and substitution)of CeO₂ surfaces on oxygen vacancy formation and CH₄ activation are studied on the basis of DFT calculations.The results indicate that the hollow,O-O-bridge and Ce-O-bridge sites are the most stable sites for Fe,Co and Ni atom adsorption on the CeO₂(111)surface,and the double O-bridge,O-top and double O-bridge sites are the corresponding most favorable sites for the CeO₂(110)surface.Most of the configurations that are generated by the transition metal modification of CeO₂(111)and(110)surfaces are accompanied by the reduction of Ce₄₊ to Ce₃₊.Based on the calculated subsurface(SS)and sublayer(SL)oxygen vacancies of the CeO₂(111)surface,the results show that the substitution of transition metals on the CeO₂(111)surface can promote SS oxygen vacancy formation spontaneously,whereas the most stable adsorption of transition metal Fe and Ni atoms on the CeO₂(111)surface can promote SL oxygen vacancy formation spontaneously.For the CeO₂(110)surface,the substitution of transition metals can facilitate plain(P)and spilt(S)-type oxygen vacancy formation spontaneously.With respect to CH₄ activation,the results show that Co atom substitution on the CeO₂(110)surface can greatly facilitate the first C-H bond activation step,with an energy barrier of 0.783 eV and a reaction energy of 0.229 eV,which were significntly lower than 0.418 eV and 1.741 eV on pure CeO₂(110)surface.However,Co atom substitution on the CeO₂(110)surface with P and S-type oxygen vacancies is not conducive to C-H activation.(3)Combined the efforts of in situ techniques(such as XRD,XAFS,AP-XPS,and DRIFTS)and DFT calculations were carried out to investigate the effects of CeO₂-supported Pd and Pd Fe bimetallic catalyst for catalytic ethane and CO₂.It was found that in the interfacial oxygen species formed between CeO₂ and supported Pd mono-metal and Pd Fe bimetallic catalysts.Due to the formation of Pd nanoparticles,electrondeficient,highly active and non-selective oxygen species are generated at the Pd/Ce Ox interface,which promotes the fracture of C-H/C=C bond to produce syngas.On the contrary,the formation of Fe Ox species on the surface of CeO₂ reduces the reduction performance of CeO₂ and the formation of Pd nanoparticles,but inhibits the production of non-selective oxygen,thus generating electron-rich and selective oxygen species to enhance the C-H bond fracture and weaken the adsorption of ethylene,thus increasing the yield of ethylene.This study reveals the mechanism that interfacial oxygen species formed between CeO₂-supported Pd mono-metal and Pd Fe bimetallic catalysts regulates the selective fracture of C-C/C-H bonds in ethane.