Study On The Mechanism Of Fe-Ni Catalyst Support Surface Interface Structure Regulating CO₂ Activation And C₂H₆ Directional Conversion Reaction

The use of CO₂ weak oxidizing property to directional reaction with ethane to produce value-added products can simultaneously realize the efficient utilization of carbon-based fuels such as shale gas and the resourceful conversion of CO₂.At present,conventional catalysts have difficult in activate CO₂ molecules with stable structure,and it is difficult to control the binding of active O^*species and the directional conversion of C_xH_yO^*,resulting in the low reaction conversion and selectivity of high-value ethylene products.In this thesis,the combination of catalytic reaction experiment,catalyst characterization analysis and catalytic reaction path DFT calculation were used to promote the selective cleavage of ethane C-C/C-H bond by regulating CO₂ activation through different catalyst surface interface structure.Firstly,the effect of metal-support interaction on the catalytic performance of ethane with CO₂ was investigated by using active metal Fe/Ni composition and carrier reducibility,and the relationship between"coordination environment-catalytic efficiency"was established,and then the exposed crystal plane and O_v concentration of the carrier were adjusted and correlated with the relationship of"crystal plane effect-vacancy defect-catalytic efficiency".Finally,the carrier metal cation substitution structure enhanced CO₂ activation to promote the directional reaction of ethane was explored,and the"substitution structure-site type-catalytic efficiency"relationship was constructed.Through the research of this thesis,a set of research methods of design-preparation-structure-performance relationship of catalyst materials is formed,which can be used to guide the resource utilization of greenhouse gas CO₂ and the efficient utilization of carbon-based energy.The relevant research work and main conclusions are as follows:（1）The mechanism of the effect of catalyst metal Fe-Ni active component on the performance of the catalytic reaction of ethane with CO₂ was obtained.（1）The formation of the Fe-Ni bimetallic active structure reduces the activation energy of ethane and CO₂,and the activation energy for the oxidative dehydrogenation reaction is also reduced to the lowest（99.5 k J/mol）,which is beneficial to improve the yield of C₂H₄.（2）The interaction between Ni⁰/Fe²⁺and Ce⁴⁺after Fe/Ni is supported on the surface of Ce O₂ oxide carrier leads to the redox cycle of Ni⁰/Fe²⁺+Ce⁴⁺?Ni²⁺/Fe³⁺+Ce³⁺,promotes the formation of oxygen vacancy and unsaturated chemical bond in the process of electron transfer,and improves the catalytic performance of CO₂ and C₂H₆.（3）The redox performance of the catalyst was improved by active metal loading,the content of lattice oxygen O_αdecreased,while the content of surface adsorbed oxygen O_βincreased,there was a synergistic effect between Fe/Ni and Ce,and additional surface adsorbed oxygen was produced by oxygen transfer.（4）The best reaction path of ethane dehydrogenation with CO₂ on Fe-Ni based catalysts is the direct activation dehydrogenation of C₂H₆ to C₂H₅^*,followed by the subsequent oxidative dehydrogenation process assisted by O^*.The formation of Fe-Ni bimetallic structure greatly reduces the energy barrier of oxidative dehydrogenation,so it has high oxidative dehydrogenation activity and C₂H₄ selectivity.（2）The strong interaction between reducible carrier（Ce O₂/Zr O₂）and active metal and its regulation on the catalytic reaction of ethane with CO₂ were revealed.（1）The strong interaction between the active metal and the support on the reducible supports Ce O₂ and Zr O₂ makes Fe-O-Ce and Fe-O-Zr covalent bond on Fe_1.5Ni_0.5/Ce O₂and Fe_1.5Ni_0.5/Zr O₂ catalysts stronger than that of Fe-O-Ti and Fe-O-Al covalent bond on Fe_1.5Ni_0.5/Ti O₂ and Fe_1.5Ni_0.5/γ-Al₂O₃.（2）The lattice oxygen on reducible support catalyst is easier to participate in the reaction,and the strong interaction between the active metal and the support makes Fe O_x to be reduced on the support,and the newly formed active center maintains the high reactivity on the catalyst.The redox cycle between active species Fe³⁺/²⁺-Ni²⁺/⁰ and reducible variable valence metal Ce⁴⁺/³⁺species promotes each other,thus improving the catalytic performance for the reaction of ethane with CO₂.（3）The effect of crystal phase structure of catalyst Zr O₂ carrier（Monoclinic phase/Tetragonal phase/Mixed phase）on the performance of CO₂-assisted ethane dehydrogenation reaction was clarified.（1）The monoclinic Fe_1.5Ni_0.5/Zr O₂-M catalyst exhibited the highest ethane（21.8%）and CO₂（25.2%）conversion and C₂H₄selectivity（80.5%）,which was suitable for oxidative dehydrogenation.（2）The monoclinic phase of Zr O₂ selectively exposes（-111）and（111）low-index surfaces with higher oxygen vacancy（O_v）density and stronger metal-support interactions,which allow the formation of more catalytically active Fe_xZr_1-xO₂ structure.（3）Oxygen vacancies O_v and Fe²⁺-O-Zr³⁺sites supplement O^*species through direct C=O bond cleavage,thereby promoting the activation of CO₂.The O^*species on oxidative Fe³⁺-O-Zr⁴⁺sites are captured and combined with the H^*of the selective C-H bond cleavage derived from ethane to form H₂O.The oxidation of the active site Fe²⁺-O-Zr³⁺by CO₂ and the cyclic transformation of Fe³⁺-O-Zr⁴⁺by H^*reduction promote the continuous and efficient operation of oxidative dehydrogenation.（4）The mechanism of the exposed crystal plane（111/110/100）structure of Ce O₂ carrier on the performance of CO₂ activation and ethane dehydrogenation reaction was revealed.（1）Ce O₂ with different morphologies exposes different crystal planes,thus showing different O_v densities.Fe_1.5Ni_0.5/Ce O₂-R has the highest Fe³⁺and Ni⁰ density,and the Fe³⁺-Ni⁰ structure formed by the combination of active metal high valence state Fe³⁺and reduced Ni⁰ is the best ODEC active site.（2）The Fe_1.5Ni_0.5/Ce O₂-R catalyst of rod Ce O₂ has lower activation energy of C₂H₆ and CO₂dissociation and oxidative dehydrogenation than Fe_1.5Ni_0.5/Ce O₂-O catalyst.The selectivity of C₂H₄ on Fe_1.5Ni_0.5/Ce O₂-R catalyst with mainly exposed（-111）surface reached the highest value of 82.51%at 650℃.（3）The surface of Ce O₂（111）has three-coordinated O and heptacoordinated Ce,which enhances the surface stability of Ce O₂（111）.The most favorable CO₂ adsorption on the surface of Ce O₂（111）is to form tridentate carbon with surface oxygen acid salt,with an adsorption energy of-0.74 e V on the reduced surface,and in the presence of oxygen vacancies（O_v）,the interaction of CO₂ with the surface is stronger,thereby promoting the adsorption and activation of CO₂.（5）The regulation mechanism of the effect of different metal cations（Zr,Ti,La）substitution structures on cubic crystal phase Ce O₂ carrier on CO₂ activation and the directional conversion of ethane was obtained.（1）The optimal substitution amount of different metal cations（Zr,Ti,La）on cubic Ce O₂-C is 2～3%,and the Fe_1.5Ni_0.5/Ce Ti-2 catalyst substituted by Ti has the highest ethane conversion rate of11.8%and CO₂ conversion rate.The highest was 21.2%and exhibited a high value product C₂H₄ selectivity of>80%.（2）The Ti/La species in the substitution defect structure of Ti and La are highly dispersed into the Ce O₂ lattice to form a Ti/La_xCe_1-xO_2-δsolid solution,and the substitution defect structure induces more（-111）and（110）crystals with high ethylene selectivity On the other hand,the ratio of Ce³⁺in the Ti-substituted catalyst is the highest,which improves the oxygen activation and transfer and makes Fe_1.5Ni_0.5/Ce Ti-C-2 exhibit higher catalytic reactivity.（3）The existence of metal cation substitution weakens the adjacent Ce-O bond strength,and the Ti substitution defect structure is beneficial to the generation of O_v as a highly active center for CO₂ reduction.The energy barrier of CO₂ dissociation process is reduced from 3.87 e V on original Ce O₂（111）to 2.17 e V,and the reaction energy endotherm is0.64 e V.The initial reductive dissociation of CO₂on Ti-Ce O₂（111）is more competitive than that on original,Zr and La substituted Ce O₂（111）,which further confirms the high reaction activity of ethane with CO₂ on Fe_1.5Ni_0.5/Ce Ti-2 catalyst.