Structure Evolution And Reaction Mechanism Of CuAl₂O₄ Spinel Catalyst For Methanol Steam Reforming

Methanol has the advantages of convenient transportation,high energy density,and low conversion temperature.Methanol reforming technology combines hydrogen production,transportation and downstream utilization.Thus methanol can be used as the carrier of hydrogen energy to produce hydrogen.Traditional supported Cu-based catalysts for methanol reforming have poor thermodynamic stability and deactivaties easily.The active copper component can be dispersed uniformly into the alumina bulk phase to form spinel structure as sustained release catalyst,thereby effectively increased the stability of the catalyst.Understanding the structure-activity relationship is the basis for catalyst design.While,it is lack of understanding of the thermodynamic basis and dynamic process of how copper atoms are released from the the Cu-Al spinel during the calcination,reduction treatment and reaction.In addition,it is lack of relevant basic research on the physical and chemical basis of Cu-Al spinel surface reconstruction and evolution in the‘sustained release’process.Aimed at the above scientific issues,the present thesis performed density functional theory calculations to investigated:（1）Simulate the preparation process of Cu-Al spinel;（2）the‘sustained release’mechanism of Cu Al₂O₄ spinel;（3）the thermodynamic stability and deactivation mechanism of the‘sustained release’active Cu;（4）the activation mechanism and adsorption state of the reactant water molecules on the Cu Al₂O₄ surface;（5）the methanol decomposition mechanism on the Cu Al₂O₄ spinel surface.（1）Simulate the preparation process of Cu-Al spinel at the molecular level.Using Cu atom doped theγ-Al₂O₃ surface Al atoms,and H is used as the ion for charge balance.It is found that the substitution of surface Al³⁺by Cu²⁺is thermodynamically accessible.Gibbs free energy calculations show that the dehydration temperature for theγ-Al₂O₃（110）surface after substitution is higher than that of on the originalγ-Al₂O₃（110）and Cu Al₂O₄ surface.In addition,the Cu dopedγ-Al₂O₃ interface could strengthen the binding of Cu with the alumina surface.Study of the microscopic process of Cu doping on the ofγ-Al₂O₃ surface to form’surface spinel’structure.The results provide molecular level insights for the understanding of the formation mechanism of spinel and the Cu/γ-Al₂O₃ interface structure with low Cu loading ratio.（2）Studying the deoxidation process of the catalyst surface structure under high temperature calcination and reaction atmosphere.It is found that the thermal movement of atoms and molecules under high temperature calcination leads to the formation and desorption of surface oxygen molecules;and the adsorption of hydrogen under hydrogen atmosphere leads to the formation and desorption of surface water.During the hydrogen reduction of spinel,single H₂ molecule is adsorbed on the perfect Cu Al₂O₄（100）and（110）surfaces via homolytical dissociative adsorption with the adsorption energies of?163 and?157 k J/mol vs.?99and?101 k J/mol for one O-defective Cu Al₂O₄（100）and（110）surfaces.The Gibbs Free energy change criterion shows the reduction temperatures of（100）and（110）surfaces under hydrogen atmosphere（100 k Pa）are 668 and 563 K vs,1947 and 2076K for（100）and（110）surfaces calcination at the oxygen partial pressure of 0.1 k Pa.The hydrogen atmosphere greatly promotes the reduction the surface Cu and the‘sustained release’of the catalyst.（3）Calculation the stability and growth mechanism of the active copper components‘sustained release’on the Cu-Al spinel.It is found that the adsorption energies for single Cu atom adsorbed on Cu Al₂O₄（100）and（110）surfaces are-430and-316 k J/mol,respectively.The adsorption energy for single Cu atom absorbed on the oxygen-defective surface is smaller than on the perfect Cu Al₂O₄surface.Bader charge and density of states analysis revel that the adsorption of Cu on the spine surface is accompanied by charge transfer.Due to the strong interaction of Cu with the Cu Al₂O₄（100）and（110）surfaces,the Cu₂ and Cu₃ prefer dissociative adsorption on the Cu Al₂O₄ surfaces.As indicated by the calculated adsorption energies,growth energies and aggregations energies,the interaction of Cu with Cu Al₂O₄ surface is much stronger than with theγ-Al₂O₃（110）surface.It shows that the spinel can effectively inhibit the growth and deactivation of the loaded copper.（4）Investigation the adsorption of n H₂O（n=1-8）on the Cu Al₂O₄（100）and（110）surfaces.The results showed that single water molecule is adsorbed on the Cu Al₂O₄（100）and（110）surfaces via dissociative adsorption with adsorption energies of?103and?170 k J/mol.On O defective Cu Al₂O₄ surfaces,the O atom of the water molecule prefers to insert into the O vacancy,leaving two isolated H atoms bonds to surface O atoms forming two in-surface hydroxyls.For n H₂O（n=2-8）on Cu Al₂O₄surfaces,molecular and dissociative water adsorption can coexist.The interaction of H₂O with Cu Al₂O₄（110）surface is much stronger than with the（100）surface.The partial density of states analysis revels that water molecules form bonds with surface Lewis acid sites through lone pairs electrons,while accompanied by charge transfer.The the work function is affected by water adsorption.As the coverage of water increases,the surface work function first decreases and then increases.（5）Investigating the methanol decomposition mechanism over Cu Al₂O₄ surface.The results show that the adsorption energies of molecular adsorption methanol on the（100）and（110）surfaces are?104 and?148 k J/mol,respectively,which is similar to the adsorption energy of molecular water.The first step of methanol decomposition is O-H breaking,the same reaction pathway was found on the（100）and（110）surfaces,expressed as CH₃OH→CH₃O→CH₂O→CHO→CO+H₂.The rate-determining step on the（100）surface is from CH₃O to CH₂O,with the energy barrier is 186 k J/mol;For the（110）surface is from CHO to CO,with the energy barrier is 305 k J/mol.It indicates that methanol decomposition on the（100）surface is easier than on the（110）surface.In addition,the elementary reaction rate and CO selectivity are calculated by molecular dynamics,and the results are in agreement with the experiment.In summary,the results confirmed the formation mechanism of Cu Al₂O₄,the catalyst surface‘sustained release’mechanism under the reaction atmosphere,and the reaction mechanism of the Cu Al₂O₄ catalyst methanol reforming to hydrogen,laying the foundation for the design,synthesis and application of the‘sustained release’spinel catalyst.