Study On The Interaction Between Pd_nB/Pd And H₂ And The Mechanism Of C₂H₂ Hydrogenation Catalyzed By Pd_nB/Pd

Palladium catalysts have attracted considerable attention because of their wide applications in different fields such as pharmaceutical synthesis,pesticide synthesis,photoelectric materials,fiber synthesis and other fine chemical fields.The methods commonly used to improve the catalytic performance of metal palladium include adding additives,modifying carriers and adding the second component metal to form alloys.Recent studies have found that palladium nanoparticles modified with interstitial B atoms(Pd-B catalyst)exhibited ultra-catalytic performance in selected partial hydrogenation reaction.Morover,Single-Atom Catalysis has the characteristic of"isolated active site"of homogeneous catalyst,which is the bridge between heterogeneous catalysis and homogeneous catalysis.Single-Atom Catalysis provides excellent guidance for researchers to synthesize better catalysts both experimentally and theoretically.Therefore,the Pd catalyst is modified with interstitial B atom and dispersed to monatom.Selective hydrogenation of acetylene will be used as a probe reaction to deeply explore the structure-activity relationship between microstructure and catalytic performance of Pd-B and Pd catalyst,and to investigate the basic problem of whether the acetylene hydrogenation catalyzed by Pd is optimal.A catalyst should have good stability firstly.To make clear the effect of interstitial B atom,the structures,stabilities and electronic properties of Pd_nB(n=1-10)clusters have been analyzed by density functional theory.B atom takes residence in octahedral positions of the Pd_n,which just are interstitial lattice sites of Pd catalyst.B atom is easily incorporated into when Pd:B?6:1.The magic numbers of Pd_nB(n=1-10)clusters is 6.The Pd₆B happens to be the unit structure of Pd-B catalyst.Based on the analysis of structure parameters,electronic properties and d-band of Pd₆B,we conclude that the improvement of Pd catalyst by interstitial B atom is mainly due to lattice expansion,strong electronic effect and hindering interstitial H atom.The effectiveness of hydrogenation catalysts depends largely on how easily hydrogen molecules could be decomposed into hydrogen atoms.The configurations of Pd₆and Pd₆B were optimized and adsorption energies of multiple of hydrogen molecules as well as the energy barriers of hydrogen molecule dissociation and hydrogen atom migration have been comparised.It is concluded that there is one hydrogen molecule can dissociate on Pd₆and two hydrogen molecules can dissociate on Pd₆B.The dissociation energy barrier of hydrogen on Pd₆is 0.61 eV and migration energy barrier of H atom is only 0.07 eV.While the dissociation energy barrier of hydrogen on Pd₆B is 0.44 eV and migration energy barrier of H atom is only0.10 eV.The second hydrogen molecule has a dissociation barrier of 0.25 eV and a migration barrier of 0.01 eV.It means that hydrogen molecules are more easily dissociated on the surface of Pd₆B.Detailed DFT calculations of C₂H₂hydrogenation on Pd₆,Pd₆⁺and Pd₆B clusters were performed to explore the effect of interstitial B on the mechanisms of C₂H₂hydrogenation catalyzed by Pd₆.The results show that the Pd₆B cluster has the lowest diffusion barriers of H atoms and dissociated barriers of H₂;the interstitial B atom can simultaneously improve both the activity and selectivity of C₂H₂hydrogenation to C₂H₄on Pd₆cluster by altering the major product and the optimal pathway.Moreover,the higher charge is conducive to enhancing the adsorption of C₂H₂and C₂H₄,rather than the selectivity of C₂H₄formation.The whole process from acetylene to ethylene catalyzed on Pd is exothermic by 37.58kcal/mol.The rate-determining step is the dissociation of the hydrogen molecule and the migration of the first H atom.The energy barrier is 33.44kcal/mol,higher than 26.49kcal/mol on the surface of Pd₆.The whole process of the formation of ethane from ethylene exothermic by 3.04 kcal/mol and the energy barrier was 44.56 kcal/mol.Although both Pd and Ti can catalyze the hydrogenation of acetylene to form ethylene,Pd is more favorable than Ti in thermodynamics to form ethylene,but the activity is weaker than Pd₆.Pd atom catalyzed hydrogenation of benzene is not allowed thermodynamically,while Ti atom could catalyze C₆H₆to Ti H₃C₆H₁₁.The binding energies between Pd,Pd₆and Pd₆B and grepheen are similar to the binding energies between clusters and hydrogen molecules.Therefore,the desorption of hydrogen molecules competes with that of Pd H₂,Pd₆H₂and Pd₆BH₂.When Pd,Pd₆and Pd₆B are loaded on graphene with a C-vacancy and N-doped defective graphene,the binding energies increases 3-5 times respectively,indicating that vacancy or N-doped defects can effectively avoid desorption competition between hydrides and hydrogen molecules.In addition,compared with Pd6,the adsorption of hydrogen molecules to pd6b was almost unaffected by defect and N-doping,which is remine at-0.45 to-0.49 eV.As a potential hydrogenation catalyst,pd/graphene has three major problems:transition metal agglomeration,H atom in lattice space,and Pd_n-H_xdesorption.In this paper,B atom is used to block interstitial of H atom,and the surrounding environment of the active site is changed by defects and N doping.The results showed that when graphene was defective,a Pd atom in the cluster would be anchored at the defect site.The defects increases the binding energy of Pd,Pd₆,Pd₆B on graphene by 5-7 times,and the N-doped defect site increases the binding energy by 2-3 times.After hydrogen adsorption,the binding ability of Pd,Pd₆and Pd₆B to defective graphene C₄₉increased by about 0.44-1.77eV,and the difference between binding energy of Pd_n-H₂on graphene and the adsorption energy between hydrogen molecule on Pd cluster were 0.05-0.42eV on C₅₀,5.56-6.90eV on C₄₉,and 1.17-2.93eV on N-doped graphene C₄₆N₃.It indicates that the defect can guarantee the structure stability of the catalyst during the adsorption of hydrogen molecules.In conclusion,defects can enhance the interaction between clusters and graphene,and overcome desorption of Pd_n-H₂from the graphene surface to ensure the structure of the catalyst remains stable.