Density Functional Study Of Adsorption Of CO₂/C₂H₆ Reaction System On Pd-based Catalyst

Using the weak oxidizability of CO₂ for the catalytic reaction of C₂H₆ is of great significance to environmental protection,industrial raw material production and energy efficient conversion.Exploring the adsorption process of the CO₂/C₂H₆ reaction system on Pd-based catalyst from the molecular scale is of great academic significance and engineering application value to the study of the microscopic reaction mechanism.In this paper,the density functional theory was used to simulate the adsorption process of the CO₂/C₂H₆ reaction system on Pd-based catalyst.The adsorption performance of CO₂/C₂H₆/CO/H₂/H₂O/C on the surface of Pd-based catalyst and its influence on the reaction were studied.Furthermore,the influence of the oxidation state of the active phase and carrier were discussed.The research results obtained in this paper are as follows:（1）The density functional theory was used to clarify the adsorption and activation of CO₂/C₂H₆/CO/H₂/H₂O/C on the surface of Pd-based catalyst and the effect for reactions was evaluated.In general,the adsorption energy of Pd_1-4/（Pd O）_1-4 surface are ranked as CO₂>C₂H₆ and CO>H₂>H₂O.In general,the activation ranks as CO₂>C₂H₆and H₂>CO>H₂O.The key step in the reaction between CO₂ and C₂H₆ is the adsorption and activation of C₂H₆.In the reactions involving intermediate reactants and products such as CO,H₂ and H₂O,the strong adsorption energy of CO and the low activation degree of CO are susceptible to toxic active sites,which is not conducive to the reaction.The suitable adsorption energy of H₂ and strong activation degree of H₂are conducive to the reaction.The low adsorption energy of H₂O and low activation degree of H₂O facilitate desorption.The adsorption energy of C on Pd_1-4/（Pd O）_1-4 is much higher than that of other adsorbates.Meanwhile,there is a variety of secondary stable adsorption sites for C.It is easy for C to seize the potential adsorption sites of other reactants,which poisons the reaction activity and reduces the overall reaction activity.（2）The effect of the support on the stability and surface adsorption performance of the Pd-based catalyst were revealed from the aspects of cluster growth energy,adsorption energy and adsorption geometry.γ-Al₂O₃carrier is benefit for the stability of Pd_1-4/（Pd O）_1-4.The growth energy of Pd_1-4/（Pd O）_1-4 before loading increases with the increase of cluster size.After loading,the growth energy of Pd_1-4/（Pd O）_1-4 peaked at Pd₃and（Pd O）₂,respectively.Then the growth slowed down.In all configurations,the growth rate of Pd_1-4/（Pd O）_1-4is lower than that before loading.The effect ofγ-Al₂O₃ for adsorption is different.In general,γ-Al₂O₃ is conducive to the adsorption of substances on Pd_1-2 and（Pd O）₄.Whileγ-Al₂O₃ is not conducive to the adsorption of substances on Pd_3-4 and（Pd O）_1-3.Surface-assisted bonding and cluster deformation is conducive to adsorption stability.The promotion effect ofγ-Al₂O₃ for the adsorption on（Pd O）₄/γ-Al₂O₃ is attributed to the free deformation promotion effect caused by partial loading.The steric hindrance and the bonding of the carrier to the cluster restrict the deformation of the clusters on the surface of Pd_3-4/γ-Al₂O₃ and（Pd O）_1-3/γ-Al₂O₃,which is detrimental to the adsorption stability.The cluster distortion in all configurations is harmful to the stability of adsorption.（3）The effects of the active phase oxidation state on the performance of Pd-based catalyst were studied in terms of adsorption energy,adsorbate activation,and cluster stability.Compared with Pd_1-4,（Pd O）_1-4 is benefit for the adsorption of H₂,H₂O and C due to the bonding of O atoms in（Pd O）_1-4 and the formation of hydrogen bond.Compared with Pd_1-4,activation degree of CO₂ and H₂is significantly promoted by（Pd O）_1-4 due to the increase in bonding strength of O atoms.Compared with Pd_1-4,the stability of（Pd O）_1-4 is lower than that of Pd_1-4.During the adsorption process,CO and C is prone to seize the O atoms in（Pd O）_1-4 for geometric distortion of（Pd O）_1-4,which is detrimental to cluster stability.