Density Functional Theory Study Into The HCN Adsorption On The Surfaces Of Fe, Co And Ni

Adsorption is an inevitable and basic step in heterogeneous catalytic reactions.And the catalysts can not speed the reaction until they can chemically adsorb the reactants. So,the mechanism of catalytic hydrogenation of nitriles is considered to start from the adsorptionand dissociation of nitriles on the surface of catalysts, and such course has already beendetermined as the reaction rate control step of catalytic hydrogenation of nitriles. Fe, CoandNi are important transition metal catalysts and have already been extensively used incatalytic reactions in recent years. Exploring the adsorption of HCN on the surface of Fe, CoandNi will have great significance for the study on the mechanism of transition metalcatalysis and the improvement of properties of catalyst. Research contents as follows:（1） Twenty kinds of adsorptions of HCN on the Fe（100）, Fe（111） and Fe（110） surfacesat the1/4monolayer coverage are found using the density functional theory. For Fe（100）, theadsorption energy of the most stable configuration where the HCN locates at the fourfold sitewith the C≡N bonded to four Fe atoms is–1.928eV. The most favored adsorption structure forHCN on Fe（111） is f-η³（N）-h-η³（C）, in which the C≡N bond is almost parallel to the surface,and the adsorption energy is–1.347eV. On Fe（110）, the adsorption energy in the most stableconfiguration in which HCN locates at the two long-bridge sites is–1.777eV. The adsorptionenergy of the parallel orientation for HCN is larger than that of the perpendicularconfiguration. The binding mechanism of HCN on the Fe（100）, Fe（111） and Fe（110） surfacesis also analyzed by Mulliken charge population and the density of states in HCN. The resultindicates that, the configurations in which the adsorbed HCN become the non-linear arebeneficial to the formation of the addition reaction for hydrogen. The nature that theintroduction of Fe into catalyst could increase the catalytic activity of the bimetallic catalystin the addition reaction of hydrogen for nitriles is revealed.（2） Thirteen kinds of adsorption adsorptions of HCN on the Co（100） and Co（110） surfaces at the1/4monolayer coverage are investigated using the density functional theory.For Co（100）, the adsorption energy of the most stable configuration where HCN locates at thefourfold site with the C≡N bonded to four Co atoms is–1.836eV. On Co（110）, the bondingenergy in the most favored adsorption configuration in which HCN locates at thefourfold-long site is–1.580eV. Similar to previous HCN/Co（111）, the parallel adsorptionconfiguration is energetically favored compared with the perpendicular mode and the formerweakens greater the strength of C≡N bond than the latter. Thus, the greater activation of theC≡N bond might be found in parallel configuration and hydrogenation reaction might beeasier in the parallel configuration in which the adsorbed HCN becomes the non-linear. Thebinding mechanism of HCN on the Co（100） and Co（110） surfaces is analyzed by Mullikencharge population in HCN.（3） The adsorption of HCN on the Ni（111）、Ni（100） and Ni（110） surface at the1/4monolayer coverage has been carried out at the level of density functional theory forunderstanding hydrogenation processes of nitriles. The most favored adsorption structure forHCN on Ni（111） is the C≡N bond almost parallel to the surface with the C≡N bondinteraction with adjacent surface Ni atoms （f-η³（N）-h-η³（C））, with the adsorption energy of-1.369eV. For Ni（100）, the most stable configuration is the HCN locates at the fourfold sitewith the C≡N bonded to four Ni atoms, and the adsorption energy is–1.932eV. In Ni（110）, theHCN adsorption has been computed, and the adsorption pattern is nearly similar to the HCNon Ni（111） and Ni（100）, respectively. The HCN locates at the two long-bridge sites and theenergy is–1.780eV. Furthermore, the binding mechanism of HCN on the Ni（111）、Ni（100）and Ni（110） surface is also analyzed. The result is that the adsorbed molecules rehybridize onthe surfaces, becoming non-linear with a bent H–C–N angle.（4） The DFT-B3LYP/6-311++G**and MP2（full）/6-311++G**calculations on thestructures and properties were carried out on the binary complex formed by M²⁺（M=Fe, Co,Ni） and HCN as well as the HCN ternary system with M²⁺(M²⁺:HCN=1:2). The cooperativityeffects between the hydrogen-bonding （N···H and π···H） and N···M²⁺as well as π···M²⁺interactions were investigated. The result shows that most of the equilibrium distances R_N···H, R_π···H, R₍N···M2+and Rπ···M²⁺in the ternary complex decrease and both the interactions arestrengthened when compared to the corresponding binary complex. The cooperativity oranti-cooperativity effect of the N···M²⁺and π···M²⁺interactions on the hydrogen-bondinginteractions is more pronounced than that of the hydrogen-bonding interactions on the N···M²⁺and π···M²⁺interactions. The nature of the cooperativity effect was revealed by the analysis ofthe atom in molecule （AIM） method. The result suggests that the cooperativity effect, in somecases, might influence the adsorption process of HCN on the M （M=Fe, Co, Ni） surface.