Stability and reactivity of monocarbon and dicarbon hydrocarbons on metals and metal alloys: A first principles study

Computational chemistry is becoming an important tool in conjunction with traditional experimental techniques for investigating the adsorbed species formed during a chemical reaction on a heterogeneous catalyst. The advent of fast computers along with tremendous advances in theoretical methods and the numerical algorithms have led to application of quantum mechanics to practical problems in catalysis.; The stability and reactivity of several small molecules on metals and metal alloys were studied using density functional theory techniques. The catalyst surface was modeled by using cluster and slab approaches. A good agreement is obtained between the calculated energetics and structures of adsorbed species and the corresponding experimental data. For example, the calculated energies of hydrogen, carbon monoxide and ethylene adsorption on platinum were within 20 kJ/mol of their respective experimental values. Importantly, theoretical methods allowed investigation of reactive intermediates, and activated complexes that are invisible to common experimental techniques. The energetic and entropic information obtained from the theory provides a reasonable parameter space for kinetic analyses. The principles of De Donder, which provide a simple means to determine the number of kinetic parameters required to calculate the overall reaction rate from a reaction scheme, were applied to study ethane hydrogenolysis on platinum and carbon monoxide methanation on nickel.; Quantum chemical methods were used to probe the fundamental surface chemistry for several catalytic systems. A correlation between the electronic structure and chemical reactivity was established based on the d-band center for the adsorption site. The defect sites simulated by the step edges of Pt(211) surface were shown to be more reactive than the terrace sites on Pt(111), which explains the structure sensitivity of ethane hydrogenolysis on platinum. It was shown that the most abundant species on the surface need not be the most reactive species. Addition of tin to platinum results in a significant electronic effect on the binding energy of ethylene. This electronic effect is dependent on the type of adsorption site. The addition of tin to Pt(111) leads to a surface heterogeneity, wherein ethylidyne species prefer to adsorb away from Sn atoms and oxygen atoms prefer to adsorb near Sn atoms.