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The purpose of this study is to predict the activation barriers and enthalpy for elementary steps in the process of Fischer-Tropsch (F-T) on the surfaces of Fe(110), Cu(111) and Fe/Cu alloys catalyst using 'Unity Bond Index-Quadratic Exponential Potential' method aimed at predicting the activity and selectivity on the basis of energy criteria. The elementary steps, such as dissociation of CO, hydrogenation of carbidic carbon, C-C chain growth by insertion of CH versus CO into the metal-alkyl bonds, and chain termination, which lead to hydrocarbons (alkanes versus α-olefins) or oxygenates are discussed in detail. The results show that metallic Fe(110) is necessary to produce the carbidic carbon from CO dissociation, but the synthesis of hydrocarbons and oxygenates can effectively proceed on Cu(111) surface. For optimum performance of F-T synthesis catalyst, these conflicting properties must be optimized. In this regard, we studied Fe/Cu alloy catalyst. On all the catalyst surfaces, the energetically preferred path to initiate the alkyl chain growth is via insertion of a CH group into the carbon-metal bond of a CH group. On FeCu catalyst surface, the activation barrier for termination of alkyl chain growth by β-elimination of hydrogen is found to be lower than that for α-addition of hydrogen and consequently for this catalyst, olefins are expected to form more readily than paraffins. The results of the model for a single metal surface are in agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Abstracts: The energetics of the water–gas shift (WGS) reaction (CO+H2O→CO2+H2) over the oxygen-preadsorbed Cu(111), Cu(100), and Cu(110) surfaces has been studied by first-principle density functional calculations together with the UBI-QEP approach. Cluster models of the surface have been employed to simulate the adsorption of H, OH, and H2O on the atomic oxygen-preadsorbed Cu(hkl) surfaces at low coverage. Optimized results show that pre-adsorbed oxygen could intensify the adsorption of H, OH, and H2O on Cu(hkl) surfaces. Interestingly, oxygen-induced spontaneous dissociation of water has been found with pre-adsorbed oxygen in the 4-fold hollow site. Furthermore, it can be observed from energetic analysis that the WGS reaction can be promoted by pre-adsorbed oxygen either by its acting as a promoter or as a reaction intermediate. This promoting role of pre-adsorbed oxygen over Cu(hkl) surfaces for the WGS reaction is in accord with the experiments. [Copyright &y& Elsevier]

The method of unity bond index–quadratic exponential potential (UBI–QEP) is employed to derive the energetic parameters associated with the steps of the pathway which we propose for the catalytic decomposition of azomethane on the Pd(111) surface. According to the energy calculations, azomethane adsorbs molecularly in trans-configuration and then decomposes to CH[sub 3]N with no activation energy. The reaction continues by tilting and dehydrogenation to the products (H[sub 2] and HCN). The calculated activation energies at various surface coverages perfectly account for the variation of relative yields of H[sub 2] and HCN with changing of the coverage of azomethane. According to the calculations, desorption of molecular azomethane at high surface coverage occurs, which is in excellent agreement with experimental data. [ABSTRACT FROM AUTHOR]

The formation of NH[sub 2,ads] and its further reactions with hydrogen and NO on the Pt(100) surface were previously studied by the methods of HREELS and TPR, in order to understand the role of amino species in the mechanism of the NO+H[sub 2] reaction. In this work the method of unity bond index – quadratic exponential potential (UBI-QEP) has been employed to rationalize the experimental findings by calculating the energies associated with the envisaged routes of reactions. It is concluded that the activation energy of formation of NH[sub 2,ads] is higher than water production. The simplicity of recombinative desorption of N[sub 2] is due to the decrease of its activation energy because of the destabilizing effect of O[sub ads] and NO[sub ads]. The rate-determining step of explosive surface reaction in the saturated coadsorption layer of NH[sub 2,ads] and NO[sub ads] is dissociation of NO[sub ads]. Autocatalytic acceleration of the explosive reactions is due to the decrease of activation energy of the rds by increasing the number adsorption vacant sites. H[sub 2]O is produced via two different processes with different activation energies. NH[sub 3] is produced via several paths. [ABSTRACT FROM AUTHOR]