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A new kinetic approach is presented to study the mechanism of the catalytic reaction of the hydrogenation of carbon monoxide on Fe–Co–Mn catalyst prepared by wet impregnation method over MgO support. The experimental data obtained by a fixed bed reactor are used for calculating the apparent activity energy of the LHHW kinetic models developed for the consumption of carbon monoxide, and formation of methane, alkenes and alkanes. Then, using the calculated activation energies and the available UBI–QEP relationships for the ternary catalyst and obtaining the relationships between the apparent activity energy of the LHHW kinetic model and the activity energy of surface elementary reactions involved in the kinetic equation extraction and by developing an appropriate software coupled with a genetic optimization algorithm, the surface binding energy of carbon, oxygen and hydrogen on the ternary catalyst are calculated. [ABSTRACT FROM AUTHOR]

The Single-Event MicroKinetic methodology has been successfully extended from Fe to Co catalyzed Fischer-Tropsch Synthesis. A total of 82 experiments were performed in a plug flow reactor with a H 2 to CO molar inlet ratio between 3 and 10, a temperature range from 483 to 503 K, CO inlet partial pressures from 3.7 to 16.7 kPa and space time varying between 7.2 and 36.3 (kg cat s) mol −1 . Via regression, statistically significant and physicochemically meaningful estimates were obtained for the activation energies in the model and the H, C and O atomic chemisorption enthalpies as required for the UBI-QEP method. A reaction path analysis allowed relating the observed deviations from the Anderson-Schulz-Flory distribution, i.e., a high methane and low ethene selectivity, to the symmetry numbers and a higher chemisorption enthalpy of the metal methyl species compared to the other metal alkyl species. Simulations at industrially relevant conditions show that, as a catalyst descriptor, the H atomic chemisorption enthalpy crucially determines both the CO conversion and the C 5+ selectivity. The higher FTS activity of Co compared to Fe is explained via the higher oxygen atomic chemisorption enthalpy on the latter compared to the former. [ABSTRACT FROM AUTHOR]

A novel approach in kinetic study of CO hydrogenation reaction over Co-Ni/AlO catalyst was described. The experimental data were collected using a fixed bed micro reactor under next conditions: pressure 2-12 bar, temperature 230-280°C, molar ratio H: CO = 1-3, space velocity 2000-2700 h. A new Langmiur-Hinshelwood (LH) kinetic model on the basis of formyl mechanism was developed. The apparent activation energy of LH kinetic model, bond energy of carbon, oxygen, hydrogen and some surface intermediate on catalyst surface, activation energies of some surface reactions and catalyst surface coverage were estimated from a new combined LHHW/UPI-QEP methodology. It was postulated that under reaction conditions used, CH formation through H-assisted CO dissociation is probably dominant mechanism of hydro-genation over Co-Ni catalyst. [ABSTRACT FROM AUTHOR]

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]

Abstract: The microkinetic of gas-solid Fischer-Tropsch synthesis on Fe–Cu–K–SiO2 catalyst was studied which was consisted of different elementary steps. The operation condition was in temperature of 523K, total pressures from 0.8 to 3.2MPa, space velocity from 0.5 to 2.0 ×10-3 Nm3kg-1s-1 and H2/CO inlet ratios from 0.5 to 2mol/mol. Unity bond indexquadratic exponential (UBI-QEP) and transition state theory (TST) was used to calculate heats of adsorption and initial estimates of some of the kinetic parameters. The pre-exponential factors were calculated based on statistical thermodynamics. The accuracy of the UBI-QEP method in calculating the energetic of the elementary reactions on catalyst surface was acceptable. The calculated activation energies to produce n-alkanes and 1-alkenes were in range of 19.57 to 25.68 and 19.53 to 20.45kcal/mol respectively. These values were used to obtain the product distributions. However the application of the microkinetic study with molecular approach can provide better insight to the analysis of the FTS reaction mechanism. [Copyright &y& Elsevier]

Abstract: Elementary reactions that are relevant to catalytic hydrogen production have been evaluated over a wide range of transition metals. The UBI-QEP formalism was used to estimate heats of chemisorption and activation energies. The reactions were evaluated on a clean surface with the primary purpose of illustrating relative differences of intrinsic properties. The results were supportive of what is typically observed experimentally, in particular with respect to the relative difference between classical combustion catalysts (Pt, Pd) and state-of-art reforming catalysts (Ni, Rh). The likely scrambling of CH x species was also predicted which is in accordance with isotopic tracer studies in the literature. [Copyright &y& Elsevier]

Abstract: Theoretical analysis of ORR on Pt (111) was carried out with the combined technique of DFT calculation and the UBI-QEP method in order to understand the overall ORR pathways, behavior of H2O2 formation, and the impact of trifluoromethane sulfonic acid (CF3SO3H and TfOH) coverage, the alternative material of Nafion®, on the reactivity on the Pt surface. The ORR scheme consisting of elementary reactions was then modeled to determine the dominant path and the limiting step based on their activation energies. The results showed that the dominant ORR path included the H2O2 formation step and OOH formation step was limiting. When TfOH covered the Pt surface, it was revealed that the adsorption energy of an O2 molecule on Pt (111) was decreased due to the lower Fermi level and the d-band center, resulting in decreasing the activation energy of the limiting step. TfOH, however, could suppress the O2 adsorption on the Pt surface. In addition, with the TfOH coverage, it was indicated that the limiting step of ORR was shifted to H2O-production step which was after the H2O2 production, resulting in the enhancement of the H2O2 formation. [Copyright &y& Elsevier]

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]

The energetics of the adsorption and decomposition of ammonia on Nb(100) surface was investigated by the method of unity bond index-quadratic exponential potential (UBI-QEP). The experimental as well as theoretically derived atomic heats of adsorptions were used as the input data and the results were compared with the findings of local density functional theory (LDFT) and non-local density functional theory (NLDFT). The method was capable of correctly predicting the fragmentation of ammonia on 4-fold hollow sites on the surface of Nb(100) and magnitudes of the heats of adsorptions and activation energies were more realistic. [ABSTRACT FROM AUTHOR]