Introduction to microkinetic modeling

This book started out as two separate documents. One was a set of exercises for the Advanced Thermodynamics and Catalysis course and the other was a method and theory section at that time envisioned for my PhD thesis. Only a very small part of the material in this book eventually made it into the thesis, as the whole would be much too elaborate

The quantum chemistry of transition metal surface bonding and reactivity

The electronic basis to the surface chemical bond of molecules and atoms chemisorbed to transition-metal surfaces is introduced. The chemical bonding features that determine preference of an adsorbate for different coordination sites are identified. This is related to a discussion of chemisorption as a function of particle size. Lateral effects relevant at high surface coverage are discussed with chemisorption-induced surface reconstruction. The analysis focuses on the relation of the surface chemical bond energy with degree of delocalization of surface transition-metal d-valence electrons and distribution of electrons over bonding and antibonding adsorbate complex fragment orbitals. The model of chemisorption as surface molecule complex formation embedded into the surface of a metal is shown to be a good approximation. This chapter concludes with an analysis of transition states of elementary surface reactions of small adsorbed molecular species. The structure and energy of such transition states are shown to relate in an interesting way to the nature of the chemical bond to be activated (p-bond vs s bond) and the topology of the reaction center

Quantum chemistry of the Fischer-Tropsch reaction catalysed by a stepped ruthenium surface

A comprehensive density functional theory study of the Fischer–Tropsch mechanism on the corrugated Ru(111) surface has been carried out. Elementary reaction steps relevant to the carbide mechanism and the CO insertion mechanism are considered. Activation barriers and reaction energies were determined for CO dissociation, C hydrogenation, CHx + CHy and CHx + CO coupling, CHxCHy–O bond scission and hydrogenation reactions, which lead to formation of methane and higher hydrocarbons. Water formation that removes O from the surface was studied as well. The overall barrier for chain growth in the carbide mechanism (preferred path CH + CH coupling) is lower than that for chain growth in the CO insertion mechanism (preferred path C + CO coupling). Kinetic analysis predicts that the chain-growth probability for the carbide mechanism is close to unity, whereas within the CO insertion mechanism methane will be the main hydrocarbon product. The main chain propagating surface intermediate is CH via CH + CH and CH + CR coupling (R = alkyl). A more detailed electronic analysis shows that CH + CH coupling is more difficult than coupling reactions of the type CH + CR because of the s-donating effect of the alkyl substituent. These chain growth reaction steps are more facile on step-edge sites than on terrace sites. The carbide mechanism explains the formation of long hydrocarbon chains for stepped Ru surfaces in the Fischer–Tropsch reaction

The optimally performing Fischer-Tropsch catalyst

Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal

The optimally performing Fischer-Tropsch catalyst

Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer-Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal-carbon and metal-oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal. State-of-the-art quantum-chemical reaction data were used in a microkinetics simulations study to elucidate the different fundamental kinetic regimes underlying Fischer-Tropsch activity and selectivity. Based on the nature of the rate-controlling steps, three regimes were identified: I) monomer formation, II) chain-growth termination, and III) water formation

The optimally performing Fischer-Tropsch catalyst

Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer-Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal-carbon and metal-oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal. State-of-the-art quantum-chemical reaction data were used in a microkinetics simulations study to elucidate the different fundamental kinetic regimes underlying Fischer-Tropsch activity and selectivity. Based on the nature of the rate-controlling steps, three regimes were identified: I) monomer formation, II) chain-growth termination, and III) water formation

Quantum chemistry of the Fischer-Tropsch reaction catalysed by a stepped ruthenium surface

A comprehensive density functional theory study of the Fischer–Tropsch mechanism on the corrugated Ru(111) surface has been carried out. Elementary reaction steps relevant to the carbide mechanism and the CO insertion mechanism are considered. Activation barriers and reaction energies were determined for CO dissociation, C hydrogenation, CHx + CHy and CHx + CO coupling, CHxCHy–O bond scission and hydrogenation reactions, which lead to formation of methane and higher hydrocarbons. Water formation that removes O from the surface was studied as well. The overall barrier for chain growth in the carbide mechanism (preferred path CH + CH coupling) is lower than that for chain growth in the CO insertion mechanism (preferred path C + CO coupling). Kinetic analysis predicts that the chain-growth probability for the carbide mechanism is close to unity, whereas within the CO insertion mechanism methane will be the main hydrocarbon product. The main chain propagating surface intermediate is CH via CH + CH and CH + CR coupling (R = alkyl). A more detailed electronic analysis shows that CH + CH coupling is more difficult than coupling reactions of the type CH + CR because of the s-donating effect of the alkyl substituent. These chain growth reaction steps are more facile on step-edge sites than on terrace sites. The carbide mechanism explains the formation of long hydrocarbon chains for stepped Ru surfaces in the Fischer–Tropsch reaction