Theoretical and kinetic assessment of the mechanism of ethane hydrogenolysis on metal surfaces saturated with chemisorbed hydrogen (original) (raw)
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Journal of Catalysis, 1989
A kinetic model for ethane hydrogenolysis over Pt, Pd, Ir, and Co was formulated in terms of essentially two chemical parameters: the strength of bonding between atomic hydrogen and the metal surface and the strength of carbon-metal bonding between hydrocarbon fragments and the surface. These two surface bond strengths were estimated by calorimetric measurements of the heats of Hz and CO adsorption, combined with bond order conservation calculations. The results of the kinetic simulations suggest that ethane hydrogenolysis over Pt, Pd, Ir, and Co takes place through irreversible C-C rupture of C2H4 and CrHr surface species. Hydrogenation of monocarbon CH, fragments is kinetically insignificant. Dissociative adsorption of hydrogen is an equilibrated process, while dissociative adsorption of ethane is slow and reversible. Finally, the role of kinetic modeling in the formulation, interpretation, and generalization of experimental research in heterogeneous catalysis is discussed. 8 1989 Academic FWSS, IK. 155
The Journal of Chemical Physics, 2003
Hydrogenation reaction, as one of the simplest association reactions on surfaces, is of great importance both scientifically and technologically. They are essential steps in many industrial processes in heterogeneous catalysis, such as ammonia synthesis (N 2 ϩ3H 2 →2NH 3). Many issues in hydrogenation reactions remain largely elusive. In this work, the NH x (xϭ0,1,2) hydrogenation reactions ͑NϩH→NH, NHϩH→NH 2 and NH 2 ϩH→NH 3) on Rh͑111͒ are used as a model system to study the hydrogenation reactions on metal surfaces in general using density-functional theory. In addition, C and O hydrogenation ͑CϩH→CH and OϩH→OH͒ and several oxygenation reactions, i.e., CϩO, NϩO, OϩO reactions, are also calculated in order to provide a further understanding of the barrier of association reactions. The reaction pathways and the barriers of all these reactions are determined and reported. For the C, N, NH, and O hydrogenation reactions, it is found that there is a linear relationship between the barrier and the valency of R ͑RϭC, N, NH, and O͒. Detailed analyses are carried out to rationalize the barriers of the reactions, which shows that: ͑i͒ The interaction energy between two reactants in the transition state plays an important role in determining the trend in the barriers; ͑ii͒ there are two major components in the interaction energy: The bonding competition and the direct Pauli repulsion; and ͑iii͒ the Pauli repulsion effect is responsible for the linear valency-barrier trend in the C, N, NH, and O hydrogenation reactions. For the NH 2 ϩH reaction, which is different from other hydrogenation reactions studied, the energy cost of the NH 2 activation from the IS to the TS is the main part of the barrier. The potential energy surface of the NH 2 on metal surfaces is thus crucial to the barrier of NH 2 ϩH reaction. Three important factors that can affect the barrier of association reactions are generalized: ͑i͒ The bonding competition effect; ͑ii͒ the local charge densities of the reactants along the reaction direction; and ͑iii͒ the potential energy surface of the reactants on the surface. The lowest energy pathway for a surface association reaction should correspond to the one with the best compromise of these three factors.
Kinetic and mechanistic features of carbon monoxide hydrogenation over supported transition metals
Theoretical and Experimental Chemistry, 1997
Data on the kinetics and mechanism of carbon monoxide hydrogenation to form alkanes, alkenes, and alcohols over supported transition metals are summarized and correlated. The observed kinetics of the overall carbon monoxide conversion can be interpreted on the basis of a mechanism that includes equilibrium adsorption of CO in the molecular form and equilibrium dissociative adsorption of hydrogen. Detailed mechanistic schemes and the corresponding kinetic models are presented for the reactions of formation of alkanes, alkenes, and alcohols. Catalytic activity and selectivity are examined in relation to the adsorptive and physicochemical properties of the catalysts. The hydrogenation of carbon monoxide over metallic catalysts can be directed toward the formation of various products methane and higher alkanes, alkenes, and alcohols. The distribution of the reaction products is greatly dependent on the nature of the metal, the fineness of metal dispersion, the reaction conditions, and other factors [1-5]. Various opinions are held on the mechanism of CO hydrogenation, both for the initial activation of the reactants (CO and hydrogen) and the formation of specific reaction products [1-3]. In this paper we will summarize and correlate results from systematic studies of the kinetics and mechanism of the overall hydrogenation of CO and the reactions through which individual products of CO hydrogenation are formed (alkanes, alkenes, oxygenates) over supported transition metals. ADSORPTION OF HYDROGEN AND CARBON MONOXIDE The adsorption of hydrogen on metals is predominantly dissociative; the initial heats of adsorption on polycrystalline Group VIII metals are 100-150 kJ/mole [1, 6-8]. In the adsorption of CO, carbonyl linear and bridge structures are characteristic; at elevated temperatures, dissociative adsorption of CO is also possible [1, 2, 6, 7]. The initial heats of adsorption of CO on metals vary within the limits of 100-200 kJ/mole, dropping off with increasing coverage of the surface with carbon monoxide [1, 6-8]. The adsorption of hydrogen and CO can be represented schematically as follows: H 2 + 2Z ~ 2ZH, CO + Z ~ ZCO. Here and subsequently, Z denotes a free site (center) on the catalyst surface, regarded as a participant in the reaction. A "site" on the surface is understood to be a structural unit of the surface (elementary area) that is capable of adsorbing one particle (molecule or atom). Let us note that with such a definition, the elementary area does not necessarily consist of a single atom of the catalyst surface layer. It may also consist of a group of mutually adjacent surface atoms, as shown in Fig. 1 for the (111) face of a metal.
Physical Review Letters, 2007
Density functional theory calculations are presented for CH x , x 0; 1; 2; 3, NH x , x 0; 1; 2, OH x , x 0; 1, and SH x , x 0; 1 adsorption on a range of close-packed and stepped transition-metal surfaces. We find that the adsorption energy of any of the molecules considered scales approximately with the adsorption energy of the central, C, N, O, or S atom, the scaling constant depending only on x. A model is proposed to understand this behavior. The scaling model is developed into a general framework for estimating the reaction energies for hydrogenation and dehydrogenation reactions.
The Journal of Chemical Physics, 2005
We determined the binding energy of hydrogen to the closest packed surface for all nine group VIII transition metals as a function of surface coverage using quantum mechanics ͑density functional theory with the generalized gradient approximation͒ with periodic boundary conditions. The study provides a systematic comparison of the most stable surfaces of the nine group VIII transition metals, leading to results consistent with available surface science studies. We then use these to develop a simple thermodynamic model useful in estimating the surface coverage under typical heterogeneous catalysis conditions and compare these results to temperature programmed desorption experiments.
Mechanistic study of the catalytic hydrogenolysis of ethane
Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1984
A mechanistic investigation of the hydrogenolysis of ethane is presented. The chemisorptions of ethane and hydrogen produce a common surface species, adsorbed hydrogen, and the coverages of the two-carbon-atom surface compound and the adsorbed hydrogen are interdependent through the partial pressures of ethane and hydrogen. The kinetically slow rupture of the C—C bond takes place in an interaction with a free site, adsorbed hydrogen or molecular hydrogen. Based on a theoretical analysis and previous experimental results, our conclusion is that molecular hydrogen is the most probable agent in the bond splitting.
The Journal of Physical Chemistry A, 2008
The adsorption of H 2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H 2 is found to be exclusively dissociative on Sc n O + , V n + , Fe n + , and Co n + , and both atomic and molecularly chemisorbed hydrogen is present in Ni n H m + complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2-or 3-fold coordinated to cationic metal clusters.
Insights into the Staggered Nature of Hydrogenation Reactivity over the 4d Transition Metals
The Journal of Physical Chemistry C, 2009
Hydrogenation reactions at transition metal surfaces comprise a key set of reactions in heterogeneous catalysis. In this paper, density functional theory methods are employed to take an in-depth look at this fundamental reaction type. The energetics of hydrogenation of atomic C, N, and O have been studied in some detail over low index Zr, Nb, Mo, Tc, Ru, Rh, and Pd surfaces. Detailed bonding analysis has also been employed to track carefully the chemical changes taking place during reaction. A number of interesting horizontal and vertical trends have been uncovered relating to reactant valency and metal d-band filling. A general correlation has also been found between the reaction barriers and the reaction potential energies. Moreover, when each reaction is considered independently, correlation has been found to improve with decreasing reactant valency. Bonding analysis has pointed to this being related to the relative position of the transition state along the reaction coordinate and has shown that as reactant valency decreases, the transition states become progressively later.
Hydrogen transfer in hydrocarbon conversions over transition metal surfaces
Surface Science, 2001
Hydrogen-transfer steps are a poorly studied and rarely proposed class of elementary reactions in heterogeneous catalysis over metal surfaces. In these steps, a hydrogen atom that is attached to the carbon in one surface species transfers to a carbon atom of another species. Unity bond index±quadratic exponential potential (UBI±QEP) calculations of the activation energies for three model surfaces representing substrates with strong, weak, and intermediate chemisorption abilities, combined with UBI±QEP/DFT data for the Pt(1 1 1) surface, suggest that there is no great energetic impediment to these steps. Therefore, they should not be arbitrarily ignored when building mechanisms of hydrocarbon conversions over metal surfaces. Moreover, kinetics simulations show that conditions are possible under which one-step hydrogen transfer is faster than the two-step process with the formation of adsorbed hydrogen.
Hydrogen Evolution from Metal–Surface Hydroxyl Interaction
The Journal of Physical Chemistry C, 2014
The redox interaction between hydroxyl groups on oxide surfaces and metal atoms and clusters deposited thereon, according to which metals get oxidized and hydrogen released, is an effective route to tune both the morphological (particle size and shape) and electronic (oxidation state) properties of oxide-supported metals. While the oxidation state of the metals can straightforwardly be probed by X-ray based methods (e.g., XPS), hydrogen is much more difficult to capture, in particular in highly reactive systems where the redox interaction takes place directly during the nucleation of the metals at room temperature. In the present study, the interaction of Pd with a hydroxylated MgO(001) surface was studied using a combination of vibrational spectroscopy, electronic structure studies including Auger parameter analysis, and thermal desorption experiments. The results provide clear experimental evidence for the redox nature of the interaction by showing a direct correlation between metal oxidation and hydrogen evolution at slightly elevated temperature (390 K). Moreover, a second hydrogen evolution pathway opens up at 500 K, which involves hydroxyl groups on the MgO support and carbon monoxide adsorbed on the Pd particles (water−gas shift reaction).