Bond Order Conservation Model Research Articles

Scaling relationships for adsorption energies of C 2 hydrocarbons on transition metal surfaces

Using density functional theory calculations we show that the adsorption energies for C 2H x -type adsorbates on transition metal surfaces scale with each other according to a simple bond order conservation model. This observation generalizes some recently recognized adsorption energy scaling laws for AH x -type adsorbates to unsaturated hydrocarbons and establishes a coherent simplified description of saturated as well as unsaturated hydrocarbons adsorbed on transition metal surfaces. A number of potential applications are discussed. We apply the model to the dehydrogenation of ethane over pure transition metal catalysts. Comparison with the corresponding full density functional theory calculations shows excellent agreement.

Theory of surface chemical reactivity

Fundamental reactivity concepts of relevance to the reactivity of transition metal surfaces are reviewed using elementary quantum-chemical concepts. The Newns-Anderson tight binding model of chemisorption is presented and subsequently used to outline the electronic structure characteristics of weak versus strong chemisorption. Fundamental concepts such as electron localization and surface complex embedding energies are defined and used to help explain surface reactivity. The emphasis here is on establishing an understanding of the surface chemical bond as a function of adatom coordination number, degree of coordinative unsaturation of the surface atoms and electron occupation of the d-type valence electron band. We derive from formal chemisorption theory the important relationships that exist between measured chemisorption properties and the average position of the d-valence electron band. The Newns-Anderson model is also used to show the relationship that exists between the d-band center and the coordinative unsaturation of the metal surface atoms. The general conclusion is that for Group VIII metals the shift of the average energy of the surface local density of states correlates with the strength of the interaction of the surface atoms with the metal atoms next to the surface layer. The same model is then used to analyze the Shustorovich bond order conservation model. The BOC or its modern version UBI-QEP is found to be consistent with a surface interaction potential comprised of a two-body repulsive term along with a constant attractive interaction independent of the number of coordinating atoms. The concepts of weak and strong chemisorption provide a very good basis for the subsequent analysis of the Bronsted-Eyring-Polanyi (BEP) relation. The extreme values of the BEP proportionality constant are related to the concept of loose and tight transition states. This proportionality constant between transition state energy and reaction energy can be expressed in parameters from the Newns-Anderson model by identifying loose transition states with intermediates in which the bond to be activated has not yet been broken, whereas in tight transition states this bond can be considered to be broken. We conclude the paper with an analysis of surface reconstruction. The power of the surface-molecule complex view of chemisorption will be quite apparent. The paper has an extensive introductory section to relate the topics of the four sections that follow with important classical catalytic notions.

First-principles-based kinetic Monte Carlo simulation of the selective hydrogenation of acetylene over Pd(111)

The kinetics for the selective hydrogenation of acetylene over Pd(111) was investigated by using first-principles-based kinetic Monte Carlo simulations. Density functional theory (DFT) calculations were carried out to obtain intrinsic kinetic data for a Horiuti–Polanyi-type reaction mechanism involving the sequential hydrogenation of acetylene. The results were subsequently used to develop a detailed intrinsic kinetics database that includes the adsorption energies of the reactants, intermediates, and products, the reaction energies and activation barriers of the elementary steps in the proposed reaction mechanism. The DFT-calculated energies and activation barriers were initially performed at lower surface coverages, to probe the intrinsic surface chemistry. Subsequent calculations were carried out at higher coverages, to capture the influence of the local environment on the reaction kinetics, and used to parameterize coarse-grained models that describe adsorbate interactions. A van der Waals force field model and a modified bond order conservation model were subsequently used within the simulation to calculate the local through-space interactions and the lateral through-surface interactions occurring between coadsorbates, respectively, and to assess the influence of the local reaction environment. The intrinsic DFT-derived kinetic data and the coarse-grained reaction environment models were used together in a variable time step kinetic Monte Carlo simulation to track the molecular transformations involved in acetylene hydrogenation over the (111) surface of Pd. The kinetic Monte Carlo simulation method [E.W. Hansen, M. Neurock, Chem. Eng. Sci. 54 (1999) 3411; E.W. Hansen, M. Neurock, J. Catal. 196 (2000) 241; E.W. Hansen, M. Neurock, Surf. Sci. 464 (2000) 91; E.W. Hansen, M. Neurock, J. Phys. Chem. B 105 (2001) 9218] used herein explicitly treats the atomic surface structure, the effects of the local reaction environment, and the reaction conditions on the surface kinetics. The simulated apparent activation energy for acetylene hydrogenation was calculated as 8.0 ± 0.6 kcal / mol at P H 2 = 100 Torr and P C 2 H 2 = 100 Torr over the temperature range of 300–500 K, in very good agreement with the value of 9.6 kcal/mol reported from experimental studies over well-defined Pd(111) surfaces [H. Molero, B.F. Bartlett, W.T. Tysoe, J. Catal. 181 (1999) 49]. The reaction orders were calculated as − 0.52 ± 0.03 for acetylene and 1.16 ± 0.03 for hydrogen, which agree very well with the experimental reaction orders by Molero et al. [H. Molero, B.F. Bartlett, W.T. Tysoe, J. Catal. 181 (1999) 49] of −0.66 and 1.04, respectively. A comparison of the simulations carried out assuming non-interacting adsorbates (hard sphere) and those that include lateral interactions between adsorbates showed that although the overall apparent activation energy was weakly sensitive to the presence of lateral interactions, the surface coverages and intrinsic rates changed considerably due to the presence of lateral interactions. The addition of lateral interactions between coadsorbates was found to be essential in simulating the correct overall selectivity behavior and appropriately predicting the apparent reaction orders with respect to hydrogen and acetylene.

First-principles-based kinetic Monte Carlo simulation of nitric oxide decomposition over Pt and Rh surfaces under lean-burn conditions

First-principles-based kinetic Monte Carlo simulation was used to track the elementary surface transformations involved in the catalytic decomposition of NO over Pt(100) and Rh(100) surfaces under lean-burn operating conditions. Density functional theory (DFT) calculations were carried out to establish the structure and energetics for all reactants, intermediates and products over Pt(100) and Rh(100). Lateral interactions which arise from neighbouring adsorbates were calculated by examining changes in the binding energies as a function of coverage and different coadsorbed configurations. These data were fitted to a bond order conservation (BOC) model which is subsequently used to establish the effects of coverage within the simulation. The intrinsic activation barriers for all the elementary reaction steps in the proposed mechanism of NO reduction over Pt(100) were calculated by using DFT. These values are corrected for coverage effects by using the parametrized BOC model internally within the simulation. This enables a site-explicit kinetic Monte Carlo simulation that can follow the kinetics of NO decomposition over Pt(100) and Rh(100) in the presence of excess oxygen. The simulations are used here to model various experimental protocols including temperature programmed desorption as well as batch catalytic kinetics. The simulation results for the temperature programmed desorption and decomposition of NO over Pt(100) and Rh(100) under vacuum condition were found to be in very good agreement with experimental results. NO decomposition is strongly tied to the temporal number of sites that remain vacant. Experimental results show that Pt is active in the catalytic reaction of NO into N2 and NO2 under lean-burn conditions. The simulated reaction orders for NO and O2 were found to be +0.9 and −0.4 at 723 K, respectively. The simulation also indicates that there is no activity over Rh(100) since the surface becomes poisoned by oxygen.

First-principles-based Monte Carlo methodology applied to O/Rh(100)

A kinetic Monte Carlo algorithm was developed to simulate the temperature-programmed desorption of oxygen on Rh(100). The interactions between surface oxygen atoms were derived by a bond order conservation (BOC) model that was parameterized from a database of ab-initio density functional quantum chemical results. The intrinsic activation barriers and heats of adsorption were taken from the DFT-calculated database. The effect of coverage on these intrinsic parameters was calculated internal to the simulation at each site throughout the simulation using the modified BOC model. The Monte Carlo simulation was subsequently used to follow the coverage-dependent desorption kinetics for O/Rh(100). The simulated temperature-programmed desorption peaks of 820, 925, and 1250 K compare favorably with experimental peak temperatures at 810, 920, and 1200 K. Below 0.3 ML, oxygen prefers the fourfold hollow sites and orders in a (2×2) arrangement. Between 0.3 and 0.5 ML, oxygen begins to reorder due to repulsive lateral interactions. The results shown here indicate that oxygen begins to occupy bridge sites. Although reconstruction is not explicitly considered, it would likely demonstrate similar effects of reducing repulsive interactions through a change in the surface structure.

Modeling surface kinetics with first-principles-based molecular simulation

A first-principles-based dynamic Monte Carlo (MC) algorithm was developed to simulate catalytic kinetics on well-defined transition metal surface. Density functional quantum chemical calculations along with fundamental experimental data were used to establish an intrinsic kinetic database. Both pairwise and bond order conservation models were parameterized from this database and used internally in the MC simulation to describe adsorbate lateral interactions. The MC algorithm is an object-oriented simulation that tracks the dynamic changes in the atomic surface structure with changes in processing conditions by following the molecular kinetics. The MC algorithm is applied herein to two relevant catalytic systems: (1) nitric oxide decomposition on Rh (1 0 0) and (2)ethylene hydrogenation over Pd (1 0 0 ). The results indicate that the ab initio modified bond order conservation model provides reliable predictions of surface kinetics. The simulated temperature programmed desorption profiles agree with the experimental measurements for both NO and ethylene. The simulated overall activation energy for the kinetics of ethylene hydrogenation was 9.2 kcal/mol that compares very well with experimental estimates of 6.5–10.7 kcal/mol.

99/01959 Energetics study on syngas (CO + H 2) methanation reaction on Ni, Cu and Ni-Cu alloy surfaces by bond-order conservation model : Xia, W.-S. et al. Huaxue Xuebao, 1998, 56, (8), 773–779. (In Chinese)

Bond - order conservation - Morse potential method has been some modified for alloy systems. Comparison with Ni(100) and Cu(100), syngas (CO + H2) methanation activity on NiCu(100) has been studied. The results show, due to the introduction of Cu, methanation on NiCu has much lower activity than on Ni, which can be explained by the formation of deposited Csfrom CO dissociation on Ni surfaces. Also, we investigate the rate - determining - step of methanation reaction on those surfaces, and the influences of the deposited surface species at reaction beginning on the reactivity of CO methanation.

Coadsorption, promotion, and poisoning effects: Analytic modeling based on bond-order conservation

Our bond-order conservation model [ J. Am. Chem. Soc. 106 (1984) 6479; Surface Sci. 150 (1985) L115, 163 (1985) L645, L730] has been extended to treat effects of an atomic modifier D, either electronegative or electropositive, on coadsorbed atoms A and diatomic molecules AB. Taking a fcc(100) surface as an example, explicit formulas have been obtained for the relevant heats of chemisorption Q A and Q AB as a function of the D coverage θ D when D occupies the hollow site, for three typical cases: (i) a single D; (ii) a p(2 × 2) structure, θ D = 1 4 ; (iii) a c(2 × 2) structure, θ D = 1 2 . The values of Q A and ( Q AB) been estimated for various on-top, bridge, and hollow sites for a realistic range of the parameter r = Q D Q A (Q AB) . Possible corrections due to direct D-A and D-AB interactions, both attractive and repulsive, are discussed. We consider coadsorption effects on the stretch frequency ν AB and dissociation activation barrier Δ E ∗ AB and project a variety of patterns depending on the nature of D, coverages θ D and θ AB, and some other parameters. The results obtained are in encouraging agreement with experiment. The complex nature of coadsorption effects is stressed.

Chemisorption phenomena: Analytic modeling based on perturbation theory and bond-order conservation

Tremendous advances in experimental studies of chemisorption revealed that many phenomena could not be understood and projected by the current theoretical constructs. We discuss some of the experimental puzzles that prompted a development of new analytic approaches to chemisorption based on general principles such as perturbation theory (PT) and bond-order conservation (BOC). The PT results concern the periodic regularities of the heat of chemisorption, the role of the antibonding adsorbate orbitals, and universal patterns of adsorbate-induced surface polarization Some of the PT findings are further corroborated within a much broader BOC approach. The BOC model and its postulates (including the use of a Morse potential) and diverse projections are thoroughly discussed. For atomic A and diatomic AB adsorbates, it is shown how the BOC model explicitly and rigorously interrelates a variety of seemingly disparate phenomena such as preferred adsorbate sites, the activation barriers for surface migration and dissociation, relations between atomic Q A ( Q B) and molecular Q AB heats of chemisorption, coverage and coadsorption effects on Q A, overlayer phase transitions and island formation, the nature of promotion and poisoning. The model also projects possible intermediates and elementary steps of surface reactions. Although some of the findings are counter to commonly held perceptions, the whole picture of chemisorption is coherent and fits experiment well. The new conceptual understanding is stressed and some comments on the theory of chemisorption are made.