Dynamics Of Dissociative Chemisorption Research Articles

Effects of vibrational and rotational excitations on dissociative chemisorption dynamics of N2 on Fe(111)

The dissociative chemisorption of N2 is the rate-limiting step for ammonia synthesis in industry. Here, we investigated the role of initially vibrational excitation and rotational excitation of N2 for its reactivity on the Fe(111) surface, based on a recently developed six-dimensional potential energy surface. Six-dimensional quantum dynamics study was carried out to investigate the effect of vibrational excitation for incidence energy below 1.6 eV, due to significant quantum effects for this reaction. The effects of vibrational and rotational excitations at high incidence energies were revealed by quasiclassical trajectory calculations. We found that raising the translational energy can enhance the dissociation probability to some extent, however, the vibrational excitation or rotational excitation can promote dissociation more efficiently than the same amount of translational energy. This study provides valuable insight into the mode-specific dynamics of this heavy diatom-surface reaction.

Fundamental invariant-neural network potential energy surface and dissociative chemisorption dynamics of N2 on rigid Ni(1 1 1)

We constructed a new full-dimensional potential energy surface (PES) for the dissociative chemisorption of N2 on rigid Ni(111). The PES was fitted using the fundamental invariant-neural network method based on DFT energy points. The resulting PES is accurately fitted with very small fitting error of about 4.5 meV, which reproduces well the minimum energy path and energies of stationary points. Based on the new PES, the effects of incidence energy, incidence angle as well as the vibrational excitations of impinging molecules on dissociation probability were investigated by quasi-classical trajectory calculations. The vibrational excitation enhances the reactivity more effectively than the same amount of translational energy, which is consistent with the very “product-like” transition state for this system. The dissociation occurs close to the bridge site, presumably due to the extremely tight and high-energy barriers of PES at top and hollow sites.

Dynamics of dissociative chemisorption of O2 on Cu(100) surface: A theoretical study

The dynamics of O2 dissociative chemisorption on Cu(100) is studied by means of quasi-classical dynamics simulation making use of a 6-dimensional potential energy surface based on density functional theory (DFT) calculations. The sticking probability is found in reasonable agreement with experiment above 0.3 eV collision energy. However, theory fails at capturing the activated behaviour experimentally evidenced for lower energies. While molecular beam experiments exhibit an energetic threshold in the sticking curve, simulations show a high dissociation probability at collision energies below this threshold. Present calculations suggest that this discrepancy is due to dynamics governed by a direct dissociation mechanism steming from several barrierless reaction paths associated with an indirect dissociation mechanism governed by dynamic trapping. These direct and indirect components are strongly related to the structure of the PES questioning the reliability of the DFT calculations and the choice of the functional used. Beyond the question of the DFT accuracy, this theoretical work addresses the still open question of experiments/theory comparison for systems involving O2 and metal surfaces such as Cu(100).

Vibrational enhancement in the dynamics of ammonia dissociative chemisorption on Ru(0001).

The vibrationally selective dissociative chemisorption dynamics of ammonia on the Ru(0001) surface is investigated using a quasi-classical trajectory (QCT) method on a new global potential energy surface (PES) with twelve dimensions. The PES is constructed by fitting 92 524 density functional theory points using the permutation invariant polynomial-neural network method, which rigorously enforces the permutation symmetry of the three hydrogen atoms as well as the surface periodicity. The PES enables highly efficient QCT simulations as well as future quantum dynamical studies of the scattering/dissociation dynamics. The QCT calculations yield satisfactory agreement with experiment and suggest strong activation of dissociation by both translational and vibrational excitations.

Dynamics of carbon monoxide dissociation on Co(112[combining macron]0).

The dissociative chemisorption dynamics of CO on rigid Co(112[combining macron]0) is investigated using a quasi-classical trajectory method on a new global six-dimensional potential energy surface (PES). The PES is fit using a neural network method to represent 24 630 density functional energies in various configurations. The reaction path features deep chemisorption wells and a late barrier for dissociation, agreeing well with previous calculations. The activation energy for dissociation ranges from 0.1 eV at the hollow site to 2.46 eV on the top site, indicating a highly corrugated PES. Effects of the incidence energy of the impinging molecule, its initial orientation, vibrational and rotational excitations, and site specificity are examined. Despite the presence of a low barrier, the initial dissociation probability is very small, even at high incident energies, as a large percentage of trajectories is either trapped or desorbed back to the gas phase. The low reactivity is attributed to inefficient energy transfer into the dissociation reaction coordinate in the chemisorption well where thermal equilibrium is not reached. This system underscores the importance of dynamics in understanding reactions at gas-surface interfaces and in kinetic modeling of catalytic processes.

First-principles quantum dynamical theory for the dissociative chemisorption of H2O on rigid Cu(111).

Despite significant progress made in the past decades, it remains extremely challenging to investigate the dissociative chemisorption dynamics of molecular species on surfaces at a full-dimensional quantum mechanical level, in particular for polyatomic-surface reactions. Here we report, to the best of our knowledge, the first full-dimensional quantum dynamics study for the dissociative chemisorption of H2O on rigid Cu(111) with all the nine molecular degrees of freedom fully coupled, based on an accurate full-dimensional potential energy surface. The full-dimensional quantum mechanical reactivity provides the dynamics features with the highest accuracy, revealing that the excitations in vibrational modes of H2O are more efficacious than increasing the translational energy in promoting the reaction. The enhancement of the excitation in asymmetric stretch is the largest, but that of symmetric stretch becomes comparable at very low energies. The full-dimensional characterization also allows the investigation of the validity of previous reduced-dimensional and approximate dynamical models.

Six-dimensional quantum dynamics of dissociative chemisorption of H2 on Co(0001) on an accurate global potential energy surface.

Cobalt is a widely used catalyst for many heterogeneous reactions, including the Fischer-Tropsch (FT) process, which converts syngas (H2 and CO) to higher hydrocarbons. As a result, a better understanding of the key chemical steps on the Co surface, such as the dissociative chemisorption of H2 as an initial step of the FT process, is of fundamental importance. Here, we report an accurate full-dimensional global potential energy surface for the dissociative chemisorption of H2 on the rigid Co(0001) surface constructed from more than 3000 density functional theory points. The high-fidelity potential energy surface was obtained using the permutation invariant polynomial-neural network method, which preserves both the permutation symmetry of H2 and translational symmetry of the Co(0001) surface. The reaction path features a very low barrier on the top site. Full-dimensional quantum dynamical calculations provide insights into the dissociation dynamics and influence of the initial vibrational, rotational, and orientational degrees of freedom.

Quantum dynamics of dissociative chemisorption of CH4 on Ni(111): Influence of the bending vibration

Two-dimensional, three-dimensional, and four-dimensional quantum dynamic calculations are performed on the dissociative chemisorption of CH(4) on Ni(111) using the multiconfiguration time-dependent Hartree (MCTDH) method. The potential energy surface used for these calculations is 15-dimensional (15D) and was obtained with density functional theory for points which are concentrated in the region that is dynamically relevant to reaction. Many reduced dimensionality calculations were already performed on this system, but the molecule was generally treated as pseudodiatomic. The main improvement of our model is that we try to describe CH(4) as a polyatomic molecule by including a degree of freedom describing a bending vibration in our three-dimensional and four-dimensional models. Using a polyspherical coordinate system, a general expression for the 15D kinetic energy operator is derived, which discards all the singularities in the operator and includes rotational and Coriolis coupling. We use seven rigid constraints to fix the CH(3) umbrella of the molecule to its gas phase equilibrium geometry and to derive two-dimensional, three-dimensional, and four-dimensional Hamiltonians, which were used in the MCTDH method. Only four degrees of freedom evolve strongly along the 15D minimum energy path: the distance of the center of mass of the molecule to the surface, the dissociative C[Single Bond]H bond distance, the polar orientation of the molecule, and the bending angle between the dissociative C[Single Bond]H bond and the umbrella. A selection of these coordinates is included in each of our models. The polar rotation is found to be important in determining the mode selective behavior of the reaction. Furthermore, our calculations are in good agreement with the finding of Xiang et al. [J. Chem. Phys. 117, 7698 (2002)] in their reduced dimensional calculation that the helicopter motion of the umbrella symmetry axis is less efficient than its cartwheel motion for promoting the reaction. The effect of pre-exciting the bend modes is qualitatively incorrect at higher energies, suggesting the necessity of including additional rotational and vibrational degrees of freedom in the model.

SIX-DIMENSIONAL DYNAMICS OF DISSOCIATIVE CHEMISORPTION OFH2ON METAL SURFACES

The theory of time-dependent quantum dynamics of dissociative chemisorption of hydrogen on metal surfaces is reviewed, in the framework of electronically adiabatic scattering from static surfaces. Four implementations of the time-dependent wave packet (TDWP) method are discussed. In the direct product pseudo-spectral and the spherical harmonics pseudo-spectral methods, no use is made of the symmetry associated with the surface unit cell. This symmetry is exploited by the symmetry adapted wave packet and the symmetry adapted pseudo-spectral (SAPS) method, which are efficient for scattering at normal incidence. The SAPS method can be employed for potential energy surfaces of general form.Comparison to experiment shows that the TDWP method yields good, but not yet excellent, quantitative accuracy for dissociation of (ν = 0, j = 0) H2if the calculations are based on accurately fitted density functional theory calculations that are performed using the generalized gradient approximation. The influence of the molecule's vibration (rotation) is well (reasonably well) described. The theory does not yet yield quantitatively accurate results for rovibrationally inelastic scattering. The theory has helped with the interpretation of existing experimental results, for instance, by solving a parodox regarding the corrugation of Pt(111) as seen by reacting and scattering H2. The theory has also provided some exciting new predictions, for instance, concerning where on the surface of Cu(100) H2reacts depending on its vibrational state. Future theoretical studies of H2reacting on metal surfaces will likely be aimed at validating GGAs for molecule-surface interactions, and understanding trends in collisions of H2with complex metal surfaces.

Six-dimensional quantum dynamics of dissociative chemisorption of H2 on Ru(0001)

Six-dimensional quantum dynamics calculations on dissociative chemisorption of H(2) on Ru(0001) are performed. The six-dimensional potential energy surface is generated using density functional theory. Two different generalized gradient approximations are used, i.e., RPBE and PW91, to allow the results to be compared. The dissociation probability for normally incident H(2) on a clean Ru(0001) surface is calculated. Large differences between the reaction probabilities calculated using the RPBE and PW91 are seen, with the PW91 results showing a much narrower reaction probability curve and a much higher reactivity. Using the reaction probabilities and assuming normal energy scaling reaction rates are generated for temperatures between 300 and 800 K. The rate generated using the PW91 results is higher by about a factor 5 than the rate based on the RPBE results in the range of temperatures relevant to ammonia production.

Influence of external stress on surface reaction dynamics.

In exploring the dynamics of dissociative chemisorption of O2 on clean and O-covered Cu(001), we demonstrate that the chemisorption is very sensitive to uniaxial tensile stress within an elastic limit. The stress enhances the dissociative chemisorption on clean Cu(001) when the translational energy of incident O2 is below 250 meV, and suppresses it when the energy is above 250 meV. In the case of an oxygen-covered Cu(001)- (2squareroot2 x squareroot2) surface, the dissociative adsorption probability of O2 decreases as the stress increases. The effect of external stress on the O2 dissociation dynamics is different between clean and O-covered surfaces.

Eigenstate-Resolved Studies of Gas-Surface Reactivity:CH4(ν3) Dissociation on Ni(100)

A new experimental technique uses state-resolved infrared laser excitation to probe a polyatomic molecule's dissociative chemisorption dynamics with previously unattainable detail. Methane molecules excited to $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ of the ${\ensuremath{\nu}}_{3}$ C-H stretching vibration are up to 1600 times more reactive on a clean Ni(100) surface than are molecules in the ground vibrational state. Over a translational energy range of 27 to 54 kJ/mol, their absolute reaction probability increases from $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ to $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, which indicates that ${\ensuremath{\nu}}_{3}$ is responsible only in part for the vibrational activation reported in previous studies.

Six-dimensional quantum dynamics of dissociative chemisorption of H2 on Cu(100)

The dissociative chemisorption of H2 on Cu(100) has been studied using a six-dimensional wave packet method. All molecular degrees of freedom are treated quantum mechanically, with no dynamical approximations. The potential-energy surface used is an analytical fit to calculations employing density functional theory (DFT), using the generalized gradient approximation (GGA) and a slab representation of the metal surface. The dependence of the reaction probability on normal incidence energy has been obtained for both (v=0, j=0) and (v=1, j=0) H2. Comparison to experiment suggests that, on average, the DFT method overestimates the barriers to dissociation by ∼0.15 eV for H2+Cu(100). In broad agreement with experiments on H2+Cu(111) and H2+Cu(110) the calculations show large vibrational inelasticity. A prediction of the survival probability of (v=1, j=0) H2 as a function of incidence energy is also presented. The measurement of this quantity is now feasible and should provide another sensitive test of the DFT potential-energy surface.

Activated adsorption of methane on clean and oxygen-modified Pt{111} and Pd{110}

Activated adsorption of CH 4 on clean and oxygen modified Pt{111} and Pd{110} has been studied using molecular beam surface scattering. The absolute dissociation probability of CH 4 was measured as a function of the incident normal energy ( E) and the surface temperature ( T s). The results from clean Pt{111} and Pd{110} are consistent with a direct dissociation mechanism. The dissociative chemisorption dynamics of CH 4 is addressed by using quantum mechanical and statistical models. The influence of adsorbed oxygen on the dissociative adsorption of CH 4 on both Pt{111} and Pd{110} shows that the dissociation probability decreases linearly with increasing oxygen coverage.

Dissociative Adsorption of O2 on Cu(110) and Cu(100): Three-Dimensional Quantum Dynamics Studies

The dynamics of dissociative chemisorption of oxygen on Cu(110) and Cu(100) has been studied using a time-dependent quantum wave-packet approach. Dissociation probabilities for O2 on both (110) and (100) surfaces of copper are calculated for ground state as well as rovibrationally excited oxygen molecules. The present calculation simulates O2 dissociation on nascent copper surfaces with no consideration for surface reconstruction. The dynamics calculation is based on a flat-surface model in which three molecular degrees of freedom are explicitly included while the lateral coordinates of the molecule are neglected. The interaction potential energy surface (PES) for the dynamics calculation is constructed using the LEPS (London−Erying−Polyni−Sato) PES form with potential parameters fitted to some available experimental and theoretical data. The barrier of the LEPS PES for oxygen dissociation on copper is 0.11 eV for Cu(110) and 0.08 eV for Cu(100). Relatively speaking, the saddle point of the O2/Cu(110) PES is located near the entrance channel, while that of the O2/Cu(100) is near the product channel. This feature is primarily responsible for the difference in calculated dissociation probabilities of oxygen on two surfaces. Specifically, the dissociation probability of O2 on Cu(110) is large and less sensitive to the vibrational excitation of the molecule, while that of O2 on Cu(100) is much smaller and more sensitive to vibrational excitation of the molecule.

Effect of molecular energy content on the dissociative chemisorption of N 2 on Re(0001)

Dissociative chemisorption dynamics of N 2 on Re(0001) was studied by employing molecular-beam methods. The dissociation probability S 0 increases three orders of magnitude from 4×10 −5 to 4×10 −2 upon increasing the normal translational energy of N 2 from 0.05 to 1.5 eV, respectively. These results imply the presence of a barrier for dissociation of 0.6±0.15 eV. The influence of vibrational excitation of the incident molecules on S 0 was investigated. A quantum-mechanical study was carried out to simulate the dissociation dynamics. The calculated results reproduce the experimental S 0 dependence on translational energy over a wide range. The vibrational energy is predicted to be less effective for the enhancement of S 0, in agreement with experiment. The quality of the agreement between the theoretical simulation and the experimental data strongly supports a tunneling process through the adiabatic barrier for dissociation as a key mechanism to explain the dynamics of N 2 chemisorption over Re.

Tunneling mechanism for the dissociative chemisorption of N 2 on metal surfaces

A quantum mechanical tunneling mechanism is found to describe accurately recent experimental study on the dissociative chemisorption dynamics of nitrogen molecules on metal surfaces. A numerical treatment of the time dependent Schrodinger equation was employed to evaluate the transition probability (S) from N 2 -metal to N-metal potential energy surfaces (PES) for two degrees of freedom on a flat and rigid surface. The rapid increase of S with incident kinetic energy, its saturation at energies above the barrier for dissociation and vibrational excitation effects were all in good agreement with the experimental results. The calculated results were found to be very sensitive to the potential topology at the crossing area (seam) between the two PES. This extreme sensitivity may serve as a calibration method for the potential shape, by using experimental data for the relative importance of vibration to translation in promoting the dissociation. Large isotope effects are predicted and confirmed by temperature programmed recombinative desorption experiments of 14 N 2 and 15 N 2 from Re(0001).

The dissociative chemisorption dynamics of N2 on catalytic metal surfaces: A quantum-mechanical tunneling mechanism

A tunneling mechanism is suggested for explaining the dissociative chemisorption of nitrogen molecules on metal surfaces. The time-dependent Schrödinger equation was numerically solved for the transition dynamics from the N2–metal to the N–metal potential-energy surfaces for two degrees of freedom. The dynamics was found to be sensitive to the topology at the crossing region between the two nonadiabatic potential-energy surfaces (PES). The resulting rapid increase of the dissociation probability (S0) with incident kinetic energy, its saturation at high energies and vibrational enhancement are in good agreement with recent experiments. A substantial isotope effect is predicted by the calculations. Recombinative desorption experiments of 14N2 and 15N2 from Re(0001) are in excellent agreement with the tunneling model.

Effect of the lattice model on the dynamics of dissociative chemisorption of H 2 on a Si(111) surface

The effects of (a) surface relaxation to the “bath” modes of the bulk, (b) the number of lattice atoms explicitly considered in the primary reaction zone, (c) lattice force constants, and (d) the functional form of the lattice potential upon the dynamics of H2 chemisorption on a Si(111) surface have been investigated. The H2 sticking probabilities, energy transfer rates out of the newly formed Si−H bond, and instantaneous surface mobilities have been computed. The sticking probabilities are virtually independent of surface relaxation, the size of the lattice model, and the nature of the lattice potential. Si−H vibrational energy transfer rates are increased and surface mobilities are decreased by inclusion of relaxation. However, the magnitude of these effects is less than a factor of two. Variation of the lattice potential is likewise found to exert a small effect upon the Si−H energy transfer rates and surface mobilities. The origin of these effects appears to be related to resonance effects between the Si−H vibrational motion and the phonon modes of the lattice. The results suggest that a three-layer lattice model with 25 atoms in the primary reaction zone is sufficient for this system. When primary zones of this size are employed, surface relaxation to the “bath” modes of the bulk are not of major importance for semiconductor systems at low temperatures.

Effect of the lattice model on the dynamics of dissociative chemisorption of H 2 on a Si(111) surface

Effect of the lattice model on the dynamics of dissociative chemisorption of H 2 on a Si(111) surface