Normal and off-normal incidence dissociative dynamics of O2(v,J) on ultrathin Cu films grown on Ru(0001).

The dissociative adsorption of molecular oxygen on metal surfaces has long been controversial, mostly due to the spin-triplet nature of its ground state, to possible non-adiabatic effects, such as an abrupt charge transfer from the metal to the molecule, or even to the role played by the surface electronic state. Here, we have studied the dissociative adsorption of O2 on CuML/Ru(0001) at normal and off-normal incidence, from thermal to super-thermal energies, using quasi-classical dynamics, in the framework of the generalized Langevin oscillator model, and density functional theory based on a multidimensional potential energy surface. Our simulations reveal a rather intriguing behavior of dissociative adsorption probabilities, which exhibit normal energy scaling at incidence energies below the reaction barriers and total energy scaling above, irrespective of the reaction channel, either direct dissociation, trapping dissociation, or molecular adsorption. We directly compare our results with existing scanning tunneling spectroscopy and microscopy measurements. From this comparison, we infer that the observed experimental behavior at thermal energies may be due to ligand and strain effects, as already found for super-thermal incidence energies.

Dissociative and non-dissociative adsorption of O2 on Cu(111) and CuML/Ru(0001) surfaces: adiabaticity takes over.

The role of spin non-adiabatic effects in the reactivity of O2 on metal surfaces has been a matter of debate for several years. By means of density functional theory with a semi-local exchange-correlation functional, and classical dynamics calculations, we show that the recently observed activated character of O2 adsorption on Cu(111) and CuML/Ru(0001), as well as the delicate interplay between dissociative and non-dissociative O2 sticking on Cu(111) at different surface temperatures, can be explained by assuming an adiabatic evolution of the molecular spin. This suggests that spin adiabaticity during O2 adsorption on metal surfaces could be a more general scenario than anticipated.

Dynamics of H2 Eley-Rideal abstraction from W(110): sensitivity to the representation of the molecule-surface potential.

Dynamics of the Eley-Rideal (ER) abstraction of H2 from W(110) is analyzed by means of quasi-classical trajectory calculations. Simulations are based on two different molecule-surface potential energy surfaces (PES) constructed from Density Functional Theory results. One PES is obtained by fitting, using a Flexible Periodic London-Eyring-Polanyi-Sato (FPLEPS) functional form, and the other by interpolation through the corrugation reducing procedure (CRP). Then, the present study allows us to elucidate the ER dynamics sensitivity on the PES representation. Despite some sizable discrepancies between both H+H/W(110) PESs, the obtained projectile-energy dependence of the total ER cross sections are qualitatively very similar ensuring that the main physical ingredients are captured in both PES models. The obtained distributions of the final energy among the different molecular degrees of freedom barely depend on the PES model, being most likely determined by the reaction exothermicity. Therefore, a reasonably good agreement with the measured final vibrational state distribution is observed in spite of the pressure and material gaps between theoretical and experimental conditions.

Chain-length and temperature dependence of self-assembled monolayers of alkylthiolates on Au(111) and Ag(111) surfaces.

We present a molecular dynamics (MD) study on the structure of self-assembled monolayers (SAMs) of alkylthiolates on various metal surfaces, with especial attention to Au(111) and Ag(111). Variations in the structure of these SAMs as a function of temperature and alkyl-chain length are systematically investigated. The MD simulations are performed by using a recently developed force field based on second-order Møller-Plesset perturbation theory calculations. Good agreement between the present results and the existing experimental data is found on Au(111). On Ag(111) the comparison between theory and experiment is also satisfactory for alkylthiolates with no more than 14 carbon atoms. The dependences of the average tilt angle of SAMs on temperature and chain length are easily understood by means of a simple single-chain model.

Electronic friction dominates hydrogen hot-atom relaxation on Pd(100).

We study the dynamics of transient hot H atoms on Pd(100) that originated from dissociative adsorption of H2. The methodology developed here, denoted AIMDEF, consists of ab initio molecular dynamics simulations that include a friction force to account for the energy transfer to the electronic system. We find that the excitation of electron-hole pairs is the main channel for energy dissipation, which happens at a rate that is five times faster than energy transfer into Pd lattice motion. Our results show that electronic excitations may constitute the dominant dissipation channel in the relaxation of hot atoms on surfaces.

Towards bond selective chemistry from first principles: methane on metal surfaces.

Controlling bond-selective chemical reactivity is of great importance and has a broad range of applications. Here, we present a molecular dynamics study of bond selective reactivity of methane and its deuterated isotopologues (i.e., CH(4-x)D(x), x=0,1,2,3,4) on Ni(111) and Pt(111) from first principles calculations. Our simulations allow for reproducing the full C-H bond selectivity recently achieved experimentally via mode-specific vibrational excitation and explain its origin. Moreover, we also predict the hitherto unexplored influence of the molecular translational energy on such a selectivity as well as the conditions under which the full selectivity can be realized for the a priori less active C-D bond.

Environment-driven reactivity of H2 on PdRu surface alloys.

The dissociative adsorption of molecular hydrogen on Pd(x)Ru(1-x)/Ru(0001) (0 ≤ x ≤ 1) has been investigated by means of He atom scattering, Density Functional Theory and quasi-classical trajectory calculations. Regardless of their surroundings, Pd atoms in the alloy are always less reactive than Ru ones. However, the reactivity of Ru atoms is enhanced by the presence of nearest neighbor Pd atoms. This environment-dependent reactivity of the Ru atoms in the alloy provides a sound explanation for the striking step-like dependence of the initial reactive sticking probability as a function of the Pd concentration observed in experiments. Moreover, we show that these environment-dependent effects on the reactivity of H2 on single atoms allow one to get around the usual constraint imposed by the Brønsted-Evans-Polanyi relationship between the reaction barrier and chemisorption energy.

Reactive scattering of H2 from Cu(100): comparison of dynamics calculations based on the specific reaction parameter approach to density functional theory with experiment.

We present new experimental and theoretical results for reactive scattering of dihydrogen from Cu(100). In the new experiments, the associative desorption of H(2) is studied in a velocity resolved and final rovibrational state selected manner, using time-of-flight techniques in combination with resonance-enhanced multi-photon ionization laser detection. Average desorption energies and rotational quadrupole alignment parameters were obtained in this way for a number of (v = 0, 1) rotational states, v being the vibrational quantum number. Results of quantum dynamics calculations based on a potential energy surface computed with a specific reaction parameter (SRP) density functional, which was derived earlier for dihydrogen interacting with Cu(111), are compared with the results of the new experiments and with the results of previous molecular beam experiments on sticking of H(2) and on rovibrationally elastic and inelastic scattering of H(2) and D(2) from Cu(100). The calculations use the Born-Oppenheimer and static surface approximations. With the functional derived semi-empirically for dihydrogen + Cu(111), a chemically accurate description is obtained of the molecular beam experiments on sticking of H(2) on Cu(100), and a highly accurate description is obtained of rovibrationally elastic and inelastic scattering of D(2) from Cu(100) and of the orientational dependence of the reaction of (v = 1, j = 2 - 4) H(2) on Cu(100). This suggests that a SRP density functional derived for H(2) interacting with a specific low index face of a metal will yield accurate results for H(2) reactively scattering from another low index face of the same metal, and that it may also yield accurate results for H(2) interacting with a defected (e.g., stepped) surface of that same metal, in a system of catalytic interest. However, the description that was obtained of the average desorption energies, of rovibrationally elastic and inelastic scattering of H(2) from Cu(100), and of the orientational dependence of reaction of (v = 0, j = 3 - 5, 8) H(2) on Cu(100) compares less well with the available experiments. More research is needed to establish whether more accurate SRP-density functional theory dynamics results can be obtained for these observables if surface atom motion is added to the dynamical model. The experimentally and theoretically found dependence of the rotational quadrupole alignment parameter on the rotational quantum number provides evidence for rotational enhancement of reaction at low translational energies.

Commensurate solid-solid phase transitions in self-assembled monolayers of alkylthiolates lying on metal surfaces.

A temperature-induced commensurate solid-solid phase transition in self-assembled monolayers (SAMs) of alkylthiolates lying on Pt(111) is predicted from molecular dynamics simulations based on ab initio potential energy surfaces. As the system cools down from room temperature to low enough temperature, SAMs of alkylthiolates with more than ~12 carbon atoms undergo an abrupt change of orientation from a nearly upright to a tilted configuration. As the initial hexagonal arrangement of the sulfur headgroups is kept fixed during the simulations, the phase transition is entirely governed by chain-chain interactions. Similar commensurate phase transitions are predicted for hexagonally arranged SAMs with lattice spacings of the order of 4.7-4.9 Å, which, among others, excludes the well-known cases of densely packed SAMs of alkylthiolates on Au(111) and Ag(111). These findings could be relevant for the design of novel electronic or optical devices controllable by temperature.

H2 dissociation on individual Pd atoms deposited on Cu(111).

We present a Molecular Dynamics (MD) study based on Density Functional Theory (DFT) calculations for H(2) interacting with a Pd-Cu(111) surface alloy for low Pd coverages, Θ(Pd). Our results show, in line with recent experimental data, that single isolated Pd atoms evaporated on Cu(111) significantly increase the reactivity of the otherwise inert pure Cu surface. On top of substitutional Pd atoms in the Pd-Cu(111) surface alloy, the activation energy barrier for H(2) dissociation is smaller than the lowest one found on Cu(111) by a factor of two: 0.25 eV vs. 0.46 eV. Also in agreement with experiments, our DFT-MD calculations show that a large fraction of the dissociating H atoms efficiently spillover from Pd (i.e. the active sites), thanks to their extra kinetic energy due to the ~0.50 eV chemisorption exothermicity. Still, our DFT-MD calculations predict a dissociative sticking probability for low energy H(2) molecules that is much smaller than the estimated value from scanning tunneling microscopy experiments. Thus, further theoretical and experimental investigations are required for a complete understanding of H(2) dissociation on low-Θ(Pd) Pd-Cu(111) surface alloys.

Theoretical study of the structure of self-assembled monolayers of short alkylthiolates on Au(111) and Ag(111): the role of induced substrate reconstruction and chain-chain interactions.

We compare the stability of various structures of high coverage self-assembled monolayers (SAMs) of short alkylthiolates, S(CH(2))(n-1)CH(3) (= C(n)), on Ag(111) and Au(111). We employ: (i) the ab initio thermodynamics approach based on density functional theory (DFT) calculations, to compare the stability of SAMs of C(1) (with coverages Θ = 3/7 and 1/3) on both substrates, and (ii) a set of pairwise interatomic potentials derived from second-order Møller-Plesset (MP2) perturbation theory calculations, to estimate the role of chain-chain (Ch-Ch) interactions in the structure and stability of SAMs of longer chain alkylthiolates. For C(1)/Ag(111) (C(1)/Au(111)) the SAM with Θ = 3/7 is more (less) stable than for Θ = 1/3 in a wide range of temperatures and pressures in line with experiments. In addition, for the molecular densities of SAMs corresponding to Θ = 3/7 and 1/3, the MP2-based Ch-Ch interaction potential also predicts the different chain orientations observed experimentally in SAMs of alkylthiolates on Ag(111) and Au(111). Thus, for short length alkylthiolates, a simple model based on first principles calculations that separately accounts for molecule-surface (M-S) and Ch-Ch interactions succeeds in predicting the main structural differences between the full coverage SAMs usually observed experimentally on Ag(111) and Au(111).

Dynamics of scattering and dissociative adsorption on a surface alloy: H2/W(100)-c(2 × 2)Cu.

H(2) scattering and dissociative adsorption on the W(100)-c(2 × 2)Cu surface alloy is studied based on DFT calculations. A strongly site dependent reactivity is observed in line with results obtained for the density of states projected onto the W and Cu atoms of the topmost layer. H(2) dissociation on a defect free terrace of W(100)-c(2 × 2)Cu is found to be a non-activated process like on W(100), despite the reduction of the number of energetically accessible dissociation pathways at low impact energies due to the presence of Cu atoms. A prominence of dynamic trapping and a reduction of the efficacy of trapping to promote dissociation is also verified, leading to a decrease of the initial sticking probability as a function of the molecular impact energy, in qualitative agreement with experimental findings. The heterogeneous reactivity is also evidenced by two different kinds of reflection events at low energies. Its combination gives rise to a broad specular peak superimposed on a cosine-like angular distribution of scattered molecules which is in good agreement with available experimental data.

Study of the interaction between short alkanethiols from ab initio calculations.

We have developed a force field to describe the interaction of alkanethiols HS(CH(2))(n-1) CH(3) (C(n) for short) by fitting a set of approximately 220 interaction energies for dimers of C(n) (with n = 1,2,...6) and CH(4) molecules obtained from second-order Møller-Plesset perturbation theory calculations. The derived force field, based on a sum of so-called exp-6 pairwise potentials and effective Coulomb interaction potential between the HS- heads, predicts very well the interaction energies for dimers and trimers of alkanethiols not included in the input database for the fit. Also the calculated minimum energy tilt angle of the alkyl chains for a hexagonal arrangement of alkanethiolates with a nearest neighbor distance of 5 A is in good agreement with the available experimental data for a sqrt [3] x sqrt [3] self-assembled monolayer (SAM) on Au(111). Thus, the derived force field might be suitable for large-scale molecular dynamics and/or Monte Carlo simulations to predict the structure, stability and/or kinetics of SAMs on other surfaces.

Reactive force fields for surface chemical reactions: A case study with hydrogen dissociation on Pd surfaces.

An approach based on reactive force fields is applied to the parametrization of potential energy surface (PES) for chemical reactions on surfaces with a benchmark system, H(2)/Pd(111). We show that a simple reactive force field based on the second moment approximation does not allow for obtaining reliable results of reaction dynamics for the considered system. With a more elaborate reactive force field, i.e., reactive bond order (REBO) force field, we succeeded in obtaining a reliable PES for H(2)/Pd(111). The accuracy of the constructed REBO force field is carefully checked through various tests including the comparison not only between energies calculated with density functional theory and those with REBO force field but also between the available results of ab initio molecular dynamics simulations and those with our force field. Moreover, our REBO force field is endowed with some transferability since the force field constructed with a database containing only information on H(2)/Pd(111) allows for obtaining also accurate results for H(2)/Pd(100) and qualitatively correct results for H(2)/Pd(110) without any refitting. With the help of our reactive force field, the molecular dynamics simulation for the dissociation of H(2) on the considered Pd surfaces is speeded up by five orders of magnitude compared to ab initio molecular dynamics method. The demonstrated reliability and the very high computational efficiency of reactive force fields open extremely attractive perspectives for studying large-scale complex reacting systems.

Chemically accurate simulation of a prototypical surface reaction: H2 dissociation on Cu(111).

Methods for accurately computing the interaction of molecules with metal surfaces are critical to understanding and thereby improving heterogeneous catalysis. We introduce an implementation of the specific reaction parameter (SRP) approach to density functional theory (DFT) that carries the method forward from a semiquantitative to a quantitative description of the molecule-surface interaction. Dynamics calculations on reactive scattering of hydrogen from the copper (111) surface using an SRP-DFT potential energy surface reproduce data on the dissociative adsorption probability as a function of incidence energy and reactant state and data on rotationally inelastic scattering with chemical accuracy (within approximately 4.2 kilojoules per mole).

Adsorption dynamics of H2 on Pd(100) from first principles.

We study H2 dissociative adsorption on Pd(100) through classical molecular dynamics (MD) calculations, using density functional theory (DFT) to describe the molecule-surface interaction potential. We employ two methods to evaluate the forces acting on the atoms along the trajectories: (i) by doing a DFT calculation (and using the Hellman-Feynman theorem) every time step, and (ii) by computing the gradient of a six-dimensional potential energy surface (PES) obtained first, by interpolation of DFT total energy results using the corrugation reducing procedure (CRP). The corresponding MD calculations, hereafter referred to as ab initio MD (AIMD) and CRP-PES-MD, respectively, provide very similar dissociative adsoption probabilities (P(diss)) as a function of the impact energy (E(i)) for initial rotational states characterized by 0 < or = J < or = 4 indicating that the interpolated CRP-PES gives a faithful representation of the underlying ab initio PES. Thus, we make use of the computationally cheaper CRP-PES-MD for a detailed analysis of rotational effects on dissociative adsorption for 0 < or = J < or = 12. In agreement with available experimental data for H2 interacting with Pd surfaces, we have found that P(diss) barely depends on J for E(i) > or = 200 meV, and that it follows a non-monotonic J-dependence at low energies. Our simulations show that two competing dynamical effects which were previously suggested based on lower-dimensional model calculations are indeed also operative at low energies in a realistic high-dimensional treatment. For low values of J, a shadowing effect prevails that entails a decrease of Pdiss when J increases, whereas for J > 6, rotational effects are dominated by the adiabatic energy transfer from rotation to perpendicular motion that provokes the increase of Pdiss with increasing J.

Theoretical study of hydrogen dissociative adsorption on strained pseudomorphic monolayers of Cu and Pd deposited onto a Ru(0001) substrate.

We present an extensive study based on the density functional theory (DFT) of the hydrogen dissociative adsorption on strained pseudomorphic monolayers of Cu or Pd deposited onto a Ru(0001) substrate. First, the full six-dimensional potential energy surfaces (PESs) are obtained by interpolation of the DFT ab initio calculations using the corrugation reducing procedure. The accuracy of these PESs is analyzed in detail. Then, the dissociative adsorption probability has been determined by means of six-dimensional (6D) quasi-classical dynamics simulations for both normal and off-normal incidence. Although H(2) dissociation is known to be non activated on Pd(111) and strongly activated on Cu(111), we have found that it is activated on both Pd/Ru(0001) and Cu/Ru(0001). Surprisingly, the minimum activation energy for H(2) dissociation is very similar on both pseudomorphic monolayers, approximately 100 meV. In both cases, H(2) dissociation occurs after the first encounter between the molecule and the surface (direct process) and, as expected, the adsorption probabilities follow, to a good approximation, normal energy scaling.

Dissociative dynamics of spin-triplet and spin-singlet O2 on Ag(100).

We study the dissociative dynamics of O(2) molecules on the Ag(100) surface. Initially, the impinging molecules are either in the spin-triplet ground state or in the spin-singlet excited state. The molecule-surface interaction is obtained in each case by constructing the six-dimensional potential energy surface (PES) from the interpolation of the energies calculated with spin-polarized and non-spin-polarized density functional theories, respectively. Classical trajectory calculations performed in both PESs show that O(2) molecules initially in the spin-triplet ground state only dissociate for incidence energies above 1.05 eV. This result is consistent with molecular beam experiments performed in this system. Interestingly, our results also suggest that for the spin-singlet O(2) dissociation occurs even for incidence energies as low as 50 meV. We propose the use of spin-singlet excited O(2) molecules to improve the otherwise low dissociative reactivity of O(2) at clean Ag(100).

Role of electron-hole pair excitations in the dissociative adsorption of diatomic molecules on metal surfaces.

We quantitatively evaluate the contribution of electron-hole pair excitations to the reactive dynamics of H2 on Cu(110) and N2 on W(110), including the six dimensionality of the process in the entire calculation. The interaction energy between molecule and surface is represented by an ab initio six-dimensional potential energy surface. Electron friction coefficients are calculated with density functional theory in a local density approximation. Contrary to previous claims, only minor differences between the adiabatic and nonadiabatic results for dissociative adsorption are found. Our calculations demonstrate the validity of the adiabatic approximation to analyze adsorption dynamics in these two representative systems.

The role of exchange-correlation functionals in the potential energy surface and dynamics of N(2) dissociation on W surfaces.

We study the dissociative adsorption of N(2) on W(100) and W(110) by means of density functional theory and classical dynamics. Working with a full six-dimensional adiabatic potential energy surface (PES), we find that the theoretical results of the dynamical problem strongly depend on the choice of approximate exchange-correlation functional for the determination of the PES. We consider the Perdew-Wang-91 [Perdew et al., Phys. Rev. B 46, 6671 (1992)] and Perdew-Burke-Ernzerhof (RPBE) [Hammer et al., Phys. Rev. B 59, 7413 (1999)] functionals and carry out a systematic comparison between the dynamics determined by the respective PESs. Even though it has been shown in earlier works that the RPBE may provide better values for the chemisorption energies, our study brings evidence that it gives rise to a PES with excessive repulsion far from the surface.