Dynamics Of Hydrogen Dissociation Research Articles

Dynamics of hydrogen dissociation on stepped platinum

We have studied the reactivity of hydrogen on the Pt(211) stepped surface using supersonic molecular beam techniques. We observe an energy dependence that is indicative of indirect adsorption below 9 kJ mol(-1) and direct adsorption between 0 and 37 kJ mol(-1). Comparison of our results to predictions based on six-dimensional quantum dynamics calculations for Pt(211) [R. A. Olsen et al., J. Chem. Phys. 128, 194715 (2008)] yields reasonable agreement. Discrepancies between theory and our experiments at low kinetic energy strongly indicate that the wells in the used potential energy surface are too shallow. Discrepancies at high kinetic energy point toward neglect of degrees of freedom vital to capture the full dynamics.

Molecular dynamics of hydrogen dissociation on an oxygen covered Pt(111) surface

The dissociation of hydrogen on a Pt(111) surface covered with a p(2 x 2) oxygen phase was investigated using quasiclassical, six-dimensional molecular dynamics. The potential energy surface (PES) used in these simulations was obtained by an iterative novelty sampling algorithm. Compared to molecular beam experiments performed under similar conditions, the simulations give an accurate prediction of the reaction probability via a direct dissociation pathway. When compared to previously reported reaction probability curves for the clean Pt(111) surface, we find that the presence of an oxygen overlayer inhibits the direct pathway to hydrogen dissociation. This inhibition is a function of incident energy and cannot be described by a simple site blocking model. An indirect pathway to dissociation, which was observed in experiments, is not properly captured by the PES. Spatially resolved "reaction maps" indicate that the preferred site for hydrogen dissociation on an oxygen covered Pt surface is the top site of the Pt atom farthest from the adsorbed oxygen atom.

The role of steps in the dynamics of hydrogen dissociation on Pt(533)

The dissociative adsorption of H2 and D2 on Pt(533) (Pt{4(111)×(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D2 from (100) step sites is observed at lowest exposures in TPD (assigned β3) at 375 K. Saturation of this peak at ΘH=0.14 corresponds to the filling of half of the available four-fold sites at the (100) step edge. At higher coverages, additional desorption takes place from the (111) terraces in a broad peak below 300 K similar to that observed (assigned β1 and β2) for the Pt(111) surface. The incident kinetic energy (Ei), surface temperature (Ts), coverage (ΘD), and incident angle (Φ) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S0) first decreases with increasing kinetic energy over the range 0<Ei(meV)<150 (low energy component), and subsequently increases (high energy component). Comparison with D2 dissociation on Pt(111), where (S0) increases linearly with Ei, leads to the conclusion that it is the step sites that are responsible for the low energy component to dissociation on Pt(533). The high energy component is a result of a direct dissociation channel on (111) terraces of the Pt(533) surface. The probability of dissociation through the direct channel on the (111) terraces is found to be independent of Ts. The probability of dissociation through the low energy component associated with the (100) steps, over most of the range of Ei where it contributes, is also shown to be independent of Ts. Only at the very lowest value (6.6 meV) of Ei investigated does S0 exhibit a (negative) temperature dependence. A (0.8-ΘD)2 dependence (where 0.8 is the measured saturation coverage) of S with ΘD is observed at Ei=180 meV where the direct channel dominates. However, the dependence of S on ΘD exhibits characteristics similar to those expected for precursor mediated dissociation at Ei=16 meV and Ei=6.6 meV where the low energy channel dominates. The angular dependence S0(Φ) scattering in a plane perpendicular to the step direction is asymmetric about the Pt(533) surface normal at both Ei=6.6 meV and Ei=180 meV. At 180 meV S0(Φ) can be understood by considering direct dissociation at the (111) terrace and (100) step plane. At 6.6 meV, S0 tends to scale with total energy. The observed characteristics of the low energy channel is discussed in the light of models [specifically the role steps and defects, precursors (accommodated and dynamical), and steering] suggested to account for the low energy component for H2/D2 dissociation and exchange on metal surfaces presenting low activation barriers. At lowest energies (Ei=6.6 meV) dissociation through a conventional accommodated precursor takes place. In addition, more significant proportion of sticking in the range 0<Ei(meV)<150 takes place through an indirect channel involving an unaccommodated precursor dissociating at step sites, and is unlikely to be accounted for through a steering mechanism.

Dynamics of hydrogen dissociation at the sulfur-covered Pd(100) surface

We report calculations of the dissociative adsorption and associative desorption of ${\mathrm{H}}_{2}$ at Pd(100) covered with 1/4 monolayer of sulfur using quantum dynamics as well as molecular dynamics and taking all six degrees of freedom of the two H atoms fully into account. The potential energy surface (PES) has been derived from density-functional theory calculations. The absolute value of the calculated sticking coefficient turns out to be at variance with a molecular beam experiment. However, the relative change of the sticking coefficient as a function of the angle of incidence as well as the mean kinetic energy and the rotational alignment of desorbing hydrogen molecules agree quite well with the experiment. This indicates that the calculated PES reproduces the most favorable reaction path, but that in the experiment the sulfur adlayer was probably not very well ordered.

Dynamics of hydrogen dissociation on Pt{100}: Steering, screening and thermal roughening effects

The normal translational energy (Ei) and surface temperature (Ts) dependence of the initial D2 sticking probability (s0) on Pt{100} has been measured using molecular beam techniques. On the hex phase s0 is found to decrease sharply with Ei between 5 and 10 meV, and to increase more gradually over the range 20⩽Ei⩽400 meV. This is interpreted as dynamical steering at low incident energies, giving way to weak translational activation at higher energies. The variation of the barrier to dissociation (Ea) across the unit cell gives rise to screening of dissociation with the impact parameter. There is less variation of Ea across the smaller (1×1) unit cell, leading to a stronger Ei dependence on this surface. On the hex phase s0 has been measured over a wide Ts range, from 150 to 1100 K. Weak thermal activation of dissociation is observed. This is well explained by a roughening model in which s is assumed to be proportional to the linear thermal displacement of surface metal atoms. The model is supported by density functional theory calculations of potential energy surfaces for a modelled rough surface. The absolute value of s0 under conditions of zero-point motion is estimated as 0.03 at Ei=69 meV and 0.06 at 342 meV.

Surface oscillator models for dissociative sticking of molecular hydrogen at non-rigid surfaces

Effects of surface atom motion on the dissociation dynamics of hydrogen at a copper model surface are studied using time-dependent quantum dynamics, the (ordinary) surface oscillator (SO) model of Harris et al., and a modified surface oscillator (MSO) model with a microscopically motivated molecule-surface coupling. For both models the dependence of zero-coverage dissociative sticking probabilities on (1) the Einstein oscillator frequency, (2) on anharmonicities of the surface vibrations and (3) on isotopomer masses, is studied. The unmodified SO model predicts an almost perfect insensitivity on the surface oscillator frequency which explains, therefore, the well-known good agreement with the surface mass model of Luntz and Harris. In contrast, the MSO model depends on the oscillator frequency and seems more realistic if applied to solids with a rapidly varying frequency spectrum. A further qualitative difference between the SO and the MSO models is an increase (relative to the rigid-surface case) of the dissociative sticking probability at a given impact energy predicted by the latter. The SO model predicts the opposite. This is explained by the observation that in the MSO case, low-barrier reaction paths become available during surface atom motion. In contrast in the SO model the barrier remains stiff, and the loss of relative velocity between the colliding partners is dominant. Anharmonicities in the lattice vibrations are found to have negligible effects within both models. Isotopic substitution of H with D and T, leads to an expected increase of the magnitude of surface-related corrections. It is finally shown that the MSO approach accounts in a way similar to the SO model, for the experimentally observed broadening of the sticking curve with increasing surface temperature.

The dynamics of hydrogen dissociation on W(100)-c(2 × 2)Cu

The dynamics of dissociative hydrogen adsorption on the W(100)-c(2 × 2)Cu surface have been studied with a supersonic molecular beam. The surface is observed to present a higher barrier to direct dissociation than W(100) where this channel is non-activated. The removal of the direct channel at low energies reveals a pure indirect channel, allowing for the first time the study of an isolated indirect route to hydrogen dissociation on a metal surface. The initial sticking probability (S0) via the indirect channel decreases surprisingly slowly with incident beam energy, decaying exponentially up to Ei ∼ 150 meV. Only a weak dependence on surface temperature and angle of incidence exists, while S0 is constant up to a hydrogen coverage of θH = 0.5. The in-plane scattering distribution is diffuse and peaked at sub-specular angles (towards the surface normal), and exhibits some memory of the incident conditions. No direct scattering component could be observed. The sticking and scattering results cannot be reconciled with the trapping of a molecular precursor through inelastic processes such as phonon excitation or electron-hole pair creation. We suggest that the indirect channel to dissociation takes place via the trapping of the molecule without full energetic accommodation, through a dynamical precursor.