Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions.

Facile C-C bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in C-C bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C2Hx adsorbates on a Co(0001) model catalyst and promote C-C bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C2Hx adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions.

Site-specific reactivity of molecules with surface defects-the case of H2 dissociation on Pt.

The classic system that describes weakly activated dissociation in heterogeneous catalysis has been explained by two dynamical models that are fundamentally at odds. Whereas one model for hydrogen dissociation on platinum(111) invokes a preequilibrium and diffusion toward defects, the other is based on direct and local reaction. We resolve this dispute by quantifying site-specific reactivity using a curved platinum single-crystal surface. Reactivity is step-type dependent and varies linearly with step density. Only the model that relies on localized dissociation is consistent with our results. Our approach provides absolute, site-specific reaction cross sections.

Eley-rideal reactions with N atoms at Ru(0001): formation of NO and N(2).

Forward-directed NO molecules with large translational energies are formed upon exposure of an O-covered Ru(0001) surface to a nitrogen (N+N_{2}) beam. This is an unequivocal experimental demonstration of the Eley-Rideal reaction for a "heavy" (i.e., nonhydrogenated) neutral system. The time dependence of prompt NO formation exhibits an exceptionally fast decay as a consequence of shifting reaction pathways and probabilities over the course of the exposure. Prompt production shuts down as the O coverage decreases due to competition from more favorable Eley-Rideal production of N_{2}.

Evidence of stable high-temperature D(x)-CO intermediates on the Ru(0001) surface.

We demonstrate the formation of complexes involving attractive interactions between D and CO on Ru(0001) that are stable at significantly higher temperatures than have previously been reported for such intermediate species on this surface. These complexes are evident by the appearance of new desorption features upon heating of the sample. They decompose in stages as the sample temperature is increased, with the most stable component desorbing at >500 K. The D:CO ratio remaining on the surface during the final stages of desorption tends towards 1:1. The new features are populated during normally incident molecular beam dosing of D(2) on to CO pre-covered Ru(0001) surfaces (180 K) when the CO coverage exceeds 50% of the saturation value. The amount of complex formed decreases somewhat with increasing CO pre-coverage. It is almost absent in the case of dosing on to the fully saturated surface. The results are interpreted in terms of both local and long-range rearrangements of the overlayer that give rise to the observed CO coverage dependence and limit the amount of complex that can be formed.

The interaction of hyperthermal nitrogen with N-covered Ag(111).

A mixed beam of hyperthermal N atoms and N(2) molecules was scattered from the N-covered Ag(111) surface held at 300 K. The angular distribution of scattered N atoms is very broad. In contrast, N(2) molecules exhibit a sharp angular distribution. Taking into account the relative mass ratio, N loses more energy at the surface than N(2). In terms of energy loss, the atoms approximately follow the binary collision model while the molecules do not. Instead, the energy curves of scattered N(2) are more comparable to the parallel momentum conservation model for near specular outgoing angles (40°-65°). For both atoms and molecules the angle-resolved intensity and final energy curves are very similar to those from the bare surface. However, the N-covered surface yields non-negligible N(2) intensity for a broad range of outgoing angles, including along the surface normal. This was not the case from the clean surface, where the measured intensity distribution was confined to the narrower angular range indicated above. Backscattering and direct abstraction reactions are evaluated as possible origins of this additional N(2) signal. Of these, an abstraction mechanism appears to be the most consistent with the measured data.

The interaction of hyperthermal argon atoms with CO-covered Ru(0001): scattering and collision-induced desorption.

Hyperthermal Ar atoms were scattered under grazing incidence (θ(i) = 60°) from a CO-saturated Ru(0001) surface held at 180 K. Collision-induced desorption involving the ejection of fast CO (∼1 eV) occurs. The angularly resolved in-plane CO desorption distribution has a peak along the surface normal. However, the angular distribution varies with the fractional coverage of the surface. As the total CO coverage decreases, the instantaneous desorption maximum shifts to larger outgoing angles. The results are consistent with a CO desorption process that involves lateral interaction with neighboring molecules. Furthermore, the data indicate that the incident Ar cannot readily penetrate the saturated CO overlayer. Time-of-flight measurements of scattered Ar exhibit two components-fast and slow. The slow component is most evident when scattering from the fully covered surface. The ratio and origin of these components vary with the CO coverage.

Scattering of hyperthermal argon atoms from clean and D-covered Ru(0001) surfaces.

Hyperthermal Ar atoms were scattered from a Ru(0001) surface held at temperatures of 180, 400 and 600 K, and from a Ru(0001)-(1×1)D surface held at 114 and 180 K. The resultant angular intensity and energy distributions are complex. The in-plane angular distributions have narrow (FWHM ≤ 10°) near-specular peaks and additional off-specular features. The energy distributions show an oscillatory behavior as a function of outgoing angle. In comparison, scattered Ar atoms from a Ag(111) surface exhibit a broad angular intensity distribution and an energy distribution that qualitatively tracks the binary collision model. The features observed for Ru, which are most evident when scattering from the clean surface at 180 K and from the Ru(0001)-(1×1)D surface, are consistent with rainbow scattering. The measured TOF profiles cannot be adequately described with a single shifted Maxwell-Boltzmann distribution. They can be fitted by two components that exhibit complex variations as a function of outgoing angle. This suggests at least two significantly different site and∕or trajectory dependent energy loss processes at the surface. The results are interpreted in terms of the stiffness of the surface and highlight the anomalous nature of the apparently simple hcp(0001) ruthenium surface.

Scattering of hyperthermal nitrogen atoms from the Ag(111) surface.

Measurements on scattering of hyperthermal N atoms from the Ag(111) surface at temperatures of 500, 600, and 730 K are presented. The scattered atoms have a two-component angular distribution. One of the N components is very broad. In contrast, scattered Ar atoms exhibit only a sharp, single-component angular distribution. There are noteworthy features in the angle-resolved energy of the scattered N when compared with Ar. Taking into account the relative masses involved, N atoms lose significantly more energy at the surface than Ar. However, there is a preferential loss mechanism that predominantly affects low-energy N atoms with small total scattering angle trajectories. The results are interpreted in terms of probing of different interaction potentials: strongly attractive and almost purely repulsive, and spin-state changes during the interaction of N with the surface appear probable.

CO blocking of D2 dissociative adsorption on Ru(0001).

The influence of pre-adsorbed CO on the dissociative adsorption of D(2) on Ru(0001) is studied by molecular-beam techniques. We determine the initial dissociation probability of D(2) as a function of its kinetic energy for various CO pre-coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D(2) dissociation and perturbs the local surface reactivity up to the nearest-neighbour Ru atoms. Non-activated sticking and dissociation become less important with increasing CO coverage, and vanish at theta(CO) approximately 0.33 ML. In addition, at high D(2) kinetic energy (>35 kJ mol(-1)) the site-blocking capability of CO decreases rapidly. These observations are attributed to a CO-induced activation barrier for D(2) dissociation in the vicinity of CO molecules.