Surface-aligned reaction of photogenerated oxygen atoms with carbon monoxide targets

Abortive reaction leads to selective adsorbate rotation

Electron-induced dissociation of a fluorocarbon adsorbate CF3 (ad) at 4.6 K is shown by Scanning Tunnelling Microscopy (STM) to form directed energetic F-atom 'projectiles' on Cu(110). The outcome of a collision between these directed projectiles and stationary co-adsorbed allyl 'target' molecules was found through STM to give rotational excitation of the target allyl, clockwise or anti-clockwise, depending on the chosen collision geometry. Molecular dynamics computation linked the collisional excitation of the allyl target to an 'abortive chemical reaction', in which the approach of the F-projectile stretched an H-C bond lifting the allyl above the surface, facilitating isomerization from 'Across' to 'Along' a Cu row.

Innovations in nanosynthesis: emerging techniques for precision, scalability, and spatial control in reactions of organic molecules on solid surfaces

The surface science-based approach to synthesising new organic materials on surfaces has gained considerable attention in recent years, owing to its success in facilitating the formation of novel 0D, 1D and 2D architectures. The primary mechanism used to date has been the catalytic transformation of small organic molecules through substrate-enabled reactions. In this Topical Review, we provide an overview of alternate approaches to controlling molecular reactions on surfaces. These approaches include light, electron and ion-initiated reactions, electrospray ionisation deposition-based techniques, collisions of neutral atoms and molecules, and superhydrogenation. We focus on the opportunities afforded by these alternative approaches, in particular where they may offer advantages in terms of selectivity, spatial control or scalability.

Reversible 1D chain-reaction gives rise to an atomic-scale Newton's cradle

An F-atom with ∼1 eV translational energy was aimed at a line of fluorocarbon adsorbates on Cu(110). Sequential 'knock-on' of F-atom products was observed by STM to propagate along the 1D fluorocarbon line. Hot F-atoms travelling along the line in six successive 'to-and-fro' cycles paralleled the rocking of a macroscopic Newton's cradle.

Direct Observation of Knock-on in Surface Reactions at Zero Impact Parameter

Reaction dynamics examines molecular motions in reactive collisions. The aiming of reagents at one another has been achieved at selected miss distances (impact parameters, b) by using the corrugations on crystalline surfaces as collimator. Prior experimental work and ab initio calculation showed single atoms aimed at chemisorbed molecules with b = 0 gave knock-on of atomic reaction products through a linear transition state. Here we report a study of b = 0 collision between directed CF₂ and stationary chemisorbed CF₃. Experiments and ab initio calculations again show linear reaction with a linear transition state, despite the additional degrees of freedom for CF₂. The directed motion of CF₂ is conserved through this linear transition state. Conservation of directionality is evidenced experimentally by the observation of a knock-on chain reaction along a line of chemisorbed CF₃.

Direct observation of knock-on reaction with umbrella inversion arising from zero-impact-parameter collision at a surface

In Surface-Aligned-Reactions (SAR), the degrees of freedom of chemical reactions are restricted and therefore the reaction outcome is selected. Using the inherent corrugation of a Cu(110) substrate the adsorbate molecules can be positioned and aligned and the impact parameter, the collision miss-distance, can be chosen. Here, substitution reaction for a zero impact parameter collision gives an outcome which resembles the classic Newton's cradle in which an incident mass 'knocks-on' the same mass in the collision partner, here F + CF₃ → (CF₃)' + (F)' at a copper surface. The mechanism of knock-on was shown by Scanning Tunnelling Microscopy to involve reversal of the CF₃ umbrella as in Walden inversion, with ejection of (F)' product along the continuation of the F-reagent direction of motion, in collinear reaction.

Electron-induced molecular dissociation at a surface leads to reactive collisions at selected impact parameters

A collimated beam of ‘projectiles’ strikes a chemisorbed ‘target’ thereby selecting the impact parameter, achieving an elusive goal of reaction dynamics.

Approaching the forbidden fruit of reaction dynamics: Aiming reagent at selected impact parameters

Collision geometry is central to reaction dynamics. An important variable in collision geometry is the miss-distance between molecules, known as the "impact parameter." This is averaged in gas-phase molecular beam studies. By aligning molecules on a surface prior to electron-induced dissociation, we select impact parameters in subsequent inelastic collisions. Surface-collimated "projectile" molecules, difluorocarbene (CF2), were aimed at stationary "target" molecules characterized by scanning tunneling microscopy (STM), with the observed scattering interpreted by computational molecular dynamics. Selection of impact parameters showed that head-on collisions favored bimolecular reaction, whereas glancing collisions led only to momentum transfer. These collimated projectiles could be aimed at the wide variety of adsorbed targets identifiable by STM, with the selected impact parameter assisting in the identification of the collision geometry required for reaction.

Two distinctive energy migration pathways of monolayer molecules on metal nanoparticle surfaces

Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here we report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. On a 5 nm platinum particle, within a few picoseconds the vibrational energy of a carbon monoxide adsorbate rapidly dissipates into the particle through electron/hole pair excitations, generating heat that quickly migrates on surface. In contrast, the lack of vibration-electron coupling on approximately 1 nm particles results in vibrational energy migration among adsorbates that occurs on a twenty times slower timescale. Further investigations reveal that the rapid carbon monoxide energy relaxation is also affected by the adsorption sites and the nature of the metal but to a lesser extent. These findings reflect the dependence of electron/vibration coupling on the metallic nature, size and surface site of nanoparticles and its significance in mediating energy relaxations and migrations on nanoparticle surfaces.

Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here, the authors employ ultrafast vibrational spectroscopy to show two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces.

Active sites in a two-step catalytic bimolecular reaction on a reconstructed platinum surface

Active sites for a thermally induced bimolecular two-step catalytic reaction (O2-->2O, O + CO-->CO2) that occurs on a Pt(113)(1 x 2) structure at around 160 K were studied by angular distribution measurements of desorbing product CO2. It was found that the intrinsic activity level of two-atom-wide (001) facets is significantly higher than that of two-atom-wide (111) facets, while the activity of (111) facets was also significant when this reaction was induced by irradiation of 193 nm photons. Possible mechanisms for the difference in activities of the two facets are discussed.

Ru-Pt core-shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen

Most of the world's hydrogen supply is currently obtained by reforming hydrocarbons. 'Reformate' hydrogen contains significant quantities of CO that poison current hydrogen fuel-cell devices. Catalysts are needed to remove CO from hydrogen through selective oxidation. Here, we report first-principles-guided synthesis of a nanoparticle catalyst comprising a Ru core covered with an approximately 1-2-monolayer-thick shell of Pt atoms. The distinct catalytic properties of these well-characterized core-shell nanoparticles were demonstrated for preferential CO oxidation in hydrogen feeds and subsequent hydrogen light-off. For H2 streams containing 1,000 p.p.m. CO, H2 light-off is complete by 30 (composite function)C, which is significantly better than for traditional PtRu nano-alloys (85 (composite function)C), monometallic mixtures of nanoparticles (93 (composite function)C) and pure Pt particles (170 ( composite function)C). Density functional theory studies suggest that the enhanced catalytic activity for the core-shell nanoparticle originates from a combination of an increased availability of CO-free Pt surface sites on the Ru@Pt nanoparticles and a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.

Kinetic measurements of hydrocarbon conversion reactions on model metal surfaces

Examples from recent studies in our laboratory are presented to illustrate the main tools available to surface scientists for the determination of the kinetics of surface reactions. Emphasis is given here to hydrocarbon conversions and studies that rely on the use of model systems, typically single crystals and controlled (ultrahigh vacuum) environments. A detailed discussion is provided on the use of temperature-programmed desorption for the determination of activation energies as well as for product identification and yield estimations. Isothermal kinetic measurements are addressed next by focusing on studies under vacuum using molecular beams and surface-sensitive spectroscopies. That is followed by a review of the usefulness of high-pressure cells and other reactor designs for the emulation of realistic catalytic conditions. Finally, an analysis of the power of isotope labeling and chemical substitutions in mechanistic research on surface reactions is presented.

Collision-induced desorption in 193-nm photoinduced reactions in (O2+CO) adlayers on Pt(112)

The spatial distribution of desorbing O(2) and CO(2) was examined in 193-nm photoinduced reactions in O(2)+CO adlayers on stepped Pt (112)=[(s)3(111)x(001)]. The O(2) desorption collimated in inclined ways in the plane along the surface trough, confirming the hot-atom collision mechanism. In the presence of CO(a), the product CO(2) desorption also collimated in an inclined way, whereas the inclined O(2) desorption was suppressed. The inclined O(2) and CO(2) desorption is explained by a common collision-induced desorption model. At high O(2) coverage, the CO(2) desorption collimated closely along the (111) terrace normal.

Surface-Aligned Ion−Molecule Reaction on the Surface of a Molecular Crystal CD3+ + CD3I → C2D5+ + DI

An ion-molecule reaction has been observed from a condensed molecular crystal of CD(3)I using the time-of-flight electron-stimulated desorption ion angular distribution technique. The CD(3)I multilayer is produced by growth on an ordered substrate. The reaction occurs between CD(3)(+) ions produced by electron-stimulated desorption and neighbor CD(3)I molecules in the topmost layer of the molecular crystal of CD(3)I, forming product C(2)D(5)(+) ions whose desorption dynamics have been measured. The normal momentum of the product ion is close to that of the reactant ion, suggesting that the reaction is dominated by a two-body collision, i.e., the momentum of the reactant CD(3)(+) ion governs the momentum of the product C(2)D(5)(+) ion. The ion-molecule reaction is of high cross section since the C(2)D(5)(+) yield is comparable to the CD(3)(+) yield. It is found that the yield and directionality of the emission of the C(2)D(5)(+) product ion is governed by the molecular order that is characteristic of the molecular crystal of CD(3)I. Destroying or modifying this order by using a spacer layer of H(2)O diminishes the C(2)D(5)(+) product ion yield relative to the reactant CD(3)(+) yield and broadens the ion emission directions.

Surface aligned ion-molecule reaction: direct observation of initial and final ion momenta

An ion-molecule reaction has been studied by measuring the momentum of both the reactant and the product ions. This is carried out in an ordered molecular film of CD3I where electron stimulated desorption causes the reaction CD+3+ CD3I--> C2D+5+DI. The close similarity of the normal momentum of CD+3 and C2D+5 indicates that a sticky collision occurs in which, to within 10%, the momentum of the reactant ion is transferred to the momentum of the product ion. The measurement represents the first use of molecularly aligned species to study momentum effects in an ion-molecule reaction.

Electron wave function at a vicinal surface: switch from terrace to step modulation

The Cu(111) surface state has been mapped for vicinal surfaces with variable step densities by angle-resolved photoemission. Using tunable synchrotron radiation to vary the k dependence perpendicular to the surface, as well as the (k) dependence, we find a switch between two qualitatively different regimes at a miscut of 7 degrees (17 A terrace width). For larger miscut angles the step modulation of the wave function dominates, and for smaller miscut angles the terrace modulation dominates. These observations resolve an apparent inconsistency between prior photoemission and STM results.