Efficient N₂ Formation on Ag(111) by Eley–Rideal Recombination of Hyperthermal Atoms

Abstract

Using molecular dynamics simulations and potential energy surfaces of ab-initio quality, we show that direct pick-up of N adsorbates by gas-phase N is a highly efficient channel for N 2 formation on Ag(111). This recombination process, called Eley-Rideal, was traditionally associated to lighter projectiles and regarded as marginal, but here we obtain reactivities for * To whom correspondence should be addressed TOC GraphicKeywords: Gas-surface interactions, Eley-Rideal reactions, scattering, potential energy surfaces, molecular dynamics. 2Among the various elementary processes that occur in everyday gas-surface reactions, those resulting in the recombination and desorption of molecules deserve a special attention, as they regulate the replenishment of the surface active sites. One of these processes, the direct Eley-Rideal (ER) recombination has been found to be a marginal phenomenon, only observed experimentally for incoming light atoms (H,D). [1][2][3][4][5][6][7][8][9] Originally postulated as an atom exchange, 10-12 the ER process is nowadays assigned to the direct recombination of an incoming gas-phase species that picks up a surface adsorbate to conform a molecular compound that desorbs during the collision. Even if the initial measurements pointed to ER cross sections of about 2 − 5 Å 2 , 4,6,9 subsequent theoretical studies demonstrated that they were around one order of magnitude smaller because most of the recombination events proceed after few collisions of the incoming (H,D) atom with the surface, i.e., through the slower hot-atom (HA) process. [13][14][15][16][17][18] In the scope of reactive ion scattering (RIS), C abstraction from a graphite surface by N + ions has been characterised by experiments, and found to happen after several collisions with the substrate 19 Recombination of neutral atomic species heavier than (H,D) has been experimentally studied in the context of CO formation and CO oxidation on metal surfaces. In all cases, CO and CO 2 formation is dominated by the Langmuir-Hinshelwood (LH) recombination in which the species involved in the reaction are fully thermalized with the surface. [20][21][22] Also in these systems, the direct ER pick-up events are accepted to play a very minor role. Within this context, the recent experiments by Ueta et al. suggesting the existence of direct recombination on the N-covered Ag(111) surface between gas-phase and adsorbed N atoms are certainly intriguing. 23,24 All the more so when recent simulations of N 2 formation on the W(100) surface also exhibit low ER probabilities. 25 In this letter we demonstrate that thermal and hyperthermal gas-phase N scattered off Ncovered Ag(111) is indeed a prototype system that fulfills all the requirements to observe a highly efficient direct ER recombination. This outcome poses a new scenario for the study of gas-surface interactions, as it shows that fast direct recombination channels may be active in contrast to the usual assumption of neglecting them, specially for large atomic projectiles. 3The comple...

Influence of the van der Waals interaction in the dissociation dynamics of N2 on W(110) from first principles

“…It is at this stage when nowadays simulations based, mostly, on density functional theory (DFT) are the necessary counterpart to fully understand the properties governing these processes and to confirm the interpretation of the experimental outcome. 4,[11][12][13] In this respect, there are various examples showing that not only the reactivity [14][15][16] but also scattering properties such as the diffraction peaks positions, 17,18 angular distributions, [19][20][21][22][23] rotational states, [24][25][26][27][28] and energy loss 21,23,29 are reasonably described by this level of theory. Considering that in all these cases, the potential energy surfaces (PESs) have been calculated with the (hitherto standard) semilocal exchange-correlation (xc) functionals that do not correctly describe the long-range van der Waals (vdW) interaction, such an agreement is somehow unexpected at first thoughts.…”

Eley-Rideal Reactions with N Atoms at Ru(0001): Formation of NO andN2

“…One of the closest examples to date is the indication of N abstraction of N from Ag(111) [20]. While the presence of a large N 2 component in the incident beam precludes definitive assignment on the sole basis of the measurements, molecular dynamics (MD) simulations validate the inference of an ER reaction [21]. A similar issue affects attribution of a possible ER process in the case of hyperthermal O interaction with graphite [22].…”

“…The latter indicates an energydependent reaction probability, leading to suppression of direct reaction in the case of the highest energy N atoms. MD simulations of N abstraction of N from Ag(111) have found a significant energy dependence, with the reaction probability decreasing at higher incident energies [21].…”

“…N/N-W(100) calculations [6] yielded values of less than 0.5 Å 2 . MD simulations in the case of N/N-Ag(111) do indicate large cross sections, but still limited to the 2.5-4.5 Å 2 range [21]. While experimentally determined ER cross sections are usually small relative to the surface unit cell area, modeling efforts tend to reveal them as overestimates.…”

Kinematics of Eley-Rideal Reactions at Hyperthermal Energies

“…Recently, the abstraction of adsorbed O and N atoms on Ru(0001) was shown with an effusive beam of neutral nitrogen species (N þ N 2 ) [16]. In computational studies of N-atom abstraction by energetic N on Ag(111), ER reactions were shown to be highly efficient [17]. Most of these studies focused on proving that the molecular product is indeed produced by an ER reaction.…”