Surface Processes Induced by Collisions

Abstract

Energetic gas-phase particles that collide with adsorbed species on solid surfaces induce a variety of processes. Collision induced processes (CIP) are important and may play a central role in the mechanism governing heterogeneous catalytic reactions at high pressures and elevated temperatures. A number of different CIPs are discussed in this article with a strong emphasis on the utilization of molecular dynamics (MD) simulations as a tool for gaining molecular level mechanistic and dynamic insight into chemical events under investigation. Collision induced desorption (CID) is the simplest CIP to be discussed. The CID of N 2 from Ru(001) is described as a test case for the effect of collisions on a polarized adsorbate and has been studied at both low and high coverages. The interpretation of the experimental data at low coverage using MD simulations has led to the introduction of a new desorption mechanism. It involves strong coupling of surface corrugation and adsorbate frustrated rotation that lead to a normal motion away from the surface. Another system for which CID was shown to provide unique information is that of water on Ru(001). Here, the enhanced CID rate was demonstrated to be selective for a specific adsorption site/structure on the surface (the A 2 site), providing a new insight into the structure of water on this surface, recently recalculated by employing ab initio methods. At the multilayer coverage range, a remarkable stability was found of the ice layer against CID, suggesting particularly efficient dissipation of the collider energy within the hydrogen bonded network at an ice thickness of 3 bilayers and above. A unique CIP to be discussed is collision induced migration (CIM), a new phenomenon that has never been considered before. Based on MD simulations, it is shown that CIM may result in migration distances of more than 150 Å at very low coverage, whereas at high coverage, these displacements are shortened significantly. The potential importance of this process for inducing novel catalytic routes on surfaces is discussed. A related example involves MD simulations that address the relation between tracer surface diffusion and the pressure of collider from the gas phase. It is predicted that by increasing the pressure in the range 0-500 atm significant changes in adsorbates surface diffusivity should take place as a result of collision induced migration. Finally, CID within the O 2 /Ag(110) system arising from photodissociation of adsorbed molecular oxygen is described. MD simulations were used to explain the experimentally determined coverage dependent phenomena such as desorption yield and angular distribution of desorbates.

Collision-Induced Migration of Adsorbates on Solid Surfaces: An Experimental Approach

“…As shown in Figure 6, the dependence of <∆r> on incident energy has a threshold: the onset for effective CIM appears to be near collision energy of 1.0 ( 0.15 eV (total energy of 3.3 eV, assuming energy transfer directly to the adsorbates is near 30% (upper limit) of the total incident colliders' energysthe rest goes to the reflected collider (about 25%) and the substrate 19 ).…”

“…Collision-induced processes were demonstrated to include desorption (CIDes) [13][14][15][16][17] and reaction (CIR). 18,19 Dissociative adsorption dynamics of energetic molecular colliders on metal surfaces has been at the focus of theoretical and experimental research for more than two decades. In these studies of the role of internal degrees of freedom of the collider, statistical vs quantum mechanical theoretical approaches to describe the dynamics were presented over the years.…”

“…In a theoretical introduction to the subject, based on molecular dynamics simulations, the potential effect of energetic collisions on adsorbates mobility at low and at high coverage and high pressures has been demonstrated. 19,23 Here we present the first experimental attempt to study CIM. We have measured the CIM of potassium atoms in the presence of CO on an Ru(001) surface under UHV conditions, using an energetic rare gas supersonic atom beam, while in-situ monitoring the surface diffusion by means of the optical second harmonic diffraction technique.…”

Chemistry at surfaces: from ab initio structures to quantum dynamics

“…We adopted different sets of coordinates in order to complement previous theoretical investigations on the Eley-Rideal reaction, considering the competitive CID process. The latter process had received some attention in the literature, both experimentally and theoretically [145,146], but very little if any was known about the role of quantum dynamics (e.g. the quantization of the target-surface initial state) even when light atoms were involved.…”

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

“…Recently, Zeiri and co-worker reported their results for a molecular-dynamics ͑MD͒ simulation which was planned to explain the O 2 desorption at 248 nm on Ag͑110͒. 26,27 In their simulations, strong coupling of surface corrugation and adsorbate-frustrated rotation as well as hot-atom energy plays a main role to control the collimation angle. The resultant O 2 desorption broadly collimates at either 20°-40°or 60°-80°off the surface normal toward the ͓110͔ direction at low O 2 density and the 10°-25°o ff normal desorption is enhanced at high density.…”