Surface chemistry: molecular cart-wheeling.


Catalysis underlies nearly every industrially important chemical process, with fuel production and the conversion of carbon monoxide to carbon dioxide in engine exhaust representing notable examples. Many catalytic processes are heterogeneous, involving molecules interacting with a solid surface, and they rely on molecular adsorption, surface diffusion and surface-catalysed chemical reactions. Often, two types of molecule must find each other on the surface before reacting, and if the surface temperature is too low then surface diffusion will not suffice for this to occur. Another important case to consider is when excess energy is liberated when a molecule undergoes a spontaneous exoergic reaction on adsorption onto a surface. The accepted view of this situation is that excess energy will either trigger a chemical reaction local to the excitation site1 or lead to desorption of the molecule from the surface through ‘desorption induced by electronic transition’ (DIET)2. Neither of these processes involve molecular motion on the surface but now, writing in Nature Chemistry, John Polanyi and co-workers report another possibility whereby such motion is indeed possible3. The experiments from Polanyi and coworkers provide compelling evidence for the long-range edgewise rolling (or ‘cart-wheeling’) motion of molecules across a surface resulting from an exoergic adsorption reaction. Density functional theory (DFT) simulations provide clear support for the experimental observations. Polanyi and colleagues used scanning tunnelling microscopy (STM) to study the molecular recoil induced by the exoergic adsorption of 1,2-dihaloethane (DXE) molecules onto a Si(100) surface. DXE molecules spontaneously undergo exoergic dehalogenation, releasing ~1.3 eV per molecule during the process, through the formation of a C=C π-bond. The beauty of this experiment is that, on reaction, the halogen atoms are left behind, resulting in an atomic footprint on the surface that indexes the reactants’ initial positions. This enables easy determination of both the starting points of the reactants and the ending points of the products for the low molecular doses used in the experiments. In other experiments the Polanyi team directly induced molecular recoil by using STM electrons to stimulate chemisorbed alkenes such as ethylene. In both experiments they determined that inplane molecular motion over long distances — up to 200 Å — occurred before final immobilization on the silicon surface. This is notable given that these molecules do not diffuse on silicon at the temperatures used in the study. Furthermore, by correlating the initial and final positions of the molecules, Polanyi and co-workers observed that the recoiling molecules negotiated paths around obstacles on the surface, and even moved up from one atomic terrace to a higher adjacent terrace (inset of Fig. 1). Although the molecules in question are planar and have a strong propensity to lie flat on the silicon surface, their recoil motion following excitation is not compatible with maintaining such a configuration. SURFACE CHEMISTRY

DOI: 10.1038/nchem.1035

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Cite this paper

@article{Lyding2011SurfaceCM, title={Surface chemistry: molecular cart-wheeling.}, author={Joseph W. Lyding}, journal={Nature chemistry}, year={2011}, volume={3 5}, pages={341-2} }