More Complex PE Curves & Multi-Dimensional PE Surfaces

As indicated in the previous section there are many more parameters which affect the energy of the system and the adsorption characteristics in addition to the distance (d) of the centre of mass of the molecule from the surface. We will now look at several examples which illustrate these effects.

(1) Adsorption of CO on Metal Surfaces.

Some of the additional factors which need to be considered are :

All these points indicate that a simple 1D PE curve can only provide a very rough first approximation to the interaction of the molecule with the surface. Indeed the situation would be hopelessly complicated if we attempted to take all these effects into account, so instead we will revert to a simpler system and consider just a couple of additional factors.

(2) Adsorption of Hydrogen on W(100)

This system retains some simplifying elements, namely that :

and we will only look at the zero coverage limit (i.e. adsorption on the clean surface) with the molecule approaching along the surface normal. We will, however, now extend the 1D model to consider the effect of two additional variables as the molecule approaches the surface

  1. the angular orientation of the molecule (with the limitation that the molecular axis will only be permitted to be either parallel or perpendicular to the surface plane)
  2. the lateral position on the surface (with the limitation that the molecule will be considered either to be approaching directly towards a W atom (approaching the "atop site") or directly towards a "bridging site" located between two surface W atoms)

The results shown below are based on computational work and are included courtesy of S.Holloway (University of Liverpool) - they show four slices though the 6D potential energy surface (with energies in eV) where the y-axis in each case is the distance between the surface plane and the molecular centre-of-mass, and the x-axis is the H-H separation.


A. Hydrogen molecule approaching the W Surface end-on.

A1. When the hydrogen molecule approaches end-on to an atop site (i.e. from directly above a W atom - see inset on figure) the potential energy increases rapidly for a molecule/surface separation of less than 2.3 Å. The molecule is therefore repelled from the surface.


A2. When the hydrogen molecule approaches end-on to a bridging site the potential energy increases slowly and although some lengthening of the H-H bond may occur with little corresponding increase in energy, there is no facile dissociation channel and the molecule is weakly repelled from the surface.
  
B. Hydrogen molecule approaching the W Surface broadside-on.

B1. When the hydrogen molecule approaches broadside-on to an atop site then the molecule may dissociate with considerable energy gain and there is no activation barrier to this process (i.e. the potential energy surface is strongly attractive). The deep well in the lower right corner corresponds to the hydrogen atoms produced by dissociation entering the bridge sites of the W surface.


B2. When the hydrogen molecule approaches broadside-on to a bridge site then the molecule may again dissociate but the energy gain as the hydrogen atoms enter the atop sites is less marked than in the previous case (B1) and there is an activation barrier of ca. 0.2 eV that must be overcome for dissociation to occur.


In summary, these calculations demonstrate that parameters such as the molecular orientation and the point of impact on the surface can have a dramatic effect on the nature of the molecule-surface interaction and hence the dissociation probability.

In the case of the H2-W system they indicate that the most favoured channel to dissociative adsorption is for a molecule approaching broadside on to an atop site, with dissociation of the hydrogen into the adjacent bridging sites.

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