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Modelling the interaction between liquids and solid surfaces

Understanding the interactions between liquids and solid surfaces is fundamental to understanding catalytic processes in which either liquids are used as reactants or liquids are formed as products. With improved understanding it should be possible to optimise the design of catalysts for specific reactions in order to increase the activity or the selectivity. In addition to experimental techniques, computer modelling can be used to help with the understanding of liquid/solid interactions. In the case of oxide-supported metal catalysts, the interaction of liquids with the support may be very important but this is rarely investigated.

γ-alumina (Al2O3) is a common oxide support for metal catalysts. For the calculations the alumina is represented as a rigid lattice with the water molecules interacting at the surface and then extending out into the pure liquid phase.


The orientation of the water molecules at the alumina surface can be computed, for the first water layer:

and for the second water layer:


Similarly, the interaction with alumina of other solvents, such as iso-propanol (propan-2-ol) can be computed:

To investigate the effect of depositing a metal on the alumina, a patch of the alumina surface, comprising 4, 12 or 24 ions,is given a zero charge. The resulting interactions with water molecules is shown below:

The calculations show a significant effect on the structure of the liquid around the edge of the patch. For other solvents, such as alcohols with more hydrophobic character, these edge effects could create regions of high hydrophobicity aound a small metal particle. The edge effect would be different for different solvents. Since it would be expected that any edge effects would affect the orientation and adsorption of reactant molecules we can use these calculations to help us to begin to understand the effect of different solvents on catalytic performance..

In a recently published paper (T. Youngs and C. Hardacre, Phys.Chem.Chem. Phys., advance paper, DOI: 10.1039/c0cp01838d, to be published in 2011) further calculations are reported which show how the structure of the liquid appears to develop at further distances from the "metal-modified" surface as shown below:

For example, in the case of propan-2-ol the calculated structures of the solvent above the solid surfaces are shown below:

Although the specific effects will depend on the nature of the solvent and of the diffusing species, strong structural effects in the solvent above the active metal are likely to affect the diffusion of reactants or products between the bulk and the interfacial regions and consequently to affect the nature of the species near to the active site.