COMPUTER-SIMULATION OF THE ADSORPTION OF ARGON ON THE SURFACE OF TITANIUM-DIOXIDE .1. CRYSTALLINE RUTILE

Computer simulation of the adsorption of Ar on the hydroxylated and dehydroxylated (110) faces and on the (100) face of rutile was carried out by the grand canonical ensemble Monte Carlo method. As monolayer adsorption proceeds, mutual forcing out of Ar atoms from the optimum adsorption sites takes place on the (100) face because the nearest distances between adsorption sites are less than the van der Waals diameter of the Ar atom. On the dehydroxylated (110) face, the area per optimum site is larger than the molecular area of Ar. However additional "adsorption sites" appear on this surface during the course of adsorption. These are not minima of the adsorption potential but minima of the total potential energy of an adsorbed molecule. In the multilayer region, the adsorption isotherm on the (100) and hydroxylated (110) faces has a partial stepwise character while the simulated isotherm on the dehydroxylated (110) face has a long linear portion in that coverage region. It is shown that the shape of the simulated isotherm in the supermonolayer region is determined by adsorption in the second layer and not by simultaneous adsorption in the first and second layers as assumed by the BET theory. It is also demonstrated that the stepwise character of adsorption in the multilayer region occurs when the surface of the first layer is smooth. If it is rough, as in the case of the dehydroxylated (110) face, a linear portion of the isotherm is observed.