Trends within the adsorption energy of alcohols over rutile TiO2(110) and (011) clusters

The non-dissociative adsorption energy of a series of alcohols over a rutile Ti8O29H26 cluster, representing the TiO2(0 1 1) surface, as well as over a rutile Ti11O42H40 cluster, representing the TiO2(1 1 0) surface, was computed by means of a semi-empirical PM3 method. Over TiO2 (0 1 1) Surface the non-dissociative adsorption energy of methanol, ethanol, n-propanol and i-propanol were 1.29, 1.46, 2.08, and 2.40 eV, respectively. This trend can reasonably be explained by gas phase acidity, as measured by their relative (to methanol) polarizability. On the other hand, the same series investigated over TiO2(1 1 0) gave the following adsorption energy values: 1.49, 1.60, 1.96, and 1.94 eV, respectively, The relatively low adsorption energy for i-propanol, shows a deviation from the expected correlation. The reason for that is most likely structural. While TiO2(1 1 0) surface contains Ti4+ cations five- and six-fold coordinated to oxygen, TiO2(0 1 1) surface contains Ti4+ all in a five-fold coordination environment. Thus, it appears that the alternating rows of bridging oxygen anions of the (I 10) surface exert a repulsive interaction over the two-methyl groups of i-propanol decreasing its adsorption energy. (C) 2002 Published by Elsevier Science B.V.