Thermal dechlorination and dehydrogenation pathways for ortho-, meta-, and para-chlorotoluene on the Pt(1 1 1) surface: C-Cl bond activation by methyl

The effects of a methyl substituent on the dechlorination pathways for ortho-, meta- and para-chlorotoluene have been characterized using temperature programmed reaction spectroscopy (TPRS) on the Pt(1 1 1) surface. All three isomer of chlorotoluene dechlorinate to form HCl, H2, and a small amount of benzene as the temperature is increased. The methyl group partially dehydrogenates at ∼300 K to yield a methylene group. Temperature differences are observed that are clearly dependent on the position of the methylene substituent relative to the chloro group. All the isomers are partially dechlorinated below HCl's desorption temperature resulting in a desorption limited HCl peak near 300 K. However, above 300 K the HCl peaks are reaction limited. ortho-Chlorotoluene's reaction limited HCl peak is at 326 K, while meta-chlorotoluene's reaction limited peaks are at 390 K and 420 K. The reaction limited HCl peak for para-chlorotoluene appears at 420 K. Clearly, the proximity of the methylene group to the chloro group decreases the temperature for HCl formation. The intensities of the primary reaction limited HCl peaks also change with chlorotoluene coverage for all the isomers. Specifically, for low chlorotoluene coverages, HCl formation near 420 K dominates for all three isomers. Initial adsorption appears to be associated with a less reactive site with a large adsorption strength. With increasing chlorotoluene exposure the low temperature HCl peaks increase in size. Similar results were obtained for hydrodechlorination of chlorobenzene on the Pt(1 1 1) surface and used to support a two site reaction model.