Removal of Carbonaceous Residues by Deuterium from Pt Catalysts

摘要

Mixtures of hexane (nH) and hydrogen (H_2) were reacted over EUROPT-1 (6.3% Pt/SiO_2) and Pt black. In additin to gas-phase products, including isomers, methylcyclopentane, fragments, and benzene, firmly chemisorbed entities were also produced. They may be C_1 units or -highly dehydrogenated-polymeric deposits. Removal of these surface carbonaceous entities was studied upon exposing them to deuterium between 293 to 673 K. Deuterium "hydrogenated off" surface CH_x species and "hydrogenolyzed" polymeric deposits. The amount and the composition in methane isotopomers was monitored as a function of time and removal temperature. The effect of the temperature and the nH/H_2 ratio during carbonization was systematically studied over EUROPT-1. The initial mixture removed initially from both catalysts contained mostly CD_3H and CD_2H_2. These methane molecules must have originated from the most reactive surface species (methylidyne and methylene). C_1 removed from Pt black contained, as a rule, more deuterium. The estimated H/C r atio in surface precursors was 0.4-0.9 (CD_4 CHD_3 > CH_2D_2 > CH_3D). The H/C ratio in the case of EUROPT-1 was, in turn, 1-1.5 (CHD_3 > CH_2D_2 approx = CD_4 > CH_3D). The amount of C_1 removed from EUROPT-1 showed a maximum as a function of the removal temperature between 350 and 450 K and further increased above 573 K. The removed methane contained more D when the catalyst was heated in the absence of hydrogen before removal. The deuterium cont ent of methanes was indicative of the degree of dehydrogenation of their surafce precursors. Dehydrogenation was significantly higher on Pt black. These observations are in good agreement with the propensity of EUROPT-1 to produce more sturated gas-phase products.