Removal of Carbonaceous Residues by Deuterium from Pt Catalysts

Abstract Mixtures of hexane (nH) and hydrogen (H2) were reacted over EUROPT-1 (6.3% Pt/SiO2) and Pt black. In addition to gas-phase products, including isomers, methylcyclopentane, fragments, and benzene, firmly chemisorbed entities were also produced. They may be C1 units or—highly dehydrogenated—polymeric deposits. Removal of these surface carbonaceous entities was studied upon exposing them to deuterium between 293 and 673 K. Deuterium “hydrogenated off” surface CHx 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/H2 ratio during carbonization was systematically studied over EUROPT-1. The initial mixture removed initially from both catalysts contained mostly CD3H and CD2H2. These methane molecules must have originated from the most reactive surface species (methylidyne and methylene). C1 removed from Pt black contained, as a rule, more deuterium. The estimated H/C ratio in surface precursors was 0.4–0.9 (CD4⪢CHD3>CH2D2>CH3D). The H/C ratio in the case of EUROPT-1 was, in turn, 1–1.5 (CHD3>CH2D2≈CD4>CH3D). The amount of C1 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 content of methanes was indicative of the degree of dehydrogenation of their surface precursors. Dehydrogenation was significantly higher on Pt black. These observations are in good agreement with the propensity of EUROPT-1 to produce more saturated gas-phase products.

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