Abstract
The hydrogenation of ethylene on thermally activated TaC were studied. Similar to the catalysis by transition metals, the hydrogenation on TaC exhibited a maximum rate at a definite reaction temperature; the activation energy of the reaction was 33.8 kJ mole −1 below 370 K, but −54.4 kJ mole −1 above 420 K. The reaction of ethylene with D 2 at 273 K yielded ethane ranging from d 0 to d 5 and highly exchanged ethylene with increased conversions. The isotope composition of gaseous hydrogen reached nearly equilibrium when the degree of conversion exceeded 40%. The chemisorption of hydrogen on TaC obeyed the dissociative Langmuir isotherm. The application of strongly adsorbed carbon monoxide and acetylene as poison showed that about 40% of the total surface was catalytically effective and that the reaction rate diminished with increasing fraction of poisoned surface, θ p , being proportional to (1 − θ p) 3. The accumulation of graphite-like carbon on the TaC surface occurred by heating the catalyst above 400 K in an C 2H 4 atmosphere, and was accompanied by a drastic decrease in the catalytic activity. X-Ray photoelectron study revealed that the characteristic features of electronic states in the activated TaC were a partial electron transfer from Ta to carbon atom. The associative mechanism of hydrogenation was proposed and the possible structure of active sites on TaC was discussed.
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