The hydrogenation of acetylene catalyzed by palladium: Hydrogen pressure dependence

The kinetics of acetylene hydrogenation catalyzed by a clean palladium foil at high pressures are measured and yield an activation energy of 9.6 +/- 0.1 kcal/mol when using hydrogen. The rate exhibits a deuterium isotope effect such that the reaction activation energy is 9.0 +/- 0.2 kcal/mol for reaction with deuterium. The hydrogen pressure reaction order is 1.04 +/- 0.02 at 300 K with an acetylene pressure of 100 Torr and the acetylene order is -0.66 at 300 K and with a hydrogen pressure of 100 Torr. These reaction kinetics closely mimic those of supported model catalysts. In addition, it is found that the rate of benzene formation is accelerated by the addition of hydrogen to the reaction mixture. This is rationalized by proposing that hydrogen enhances the coverage of acetylene under catalytic conditions. This notion can be used to successfully calculate the hydrogen pressure dependence for acetylene hydrogenation as a function of temperature, a value which varies between similar to 1.05 and 1.3 as the temperature changes from 300 to 380 K. Possible origins for this effect are discussed. (C) 1999 Academic Press.