THE DISSOCIATIVE ADSORPTION OF H-2 AND D2 ON CU(110) - ACTIVATION BARRIERS AND DYNAMICS

A true Arrhenius activation energy for the dissociative adsorption of H-2 on Cu(110) was measured using a Boltzmann distribution of H-2 gas at the surface temperature and found to be 14.3 +/- 1.4 kcal/mol with a pre-exponential factor of 10(0.03 +/-0.16) per H-2 collision with the surface. The temperature of the H-2 gas impinging on the heated Cu(110) surface was brought to the surface temperature (approximately 623 K) from the cold wall temperature (approximately 300 K) by increasing the total pressure in the reaction vessel with inert N2. This increases the translational and internal energy of H-2 by collisional energy transfer near the surface. The rate of dissociative H-2 adsorption was found to increase strongly with the addition of N2 UP to approximately 2 Torr, but to increase slowly above that, consistent with a "direct" mechanism for dissociative adsorption where the H-2 translational energy is most effective in scaling the activation barrier. Gas-phase collisional energy transfer was computer-simulated using known energy transfer rates to determine the average translational and internal energies of H-2 impinging on the copper surface as a function of N2 pressure. The translational, rotational, and vibrational temperatures approach the surface temperature at widely different N2 pressures, allowing assessment of the relative effectiveness of these degrees of freedom in assisting H-2 adsorption. Comparison of the activation energy with the desorption energy of adsorbed hydrogen indicates that the adsorption of hydrogen is nearly thermoneutral. Similar results are also reported here for D2 adsorption, where no significant differences between D2 and H-2 could be seen.