ENERGY-REQUIREMENTS FOR THE DISSOCIATIVE ADSORPTION OF HYDROGEN ON CU(110)

The activation energy for the dissociative adsorption of H-2 on Cu(110) is determined to be 14.3 +/- 1.4 kcal/mol (95% confidence limit). This value represents a true Arrhenius activation energy from Boltzmann distributions of impinging H-2 gas at approximately 623 K. The results were obtained by titrating atomic oxygen from a Cu(110) surface under conditions where the rate of titration was equal to the rate of dissociative hydrogen adsorption and where the oxygen coverage did not affect the rate. The hot copper surface was exposed to approximately 10(-4) Torr H-2 in a microreactor whose walls were at 300 K. The degree to which the H-2 translational, rotational, and vibrational populations approach the surface temperature was controlled by adding N2 as an energy transfer medium. The dissociative adsorption probability at 623 K increased from approximately 5 x 10(-7) to approximately 10(-5) when the N2 pressure was increased from 0 to approximately 80 Torr. About 70% of this increase occurred below 2 Torr. Known gas-phase energy transfer rates suggest that the translational temperature of H-2 hitting the Cu surface was already at the surface temperature after addition of only 2 Torr N2, but that the v = 1 vibrational population was still unchanged. It approached the population of the surface temperature only at 80-300 Torr N2. These results together with beam experiments suggest either translational or vibrational energy can dominate in overcoming the activation barrier to dissociation of an H-2 molecule.