Change in the Thermionic Emission Current of Palladium Due to Chemisorption of Atomic and Molecular Hydrogen

The thermionic emission current from a palladium wire at 1000°C was measured while the wire was exposed to molecular- and atomic-hydrogen beams. The molecular-beam source was at 300°K; the atomic-beam source was at 2400°K. Ultrahigh-vacuum techniques were used, and background pressures were 10−9 torr. The average sticking coefficient, for chemisorption, was found to be larger for molecular hydrogen than for atomic hydrogen. This may be understood in terms of the energy-exchange phenomena between the gas and the surface. The slow moving molecules (300°K) transfer enough kinetic energy to the lattice, during collision, to be adsorbed. The energetic atoms (2400°K) are not so successful in this energy transfer, and a considerable fraction of them are reflected from the surface. Based upon heat of adsorption considerations, one would expect atomic hydrogen to have a higher sticking coefficient than molecular hydrogen. The data show the opposite, and it is concluded that the kinetic energy of the incoming particle affects the sticking coefficient more strongly than the heat of adsorption, for the conditions studied here.