Chemisorption of Hydrogen on the (111) Plane of Nickel

The adsorption of hydrogen on the (111) plane of nickel at room temperature has been investigated using the technique of flash desorption spectroscopy. The residual CO partial pressure has been reduced below 1 × 10−11 torr to minimize the effect of H2 and CO coadsorption. The experimental apparatus and crystal cleaning procedure are described. Hydrogen was observed to desorb in a single pressure burst for all the experimental conditions examined. Four techniques have been used to analyze the experimental results and a coherent picture emerges for low surface populations. The adsorption energy, Arrhenius preexponential factor and sticking coefficient have been determined for surface hydrogen populations less than 1013 atoms/cm2, the values obtained being 22.7 kcal/mole, 2×10−1 cm2 atoms−1 · sec−1, and 0.1, respectively. Furthermore it is shown that the adsorption energy is constant at low coverages. For surface populations greater than 1013 atoms/cm2, we have been able to reproduce the desorption spectra with a model in which either the adsorption energy decreased with adsorbed population or the pre-exponential factor increased with population. It is not possible to distinguish between the two possibilities. Such a model does not however describe very well the observed adsorption kinetics. We have proposed that this discrepancy may be due to diffusion of hydrogen into the nickel when the crystal is heated.