Chemisorption of H2 on W(211)

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Abstract
Adsorption, desorption, and work function data for hydrogen adsorption and mixed adsorption of deuterium and ethylene show that multiple binding states exist for hydrogen on the (211) face of tungsten. The desorption spectrum consists of two peaks: β1 with peak maximum at 330°K and β2 with peak maximum at 675°K. Both follow second order desorption kinetics with activation energies of 16 kcal/mole and 35 kcal/mole, respectively. The rate of adsorption into the β1 state is proportional to the fraction of unoccupied β1 sites with a sticking probability of 0.05. Until the β2 sites are nearly filled, adsorption into this state is independent of coverage and occurs with a sticking probability of 0.57. For both states the work function change is linear with coverage, but with a maximum increase of 0.61 eV for the β2 state and a maximum decrease of 0.27 eV for the β1 state. Consideration of the topography of the (211) surface leads to the conclusion that the hydrogen sites on this face are associated with the close packed rows of tungsten atoms and with the troughs between the rows. With these two possible sites both the adsorption kinetics and the results for mixed adsorption of deuterium and ethylene are shown to be consistent with the assignment of the β1 state to the rows and the β2 state to the troughs.