Chemisorption of carbon monoxide on polycrystalline tungsten

Abstract

The effect of the individual adsorbed states of CO on the work function of polycrystalline tungsten has been examined. The β₂ and β₃ states are adsorbed simultaneously and together increase the work function ϕ by 0.33 V at 300°K. At this temperature, the α state begins to fill before the completion of the β₁ state, but the total effect of the three β states on ϕ is estimated as +0.6 V. At 700°K, where the α state is absent, addition of the β₁ state to a surface previously filled with β₂ and β₃ gives a composite layer of lower mean dipole moment, corresponding to a work function increase of 6.3 × 10^–16 V/mol. cm², compared with 14.9 × 10^–16 V/mol cm² for the β₂+β₃ states. It is proposed that the β₁ state is involved in a clustering process. The α state changes the work function by approximately –0.1 V (–2.8 × 10^–16 V/mol cm²) and at high ambient pressures can compete for β₁ sites, thereby reducing the observed maximum work function change.

Initial sticking probabilities of unity are observed at 300°K. Reasons for the discrepancy with earlier work are discussed and a mechanism for the filling of the various adsorbed states is proposed. As the adsorption temperature increases the initial sticking probability falls, showing marked decreases at temperatures where the α and β₁ states become depopulated.