The co-adsorption of hydrogen and carbon monoxide on the (110) surface of iridium†

Abstract

The nonreactive co-adsorption of hydrogen and carbon monoxide on Ir(110) was studied under ultra-high vacuum conditions with thermal desorption mass spectrometry, LEED, contact potential difference measurements and UPS. No changes in the Ir(110)-(1 × 2) reconstructed surface structure or ordering of the adsorbed layers were observed with LEED due to the presence of hydrogen and CO co-adsorbed. The adsorption of hydrogen on pre-adsorbed CO, or vice versa, causes less hydrogen to occupy the high temperature β 2 state and shifts the occupancy to the low temperature β 1 state preferentially. An apparent increase in the sticking probability of hydrogen for adsorption in the β 1 state for small CO coverage is discussed. At high CO coverage, the Ir(110) surface is poisoned to hydrogen adsorption. Exposing CO to preadsorbed hydrogen causes the binding energy of hydrogen to decrease with increasing CO exposure. Eventual displacement of hydrogen from the surface occurs for large CO exposures. The contact potential difference (CPD) as a function of hydrogen exposure on pre-adsorbed CO complements the desorption data for both the β 1 and β 2 states of hydrogen. For low coverages of CO, relating the CPD to hydrogen coverage shows the induced dipole of hydrogen is unchanged from the clean surface. Furthermore, the HeI UP spectra of small coverages of hydrogen and CO indicate the valence orbitals of CO are not affected detectibly by the presence of hydrogen. The results indicate CO poisons β 2 sites for hydrogen by a simple site blocking mechanism and may exclude β 1 sites at high CO coverages by a hydrogen -CO repulsive action.