The role of exposed silver in CO oxidation over MgO(0 0 1)/Ag(0 0 1) thin films

• The catalytic activity of ultrathin and thick MgO(0 0 1)/Ag(0 0 1) thin films has been examined.
• Ultrathin MgO(0 0 1)/Ag(0 0 1) films are more reactive than thick films, but less reactive than Ag(0 0 1).
• Ag remains exposed on the surface for MgO film thicknesses of at least 10 monolayers.
• Ultrathin films are highly reactive and susceptible to surface poisoning by carbonate formation.

The reactivity of MgO(0 0 1) films deposited on Ag(0 0 1) and Mo(0 0 1) in CO oxidation as a function of oxide film thickness was investigated experimentally at ambient pressure reaction conditions. MgO films grown on Mo(0 0 1) were found to be inactive in CO oxidation, whereas activity enhancement with decreasing oxide film thickness was observed for MgO(0 0 1)/Ag(0 0 1). In situ infrared and post-reaction X-ray photoemission data showed that ultra-thin MgO films interact much more strongly with the reactants and residual water than bulk-like MgO. Poisoning of the MgO surfaces by the resultant accumulation of carbonate and hydroxyl species is suggested to inhibit the reactivity in CO oxidation. Detailed investigations of the surface structure of MgO(0 0 1)/Ag(0 0 1) films indicate that silver ad-islands on Ag(0 0 1), which are formed during MgO growth, are responsible for the enhancement of CO oxidation activity over ultrathin MgO(0 0 1)/Ag(0 0 1) films.

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