COADSORPTION OF NA AND CO2 ON THE FE3O4(111) TERMINATION OF ALPHA-FE2O3(0001) - RELATIONS BETWEEN STRUCTURE AND ACTIVATION

The coadsorption of CO2 and Na on the Fe3O4(111) termination of alpha- Fe2O3(0001) has been investigated by, synchrotron-radiation-based core -level and valence band photoemission and low-energy electron diffract ion. We find that a 1 x 1 structure ideally is formed at 1/4 ML Na cov erage. Based on a simple adsorption model. Na adsorption in 3-fold hol low sites equivalent to bulk octahedral interstices is proposed. A per fect structure is lot obtained owing to occupancy of an additional ads orption site above theta(Na)approximate to 0.16 ML. The Na adsorption below 0.25 ML is accompanied by a substantial No-tu-substrate charge t ransfer leading to reduction of the uppermost 1/4 ML Fe3+ ions. The Na -induced valence band emission observed in the submonolayer region is characteristic of alkali-oxygen compounds. Occupancy of the second ads orption site also marks the onset of reactivity towards CO2 of the Na/ Fe3O4(111) interface at roam temperature. This is believed to be the f irst observation of site-specific activation in this type of systems. The resulting adsorbate is identified as a carbonate species. Especial ly near the threshold of activation both types of Na are involved in b onding which is attributed to the existence of Na2CO3 units on the sur face. The close connections between electronic aid geometric structure and reactivity are discussed and general aspects of the observed acti vation and bonding mechanisms are suggested. (C) 1998 Elsevier Science B.V. All rights reserved.