AVS 57th International Symposium & Exhibition
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM2
Hydroxylation of the MgO(001) Surface and its Effect on Metal Nucleation

Wednesday, October 20, 2010, 8:20 am, Room Picuris

Session: Oxide Surface Structure
Presenter: M. Brown, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
Authors: M. Brown, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
E. Carrasco, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
M. Sterrer, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
H.-J. Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
Correspondent: Click to Email
The water-oxide interaction is of great importance in a number of technologically relevant fields, among them heterogeneous catalysis. Several studies report on the promoting effect of water in catalytic reactions, the participation of surface hydroxyls in reaction mechanisms, and the influence of hydroxylation on the binding of metals to oxide surfaces. Achieving a fundamental atomic scale understanding of water-oxide interaction at environmentally and catalytically relevant conditions (e.g. ambient pressure) represents, therefore, a challenge for surface science studies related to heterogeneous catalysis. Here, we present a detailed study of water interaction with the surface of thin MgO films in a wide pressure range and the effect of surface hydroxylation on metal nucleation.
Adsorption of water on the surface of MgO(001) thin films of different thickness (2-20 ML) has been studied extensively from UHV to mbar pressure conditions using IRAS, XPS and TDS. Hydroxylation studies at room temperature were carried out in a high pressure cell up to water pressures of 1 mbar. The hydroxylation behavior of thick MgO films obtained from analysis of the O1s photoemission signal is comparable to MgO(001) single crystal studies showing a threshold pressure of 10-3 mbar H2O for hydroxylation. With decreasing MgO film thickness the threshold pressure gradually shifts to lower H2O pressures. In the limit of 2 ML MgO it is three orders of magnitude lower than for thick MgO films. Corresponding IR spectra confirm the enhanced reactivity of thin MgO films at high pressure conditions. The influence of surface hydroxylation on nucleation and sintering of gold nanoparticles was studied by comparison of Au deposition and thermal sintering on ideally terminated MgO(001) and hydroxylated MgO thin films, respectively. A combination of different spectroscopic techniques (TDS, XPS, IRAS) allows to establish a correlation between the degree of surface hydroxylation and the stability of gold atoms and clusters. Hydroxyl groups destabilize gold atoms on the MgO surface at 90 K; however, they enhance the stability of small gold clusters towards sintering at higher temperatures.