Reactions of MgO Species Supported on Cryogenic Matrices

Research project P 14730 Reactions of MgO Species Supported on Cryogenic Matrices Erich KNÖZINGER 27.11.2000 In this project the reactivity of molecules and clusters of magnesium oxide with respect to hydrogen, oxygen, and methane in the dark and - in order to overcome potential activation barriers - under UV irradiation is to be investigated. The evolution of the reactivity with the average cluster size and the trend of this evolution is of particular interest. Reference data related to the reactions of the above mentioned molecules with the surface defects on MgO nanoparticles are already available in the group of the applicant. The standard state of magnesium oxide is the solid ionic crystal, which does not exhibit a significant partial vapour pressure below 1000 `C. Therefore, it is not an easy task to provide molecules and clusters of this material. According to a special procedure developed in our group we intend to deposit Mg metal vapour on the surface of a highly dispersed inert gas matrix (e.g., Ar). Depending on the deposition conditions either monomers or clusters of a certain size distribution will be isolated on the surface. Their oxidation then requires the addition of molecular oxygen or oxygen atoms. The characterization of the resulting MgO species and of the subsequently formed reaction products will be based mainly on FTIR spectroscopy. The principal aim of the project is to produce isolated MgO species in the size range from the monomer MgO to MgO nanoparticles and to establish trends in the reactivity with respect to small probe molecules such as molecular hydrogen, molecular oxygen and methane. We expect to contribute therewith to a reliable description of reactive properties of defects on MgO surfaces and a better understanding of their catalytic activity.

Millions of tons of highly dispersed material of outstanding scientific and technological relevance are annually produced world-wide by non-equilibrium techniques such as precipitation or vapour deposition. The particle size ranges down to the micrometer and nanometer scale. Some of these materials successfully act as catalyst support, i.e., stabilize tiny catalytically active species such as metal clusters on their surface. A similar sort of surface isolation would be helpful in the study of very specific intermolecular interactions which are of relevance for nucleation processes and chemical reactions between isolated species. For this purpose essentially inert surfaces, e.g., those of solid rare gases as argon, are required. Different from cluster growth or chemical reactions after isolation of the interacting partners in the bulk of an Ar matrix, isolation of one of them on the surface of the matrix facilitates an easy access for the other partner from the gas phase. Of course this necessarily implies the application of highly dispersed matrices, in order to ensure characterizing measurements well above the respective detection limit. In fact, the project showed that the production of highly dispersed rare gas solids is feasible by non-equilibrium procedures. However, the subsequent surface isolation of molecular species for reaction or nucleation studies is still in the state of being explored. Our attempts in the course of this project were based on the deposition of the molecules under study in the form of a highly diluted gas mixture with Neon on the argon substrate at 6.5 K and subsequent evaporation of the neon gas. On slightly irregular argon surfaces this process ends up with the formation of a molecular film. In porous solid argon the emanating neon takes the isolated molecules along and thus removes them completely from the argon substrate. In order to verify whether this is an argon specific behaviour, similar experiments are envisaged with Kr as support.