Schering Preis 2003
Dr. Konrad Koszinowski
Department of Chemistry, Stanford University, USA
"Gaseous platinum clusters-versatile models for heterogeneous catalysts"
Cationic platinum clusters are proposed as models for heterogeneous catalysts. In the gas-phase, the reactivity of the cluster ions can be thoroughly investigated by means of Fourier-transform ion-cyclotron-resonance mass spectrometry.
The reactions of Ptm+ clusters, m - 5, with several simple substrates exhibit distinct similarities to the analogous processes of heterogeneous catalysts. Encouraged by the thus demonstrated validity of the model approach chosen, methane activation by Ptm+ clusters is studied in detail. Kinetic isotope effects associated with dehydrogenation, hydrogen-deuterium exchange processes, and energy-dependent collision-induced dissociation experiments provide insight into the potential-energy surfaces of the surprisingly complex Ptm+/CH4 systems. The main difference between mononuclear platinum carbene PtCH2+ and the larger homologues PtmCH2+ concerns the interaction of carbon with more than a single metal center in the case of the clusters.
The resulting stronger platinum-carbon binding leads to a drastic change in reactivity. Instead of C-N coupling as observed for mononuclear PtCH2+, the clusters PtmCH2+ yield carbide complexes PtmC(NH3)+ upon reaction with ammonia. Whereas the first type of reactivity corresponds to the key step of the Degussa process for the large-scale synthesis of hydrogen cyanide from CH4 and NH3, the deviating behavior of the clusters can be considered as gas-phase analogue of undesired soot formation on the heterogeneous catalyst.
In the gas-phase model, reactivity can be controlled if bimetallic clusters are included. Unlike Pt2+, the heteronuclear cluster ions PtCu+, PtAg+, and PtAu+ efficiently mediate coupling of CH4 and NH3. On the basis of these results, the corresponding bimetallic catalysts are suggested for potential improvement of the Degussa process.