CLUSTER CHEMISTRY ON SURFACES - CHARACTERIZATION AND CATALYTIC STUDIES OF PHOSPHINE-STABILIZED PLATINUM-GOLD CLUSTERS ON SILICA AND ALUMINASUPPORTS

Cationic, phosphine-ligated Pt-Au cluster compounds, [Pt(AuPPh(3))(8)] (NO3)(2) (1) and [(PPh(3))Pt(AuPPh(3))(6)](NO3)(2) (2), have been immo bilized on silica and alumina supports. Characterization of the suppor ted clusters by P-31 MAS NMR, IR, and W-visible spectroscopy and study by chemisorption and temperature-programmed desorption of CO show tha t the clusters are immobilized intact without measurable fragmentation or irreversible transformation. This is confirmed by quantitative des orption of the clusters. The reactivity of the supported clusters with CO and H-2 was found to be similar to the known reactivities in solut ion and molecular solid phases. The cluster [(PPh(3))-Pt(H)(AuPPh(3))( 7)](NO3)(2) (3) was also immobilized on silica and found to be intact by MAS P-31 NMR and desorption experiments. The turnover rate (TOR) fo r H-2-D-2 equilibration was determined for these supported clusters at room temperature. The clusters 1 and 2 were good catalysts on silica and alumina supports after treatment at 135 degrees C under vacuum (TO R = 10-20 s(-1)), but showed significant activation by treatment at 11 0 degrees C under H-2 (TOR = 85-220 s(-1)). Cluster 3/SiO2 gave a simi lar rate (TOR = 170 s(-1)) with no thermal activation. All samples sub jected to the thermolysis conditions given above were found to be inta ct by P-31 MAS NMR and could be quantitatively desorbed. The high acti vities of the supported Pt-Au clusters are believed to result from sup port-promoted, partial PPh(3) dissociation. Desorption experiments sho w that this dissociation is reversible.