Synthesis and characterization of bimetallic cluster-derived platinum-gold catalysts and gold supported on mesoporous supports

The properties of SiO2- and TiO2-supported Pt-Au bimetallic catalysts prepared by co-impregnation from individual salt precursors were compared with those of samples of similar composition synthesized from a Pt2Au4(C≡CtBu)8 bimetallic cluster precursor. FTIR results indicate that highly dispersed Au crystallites in the cluster-derived samples, presumably located in close proximity to Pt, are able to chemisorb CO. Furthermore, the cluster-derived Pt2Au 4/TiO2 sample was the only one exhibiting low temperature CO oxidation activity, indicating that both the high dispersion of Au and the nature of the support are important factors affecting the catalytic activity for this system. C3H6 hydrogenation was used as a probe reaction to determine the composition of the active catalytic surface. Similar TOF values for C3H8 formation (on the order of 30 sec -1) were observed for the Pt/TiO2 sample and the Pt-Au/TiO2 sample prepared by co-impregnation. However, the TOF for C3H8 formation over the Pt2Au4/TiO 2 sample, was decreased to 0.07 sec-1 suggesting that the highly diluted Pt surface ensembles present on this sample are too small to effectively catalyze C3H6 hydrogenation, although ligand (or electronic) effects induced by the presence of Au adjacent to Pt sites can not be excluded. This result further indicates that the well known structure insensitivity of Pt for C3H6 hydrogenation "breaks down" when the size of the Pt ensemble is reduced to a small number (i.e., 3--4) of Pt atoms.; Au catalysts supported on mesoporous SiO2 and TiO2 were synthesized and tested for the oxidation of CO. Au/TiO2 exhibited a much higher activity for CO oxidation, even though the Au particle sizes were similar on the TiO2 and SiO2 supports. These results once again suggest that the presence of 2--3 nm Au particles alone is not sufficient to achieve high activity in CO oxidation. Instead, nature of the support also influences the activity for this reaction. Finally, Au catalysts supported on mesoporous TiO2 with different Au loadings (i.e., 5% and 10%) were tested for the epoxidation of C3H6. Over these catalysts the conversion of C3H6 was approximately 2% at a SV of 60,000ml/gcat-hr with selectivities to PO equivalents of only 7% while the selectivity towards CO2 was higher than 70%. Addition of 1%PO to the reaction feed at 175°C resulted in an increase in the formation of isomerization products (i.e., EtCHO and acetone) confirming that these two products are formed from pathways involving PO during the reaction. TEM data combined with kinetic results on the oxidation of CO confirmed that severe Au sintering is taking place on both catalysts when these samples were treated in H2 at 150°C. The formation of big Au particles may be responsible for the high production of H2O observed during the C3H6 epoxidation reaction.