(127b) Unsupported and Supported Au1-Ox-(OH)y-Naz Clusters As Stable, Single-Site Gold Catalysts
Mononuclear metal complexes enable the maximum utilization of a metal as a catalyst. Inorganometallic clusters with a single gold cation coordinated with âO linkages to a shell of Na cations are shown here to be stable in aqueous solutions, and were used in dried powder (unsupported) form, and on a support, such as titania, for several catalytic reactions. A new facile, one-pot green route was used to synthesize the single-gold atom clusters, using aqueous solutions of Au(OH)3 and NaOH. Incipient wetness impregnation was used to prepare supported catalysts. The single-site gold species catalyze a number of reactions, such as the water-gas shift reaction, the steam reforming of methanol, and the selective dehydrogenation and coupling of methanol to methyl formate (MF) and hydrogen products. Stability up to 250-300 oC reaction temperatures is demonstrated in cyclic tests over the course of many hours. The preparation method allows for up to 1.0 wt% Au loading as single âsite isolated species on TiO2, as shown by STEM and EXAFS. The catalyst is 100% selective for the dehydrogenation of methanol to MF and H2 below 170 oC. As a counterpart, 4.8 wt% Au/TiO2 made by conventional deposition/precipitation methods shows less than 1/10 of 1.0wt% Au1-Ox-(OH)y-Naz/TiO2. However, the apparent activation energy of the reaction is similar on the two catalysts, around 95 kJ/mol, which is close to that of methanol steam reforming on Au catalysts on various supports. Both supported and unsupported Au1-Ox-(OH)y-Naz are active for methanol coupling, which is demonstrated by in-situ methanol DRIFTS. XPS and XANES analyses show that the gold state in the used catalyst is in the Au(I) state for both supported and unsupported Au1-Ox-(OH)y-Naz.
Acknowledgments: The financial support by the DOE/BES under Grant # DE-FG02-05ER15730 is gratefully acknowledged.