(127b) Unsupported and Supported Au1-Ox-(OH)y-Naz Clusters As Stable, Single-Site Gold Catalysts

Cao, S., Tufts University
Yang, M., Tufts University
Wang, C., Tufts University
Liu, J., Tufts University
Shan, J., Tufts University
Lee, S., Argonne National Laboratory
Allard, L., Oak Ridge National Laboratory
Flytzani-Stephanopoulos, M., Tufts University
Elnabawy, A., University of Wisconsin-Madison
Mavrikakis, M., University of Wisconsin-Madison
Li, M., Tufts University
Trimpalis, A., Tufts University
Mononuclear metal complexes enable the maximum utilization of a metal as a catalyst. Inorganometallic clusters with a single gold cation coordinated with –O 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 –site 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.