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 –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-O_x-(OH)_y-Na_z/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-O_x-(OH)_y-Na_z 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-O_x-(OH)_y-Na_z.

Acknowledgments: The financial support by the DOE/BES under Grant # DE-FG02-05ER15730 is gratefully acknowledged.