Reactivity studies with gold-supported molybdenum nanoparticles

The reconstructed (22 × √3)-Au(1 1 1) surface was used as a template and inert support for depositing Mo nanoparticles for reactivity studies of desulfurization and the formation of MoS _x nanoparticles. Nanoparticles of Mo were prepared on the Au(1 1 1) substrate by two methods: physical vapor deposition (PVD) of Mo and UV-assisted chemical vapor deposition (UV-CVD) through a molybdenum hexacarbonyl precursor. STM studies have shown that the Mo nanoparticles are thermodynamically unstable on the Au(1 1 1) surface, and that gold encapsulates Mo at temperatures above 300 K. Reactivity studies using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) show that bare Mo nanoparticles are very reactive and can cause complete dissociation of hydrogen sulfide, methyl mercaptan, and thiophene. The presence of gold atoms on the Mo nanoparticles modifies their reactivity. In the case of H ₂S and CH ₃SH, the overall activity for desufurization is unaffected by gold encapsulation; however, the selectivity to form methane from CH ₃SH increased from 20% on bare Mo particles to 60% on gold-covered Mo particles. In contrast, gold-encapsulated Mo nanoparticles are relatively inert towards dissociation of thiophene. We believe that the interaction of R-SH compounds with Au-encapsulated Mo nanoparticles proceeds through intermediacy of surface gold thiolates.