The reactivity of cyclohexene on Au(111)-supported molybdenum carbide nanoparticles was studied using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) techniques. The surfaces were prepared by reactive layer-assisted deposition (RLAD), in which Mo metal is deposited onto ethylene adsorbed on a Au(111) surface, followed by annealing to 700 K. We have shown that gold encapsulates the MoCx nanoparticles to some extent on annealing. This encapsulation can be removed by ion sputtering at room temperature. Although the sputtered MoCx/Au(111) surface was very reactive toward adsorbed cyclohexene, the selectivity for partial dehydrogenation to benzene was very poor (0.06), with most of the reacting cyclohexene decomposing to surface carbon and molecular hydrogen. In contrast, the Au-encapsulated MoCx nanoparticles exhibit low total activity, but very high (>0.95) selectivity for partial dehydrogenation to benzene, with no detectable decomposition to surface carbon. The low overall reactivity and high selectivity of Au-encapsulated MoCx surfaces for partial cyclohexene dehydrogenation are explained by gold selectively blocking high reactivity sites on the MoCx clusters. (c) 2005 Elsevier Inc. All rights reserved.