The Reconstruction of Supported Platinum Particles Monitored by Methylcyclohexane Dehydrogenation and H2 TPD

Alteration of the dehydrogenation activity of α-alumina-supported platinum particles induced by a high-temperature (723 K) treatment in argon atmosphere has been investigated. The methylcyclohexane dehydrogenation rate over reduced platinum particles is highly dependent on the argon treatment applied at 723 K after hydrogen treatment at the same temperature. With an increasing exposure time to the argon stream, the reaction rate increases sharply to a maximum and gradually diminishes afterwards to a stable value for a prolonged exposure time. This change is the result of two modifying actions induced by inert gas: (1) the desorption of tightly bound hydrogen species, responsible for the low initial activity, and (2) the reconstruction of the platinum crystallite surface toward a more stable active surface. The presence of "high temperature hydrogen" is systematically observed on small particles after hydrogen treatment at 723 K and seems to be related to the specific electronic properties of small particles and not to their surface structure. The reconstruction of the surface of the particles is by contrast very important for obtaining stable catalytic activity. Highly dispersed (80-90%), restructured catalysts were found to be similar in stability to mediumly dispersed (45%) catalysts.