Catalytic formation of carbon-carbon bonds in ultrahigh vacuum: Cyclotrimerization of alkynes on reduced TiO2 surfaces

The steady-state reaction of a molecular beam of methylacetylene impinging on a reduced TiO2(001) crystal was examined under UHV (10-9to 10-8mbar) conditions. Cyclotrimerization to form trimethylbenzene took place at steady-state. The reaction is truly catalytic; more than 20 turnovers could be obtained without deactivation of the single crystal catalyst. Experiments examining the cyclotrimerization rate as a function of pressure showed that, in the region from 3.0 10-9to 2.4 10-8mbar, the reaction was approximately first order in alkyne partial pressure. Temperature variation experiments showed a small decrease in rate as temperature was increased, indicating a slightly negative overall activation energy between 400 K and 535 K. A four-step kinetic model of the cyclotrimerization reaction was constructed using information derived from TPD experiments. These results demonstrate that steady-state catalytic studies of carbon-carbon bond forming reactions, characterized by an overall decrease in number of moles, can be performed on single crystal metal oxides under UHV conditions. 1996 Elsevier Science B.V. All rights reserved.

Catalytic formation of carbon-carbon bonds in ultrahigh vacuum: Cyclotrimerization of alkynes on reduced TiO2 surfaces Conference Paper