Catalyst characterization by a probe reaction. Cyclopropane hydrogenolysis and benzene hydrogenation on platinum-alumina catalysts

• Published in: Journal of molecular catalysis 1992-02, Vol.72 (1), p.85-95
• Main Authors: Choren, Eduardo, El-Chaar, Lina, Hernández, José O., Arteaga, Geomar, Arteaga, Arnedo, Sánchez, Jorge
• Format: Article
• Language: English
• Subjects: Catalysis; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0304-5102
• DOI: 10.1016/0304-5102(92)80033-D

Ethylene hydrogenation by pulse is used as a probe reaction for the study of cyclopropane hydrogenolysis and benzene hydrogenation using platinum on alumina catalyst. It is shown that cyclopropane hydrogenolysis is a non-demanding reaction, because all the irreversibly adsorbed hydrogen atoms which are active for ethylene hydrogenation are also active for cyclopropane hydrogenolysis. The number of surface hydrogen atoms exceeds that of total platinum atoms in the catalyst by a factor of about 2.5; benzene, on the contrary, at room temperature, reacts with only a small fraction of them. A pulse technique is used to study the mechanism of benzene hydrogenation. The results suggest the following mechanistic features: (a) Benzene hydrogenation is a demanding reaction. (b) The rate-determining step of the reaction is the formation of a σ-complex between the benzene ring and a metal atom in the presence of irreversibly adsorbed hydrogen. (c) The formation of a π-complex cannot be an initial condition for the benzene hydrogenation. (d) Addition of hydrogen to the σ-complex is fast and requires hydrogen in the gas phase.