Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases p. 1667 - 1684 (1987)
Update date:2022-08-11
Topics:
Vong, Mariana S. W.
Sermon, Paul A.
Isothermal alkene titrations (AT) of Pt/SiO2 have been studied on a continuous basis with simultaneous measurement of hydrogen, cyclohexene, cyclohexane and benzene concentrations.Cyclohexene either donates H atoms to the catalyst at 423 K or extracts them from the catalyst at 293-353 K, the temperature being determined by the kinetics and thermodynamics of the titration.Agter optimisation of titration parameters, standard temperatures for the dehydrogenation and hydrogenation steps in the titration were chosen to be 423 K and 310 K.At these temperatures rates of donation of H atoms (dehydrogenation) increased with increasing cyclohexene partial pressure, but subsequent rates of extraction of H atoms passed through a maximum at 500-630 Pa; the optimum partial pressure of cyclohexene selected for standard titrations was, therefore, ca. 460 Pa.Titration mechanisms are considered in detail and suggest that Pt surfaces areas can be estimated from the first phase of H-atom donation by cyclohexene; indeed these (51.8 m2 per g Pt).However, alkene adsorption can affect results from numerous repeated titrations.The alkene titrations when operated in a dehydrogenation mode (ATO) with hydrogen donation to an oxygen-covered catalyst appeared far more satisfactory than previously used titrations involving alkene hydrogenation (ATh) by preabsorbed hydrogen.Since a complete differentiation of Pt-held from support-held hydrogen could not be achieved under isothermal conditions a temperature-programmed titration is proposed.
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