Applied zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Catalysis, 15 (1985) 69-18 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 69 zyxwvuts REACTION KINETIC APPROACH TO STUDY ACTIVITY, SELECTIVITY AND DEACTIVATION OF Pt/A1203 IN n-HEXANE CONVERSION J. Margitfalvi, P. Szedlacsek, M. Hegedtis and F. Nagy Central Research Institute for Chemistry of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary ABSTRACT In this work factors influencing the activity and selectivity of the Pt/Al 0 catalyst were investigated in n-hexane conversion. Based on steady state and {ran- sient reaction kinetic data it was concluded that the main factors controlling the selectivity of this catalyst in n-hexane conversion are: (i) the amount of substrate (n-hexane) chemisorbed, i.e. the substrate coverage and (ii) the mobility of all sur- face species including chemisorbed hydrogen. These two factors strongly influence both the extent of dehydrogenation and the geometry of the surface species originat- ed from the hydrocarbon. It is suggested that on the Pt sites there are at least two types of surface intermediates involved in the formation of different reaction pro- ducts. Surface carbon, responsible for the catalyst deactivation, appeared to be a selective poison, which strongly suppressed the formation of benzene. INTRODUCTION Supported mono- and bimetallic platinum catalysts have received considerable sci- entific and industrial interest in hydrocarbon reactions. Despite tremendous effort in these two fields there were only few attempts to study reaction mechanism at high temperatures close to those used in industrial practice. Most of the kinetic studies at high temperatures were carried out in order to obtain the mathematical model or the formal rate equations of reforming reactions. The reason for the scarcity of in- formation about the mechanism of hydrocarbon reactions at high temperature can be attributed to difficulties in measuring the reaction rates above 300°C. Most of the problems are connected with the very fast catalyst ageing process to which the for- mation of a carbonaceous overlayer has the major contribution (I(. This overlayer, often called coke, changes the selectivity of hydrocarbon reaction 12,31. In this contribution we are presenting a reaction kinetic approach obtained in the course of study of n-hexane conversion, the aim of which is to answer the ques- tion as to what main factors control the activity and selectivity of the Pt/Al,03 catalyst. Based on literature data and on results obtained in this work, an emphasis was laid to demonstrate that the changes in the product selectivities can be attrib- uted to the alteration in the hydrogen and hydrocarbon coverages on the catalyst. These coverages control both the geometry and the mode of chemisorption of the 0166-9834/ 85/ $03.30 0 1985 Elsevier Science Publishers B.V.