JOURNAL OF CATALYSIS 160, 269–278 (1996) ARTICLE NO. 0145 Turnover Rate Enhancement of Reforming Reactions on Polycrystalline Pt–Ir Foils Adrian L. Bonivardi, 1 Fabio H. Ribeiro, ∗ and Gabor A. Somorjai Department of Chemistry, University of California at Berkeley, Berkeley, California 94720; and ∗ Center for Advanced Materials, Materials Sciences Division, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received August 29, 1995; revised December 18, 1995; accepted January 4, 1996 The reactions of n-hexane over bimetallic Pt–Ir polycrystalline foils were studied at 720 K, H 2 /n-hexane = 67, and a total pressure of 0.9 MPa. The addition of Ir to Pt increases the hydrogenolysis turnover rate by up to two orders of magnitude and decreases the dehydrogenation turnover rate to 1-hexene by up to one order of magnitude. The rates of isomerization to 2- and 3-methylpentane and cyclization to methylcyclopentane show a volcano-type correla- tion when plotted against the surface Ir composition. The maximum rate occurs at a surface composition of Pt 2 Ir and the rate is about three times higher than the rate for pure Pt. Addition of sulfur to the surface decreases the rates for isomerization, cyclization, and especially the rate for hydrogenolysis, resulting in a substantial de- crease in the initial selectivity for hydrogenolysis. However, sulfur addition increases the dehydrogenation rates for the monometallic and bimetallic surfaces. It is observed that Pt–Ir shows a surface chemistry for the reactions of n-hexane similar to the surface chem- istry of Pt when the bimetallic surface is sulfided. The turnover rates, however, are higher for Pt 2 Ir than for Pt in the presence and absence of sulfur. c  1996 Academic Press, Inc. INTRODUCTION The reforming of naphtha is one of the largest volume processes in the petroleum industry. The catalyst consists of either Pt–Re, Pt–Sn, or Pt–Ir clusters supported on a high surface area chlorinated alumina. The bimetallic Pt–Ir and Pt–Re catalysts have shown a superior performance, higher activity and, much improved activity maintenance over the monometallic first generation Pt catalysts (1). For this reason it is important to understand the role of the second metal. Of these three systems, Pt–Ir seems to have the highest rate per unit of volume of packed bed for a simi- lar metal loading (2, 3). In addition, the reforming process can be more effective by using the Pt–Ir catalyst in the tail zone of a reforming unit for the better dehydrocyclization of paraffins of supported Pt–Ir versus Pt (1, 4). 1 Present address: INTEC, G ¨ uemes 3450, 3000 Santa Fe, Argentina. The advantages of these multimetallic systems are on their higher stability to deactivation and higher rates of reaction. A number of explanations for the increased sta- bility are proposed. Cartel et al. (2) suggested that the lower rate of deactivation of Pt–Ir is due to a lower rate of car- bonaceous deposits due to the higher rate of hydrogenolysis while Ramaswany et al. (5) suggested that the greater stabi- lity is due to a decreased rate of dehydrogenation on Pt–Ir catalysts. In our studies we found no evidence that Pt–Ir is more stable than Pt. However, our studies were geared to study the higher rates of Pt–Ir catalysts as compared to other Pt-based reforming catalysts. This increased acti- vity seems to be one of the great commercial advantages of this system since production can be increased by changing to Pt–Ir catalyst on existing units instead of building new units. Our approach to understand the role of the second metal- lic component has been to study the bimetallic system with- out the interference of the alumina support in the same way as we did before for Pt–Re (6). The model reaction for studying catalytic naphtha reforming is the conversion of n-hexane in the presence of excess hydrogen to other molecules by reactions involving dehydrogenation, isomer- ization, cyclization, and hydrogenolysis. We used a model catalyst system that consists of a polycrystalline Pt or Ir foil of small (∼1 cm 2 ) surface area. Iridium was deposited on to Pt foil from a pulsed metal plasma gun; or conversely, Pt was deposited on an Ir foil in the same manner. The surface concentration of the two metals was determined by Auger electron spectroscopy (AES) and by the n-hexane dehy- drogenation reaction which is particularly sensitive to Pt coverage. Then the bimetallic system was sulfided, both by sulfur deposition and by the addition of thiophene during the n-hexane reaction. The presence of sulfur on the metal surface markedly reduced the hydrogenolysis activity of Ir (by two orders of magnitude). The rates and selectivities were the same whether Ir was deposited on Pt or Pt was deposited on Ir, indicating that the surface composition of the bimetallic 269 0021-9517/96 $18.00 Copyright c  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.