JOURNAL OF CATALYSIS 158, 193–198 (1996) ARTICLE NO. 0018 Liquid-Phase Hydrogenation of Cyclohexene over Pt Foil Catalysts D. E. Gardin, Xingcai Su, Paul S. Cremer, and G. A. Somorjai Department of Chemistry, Materials Science Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received December 13, 1994; revised June 13, 1995; accepted July 20, 1995 energies are 15 kcal/mol and 6 kcal/mol in the diffusion- A liquid-phase reaction cell, coupled to an ultrahigh-vacuum controlled and surface reaction-controlled regimes, respec- surface analysis chamber, was built to study liquid-phase hydro- tively. genation reactions on small area model catalyst surfaces. The hydrogenation of cyclohexene to cyclohexane was studied at 2. EXPERIMENTAL about 1.5 atm total pressure of H 2 and as a function of tempera- ture in the 313–333 K range in liquid cyclohexene. The hydroge- The apparatus and procedure we used to perform the nation rate of cyclohexene on a clean Pt surface increased with experiments are described in detail in Ref. (5). The main the increasing circulation velocity of the liquid, indicating that characteristics of our apparatus (shown in Fig. 1) are as the reaction rate was controlled by hydrogen diffusion to the follows: surface. When the surface reaction rate was reduced by the deposition of hydrocarbon fragments on the platinum surface, (a) The UHV chamber had a base pressure of 1  10 -9 the rate of cyclohexene hydrogenation became independent of Torr, which remained constant during the liquid–phase hydrogen diffusion and became controlled by the kinetics of reaction. The sample could be cleaned in the UHV cham- the surface reaction. The estimated activation energy of the ber by argon sputtering and oxygen heat treatments. reaction is 6 kcal/mol for cyclohexene hydrogenation on a (b) The sample was mounted at the end of two long platinum foil that was partly covered with carbonaceous depos- its.  1996 Academic Press, Inc. -in.-o.d. stainless-steel tubes. It could be resistively heated or cooled by circulating liquid nitrogen through the tubes. (c) During the transfer from UHV to the liquid reaction 1. INTRODUCTION cell, the sample was moved vertically down to the cell in two steps. First, a hydraulic system closes a cylinder around Liquid-phase reactions carried out at or near 300 K, like the sample, isolating it from the rest of the UHV chamber. the hydrogenation of olefins or nitriles, are important to The inside of the cylinder was then pumped by a diffusion chemical technology. It is also of considerable interest to pump, keeping the pressure in the 10 -8 Torr range. After compare and correlate rates and selectivity of catalytic opening the gate valve at the bottom of the cylinder to reactions carried out at solid–liquid vs solid–gas interfaces access the cell, the sample could be lowered to its reaction under otherwise identical experimental conditions. Such position by an 8-in.-long transfer arm. comparisons and correlations allow us to learn about (d) To allow good mass transfer at the liquid–solid inter- changes of reaction mechanism with changes in the re- face, a small gear pump produced a liquid jet (velocity of actant phase. We designed gas-phase (1–4) and liquid- up to 6 m/s) which was impinged on the surface, thus phase (5) reaction cells in combination with ultrahigh- creating a thin liquid film. vacuum surface analysis chambers to study catalytic (e) The total pressure during the reaction could go up reactions over low-surface-area model catalysts that were to 2 atm, while the temperature of the liquid could be properly cleaned and characterized before and after the adjusted up to 70°C. reaction by a combination of surface science techniques. (f ) A septum on the side of the cell allowed sampling for In this article we report the hydrogenation of cyclohexene chromatographic analysis which monitored the progress of using a polycrystalline platinum foil as the catalyst. The the reaction. hydrogenation of cyclohexene on the clean metal surface is so rapid that the rate is controlled by hydrogen diffusion The 0.1-mm-thick Pt foil (99.995% pure) of 1 cm 2 was spotwelded to the sample holder with Pt wires. Sample to the surface at about 300 K. However, the rate of cyclo- hexane production on a partially contaminated platinum cleaning was achieved by cycles of Ar + sputtering at 900– 1000 K (5  10 -5 Torr Ar, 1.0 keV, 30 min) followed by surface became surface reaction limited. The activation 193 0021-9517/96 $12.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.