A Dynamic Study of CO Oxidation on Supported Platinum zyx Catalytic oxidation of CO on alumina-supported platinum was studied in a tu- bular isothermal reactor with applied concentration transients. A completely au- tomated reactor system capable of switching frequencies as high as 0.2 Hz was constructed. Concentration cycling resulted in reaction rates that were much higher than the maximum rate achievable at steady-state operation. Mapping of the time-averaged carbon dioxide production rate in the timeperiod duty-fraction plane, resulted in a unique global maximum. We find that the maximum in rate occurs when the switching times are comparable to the characteristic time of the surface reaction. Results of our experiments also rule out significant contribution from the Eley- Rideal mechanism at the temperature range of this work (below 120OC). Yoav Barshad and Erdogan Gulari Department of Chemical Englneerlng The University of Mlchlgan Ann Arbor, MI 48109 SCOPE The fact that periodic operation of catalytic reactors may result in better overall performance has been suggested by many researchers including Douglas (1972) and Bailey (1973,1977). Most of the work in this area has been theoretical, with the ex- ception of a few experimental studies, among which Cutlip's (1979) work showed the most dramatic rate enhancement. In this paper the possibility of using a tubular reactor was explored in order to obtain a system time constant of the same order of magnitude as the surface processes. In order to achieve this very short time response a completely automated reactor system was built and the oxidation of CO on supported platinum was studied with relatively short switching periods (as low as 10 seconds). CONCLUSIONS AND SIGNIFICANCE Periodic operation of a catalytic reactor was extended to switching times significantly shorter than previously reported for this reaction. It is shown that the maximum in conversion occurs at a point where the time period and duty-fraction are comparable to characteristic times of surface phenomena rather than the reactor time constant. The most significant findings are: 1. Periodic feed switching results in time-averaged oxidation rates much higher than the maximum achievable by steady-state operation. The increase can not be explained as being due only to increased surface coverage of oxygen. 2. Experimentally, it is verified that there is a global maxi- mum in conversion in the timeperiod, duty-fraction plane. The locus of the maximum in conversion moves in a predictable INTRODUCTION Deliberate periodic operation of chemical reactors for im- provements in performance and/or selectivity was discussed by Douglas (1967, 1972),Douglas and Rippin (1966)and extensively studied and reviewed by Bailey (1973, 1977). While most of the initial papers in periodic operation were theoretical, in recent years Correspondence zyxwvutsrqpo concerning this paper should be addreared to Erdogan zyxwvutsrqp Gulari. AlChE Journal (Vol. 31, No. 4) fashion with feed concentration and temperature. 3. Increased rate of reaction is only due to periodic switching of the carbon monoxide feed. We believe that periodic on-off switching of the feed optimizes the reactive CO surface species at the expense of nonreactive stable species. 4. At the temperatures used in this work, oxidation of CO proceeds by the Langmuir-Hinschelwood mechanism. We have also observed that the dependence of the reaction rate on the surface concentration of CO and zy 02 is not the same. Sustained oscillations in the conversion occur both during steady-state and periodic operation of the tubular reactor. This study shows the benefits of automated reactors in collecting and analyzing enormous amounts of kinetic data in real time. the number of experimental studies has increased considerably. One of the first experimental studies in this area was by Denis and Kabel(l970) who studied the periodic operation of a heterogeneous reactor for the vapor phase dehydrogenation of ethanol and ob- served that adsorption-desorption played a predominant role in the system transients. Oxidation of zyxw SO2 on V205 was studied by Unni et al. (1973), where rate improvementsof 30% were achieved. Renken et al. (1974)found the periodic operation of a tubular re- actor for ethylene oxidation to be superior to steady state operation. April, 1985 Page 649