A Study of Catalytic Ignition and Kinetic Phase Transitions (1996)

Catalytic ignition and kinetic phase transitions are two critical phenomena which can occur in heterogeneously catalyzed reaction systems. They have in common that the system behavior changes qualitatively when a control parame-ter passes through a critical value. Catalytic ignition, which can occur in exothermic catalytic reactions, is the discontinuous transition from a state where the reaction rate is primarily determined by the reaction kinetics, to a state pri-marily determined by mass transport. It occurs due to an imbalance between the chemical power generation and the heat losses in the system, occurring at a critical temperature called the ignition temperature. Kinetic phase transitions are, in contrast, of purely kinetic origin. They consist of concerted changes in reactant coverages and reactant rate, occurring at some critical value of, e.g., the reactant mixing ratio or temperature. The two phenomena are of both technological and scientific interest. Catalytic ignition is related to light-off and extinction in catalytic reactions. Kinetic phase transitions are scientifically interesting as critical phenomena, and are related to nucleation and propagation of chemical waves, and spatio-temporal chemical oscillations. Technologically they are related to self-poisoning, reaction rate multiplicity, and instabilities in chemical reactors. In this work both phenomena have been studied for the H2+O2 and CO+O2 reactions on Pt. They are well known model reactions for which explicit kinetics can be included in simulations of the two critical phenomena. Catalytic ignition was experimentally studied at atmospheric pressure with the heated wire technique, simple calorimetry, and mass spectrometry. Experimental ignition temperatures were measured for all mixing ratios of H2/O2 and CO/O2 where ignition occurs. The experimental results were analyzed theoretically and by computer simulations. A detailed understanding has been obtained of the basic physical principles underlying catalytic ignition. This is e.g. illustrated by experimental and theoretical verifications of the so called Frank-Kamenetskii criterion for catalytic ignition. The ignition temperature is determined almost entirely by the adsorption/desorption kinetics and the heat transport characteristics of the system. By incorporation of mass transport, also the transient ignition behavior is described. Kinetic phase transitions were studied during oxidation of H2 and CO at 1 atm., at temperatures ranging from 300 to 800 K over Pt-foils, by utilizing adsorbate induced work function changes measured with a Kelvin probe. Mass spectrome-try was used simultaneously to measure the local gas composition close to the sample. Kinetic phase transitions were observed as sudden changes of work function for both H2+O2 and CO+O2, but their appearances deviate strongly from what is observed or predicted under molecular flow conditions. These ex-perimental data have been analyzed by combining the same kinetic models as in the catalytic ignition studies, with a model that incorporates mass transport ef-fects. The simulations demonstrate that the major differences between the high pressure and the low pressure kinetic phase transitions are due to mass transport effects, and not due to different kinetics in the two pressure regimes.