Kinetic phase transitions in the reaction CO+O→CO2 on Ir(111) surfaces

Abstract

The oxidation of CO on Ir(111) surfaces was investigated under UHV conditions in the temperature range 360 K to 700 K by CO2 rate measurements utilizing mass spectroscopy. Steady-state CO2 rates were measured at constant total CO+O2 gas flux and variable gas composition (YCO=Y, YO2=1−Y) using mass flow controllers which allowed changes in the CO/O2 gas composition down to 0.1%. Between 360 K and 450 K the CO2 rates initially increase proportional to Y (T<400 K) or to Y1.5 (420 K<T<450 K) and exhibit a sudden drop to a negligible value at a temperature-dependent critical value Y*. The rate drop indicates a kinetic phase transition induced by CO poisoning of the surface. This behavior is similar to the features described by the ZGB and more recently developed lattice gas (LG) models of the CO+O reaction on surfaces. However, in contrast to the ZGB model but in accordance with LG models and experimental results on other platinum metal surfaces, no oxygen poisoning was observed at small Y, i.e., the surface was reactive even at the lowest attainable values of Y. Between 450 K and 530 K the initial CO2 rates remain proportional to Y1.5 up to critical Y* values but the kinetic phase transition softens due to the onset of CO desorption. Accordingly, CO poisoning is not complete and the CO2 rates do not attain the zero level beyond the transition. Above 530 K a kinetic phase transition is no longer seen since substantial CO desorption prevents poisoning, in accordance with conclusions from LG modeling. The kinetic phase transitions, their dependence on Y and temperature, and the measured CO2 rates can be excellently reproduced by simple kinetic modeling.