Kinetic interactions between hydrogen and carbon monoxide oxidation over platinum

Abstract

The heterogeneous chemistry coupling of H2 and CO over platinum was investigated experimentally and numerically for H2/CO/O2/N2 mixtures with overall lean equivalence ratios φ=0.13–0.26, H2:CO molar ratios 1:5–3:1, and a pressure of 5bar. Experiments were performed in an optically accessible channel-flow reactor at surface temperatures 510–827K and involved in situ Raman measurements of major gas-phase species concentrations and thermocouple measurements of surface temperatures. Emphasis was placed on the low temperature range 510–600K, whereby H2 inhibited the CO oxidation, and which was of particular relevance to gas turbine idling and part-load operation. Comparisons of measurements with 2-D simulations attested the aptness of the employed kinetic scheme, not only for H2/CO fuel mixtures but also for pure CO. Measured and predicted transition temperatures below which H2 inhibited CO oxidation agreed well with each other, showing a main dependence on the overall equivalence ratio (550±5K at φ=0.13 and 600±5K at φ=0.26) and a weaker dependence on the H2:CO ratio. Furthermore, this inhibition was non-monotonically dependent on the H2:CO ratio, becoming higher at a value of 1:1. The inhibiting kinetic effect of H2 was an outcome of the competition between H2 and CO/O2 for surface adsorption and, most importantly, of the competition between the adsorbed H(s) and CO(s) for surface-deficient O(s). Finally, transient simulations in practical catalytic channels revealed the interplay between kinetic and thermal effects. While at φ=0.13 the H2/CO reactive mixture exothermicity was insufficient to overtake the kinetic inhibition, at φ=0.26 catalytic ignition could still be achieved at temperatures well-below the transition temperature. The effect of H2:CO molar ratio on the light-off times was quite strong, suggesting care when designing syngas catalytic rectors with varying compositions.