Mechanism of Oxidation of Carbon Monoxide in Presence of Chromium Carbonyl

Abstract

The catalytic effect of small concentrations of chromium vapor, added in the form of Cr(CO)6, on the oxidation of CO is investigated at temperatures between 1100 and 2200°K in shock waves using two different methods of product detection. The formation of CO2 in incident shock waves of CO/O2/Ar mixtures containing between 100 and 500 ppm Cr(CO)6, at a total gas density of about 5.0·1017 particles/cc, was followed by ir emission over a time scale of 1500−μsec particle time. The rate of CO2 formation was much larger than in the absence of Cr(CO)6, and the CO2 concentration was observed to rise linearly with time. A complete intermediate and product analysis was performed by shock heating various CO/O2/Ne mixtures containing 0.4% Cr(CO)6 and observing the reaction species in reflected shock waves by means of a time-of-flight mass spectrometer during 180 μsec. The thermal decomposition of Cr(CO)6 is very rapid at all experimental temperatures. The oxides of chromium, CrO, CrO2, and CrO3, were observed together with Cr. A chain mechanism involving these oxides is proposed to account for the catalysis of CO oxidation by Cr(CO)6, and rate constants for the chain reactions are estimated. A rather sudden cessation of the catalytic effect was observed under varying conditions of (Cr) and temperature. Mass spectrometric analysis indicated the disappearance of atomic Cr and its oxides from the gas phase under these conditions. A very rapid condensation of Cr atoms when chromium vapor becomes highly supersaturated is proposed to account for this phenomenon.