A mechanistic study of the low-temperature conversion of carbon monoxide to carbon dioxide over a cobalt oxide catalyst

A nanocrystalline cobalt oxide (Co3O4)-based catalyst formed by heating a basic cobalt(II) carbonate precursor in air at 250–300 °C has been shown to exhibit much greater catalytic activity than Co3O4 calcined at higher temperatures. In a highly exothermic reaction, the properly calcined catalyst rapidly oxidizes carbon monoxide to carbon dioxide at room temperature. X-ray diffraction, X-ray photoelectron spectrometry, Brunauer–Emmett–Teller surface area measurements, and two FT-IR techniques were used to investigate the mechanism of the CO oxidation reaction. Diffuse reflection (DR) infrared spectrometry of the catalyst was used to monitor the gases adsorbed on the catalyst surface. The observation of a CO band at 2006 cm−1 indicates that CO is adsorbed onto cobalt atoms in a low oxidation state. The highest catalytic activity appears to be achieved when a specific ratio of Co(II) to Co(III) is found on the surface and the particle size is small (i.e., the surface area is large). Poisoning of the catalyst is evidenced in the DR spectra by the geminal adsorption of two CO molecules onto a cobalt atom in a high oxidation state, giving rise to a doublet at ∼2180 cm−1. A strong band at 2343 cm−1 indicates that CO2 is physisorbed onto the catalyst when it is poisoned. A unique ultra-rapid scanning FT-IR spectrometer was used to measure the concentration of the CO2 formed, as well as that of the unreacted CO within 5 ms after the gas was passed through the catalyst. The spectra indicate that CO2 is formed in a vibrationally excited state.