Carbon Monoxide Adsorption on Manganese Oxide/Cobalt: an Ambient Pressure X-ray Photoelectron Spectroscopy Study

MnOx enhances the catalytic activity of Co during Fischer-Tropsch synthesis, increases selectivity toward C5+ products, and decreases methane formation. These desired traits are thought to result from a higher CO adsorption energy and, thus, potentially higher CO coverage. To investigate this, ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to probe the CO coverage of Co foil with increasing MnOx amounts at room temperature. The technique permits the quantification of chemical species on a surface from ultrahigh vacuum to the mbar pressure regime. CO was found to adsorb at both Co and MnOx sites. The electronic effect which results in the promotion of CO adsorption also promotes the adsorption of OH groups from background water vapor pressures. This process competes with CO adsorption, despite the water pressure being similar to 8 orders of magnitude lower than the CO pressure at 1 mbar. Because water is a product of Fischer-Tropsch synthesis, this result has relevance to the understanding of MnOx as a promoter. This finding highlights the importance of considering unexpected contributions of background impurities in APXPS and other ambient pressure surface science techniques.