Infrared study of CO adsorption on magnesium oxide

Abstract

The adsorption of CO at room temperature on well outgassed specimens of MgO gives rise to a large number of bands in the 2200–1000 cm^–1 range, which can be divided into two main groups. The bands of the former group are destroyed by oxygen at room temperature: some react instantaneously and are associated with a marked pink colour of the sample; the others are less reactive as they require prolonged contact time in order to be completely oxidized at room temperature (r.t.). The bands of the latter group, far from being destroyed by oxygen, grow when the oxygen-sensitive species are depleted. The oxygen-sensitive species are thought to be negatively charged polymeric CO structures (CO clusters) of the type (CO)^x–_n, where x= 2 or 4 and n is > 2. The simplest Co clusters (dimers) can be transformed into larger polymers by further CO addition. Under the correct conditions the reverse process can also be carried out. The oxygen-insensitive species have a carbonate-like structure and are present on the surface in fairly constant ratios with respect to the former group species. A chemisorption mechanism leading both to oxidized (carbonate-like) and to reduced (negative CO polymers) species is proposed. The active centres for CO chemisorption consist of groups of ions (both positive and negative) in strongly uncoordinated situations.