Surface-chemistry of Oxidized and Reduced Chromia - A Fourier-transform Infrared-spectroscopy Study

CO adsorption of reduced chromia leads to formation of Cr3+-CO carbonyls with an absorption band at 2167 cm(-1). Surface carbonates and formates appear with time and, simultaneously, the 2167-cm(-1) band is shifted to 2186 cm(-1) (after passing through an intermediate position at ca. 2179 cm(-1)). This shift is interpreted as due to the inductive effect of the carbonate structures on the nu(CO) position. With a decrease of the equilibrium pressure, the maximum of the 2186-cm(-1) band is gradually shifted to 2196 cm(-1) due to the decrease of the adsorbate-adsorbate interaction. When chromia is deeply reduced, a band at 2101 cm(-1), interpreted as due to Cr2+-CO carbonyls, also appears after CO adsorption. Adsorption of ammonia on oxidized chromia shows the presence of Lewis and Bronsted acidity on the sample surface. On the contrary, CO does not form any Cr-n+-CO carbonyls due to its weak basicity. However, reactive adsorption of CO (180 degrees C and above) leads to formation of carbonates, bicarbonates, and formates. Simultaneously, Cr3+-CO carbonyls (nu(CO) at 2196 cm(-1)) are detected as a result of the surface reduction. CO2 adsorption on a reduced sample gives rise to carbonates, formates, and bicarbonates that are almost absent after adsorption on an oxidized sample. However, the same kinds of species are observed after reactive CO adsorption on oxidized chromia. In addition, two kinds of Lewis acid sites are observed, where CO2 is linearly adsorbed. However, on oxidized chromia, a very low concentration of one of these sites is detected.