FTIR study of low-temperature CO adsorption on reduced ceria nanoparticles with different morphology: A comparison with oxidized samples

Abstract

• CO is bound to Ce 3+ and Ce 4+ sites on ceria without back π-donation. • Bands below 2140 cm −1 are not due to carbonyls, but to Ce 3+ electronic transition. • Stabilities and ν(CO) of the carbonyls of Ce 3+ and Ce 4+ do not differ significantly. • Stability and ν(CO) of adsorbed CO decrease after reduction of {100} and {110} facets. • Stability and ν(CO) of adsorbed CO increase after reduction of {111} facets. The unique catalytic properties of ceria and ceria-based materials are primarily determined by the easy switching between Ce 3+ and Ce 4+ oxidation states on the catalyst surface. This redox process is also important for other applications of ceria-based materials, such as their biomedical use. Therefore, in situ determination of the oxidation state of cerium cations located on different ceria planes is important. One of the most commonly used and informative techniques in this regard is IR spectroscopy of adsorbed CO. However, there is still strong disagreement in the interpretation of the carbonyl bands on ceria. In this work, we report the results of CO adsorption on reduced ceria nanoshapes (cubes, polyhedra and rods having different proportions of exposed {100}, {110} and {111} facets) and compare them with the results obtained with non-reduced samples. The reduction degree was monitored by the 2 F 5/2 → 2 F 7/2 spin–orbit electronic transition of Ce 3+ at 2133–2095 cm −1 . In addition to the experiments, DFT simulations were performed. The ν(CO) frequency of adsorbed CO was found to undergo various shifts after reduction of cerium cations on different planes. Thus, for an intermediate coverage, ν(CO) on stoichiometric ceria {100}, {110} and {111} facets is ca. 2162, 2171 and 2152 cm −1 , respectively, while for reduced surfaces, at 2151, 2146 and 2160 cm −1 , respectively. The reasons for the different shift values and directions are discussed. Adsorption of CO on reduced samples also causes a red shift of some components of the Ce 3+ 4f electronic band which could be misinterpreted to formation of carbonyl species. In conclusion, CO as an IR probe molecule can provide information about the existence of Ce 4+ and Ce 3+ sites on the surface. However, the regions of the carbonyl bands of Ce 4+ and Ce 3+ overlap in the range 2162–2150 cm −1 . For a correct interpretation of the bands in this region, further analysis should be performed.

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