Surface basic properties of metal oxides can generally be ascribed to the presence of basic hydroxyl groups (OH) and basic surface oxygens (O
2–), which can react directly with CO
2 producing, respectively, hydrogen carbonate and carbonate species. FTIR technique provides a powerful means of investigation of the surface properties of the metal oxides [
59,
62–
64], by studying the interaction with probe molecules. FTIR spectra in the region 2000–1190 cm
–1, collected at room temperature on reduced NiO, CeO
2, and selected
xNiCe samples (0.3NiCe and 2.5NiCe) in contact with CO
2 (CO
2 pressure equal to 5×10
3 Pa), are shown in Fig. 7 (solid lines). The figure also reports spectra obtained after outgassing for 30 min at the same temperature (dashed lines). The FTIR spectra of 2.5NiCe, 0.3NiCe and CeO
2 display broad signals characterized by the presence of distinct bands with different intensities, indicating, in agreement with the microcalorimetric measurements, the presence of a heterogeneous surface, in which sites of different nature and strength exist [
63]. After CO
2 adsorption, the 2.5NiCe sample shows a spectrum with at least five partially superimposed contributions. According to the literature [
64], the bands centered at about 1614, 1405 and 1214 cm
–1 can be mainly ascribed to the presence of hydrogen carbonates (HC). However, a contribution of weaker hidden signals due to bidentate and monodentate carbonates (MC, typically observed at about 1580 and 1360 cm
–1, respectively) is suggested by the appearance of two shoulders at about 1510 (MC) and 1315 cm
–1 (bidentate carbonates (BC)), due to the interaction of CO
2 with basic surface O
2– species. As expected, a decrease in the intensity of the bands is observed because of the outgassing procedure, though to a lesser extent in the case of the MC, for which a higher stability can be supposed. Similar features appear in the spectra of the 0.3NiCe sample, which exhibit bands very similar to those observed for 2.5NiCe, in terms of both frequencies and relative intensities. Concerning the pure CeO
2 oxide, the most intense peak observed after CO
2 adsorption is centered around 1594 cm
–1 and this can be reasonably explained by the superimposition of the signals of HC and BC. The presence of HC is also clearly pointed out by the observation of bands at about 1409 and 1216 cm
–1, the former characterized by an evident tail towards higher frequencies, possibly associated with polydentate carbonates (PC) [
64]. In addition, a well-defined peak ascribable to BC can be observed around 1300 cm
–1, while the little hump at about 1510 cm
–1 accounts for the presence of MC. The contribution at about 1510 cm
–1 becomes slightly more visible after outgassing, confirming the higher stability of the monodentate species with respect to the other carbonate types. In particular, it has been reported in the literature that the formation of monodentate or BC strictly depends on the basicity of the surface oxygen atoms, with stronger sites associated with the formation of monodentate species [
63]. A closer inspection of the CeO
2 spectra in comparison with those of 0.3NiCe and 2.5NiCe reveals a different distribution between monodentate and BC. The contributions of the monodentate species become more evident in the presence of Ni, in particular in the spectra recorded after outgassing, suggesting that the presence of Ni gives rise to a higher concentration of stronger sites. It is worthy of note that this observation is in agreement with the microcalorimetric results, from which an increase in the amount of medium- and high-strength basic sites with the increase in nickel content was observed up to a Ni/Ce ratio equal to 2.5. Compared to the other oxides, NiO spectra point out a lower amount of carbonate species (bands with low intensity) and the absence of MC, suggesting a poor basic character, as already revealed by microcalorimetry.