## 3 Results
### Chromate Anion  
The Cr 2p high-resolution spectrum of the chromate intercalated LDH differs from the other anionic groups shown earlier in this paper in the sense that not one but two Cr 2p3/2 peaks are present (Figure 8 left). The first is a rather broad peak with a BE of 579.0 eV, while the second is much sharper and has a BE of 580.2 eV. The second peak is characteristic of Cr\({}^{6+}\) in the CrO\({}_{4}\)\({}^{2-}\) anion, while the first peak at lower BE is associated with the presence of Cr\({}^{3+}\), indicative of substantial reduction of the CrO\({}_{4}\) with time. Approximately 60% of all Cr has been reduced based on the peak surface area ratio, which is directly proportional to the amounts present. The mineral croceic PbCr\({}^{73}\)O\({}_{4}\) showed a Cr 2p3/2 at 579.3 eV [45], while a compound such as BaCr\({}^{73}\)O\({}_{4}\) has a BE of 579.1 eV. In contrast, Cr\({}^{73}\)(OH)\({}_{3}\) has a lower BE of 577.1-577.4 eV [51]. [PERSON] et al. [82] observed that upon adsorption of chromate using LDH, complete reduction of the Cr\({}^{73}\) to Cr\({}^{73}\) took place with Cr 2p3/2 peak at a BE of 277.9 eV. A similar shift was also observed for the reduction of sodium chromate during ion etching in the XPS instrument by [PERSON] and [PERSON] [83]. The Cr\({}^{3+}\) is probably initially present in the form of Cr(OH)\({}_{6}\)\({}^{3-}\) but this anion shows a strong affinity for carbonate [84], so the exchange of OH\({}^{-}\) for CO\({}_{3}\)\({}^{-}\) in the anion over time is well possible within the LDH interlayer space given that LDH itself also has a strong affinity for carbonate adsorption. The O 1s peak for the chromate is found around 530.2 eV, which is the same as that of crocoite, PbCrO\({}_{4}\)[45], and comparable to compounds such as Na\({}_{2}\)CrO\({}_{4}\) at 530.0-530.3 eV, CaCrO\({}_{4}\) at 529.5 eV and Li\({}_{2}\)CrO\({}_{4}\) at 530.3 eV [51]. The O 1s peak for Cr\({}^{\text{III}}\) compounds occurs at slightly high BE and can not be distinguished from the O 1s OH peak of the LDH layer structure (Figure 8 right). The NIST database reports BE values of 530.8 to 531.6 for Cr(OH)\({}_{3}\)[51]. Vibrational spectroscopy (Raman and FTIR) results on the fresh sample showed that the initial chromate anions were not polymerized to Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\) in the LDH interlayer space [16]. The Raman spectrum of the chromate anion in solution reveals distinct vibrational modes: the symmetric stretching mode (\(\
u_{1}\)) is observed at 848 cm\({}^{-1}\), the bending mode (\(\
u_{2}\)) at 348 cm\({}^{-1}\), the asymmetric stretching mode (\(\
u_{3}\)) at 884 cm\({}^{-1}\), and the out-of-plane bending mode (\(\
u_{4}\)) at 363 cm\({}^{-1}\)[80]. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [18] reported the Raman spectrum of the same freshly prepared sample as later used in this study for the XPS analyses. They observed a strong band at 848 cm\({}^{-1}\) attributed to the \(\
u_{1}\) symmetric stretching vibrational mode, while two bands at 884 and 928 cm\({}^{-1}\) were assigned to the \(\
u_{3}\) antisymmetric stretching vibrational mode. The band at 474 cm\({}^{-1}\) was assigned to the \(\
u_{4}\) bending mode. Finally, two bands at 363 and 237 cm\({}^{-1}\) were interpreted as being the \(\
u_{2}\) bending modes. The splitting of the \(\
u_{3}\) and \(\
u_{2}\) modes is suggestive of an initial lowering of the symmetry of the chromate anions in the LDH interlayer space prior to partial reduction. The symmetry lowering must be taken into account through the interaction of the CrO\({}_{4}\)\({}^{2-}\) anions with both interlayer water and the LDH hydroxide layer surfaces on both sides of the interlayer space. They did not give an explanation for a broad band at 821 cm\({}^{-1}\). Although this is lower than reported for fresh Cr(OH)\({}_{3}\) around  
Figure 8: XPS high-resolution scan of Cr 2p and O 1s of LDH with chromate as the interlayer anion.