## 3 Results
### Molybdate Anion  
The MoO\({}_{4}\)-LDH is characterized by a single peak for the Mo 3d5/2 at a BE of 229.6 eV (Figure 7 left). This BE value is lower than, e.g., molybden, MoS\({}_{2}\), with a BE of 230.1 eV or wulfenite, PbMoO\({}_{4}\), with a BE of 231.9 eV [3]. Several molybdates, such as Al\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) (232.5 eV), CaMoO\({}_{4}\) (232.8 eV), NiMoO\({}_{4}\) (233.0), etc. all have higher BE than observedfor intercalated molybate ions in LDH. This is similar to what has been observed before, an indication that the interactions between the molybate anion and the LDH hydroxide layers are not as strong as when the molybate is part of a rigid crystal structure. The corresponding O 1s peak is observed at 530.3 eV (Figure 7 right), which compares well with the value observed for wulfenite at 530.0 eV. The NIST database reports for H\({}_{2}\)MoO\({}_{4}\) a BE of 530.7 eV, CaMoO\({}_{4}\) at 530,6 eV, and NiMoO\({}_{4}\) at 530.9 eV, while for Al\({}_{2}\)(MoO\({}_{4}\)), the BE ranges from 530.7 to 531.0 eV [51]. [PERSON] et al. [73] assigned the Mo 3d5/2 peak at 232.6 eV to Mo(VI) in MoO\({}_{4}\)\({}^{2-}\). The hardly visible shoulder at 231.7 eV was attributed to Mo(V) species, possibly caused by a charge transfer between (Mo\({}^{6+}\) + O\({}^{2-}\)) and (Mo\({}^{5+}\) + O\({}^{-}\)) [74; 75]. This shoulder was not observed in this study. [PERSON], [PERSON] and [PERSON] [73] also reported the BE value of O 1s at 532.4 eV for the fresh molybate-LDH assigned to the O\({}^{2-}\)[35; 38; 45]. They concluded that it is probably composed of two overlapping photoelectron peaks at 532.4 and 531.5 eV. The peak at 532.4 eV was assigned to the metal hydroxides in the LDH layers, while the other peak at 531.6 eV was attributed to oxygen O\({}^{-}\) in the oxomolybdenum. Both the Mo 3d5/2 and O 1s peaks for the MO\({}_{4}\)\({}^{2-}\) anion are at slightly higher BE values than observed in this study and in comparison with other molybate structures such as those listed in [PERSON] and Wood [45] and the NIST database [51]. Previously, infrared and Raman spectroscopy on the same sample studied here have shown that molybate anions are not polymerized in the interlayer space of the LDH because of the synthesis under alkaline conditions. The splitting of the vibrational modes of the molybate anion in the vibrational spectra suggests a symmetry lowering due to interactions of the molybate anion with the LDH hydroxide surfaces and interlayer water [16]. [PERSON], [PERSON] and [PERSON] [21] reported a sharp Raman band at around 900 cm\({}^{-1}\), which was assigned to the MoO\({}_{4}\)\({}^{2-}\) symmetric stretching mode. A single broad IR band at approximately 830 cm\({}^{-1}\) was reported as characteristic of the MoO\({}_{4}\)\({}^{2-}\) anion, by [PERSON] et al. [76]. [PERSON] and [PERSON] [77] confirmed the finding of [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [16] that at pH above 7-8, the prevalent molybate species was the MoO\({}_{4}\)\({}^{2-}\) ion characterized by an IR band at ca. 830 cm\({}^{-1}\)  
Figure 7: XPS high-resolution scan of Mo 3d and O 1s of LDH with molybate as the interlayer anion. while LDH prepared at pH 4.5 and with high molybate loadings had an IR band at ca. 920-930 cm\({}^{-1}\) and bands or shoulders near 890 cm\({}^{-1}\) suggesting the presence of polymolybdate. [PERSON] et al. [78] reported a single band in the IR spectrum at 806 cm\({}^{-1}\) assigned to the antisymmetric stretching mode of Mo-O. [PERSON] et al. [79] observed a similar IR band attributed to the antisymmetric mode of Mo-O-Mo in MoO\({}_{4}\)\({}^{2-}\) at 834 cm\({}^{-1}\). Their Raman spectrum showed the Mo-O symmetrical stretching mode in MoO\({}_{4}\)\({}^{2-}\) at 910 cm\({}^{-1}\), while two bands at 320 and 220 cm\({}^{-1}\) were attributed to the Mo = O bending and Mo-O-Mo deformation mode, respectively. A tetrahedral ion like MoO\({}_{4}\)\({}^{2-}\) has four vibrational modes if it has full Td symmetry.