Based on the above results, it is evident that the active sites of the two samples should have distinct characteristics, thus further research was conducted into the state of the Co species in the sample active sites. UV–vis spectroscopy was used to probe the oxidation states and coordination geometry of the Co species in different forms. As shown in
Fig. 5a, both samples exhibit two absorption peaks at 220 nm and 310 nm, corresponding to the MFI framework structure of the zeolite. The 0.5Co/SN-1–0.05 catalyst shows characteristic peaks around 520 nm, 580 nm, and 660 nm, attributed to the d-d transitions of tetrahedrally coordinated Co(Ⅱ) (
4A
2(F) →
4T
1(P)), indicating the presence of Co
2+ in a tetrahedral coordination within the zeolite framework. In contrast, the 0.5Co/S-1 sample shows a weak peak at 675 nm, suggesting the presence of only a partial amount of Co
2+ on the surface of 0.5Co/S-1. Additionally, broad absorption bands at 425 nm and 750 nm are observed, associated with the O
2- → Co
2+ and O
2- → Co
3+ charge-transfer transitions in Co
3O
4 [
25–
27]. Therefore, the primary form of Co species in 0.5Co/SN-1–0.05 is tetrahedrally coordinated Co
2+, while in 0.5Co/S-1, Co species mainly exist in the form of Co
3O
4. Similarly, Raman spectroscopy results (
Fig. 5b) show comparable findings. Two distinct Raman bands at approximately 370 cm
-1 and 800 cm
-1 correspond to the MFI topology of the zeolite. New Raman bands at 465 cm
-1, 593 cm
-1, and 680 cm
-1 in the 0.5Co/S-1 catalyst are attributed to Co
3O
4 species. The Raman band at 1046 cm
-1 corresponds to the asymmetric stretching vibration of Co
δ-O-Si in the framework, while the Raman band at 1160 cm
-1 in the 0.5Co/SN-1–0.05 catalyst indicates successful incorporation of Co atoms into the zeolite framework [
28,
29]. XPS analysis of Co 2p (
Fig. 5c) reveals two broad and asymmetric main peaks in both samples at 775–790 eV and 790–810 eV, corresponding to Co 2p
3/2 and Co 2p
1/2, respectively [
30]. The 0.5 Co/S-1 catalyst shows two spin-orbit double peaks for Co
2+ and Co
3+ and a broad satellite peak, with 781.5 eV assigned to Co
2+, 780.0 eV to Co
3+, and a Co
2+/Co
3+ ratio of 1.51. The 0.5Co/SN-1–0.05 catalyst also shows characteristic peaks corresponding to Co
2+ and Co
3+, with a Co
2+/Co
3+ ratio of 3.39. This indicates that the Co species in the 0.5 Co/SN-1–0.05 catalyst mainly exist in the form of tetrahedral Co
2+ anchored to the zeolite framework, while the Co species in the 0.5 Co/S-1 catalyst are mostly in the form of Co
3O
4 aggregates [
31,
32]. Furthermore,
in-situ FTIR spectroscopy (
Fig. 5d) was used to analyze the Co species under CO adsorption conditions. The double peaks in the range of 2100 cm
-1 to 2300 cm
-1 represent the characteristic peaks of free CO gas, while the peak at 1900 cm
-1 is associated with the zeolite. The peak at 2030 cm
-1 is attributed to CO adsorption on the linear coordination of Co
2+ species in the zeolite framework [
32]. These results indicate that the Co species in 0.5Co/SN-1–0.05 are predominantly in the form of Co
2+ anchored to the zeolite framework, while the Co species in 0.5Co/S-1 are mainly in the form of aggregated Co
3O
4. The differences in the active sites account for the distinctly different performances observed in the PDH reaction. The reason for this difference in active sites may be due to the fact that [Co(H
2O)
6]
2+ generated from Co(NO
3)
2 dissolution in water has difficulty entering the untreated S-1 zeolite channels. In contrast, alkali treatment generates mesopores that allow [Co(H
2O)
6]
2+ to easily penetrate the zeolite and become anchored by hydroxyl nests[
32]. Consequently, only a minimal amount of Co species enters the S-1 zeolite, with most remaining on the surface, resulting in Co
3O
4 dominance in 0.5Co/S-1. Conversely, the abundant mesopores and hydroxyl nests in SN-1–0.05 facilitate the good dispersion and anchoring of Co species within the zeolite, leading to a predominance of unitary Co
2+ in 0.5Co/SN-1–0.05.