Figure 2. Fe-2p XPS spectra of (a) as-prepared and(b) spent K-ZnFe2O4 and Na-ZnFe2O4; (c) the XANES spectra of Fe K-edge in spent ZnFe2O4;(d) SEM images of K-ZnFe2O4@K-ZSM-5; (e) the mapping of all the elements over the above section; (f) Fe elemental distribution; (g) Zn elemental distribution;(h) Al elemental distribution; (i) Si elemental distribution.
Compared with as-preapred ZnFe2O4@ZSM-5 catalyst, phase composition of spent capsule catalyst still remains the MFI structure of ZSM-5, which illustrates that the core-shell structure of the capsule catalyst can maintains excellent physical stability (Figure 1). After reaction, the Zn and Fe in ZnFe2O4 are transformed into ZnO into Fe3O4 and Fe5C2, respecively. Thereinto, Zn acts as a structure and electronic promoter can enhance the basicity and thus increases light olefins selectivity. In general, Fe3O4, as the active phase for RWGS reaction, promotes the CO2 molecules into CO intermediates, while the co-existence of Fe5C2 as crucial active phase makes the chain propagation to form hydrocarbons. As a consequence, different configuration compositions of Fe3O4 and Fe5C2 can regulate CO2conversion performance, leading to clear distinction in activity and selectivity. Besides, the SEM images of Na-ZnFe2O4 and K-ZnFe2O4 were shown in Figure S1. As shown in Figure 2b, spent ZnFe2O4catalysts were performed by a X-ray photoelectron spectroscopy (XPS). The binding energy peaks at 706.4 eV, 710.6 eV, and 712.3 eV are attributed to Fe5C2 species, Fe(II), and Fe(III), respecitively. Compared with Fe-2p XPS spectra of as-prepared, highly valenced iron oxide species are converted to carbides and Fe3O4, crucial active phases for CO2 conversion (Figure 2a). It is worth noting that the introduction of Na causes the Fe-C peak shift to a direction with low binding energy. According to the relative content of Fe-2p in different XPS, the spent Na-ZnFe2O4 have more iron-carbon bonds content than spent K-ZnFe2O4 (Table S1). HR-TEM images of spent K-ZnFe2O4 and Na-ZnFe2O4 catalyst were shown in Figure S2. The 4.64Å and 2.65 Å are belong to Fe3O4 (111) and Fe5C2 (311), which coincides with the results of XRD and XPS. Obviously, the co-existence of Fe3O4 and Fe5C2 together push the reaction forward. The morphologies and structures of the core Fe-based catalyst before and after the reaction were also explored. After reaction, the structure of bulk ZnFe2O4 present uniform dispersion of small particles, assigning to the dynamic transformation of ZnFe2O4 spinel structure (Figure S1). Besides, after K ions exchange, the morphologies and structures of zeolite change slightly (Figure S3). For a zeolite treated by Ce ions, there are a few crystals on the surface of the zeolite (Figure S4). Moreover, TEM images of zeolites with different ions exchange strategies were compared in Figure S5. It can be clearly found that the exchanged metal ions are evenly distributed in the zeolite. Moreover, the content of the exchanged ions in the zeolite is relatively low (Table S2).
Fe K-edge XANES was used to investigate the nature and coordination properties of Fe species in the spent ZnFe2O4 catalyst under the relevant operating conditions (Figure 2c and S6). The normalized XANES spectra of the Fe K-edge in ZnFe2O4 are given in Figure 2c; and the data for Fe foil, ZnFe2O4, Fe2O3, Fe3O4 and Fe5C2 are also presented. In the XANES spectra, the K-ZnFe2O4 shifts to a higher energy than Na-ZnFe2O4, illustrating that K promoter is conducive to the transition of Fe phase to a high valence state of Fe species. The introduction of additives can enhance the electronic transition between active phases and raw molecules, and then achieve the regulation of product selectivity during catalytic reactions. In the wavelet detail of spent ZnFe2O4 ,the Fe has the same coordination in Figure S6c and S6d. The difference is that the introduction of K promoter lengthens the number of wave vectors and strengthens the degree of transition of the catalyst to Fe-C during the reaction.
Besides, H2-TPR patterns of as-prepared Na-ZnFe2O4 and K-ZnFe2O4 were compared in Figure S7a. Compared with Na modification, the introduction of K promoter is silghtly conducive to the reduction behavior of iron species. Meanwhile, the CO2-TPD profiles were shown in Figure S7b. It can be found that all the catalysts of K-ZnFe2O4 and Na-ZnFe2O4 have obvious weak adsorption and moderate adsorption. K-ZnFe2O4exhibits a better absorption of CO2 than Na-ZnFe2O4, which because of K has a stronger adsorption capacity for CO2 than Na or more ZnFe2O4 phases, which will adsorb more CO2 than single Fe2O3phase. It demonstrates that K-ZnFe2O4have a high active capability for CO2 conversion. K-ZnFe2O4 also exhibits a better absorption of CO than Na-ZnFe2O4 (Figure S7c).