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).