2. Results and Discussion
The textural properties and chemical composition of the studied samples are presented in
Table 1 and
Table 2, respectively. The textural properties (see
Table 1) show minimal changes after the first and second steps of ion exchange. The chemical composition demonstrates slight dealumination after the first steps of ion exchange; moreover, there is an excess of Na
+ in the initial commercial mordenite, as was also observed before [
20]. The ion exchange abilities of zeolites are based on the negative charge induced by the presence of aluminum in the neutral silicate framework, which is compensated by some exchanged cation. These exchangeable cations are located on the surface of channels and pores, which makes it possible to efficiently use a large surface in the exchange, adsorption or catalytic process and avoid diffusion hindrances. The equilibrium ion exchange modulus (EIEM) indicates how complete the ion exchange is. It can be calculated as the sum of the charges of all cations present divided by the aluminum content and should be equal to 1. In this study, the utilized cations were Ag
+, Fe
2+, Fe
3+ and H
+ (from the acidified iron precursor solution), along with residual Na
+ of the initial mordenite (if any):
The chemical composition of the samples, as measured by ICP-OES, is presented in
Table 2. By comparing the Si/Al ratio of the monometallic and bimetallic samples with the initial value for NaMOR, all samples, except AgMOR, exhibit a weak dealumination, which is attributed to the acidic pH of the iron solution used in the exchange. EIEM calculations based on elemental analysis for these materials are complicated by four facts at once. First, as it was found [
12], Fe
2+ and Ag
+ have red-ox interaction, leading to precipitation of some amount of Ag
+ in Ag
0, which does not participate in the ion exchange, but it is present in the quantitative data on the component contents. Second, the method does not allow estimating separately Fe
2+ and Fe
3+, which invariably introduces an error in the calculations by equation (1). Third, as was shown [
19,
20], some part of Fe
3+ is incorporated into the mordenite framework instead of aluminum rather than taking part in the process of ion exchange, which leads to dealumination. This also means the participation of Fe
3+ in ion exchange in a new capacity and adds unknowns to Equation (1). Finally, the amount of H
+ cannot be estimated by ICP-OES, but as can be judged from thermodynamic parameters considered here [
20], H
+ participates in competitive ion exchange in the presence of Ag
+ and Na
+ and must be considered.
Thus, the EIEM was calculated from ICP-OES results for all samples with several assumptions (see
Table 2). Different values were observed between NaMOR, AgMOR and three Fe-containing samples. Since it is impossible to estimate the number of iron cations in the framework and evaluate Fe
2+ to Fe
3+ ratio [
20] from ICP-OES data, the limit values of EIEM-Fe
2+ and EIEM-Fe
3+ were calculated, considering that all the iron is entirely as cations of one of the possible oxidation states (Fe
2+ or Fe
3+). These values represent the interval within which the real value of the modulus lies:
As is seen from
Table 2, EIEM for most samples is close to one, with the exception of NaMOR and FeMOR. The high EIEM value for NaMOR was previously associated with the fact that NaOH, which is present during the synthesis of zeolite in the industrial process, was not completely removed by washing the resulting material [
20]. For FeMOR, the EIEM has a value of less than 1. This can be explained by the participation of H
+ in the process of ion exchange.
In
Figure 1, the experimental XRD patterns of the studied samples are presented. The angular positions (2θ) of the peaks remain unchanged after the replacement of compensating Na
+ ions by exchanged Ag
+ and/or Fe
2+ ions. However, relative intensities of peaks of the NaMOR zeolite are affected depending on the nature of the introduced exchangeable cation. This is evident when comparing FeMOR and AgMOR patterns with NaMOR X-ray diffraction patterns. It appears that Fe
2+ affects the intensity of the peak associated with the (110) plane, while Ag
+ cations affect the amplitude of several peaks in the range 8 < 2θ < 30. The effect of Ag
+ cation exchange is so great that the amplitudes of some peaks, such as those associated with planes (020), (200) and (202), are very strongly suppressed. Of course, Fe
2+ and Ag
+ cations are of different natures, but these results show that they are hosted in significantly different positions accessible for the exchangeable zeolite cations.
In the case of bimetallic samples, except for some differences, the diffraction patterns of FeAgMOR and AgFeMOR show a noticeable similarity with the pattern for the monometallic AgMOR sample. These differences show that, regardless of the order in which a second exchangeable cation is introduced, in all cases, the Ag
+ cation affects the intensity of the peak associated with the (110) plane, while the Fe
2+ cation causes a change in the intensity of the peak associated with the (202) plane. Such an increase in the intensity of only some planes suggests the coexistence of Ag
+ and Fe
2+ cations in a certain region of the zeolitic matrix and the presence of preferred positions for each of the cations. In addition, the more pronounced effects of Ag
+ cations on the diffraction pattern suggest they can access a larger number of exchange sites than Fe
2+ cations. This is consistent with the results presented in
Table 1, which show that Ag is always found in higher amounts than Fe, regardless of the sample type. The phase peak of metallic Ag (Ag 00-003-0921) was observed in the XRD pattern of only one sample: AgFeMOR. The crystallite size, defined as the size of the region of coherent X-ray scattering, was equal to 6 nm. The formation of Ag
0 was caused by the redox interaction between Ag
+ and Fe
2+, as was explained earlier for bimetallic Ag-Fe samples on mordenite prepared from iron (II) perchlorate [
12]. For FeAgMOR, redox interaction between Ag
+ and Fe
2+ also took place, but XRD did not register the product of this reaction, the Ag
0-containing phase. This may indicate that the order of deposition of Ag
+ and Fe
2+ ions contributed to the formation of this phase. A similar effect of the order of deposition of Ag
+ and Fe
2+ was observed for AgFeMOR, prepared at varying synthesis parameters [
20].
Micrographs and particle size distribution (PSD) of silver NPs are presented in
Figure 2. The average particle diameter for all samples is approximately 4 nm (from 3.97 to 4.36 nm), and only the PSD widths represent differences between them. In all cases, PSD has a single intense peak; however, only the PSD of AgMOR can be called unimodal. For bimetallic samples, a second maximum was observed at 9 nm; for AgFeMOR, another maximum was also seen at 13 nm. In addition, the monometallic AgMOR (see
Figure 2a) has the narrowest PSD: from 2 to 8 nm, whereas the bimetallic samples have a wider range PSD: 2–21 nm for AgFeMOR (
Figure 2b) and 2–15 nm for FeAgMOR (
Figure 2c).
Nevertheless, for all three samples, at least 30% of the Ag NPs are greater than 5 nm, which means they are large and numerous enough to be detected by XRD. According to XRD results, the presence of a crystalline phase of metallic silver was confirmed only for AgFeMOR (see
Figure 1). When considering that the presence of Fe increases the wide distribution and the fact that XRD analysis confirmed the presence of a metallic silver phase only in AgFeMOR, it is logical to conclude that the large Ag NPs in AgFeMOR sample have a metallic structure due to reduction in silver ions by iron (II). Thus, it can be assumed that Ag NPs in FeAgMOR could also be metallic due to the presence of Fe
2+, but the particles are simply too small to be registered by XRD. In the case of AgMOR, the nature of silver species that we observe with HRTEM should be additionally studied.
In the micrographs of
Figure 2, for all three Ag-containing samples, in the areas highlighted by the dotted line, except for Ag NPs agglomerated on the zeolite surface, all three samples contain ordered domains of Ag species of equally small size. The diameter of these species is approximately 0.7 nm, which coincides with the size of the cross-section of the main mordenite channel. These clusters were not included in the particle size distribution counts because they are monodispersed and because such a size is beyond the limits of reliable diameter measurement due to image quality limitations. The parameters of such highly ordered structural domains were analyzed previously using a specially developed image processing program that uses the Fast Fourier Transform to detect periodic textures in HRTEM images [
21,
22]. However, these 0.7 nm Ag nanospecies are widely represented: their amount in each sample is several times greater than the number of Ag NPs in the corresponding histogram. Since the distances between Ag nanospecies in each domain are similar to the distances between the main mordenite channels, and the size is quite close to the diameter of the main mordenite channels, we can conclude that we observed intra-channel silver clusters. Detection of silver clusters formed inside the zeolite cavities due to a reduction in hydrogen flow at elevated temperatures of Ag
+ inside mordenite channels has been described earlier [
23,
24,
25,
26,
27]. However, to our knowledge, before the present work, these clusters had never been registered by HRTEM. In addition, we did not apply reductive treatments. This species may represent another type of active center [
13,
14]. The study of their physicochemical and catalytic properties requires additional research, with adequate methods to study the clusters.
DRS UV-Vis was applied to determine the presence of various silver species in studied samples, and spectra are presented in
Figure 3. The broad absorption band at 375 nm for both bimetallic samples can be easily interpreted as the plasmon resonance peak of metallic silver subcolloidal NPs with sizes in the range of ca. 1–3 nm [
28,
29]. In a work conducted by Mulvaney [
30], a peak around 380 nm was reasonably attributed to Ag NPs with a diameter of 3 nm in an aqueous medium. It should be considered that, in our case, the NPs are located not in an aqueous environment but on the surface of mordenite. In our experimental spectra, attention is drawn to an almost complete absence of plasmon of Ag NPs in the 410–450 nm range, which usually appears in all systems where reduction and aggregation of silver ions take place [
29]. It should be noted that in [
28], the appearance of a 380 nm peak was observed under the mildest conditions of reduction in a hydrogen flow (temperature in the range of 20–50 °C), and a further increase in temperature resulted in the formation of a peak at 420 nm. This is because of their size, which exceeds the available diameters of regular mordenite channels; both subcolloidal (absorption peak at 370–380 nm) and larger silver particles (peak at λ > 380 nm) can only be found on the outer surface of microcrystals or in existing mesopores [
31].
The position of this peak changes when the size of Ag NPs is adjusted [
32]. It can also be seen that FeAgMOR has a much lower content of Ag NPs with a size of 1–3 nm when compared to AgFeMOR (see peak “metallic silver” intensities), so we suppose the number of metallic NPs with a size ≥5 nm was not enough to appear on XRD patterns. Thus, according to DRS UV-Vis data, both AgFeMOR and FeAgMOR bimetallic samples exhibited a redox interaction between Ag
+ and Fe
2+ with the formation of metallic Ag NPs. However, the order of cation deposition definitely affects the final content and ratio of silver species.
For all Ag-containing samples, a weak and broad structure with low absorption at 300–650 nm corresponding to the silver-black—non-crystalline silver colloids are observed [
29,
33,
34]. AgMOR demonstrated the most intense absorption in this range. It is assumed that the silver-black particles are Ag NPs, which we see in the AgMOR micrographs (see
Figure 2a) and which could not be identified by XRD. Unfortunately, the intense absorption of mordenite in the 200–250 nm wavelength range and the absorption peak with a maximum at 272 nm corresponding to the charge transfer of O → Fe(III) in tetrahedral coordination [
35,
36,
37] makes this range unavailable for identifying any of Ag ions or clusters.
Thus, according to XRD, HRTEM and DRS UV-Vis results, the studied samples demonstrated a significant difference in the formation of relatively large silver species on MOR.
For all Ag-containing samples, colloidal non-structured Ag NPs up to 15 nm in size were detected; AgMOR exhibited the largest amount of them. Additionally, colloidal Ag NPs for both bimetallic samples because of redox interaction between Ag
+ and Fe
2+ metallic Ag NPs were also observed; however, for FeAgMOR, it was in insignificant amounts. As it is known, the formation of large Ag NPs on the outer mordenite surface for these samples is a result of the Ag
+-Fe
2+ redox interaction and ion exchange selectivity, and cation deposition order has a decisive influence on what form Ag
0-base species will take: clusters, colloids or metallic particles [
20]. The present paper is dedicated to developing this mechanism and understanding the formation of small silver species, such as clusters and ions, on the inner surface voids of mordenite. In order to characterize such species, XPS, Raman spectroscopy and XAFS methods were applied. The XPS results are presented in
Figure 4 and in
Table 3.
The electronic state of silver with the largest contribution is characterized by BE = 369 eV and is related to silver clusters less than 2 nm in diameter [
38], which could be those intrachannel silver clusters with a size of ~0.7 nm organized into domains and registered by HRTEM (see
Figure 2). Spectra also show a small Ag 3d
3/2 peak, with a BE of about ~366.6–367.7 eV. According to the literature data, the peak at BE = 366.2 eV corresponds to silver ions in solution, and the peak at 367–368 eV is interpreted as the electron state of silver interacting with Si and O in non-zeolitic Si-containing supports [
39,
40,
41]. We may only suggest the presence of Ag
+ supported on mordenite, according to the XPS data. Thus, we can conclude that most of the silver is in the form of clusters Ag
n < 2 nm, which, the most probable, are those species observed on micrographs as the ones organized in domains (see
Figure 2).
XAFS methods were applied to study the interaction of cations with mordenite at the atomic level. XANES spectra (
Figure 5a) show practically no differences between Ag-containing samples. The shape of all XANES spectra differs from bulk reference samples with simple structures, such as Ag foil, Ag
2O, AgCl, etc. (not shown here). However, similar XANES results were obtained for ionic silver in zeolite A [
42]. Therefore, one may assume Ag in the studied samples to be mainly ionic. The molar fraction of crystalline Ag observed by XRD in the case of AgFeMOR is exceedingly small and does not significantly affect the shape of the XANES spectrum. Since XANES spectra are almost coincident for all samples, we assume that almost all Ag is located in the MOR framework, forming very similar local structures for AgMOR, FeAgMOR and even AgFeMOR.
The Ag K-edge EXAFS curves (see
Figure 5b) include multiple sharp peaks between 1 and 2.4 Å, which are similar for all samples, and a wider peak at 2.6 Å. This last peak is clearly observed only in the case of AgFeMOR and can be attributed to the first coordination sphere in metallic silver. The observation of an Ag-Ag peak for this sample is in good agreement with XRD results confirming the formation of a crystalline phase of metallic silver in the AgFeMOR sample. For all samples at shorter distances, a series of maxima with slight variations in intensity are observed. Based on the XANES data, it can be assumed that these maxima correspond to the local environment of silver ions in the mordenite structure. Therefore, these peaks can be attributed to the Ag-O, Ag-Si or Ag-Al interatomic distances. Nevertheless, since the exact numbers and positions of the maxima vary sharply with the k-range, we suggest that the so-called «EXAFS interference phenomenon» occurs.
The phenomenon of EXAFS interference is sometimes observed when two or more interatomic distances in a local structure are close but not equal. Usually, in k-space, each scattering path is represented by a sine wave with variable amplitude. The lighter the atom, the smaller the k value at which the amplitude has a maximum, after which the wave attenuates. However, the addition of two sinusoidal contributions of light atoms (e.g., O) with very close (but not equal) frequencies in k-space leads to the appearance of a sinusoidal wave with a similar frequency, the amplitude of which first decreases but then increases again at large k values, especially in the case of large k-weights. Such a shape of the EXAFS spectrum may be misinterpreted as the presence of heavy atoms at very short interatomic distances. A similar problem was described for a bimetallic Pt-Sn sample on SiO
2 in [
43]. When the EXAFS functions of pure Pt and PtSn alloy were combined, the resulting Fourier transform (FT) exhibited two main peaks at 2.18 and 2.78 Å, which is in good correspondence with the experimental FT. By using references to theoretical works, the authors demonstrated this peak at a short distance to arise from interference phenomenon in the EXAFS spectra, resulting from the coexistence of pure Pt clusters and PtSn alloy. The presence of this peak is not only a fingerprint for the Pt-Sn interaction but also evidence for the existence of undoped Pt.
In terms of EXAFS FT, the interference phenomenon leads to a rise in additional peaks. The positions and intensities of those peaks may not correspond to the real electron density distribution over the coordination spheres. Since the intensities, exact positions and even the number of peaks may gradually change depending on the k-range in which the FT is taken, we propose to call these peaks (peaks at short distances such as 1.27, 1.66, 2.03 Å) an “interference crest”. In order to fit the crest, we took into consideration a high-k part of the EXAFS spectrum by extending the FT range to 15 Å and using kw = 3. In order to model the local surroundings of Ag, we introduced four scattering paths of Ag-Si, assuming the Ag ion to be attached to the mordenite framework, and the Ag-Si (or Ag-Al) distances were close but not equal, giving rise to the interference crest. However, the two main maxima of the interference crest were remarkably similar to those observed for Ag coordinated with O (see Ag
2O reference in
Figure 5b). Therefore, we also introduced four Ag-O scattering paths, imposing silver to interact with O atoms. The Debye factors were equalized for all Ag-O and Ag-Si paths to reduce the number of parameters. One Ag-Ag path was also included to account for metal particles in AgFeMOR.
As seen from
Table 4, observing a reduction in the interatomic distances and coordination numbers of the studied samples if compared with the references, it may be concluded that the silver, in all three samples, is mostly in the form of monoatomic species attached to the mordenite framework. Moreover, based on the fit results, it can be concluded that the coordination of Ag in AgMOR is different from that for bimetallic samples. For this sample, 4 O and 4 Si atoms were not enough to describe the low-distance part of the interference crest, which is more intense for AgMOR if compared with the spectra of bimetallic samples. By varying the coordination numbers, we assumed the presence of an additional O atom next to Ag in the AgMOR. However, the addition of the Ag-Ag path does not improve the model and results in either higher R factors or zero coordination numbers of Ag-Ag. This may mean that silver exists in AgMOR as a single-atom oxidized silver species in the form of Ag
+.
For bimetallic samples, the formation of Ag clusters might not be ruled out. In accordance with data in
Table 4, the presence of Ag-Ag distance in the FeAgMOR sample reduces the R factor, which is speaking of the formation of some amount of metallic silver species, while most of the silver in FeAgMOR is in the form of dimers. In the case of AgFeMOR, the Ag-Ag peak is intense due to the presence of metal particles (see
Figure 5b), while the presence of silver dimers is not registered. However, since the shape of the interference crest is similar to that for FeAgMOR and the “4 O + 4 Si/Al” model is sufficient to fit the spectrum, we assume that the coordination of Ag is included in the MOR for both FeAgMOR and AgFeMOR equally. Thus, in accordance with the EXAFS results, the position of Ag in the bimetallic samples does not depend on the cation deposition order, even though this order affects the final product: formation of silver dimers in FeAgMOR and formation of silver metal phase in AgFeMOR. Deposition of Ag leads to similar changes in the relative intensities of Bragg peaks, regardless of whether silver was deposited before or after iron, which is supported by XRD data (see
Figure 1). It is also worth noting that for AgFeMOR and FeAgMOR, the two shortest Ag-Si/Al distances coincide with the third decimal place, i.e., 2 of 4 Si/Al atoms around Ag are chemically equivalent. This observation may help to establish the exact structure of Ag metal centers.
As the study of the surface by XPS methods showed, most of the small silver species on mordenite are represented by partially charged small silver clusters Agnδ+. Following HRTEM data, those clusters are organized into domains, in which the arrangement of clusters coincides with the symmetry of location and size of the main mordenite channels. The presence of some amount of Ag+-MOR on the surface was reported by the XPS method, while by the XANES method, it was shown that most of the silver in the bulk of the samples was present as Ag+-MOR. By the EXAFS method, it was found that the Ag+ ion was coordinated with the mordenite framework by four bonds, which can be Ag-O, Ag-Si or Ag-Al; the interatomic distances in all of them are close but not equal. The coordination of Ag in the zeolite bulk is similar for the two bimetallic samples and does not depend on the order of metal deposition, while the coordination of silver in the monometallic one differs from them; that is, iron significantly affects the state of silver. The results of EXAFS fit show silver in AgMOR to be mostly present in monoatomic form; in FeAgMOR, it is in the form of silver dimers, and in AgFeMOR, it is in the form of metal clusters, confirming the influence of the exchange order on the local structure of the samples.
Spectra of micro-Raman scattering of powder samples, obtained by excitation with the 784.29 nm line, are shown in
Figure 6. The set of these results has the finality of demonstrating evidence that the micro-Raman spectra of the AgFeMOR, an isomorphous substitution of Fe
3+ ions in the framework of mordenite, is observed. In order to validate this hypothesis, we measured the Raman spectra for each sample in at least three different sites with different conditions of incident excitation power. In
Figure 6, for NaMOR, we presented the Raman spectra after subtracting the fluorescence background. For each one of these Raman spectra, we have characteristic Raman bands, coinciding with those previously published and measured by conventional Raman equipment, with a source line of 514.5 nm, for the same commercial NaMOR from Zeolyst International with SiO
2/Al
2O
3 = 13 [
45]. Raman bands of NaMOR at 315 and 397 cm
−1 correspond to 8- and 5-membered rings (8-MR and 5-MR), respectively; a band at 509 cm
−1 and a pair at 447 and 452 cm
−1 were assigned to 4-MR. The band around 639 cm
−1 corresponded to asymmetric stretching motions of T-O bonds [
46,
47]. Raman spectra of NaMOR reported by [
48] show similar changes in the location and intensity of bands due to ion exchange: 397 associated with 5-MR vibrations, 447, 470 and 510 cm
−1, to 4-MR vibrations. Visual identification of the different rings mentioned above are given below.
As it is known, changes within the range of 250–600 cm
−1 are attributed to large deformation of the rings in the mordenite structure, and it may take place due to ion exchange [
48]. It can be seen from
Figure 6a that the bands of FeMOR are practically the same as for NaMOR, with a relative increase in the band intensity at 637 cm
−1 associated with asymmetric stretching motions of T-O bonds. In addition, on the FeMOR and FeAgMOR, the position of the band at 509 cm
−1, corresponding to 4-MR vibrations, shifted to 515 cm
−1. The micro Raman spectrum of FeAgMOR does not demonstrate any drastic changes in its spectrum after the ion exchange of FeMOR with Ag
+.
On the contrary, with FeMOR-based samples, micro-Raman spectra of AgMOR-based ones (AgMOR and AgFeMOR) demonstrate obvious changes in the range of 250–650 cm
−1 if compared with NaMOR. From
Figure 6b, bands corresponding to 4-MR vibration were affected: the band at 509 cm
−1 disappeared, and instead of a pair at 447 and 469 cm
−1, a single band at 456 cm
−1 appeared. A band at 637 cm
−1 has shifted to 629 cm
−1, indicating changes in asymmetric stretch T-O motion. Thus, NaMOR after an exchange with Ag
+ led to the deformation of rings in the framework, while spectra showed minor changes after an exchange with Fe
2+. Results of micro-Raman spectroscopy agree with the XRD (see description in
Figure 1). Moreover, it must be concluded that the cation of the first step of ion exchange defines the framework changes (in our case, Fe
2+ saves the structure while Ag
+ leads to large deformation in the mordenite structure), which confirms the cation deposition order to be a crucial factor to properties of the resulting materials.
Expected framework deformation in AgFeMOR is connected to Fe
3+ incorporation into mordenite framework instead of Al
3+, proved by Mössbauer spectroscopy [
19] and by EXAFS [
20] for samples studied in the present work. The new band at 570 cm
−1 appearing on the AgFeMOR is ascribed to ions of Fe
3+ in tetrahedral positions in the mordenite framework, also confirming it. Similar Raman bands at ca. attributed to a symmetric stretch of tetrahedrally coordinated Fe-O-Si were observed in the literature: at 516 cm
−1 for Fe-ZSM-5 [
49,
50,
51], at 530 cm
−1 for Fe-ZSM-35 [
52], at 510 cm
−1 for Fe-SBA-15 [
53], at 605 cm
−1 for the Fe/MFI-15 and at 418 and 596 cm
−1 for Fe/MFI-37 [
54]. The band registered at 570 cm
−1 for AgFeMOR is characterized by strong broadening; it could be due to (1) overlap because of a presence of a new family of 5- or 4-MR containing the Fe
3+ ion in the mordenite framework or (2) iron incorporation into the mordenite framework, which induces a new framework disorder [
55].
Our previous work considered the formation mechanism of bimetallic Ag-Fe samples deposited on mordenite [
20]. As was already mentioned, a different order of cation deposition was applied, and two independent processes were considered: ion exchange on the mordenite surface, including the outer surface of crystals and the inner surface of the porous system, and the parallel redox interaction between Ag
+ and Fe
2+ ions. In that paper, the main attention was paid to the state of iron ions. However, the logic of further investigation with a focus on the peculiarities of Ag species formation leads to the need to consider the specific location of exchange cations on the inner surface of mordenite channels. In order to compare theoretical and experimental results and thus shed light on the formation of all occurring types of silver species, in the present study, a theoretical model was considered for Si/Al = 7 ratio [
11]; it is a model with the closest value of the chemical composition of mordenite used in our experimental work.
Figure 7 shows this model of the distribution of different types of atoms in the a-b plane for the NaMOR framework; the unit cell was enlarged 2 × 2 times to appreciate better the distribution of Al and Na atoms and the main channels in the zeolite. Two potential ion exchange centers were considered: the channels in yellow color are associated with the 12-MR and those in green color with the 8-MRs. It also means that cation location in the side pockets leaves channels of zeolite free for reactant transporting, which is beneficial for applying studied materials in catalysis and adsorption.
As it was mentioned, micro-Raman spectroscopy presents evidence of isomorphic substitution of Fe
3+ for Al
3+ cations in the zeolite matrix, which affects the 4-MR and 5-MR vibrations, demonstrated on the insert image of
Figure 8 by cyan and green colors, respectively. Although this case was not considered in the model construction in
Figure 7, recent results (not yet published) show that this type of substitution occurs preferentially of Al
3+ by Fe
3+. That is, Fe
3+ cations substitute for aluminum cations at existing positions rather than Si
4+ cations at random positions in the lattice. Consequently, the ion exchange sites are unaltered, respecting the ion exchange rules just discussed without the presence of Fe
3+ in the lattice, which will be published elsewhere.
Based on this model, the possible exchange mechanism for different cation deposition orders is suggested in
Figure 8. In each process, the arrows and their color indicate the channel where the respective cation is exchanged. Additionally, in each case, the Roman numerals indicate the number of possible exchange sites of a particular ion.
The main feature of zeolite structures is that when a silicon atom is isomorphically substituted with an aluminum atom, such a tetrahedron acquires a negative charge that requires cations outside the crystal structure for neutralization. Since the Ag
+ cation has the same valence as Na
+,
Figure 8a shows that the one-to-one ion exchange can occur in three ways: (a) exchange in the 12-MR channels (position labeled I), (b) exchange of one or two Na
+ cations for Ag
+ (positions labeled II and III). Meanwhile, 12-MR channels have the largest dimensions; Ag
+ cations diffuse predominantly through this channel, and consequently, their exchange occurs preferentially in this channel. According to
Table 1, the ion exchange of Na
+ for Ag
+ in the AgMOR sample is not 100%, indicating that there are still sites (but in a much smaller proportion than in starting NaMOR) with Na
+ cations available for exchange.
Due to valence 2+ of iron cation, its exchange only occurs within the 8-MR channels, as shown in
Figure 8b. This is because the Fe
2+ cation necessarily requires both single-charge sites in the 8-MR channels for its exchange, and the presence of at least one Ag
+ cation in this channel makes its lodging impossible. Therefore, if the zeolite is first exchanged with Ag
+ ions, these lodge in as many 8 MR rings as possible (
Figure 8c), thus decreasing the number of accessible sites for Fe
2+ (
Figure 8d). Indeed, the silver content in the AgFeMOR sample is the lowest for our system (see
Table 1). As already discussed, the Ag
+ ion alone has a marked effect on planes located in the range 8 < 2θ < 30, whereas the iron ion alone directly affects the (110) plane. XRD pattern of the AgFeMOR sample (see
Figure 1) is still dominated by the influence of Ag and, to a lesser extent, Fe. This agrees with the described ion exchange model.
Figure 8c shows the channels through which the exchange of Fe
2+ in NaMOR zeolite is possible. As mentioned above, it corresponds to 8-MR channels only. Without the prior presence of Ag
+ cations, the number of 8-MR channels in which Fe
2+ cations can be accommodated is relatively greater. Starting from this configuration (which is associated with the FeMOR sample),
Figure 8d shows four possible sites where Ag
+ cations can be located, giving rise to the expected configuration in the FeAgMOR sample: (a) in 12-MR (indicated by I), (b) in 8-MR channels where no Fe
2+ is present (indicated by II and III, respectively) and (c) ionic substitution of Fe
2+ cations by Ag
+ cations in the 8-MR channels (indicated by IV). Evidence of the substitution of Fe cations for Ag can be seen in
Table 1, which shows the percentage of Fe in the FeAgMOR sample to be lower than in FeMOR. A comparison of XRD patterns of FeMOR and FeAgMOR samples (see
Figure 1) shows the order of Ag introduction to increase the intensity of the peak associated with the Mordenite (110) plane, which also points to partial removal of Fe from the sample. Additionally, the XRD patterns of AgMOR, FeAgMOR and AgFeMOR samples show that the peaks in the range 8 < θ < 30 present similar behaviors. This indicates that Ag takes up the exchange sites, removing Fe, which can only be found in the 8-MR channels. This type of interaction has already been observed in other studies [
12,
20].