*2.4. La0.3Sr0.55Ti1-x-yFexNiyO3*±<sup>δ</sup> *(x = 0, 0.025, 0.05; y = 0, 0.05)*

Catalytic activity and sulfur uptake data indicated that Fe might be a suitable candidate to improve sulfur tolerance of LSTN while maintaining the self-regeneration property. In order to explore the potential segregation of both Fe and Ni from the perovskite-type host, both Ni and Fe were introduced at the perovskite B-site of LST to obtain compositions of the type La0.3Sr0.55Ti1-x-yFexNiyO3±<sup>δ</sup> (x=0, 0.025, 0.05; y = 0, 0.05) the denotations of which are summarized in Table 1. Figure 5 displays a summary of XRD patterns collected on calcined materials, as well as after 15 h reduction at 800 ◦C (10 vol.% H2/Ar).

All calcined samples exhibited only reflections, which could be attributed to the perovskite host. After reduction, weak reflections belonging to metallic phases were observed. Reduced La0.3Sr0.55Ti0.95Ni0.05O3±<sup>δ</sup> (LSTN-5Ni) showed a reflection centered at 44.40◦, which corresponds to the Ni (111) reflection. Reduced La0.3Sr0.55Ti0.925Fe0.025Ni0.05O3±<sup>δ</sup> (LSTFN-2Fe5Ni) and La0.3Sr0.55 Ti0.9Fe0.05Ni0.05O3±<sup>δ</sup> (LSTFN-2Fe5Ni) exhibited a weak reflection at 43.77◦, which is higher than one would expect for the (111) reflection of metallic Fe (43.6◦ 2θ), but certainly at lower angles than the Ni (111) reflection. This can be regarded as evidence for Ni/Fe alloy particle formation upon reduction of LSTFN-type materials. La0.3Sr0.55Ti0.925Fe0.025Ni0.05O3±<sup>δ</sup> (LSTF-5Fe) did not show significant reflections of metallic Fe after reduction. However, a new reflection appeared at 38.27◦, which could not be assigned to any metallic phase. While metallic Ti is expected to display a reflection at around 38.4◦, it is highly unlikely that it formed under these pretreatment conditions, due to the inherent stability of Ti4+. The missing reflection at 35.2◦ excludes this possibility conclusively. However, the observed reflection could be explained by the formation of a new perovskite-type phase of lower symmetry. Orthorhombic perovskites (such as A-site stoichiometric LaSrFeO3±δ) possess a reflection at ca. 38◦ corresponding to the (113) lattice planes. All other reflections might be either hidden below the dominant original perovskite phase or too weak to be observed.

**Table 1.** Sample denotations and metal content of La0.3Sr0.55Ti1-x-yFexNiyO3±<sup>δ</sup> -type samples.

**Figure 5.** Powder XRD patterns of calcined and reduced LSTN-5Ni, LSTFN-2Fe5Ni, LSTFN-5Fe5Ni and LSTF-5Fe (10 vol.% H2/Ar, 800 ◦C, 15 h). Diffractograms are magnified with respect to intensity (2×) to emphasize Ni, Fe and Ni/Fe reflections.

The materials were subjected to TPR and reoxidation cycles to verify the structural reversibility and to determine the temperature at which segregated metals are reversibly incorporated into the host perovskite lattice. In these experiments, TPR profiles were followed by an isothermal reduction at 800 ◦C. The materials were then subsequently reoxidised at the indicated temperature (700 ◦C, 750 ◦C, 800 ◦C, 850 ◦C, 900 ◦C and 950 ◦C) before the next TPR profile was collected. The TPR profile is sensitive to the nature and coordination environment of reducible metal species and such experiments can, therefore, be exploited to determine the reoxidation temperature needed to reestablish the state of the reducible metal species in the initial calcined material [2,23]. Figure 6a displays the TPR redox cycles obtained on LSTN-5Ni. The reduction feature of NiO (ca. 370 ◦C) disappears after reoxidation at Treox ≥ 800 ◦C, thus indicating successful and complete Ni reincorporation at this temperature [2]. The initial TPR of calcined LSTFN-2Fe5Ni (Figure 6b) was not as well defined as the one recorded for LSTN-5Ni. Instead of the distinct double feature, the sample exhibited a broad reduction peak between 400 ◦C and 650 ◦C. After reduction and subsequent reoxidation at low temperatures (700 ◦C and 750 ◦C) the sample exhibited a low temperature feature peaking at around 475 ◦C. This feature then

transitioned into the previously observed double feature for reoxidation above 850 ◦C, which could be interpreted as the temperature at which both Fe and Ni are reincorporated into the perovskite host.

**Figure 6.** TPR reduction-reoxidation cycles for (**a**) LSTN, (**b**) LSTFN-2Fe5Ni, (**c**) LSTFN-5Fe5Ni and (**d**) LSTF-5Fe. Hydrogen consumption values were normalized by molar quantity of sample. Reoxidized samples were subjected to pre-reduction (10 vol.% H2/Ar, 800 ◦C, 1 h) before reoxidation (20 vol.% O2/N2, 2 h) at the temperature indicated for each row.

The sample with higher Fe content (LSTFN-5Fe5Ni; Figure 6c) exhibited the reduction feature, which was attributed previously to a two-step reduction process of Ni [24]. However, peak reduction temperatures were shifted to lower temperatures by 22 ◦C compared to the ones recorded for LSTN-5Ni in Figure 6a. Reoxidation at lower temperatures caused the formation of a new reduction feature between 350 ◦C and 550 ◦C also on this sample, which can be attributed to the reoxidation of Ni/Fe oxides at the perovskite surface. Reestablishment of the initial reduction profile was achieved after reoxidation at 850 ◦C. Interestingly, LSTF-5Fe (Figure 6d) did not exhibit as an extensive reduction as the other samples; the reduction features were broad and attenuated with increasing reoxidation temperature. In this case, TPR-redox cycling seemed unsuitable to accurately trace the reoxidation temperature necessary for Fe reincorporation and demonstrates the limited reducibility of Fe in LSTF-5Fe in absence of Ni.

Even though TPR provides important insight in the reducibility of the materials, only an element specific method, such as X-ray absorption spectroscopy (XAS) can be used to ultimately differentiate between individual contributions of two or more reducible species. Therefore, XAS was applied to investigate the effect of the simultaneous presence of both Fe and Ni in LSTFN-type samples on the reduction and reoxidation of the individual metals. Figure 7 displays the Ni K-edge (8.333 keV) X-ray absorption near edge structure (XANES) spectra of the Ni-containing samples LSTN-5Ni (Figure 7a), LSTFN-2Fe5Ni (Figure 7b) and LSTFN-5Fe5Ni (Figure 7c) in their calcined state, as well as after reduction (10 vol.% H2/Ar, 800 ◦C, 15 h) and reoxidation (20 vol.% O2/N2, 800 ◦C, 2 h).

**Figure 7.** Normalized Ni K-edge (8.333 keV) XANES spectra of (**a**) calcined and reduced (10 vol.% H2/Ar, 800 ◦C, 15 h) LSTN-5Ni, (**b**) LSTFN-2Fe5Ni and (**c**) LSTFN-5Fe5Ni. Plots of the first derivative of the normalized spectra are shown in the bottom panels. The spectra of Ni foil and NiO reference materials are included for comparison (dashed lines).

The Ni K-edge spectra of the calcined materials displayed an intense whiteline with a high energy shoulder that can be taken as characteristic of Ni adopting the coordination environment of Ti [2,25,26]. The edge energy was also higher than in the case of Ni2+ in NiO. After reduction, the spectra corresponded to a linear combination of the spectra of Ni adopting the coordination environment of the B-site after calcination and Ni0. Although TPR analysis showed reversibility only after reoxidation at 850 ◦C (Figure 6), the shape of the XANES spectra and thus the state of Ni were completely reversible in this redox cycle, which was carried out at lower temperatures. It was also observed that the contribution of the Ni<sup>0</sup> reference to the spectra of the reduced samples increased with increasing Fe concentration. This was confirmed by linear combination fit (LCF) of the spectra indicating that the amount of Ni<sup>0</sup> increased from 52% in LSTN-5Ni to 62% in LSTFN-2Fe5Ni and to 67% in LSTFN-5Fe5Ni. LCF results are summarized in Figure S1 and the corresponding fit results are shown in Figure S2.

Evidence that Ni was not only reduced, but also segregated and formed metallic particles is provided by the extended X-ray absorption fine structure of the Ni K-edge. The k3-weighted data is shown in Figure S3 for all samples, as well as Ni references. The radial distances of coordination shells become obvious through Fourier transformation of this data, as shown in Figure 8. After reduction, the feature attributed to a Ni-Ni coordination shell appeared at 2.15 Å. This feature was present for all samples so that metal particle formation can be assumed likewise.

Fe K-edge (7.112 keV) XANES data was obtained on the Fe-containing samples LSTFN-2Fe5Ni, LSTFN-5Fe5Ni and LSTF-5Fe and spectra of calcined, reduced and reoxidized samples are displayed in Figure 9 along with spectra of Fe0, FeO, Fe3O4 and Fe2O3 reference compounds. Significant changes in the shape of the XANES could be observed also for Fe K-edge absorption spectra over the redox cycle. Clear Fe reduction could be observed by a decrease in whiteline intensity, as well as a shift in absorption edge energy (E0). This shift is best determined through the position of the first maximum in the derivative of the absorption curves, which changes from 7.128 keV for calcined materials to 7.125 keV for reduced materials, corresponding to a decreased ionization energy, which is typically observed for reduced states. However, in the spectra of the reduced samples, the whiteline did not correspond to a simple linear combination of the reference spectrum of Fe0 and the spectrum of calcined LSTFN- or LSTF-type materials, thus suggesting other states of Fe.

**Figure 8.** Fourier transformed k3-weighted Ni K-edge EXAFS data obtained for (**a**) Ni<sup>0</sup> (Ni foil) and NiO reference materials, as well as calcined, reduced (10 vol.% H2/Ar, 800 ◦C, 1 h) and reoxidized (20 vol.% O2/N2, 800 ◦C, 2 h) (**b**) LSTN-5Ni, (**c**) LSTFN-2Fe5Ni and (**d**) LSTFN-5Fe5Ni. Features are labelled on the reduced materials according to the underlying Ni-Ni scattering paths.

**Figure 9.** Normalized Fe K-edge (7.112 keV) XANES spectra of (**a**) calcined, reduced (800 ◦C, 10 vol.% H2/Ar, 15 h) and reoxidized (800 ◦C, 20 vol.% O2/N2, 2 h) LSTFN-2Fe5Ni, (**b**) LSTFN-5Fe5Ni, (**c**) LSTF-5Fe. (**d**) XANES spectra of LST-5Fe, Fe0, Fe2O3, Fe3O4 and FeO reference materials. The first derivative of the normalized spectra is shown in the bottom panels.

No suitable fits could be obtained through LCF analysis using all displayed Fe reference spectra. This may be linked to the presence of other Fe-containing phases as was already suggested from the XRD patterns in Figure 1. Furthermore, the XANES of LSTFN-2Fe5Ni and LSTFN-5Fe5Ni was not completely restored in the reoxidized materials as can be seen in the region of the local minimum at around 7.135 keV. LCF of the spectra of the reoxidized materials indicated the presence of Fe3O4 (ca. 17%) and thus incomplete reincorporation of Fe under the applied reoxidation conditions.

The k3-weighted Fe K-edge (7.112 keV) EXAFS data of LSTFN-2Fe5Ni, LSTFN-5Fe5Ni and LSTF-5Fe is shown in Figure S4, whereas the Fourier transformed data is shown in Figure 10. Compared to the Ni0 reference in Figure 8, Fe0 in the Fe foil displayed the first coordination shell at a slightly longer radial distance (2.23 Å) and similar to the Ni K-edge data contributions of this feature, could be found in the spectra of the reduced samples. This indicates that besides Ni Fe was also partially reduced to Fe0 and was present in the form of metal particles. Interestingly, the contribution of this feature to the spectra of the reduced samples decreased with decreasing Ni/Fe ratios along the series LSTFN-2Fe5Ni > LSTFN-5Fe5Ni > LSTF-5Fe, which suggests that larger Ni content favors Fe reduction. Hence, the positive influence of one metal on the reducibility and segregation of the other metal could be observed.

**Figure 10.** Fourier transformed k3-weighted Fe K-edge EXAFS data obtained for (**a**) Fe<sup>0</sup> (Fe foil) reference, as well as (**b**) LSTFN-2Fe5Ni, (**c**) LSTFN-5Fe5Ni and (**d**) LSTF-5Fe after calcination, reduced (10 vol.% H2/Ar, 800 ◦C, 1 h) and reoxidized (20 vol.% O2/N2, 800 ◦C, 2 h). Features are labelled on the reduced materials according to the underlying Fe-Fe scattering paths.

Figure 11a shows CO conversion curves for the Ni-containing samples LSTN-5Ni, LSTFN-2Fe5Ni and LSTFN-5Fe5Ni. It is apparent that the conversion curves shifted to higher temperatures with increasing Fe concentration, which corresponds to a decrease in catalytic activity. This is in contrast to the previous observation that the presence of Fe increases Ni reducibility (Figure S1). The expected effect would be a larger amount of active Ni0 and thus higher catalytic activity. On the other hand, the observation is in line with the decrease in WGS activity observed for LSTN-5Fe compared to LSTN in Figure 3b and the indication of Ni/Fe alloy formation during reduction provided by this catalytic activity data, as well as XRD (Figure 5). Since the addition of small quantities of Fe did not appear to be detrimental for catalytic activity, LSTFN-2Fe5Ni was selected for testing catalytic activity with respect to its redox stability, as well as sulfur tolerance. However, it can be seen in Figure 11b that CO conversion decreased over the number of redox cycles, which could be a consequence of Ni/Fe particle growth, due to the incomplete Fe reincorporation over the redox cycles observed by XANES (Figure 9). Despite the fact that Ni re-incorporated completely during reoxidation of reduced

LSTFN-2Fe5Ni (Figure 7), the remaining Fe oxide at the surface may have caused particle growth and thus the observed catalyst deactivation over the consecutive redox cycles.

**Figure 11.** WGS activity of (**a**) reduced LSTN-5Ni, LSTFN-2Fe5Ni, LSTFN-5Fe5Ni (20 vol.% H2/Ar, 800 ◦C, 15 h) and (**b**) LSTFN-2Fe5Ni after eight redox cycles, as well as after poisoning with 50 ppm H2S under reaction conditions (poisoned) and after subjecting the material to one further redox cycle (regenerated). Feed gas composition: 15 vol.% CO/15 vol.% H2O/7.5 vol.% H2/Ar; 30,000 mLh<sup>−</sup>1g−<sup>1</sup> at STP. The calculated theoretical conversion equilibrium is indicated by the dashed red curve.

The small amounts of Fe (2.5 mol.% or 0.75 wt.%) also did not provide additional stability against poisoning by sulfur, which can be realized from the CO conversion measured after catalyst poisoning by H2S (Figure 11b). CO conversion was as low as in poisoned LSTN. A complete redox cycle recovered catalytic activity, which was even improved compared to the catalytic activity measured before poisoning. The reason behind this phenomenon was not further investigated as any obvious improvement in performance in terms of the catalytic activity of LSTFN-2Fe5Ni in neither sulfur-free nor sulfur-containing reaction gas feeds could be observed. Nevertheless, it indicates that sulfur can also be successfully removed from LSTFN-type oxides over oxidation-reduction cycles at 800 ◦C, which could be potentially exploited to completely regenerate these materials if catalyst redox stability can be achieved. The conditions to achieve full reversibility of activity after poisoning should be the aim of future work.
