Exploring Reversible Redox Behavior in the 6H-BaFeO3−δ (0 < δ < 0.4) System: Impact of Fe3+/Fe4+ Ratio on CO Oxidation

This work is devoted to evaluating the relationship between the oxygen content and catalytic activity in the CO oxidation process of the 6H-type BaFeO3−δ system. Strong evidence is provided about the improvement of catalytic performance with increasing Fe average oxidation state, thus suggesting the involvement of lattice oxygen in the catalytic process. The compositional and structural changes taking place in both the anionic and cationic sublattices of the catalysts during redox cycles have been determined by temperature-resolved neutron diffraction. The obtained results evidence a structural transition from hexagonal (P63/mmc) to orthorhombic (Cmcm) symmetry. This transition is linked to octahedra distortion when the Fe3+ concentration exceeds 40% (δ values higher than 0.2). The topotactical character of the redox process is maintained in the δ range 0 < δ < 0.4. This suggests that the cationic framework is only subjected to slight structural modifications during the oxygen exchange process occurring during the catalytic cycle.


X-Ray photoelectron spectroscopy
Figure S1 shows the core level spectra of Ba 3d, O 1s and Fe 2p of the three Ba-Fe samples.The Ba 3d spectra reveal clear splitting of the signals in BaFeO2.90 and BaFeO2.96samples, suggesting carbonation of every sample's surface.The formation of carbonates due to the capture of atmospheric CO2 is common in barium and strontium-containing materials due to their high basicity [2,3].The O 1s spectra confirm the presence of barium carbonate by a major peak present at 531.4 eV [4,5] and surface water physisorption is also evidenced by the presence of a small peak (533.3 eV) [6]. of the signal at ≈+1 eV O 2-O1s signal assigns it to oxygen vacancies [9], new calculations have determined that such assignment is not correct [10,11].As these authors also explain, this band may be assigned to surface hydroxyl groups generated by absorption of water in the previous oxygen vacancies sites and that may or may not be quantitively related to oxygen vacancies.We have decided to label our O 1s signal at ≈ 530 eV as O* 1s.It is also noticeable that in our samples' spectra, the intensity of this peak (both with and without deconvolution) can be qualitatively related to the oxygen content detected in the bulk.
. Final Rietveld refinement of the NPD data for the revisited 6H-BaFeO2.78[1] in Cmcm space group.The observed patterns (red circles), calculated patterns (continuous black line) and difference curves (continuous blue line) are shown..87 and c=14.18Å) using as input the highly distorted 6H framework previously described for SrIrO3 [12] and introducing anionic vacancies to get the threefold order in a and b axes experimentally observed (marked by red arrows).In this model, the octahedral arrangement is distorted in a crooked way comparing with the pristine 6H structure.

Morphological and textural properties
Figure S10 corresponds to SEM images of the BaFeO2.78,BaFeO2.90 and BaFeO2.96samples.The first two micrographs show particles of similar size, ranging from 100 to 200 nm.However, the most oxidized BaFeO2.96sample clearly shows a higher degree of sintering thus, the particle size is larger and not in the same range as the other samples.
Table S6 summarizes the main parameters derived from the measured isotherms.Low values of BET surface and pore volume are related to the synthesis method employed to prepare the solids.The presence of micropores can be excluded in all the samples studied.In all cases, the isotherms can be described as type II according to the IUPAC indications.Considering the results in Fig. S11, these pores can be described as mesopores.

Catalytic activity
Figure S12 shows the CO conversions for BaFeO2.78,BaFeO2.90, and BaFeO2.96perovskites.BaFeO2.96perovskite was obtained by high-temperature oxidation at the expense of a low specific surface area.Therefore, despite having a higher Fe 4+ content, the CO conversion was similar to that of the BaFeO2.78perovskite.

S12 5 .
Figure S6.Electron diffraction patterns along [010] of the H-BaFeO2.96sample recorded at (a) 40 °C and (b) 550 °C.Corresponding TEM images are shown in Figures (c) and (d).At 550 °C the 6H packing sequence along the c axis (d001 = 14.1 Å) is lost irreversibly.Notice the drift in both images because of the vibration due to the water flow of the heating stage.

Figure S8 .
Figure S8.TEM image along [010] of the H-BaFeO2.96sample recorded at 25 °C after cooling.Distances of 14.1 and 4.9 Å can be measured along 001H and 100H respectively.Corresponding FFT is shown as inset.The enlarged image of the thinnest region of the crystals (marked in orange) allows to identify thecchcch-sequence of the 6H polytype.

Figure S9
Figure S9 Simulated (c) [001] and (d) [010] SAED diagrams are obtained considering a tentative structural model (a, b) (C2 space group; a=16.81,b=28.87 and c=14.18Å) using as input the highly distorted 6H framework previously described for SrIrO3[12] and introducing anionic vacancies to get the threefold order in a and b axes experimentally observed (marked by red arrows).In this model, the octahedral arrangement is distorted in a crooked way comparing with the pristine 6H structure.

Table of contents: 1. Chemical analysis and XRD of prepared samples 2. X-Ray photoelectron spectroscopy 3. EELS spectra 4. Neutron diffraction 5. SAED and Electron Microscopy 6. Morphological and textural properties 7. Catalytic activity 8. References S3 1. Chemical analysis and XRD of prepared samplesTable S1 .
Chemical composition and cell parameters of prepared samples.

Table S3 .
Crystallographic parameters refined from the NDP data for samples O-
a Calculated by BET method.b Determined by BJH method using the desorption branch.c