**3. Materials and Methods**

Mixed metal oxides with nominal composition La0.3Sr0.55TiO3±<sup>δ</sup> (LST) and La0.3Sr0.55Ti0.95 Ni0.05O3±<sup>δ</sup> (LSTN) were synthesized according to the synthesis procedure described earlier and including a final calcination step at 960 ◦C (6 h) [27]. Aliquots of these two parent materials were then loaded with Fe, Cr, Mn and Mo precursors by wet impregnation with aqueous precursor solutions followed by drying at 120 ◦C for around 12 h and subsequent calcination at 500 ◦C for 2 h. Metal loading was chosen such that the final loading resembled the metal concentration in hypothetical La0.3Sr0.55Ti0.95Me0.05O3±<sup>δ</sup> and La0.3Sr0.55Ti0.9Ni0.05Me0.05O3±<sup>δ</sup> (Me = Fe, Cr, Mn and Mo). Table S1 contains all information regarding type and quality of metal precursors used, as well as denotations of the corresponding samples, which are used throughout the text. Furthermore, La0.3Sr0.55Ti0.925Ni0.05Fe0.025O3±<sup>δ</sup> (LSTFN-2Fe5Ni), La0.3Sr0.55Ti0.9Ni0.05Fe0.05O3±<sup>δ</sup> (LSTFN-5Fe5Ni), La0.3Sr0.55Ti0.95Fe0.05O3±<sup>δ</sup> (LSTF-5Fe), and La0.3Sr0.55Ti0.95Ni0.05O3±<sup>δ</sup> (LSTN-5Ni) powders were also synthesized according to the same procedure, but were calcined at 860 ◦C for 6 h. After calcination, the powders are referred to as "calcined".

The catalytic activity towards the water gas shift (WGS) reaction was measured on powders in a quartz reactor of plug flow geometry (6 mm ID). Mass flow controllers (Brooks) were used to dose the reactant gases and a K-type thermocouple, which was placed in the middle of the catalyst bed, was used to monitor catalyst bed temperature. To avoid back pressure, all calcined powders were pelletized (4 MPa), crushed and sieved to 100−150 μm before use. The sample (100 mg) was diluted with cordierite powder (200 mg, 75−100 μm) to achieve a thoroughly mixed catalyst bed of ca. 15 mm in length. Catalytic tests were conducted on pre-reduced samples (20 vol.% H2/Ar, 800 ◦C, 1 h) after an initial pretreatment of a single redox cycle. This treatment was found to activate LSTN [2] and has therefore been adopted in this work. Catalytic activities were measured at a weight hourly space velocity (WHGS) of 15,000 mL·g−1·h−<sup>1</sup> at STP (200 mg catalyst, 50 mL·min−1). A Pfeiffer OmniStar GSD 320 quadrupole mass spectrometer equipped with a heated stainless steel capillary was used for compositional analysis of the exhaust gas. CO conversion (XCO) was calculated using Equation (1),

$$\chi\_{\rm CO} = \frac{[\rm CO]^{\rm in} - [\rm CO]^{\rm out}}{[\rm CO]^{\rm in}} \times 100\% \tag{1}$$

where [CO]in is the initial concentration of CO and [CO]out is the concentrations of CO at the reactor outlet.

Sulfur loading of the catalyst samples was conducted under reaction conditions using the reaction gas mixture (15 vol.% H2O/15 vol.% CO/7.5 vol.% H2/Ar), including 50 ppm H2S. Sulfur loading was conducted at 800 ◦C for 60 min during which H2S concentration in the reactor exhaust was monitored using the mass spectrometer signal at M/Z = 34.

CO conversions of LSTN-5Ni, LSTFN-2Fe5Ni, LSTFN-5Fe5Ni were determined on powder samples after an initial activating redox cycle at WGHS= 30,000 mL·g−1·h−<sup>1</sup> (at STP).

The crystal structure of the powder catalysts was investigated by powder X-ray diffraction (XRD, Bruker D8 Advance) equipped with Ni-filtered Cu Kα-radiation, variable slits and an energy sensitive line detector (LynxEye). Diffractograms were collected at an acquisition time of 4 s and a step size of Δ2θ = 0.03◦ between 15◦ and 80◦. Aliquots of the samples listed in Table S1 were reduced (10 vol.% H2/Ar, 800 ◦C, 1 h) prior to XRD analysis. After reduction, samples were cooled down in Ar (20 ◦C·min−1). XRD was also recorded on reduced samples at an increased resolution in the angular range 40◦–50◦ (step size 0.005◦). The XRD was also recorded for the Ni and Fe containing perovskite-type oxides LSTN-5Ni, LSTFN-2Fe5Ni, LSTFN-5Fe5Ni and LSTF-5Fe after prolonged reduction (10 vol.% H2/Ar, 800 ◦C, 15 h).

Temperature programmed reduction (TPR) experiments were conducted using a bench top TPDRO-1100 (ThermoElectron) instrument equipped with mass flow controllers and a thermal conductivity detector. The samples (100 mg) were loaded into the quartz reactor tube and heated to 500 ◦C under a constant flow of 20 vol.% O2 before cooling to room temperature. TPRs were recorded in 10 vol.% H2/Ar (20 mL·min−<sup>1</sup> at STP) and at a heating rate of 5 ◦C·min<sup>−</sup>1. The reoxidation temperature at which Ni is reversibly reincorporated into the perovskite lattice was estimated by TPR redox experiments. A TPR profile was recorded on the calcined sample up to 800 ◦C followed by an isothermal reduction for 1 h at the same temperature. The sample was then cooled in Ar to room temperature (25 ◦C) before reoxidation at 700 ◦C in 20 vol.% O2/N2 for 2 h. The sample was again cooled in Ar to 25 ◦C before starting the second TPR on the now reoxidised material. Such TPR-reduction-reoxidation-TPR cycles were repeated five times with increasing reoxidation temperature (700 ◦C, 750 ◦C, 800 ◦C, 850 ◦C and 900 ◦C). The heating rate during reoxidation and cooling after all experiments was 10 ◦C·min<sup>−</sup>1.

Ni K-edge (8.333 keV) and Fe K-edge (7.112 keV) X-ray absorption spectra were acquired ex situ on pelletized samples in fluorescence mode at the X10DA (SuperXAS) beamline of the Swiss Synchrotron Light Source (SLS, Villigen, Switzerland) using a 5 element SD detector. The required X-ray energies were scanned using a Si(111) monochromator. The Demeter software package (version 0.9.24) [28] was used to reduce and model all data. The radial distribution function (R) was obtained by Fourier transforming k3-weighted k-functions typically in the range of 3.0−12.0 Å−<sup>1</sup> using a Hanning window function. NiO (99.99% trace metals basis, Sigma, Buchs, Switzerland), FeO (99.7% trace metal basis, Sigma), Fe2O3 (puriss. ≥97%, Sigma, Buchs, Switzerland), Fe3O4 (99.99% trace metal basis, Sigma, Buchs, Switzerland), Fe foil and Ni foil references were measured in transmission mode using ionization chamber detectors. Spectra were recorded on calcined powder samples, after pre-reduction (10 vol.% H2/Ar, 800 ◦C, 15 h), as well as after reoxidation (20 vol.% O2/Ar, 800 ◦C, 2 h).

Linear combination fitting (LCF) of Ni K-edge X-ray absorption near edge structure (XANES) spectra were performed in the spectral range −20 eV < E0 < 30 eV around the absorption edge to quantify the fraction of each Ni species present in the samples. Reference compounds for each fit included Ni foil, NiO and calcined La0.3Sr0.55Ti0.95Ni0.05O3±<sup>δ</sup> representing Nin+oct (n > 2) in the perovskite coordination.
