**4. Conclusions**

Four transition metals (Cr, Fe, Mn and Mo) were screened for potential benefits towards the activity for the water gas shift (WGS) reaction and the stability improvement against sulfur poisoning of La0.3Sr0.55TiO3±<sup>δ</sup> (LST) and La0.3Sr0.55Ti0.95Ni0.05O3±<sup>δ</sup> (LSTN). While Cr, Mn and Mo impregnation on LST did not result in active catalysts, Fe exhibited significant WGS activity. Impregnation with Ni produced the most active catalysts. All other metals decreased the intrinsic activity of LSTN suggesting the presence of Ni/metal interactions. Sulfur stability compared to LSTN was improved only in the case of Fe-impregnated LSTN.

Implementing structural reversibility of Fe and Ni was attempted and both metals enhanced reciprocally their reduction behavior. Catalyst oxidation at 800 ◦C led to complete incorporation of Ni into the host perovskite, whereas Fe incorporation was found to be incomplete under these conditions and resulted in decreased catalyst redox stability at this temperature. Furthermore, WGS activity in the absence of sulfur was reduced compared to LSTN.

Although no beneficial consequences of bimetallic particle segregation were observed, the data demonstrate that Ni catalyst properties towards the WGS reaction at SOFC operation temperatures may be influenced significantly by the presence of other transition metals and that more than one metal can be segregated from LST-type host perovskites. However, the reincorporation behavior may be different for each metal, which has to be taken into account to exploit full structural reversibility of complex systems. Hence, further work is required to optimize regeneration conditions and to exploit the full potential of such materials.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/4/332/s1. Figure S1: Ni speciation from a fit of XANES spectra, Figure S2: Ni K-edge XANES linear combination fit results, Figure S3: k3-weighted χ(k) functions at the Ni K-edge, Figure S4: k3-weighted χ(k) functions at the Fe K-edge, Table S1: sample list.

**Author Contributions:** All authors were involved in the conceptualization of this work. P.S. carried out the experiments, analyzed and interpreted the data and wrote the manuscript. D.B. synthesized and provided the perovskite-type materials. O.K. discussed results. A.H. and D.F. analyzed and interpreted the data and served as project leaders. D.F. contributed to writing the manuscript.

**Funding:** This research was funded by the Competence Center for Energy and Mobility (CCEM) and the Swiss National Science Foundation (SNF, No. 200021\_159568).

**Acknowledgments:** The work was financially supported by the Competence Center for Energy and Mobility (CCEM), the Swiss National Science Foundation (SNF) and the Swiss Federal Office of Energy (SFOE). The work was conducted in the context of the Swiss Competence Center for Energy Research (SCCER BIOSWEET) of the Swiss innovation agency Innosuisse. The X10DA (SuperXAS) beamline at the Swiss Light Source (SLS) in Villigen (Switzerland) and M. Nachtegaal are thanked for kindly providing the beam time and support during measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.
