**1. Introduction**

In recent years perovskite-type metal oxide (PMO) derived metal catalysts have attracted great attention for their high redox stability, due to the reversible segregation of catalytically active metals from the bulk of the oxide in reducing atmospheres and their reincorporation during oxidative treatments [1]. It was demonstrated that this property allows for the regeneration of catalysts, which have suffered from active metal particle sintering, as well as the recovery of catalysts poisoned by coke or sulfur through simple redox cycling [2,3]. However, achieving catalyst stability while maintaining high catalytic conversion rates, thus decreasing the necessary frequency of catalyst regeneration cycles, appears to be as propitious as increasing catalyst regenerability. This is especially important in redox-sensitive electrochemical devices, such as solid oxide fuel cells (SOFCs), where metallic Ni is typically applied as the active phase in the anode for fuel oxidation, but also for its activity towards the water gas shift reaction (WGS) when the device is operated on CO-rich feeds [4,5]. Prominent examples of sulfur poisoning in heterogeneous catalysis include exhaust gas after-treatment reactions in the three-way catalytic converters and the selective catalytic reduction of NOx compounds [6], methanation

of carbon oxides [7], reforming of methane and higher hydrocarbons [8,9], Fischer-Tropsch [10] and methanol synthesis from syngas [11]. In the case of all metal-catalyzed reactions, sulfur tolerance may generally be increased in three ways: (i) Increasing the number of catalytically active sites, which leaves higher number of free active sites at equal sulfur surface coverage, (ii) a sacrificial species may be introduced on the catalyst, which preferentially interacts with sulfur leaving the active species available for the reaction and (iii) the electronic effect on the active metal caused by the introduction of a second metal may result in decreased metal-sulfur interactions [12]. In the case of the Ni-catalyzed WGS reaction, promising results have been reported regarding improved sulfur stability of Ni reforming catalysts by the addition of Mo, Co and Re [13]. The beneficial effect of Re was attributed to the formation of a sulfur tolerant alloy, whereas in Ni-Mo metal combinations Mo acted as the sacrificial element [14]. The interaction between Ni and Mo was found to also increase the electron density on Mo thus facilitating its interaction strength with electronegative sulfur. Re-doping was also applied to improve sulfur tolerance of a Ni-Sr/ZrO2 catalyst for the reforming of hydrocarbons [15]. Metal-metal interactions were also exploited to reduce the electron donor capacity of Pd by Mn addition thus decreasing its interaction strength with sulfur [16–18]. It is likely that doping of the Ni phase with other transition metals may also change sulfur adsorption properties on Ni.

It is the aim of the present work to combine the excellent regenerability of PMO-derived Ni catalysts for the WGS reaction with the possibility to alter the adsorption properties by transition metal doping of La0.3Sr0.55Ti0.95Ni0.05O3±δ, a self-regenerable SOFC anode material [2]. Sulfur sensitive elements, such as Cr, Mn, Fe and Mo were selected as potential sacrificial agents for screening towards a Ni-metal combination resistant to sulfur. Molybdenum, Cr and Mn are also of great importance to industrial high-temperature WGS catalysts [19].

## **2. Results**

The work is structured as follows. We start showing the characterization of all materials (Table S1 for sample denotation) using ex situ and in situ X-ray diffraction (XRD) and temperature programmed reduction (TPR). Then the catalytic activity of the materials towards the water gas shift reaction (WGS) in the absence and presence of H2S is presented. Because La0.3Sr0.55Ti0.95Ni0.05O3±<sup>δ</sup> impregnated by Fe resulted in the most promising in terms of sulfur uptake and resistance to poisoning, further samples were prepared with various Fe/Ni ratios (see Materials and Methods and Table 1 for sample denotation), which were characterized also for the local environment of Ni and Fe using X-ray absorption spectroscopy and were tested for reaction and poisoning.
