Abstract
Perovskite derived Ni catalysts offer the remarkable benefit of regeneration after catalyst poisoning or Ni particle growth through the reversible segregation of Ni from the perovskite-type oxide host. Although this property allows for repeated catalyst regeneration, improving Ni catalyst stability towards sulfur poisoning by H2S is highly critical in solid oxide fuel cells. In this work Mn, Mo, Cr and Fe were combined with Ni at the B-site of La0.3Sr0.55TiO3±δ to explore possible benefits of segregation of two transition metals towards sulfur tolerance. Catalytic activity tests towards the water gas shift reaction were carried out to evaluate the effect of the additional metal on the catalytic activity and sulfur stability of the Ni catalyst. The addition of Fe to the Ni perovskite catalyst was found to increase sulfur tolerance. The simultaneous segregation of Fe and Ni from La0.3Sr0.55Ti0.95-xNi0.05FexO3±δ (x ≤ 0.05) was investigated by temperature programmed reduction, X-ray diffraction and X-ray absorption spectroscopy and catalytic tests after multiple redox cycles. It is shown that catalytic properties of the active phase were affected likely by the segregation of Ni/Fe alloy particles and that the reversible segregation of Ni persisted, while it was limited in the case of Fe under the same conditions.
Highlights
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]
Achieving catalyst stability while maintaining high catalytic conversion rates, 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, and for its activity towards the water gas shift reaction (WGS) when the device is operated on CO-rich feeds [4,5]
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]
Summary
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]. Achieving catalyst stability while maintaining high catalytic conversion rates, 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, and for its activity towards the water gas shift reaction (WGS) when the device is operated on CO-rich feeds [4,5]. The interaction between Ni and Mo was found to increase the electron density on
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