Ni is widely used as active phase in hydrogenation catalysts for its high activity and relatively low cost compared to noble metals. The the presence of unsaturated hydrocarbons (especially in biomass derived feed gases) are known to promote coking of the catalyst . Ni-containing catalysts may be oxidatively regenerated at elevated temperatures by burning off carbon and cleaning the catalyst surface from coke and/or poison species. However, during regeneration current Ni catalysts with high metal loadings undergo microstructural changes like sintering. The aim of this work is to develop a redox stable and catalytically active material allowing catalysts that suffered from the above mentioned deactivation mechanisms to be oxidatively regenerated without unwanted activity loss due to Ni particle sintering. Incorporation of Ni in perovskite-type mixed oxide (PMO) LaFe1-xNixO3±δ (LFNO) allows to exploit the self-regenerating property of PMOs [2-4]. In the current study, we explore the structural reversibility of Ni in LFNO for CO2 hydrogenation. It is shown that Ni can be selectively reduced and segregates from the perovskite lattice at elevated temperatures forming dispersed and active metallic Ni particles on the surface. Reoxidation causes Ni to reenter the support lattice. This principle allows C-deposits and poison species to be removed simultaneously, whereas inhibition of Ni sintering by re-dispersion of the active phase on the surface upon reduction is a beneficial consequence.
It was demonstrated that LFNO exhibits the exceptional property of reversible nickel segregation. After reduction in 10 vol% H2/Ar at 600 °C Ni for 1 h, Ni was preferentially reduced and segregated to the PMO surface where it formed catalytically active, metallic Ni particles of few nm in size. These Ni particles were again fully reincorporated into the perovskite lattice after reoxidation in air at 650 °C. Amongst other methods like X-ray diffraction and temperature programmed reduction, the state of nickel was assessed over a redox cycle using X-ray absorption spectroscopy around the Ni K-edge. After reduction 50 % of all Ni was reduced and all Ni was reincorporated into the host lattice after reoxidation at 650°C for 2 h from which it could be segregated again at no loss of catalytic activity. The stability could be attributed to the constant particle size due to the Ni incorporation and segregation mechanism. Other Ni catalysts which do not show this mechanism suffer from particle sintering over the number of redox cycles (e.g. Ni/Al2O3). It is shown in this work that the preservation of catalytic activity over redox cycles at high temperatures allows the material to be regenerated after it has suffered from severe coking during reaction.
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4. Burnat, D., et al. A. J. Mater. Chem. A 4 (2016)