Metal-support interaction effects and their consequences in CO 2 /CO methanation and methane steam reforming have been exemplarily studied on two complex Ni-perovskite powder catalyst systems, namely Ni-La 0.6 Sr 0.4 FeO 3-δ (lanthanum strontium ferrite, LSF) and Ni-SrTi 0.7 Fe 0.3 O 3-δ (strontium titanium ferrite, STF). Pre-reduction in hydrogen and treatment in catalytic gas mixtures cause a variety of structural effects, including exsolution of iron particles and formation of Ni-Fe alloy particles. These manifestations strongly depend on the reducibility of the perovskite and are hence much more pronounced on LSF. Reactivity differences are strongly influenced by the chemical properties of the respective perovskite support. The more reducible the perovskite support, the stronger the deviation from the catalytic behaviour of a Ni/Al 2 O 3 reference catalyst, rendering establishments of direct structure-activity/selectivity relationships difficult. The studies show the extreme variety of the metal-perovskite interface, which helps in judging similar systems of recent high catalytic importance, e.g. metals supported on spinel or other perovskite phases.
Formation
of uniform Fe and SrO rods as well as nanoparticles following controlled
reduction of La0.6Sr0.4FeO3−δ (LSF) and Ni-LSF samples in dry and moist hydrogen is studied by
aberration-corrected electron microscopy. Metallic Fe and SrO precipitate
from the perovskite lattice as rods of several tenths of nm and thicknesses
up to 20 nm. Based on a model of Fe whisker growth following reduction
of pure iron oxides, Fe rod exsolution from LSF proceeds via rate-limiting
lattice oxygen removal. This favors the formation of single iron metal
nuclei at the perovskite surface, subsequently growing as isolated
rods. The latter is only possible upon efficient removal of reduction-induced
water and, subsequently, reduction of Fe +III/+IV to Fe(0). If water
remains in the system, no reduction or rod formation occurs. In contrast,
formation of SrO rods following reduction in dry hydrogen is a catalytic
process aided by Ni particles. It bears significant resemblance to
surface diffusion-controlled carbon whisker growth on Ni, leading
to similar extrusion rods and filaments. In addition to SrO rod growth,
the exsolution of Fe nanoparticles and, subsequently, Ni–Fe
alloy particles is observed. The latter have also been observed under
static hydrogen reduction. Under strict control of the experimental
parameters, the presented data therefore open an attractive chemically
driven pathway to metal nanoarchitectures beyond the formation of
“simple” nanoparticles.
Comparative (electro)catalytic, structural,
and spectroscopic studies
in hydrogen electro-oxidation, the (inverse) water-gas shift reaction,
and methane conversion on two representative mixed ionic–electronic
conducting perovskite-type materials La0.6Sr0.4FeO3−δ (LSF) and SrTi0.7Fe0.3O3−δ (STF) were performed with the
aim of eventually correlating (electro)catalytic activity and associated
structural changes and to highlight intrinsic reactivity characteristics
as a function of the reduction state. Starting from a strongly prereduced
(vacancy-rich) initial state, only (inverse) water-gas shift activity
has been observed on both materials beyond ca. 450 °C but no
catalytic methane reforming or methane decomposition reactivity up
to 600 °C. In contrast, when starting from the fully oxidized
state, total methane oxidation to CO2 was observed on both
materials. The catalytic performance of both perovskite-type oxides
is thus strongly dependent on the degree/depth of reduction, on the
associated reactivity of the remaining lattice oxygen, and on the
reduction-induced oxygen vacancies. The latter are clearly more reactive
toward water on LSF, and this higher reactivity is linked to the superior
electrocatalytic performance of LSF in hydrogen oxidation. Combined
electron microscopy, X-ray diffraction, and Raman measurements in
turn also revealed altered surface and bulk structures and reactivities.
Metal–support interaction in rhodium–perovskite systems was studied using LSF (La0.6Sr0.4FeO3−δ) and STF (SrTi0.7Fe0.3O3−δ) supports to disentangle different manifestations of strong or reactive metal–support interaction. Electron microscopy and catalytic characterization in methane steam reforming/CO2 methanation reveal that reduction in hydrogen at 673 K and 873 K causes different extents of Fe exsolution. Depending on the perovskite reducibility, Fe–Rh alloy particles are observed. No signs of strong metal–support interaction (i.e., encapsulation of metal particles) by reduced oxide species were observed. As re‐oxidation in oxygen at 873 K did not fully restore the initial structures, the interaction between Rh and the perovskites manifests itself in irreversible alloy formation. Catalytic effects are the suppression of methane reactivity with increasing prereduction temperature. The results show the limits of the strong metal–support interaction concept in complex metal–oxide systems.
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