(499c) Colloidal Iron Nanoparticles Provide Size-Control in the Catalytic CO Hydrogenation Reaction

Recent
advances in synthetic nanotechnology to produce uniform metal nanoparticle
catalysts and to use such “monodisperse” particles in
catalytic reactions clearly allowed establishing correlations between the
catalyst activity/selectivity patterns and the size and shape of metal catalyst
particles. The rationale behind these efforts is the quest for high activity
and selectivity in multi-path catalytic reactions, with little to no waste or
polluting by-products. Among the various methods employed to prepare
well-defined metal nanoparticles, those based on colloidal recipes have gained
major importance. While the use of stabilizers (“capping agents”, sometimes
simply “ligands”) to obtain stable colloidal dispersions with narrow particle
size distribution is considered mandatory, there is ample evidence that these
stabilizers cause alterations in the catalytic properties of the metal
nanoparticles. On the other hand, an intriguing reaction for various green
feedstock production on an industrial scale is the
hydrogenation of carbon monoxide according to Fischer-Tropsch.
Fe-, Co- and Ru-based catalysts have been most
commonly investigated in the past for their capability to produce paraffins, olefins, and oxygenates with varying hydrocarbon
chain length. Tuning these catalysts for high activity and selectivity to
specific product classes is therefore of utmost interest. In this research we
demonstrated that monodisperse Fe(0) particles in
silica could be successfully prepared and tested in the catalytic CO
hydrogenation (so-called Fischer-Tropsch process)
without ligand effects to play any visible influence.¹ Different
from all of the previous studies, which use iron oxide as precursor to prepare
catalysts, we applied metallic (0) iron particles. This Fe(0)
metal can be easily converted into iron carbide during Fischer-Tropsch synthesis since it is much more active than iron
oxide. Palmitic acid and hexadecylamine
used as capping agents in suitable ratios, were
quantitatively eliminated through the reaction with hydrogen. We further reveal
the occurrence of pronounced particle size effects with respect to catalytic
activity and selectivity. In particular, an increase in the particle size
causes increasing TOF (activity). TOF values of the same magnitude and particle
size dependence have been found when relating the geometry-derived TOF to the
active Fe(0) metal surface as determined through
hydrogen-deuterium exchange. As to the selectivity signatures of Fe/MCF-17,
methane and short-chain (terminal) olefin formation significantly increase with
decreasing Fe particle size. Quite differently, both the long-chain C₅₊
olefin and (total) oxygenate slates show the opposite behavior. As to the C₂₊paraffins, it seems that small particles are less
efficient in chain lengthening than larger ones. According to XRD results, bulk
Fe(0) of the as-prepared catalyst is chemically
reconstructed into iron-carbides, ε- and χ-Fe₅C₂,
after reaction. The occurrence of iron-oxide phases, though not clearly
detectable here, cannot be ruled out either. Reaction-induced nanoparticle
sintering was limited according to TEM and microtome studies.

Summarizing,
the methodological approach to study the catalytic CO hydrogenation over monodisperse Fe nanoparticles intercalated in the mesopores of silica provided new insights into the
structure-sensitivity of the reaction and is anticipated to help design
catalysts with specific selectivity signatures at high reaction rates.

1.      
V. Iablokov, Y. Xiang, A. Meffre,
P.F. Fazzini, B. Chaudret,
N. Kruse ACS Catalysis 2016, 6, 2496-2500