Stability of Colloidal Iron Oxide Nanoparticles on Titania and Silica Support.

Using model catalysts with well-defined particle sizes and morphologies to elucidate questions regarding catalytic activity and stability has gained more interest, particularly utilizing colloidally prepared metal(oxide) particles. Here, colloidally synthesized iron oxide nanoparticles (Fe x O y -NPs, size ∼7 nm) on either a titania (Fe x O y /TiO2) or a silica (Fe x O y /SiO2) support were studied. These model catalyst systems showed excellent activity in the Fischer-Tropsch to olefin (FTO) reaction at high pressure. However, the Fe x O y /TiO2 catalyst deactivated more than the Fe x O y /SiO2 catalyst. After analyzing the used catalysts, it was evident that the Fe x O y -NP on titania had grown to 48 nm, while the Fe x O y -NP on silica was still 7 nm in size. STEM-EDX revealed that the growth of Fe x O y /TiO2 originated mainly from the hydrogen reduction step and only to a limited extent from catalysis. Quantitative STEM-EDX measurements indicated that at a reduction temperature of 350 °C, 80% of the initial iron had dispersed over and into the titania as iron species below imaging resolution. The Fe/Ti surface atomic ratios from XPS measurements indicated that the iron particles first spread over the support after a reduction temperature of 300 °C followed by iron oxide particle growth at 350 °C. Mössbauer spectroscopy showed that 70% of iron was present as Fe2+, specifically as amorphous iron titanates (FeTiO3), after reduction at 350 °C. The growth of iron nanoparticles on titania is hypothesized as an Ostwald ripening process where Fe2+ species diffuse over and through the titania support. Presynthesized nanoparticles on SiO2 displayed structural stability, as only ∼10% iron silicates were formed and particles kept the same size during in situ reduction, carburization, and FTO catalysis.

Attachment of iron oxide nanoparticles to carbon nanofibers studied by in-situ liquid phase transmission electron microscopy.

By using liquid phase transmission electron microscopy (LP-TEM), the dynamics of iron oxide nanoparticle (Fe-NP) attachment to carbon nanofibers (CNFs) and oxygen functionalized CNFs (CNF-Ox) were studied in-situ. The beam effect on the stability of the sample in various liquids was examined, and it was found that toluene provided the highest stability and resolution to image both CNF supports and Fe-NPs. Flowing particles dispersed in toluene through the liquid cell allowed direct monitoring of the attachment process at ambient temperature. Using CNF-Ox as a support led to a large extent and irreversible attachment of iron nanoparticle compared to a lower extent and reversible attachment of Fe-NPs to pristine CNF, indicating the influence of surface functionalization on colloidal particle attachment. The results were confirmed by lab-scale experiments as well as experiments performed with the electron beam switched off, verifying the notion that beam effects did not affect the attachment. This study revealed previously unknown phenomena in colloidal particle - support interactions and demonstrates the power of LP-TEM technique for studying such nanoscale processes.

Preparation of Cobalt Nanocrystals Supported on Metal Oxides To Study Particle Growth in Fischer-Tropsch Catalysts.

Colloidal synthesis of nanocrystals (NC) followed by their attachment to a support and activation is a promising route to prepare model catalysts for research on structure-performance relationships. Here, we investigated the suitability of this method to prepare well-defined Co/TiO2 and Co/SiO2 catalysts for the Fischer-Tropsch (FT) synthesis with high control over the cobalt particle size. To this end, Co-NC of 3, 6, 9, and 12 nm with narrow size distributions were synthesized and attached uniformly on either TiO2 or SiO2 supports with comparable morphology and Co loadings of 2-10 wt %. After activation in H2, the FT activity of the TiO2-supported 6 and 12 nm Co-NC was similar to that of a Co/TiO2 catalyst prepared by impregnation, showing that full activation was achieved and relevant catalysts had been obtained; however, 3 nm Co-NC on TiO2 were less active than anticipated. Analysis after FT revealed that all Co-NC on TiO2 as well as 3 nm Co-NC on SiO2 had grown to ∼13 nm, while the sizes of the 6 and 9 nm Co-NC on SiO2 had remained stable. It was found that the 3 nm Co-NC on TiO2 already grew to 10 nm during activation in H2. Furthermore, substantial amounts of Co (up to 60%) migrated from the Co-NC to the support during activation on TiO2 against only 15% on SiO2. We showed that the stronger interaction between cobalt and TiO2 leads to enhanced catalyst restructuring as compared to SiO2. These findings demonstrate the potential of the NC-based method to produce relevant model catalysts to investigate phenomena that could not be studied using conventionally synthesized catalysts.

Promoted Iron Nanocrystals Obtained via Ligand Exchange as Active and Selective Catalysts for Synthesis Gas Conversion.

Colloidal synthesis routes have been recently used to fabricate heterogeneous catalysts with more controllable and homogeneous properties. Herein a method was developed to modify the surface composition of colloidal nanocrystal catalysts and to purposely introduce specific atoms via ligands and change the catalyst reactivity. Organic ligands adsorbed on the surface of iron oxide catalysts were exchanged with inorganic species such as Na2S, not only to provide an active surface but also to introduce controlled amounts of Na and S acting as promoters for the catalytic process. The catalyst composition was optimized for the Fischer-Tropsch direct conversion of synthesis gas into lower olefins. At industrially relevant conditions, these nanocrystal-based catalysts with controlled composition were more active, selective, and stable than catalysts with similar composition but synthesized using conventional methods, possibly due to their homogeneity of properties and synergic interaction of iron and promoters.

Effects of the Functionalization of the Ordered Mesoporous Carbon Support Surface on Iron Catalysts for the Fischer-Tropsch Synthesis of Lower Olefins.

Ordered mesoporous carbon (CMK-3) with different surface modifications is applied as a support for Fe-based catalysts in the Fischer-Tropsch to olefins synthesis (FTO) with and without sodium and sulfur promoters. Different concentrations of functional groups do not affect the size (3-5 nm) of Fe particles in the fresh catalysts but iron (carbide) supported on N-enriched CMK-3 and a support with a lower concentration of functional groups show higher catalytic activity under industrially relevant FTO conditions (340 °C, 10 bar, H2/CO=2) compared to a support with an O-enriched surface. The addition of promoters leads to more noticeable enhancements of the catalytic activity (3-5 times higher) and the selectivity to C2-C4 olefins (≈2 times higher) than surface functionalization of the support. Nitrogen surface functionalization and removal of surface groups before impregnation and calcination, however, further increase the activity of the catalysts in the presence of promoters. The confinement of the Fe nanoparticles in the mesopores of CMK-3 restricts but does not fully prevent particle growth and, consequently, the decrease of activity under FTO conditions.