The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.

Metal (oxide) nanoparticles smaller than about 20 nm have received widespread interest recently because of their envisioned applications in electronics, optics, and magnetic storage devices.[1] They are currently used as catalysts for the production of fuels and chemicals and the reduction of environmental pollution.[2] High surface-to-volume ratios are important for these particles since catalytic processes take place at the metal (oxide) surface; therefore supports such as SiO 2 and Al 2 O 3 are generally used to obtain small and thermally stable particles. Furthermore, the use of inert matrices allows the design of materials for specific applications, such as drug-delivery systems. [3] Small particles on a support material can be obtained by deposition from the vapor or liquid phase, [4] and the most widely used method is based on impregnation of a porous support with a precursor-containing solution, followed by drying. Subsequent thermal treatment in air converts the precursor into the desired metal oxide or metal if followed by high-temperature reduction. Particles with diameters of 1-3 nm can be deposited from organic precursor complexes, but their limited solubility allows only moderate loadings ( 10 wt %) by single-step impregnations;[5] therefore inorganic salts are typically used to achieve higher metal oxide loadings. Nitrates, in contrast to chlorides and sulfates, are the most commonly used salts, because they can be fully converted into the corresponding oxides. However, supported metal oxides prepared from nitrates generally display relatively large particle sizes. [5][6][7] Herein we present a new method that allows the preparation of uniform and small metal oxide particles based on impregnation with aqueous metal nitrate solutions. We describe nickel on silica as an example, but also show the relevance of this method for other systems. Moreover, the significance of these nanoparticles for catalysis is illustrated by the activity of Co/SiO 2 in the Fischer-Tropsch synthesis of hydrocarbons. 3 ½NiðOH 2 Þ 6 ðNO 3 Þ 2 ðaqÞ T¼120 C

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We explored melt infiltration of mesoporous silica supports to prepare supported metal catalysts with high loadings and controllable particle sizes. Melting of Co(NO(3))(2)·6H(2)O in the presence of silica supports was studied in situ with differential scanning calorimetry. The melting point depression of the intraporous phase was used to quantify the degree of pore loading after infiltration. Maximum pore-fillings corresponded to 70-80% of filled pore volume, if the intraporous phase was considered to be crystalline Co(NO(3))(2)·6H(2)O. However, diffraction was absent in XRD both from the ordered mesopores at low scattering angles and from crystalline cobalt nitrate phases at high angles. Hence, an amorphous, lower density, intraporous Co(NO(3))(2)·6H(2)O phase was proposed to fill the pores completely. Equilibration at 60 °C in a closed vessel was essential for successful melt infiltration. In an open crucible, dehydration of the precursor prior to infiltration inhibited homogeneous filling of support particles. The dispersion and distribution of Co(3)O(4) after calcination could be controlled using the same toolbox as for preparation via solution impregnation: confinement and the calcination gas atmosphere. Using ordered mesoporous silica supports as well as an industrial silica gel support, catalysts with Co metal loadings in the range of 10-22 wt % were prepared. The Co(3)O(4) crystallite sizes ranged from 4 to 10 nm and scaled with the support pore diameters. By calcination in N(2), pluglike nanoparticles were obtained that formed aggregates over several pore widths, while calcination in 1% NO/N(2) led to the formation of smaller individual nanoparticles. After reduction, the Co/SiO(2) catalysts showed high activity for the Fischer-Tropsch synthesis, illustrating the applicability of melt infiltration for supported catalyst preparation.

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