Supported heterogeneous catalysts : What controls cobalt nanoparticle dispersion on alumina ?

We investigate how a number of physical parameters control the rate and pattern of nanoparticle assemblage onto a commercially available alumina surface. 8 nm e-Co nanoparticles supported on polycrystalline alumina are found to have areas of both good dispersion and areas of aggregation. A similar pattern of dispersion was also observed for larger (∼30 nm) polycrystalline ferromagnetic e-Co nanoparticles. Acid and base treatment of the amphoteric support material prior to the assemblage process is found to have little impact on dispersion of the particles. Using a nonpolar solvent for the assemblage process eliminates the effect of zeta potential and allows for rapid attachment of particles to the support. Performing the assemblage in a polar solvent is found to significantly decrease the rate of the particle attachment to the support. Despite the slower attachment of particles, there is no impact on the nanoparticle distribution pattern. In contrast to the mixed dispersion observed when assembling nanoparticles on an alumina support, e-Co nanoparticles are found to disperse uniformly across an ordered mesoporous MCM-41 silica support. It seems likely that a specific chemical interaction between the support surface and nanoparticle are dictating the assemblage process.

It is noted that passive oxidation of the surface to CoO has occurred on exposure of the particles to air during characterization.Oxidation of the particles is likely to only occur at the surface of the particle < 2 nm.S3.S4.
Cobalt particles are not detected by these methods.XRF has been used to determine cobalt loadings.These along with the BET and BJH measurements are contained in Table J6.Surface area and pore size values are similar to the support material values before impregnation with nanoparticles.

Fig. S2 :Fig. S3 :
Fig. S2: The distribution of the nanoparticles on the alumina support is difficult to see in the Bright field TEM images.EFTEM analysis is used (a-d) to highlight the cobalt particles.(a) Unfiltered image with all elemental energies shown; (b) a Co filtered map; (c) O filtered map and (d) is an overlay of (a), (b) and (c).

Fig. S5 :
Fig. S5: Bright field TEM images of MCM-41.(left) The material appears to be well order with a regular particles size and shaped.(right) The pores/channels viewed perpendicular to the pore axis result in alternate black and white lines.

Fig. S9 :
Fig. S9: TEM characterisation of Co nanoparticle assembled onto acid (pH 4) and base (pH 8) treated supports (a)(c) Bright field TEM images of Co nanoparticles (b) size distribution (7.4 ± 1.2 nm) (d) EDX spectrum.Note TEM images have aggregations of monodisperse samples.Aggregation is a side effect of TEM sample preparation; solvent used has evaporated to rapidly leaving particles clustered on the grid.

Table S2 :
Textural properties of the chemically-treated alumina supports.

Table S9 :
Textural properties of the treated support nanocatalysts