(492e) Fundamental Studies On Impregnation and Drying for Supported Catalyst Preparation Using DSC and Electron Tomography

1.      Introduction

Supported metal catalysts generally owe their high activity and selectivity to a highly dispersed and homogeneously distributed active phase. Control over nanoparticle size, shape and distribution inside a porous support is therefore the ultimate goal for supported catalyst preparation. Nowadays many preparation routes are available and the limits and possibilities of the classical routes, impregnation-drying,[1] electrostatic-adsorption[2] and deposition-precipitation[3] are now better understood.

Impregnation-drying with transition metal nitrates is still the most commonly applied route, due to its convenience, the possibility for high loadings and low costs. However, due to low support-metal precursor interactions, redistribution of the active phase during preparation is often observed. The use of organic additives,[4] other precursor salts[5] as well as different calcination atmospheres[6] has led to an improved distribution after calcination. Nevertheless, a fundamental understanding of the redistributions occurring during the preparation steps is still desirable.

 With ordered mesoporous silica as model supports the effect of the individual preparation steps can be disentangled.[7] On SBA-15 impregnated with Ni(NO₃)₂ (aq) redistribution had taken place after drying which led to filled and empty pores. Indeed, also after calcination under 1% NO in N₂, locally high NiO nanoparticle loadings were observed, while other pores were completely empty.[8] Here we present a study on impregnation and drying by differential scanning calorimetry and cryogenic electron tomography. First the extent of pore filling after calcination was quantified using the melting behaviour of the impregnation solution in the presence of a mesoporous support. Second, the distribution of the precursor solution directly after impregnation and after drying on a single support particle was visualised in three dimensions.

2.       Results and Discussion

Intraporous and extraporous impregnation solution can be distinguished with DSC due to the depressed melting point of the confined liquids.[9] After impregnating SBA-15 with increasing amounts of an aqueous Ni(NO₃)₂ solution the melting behaviour is studied. Only  for a pore filling higher than 85% of the microporous and mesoporous volume melting was observed between -35 ºC to -30 ºC, which corresponds to melting of extraporous solution. This indicates that impregnation leads to a by and large homogeneous distribution of the precursor solution over the support.

 With cryogenic electron tomography, the distribution of the precursor phase after impregnation was visualised in a single support particle. Rod-like SBA-15 was impregnated with a saturated aqueous Co(NO₃)₂ solution. Cryogenic electron tomography was performed on the same support particle directly after impregnation and after performing a drying treatment. Figure 2 shows slices of the reconstruction perpendicular to the pores for an impregnated particle at different heights. Emtpy pores are observed at the tip of the particle (A) and at the edges of the particle (B and C). This confirms that indeed by and large the pore volume is filled with impregnation solution, but that some pore sections, up to 10%, remain empty, most likely due to inhomogeneities caused by mixing of the impregnation solution and the support.

The effect of drying treatments on the precursor distribution will be discussed in the full paper. To this purpose the impregnated particles were subjected to a heat treatment after acquisition of a tilt series. Subsequently, the same single support particle was visualised. A freeze-drying treatment or a heat treatment at 60 ºC under stagnant conditions in air was applied. The distribution after the treatment was compared to drying on lab-scale. In addition, the effect on the final metal oxide distribution was determined with electron tomography after calcination. 

3. Conclusion

Impregnation-drying is a commonly used preparation method. Therefore, fundamental knowledge on the distribution of the precursor solution over the support by the first two steps of this preparation route, impregnation and drying, is essential for supported catalyst preparation. DSC was used to quantitatively show that an aqueous impregnation solution fills the pores of a mesoporous silica support for 90%. The distribution of the precursor solution over the support was also visualised with cryogenic electron tomography. Empty pores were detected mainly at the tip of the particle and at the edges.

In addition to impregnation, the effect of drying was also visualised with cryo-ET and will be reported upon in the full paper. It allowed the direct comparison of the precursor solution distribution before and after drying on a single support particle. Freeze-drying is compared to heating at 60 ºC. Finally, we will show the effect of the drying step on the final metal oxide distribution after calcination.

Figure 1. Numerical cross sections from tomogram on rod-like SBA-15 impregnated with Co(NO₃)₂, slices in xz plane.

4. References

[1] Neimark, A.V., et al., Ind. Eng. Chem. Prod. Res. Dev., 1981, 20, 439.

[2] Jiao, L., et al., Journal of Catalysis, 2008, 260, 329.

[3] van der Lee, M.K., et al., Journal of the American Chemical Society, 2005, 127(39), 13573.

[4] Borg, O., et al., Journal of Catalysis, 2008, 259, 161.

[5] van de Loosdrecht, L., et al., Applied Catalysis a-General, 1997, 150(2), 365.

[6] Sietsma, J.R.A., et al., Angewandte Chemie-International Edition, 2007, 46(24), 4547.

[7] Sietsma, J.R.A., et al., Chemistry of Materials, 2008, 20(9), 2921.

[8] Friedrich, H., et al., Journal of the American Chemical Society, 2007, 129(33), 10249.

[9] Eggenhuisen, T.M., et al., Journal of Physical Chemistry C, 2009, 113(38), 16785.