Ceria nanoparticles exhibiting controlled morphologies have been studied as model supports of gold nanoparticles to establish correlations between surface crystallography and catalytic performance [1]. It is generally assumed that the contacts between Au nanoparticles and support involve the expected ones, i.e. Au//CeO₂ {100} in the case of nanocubes, Au//CeO₂ {110} in nanorods and Au//CeO₂ {111} for nano-octahedra. Nevertheless, it is important to recall at this respect that: (1) model crystallites involve more than one type of surface plane [2]; (2) previous reports have evidenced that gold nanoparticles anchor preferentially at sites allowing to maximize contact with {111} facets [3]; (3) the surface structure of the model support crystallites may strongly depends on the activation treatments performed on the catalyst prior to the catalytic assay. Therefore, as illustrated in this contribution, a combined 2D and 3D (S)TEM investigation is necessary to reveal the actual nature of the Au//ceria interfaces on these model supports.

CeO₂ nanocubes (CeO₂NC) were prepared using a hydrothermal method. This support was then treated in an O₂(5%)/He atmosphere at 600^oC for 1 hour to simulate the influence of activation at high temperatures (CeO₂NC600). Gold was deposited over the two supports by Deposition-Precipitation method with the objective to anchor 1.5 wt. % of Au on each of these materials. The final gold loadings on those samples were 0.4% and 1.0% respectively (0.4% Au/CeO₂NC and 1.0% Au/CeO₂NC600 samples), which clearly indicated a large influence of the activation treatment on the capability of the model support crystallites to anchor the metallic phase on its surface.

HREM and HAADF-STEM tomography confirmed that the CeO₂NC surface was a mixture of {100}, {110} and {111} facets, the latter two resulting from truncations of the cubes at edges and corners. In the CeO₂NC600 support, the {110} surfaces on the edges of the nanocubes, which appeared flat in the CeO₂NC sample, were fully transformed into a system of {111} nanofacets. Additionally, those edges grew in extent after the calcination process, from less than 1% up to 25% after the thermal treatment. These results suggest that surface nanofaceting significantly promotes the deposition of gold.

In contrast to expectations, STEM studies revealed that Au nanoparticles were always preferentially anchored on {111} surfaces: on the vertices of 0.4% Au/CeO₂NC (Figures 1a,2a) and on the {111}-nanofaceted edges in the 1.0% Au/CeO₂NC600 (Figures 1b, 2b). This result does not only explain the larger metal loading observed on the calcined support but also warns about simplifications about the nanostructure of these systems as well as on the importance of their detailed characterization by STEM.   

References:

[1] Wu, Z. et al., J. Catal. 285 (2012), p. 61-73.

[2] Florea, I.et al., Cryst. Growth Des. 13 (2013), p. 1110-1121.

[3] González, J. C. et al., Angew. Chem. Int. Ed. 48 (2009), p. 5313-5315.

Figures:

Figure 1. (S)TEM images of gold nanoparticles on the 0.4% Au/CeO2NC (a) and the 1.0 % Au/CeO2NC600 (b) samples.

Figure 2. Electron tomography reconstruction of a) 0.4% Au/CeO2NC and b) 1.0 % Au/CeO2NC600 samples respectively.

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