Faceted Particles

As was for example seen with Pd and Pt (Section 2.6.1, crotonaldehyde), dense (111) metal faces are not favorable to C=C coordination. Accordingly, greater participation of this face in the catalyst surface, with large faceted particles or by support epitaxy, can favor the unsaturated alcohol [159]. 

Fig. 8 a Facetted particles formed without reducing agent, b spherical particle produced by the use of a reducing agent, c irregular shaped particles obtained by free precipitation... 

Transmission electron micrographs (TEM) of submicrometer-size particles show faceted particles, and selected area electron diffraction (SAED) patterns of isolated particles show that they are formed by a small number of crystallites (Fig. 9.2.14a), This result is consistent with the mean size of the crystallites, which can be inferred from the x-ray diffraction lines broadening analysis using a William-son-Hall plot (35) in order to take into account the contribution of microstrains to the line broadening. Over the whole composition range, the mean crystallite size is in the range 40-60 nm for particles with a mean diameter in the range 200-300 nm (Table 9.2.5) (33). 

Fig. 26. Surface morphology of a platinum sample after 4 h of oxygen plasma treatment. The platinum surface is partially covered by faceted particles 5 pm in diameter (245).

TEM images were also taken after 15 h of reaction. For the INi sample, a monomodal MPSD is observed but already shifted towards larger sizes (dm = 78 A). The DPNi sample exhibits a similar MPSD (dm = 60 A) as before reaction (dm = 57 A) but very few faceted particles (600 A) are also observed, much less numerous than after 75 h of reaction. 

The metallic surface area of the INi sample, before and after reaction, was estimated from the micrographs, assuming that i) there is no loss of Ni and ii) the Ni particles are spherical. The metallic surface area was found to decrease from 153 m (gNi) l before reaction to 21 m (gNi) after 75 h of reaction, i.e., an 85 % decrease. For the DPNi sample, the change in the metallic surface area cannot be easily calculated because of the presence of faceted particles after the reaction. 

Only a handful of the identified types of mesoporous materials have been extensively explored and used. A noteworthy occurrence has been that some of the preparations afforded well developed faceted particles, a la ordered crystals. The examples are MCM-48, SBA-1 and SBA-16 [36]. 

Figure 3.14 Bright field TEM images of y-AhOs samples from Alfa Aesar (top row) and Johnson Matthey (bottom row). Black arrows indicated facetted particles. Insets selected area diffraction patterns. Reprinted with permission from Ref. (194).

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