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The largest error in the analysis of transmission electron micrographs is beyond doubt introduced by the limited number of TEM projections that are analyzed and assumed to be representative. Samples used for TEM had been exposed to air. EXAFS analysis revealed that platinum particles of about 1 nm are completely oxidized under these conditions, whereas particles of a few nanometers are only partially oxidized. Because with TEM analysis no discrimination can be made between the oxidic and metallic state, and the mass of the particles is calculated using their sizes, an error is introduced because of the unknown platinum atom density. As the observed platinum particles are assumed metallic, this error is an overestimation of the platinum mass of maximal 30% when the actual particles would be completely oxidized (22, Table 6). In fact for the analysis of particles in the range 4–9 nm this potential error will be insignificant because in this size range only surface oxide is formed. The modeling of the size functions is based on spherically shaped particles. The shape of the particle determines the relation between the number of surface atoms (dispersion) and their average coordination number (NPt–Pt). The 2D TEM projections of catalysts calcined at 1±C/min (Figs. 5A, 5B) show the 4- to 9-nm particles (partially) occluded in the zeolite to have a spherical shape. By using HRTEM a spherical shape was found for small platinum particles dispersed in zeolite Y of a size 1–2 nm also (14). The stability of noble metal particles is driven largely by maximization of the average coordination number of the atoms. This implies for very small particles (below 1 nm) the formation of spherical shapes, as shown with embedded atomic modeling (23). In the case of (strong) metal–support interaction the formation of raft-like particles has been reported (3). For Pt on Al2O3 this is observed after reduction at 450±C. EXAFS analysis revealed a short Pt–O distance of 2.2A a on this catalyst. Three–dimensional particles are formed by reduction at lower temperature and then a Pt–O distance of 2.7 A is observed (24). For all catalysts investigated in this paper a Pt–O distance of 2.6 A was found (Table 3). Obviously zeolite Y has a tendency to preserve three dimensional metal particles within its spherical shaped micropores (24). To estimate the reliability of the procedure of combining TEM and EXAFS results and the modeled size functions, the calculated particle sizes are correlated with the H/Pt values as determined from the hydrogen chemisorption results. TEM and EXAFS analysis on catalyst Pt/NaY(1.1– IE–1) revealed 5% of the metal to be present as 4- to 9-nm particles with an average calculated coordination number of 6.7 for the other 95%. With this catalyst a H/Ptstrong value of 0.44 is found (Table 4). Suppose the TEM analysis failed and the actual mass percentage of metal as 4- to 9-nm particles had been 25% instead of 5%. In that case, using the EXAFS coordination number for this catalyst and Eq. [3], the remaining 75% of the metal should have been particles with an average coordination number of 5.4.With such small particles, a high chemisorption of hydrogen is expected: for instance with catalyst Pt/NaY(1.1– IE–0.2) an H/Ptstrong value of 1.2 and an average coordination number of 5.4 were found. The dispersion of particles in the size range 4–9 nm is about 0.2 (Fig. 9). Hence, if the actual mass percentage of metal in the size range 4– 9 nm on Pt/NaY(1.1–IE–1) had been 25% instead of 5% as supposed above, the expected H/Ptstrong value should have been 0.92 [0:75￡1:2C0:25￡0:2￡0:44] and not the obtained value of 0.44. Similar reasoning holds for the Pt/HUSY( 1.05–IE–0.2) catalysts. From these results it can be concluded that the hydrogen chemisorption data support the followed procedure of combining TEM, EXAFS, and particle modeling. |
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