EXAFS particle size determination

Results of XAFS measurements are compiled in Table 1 with the results of the estimates of the TEM measurements (for details on TEM results see ref. [ 13 ]). The catalysts are named according to the stabilizing polymer and particle size determined by TEM. Except for the PVP stabilized catalyst reduced with methanol, an excellent agreement exists between the size of the metal particles estimated from TEM and EXAFS. The methanol reduced sample shows the smallest particle size when estimated from EXAFS, while it appears to be much larger in TEM. At present we would like to speculate that this is due to the formation of agglomerates (diameter 3.5 nm) formed from very small particles (diameter 1.2 nm). Consequendy, the eatalyst is named Rh/PVP/3.5-a to indicate the difference with the sol containing primary particles of 3.5 nm in diameter. 

Owing to the excellent dispersion attainable in ALE, other means than XRD are needed to evaluate the size of the surface structures formed. In the following we review a TEM and EXAFS (extended X-ray absorption fine structure) study of CuO processed on silica and alumina from three reaction cycles of Cu(thd)2 and air [57]. The Cu concentration was 4.1 wt-% on alumina and 4.3 wt-% on silica corresponding to 2.0 and 1.4 Cu atoms/nm, respectively. From the TEM pictures of the supported CuO samples it was apparent that the particles must be very small, because they could not be seen as separate particles. This was further supported by the EXAFS results for the samples indicating that the CuO particles are extremely small, probably less than 5 A. The CuO particles on silica seemed to be somewhat larger than those on alumina. This also indicated that the interaction of the precursors is stronger with alumina than with silica. The small particle size determined by EXAFS further confirmed the excellent dispersion achievable by the ALE method when proper reaction conditions are applied. 

The interesting information is the correlation between first shell coordination numbers from EXAFS and H/M values from chemisorption, shown in Fig. 6.18. The correlation is as expected high dispersions correspond to low coordination numbers. Of course, what we really need is a relationship between particle size and H/M values. The right hand panel of Fig. 6.18 translates the experimentally determined H/M values of the catalysts into the diameter of particles with a half-spherical shape. Similar calibrations can be made for spherical particles or for particles of any other... 

Fig. 12. Cu(lll) diffraction line from a reduced Cu/Si02 catalyst acquired by using the combined XRD/EXAFS setup. The dashed line shows the calculated broadening of the Cu(111) peak corresponding to particles with the mean size determined by use of the standard EXAFS analysis. The full line shows the results when the new procedure (34) was used to estimate the copper particle size [adapted from Clausen et at. (34)].

The catalysts were all calcined and prereduced. For the calcination/reduction, a sieve fraction (225 < dp < 450 im) was placed in a downflow fixed-bed reactor. The prereduced catalysts were stored in air. The Pt particle size was determined by H2 chemisorption, high resolution transmission electron microscopy and EXAFS analysis. Details of H2 chemisorption25, HRTEM26 and EXAFS25 are given elsewhere. 

The Pt/LTL [. ], Pt/LTL [0.47, small], Pt/ASA and Pt/HT catalysts all have highly dispersed Pt particles. Based on the Htotai/Pt and Nptpt results, the average particle size for all these catalysts was estimated < 1 nm. The particle size for the Pt/ASA catalyst as revealed with HRTEM (1.5 nm) seems in contradiction with the other techniques. However, it has to be noted that with HRTEM the lower detection limit for Pt/ASA is approximately 8-10 A, and that on the HRTEM pictures taken only a small amount of particles was visible. In other words, with HRTEM the smallest particles, which make up the majority of all Pt in the Pt/ASA catalyst, are invisible. The relation between particle sizes as determined with HRTEM, H2 chemisorption and EXAFS was extensively described by de Graaf el a/35. 

Figure 10 The effect of particle size on the change in the normalized Pt J-band vacancies d-band vacancies/% surface atoms) (...) in going from 0.54 to 0.0 V. The change in the Pt-Pt coordination number as determined from the Pt L3 edge EXAFS analysis is also shown (—). The ordinate axis refers to the change in the J-band vacancy/atom determined from XANES spectra normalized with respect to total number of surface atoms present (based on cluster calculations on a cubo-octahedron model). 

EXAFS is also well suited for the study of finely divided metal (or metal oxide or metal sulfide) clusters supported within the pore structure (see Chapter 6). These particles are readily observed by X-ray spectroscopy, even if they are disordered throughout the solid. Analysis can even determine the average particle size of such clusters, which is of vital importance in catalytic preparation. Typically, for example, platinum supported on zeolites (and other solid acids) is a highly effective catalyst in the reforming of hydrocarbons. 

Stoupin et al. assume a core stracture surrounded by a more amorphous shell rich in Ru, and therefore determine a parameter Sru = (n - (1-D)/xru)/D, where D is the dispersion (fraction of atoms at the surface) determined assuming spherical particles and based on the estimated particle size from the EXAFS (or XRD) results, and n is the nominal molar ratio (R Ru, perhaps determined from XRF). Here Sr represents the relative composition of the snrface (i.e., it is the fraction of Ru atoms relative to the total on the surface), and xr is an estimate of the relative composition of the core using in situ EXAFS results. Their final results suggest that the surface of a PtRu ETEK cluster is rich in Ru, and is heavily oxidized, similar to Hwang s and our conclusions. 

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