Platinum catalysts diffraction pattern

Diffraction patterns can be used to identify the various phases in a catalyst. An example is given in Fig. 10.3b, where XRD is used to follow the reduction of alumina-supported iron oxide at 675 K as a function of time. The initially present oc-Fe2C>3 (haematite) is partially reduced to metallic iron, with Fe3C>4 (magnetite) as the intermediate. The diffraction lines from platinum are due to the sample holder [10]. 

In studies of the platinum-iridium system in the bulk, Raub and Platte (47) have reported a miscibility gap at temperatures lower than about 975°C. At 500°C the gap extends over the composition range from 7 to 99% iridium. Nevertheless, the results presented in Figure 4.23 indicate that it is possible to prepare platinum-iridium catalysts for which X-ray diffraction patterns do not reveal separate lines for platinum-rich and iridium-rich phases, despite the fact that the catalysts have been heated to only 500°C in their preparation. Instead, single diffraction lines are observed. 

Figure 4.23 X-ray diffraction patterns for a platinum-iridium bimetallic cluster catalyst and for reference materials consisting of physical mixtures of platinum and iridium in the form of large crystals or dispersed monometallic clusters (4). (Reprinted with permission from Academic Press, Inc.)...

While the X-ray diffraction data considered earlier for the catalyst containing 10 wt% each of platinum and iridium do not eliminate the possibility of differences in the average environments about the two types of atoms in the clusters, they also do not provide evidence for it. The diffraction data are entirely consistent with the platinum and iridium being present as homogeneous bimetallic clusters. This interpretation is reasonable in the absence of other information. The results of the EXAFS studies, however, provide evidence that the catalyst consists of platinum-iridium clusters that are not homogeneous. Hence the EXAFS data provide more detailed structural information than can be obtained from a diffraction pattern. 

When the platinum-iridium clusters are still more highly dispersed, and are supported on an alumina carrier instead of silica, the results shown in the right-hand sections of Figure 4.30 are obtained (48). The metal dispersion of the clusters as determined by hydrogen chemisorption is 0.93. If the clusters were spherical, the average diameter calculated from the chemisorption data would be about 12 A. Again the clusters are too small to give a satisfactory X-ray diffraction pattern. As with the previous two catalysts, the values... 

Pair-density function analysis of X-ray diffraction patterns shows that active iron oxide (FeOx) catalysts are disordered at the nanoscale by the platinum and also have micropores, while inactive catalysts do not share these features. 

Next question concerns the oxidation state of platinum in Pt/SDB catalysts. Admittedly, X-ray diffraction pattern presented in Ref [12] clearly indicated the presence of Pf, but small amounts of other platinum species (undetected by X-ray technique) possibly can also exist on the catalyst surface. Temperature-programmed reduction profile of SDB-supported R(acac)2 contained two peaks, the first of which corresponded to the reaction of hydrogen with of platinum(Il) acetylacetonate and the second one to the reaction between hydrogen and products of partial decomposition of R(acac)2. X-ray photoelectron spectroscopy (XPS) measurements carried out in the present study have shown the presence of platinum species in the +2 oxidation state, in addition to those in zero oxidation state (Table 15.4). 

The electron beam was focussed on individual particles of the bimetallic catalysts. We obtained diffraction patterns of monocrystals which is a proof that each metallic particle is a monocrystal but we were not able to have a sufficient precision on the lattice spacings to make a difference between pure platinum and bimetallic particles. 

The most direct evidence for the development of metallic platinum and PtSn alloy, by reduction of a catalyst prepared by the incipient wetness technique on low area Degussa alumina (110 irf/g) / was presented by Davis, et al. (11). Their conclusions were based on detailed XRD patterns, recorded in situ at elevated temperatures under flowing hydrogen. With 0.68% platinum samples containing tin, only Pt/Sn alloy diffraction lines were observed. 

The crystal structure of Ag and Pt are FCC while ruthenium is HCP. Additionally, ruthenium forms a soUd solution in platinum in excess of 60 atomic percent platinum (Figure 9-10). The amount of silver used was almost 20 times more than that of the catalysts. The selected area electron diffraction analysis was performed on the electrodes, demonstrating both a ring pattern of silver and spot patterns indicative of platinum (Figure 9-9). No individual spots/rings were observed for ruthenium, as expected from the phase diagram. 

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