Chemisorption studies Mossbauer

Mossbauer (and Mossbauer is only applicable for Fe, Ir, Eu, Au and Sn). Powder X-Ray diffraction and chemisorption studies are not as useful for characterizing such bimetallic particles due to their small size and poorly understood synergistic chemisorption properties. 

An indication of growing interdisciplinary interest in the field is illustrated in a review on new perspectives in surface chemistry and catalysis by Roberts 160), who discussed the interaction of N with iron surfaces. In so doing, he referred to the Fe (N2), matrix Mossbauer work of Barrett and Montano (7), which showed that molecular nitrogen only bonds to iron when the latter is present as a dimer. As the chemisorption studies 161) indicated that N is absorbed on singleatom sites, Roberts suggested 160), of the matrix data (7), "if this is correct, then our assignment of the N(ls) peak at 405 eV to end-on chemisorbed Nj will require further investigation. Other reviews that consider matrix-isolation techniques for chemisorption simulation are collected in footnote a. 

The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

It is through the changes in the electronic structure of the chemisorbed species or the surface that MGssbauer spectroscopy can be used in the study of surface interactions between gases and surface sites. Because a Mossbauer spectrum represents a sum of the contributions from the various interactions present, in contrast to an average value, information may also be deduced about the nonuniformity of the surface for the studied chemisorption or catalytic process. In such studies, the Mossbauer isotope may be part of the catalytic surface and/or present in the chemisorbed species, as illustrated in the following examples. 

The interaction of CO and C02 with the surface of Fe203 (for 8-nm particles) was also studied by Suzdalev et al. (247) using the Mossbauer effect. Due to the small size of these particles, more than 50% of the iron atoms appear superparamagnetic in the Mdssbauer spectrum at room temperature, and the effects of chemisorption were investigated through analysis... 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

The structure and metal compositions of RhFe and PdFe bimetal carbonyl cluster-derived catalysts have been studied by means of EXAFS, Mossbauer, and Fourier transform IR of CO chemisorption. The Fourier transform spectra of Rh-K and Fe-K EXAFS on the H2-reduced catalysts prepared from Si02-supported [Rh4Fe2(CO) s][TMBA]2 (4 wt% Rh) suggest that RhFe bimetal ensembles are formed in high dispersion (10 A in size), having direct Rh—Fe—O bonding [Rh—Rh CN = 5.6, r = 2.70 A Rh—Fe CN = 1.8, r = 2.54 A (137) as shown in Table XVIII. 

The spectrum of sample D-600 resembles that of sample B in Figure 4.32 more closely than it resembles the spectrum of sample D. The Mossbauer parameters in Table 4.2, especially the ratios A 2M, and W2IWU show substantial differences between samples D and D-600. The iridium in sample D-600 is largely present in the same poorly dispersed form as the iridium in sample C-600, as has been found from X-ray diffraction studies of similar samples. Sample D-600 therefore can be characterized approximately as consisting of highly dispersed platinum clusters incorporating iron atoms and separate iridium crystallites of much lower dispersion that are not significantly associated with iron atoms. This characterization is consistent with chemisorption data (41). 

Supression of hydrogen chemisorption at 25°C and increased uptake at 150°C by an activated process observed in the present case are indicative of partial alloying between Pt and Sn. An XPS study (Fig.l) on reduced sample (Sn/Pt = 2) shows the presence of Sn which could take part in alloying. Alloying in Pt-Sn /AI2O3 systems has been studied by Davis and co-workers (7) and identified by in-situ XRD, Mossbauer and electron microdiffraction technoques. In the present case, due to partial alloying, the catalyst could possibly contain unalloyed Pt, unalloyed Sn and alloyed Pt-Sn (Pt Sn, ) on the surface, which could react with H2-O2 in the following manner ... 

The Mossbauer effect has given useful information in the study of chemical reactions which involve surface states and chemisorption. Reproducibility of data and their interpretation are difficult in this type of work because the adsorbing material may be affected by its pre-history. Comparisons have to be made with iron cations in compounds of known site symmetry. 

The behavior of the CO/H2 synthesis reaction has been studied over silica-supported Ru-Fe catalysts, and an optimum range in the Ru Fe ratio was found to exist in which olefin production was maximized and methane formation was minimized. The catalyst samples were characterized by hydrogen and CO chemisorption, x-ray diffraction measurements, and Mossbauer spectroscopy. Alloy formation was verified at different Ru Fe ratios, and changes in specific activity and selectivity were observed as this ratio varied. Between Ru Fe ratios of 1/2 to 2, 45 mol% of the total hydrocarbon product was C2-C5 olefins while less than 40 moWo was comprised of methane. 

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