Small particles spectroscopy

The example is typical for many applications of Mossbauer spectroscopy in catalysis a catalyst undergoes a certain treatment, then its Mossbauer spectrum is measured in situ at room temperature. Flowever, if the catalyst contains highly dispersed particles, the measurement of spectra at cryogenic temperatures becomes advantageous as the recoil-free fraction of surface atoms increases substantially at temperatures below 300 K. Secondly, spectra of small particles that behave superparamagne-... 

Mossbauer spectroscopy has been extensively used for studies of nanostructured materials and several reviews on magnetic nanoparticles have been published, see e.g. [6-8, 46 8]. The magnetic properties of nanoparticles may differ from those of bulk materials for several reasons. The most dramatic effect of a small particle size is that the magnetization direction is not stable at finite temperatures, but fluctuates. 

Fe/Ir catalysts on silica and alumina Fe and Ir Mossbauer spectroscopy silica- and alumina-supported Fe-Ir catalysts formed by calcination in air contain mixtures of small particles of Fe(III) oxide and Ir(IV) oxide. IrOz is reduced in hydrogen to metallic Ir. a-Fe203 on SiOz is reduced in hydrogen to an Fe-Ir alloy, whilst supported on alumina stabilizes in hydrogen as Fe(II). Possible use for methanol formation is discussed... 

It has been observed that cobalt may undergo large-scale reconstruction under a synthesis gas environment.27 Reconstruction is a thermodynamically driven process that results in the stabilization of less reactive surfaces. Recent molecular modeling calculations have shown that atomic carbon can induce the clock reconstruction of an fee cobalt (100) surface.28 It has also been postulated and shown with in situ x-ray adsorption spectroscopy (XAS) on cobalt supported on carbon nanofibers that small particles (<6 nm) undergo a reconstruction during FTS that can result in decreased activity.29... 

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

S. Arnold, Spectroscopy of single levitated micron sized particles, in Optical Effects Associated with Small Particles (P. W. Barber and R. K.. Chang, eds.), World Scientific, New York (1988). 

The development of the electrodynamic balance and other particle traps has made it possible to perform precise measurements of the properties of small particles by focusing on the single particle. The variety of processes and phenomena that can be investigated with particle traps is quite extensive and includes gas/liquid and gas/solid chemical reactions, chemical spectroscopies, heat and mass transfer processes, interfacial phenomena, thermodynamic properties, phoretic forces, and other topics of interest to chemical engineers. 

Hoener CF, Allan KA, Bard AJ, Campion A, Fox MA, Mallouk TE, Webber SE, White JM (1992) Demonstration of a Shell Core Structure in Layered CdSe-ZnSe Small Particles by X-Ray Photoelectron and Auger Spectroscopies. J Phys Chem 96 3812-3817... 

Although a number of secondary minerals have been predicted to form in weathered CCB materials, few have been positively identified by physical characterization methods. Secondary phases in CCB materials may be difficult or impossible to characterize due to their low abundance and small particle size. Conventional mineral identification methods such as X-ray diffraction (XRD) analysis fail to identify secondary phases that are less than 1-5% by weight of the CCB or are X-ray amorphous. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with energy dispersive spectroscopy (EDS), can often identify phases not seen by XRD. Additional analytical methods used to characterize trace secondary phases include infrared (IR) spectroscopy, electron microprobe (EMP) analysis, differential thermal analysis (DTA), and various synchrotron radiation techniques (e.g., micro-XRD, X-ray absorption near-eidge spectroscopy [XANES], X-ray absorption fine-structure [XAFSJ). 

At infrared wavelengths extinction by the MgO particles of Fig. 11.2, including those with radius 1 jam, which can be made by grinding, is dominated by absorption. This is why the KBr pellet technique is commonly used for infrared absorption spectroscopy of powders. A small amount of the sample dispersed in KBr powder is pressed into a pellet, the transmission spectrum of which is readily obtained. Because extinction is dominated by absorption, this transmission spectrum should follow the undulations of the intrinsic absorption spectrum—but not always. Comparison of Figs. 10.1 and 11.2 reveals an interesting discrepancy calculated peak extinction occurs at 0.075 eV, whereas absorption in bulk MgO peaks at the transverse optic mode frequency, which is about 0.05 eV. This is a large discrepancy in light of the precision of modern infrared spectroscopy and could cause serious error if the extinction peak were assumed to lie at the position of a bulk absorption band. This is the first instance we have encountered where the properties of small particles deviate appreciably from those of the bulk solid. It is the result of surface mode excitation, which is such a dominant effect in small particles of some solids that we have devoted Chapter 12 to its fuller discussion. 

These criteria for an ideal spectroscopic technique have been met, to a large extent, by Mossbauer spectroscopy. UHV conditions are not required since one utilizes y rays with energies in the keV range, and the technique lends itself easily to in situ studies. Furthermore, the technique is ideally suited for studies of small particle systems, and in several instances information about particle size can also be obtained. The unique feature of Mossbauer spectroscopy is the extremely high energy sensitivity of the technique. This allows detailed chemical, structural, and magnetic information to be obtained about atoms on the surface or in the bulk phase. 

Soft X-ray spectroscopy has been used to investigate the 3d electron distribution in small Ni particles.179 180 In addition to showing modified absorption and emission spectra, the small Ni particles (< 2 nm) have two satellite peaks in the absorption spectrum missing. This is attributed to a loss of long range order due to small particle size. 

Similar considerations hold for the Mossbauer spectroscopy of gold, although with the recently increased interest in small particles of gold in catalysis, several investigations have been undertaken one such study is reviewed in the following section. 

One of the changes between small metal particles and bulk metal is the change in conductivity bulk metal is a conductor, small particles are insulators. In chapter 7 the development of a new in situ probe of metallic character in supported metal clusters utilizing X-ray absorption spectroscopy is described. A very strong support effect on the metal-insulator transition with cluster size in supported Pt clusters is found. Pt particles with basic supports show metallic screening for sizes as small as 6A. In contrast, with acidic supports the Pt particles do not show metallic behavior below 10A. 

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