Catalyst Mossbauer spectroscopy

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

The phase composition changes for the ultrafine catalyst were also very similar to those seen for the precipitated catalyst. Mossbauer spectroscopy... 

Keywords carbon nanomaterials, Fe-MgO catalyst, Mossbauer spectroscopy, pyrolytic... 

Garten, R.L. and Sinfelt, J.H. "Structure of Pt-lr catalysts Mossbauer spectroscopy studies employing S7Fe as a probe."/. Catal. 62 127-139 1980. 

Figure 4.17 illustrates how Mossbauer spectroscopy reveals the identity of the major iron phases in a supported iron catalyst after different treatments. The top spectrum belongs to a fresh Fe/Ti02 catalyst, i.e. after impregnation and drying. 

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-... 

The second approach is to study real catalysts with in situ techniques such as infrared and Mossbauer spectroscopy, EXAFS and XRD, under reaction conditions, or, as is more often done, under a controlled environment after quenching of the reaction. The in situ techniques, however, are not sufficiently surface specific to yield the desired atom-by-atom characterization of the surface. At best they determine the composition of the particles. 

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... 

Fe/Ir catalysts In situ Fe and Ir Mossbauer spectroscopy of silica-supported Fe/Ir catalysts with different iron to iridium ratios following pretreatment in hydrogen show that the reduction of the Fe component is enhanced by the presence of Ir metal. The presence of Ir was found to increase the catalytic activity in hydrogenation of carbon monoxide and also to influence selectivity... 

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

An understanding of the residual catalyst content of prepared materials is important as it could potentially influence materials ageing and reliability. For FeCl3, the co-ordination by water to the Fe3+ will result in the formation of acidic species that could induce hydrolysis and scission of the Si—O—Si linkage. We have utilised Mossbauer spectroscopy to assess the nature of the FeCl3 catalyst in the final polymer. Our work in this area is quite original, as there are no reports of any such studies being performed previously on these materials. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

The great advantage of Mossbauer spectroscopy for catalyst research is that it uses 7-radiation of high penetrating power such that the technique can be applied in situ. An economic advantage is that the technique is relatively inexpensive. The price is about a factor of ten less than equipment for electron microscopy or photoelectron spectroscopy. 

In this chapter, we will first describe what the Mossbauer effect is, then explain why it can only be observed in the solid state and in a limited number of elements. Next we discuss the so-called hyperfine interactions between the nucleus and its environment, which make the technique so informative. After a few remarks on spectral interpretation we go systematically through a number of examples which show what type of information Mossbauer spectroscopy yields about catalysts. 

The application of Mossbauer spectroscopy to the investigation of catalysts began around 1970. By 1990, over 600 scientific papers had been published [18]. Most applications of Mossbauer spectroscopy to catalysts fall in one of the following categories ... 

These two examples illustrate how Mossbauer spectroscopy reveals the identity of iron phases in a catalyst after different treatments. The examples are typical for many applications of the technique in catalysis. A catalyst is reduced, carburized, sulfided, or passivated, and, after cooling down, its Mossbauer spectrum is taken at room temperature. However, a complete characterization of phases in a catalyst... 

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. 

Alternative ways to study the kinetics of bulk transformations would be to monitor changes in weight or in magnetization. Such methods, however, are less specific about the initial and final state of the catalyst than Mossbauer spectroscopy is. 

---