Mossbauer spectroscopy catalysis

Although relatively little used in catalysis, Mossbauer spectroscopy has given important information on the state of iron and cobalt in Fischer-Tropsch and hydrodesulphurization catalysts. Mossbauer spectroscopy provides the oxidation state, the magnetic field and the lattice symmetry of a number of elements such as iron, tin, iridium, and cobalt, and can be applied in situ. We will first describe the theory behind the Mossbauer effect and explain how a nuclear technique gives information on the state of atoms. 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

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 is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

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... 

W. M. H. Sachtler and R. A. van Santen Mossbauer Spectroscopy Applications to Heterogeneous Catalysis James A. Dumesic and Henrik Tops0e Compensation Effect in Heterogeneous Catalysis... 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

Mossbauer Spectroscopy Applications to Heterogeneous Catalysis James A. Dumesic and Henrik... 

There has been to some degree the belief that Mossbauer spectroscopy, although in principle an ideal technique for catalyst studies, for practical purposes can only be applied to problems in catalysis if the catalyst contains either iron or tin. Therefore, one of the main purposes of this review is to show how Mossbauer spectroscopy can be directly extended to many additional Mossbauer atoms or isotopes (such as antimony, europium, nickel, ruthenium, gold, and tungsten) and, perhaps more importantly, how the technique can be extended to obtain information about systems that do not contain a Mossbauer atom. ... 

In this article, various chemical phenomena that are pertinent to catalysis and can be studied through Mossbauer spectroscopy will be discussed. To do this, the physical basis for Mossbauer spectroscopy and the resulting Mossbauer parameters will first be briefly introduced (Section I, A, 4). 

In Section I, C the different Mossbauer parameters were individually discussed with reference to possible catalytic applications. The purpose of that discussion was to provide a physical feeling for the parameters and an appreciation of their possible uses in catalysis. In general, however, for the study of a particular catalytic phenomenon the decision whether also to employ Mossbauer spectroscopy is not based only on the consideration of a single Mossbauer parameter. Thus, in the next sections we discuss, based on a number of examples, the manner in which various catalytic phenomena can be investigated through the systematic employment of the Mossbauer parameters. 

Mossbauer spectroscopy is a technique that spans many different disciplines. We hope that this has been reflected in the present paper, in which we have discussed catalytic problems in terms of the physical principles that form the basis for Mossbauer spectroscopy. Certainly, this technique has great potential as a tool in catalytic research. However, in order to take full advantage of this potential, strong ties between researchers in catalysis on one hand and physicists and theoretical chemists on the other hand are necessary, and this will undoubtedly lead to further Advances in Catalysis. 

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. 

Adsorption (Chemical Engineering) Batch Processing Catalysis, Homogeneous Catalysis, Industrial Electrochemistry Infrared Spectroscopy Mossbauer Spectroscopy Nuclear Magnetic Resonance Raman Spectroscopy Scanning Electron Microscopy Surface Chemistry... 

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 example is typical for many applications of Mossbauer spectroscopy in catalysis a catalyst undergoes a certain treatment next its Mossbauer spectrum... 

Application of the Mossbauer effect, which is essentially a bulk phenomenon, to the study of surfaces has received significant attention in recent years. The usefulness of this technique lies in its ability to determine the electronic environment and symmetry of the surface nucleus, and it offers a method of investigation that is clearly complementary to other physical methods for the characterization of solid surfaces. Mossbauer spectroscopy has the attractive advantage that it may be used at a variety of pressures and can be applied to the study of heterogeneous catalysis and adsorption processes to probe the nature of the solid surface and its electronic modification when holding adsorbed species. 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

The nature and breadth of the physical techniques used to investigate solid catalysts continue to increase rapidly in complexity. This statement pertains specifically to Mossbauer spectroscopy, which was applied to the characterization of solid catalysts as early as 1971 (7). A retrospective analysis of the use of Mossbauer spectroscopy in catalysis showed that it has consistently accounted for 3-10% of the communications presented at the International Congresses on Catalysis (ICQ (7%t at the ICC in Paris in 2004). Such continuity over the years reflects the high value of this technique in catalyst characterization. 

The theoretical basis of Mossbauer spectroscopy as well as its applications to catalyst characterization was reviewed in Advances in Catalysis in 1989 (2). This thorough article summarizes the physical basis of the technique and significant contributions to the characterization of solid catalysts. Since 1989, Mossbauer spectroscopy has not underwent major developments, and its applications to catalysis have been largely limited to catalysts that were not in reactive atmospheres, notwithstanding the impressive advances that have been made with other techniques in characterizing catalysts under working conditions. 

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