Catalyst, Mossbauer effect

When I obtained my Masters Degree in experimental physics at the Free University in Amsterdam in 1978,1 was totally unaware that as interesting an area as catalysis, with so many challenges for the physicist, existed. I am particularly grateful to Adri van der Kraan and Nick Delgass who introduced me, via the Mossbauer Effect in iron catalysts, to the field of catalysis. My Ph.D. advisors at Delft, Adri van der Kraan, Jan van Loef and Vladimir Ponec (Leiden), together with Roel Prins from Eindhoven, stimulated and helped me to pursue a career in catalysis, now about seventeen years ago. 

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. 

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. 

In conclusion, Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the... 

From these results, it is concluded that, in a fully reduced catalyst, FeAl204 is not present furthermore, the aluminum inside the iron particle is present as a phase that does not contain iron (e.g., A1203), and this phase must be clustered as inclusions 3 nm in size. These inclusions may well account for the strain observed by Hosemann et al. From the Mossbauer effect investigation then, the process schematically shown in Fig. 17 was suggested for the reduction of a singly promoted iron synthetic ammonia catalyst. Finally, these inclusions and their associated strain fields provide another mechanism for textural promoting (131). 

The advantage of the technique is that the particle size may be determined with the sample in a controlled atmosphere and at a temperature different from 300 K, i.e., in situ particle size measurement, and measurement of changes in particle size may be possible. The problem, however, is that the quantitative relation between the Mossbauer parameters and particle size is rather complex and in some cases not theoretically available. Therefore, the application of the Mossbauer effect to particle size measurement is often facilitated through an experimental calibration of the Mossbauer parameters to particle size for the particular catalyst system of interest, i.e., the measurement of the parameters for a set of samples of known particle size as determined by other experimental methods. This point will become clearer below, as the effects of particle size on the Mossbauer parameters are discussed. 

Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the kinetics of bulk reactions. Mossbauer spectra of super-paramagnetic iron particles in applied magnetic fields can be used to determine particle sizes. In favorable cases, the technique also provides information on the structure of catalysts. The great advantage of Mossbauer spectroscopy is that its high-energy photons can visualize the insides of reactors in order to reveal information on catalysts under in-situ conditions. 

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. 

Mossbauer Effect Spectroscopy Studies (41). Another physical probe which has been used in the characterization of platinum-iridium catalysts is Mossbauer effect spectroscopy (3,4), 52). It can be applied to catalysts in which virtually all of the metal atoms are surface atoms. In Mossbauer effect spectroscopy one is concerned with a transition between a ground state and an excited state of a nucleus (53). 

In conducting Mossbauer effect spectroscopy experiments on platinum-iridium catalysts, one might incorporate the Mossbauer nuclides 195Pt and/or 193lr in the catalysts. However, experiments with these nuclides are more difficult because of short-lived sources and the requirement for measurements... 

The reaction of H2 and N2 to form ammonia is a reaction for which an enormous literature exists, dating to the beginning of the century. Recent reviews are those of Boudart (309) and Ertl (310). Industrial fused iron catalysts present iron particles of 25 nm or higher, and it has not been possible to prepare well-reduced small iron particles (1-10 nm) on conventional silica and alumina supports. Dumesic et al. (311a-c) were able to prepare iron crystallites of diameters from 1.5 to 30 nm by using MgO as the support. The state of the iron was monitored by Mossbauer-effect spectroscopy. The synthesis rates were measured from 300 to 405°C at atmospheric pressure the conversion was small enough so that the reverse reaction could be neglected. It was found that the reaction showed antipathetic structure sensitivity. 

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