EXAFS in the Study of Catalysts

BY J. EVANS

1. Introduction

EXAFS (Extended X-Ray Absorption Spectroscopy) offers a means of deriving interatomic distances and coordination numbers about a chosen element in either ordered or totally amorphous media. As such, it has very considerable applicability to the field of catalysis, particularly by metals. Many operating catalysts are either disordered solids or in solution and obtaining structural information on them by diffraction techniques is difficult. Without any clear structural model for the metal site then any account of mechanism or of the role of promoters or poisons can at best be hazy. Often research on model systems is performed on ordered arrays (either bulk crystalline solids or single-crystal surfaces) to reduce the complexity of the problem and to allow the use of single crystal diffraction (X-ray and neutron on bulk solids, Low Energy Electron Diffraction, LEED, on the surface of a gas-solid interface) . So because EXAFS can be applied to all of these types of materials, it can act as a bridge technique correlating information between model and "real" catalysts.

EXAFS may now be a vaunted acronym, but it represents only part of the information available in X-ray absorption spectroscopy, albeit the most understood part. Since this is the first article in this Specialist Periodical Review series on the technique, the first part of the chapter will be devoted to an introduction into the basis, methodology, information content of "mainstream" X-ray absorption spectroscopy. Then examples will be presented to illustrate its application to research in catalysis. Finally, some relatively new techniques will be described which seem to offer very substantial promise for catalysts investigations. Earlier articles by Joyner, Cox and Pettifer on the application of EXAFS to catalyst characterisation have been published in the monograph of Thomas and Lambert.

2. X-Ray Absorption Spectroscopy

2.1 X-Ray absorption spectra. – The absorption coefficient of a sample generally decreases as the frequency of X-radiation increases until there is a sharp rise in this value as the energy of an absorption edge of an element in the sample is reached. An example of this is shown in Figure 1 viz. the L (III)-edge spectrum of osmium powder.

This increase in absorption corresponds to the photoejection of a core electron. For EXAFS purposes generally the most convenient absorption edge is the K-edge, due to the ejection of a Is electron. A list of absorption edge energies and wavelengths of some elements of interest in catalysis are presented in Table 1. These should be related to the mass absorption coefficients for five representative matrix elements given in Table 2. Nitrogen, aluminium, nickel, silver and platinum are chosen to estimate the degree of absorption due to the atmosphere (or solvent.), an oxide support and metals in the 3 transition series respectively.

The first three elements listed in Table 1 (C, N and O) are best studied under UHV conditions. The absorption due to all other media is very high and contamination problems will also be severe. Light attenuation due to absorption by the atmosphere or window materials is also a problem in the soft X-ray region. This includes the K-absorpti on edges of the second row elements (Si to Cl) which are relevant to many catalyst, support, materials, or, in the case of chlorine, ate present in many preparations. Nevertheless, useful experiments can be performed on some systems in this wavelength band. Care must be taken in the design of the experiment though to avoid interference due to edges of heavier elements in the system (e.g, the near coincidence of the Cl K and Ru L (III) edges).

However, for the K-edges of virtually all the 3d and 4d metals and the L(III) edges of the 5d transition elements media absorption is a relatively minor problem. Measurements can be made under chosen atmospheric environments in self-contained reactors with suitable window materials (normally beryllium), and also in solution. For the harder edges (> 12keV), the absorption of a catalyst support also becomes a minor problem.

So X-ray absorption spectroscopy is applicable to a wide range of catalytic systems. Observing the edges due to common catalytic feedstocks (generally organics) is applicable only to surface science experiments. Some promoters and poisons (P, S and Cl) may be investigated in well chosen systems, but the entire transition element block is observable for single-crystal, metal film and supported catalyst samples.

The spectra themselves contain varied features viz. the energy of the absorption edge, some sharp peaks at or near the edge itself, and finally the broader, weaker oscillatory absorption changes which may extend for several hundred eV to the high energy side of the edge (EXAFS) . All contain different information which will be considered in turn.

2.1.1. Edge Positions. – There are many possible empirical prescriptions for the definition of the edge position within the experimental spectrum eg, the onset of the edge, the point of steepest slope, or the energy at half-height. The complex structure that may be associated with the edge makes an absolute statement impossible, but the point of steepest slope is clearly evident in the first derivative and is probably the most widely used definition. In principle the edge position might be related to the effective atomic number at the metal centre, and thus be correlated with oxidation state, as for the chemical shifts of x.p.s. spectra. Such correlations have been observed for early transition elements but are absent in, for example, some cobalt complexes and in the L(III) edge data of lead compounds. Changes in the coordination number, symmetry and the metal-ligand bonding can have a significant effect on the apparent edge position which can make it an unreliable indicator of oxidation state without corroborating evidence.

The relationship between the empirically determined edge position to the energy of the orbitals at the metal centre is also often unclear. While the absorption edge position is sometimes considered to represent the energy of the vacuum level (the position of Eo which is the onset of the continuum) , this seems by no means general. In cases where the absorption edge energy can be compared to the ionisation potential as measured by x.p.s., then the edge energy may be approximately 10 eV to lower energy; this has been observed from the carbon K-edge in ethene and ethyne. But in contrast to this, calculations on the [MO4] 2- (M = Cr and Mo) ions have indicated that the onset of the continuum is at the base of the metal K-absorption edges, and therefore at a lower energy than the normal definition of the edge position. Clearly care must be taken in considerations of absorption edge positions, but in some cases they may provide an indication of oxidation...