Introduction by J. Evans (University of Southampton, UK)

Most chemistry is performed in disordered media, such as in solutions in reaction flasks, within cells and other biological environments and on the surfaces of solid particles, be they in interstellar space, in the stratosphere or within a reactor in a petrochemical process. The last of these examples is effected by a heterogeneous catalyst, with the mediating agent localised on the surface of a solid framework, and the reagents and the resulting products in a second phase (gaseous or liquid). Over 80% of all chemicals require at least one catalytic step in their production (providing a revenue of 1000 ­ 10000 billion euros), and this proportion is increasing with the drive towards atom-efficient and "green" chemical processes.

Consequently, understanding the relationships between structure, catalytic activity and selectivity has substantial value beyond its scientific merit. When viewed from the active site, a catalytic reaction can be considered as a cycle of steps (with side reactions) involving transformations between a series of intermediates. The most long-lived of these species is the resting state of the catalyst, but a thorough understanding of the system encompasses all these transformations. All effective catalysts turn over on a sub-second timescale, and thus the structure at the active site will be modified in a split second.

X-ray Absorption Fine Structure (XAFS) provides structural information about the local order of an element in any phase, and thus is the appropriate technique for probing active sites in metal catalysts. In its normal scanning mode, the acquisition time is generally too long to track the rapid processes involved in active catalysts. On ID24, a bent crystal monochromator is used to extract a span of X-radiation sufficient to obtain an XAFS spectrum with a multi-element detector sampling the steps in the spectrum simultaneously. Currently the CCD detector can operate with 150 µs as the fastest time resolution. It is thus feasible to track both rapid changes of the resting state under operating conditions and also to monitor transient steps.

This report includes two applications on ID24 involving in situ characterisation of alumina-supported catalysts. In one, the X-ray Absorption Near Edge Structure (XANES) is used to probe the oxidation state of the copper sites during the oxychlorination of ethane, whilst the other utilises the Extended X-ray Absorption Fine Structure (EXAFS) to probe the coordination site of rhodium during the reduction of nitric oxide by hydrogen. In both cases, the nature of the metal site is shown to respond rapidly to changes in reaction conditions (temperature and gas composition). Both of these experiments used a continuous flow of gas, and thus will probe changes in the resting state of the metal. In order to probe transients, the system has to be rapidly perturbed, for example by pulsed or modulated gas flow, and these are the basis of future experiments. Transient processes have been successfully studied already on ID24 in the liquid phase, using stopped flow and electrochemical techniques. Characterising transients in chemical reactions is an exciting prospect for the future of ID24.