Heterogeneous catalysts are frequently used in the chemical industry and form the bedrock of environmental clean-up processes. The ease of separation of the solid phase catalyst from the gaseous or liquid reagents and products is the key advantage of these materials. However, such catalysts generally consist of a dispersion of metal particles supported on a disordered oxide surface with a high area. This combination of disorder and site dispersion obscures the fundamental chemistry of catalyst formation and operation.

EXAFS is one of a few techniques which can provide direct structural information on such materials under reaction conditions. The development of Energy Dispersive EXAFS on ID24 provides a way of acquiring analysable EXAFS data on sub-second timescales even on dilute materials (ca 3% metal) within a tubular reactor. The Laue monochromator allows high resolution spectra to be obtained over a substantial k range for the Rh K-edge. By combining in situ EXAFS with diffuse reflectance FTIR data, it has been shown that the complex [Rh2Cl2(CO)4] reacts cleanly with high area (100 m2/g) alumina to form the surface site [Rh(CO)2Cl(O-Al)]; multiple scattering analysis of the EXAFS data confirms the presence of the two linear carbonyl groups on each rhodium. The selectivity of this surface reaction provides the opportunity of describing the reactivity of the surface organometallic centre at a molecular level.

The optics on ID24 with the Laue monochromator yield a focal spot of less than 0.1 mm in size, and this was focussed on a quartz tube micro-reactor fitted with gas mass-flow controllers and on-line multiple ion monitoring by a quadrupole mass spectrometer. This has been used to follow the process of metal particle formation under reactive and inert gases. Under hydrogen and a temperature ramp (5°/min), a rapid transformation into small metal particles was apparent at 330 K. Sintering of these clusters occurs at above 373 K, eventually leading at 673 K to large fcc particles with high local order. For the first time the formation and sintering of metal particles from mononuclear rhodium centres has been observed and found to be more rapid than normally thought.

Since rhodium is one of the components in the automotive exhaust catalyst, the reaction of [Rh(CO)2Cl(O-Al)] with NO is of some interest. On a hydroxylated surface of Al2O3 there is a rapid reaction at room temperature to form a mononitrosyl species. A stack plot of the Rh K-edge EDE spectra obtained during this reaction is shown in Figure 29. Concomitantly, the mass spectral data shows that CO evolution is rapid and the flow of NO achieves its limiting value once the evolution of CO is complete. Again, multiple scattering EXAFS analysis was required, this showing a surface co-ordination centre of [Rh(NO)Cl(O-Al)2]. The Rh-N-O angle of ca 120° indicates a Rh-(NO)­ formulation. The second mass-time plot shows that the NO can be displaced by CO, albeit more slowly than the forward reaction. The in situ EDE data shows that the [Rh(CO)2Cl(O-Al)] centres are reformed. Hence, the molecular chemistry of such surface organometallic centres can be followed in real time.

Authors
D.G. Burnaby (a), A.J. Dent (b), J. Evans (a), T. Neisius (c), M.A. Newton (a).

(a) University of Southampton (UK)
(b) Daresbury Laboratory (UK)
(c) ESRF