To follow the behaviour of the Pd component of this material in both time and space (axially along the catalyst bed), a single pass plug-flow reactor was used at BM31. Figure 124a schematically illustrates the experiment. The sample bed is heated to 353 K under the solvent flow while Pd K-edge XAS is continuously collected at 10 axial locations along the 5 mm-long bed, building up axial maps of the Pd speciation. Figure 124b shows typical Pd K-edge spectra that reveal two significant aspects of the behaviour of Pd occurring only in the presence of the base (K2CO3).
Pd K-edge XAS shows that the Pd, which initially resides in an oxidised form within the perovskite- type material, is subject to a segregative reduction to yield Pd0 nanoparticles at 353 K. However, since the magnitude of the Pd K-edge jump itself also decreases along with reduction, some Pd is also mobilised. As Figure 124c shows, this occurs to varying degrees within the catalyst bed. Towards the reactor inlet, Pd is most efficiently stripped, whereas at the outlet, the intensity of the edge jump grows slightly. This shows that re-deposition also occurs along
the axial dimension, alongside the processes that cause Pd solubilisation and transport. In the absence of the base, none of these changes in the nature of Pd are observed to occur.
Pd K-edge XAS, however, cannot elucidate what may be happening in the rest of the material. Therefore, ex-situ high-energy X-ray scattering on ID15A and fluorescence Fe K-edge XANES on BM28 was performed to investigate the other major components of the perovskite sample. Fe K-edge XANES (Figure 124e) shows that, along with the reduction and solubilisation of Pd, a significant proportion of Fe3+ is also reduced to Fe2+. The X-ray scattering data (Figure 124d), from which Rietveld refinement and pair distribution functions (PDF) may be derived, further reveal the very significant effects that the basified solvent has on the perovskite- type oxide. In the presence of aqueous ethanol, the pristine structure of the perovskite remains intact (Figure 125). However, in the presence of the base, this structure is largely destroyed. What remains is composed of a shell of predominantly Fe2O3, upon which the Pd that has not been solubilised remains in the form of
Fig. 124: a) A schematic description of the time- and space-resolved XAS experiment. b) Pd K-edge XAS obtained during heating of the sample in basified aqueous ethanol. c) Behaviour of the Pd K-edge jump as a function of axial map number and temperature for different positions within the sample bed. d) HXRD patterns derived from the fresh sample (red) and post-reaction with the basified solvent (black). e) Fe K-edge XANES for the fresh sample (black) and post-reaction with the basified solvent (red).