STRUCTURE OF MATERIALS

136 ESRF

homogenously grown metallic particles of 100-150 µm size were dispersed throughout the bulk of perovskite oxides (Figure 118).

The high dispersion achieved through this approach yields nanoparticles within nanoscale proximity to each other, inducing strain in both the metal and host lattices. In turn, this greatly enhances oxygen exchange within the nanocomposite, seamlessly connecting even the deepest embedded particles to the gas phase environment [2]. Finally, this method produces materials with high oxygen capacity, since the newly formed structures are essentially composites with high metal loadings that can withstand the reversible incorporation of oxygen within the materials. This, coupled with surface active sites, has enabled the bypassing of conventional degradation and poisoning mechanisms in order to tackle challenging catalytic transformations such as CH4 conversion to syngas [3].

Beamline ID22 was used to reveal the fundamental key differences of the particles exsolved on the surface and in the bulk in terms of size, population, unit cell parameter and strain. The high resolution of synchrotron data permitted the operando detection of very small changes in the unit cell of the perovskite (Figure 119). This, coupled with advanced Rietveld refinement of the data, allowed the identification of the expansive/compressive strain as well as the repeated change of the system between the reduced and oxidised phase under reaction stream.

In summary, this new class of materials displays a very large oxygen storage capacity (about one order of magnitude higher than conventional perovskites), important in fields such as chemical looping, conversion of thermochemical solar to fuels, or three-way catalysts, and a reversibility that unlocks new ways of stabilising nanocomposites for redox cycling applications.

Fig. 119: Operando mechanistic insight into CH4 conversion with endo-/exo- nanoparticles. a) 2D plot of the XRD pattern of the material in operando while cycling the material between redox feeds at 650°C.

b) CH4, O2, H2, CO, CO2 gas composition at the outlet of the reactor during chemical looping methane reforming experiments and rABO3 microstrain during cycling calculated from Rietveld refinement. c) Ni and NiO content and

perovskite unit cell parameter calculated by Rietveld refinement and plotted as a function of time. d) Schematic of methane conversion mechanism.