Nanoparticle behaviour in changing environments: dynamic structural reorganisation in supported Pd nanoparticles during redox cycling

Supported nanoparticulate noble metals, such as palladium (Pd), are widely used as heterogeneous catalysts - perhaps the most well-known application being in the cleaning up of exhaust gas emissions. As oxygen is omnipresent within this and other important catalytic scenarios where Pd is used, the behaviour of oxidic and metallic nanosize Pd species is of great interest. The environment these catalysts work within often changes very rapidly (for instance in a car exhaust), hence understanding how these phases might form and interconvert in response to this variability is potentially very important indeed. Using dispersive EXAFS, a direct, time resolving (in this case 3-4 Hz) probe of local structure, we can investigate these processes in situ.

Experiments were carried out at beamline ID24.  Dispersive EXAFS data were recorded for a 1 wt% Pd catalyst during a cycling experiment between flows of 5%CO and 5%NO/He. Figure 1 presents 300 millisecond spectra at different stages in a single redox cycle. Analysis of such spectra leads to Figure 2, where the changes in Pd-Pd co-ordination number and the levels of oxidised Pd (fraction of Pd²⁺) formed during a single redox cycle are plotted for two oxidising environments.

Figure 1: Normalised dispersive EXAFS data from a 1 wt% Pd catalyst during a cycling experiment between flows of 5%CO and 5%NO/He (300 milliseconds each). A spectrum derived from the starting catalysts (nanoparticulate PdO) is given for comparison.

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Figure 2: Changes in Pd-Pd co-ordination number and the levels of oxidised Pd (fraction of Pd2+) formed during a single redox cycle for a 2wt% Pd/Al2O3 catalysts at 673K. Changing gas composition from 5%CO/He to 5%NO/He is represented in blue (open circles and a dashed line); whereas changing from 5%CO/He to 5%(5O2:1NO)/He is represented in red (filled circles and a solid line). The yellow shaded area highlights t < 3 seconds after the switch to the an oxidising environment wherein large changes in Pd-Pd coordination occur with no apparent concomitant formation of oxidised Pd (Pd2+).

Using this approach we can clearly distinguish events related to size and shape change from those due to Pd oxidation.  The co-ordination number (CN) is, in this case, indicative of the number of Pd atoms present in an average nanoparticle and its variation shows that supported Pd particles of up to 3 nm in diameter rapidly (within 3 seconds) and reversibly alter their size/shape distribution in response to their environment. What is further remarkable is that oxygen actually promotes this very significant structural change before any significant oxidation of the Pd (to Pd2+) is observed. Figure 3 summarises the results obtained.

Figure 3: Size/shape changes of the Pd nanoparticles under the influence of the gasous composition of their atmospheres.

Such rapid redispersion of metallic nanoparticles that proceeds via an apparently non oxidative route, and indeed prior to oxidation itself (shaded region of Figure 2), has not previously been demonstrated.

How these fundamental nanoscale rearrangements contribute to the overall chemistry mediated by such catalysts, and whether these new processes can in themselves be manipulated for practical benefit, are themes of continuing study.

Principal Publications and Authors
M.A. Newton (a), C. Belver-Coldeira (b), A. Martínez-Arias (b), M. Fernández-García(b),  Nature Materials, 6, 528-532 (2007); and Angewandte Chemie (2007), in press.
(a) ESRF
(b) Instituto de Catalisis y petroleoquimica, Madrid (Spain)