Better understanding the structural dynamics of Pt/CeO2 catalysts
Résumé
Context and purpose
Pt/CeO2 catalysts are widely used in many redox reactions such as the oxidations of CO and hydrocarbons for automotive exhaust depollution and the water gas shift reaction. Their catalytic properties strongly depend on the coordination environment of Pt atoms. Pt can be dispersed during an oxidative treatment at moderate temperature while a reducing treatment triggers 3D particle formation. Even if such a structural dynamics has been extensively evidenced at the nanometer (crystallite) scale by transmission electron microscopies, its intra (surface, subsurface, bulk) and inter-crystallite Pt diffusion (grain, aggregate) length scale is poorly known.
In this work, the latter has been investigated by an original method using mechanical mixtures of Pt/CeO2 and CeO2 to study inter-crystallite Pt diffusion scale.
Material and methods
A 0.83 wt% Pt/CeO2 catalyst was prepared by wet impregnation using Pt(NH3)4(NO3)2 as a precursor and high surface area CeO2 as a support, followed by calcination in air at 500 °C. Different mechanical mixtures of this solid with pure CeO2 were prepared by sieving the powders between 10 and 50 µm and treating them under O2 flow at 500 °C (OX500), then under H2 flow at 500 °C (RED500). They were characterized before, during and after redox treatments by H2-TPR and spatial characterization techniques such as SEM-EDS, in situ microRaman mapping and XPS.
Main results
As expected from the difference of Pt loading, energy-dispersive X-ray spectroscopy analysis of 50%Pt/CeO2-50%CeO2 revealed, at the initial state, a bimodial distribution of the Pt concentration (Fig. 1A). It validated the choice of the granulometry and the mixture composition to follow Pt diffusion. After redox cycles (OX500/RED500/OX500 for 1 h each), both populations were still observed, revealing the absence of Pt diffusion (no homogenization) at the ceria grain/aggregate scale. This feature was confirmed for a longer treatment (OX500/RED500/OX500, for 10 h each, Fig. 1A), showing a thermodynamic or a kinetic limitation of Pt diffusion.
As SEM-EDS results, in situ microRaman mapping of 50%Pt/CeO2-50%CeO2 showed, upon redox treatments at 500 °C, a bimodal distribution of the intensity of the (Pt-O) band at the oxidized state (Fig. 1B) and the Ce3+ electronic Raman band at the reduced state. A statistical treatment confirmed that the spatial repartition of Pt/CeO2 and CeO2 did not change. However, the presence of peroxo species was systematically evidenced on bare CeO2 grains after reduction at 500 °C (map in Fig. 1C), which implies a slight reduction of CeO2.
Moreover, H2-TPR curves of different Pt/CeO2 and CeO2 mechanical mixtures presented two reduction peaks, corresponding (at increasing temperature) to Pt/CeO2 and CeO2 surface reduction. The latter is attributed to long-range hydrogen spillover, even if a slight (under the detection limit) Pt diffusion at the grain scale cannot be ruled out.
Major conclusions
This original methodology revealed that Pt diffusion within Pt/CeO2 catalysts does not occur at the grain scale or is very limited after redox treatments at 500 °C. However, a long-range hydrogen spillover is likely occurring from Pt/CeO2 to bare ceria grains, highlighting the complexity of structural dynamics.