A great attention is dedicated to the development of ceria-based catalysts designed at nanoscale due to the influence on the redox exchange capacity that can be achieved by tailoring the morphologies of ceria particles and their interaction with active metal/oxide phase. Here we want to show how the use of mechanical energy can strongly alter the interaction between different catalyst components, resulting in materials with unpredicted functionalities and morphologies. In one example we show that dry mechanical milling at high energy of ceria nanorods and nanocubes in the presence of ZrO2 originates small nanoparticles that does not resemble the original shape and that are constituted by a ceria core and decorated with very small ceria-zirconia nanoparticles showing an overall enhancement of low temperature redox and oxygen storage capacity. A combination of HAADF-STEM with EDX spectra clearly shows that Zr is located mainly at the periphery and CeO2 at the core of the particles. Also, a series of energy-filtered images (EFTEM) recorded using EELS, discloses that Zr is located forming a rim over the ceria particles. Redox behavior is investigated by H2-TPR and evidenced that ceria milled rods show higher hydrogen consumption compared to the other investigated materials. This behavior is confirmed by OSC at 250 °C in H2. The presence of small ceria-zirconia particles that decorate larger CeO2 particles is responsible for the enhancement of the redox and oxygen storage capacity of nanoshaped ceria; this is not observed if standard polycrystalline ceria particles are subjected to the same treatment. In a second example we report how the use of dry milling methods can be used to prepare robust bimetallic Pt-Pd/CeO2 methane oxidation catalysts which are not only stable during lean methane oxidation in presence of water, but also increase their initial activity during time-on-stream. The improvement compared to catalysts prepared by traditional methods is tentatively attributed to a peculiar nanoscale morphology, as revealed by HRTEM. The dynamics of these bimetallic architectures has been studied by operando AP-XPS and EXAFS/XANES to infer on the modification of the catalysts during reaction. Overall we show that the use of mechanical energy can profoundly alter the properties of ceria at nanoscale and the way ceria interacts with other catalyst component (noble metals/oxides). This can offer a powerful tool in the design of materials with unexpected properties.

Nanoscale morphology and improved activity of ceria-based catalysts prepared by mechanical methods

2019 Fall National Meeting

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