2021 Annual Meeting

(206c) Facet Effects Define Structure-Activity Relationships of Ultra-Low Loading Pt/CeO2 Catalysts with Atomic Dispersion

Authors

Song, B. - Presenter, University of Florida
Hagelin Weaver, H., University of Florida
A well-defined surface structure is key to exploring structure-activity relationships in heterogeneous catalysis. By combining single atom deposition and metal oxide nanoshape supports, well-defined Pt/CeO2 catalysts with ultra-low loading were prepared using a modified Atomic Layer Deposition (ALD) technique to investigate surface facet effects of ceria on single atoms Pt under reduction conditions. Three CeO2 shape supports, including octahedra, rods, cubes, were synthesized by a surfactant-free hydrothermal method [1]. The benefit of using CeO2 nanoshapes is that the surface facets of each shape are well defined, as the CeO2 octahedra have (111) surface terminations, the CeO2 cubes expose (100) surface facets, while the CeO2 rods have (100) and (110) surface facets. Therefore, this allows us to not only determine the influence of the support oxide surface structure on the active metal, but also facilitate identifying structure-activity relationships of reactions under investigation.

By tuning the ALD deposition conditions, such as low deposition temperature (120 °C) with a short dose time (5s), we can control the Pt loading on the CeO2 nanoshapes down to a few ppm [2]. At these ultra-low loadings, the Pt/CeO2 catalysts after calcination in air at 350 °C have only isolated Pt ions dispersed on CeO2 surface as confirmed by scanning transmission electron microscopy (STEM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It is shown that mild reduction temperature 150 °C reduces initial Pt ions and enables isolated Pt atoms to stay intact on the three different ceria facets. In contrast, at a reduction temperature of 350 °C, the Pt-ceria interactions are distinctively dependent on the specific ceria surface facet, and lead to three types of Pt ensembles, including Pt single atoms on ceria (111) facet, flat Pt clusters on oxygen vacancy rich (110) facets and Pt nanoparticles on (100) facet.

The Pt flat cluster structures which formed on the ceria rod support show significant activity enhancement in the CO oxidation reaction even at the ppm level Pt loading. Due to the ultra-low Pt loading, the turnover frequency of the Rod-5.5ppm catalysts in the current study is orders of magnitude higher than those commonly reported in the literature [3-5]. In situ characterization using (DRIFTS) demonstrates that the electron-deficient Pt species on the ceria octahedra result in stronger CO bonding, and this together with very limited reverse oxygen spillover from the CeO2(111) surface to Pt yield a low activity in the CO oxidation reaction. The highly reducible CeO2(100) surface of the cubes instead triggers Pt agglomeration into nanoparticles which leads to lower Pt atomic efficiency and poor stability.

This work demonstrates that by utilizing ultra-low loadings of precious metals together with well-defined oxide supports atomic level information on the influence of support structure on heterogeneous catalysts can effectively be probed and this offers guidance for future catalyst design and synthesis.

Reference List

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