Uncovering the 3 D Structure of Combustion‐Synthesized Noble Metal‐Ceria Nanocatalysts

Abstract

With its unique redox properties, ceria is an oxide with a range of applications, including automotive catalytic converters, which consist of platinum‐group metal nanoparticles on ceria‐containing supports. In this work, the 3 D architecture of a ceria‐based material synthesized by the widely employed glycine‐nitrate solution combustion method is revealed for the first time. Together with N2 adsorption volumetry, scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM), STEM tomography provides a comprehensive picture of the multimodal porous network of a pre‐reduced Pt‐CeO2 catalyst, from the nanometer to the micrometer scale. This material consists of ceria nanocrystallites forming 3 D aggregates and puzzle‐like 2 D walls separating large roundish mesopores and macropores. The small voids between imperfectly assembled crystallites give rise to some microporosity. In addition, it is demonstrated that a significant proportion of platinum nanoparticles (3–4 nm) are not located at the ceria surface following the one‐step synthesis process, about half of them are buried within ceria. This result is valid for another metal (Rh) and another fuel (oxalyl dihydrazide), and has important implications for heterogeneous catalysis.

Restructuring effects of the chemical environment in metal nanocatalysis and single-atom catalysis

“…Two advantages of this type of instrument are the ultimate spatial resolution attainable, similar to the one in high-vacuum analysis [64], and the possible use of any type of specimen holder, allowing e.g. heating, cooling, biasing, or tomographic imaging [65,66].…”

In Situ Preparation of Pr1-xCaxMnO3 and La1-xSrxMnO3 Catalysts Surface for High-Resolution Environmental Transmission Electron Microscopy

“…In a dedicated ETEM instrument, a standard specimen chamber is replaced with a differently pumped environmental cell (E-cell) equipped with inlet and outlet lines [6]. The E-cell allows performing tomography [7], collecting high resolution images [8] and provides an easy access to use analytical techniques such as energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) [9,10], although the maximal pressure is limited to 20 mbar [11], or 100 mbar [12]. In its turn, dedicated gaseous holders [13] allow performing studies at pressures up to 4-5 bars [14], but field of view, spatial resolution, quality of images and use of analytical tools are limited because of silicon nitride membranes especially at low accelerating voltages [8].…”

Electron Microscopy Studies of Soft Nanomaterials

“…169−172 For example, the distribution of Pt NPs within a porous ceria nanocrystallite aggregate was characterized. 173 The 3D reconstruction revealed two types of pores existing in the ceria crystallites: micron-sized mesopores and nanosized micropores, contributing to their imperfect assembly. Furthermore, nearly half the 3−4 nm Pt NPs were found to be embedded within the ceria aggregate, revealing that the fraction of the Pt NPs exposed on the surface was lower than expected and a significant number of active sites for catalysis were lost.…”

“…The 3D reconstruction revealed two types of pores existing in the ceria crystallites: micron-sized mesopores and nanosized micropores, contributing to their imperfect assembly. Furthermore, nearly half the 3–4 nm Pt NPs were found to be embedded within the ceria aggregate, revealing that the fraction of the Pt NPs exposed on the surface was lower than expected and a significant number of active sites for catalysis were lost . Another example involves the leverage of HAADF-STEM tomography to reveal the size-dependent distribution of Au NPs on a TiO 2 dielectric matrix .…”

“…One notable example is the 3D reconstruction of Pd–Ru bimetallic NPs supported on mesoporous silica by Midgley et al, where HAADF-STEM tomography was first introduced. Since then, this technique has been applied to imaging the 3D structure of a variety of inorganic systems. − For example, the distribution of Pt NPs within a porous ceria nanocrystallite aggregate was characterized . The 3D reconstruction revealed two types of pores existing in the ceria crystallites: micron-sized mesopores and nanosized micropores, contributing to their imperfect assembly.…”