Atomic Scale Studies of Heterogeneous Catalysts


Nigel D. Browning Department of Chemical Engineering and Materials Science, University of California--Davis

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In this paper, we will describe an analytical atomic-resolution scanning transmission electron microscopy (STEM) study of supported nanoscale systems. The combination of high-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) provides an analytical tool with unprecedented chemical and spatial sensitivity that is vital for studying interfaces in heterogeneous catalyst systems. We study three examples of heterogeneous catalyst systems: Pt/SiO2, Pd/Al2O3, and Cu/Al2O3 and the in situ reduction of PdO. In Pt/SiO2, the presence of a few monolayers of platinum oxide and changes in the chemistry of the SiO2 support are characterized as a function of the catalyst preparation conditions. In Pd/Al2O3, electron transfer from the alumina toward the Pd particles appears to be dependent on the metal particle size that is observed. The Cu/Al2O3, reduced at various temperatures, exhibits increasing oxidation of the Cu particles upon higher temperature reduction. The in situ reduction of PdO upon heating in the microscope column confirms that the reduction to metallic Pd occurs through a shellwise mechanism, and the effects of beam damage are studied briefly on this system.

Heterogeneous metal catalysts are routinely used in a variety of chemical processes, including dehydrogenation, naphtha reforming, oxidation and automotive exhaust catalysis (e.g., see Ref. ). Although they have been successfully used for many years, a fundamental understanding of the mechanism behind their activity and selectivity remains elusive. This is primarily because the bulk materials typically contain a wide range of metallic cluster sizes, and the majority of experimental techniques, such as infrared absorption spectroscopy extended X-ray absorption fine structure (EXAFS) do not have the required combination of spatial resolution (to see individual clusters) and chemical sensitivity (to see changes in composition and bonding). Although accurate information on the electronic structure can be obtained from these techniques, they all lack the ability to define the spatial location from which the information is obtained. The ability to characterize the structure, composition, and properties of individual clusters represents a key advance in the development of a fundamental understanding of existing heterogeneous catalysts and the future engineering of new and improved catalyst systems, as the chemical activity/selectivity in many systems is known to be intimately related to the cluster size.

Since it has become increasingly clear that in many cases the core phenomena occur at interfaces between the metal clusters and the support, the traditional method of investigating interfaces, transmission electron microscopy (TEM), is not sensitive to small local changes in composition and electronic structure. There is therefore an “information gap” between what can be learned from spectroscopy and what can be obtained from microscopy. The combination of Z-contrast imaging and EELS can potentially fill this information gap. Here, we describe the combination of Z-contrast imaging and EELS in the STEM that permits the characterization of individual nanoclusters in heterogeneous catalyst systems on the atomic scale.

The Z-contrast technique routinely provides atomic resolution images of the interface between the support and the metal cluster. Using this image to position the probe, one can acquire an energy loss spectrum from any location in and around the interface, allowing local changes in electronic structure to be correlated directly with the size and composition of the metal cluster. If this analysis is performed on catalysts whose preparation and reaction history is known, then this characterization tool can be used, in conjunction with the surface techniques mentioned above, to derive a fundamental understanding of the mechanism behind activity/selectivity in heterogeneous catalysts.

The metal–support interaction (MSI) is studied in three different systems, Pt/SiO2, Pd/Al2O3, and Cu/Al2O3. There are two reasons to choose these three systems. One is that they are all important catalysts used in environmental catalytic technologies. Another is that there are still controversies on some main issues of these catalysts. For example, the activity and selectivity of these Pd catalysts are strongly affected by the support material used. Hence, the interaction with the support can alter the electronic properties of palladium. Although a strong metal support interaction (SMSI) was not seen, it was suggested that the MSI originates from the formation of Pd–Al alloys at the metal–support interface.

Furthermore, the reduction behavior of PdO is investigated in situ by EELS to clarify its reduction mechanism. In addition, it is treated as a model system to study the beam effect on the metal oxides in our experiments.