STEM Analysis of Surface Phases in Nanocatalysts
Challenges for Heterogeneous Catalysis in the twenty first century include reducing the use of active elements (e.g. noble metals or lanthanides) while, at the same time, making catalysts more active, selective and stable. While a fine control of nanostructure is currently foreseen as a route to approach these goals, a rational design of the successful nanostructures calls necessarily for a detailed understanding of structure-function correlations. This is a major question in which modern STEM techniques are expected to make a large contribution.
From the materials point of view, the growth of thin, subnanometer-thick, layers of active components onto the surface of a carrier support offer an opportunity to reduce their loadings. Moreover, by adjusting the structure, texture and chemistry of the underlying carrier and using thermochemical treatments, opportunities to modulate the chemical and catalytic response of the layer arise.
In our lab we have used the approach described above to mimic the redox and catalytic behavior of catalysts based on ceria, CeO2; a widely used oxide in a variety of catalytic applications, most of them related to environmental protection. The key feature of materials prepared using this approach is the presence of just a few atomic layers of ceria oxides, Figure 1. These structures, which feature unconventional chemical properties, Figure 2, pose serious characterization challenges in terms of structure and composition, since most of the atoms they involve are located at surfaces and interfaces (with the carrier). In any case, they constitute a unique case of nanostructures whose exact nature can only be reached through the use of advanced STEM techniques.
In this contribution we will illustrate this novel topic, through the consideration of a series of ceria catalysts supported on MgO, ZrO2, YSZ and TiO2. With regards to the symposium, the correlations of functionality with structure determined by STEM will be highlighted.
Figure 1.- (left) HAADF image of a 6% mol. CeO2/MgO catalyst. Note the presence of an extended two layer (0,3 nm) structure on the (111) surface of a MgO support crystallite. The high intensity of the contrasts suggests the incorporation of CeO2 into this surface structure. This is confirmed by direct XEDS analysis on one of these structures (right).
Figure 2.- Water evolution (oxygen exchange capability) as a function of temperature from two catalysts: (top) a Rh/Ce2Zr2O8 catalyst and (bottom) a pyrochlore structured Ce2Zr2O8/YSZ catalyst. Structures at the right illustrate the characteristics of the two systems.
Note that the material in which the noble metal (Rh) nanoparticles have been substituted by nanosized patches of a ceria-zirconia mixed oxide with the appropriate structure (pyrochlore) depicts the same oxygen exchange properties (water evolution peaking at roughly 450 K).
Furthermore, the large amount of ceria involved in the catalyst above is largely reduced in the catalyst in which this oxide is present as nanostructured patches.