  The design of catalysts with desired chemical and thermal properties is
viewed as a grand challenge for scientists and engineers. For operation at high
temperatures, stability against structural transformations is a key
requirement. Although doping has been found to impede degradation, the lack of
atomistic understanding of the pertinent mechanism has hindered optimization.
For example, porous gamma-Al2O3, a widely used catalyst and catalytic support,
transforms to non-porous alpha-Al2O3 at ~1,100C. Doping with La raises the
transformation temperature to ~1,250C, but it has not been possible to
establish if La atoms enter the bulk, adsorb on surfaces as single atoms or
clusters, or form surface compounds. Here, we use direct imaging by
aberration-corrected Z-contrast scanning transmission electron microscopy
coupled with extended X-ray absorption fine structure and first-principles
calculations to demonstrate that, contrary to expectations, stabilization is
achieved by isolated La atoms adsorbed on the surface. Strong binding and
mutual repulsion of La atoms effectively pin the surface and inhibit both
sintering and the transformation to alpha-Al2O3. The results provide the first
guidelines for the choice of dopants to prevent thermal degradation of
catalysts and other porous materials.
