Some of Al₂O₃ polytypes (the so-called transition aluminas) are widely used as catalytic support materials for transition metal catalysts (Pt, Pd, Rh, Cr, etc.) because of their high porosity and large surface area. We employ a unique combination of first-principles density-functional calculations, Z-contrast scanning transmission electron microscopy (Z-STEM), and extended X-ray absorption fine structure (EXAFS) measurements to investigate the interaction between adsorbed metal atoms and alumina surfaces. We find that degradation of Cr catalyst on g- alumina surface is related to the trapping of Cr atoms in subsurface sites where they become catalytically inactive. Some transition metal atoms (e.g., Mn) also get trapped, but the other catalytic elements (Pt, Pd, Rh, Mo) do not. The trapping of adsorbed transition metal atoms as well as their clustering at the alumina surface are defined by a delicate balance of several factors, the bulk point-defect distribution being one of the most crucial of them.¹ We show that La atoms do not exhibit a tendency to cluster at alumina surfaces or to diffuse into the bulk. The single La atoms effectively pin the surface, thus inhibiting both sintering and structural transformation of the porous catalyst. This explains why lanthanum is raising the temperature at which the transformation from a catalytic γ- alumina structure to a non-catalytic structure occurs.² These findings have strong implications for the preparation and durability of commercial catalysts. This work was supported in part by NSF and DOE.

¹S. N. Rashkeev et al. Phys. Rev. B 67, N 11, 115414 (2003).

²S. Wang, A. Y. Borisevich, S. N. Rashkeev et al. Nature Materials 3, N 3, 143-146 (2004).