Synchrotron EXAFS and XANES spectroscopy studies of transition aluminas doped with La and Cr for catalytic applications

Transition aluminas doped with Cr find widespread application in the dehydrogenation catalysis industry, while La-stabilized transition aluminas are used extensively for high-temperature application as catalytic supports (Wefers and Misra in Oxides and hydroxides of aluminum, Alcoa Laboratories, Pittsburgh, 1987). In this work, detailed synchrotron XAFS spectroscopy studies were conducted to shed light upon the atomic mechanisms of surface and subsurface reconstructions and/or catalytic support stabilization of doped aluminas. It was demonstrated that in four transition aluminas doped with Cr, it is the atoms which are mostly in the state of oxidation Cr³⁺ and enter nanoparticles of Cr-bearing phases (Cr₂O₃ in the case of gamma- and chi-alumina). In the transition series aluminas: “gamma- chi- theta- eta-alumina,” the change of properties (in particular, the dramatic increase in dehydrogenation catalytic activity and catalyst longevity and the coloration of samples) takes place because of the reduction in the average size of Cr clusters and their appearance on the Al₂O₃ surface, probably responsible for change in catalytic activity. It was demonstrated that in the samples of gamma-alumina doped with La any substantial change in the local coordination of the La atoms takes place only upon heating up to 1400 °C. This makes the La-stabilized gamma-alumina a perfect catalytic support for the numerous applications, e.g., catalytic three-way conversion of automobile exhaust gases. This change manifested itself in the form of increased La–O bond lengths and the La coordination number (from 8 to 12). Furthermore, it was demonstrated that the local environment of La in this new La-bearing phase cannot be explained in terms of the LaAlO₃ formation. The absence of the La atoms in the second coordination sphere favors monoatomic distribution of La atoms on grain boundaries, proving that only very small amount of this rare earth material is required to achieve full stabilization. It is inferred that the tendency of La atoms to get surrounded by oxygen atoms, and also the impossibility of going into the alumina bulk, could be a major reason of the increased thermal stability of gamma-alumina doped with lanthanum.