Synchrotron EXAFS and XANES spectroscopy studies of transition aluminas doped with La and Cr for catalytic applications
Abstract Transition aluminas doped with Cr find widespread application in the dehydrogenation industry, while La-stabilized transition aluminas are used extensively for high temperature application as catalytic supports. In this work, a detailed synchrotron XAFS-spectroscopy studies were conducted to shed light upon the atomic mechanisms of catalysis and/or catalytic support stabilization. It has been demonstrated that in the samples of different transition aluminas doped with Cr, the atoms of chromium 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 row “gamma – chi – theta – eta-alumina” the change of properties (in particular, of the coloration of different samples) takes place because of dramatic reduction in the average size of Cr clusters and, possibly, their appearance on the Al2O3 surface. It has been also demonstrated that the substantial change in the local coordination of the La atoms in the samples of gamma-alumina doped with La, takes place only upon heating up to 1400°C, thereby making 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). It has been proved 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 amounts of this expensive 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 bulk of alumina crystal, could be a major reason of the increased thermal stability of gamma alumina doped with lanthanum.
- Research Organization:
- Idaho National Laboratory (INL), Idaho Falls, ID (United States)
- Sponsoring Organization:
- USDOE Office of Nuclear Energy (NE)
- Grant/Contract Number:
- AC07-05ID14517
- OSTI ID:
- 1358425
- Report Number(s):
- INL/JOU-15-34572; PII: 9737
- Journal Information:
- Applied Physics. A, Materials Science and Processing, Vol. 122, Issue 4; ISSN 0947-8396
- Publisher:
- SpringerCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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