Surface acidity and basicity of La 2O 3, LaOCl, and LaCl 3 characterized by IR spectroscopy, TPD, and DFT calculations

Olga V. Manoilova, Simon G. Podkolzin, Balarishna Tope, Johannes Lercher, Eric E. Stangland, Jean Michel Goupil, Bert M. Weckhuysen

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Abstract

Adsorption sites of La 2O 3, LaOCl, and LaCl 3 catalysts were characterized with probe molecules using infrared spectroscopy, temperature-programmed desorption (TPD), and density-functional theory (DFT) calculations. Surface acid sites were probed with CO, pyridine, and 2,6-dimethylpyridine (DMP), and basic sites were probed with CO 2. Shifts of the CO vibrational frequency at low surface coverage at 77 K suggest that the strength of Lewis acid sites increases with the concentration of Cl in the material; i.e., La 2O 3 < LaOCl < LaCl 3. DFT estimates for CO adsorption energies and LUMO energies were consistent with this ranking. On the basis of a downward shift of the surface OH stretching bands interacting with CO, pyridine, and DMP spectra at room temperature (RT) and TPD results, and confirmed by DFT calculations, the strength of Brønsted acid sites was concluded to increase in the same order. Additional DFT calculations with a frequency analysis were used to elucidate CO 2 adsorption modes. DFT calculations and IR spectra of CO 2 adsorbed on LaOCl suggest that CO 2 forms coupled bridged species. Proton affinity calculations were used to rank the basicity strength of surface O and Cl sites. The amount of CO 2 adsorbed on LaCl 3 was negligibly small, confirming the requirement of lattice O adsorption sites. IR spectra of CO 2 adsorbed on La 2O 3 at RT were similar to those of bulk La 2(CO 3) 3 and, accordingly, were assigned to the formation of polydentate and bulk carbonates. CO 2 evolution from La 2O 3 in TPD experiments closely matched the reported thermal stability of La 2(CO 3) 3.

Original languageEnglish
Pages (from-to)15770-15781
Number of pages12
JournalJournal of Physical Chemistry B
Volume108
Issue number40
DOIs
StatePublished - 7 Oct 2004

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