Relative activity of La 2O 3, LaOCl, and LaCl 3 in reaction with CCl 4 studied with infrared spectroscopy and density functional theory calculations

Simon G. Podkolzin, Olga V. Manoilova, Bert M. Weckhuysen

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Abstract

Relative activity of La 2O 3, LaOCl, and LaCl 3 in the destructive adsorption of CCl 4 to CO 2 was studied with density-functional theory calculations and temperature-programmed reaction experiments monitored with IR spectroscopy. Integral absorbance of the IR peak for phosgene, which is a reaction intermediate, was obtained as a function of temperature, and initial reaction temperatures were compared for different sample amounts of La 2O 3 and LaOCl. The initial reaction temperatures of about 390 K for La 2O 3 and 365 K for LaOCl were practically independent of the tested sample weights, and the lower temperature for LaOCl was attributed to a higher activity of surface sites on this material. Calculations suggest that CCl 4 decomposition proceeds through a stepwise Cl donation from CCl 4 to the surface and that the overall rate is controlled by the first step: CCl 4 splitting into a Cl anion and CCl 3 cation over an acid-base pair of surface sites. A lanthanum acid site in the pair initiates the split by interacting with one of the chlorine atoms in CCl 4, and an oxygen base site stabilizes the remaining CCl 3 fragment. Transition state estimates suggest that the relative activity of surface sites can be ranked in the following order: LaOCl > LaCl 3 with a partially dechlorinated surface > La 2O 3. Surface Lewis acidity and basicity of these materials are summarized in terms of the vibrational frequency for adsorbed CO, energy of the lowest unoccupied molecular orbital, and proton affinity. Higher activity of LaOCl is attributed to the higher acidity of the lanthanum site, the higher basicity of the oxygen site, and the geometry of the acid-base pair of sites that allows them to interact with CCl 4 simultaneously.

Original languageEnglish
Pages (from-to)11634-11642
Number of pages9
JournalJournal of Physical Chemistry B
Volume109
Issue number23
DOIs
StatePublished - 16 Jun 2005

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