TY - JOUR
T1 - Surface acidity and basicity of La 2O 3, LaOCl, and LaCl 3 characterized by IR spectroscopy, TPD, and DFT calculations
AU - Manoilova, Olga V.
AU - Podkolzin, Simon G.
AU - Tope, Balarishna
AU - Lercher, Johannes
AU - Stangland, Eric E.
AU - Goupil, Jean Michel
AU - Weckhuysen, Bert M.
PY - 2004/10/7
Y1 - 2004/10/7
N2 - 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.
AB - 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.
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U2 - 10.1021/jp040311m
DO - 10.1021/jp040311m
M3 - Article
AN - SCOPUS:6044274485
SN - 1520-6106
VL - 108
SP - 15770
EP - 15781
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
IS - 40
ER -