Surface Acidity and Basicity of La_2O_2,LaOCl,and LaCl_3 Characterized by IR Spectroscopy,TPD,and DFT Calculations

摘要

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 CO2.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 Br0nsted 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.