Surface Acidity and Basicity of La₂O₃, LaOCl, and LaCl₃ Characterized by IR Spectroscopy, TPD, and DFT Calculations

Adsorption sites of La₂O₃, LaOCl, and LaCl₃ 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., La₂O₃ < LaOCl < LaCl₃. 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 CO₂ adsorption modes. DFT calculations and IR spectra of CO₂ adsorbed on LaOCl suggest that CO₂ forms coupled bridged species. Proton affinity calculations were used to rank the basicity strength of surface O and Cl sites. The amount of CO₂ adsorbed on LaCl₃ was negligibly small, confirming the requirement of lattice O adsorption sites. IR spectra of CO₂ adsorbed on La₂O₃ at RT were similar to those of bulk La₂(CO₃)₃ and, accordingly, were assigned to the formation of polydentate and bulk carbonates. CO₂ evolution from La₂O₃ in TPD experiments closely matched the reported thermal stability of La₂(CO₃)₃.