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.