Assignments of Bending Vibrations of Ammonia Adsorbed on Solid Surfaces

Bending vibrations in the infrared (IR) spectra of ammonia adsorbed on Lewis acidic metal oxides, i.e., Al2O3, ZrO2 and TiO2, and zeolite were analyzed with an aid of density functional theory (DFT) calculations. The results by DFT methods reveal the wavenumbers of the vibration modes (ν4 and ν2) of NH4 bonded to Brønsted acid site and the vibration modes (δs and δd) of NH3 species coordinated to a Lewis acidic metal center (M = Al, Zr or Ti). The wavenumbers calculated based on DFT were reasonably in agreement with the experimentally observed values. The estimation of wavenumbers suggests that the δs vibration of NH3 hydrogen-bonded is invisible on a zeolite, because it is hidden by an intense absorption due to skeletal vibration. On the other hand, multiple bands of asymmetric bending modes (δd and ν2) observed on a zeolite were assigned. A quantification method of Brønsted and Lewis acid sites, and hydrogen-bonded NH3 is provided based on the peak assignments.