Vibrational study of ammonia adsorption on Pt/SiO2

Vibrational properties of surface species formed upon NH3 adsorption on Pt/SiO2, model system for the gas sensitive part in ammonia sensors based on field effect devices, have been investigated with in situ DRIFT spectroscopy. Experiments have been performed for a series of samples with different Pt loading at three temperatures, 50, 150 and 300 degreesC, and in the absence and presence of oxygen. In addition, electronic structure calculations and vibrational analysis have been performed within the density functional theory (DFT) for NH3 and NH2 species adsorbed on platinum and hydroxylated silica model systems. Observations from both DRIFT spectra and DFT calculations indicate that NH3 is more strongly bound to platinum than to silanol groups on the SiO2 support. Vibrational modes assigned to NH2 appeared in the DRIFT experiments, indicative of NH3 dissociation, an interpretation supported by the calculations. Exposure of O-2 was found to have minor effect on the vibrational spectrum at 50 degreesC. However, at 150 degreesC an increase of the vibration band assigned to the NH2 surface species was observed together with formation of gas phase N2O for samples with high platinum content. Thus, ammonia is oxidised over Pt at this temperature and oxygen is most likely facilitating ammonia dissociation.