Ammonia adsorption and decomposition on silica supported Rh nanoparticles observed by in situ attenuated total reflection infrared spectroscopy

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) is applied to study NH3, adsorbed from the gas phase, and its decomposition products, i.e. NHx species, on Rh nanoparticles, produced by spincoating from a RhCl3 solution in water followed by reduction. A silicon ATR crystal with a hydroxilated SiO2 layer acts as the support for the nanoparticles. Upon exposure to NH3 in the vacuum chamber, NH3 adsorbed to both silica and Rh is detected (sensitivity ∼5 × 10−5 absorbance units). Interaction of the NH3 with the silica OH groups is observed around ∼2840 cm−1 in combination with peaks showing the disappearance of unperturbed OH vibrations between 3500 and 3700 cm−1. In addition, NH bend vibrations at 1634 cm−1 and NH stretch vibrations at 3065 and 3197 cm−1 are observed for substrate temperatures between 20 and 100 °C. The latter two correspond to NH on Rh, as verified with a sample without Rh, and probably correspond to undecomposed NH3. Moreover, they remain after evacuation, suggesting strongly bound species. For a substrate temperature of 75 and 100 °C, additional NH stretch peaks at 3354 and 3283 cm−1 are observed, possibly due to NH2 intermediates, indicating NH3 decomposition. It is shown that ATR-FTIR can contribute to the sensitive detection of adsorption and decomposition of gaseous species on realistic planar model catalysts.