FTIR spectroscopic characterization of the adsorption and desorption of ammonia on MgO surfaces

In the present paper an attempt is made to disentangle the complex IR spectra obtained in the course of the adsorption of NH₃ on high surface area MgO. For this purpose two simplifications were introduced in comparison to previous studies: (a) Only uniformly hydroxylated MgO surfaces (degassing temperature: 200 °C) and completely dehydroxylated MgO surfaces (degassing temperature: 1000 °C) were used as adsorbent, (b) The equilibrium pressure of the adsorbate NH₃ was generally maintained below 200 Pa in all adsorption and desorption experiments. Under these conditions the position and the pressure dependence of the resulting bands may consistently be interpreted in terms of characteristic surface species. Only the completely dehydroxylated MgO surface gives rise to a heterolytic dissociation into neighbouring NH₂ and OH groups. In addition, NH₃ is physically adsorbed on less reactive sites via diverse types of hydrogen bonds. Less well defined hydroxylation states give rise to considerably more complicated spectra. In a first approximation they may be constructed by linearly combining two spectral patterns, one related to a uniformly hydroxylated and the other to a dehydroxylated surface. NH₃ pressures around and beyond 1 kPa give rise to significant changes of band positions and band shapes compared to those observed below 200 Pa. The interpretation of these effects which have previously been ignored must necessarily rely on more complicated models.