USE OF PYRROLE AS AN IR SPECTROSCOPIC MOLECULAR PROBE IN A SURFACE BASICITY STUDY OF METAL-OXIDES

Pyrrole has been adsorbed on various metal oxides: reduced or unreduce d ceria, alumina dehydroxylated to varying extents and an NaX zeolite. Pyrrole adsorption was found to be either non-dissociative or dissoci ative (on the more basic adsorption sites), the pyrrolate anion being formed in the latter case. When bound to surface hydroxy groups throug h an easily polarizable H bond, the pyrrolate anion was thought possib ly to be non-planar; complex progressions of IR bands were then observ ed involving the combination of CH-stretching modes with a ring-deform ation vibration. When pyrrole was non-dissociatively adsorbed through an NH ... O bridge with O2- moderately basic surface centres, the shif t of the NH-stretching frequency reflected the O2- basicity. Interacti on with surface hydroxy groups led to NH ...(OH) hydrogen-bridged spec ies, which have been found to be either cyclic or linear according to whether the hydroxy groups are monodentate or bidentate, respectively.