Use of pyrrole as an IR spectroscopic molecular probe in a surface basicity study of metal oxides

Abstract

Pyrrole has been adsorbed on various metal oxides: reduced or unreduced ceria, alumina dehydroxylated to varying extents and an NaX zeolite. Pyrrole adsorption was found to be either non-dissociative or dissociative (on the more basic adsorption sites), the pyrrolate anion being formed in the latter case. When bound to surface hydroxy groups through an easily polarizable H bond, the pyrrolate anion was thought possibly to be non-planar; complex progressions of IR bands were then observed involving the combination of CH-stretching modes with a ring-deformation vibration. When pyrrole was non-dissociatively adsorbed through an NH⋯O bridge with O^2– moderately basic surface centres, the shift of the NH-stretching frequency reflected the O^2– basicity. Interaction with surface hydroxy groups led to NH⋯(OH) hydrogen-bridged species, which have been found to be either cyclic or linear according to whether the hydroxy groups are monodentate or bidentate, respectively.