Thermal desorption and infrared studies of ammonia, amines and pyridines chemisorbed on chromic oxide

Abstract

The thermal desorption spectra of ammonia, amines and pyridines chemisorbed on Cr₂O₃ as well as the infrared absorption spectra have been measured in order to elucidate the acidic properties of the surface. The adsorption of ammonia on the bare Cr₂O₃ surface at room temperature leads to the formation of two kinds of coordination species giving rise to desorption peaks at 443 and 543 K, and the bonding of the latter species to the surface is strengthened to cause a shift in the peak to 593 K when the adsorption temperature is raised to 473 K. Dimethylamine bonded to Lewis-acid sites on Cr₂O₃ causes the N—H bond-cleavage reaction when the sample is heated at 473 K. Pyridine is strongly held on the surface: half the pyridine molecules absorbed on Cr₂O₃ at room temperature remain on the surface even after evacuation at 773 K. The infrared study of these molecules suggests the formation of an α-pyridone-like complex.