Polymerization of ethylene on chromium oxide catalysts. Part 4.—Infra-red study of the adsorption of nitric oxide and ammonia on active catalyst
Nitric oxide adsorbs on reduced supported chromium(VI) oxide catalyst to give infra-red bands at 1740, ca. 1800, and ca. 1860 cm–1. The band at ca. 1800 cm–1 has been assigned to nitric oxide molecules coordinatively chemisorbed on Cr4+ ions in the surface. The band at 1740 cm–1 is considered to result from a combination of separate bands at 1743 and 1738 cm–1 due to nitric oxide covalently bound to Cr3+ and Cr5+ sites respectively. The band at ca. 1860 cm–1 is similarly assigned to nitric oxide chemisorbed on two types of site.
The adsorption of ammonia gave infra-red bands due to coordinatively bound ammonia at 3400 and 3300 cm–1 which were insensitive to the extent of reduction of the catalyst.
Correlations between the intensities of the bands due to adsorbed nitric oxide and the activities of catalyst samples for the polymerization of ethylene support the conclusion that Cr5+ ions are the active sites in the polymerization process. The polymerization reaction was poisoned by the pre-adsorption of nitric oxide or ammonia.