The chemisorption of O 2 and NO on reduced and sulfided molybdena-alumina catalysts

Abstract

The chemisorptions of O 2 and NO were investigated at −78 °C and at room temperature using pulse-flow and volumetric techniques. The oxygen chemisorption was fast and irreversible while the NO chemisorption, after a fast initial process, increased slowly with time. This latter portion could be removed by evacuation and readsorbed or replaced with O 2. The strong NO chemisorption was approximately four times larger than that of O 2. Infrared spectra showed that the strongly chemisorbed NO was present in pairs, suggesting that the oxygen chemisorption was as atoms. The interference of one chemisorption by the other was studied. The introduction of O 2 did not affect the strongly chemisorbed NO whereas approximately one NO molecule was chemisorbed per oxygen atom on the oxygen saturated surface. This process was also observable in the infrared spectra. A characteristic pair of bands at 1817 and 1713 cm −1 which appeared on chemisorption of NO on the reduced catalyst (and at 1795 and 1704 cm −1 on the sulfided catalyst) have been assigned to a dinitrosyl species or NO dimer. These bands were unaffected when O 2 was admitted to the system. On the other hand, when the NO was admitted to an oxygen-covered surface the same bands appeared but with much lower intensities. As previously reported, both chemisorptions were correlatable with the anion vacancy concentration. The results could be rationalized by assuming that these chemisorptions took place on Mo 2+ centers (two anion vacancies on the same Mo). Sulfided catalysts produced nearly identical results.