Adsorption of NO on promoted Ag/alpha-Al_2O_3 catalysts

摘要

Nitric oxide was used to probe Ag/alpha-Al_2O_3 epoxidation catalysts. NO dissociated on reduced silver catalysts to oxidize metallic silver sites and desorb N_2O at 300 K. Uptake measurements and DRIFTS results showed that Ag-O sites were the principal surface species formed after NO adsorption at 300 K on reduced, unpromoted Ag catalysts, as indicated by the formation of gas-phase N_2O as well as a value of unity for the NO/O uptake ratio. The formation of nitrite species was time dependent and a weak band due to chelating nitrite was observed after long exposure times for the unpromoted catalyst after reduction at 473 K. On the unpromoted, O-covered Ag surface, NO adsorption produced a band at 1396 cm~(-1) which was assigned to a nitrate species. Bands at 1238 and 1342 cm~(-1), assigned to chelating nitrito and nitrate species, respectively, were detected after NO adsorption on Cs-promoted Ag surfaces reduced at 473 K or covered with chemisorbed oxygen following reduction at 673 K. Only the 1238 cm~(-1) band was observed after NO adsorption on the Cs-promoted catalyst following reduction at 673 K, and this band was the only absorption feature after NO adsorption on a Ag surface promoted with both Cs and Cl, regardless of the pretreatment. In the presence of Cs, all results indicated that the chemistry of oxygen at the Ag surface was different and more oxygen existed on the surface. The presence of surface oxygen enhanced NO_x adsorption, and the presence of both Cl and Cs may decrease the surface concentration of nucleophilic oxygen, which forms nitrate species. After O_2 and NO coadsorption on unpromoted Ag, the O_2 desorption peak was shifted 100 K higher to 660 K. When NO was adsorbed in the presence of gas-phase O_2, bulk Ag nitrate was a dominant species.