Selective Catalytic Ammonia Oxidation to Nitrogen by Atomic Oxygen Species on Ag(111)

Abstract

Ammonia-selective catalytic oxidation was studied on the planar Ag(111) single-crystal model catalyst surface under ultra-high-vacuum (UHV) conditions. A variety of oxygen species were prepared via ozone decomposition on pristine Ag(111). Surface coverages of oxygen species were quantified by temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy techniques. Exposure of ozone on Ag(111) at 140 K led to a surface atomic oxygen (O_a) overlayer. Low-energy electron diffraction experiments revealed that annealing of this atomic oxygen-covered Ag(111) surface at 473 K in UHV resulted in the formation of ordered oxide surfaces (O_ox) with p(5×1) or c(4×8) surface structures. Ammonia interactions with O/Ag(111) surfaces monitored by temperature-programmed reaction spectroscopy indicated that disordered surface atomic oxygen selectively catalyzed N–H bond cleavage, yielding mostly N₂ along with minor amounts of NO and N₂O. Higher coverage O/Ag(111) surfaces, whose structure was tentatively assigned to a bulklike amorphous silver oxide (O_bulk), showed high selectivity toward N₂O formation (rather than N₂) due to its augmented oxygen density. In contrast, ordered surface oxide overlayers on Ag(111) (where the order was achieved by annealing the oxygen adlayer to 473 K) showed only very limited reactivity toward ammonia. The nature of the adsorbed NH₃ species on a clean Ag(111) surface and its desorption characteristics were also investigated via infrared reflection absorption spectroscopy and TPD techniques. Current findings demonstrate that the Ag(111) surface can selectively oxidize NH₃ to N₂ under well-defined experimental conditions without generating significant quantities of environmentally toxic species such as NO₂, NO, or N₂O