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-highvacuum (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. Lowenergy 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 N 2 along with minor amounts of NO and N 2 O. Higher coverage O/Ag(111) surfaces, whose structure was tentatively assigned to a bulklike amorphous silver oxide (O bulk ), showed high selectivity toward N 2 O formation (rather than N 2 ) 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 NH 3 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 NH 3 to N 2 under well-defined experimental conditions without generating significant quantities of environmentally toxic species such as NO 2 , NO, or N 2 O.

Progress on Noble Metal-Based Catalysts Dedicated to the Selective Catalytic Ammonia Oxidation into Nitrogen and Water Vapor (NH3-SCO)

“…The dissociation of oxygen was believed to be the rate-controlling step for ammonia oxidation, while low surface coverage favors N 2 formation. Similar conclusions were given by Karatok et al [85] The exposure of ozone on Ag(111) surfaces at −133 • C led to a disordered surface atomic oxygen overlayer (confirmed by LEED). Such oxygen species selectively catalyzed N-H bond cleavage, yielding mostly N 2 and minor amounts of by-products (NO and N 2 O).…”

Enhancement of Formic Acid Dehydrogenation Selectivity of Pd(111) Single Crystal Model Catalyst Surface via Brønsted Bases

“…The first state is the low-coverage α 1 state, which has a desorption maximum at 279 K. With an increasing θ NH 3 , the α 1 state starts to broaden toward lower temperatures. The second submonolayer desorption state which is denoted as α 2 starts to appear around 0.6 MLE and reaches its desorption maximum around 164 K. A further increase in θ NH 3 leads to the formation of the α 3 111), 44 Au(111), 45 Pt(111), 46 Ag(111), 47 Ni(111), 48 and Ru(0001). 49 The broadening of the TPD signals in Figure 1 toward low temperatures can be explained by the presence of the repulsive interactions between the adsorbed ammonia molecules on Pd(111) and the existence of dissimilar adsorption sites and adsorption configurations.…”

Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

“…In former studies, oxygen coverage for a single layer of silver oxide with various oxide structures [i.e., p(4 × 4), p(4 × 5r3)] was calculated to be 0.375 ML. 13 Thus, in the current work, the saturation oxygen coverage obtained at the top layer of Ag(111) in the oxygen TPD experiments 35 was assumed to be 0.375 ML and used as a reference.…”