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

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

XPS Study of Stability and Reactivity of Oxidized Pt Nanoparticles Supported on TiO₂

The method of X-ray photoelectron spectroscopy was used to study the interaction of the model Pt/TiO₂ catalysts with NO₂ and the following reduction of the oxidized Pt nanoparticles in vacuum, hydrogen, and methane. It was shown that, while interacting with NO₂ at room temperature, the metal Pt nanoparticles transform, first, into the phase which was tentatively assigned as particles containing subsurface/dissolved oxygen [Pt-O_sub], and then, into the PtO and PtO₂ oxides. If only the first state of platinum [Pt-O_sub] is formed, it demonstrates exclusively high reactivity toward hydrogen. For the samples containing simultaneously [Pt-O_sub], PtO, and PtO₂, the highest reaction ability was demonstrated by PtO₂; contrary to the other two oxidized states, it is reducing while kept in vacuum under X-ray irradiation. All three coexisting states of the oxidized platinum can be reduced when heated in vacuum as well as while interacting with hydrogen at room temperature. First, PtO₂ is reduced to PtO. PtO and [Pt-O_sub] begin being reduced after the complete consumption of PtO₂. We propose that, when a sample contains simultaneously all three states of oxidized platinum, the supported particles have a core–shell structure with a nucleus of perturbed platinum containing oxygen atoms, which are covered with a film of Pt oxides. It was shown that none of the oxidized states of platinum react with methane at room temperature