2 research outputs found
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<sub>a</sub>) 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<sub>ox</sub>) 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<sub>2</sub> along with
minor amounts of NO and N<sub>2</sub>O. Higher coverage O/Ag(111)
surfaces, whose structure was tentatively assigned to a bulklike amorphous
silver oxide (O<sub>bulk</sub>), showed high selectivity toward N<sub>2</sub>O formation (rather than N<sub>2</sub>) 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<sub>3</sub> 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<sub>3</sub> to N<sub>2</sub> under well-defined experimental conditions
without generating significant quantities of environmentally toxic
species such as NO<sub>2</sub>, NO, or N<sub>2</sub>O
XPS Study of Stability and Reactivity of Oxidized Pt Nanoparticles Supported on TiO<sub>2</sub>
The method of X-ray photoelectron
spectroscopy was used to study
the interaction of the model Pt/TiO<sub>2</sub> catalysts with NO<sub>2</sub> and the following reduction of the oxidized Pt nanoparticles
in vacuum, hydrogen, and methane. It was shown that, while interacting
with NO<sub>2</sub> 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>sub</sub>], and then, into the PtO and PtO<sub>2</sub> oxides. If only the
first state of platinum [Pt-O<sub>sub</sub>] is formed, it demonstrates
exclusively high reactivity toward hydrogen. For the samples containing
simultaneously [Pt-O<sub>sub</sub>], PtO, and PtO<sub>2</sub>, the
highest reaction ability was demonstrated by PtO<sub>2</sub>; 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, PtO<sub>2</sub> is reduced
to PtO. PtO and [Pt-O<sub>sub</sub>] begin being reduced after the
complete consumption of PtO<sub>2</sub>. 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