A combination of first-principles thermodynamics and density functional theory (DFT) was applied for the prediction of sulfur-poisoned monomeric Cu/Fe species formed in the SSZ-13 catalyst framework under selective catalytic reduction (SCR)-relevant conditions in the presence of sulfur dioxide, ammonia, oxygen, and water. Differences in fresh and sulfurpoisoned species were found for Cu-and Fe-SSZ-13 catalysts containing one Al (1Al sites) or two Al (2Al sites) in 6-membered rings (6MRs) or 8membered rings (8MRs). The impact of ammonia concentration during lowand high-temperature sulfur-poisoning on Cu-and Fe-speciation was also investigated. SCR-relevant concentrations of ammonia in the gas mixture led to the formation of ammonium sulfates over copper in 2Al and 1Al sites of Cu-SSZ-13, while bisulfate and sulfuric acid species were predicted at these copper sites either in the absence of ammonia or at negligible concentrations of ammonia during low-and high-temperature poisoning. The absence of ammonia in the gas mixture led to the formation of iron-bisulfates at 2Al sites of Fe-SSZ-13 during lowtemperature poisoning, while the formation of ammonium sulfates was favorable under SCR-relevant conditions. In contrast to the facile formation of ammonium sulfates at copper sites of Cu-SSZ-13, only ammonium-free iron-sulfates formed at 1Al sites in Fe-SSZ-13 under realistic operational conditions. The regeneration of 2Al sites of Cu-SSZ-13 was predicted to occur at higher temperatures compared to 2Al sites in Fe-SSZ-13, whereas the opposite was predicted for 1Al sites. The analysis of fresh and regenerated Cu/Fe species was carried out as well. These theoretical results on model catalysts provide a first step in the understanding of sulfur-poisoning in Fe-SSZ-13 catalysts, supporting further experimental investigations to improve NH 3 -SCR catalysts for meeting future emission standards.
Cobalt-doped
anatase Ti1–x
Co
x
O2 (0 < x ≤
0.04) nanopowders (with a particle size of 30–40 nm) were produced
by the hydrothermal synthesis method. Morphology, structure, and thermal
stability of the synthesized compounds were examined using transmission
electron microscopy, infrared spectroscopy, and X-ray diffraction
analysis. Using X-ray photoelectron spectroscopy, cobalt ions are
shown to have an oxidation state of 2+, with titanium ions having
a tetravalent state of Ti4+. In the as-prepared state,
all investigated compounds of Ti1–x
Co
x
O2 are paramagnetic, with
the value of paramagnetic susceptibility growing in proportion to
cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals
that doping TiO2 with cobalt (up to 2%) is accompanied
by a significant increase in the concentration of F+ centers.
Further growth of the cobalt content results in a relatively wide
line (nearly 600 Oe) in the spectrum, with a g-factor
of about 2.005, demonstrating exchange-coupled regions being formed,
the fraction of which increases with cobalt content, while the intensity
of F+-center signals is reduced appreciably. Annealing
of Ti0.96Co0.04O2 in vacuum at 1000
K is shown to have resulted in the substantial localization of cobalt
atoms in the subsurface layers, resulting in an approximately 3-fold
increase in the Co atoms content on the surface of nanoparticles as
compared with that in the bulk. This is shown to be accompanied by
appearance of spontaneous magnetization at room temperature, the value
of which depends on the cobalt content in TiO2 nanopowders.
The value of magnetic moment per Co atom decreases monotonically down
to a value of ≃1 μB with cobalt content increasing.
A core–shell model proposed to be the most adequate for describing
the magnetic properties of TiO2:Co after the reducing annealing.
A hypothesis is put forward suggesting that the defect surface enriched
with Co atoms and vacancies is described with itinerant type magnetism,
allowing for the delocalized nature of electrons near vacancies.
Co-doped
TiO2 is one of the most extensively studied
oxides for applying as dilute magnetic semiconductors due to its room
temperature magnetism. Here we present results of the studies of Ti0.97Co0.03O2 nanopowders synthesized
by microwave-assisted hydrothermal method by means of X-ray diffraction,
soft X-ray absorption spectroscopy (Ti L2,3 and Co L2,3 spectra), hard X-ray absorption spectroscopy (Co K spectra),
and 1s3p resonant inelastic X-ray scattering at the Co K edge. According
to X-ray diffraction data and Ti L2,3 X-ray absorption
spectra, all the samples before the thermal treatment exhibit anatase
structure with substantial amount of amorphous phase. After annealing
the Ti0.97Co0.03O2 samples in vacuum
or hydrogen at 700 °C, the anatase structure persists while amorphous
phase contribution is eliminated. Surface-sensitive soft X-ray absorption
Co L2,3 spectroscopy revealed only Co2+ ions
tetrahedrally coordinated by oxygen ions and no sign of metallic Co.
Co2+ tetrahedral sites (instead of typical octahedral ones)
are an additional evidence for Co2+ localization at the
distorted TiO2 particle surface. Bulk-sensitive X-ray diffraction,
Co K X-ray absorption spectroscopy, and 1s3p resonant inelastic X-ray
scattering at the Co K edge revealed clustering of metallic cobalt
inside of the large agglomerates formed by TiO2 nanoparticles
in annealed TiO2:Co nanopowders.
In situ Cu and S K-edge X-ray absorption spectroscopy (XAS) was used for the investigation of sulfur-poisoned and regenerated Cu-SSZ-13 selective catalytic reduction (SCR) catalysts. Sulfur in the oxidation state...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.