3 research outputs found
Predicting NO<sub><i>x</i></sub> Catalysis by Quantifying Ce<sup>3+</sup> from Surface and Lattice Oxygen
Our
work introduces a novel technique based on the magnetic response of
Ce<sup>3+</sup> and molecular oxygen adsorbed on the surface of nanoceria
and ceria-based catalysts that quantifies the number and type of defects
and demonstrates that this information is the missing link that finally
enables predictive design of NO<sub><i>x</i></sub> catalysis
in ceria-based systems. The new insights into ceria catalysis are
enabled by quantifying the above for different ceria nanoparticle
shapes (i.e., surface terminations) and O<sub>2</sub> partial pressure.
We used ceria nanorods, cubes, and spheres and evaluated them for
catalytic reduction of NO by CO. We then demonstrated the quantitative
prediction of the reactivity of nanomaterials via their magnetism
in different atmospheric environments. We find that the observed enhancement
of reactivity for ceria nanocubes and nanorods is not directly due
to improved reactivity on those surface terminations but rather due
to the increased ease of generating lattice defects in these materials.
Finally, we demonstrate that the method is equally applicable to highly
topical and industrially relevant ceria mixed oxides, using nanoscale
alumina-supported ceria as a representative case–a most ill-defined
catalyst. Because the total oxide surface is a mixture of active ceria
and inactive support and ceria is not likely present as crystallographically
well-defined phases, reactivity does not easily scale with surface
area or a surface termination. The key parameter to design efficient
NO reduction in ceria-based catalysts is knowing and controlling the
surface localized excess Ce<sup>3+</sup> ion areal density
Acceptorless Dehydrogenative Coupling of Neat Alcohols Using Group VI Sulfide Catalysts
Group VI sulfides
were synthesized via coprecipitation of elemental
sulfur and metal hexacarbonyl and characterized with XRD, XPS, and
TEM. These materials were then demonstrated as active catalysts for
the acceptorless dehydrogenative coupling of neat ethanol to ethyl
acetate, rapidly reaching equilibrium conversion and up to 90% selectivity.
Other primary alcohols form the corresponding esters, while diols
formed the corresponding cyclic ethers and oligomers
Isolated Fe<sup>II</sup> on Silica As a Selective Propane Dehydrogenation Catalyst
We report a comparative study of
isolated Fe<sup>II</sup>, iron
oxide particles, and metallic nanoparticles on silica for non-oxidative
propane dehydrogenation. It was found that the most selective catalyst
was an isolated Fe<sup>II</sup> species on silica prepared by grafting
the open cyclopentadienide iron complex, bisÂ(2,4-dimethyl-1,3-pentadienide)
ironÂ(II) or FeÂ(<i>o</i>Cp)<sub>2</sub>. The grafting and
evolution of the surface species was elucidated by <sup>1</sup>H NMR,
diffuse reflectance infrared Fourier transform spectroscopy and X-ray
absorption spectroscopies. The oxidation state and local structure
of surface Fe were characterized by X-ray absorption near-edge structure
(XANES) and extended X-ray absorption fine structure. The initial
grafting of iron proceeds by one surface hydroxyl Si–OH reacting
with FeÂ(<i>o</i>Cp)<sub>2</sub> to release one diene ligand
(<i>o</i>CpH), generating a SiO<sub>2</sub>-bound Fe<sup>II</sup>(<i>o</i>Cp) species, <b>1-Fe</b><i><b>o</b></i><b>Cp</b>. Subsequent treatment with
H<sub>2</sub> at 400 °C leads to loss of the remaining diene
ligand and formation of nanosized iron oxide clusters, <b>1-C</b>. Dispersion of these Fe oxide clusters occurs at 650 °C, forming
an isolated, ligand-free Fe<sup>II</sup> on silica, <b>1-Fe</b><sup><b>II</b></sup>, which is catalytically active and highly
selective (∼99%) for propane dehydrogenation to propene. Under
reaction conditions, there is no evidence of metallic Fe by in situ
XANES. For comparison, metallic Fe nanoparticles, <b>2-NP-Fe</b><sup><b>0</b></sup>, were independently prepared by grafting
FeÂ[NÂ(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> onto silica, <b>2-FeN*</b>, and reducing it at 650 °C in H<sub>2</sub>. The
Fe NPs were highly active for propane conversion but showed poor selectivity
(∼14%) to propene. Independently prepared Fe oxide clusters
on silica display a low activity. The sum of these results suggests
that selective propane dehydrogenation occurs at isolated Fe<sup>II</sup> sites