4 research outputs found
ReO<sub><i>x</i></sub>/TiO<sub>2</sub>: A Recyclable Solid Catalyst for Deoxydehydration
Deoxydehydration (DODH) enables the
transformation of two adjacent
hydroxyl functions into a C–C double bond: e.g., facilitating
synthesis of 1,3,5-hexatriene from sorbitol. Here we report the first
stable heterogeneous catalyst for DODH based on ReO<sub><i>x</i></sub> supported on TiO<sub>2</sub>. ReO<sub><i>x</i></sub>/TiO<sub>2</sub> exhibits not only catalytic activity and selectivity
comparable to those of previously described molecular rhenium catalysts
but also excellent stability without deactivation over at least six
consecutive runs. X-ray absorption spectroscopy (XAFS) measurements
indicate a mixture of ReÂ(VII), ReÂ(IV), and Re(0) species at a ratio
of 0.47:0.27:0.25, remaining comparatively stable during catalysis
Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO<sub>2</sub> Hydrogenation
Highly
monodispersed CuO nanoparticles (NPs) were synthesized via
continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts
for CO<sub>2</sub> hydrogenation. The catalytic behavior of unsupported
11 nm sized nanoparticles from the same batch was characterized by
diffuse reflectance infrared fourier transform spectroscopy (DRIFTS),
extended X-ray absorption fine structure (EXAFS), X-ray diffraction
(XRD), and catalytic testing, under CO<sub>2</sub>/H<sub>2</sub> in
the temperature range 25–500 °C in consistent experimental
conditions. This was done to highlight the relationship among structural
evolution, surface products, and reaction yields; the experimental
results were compared with modeling predictions based on density functional
theory (DFT) simulations of the CuO system. In situ DRIFTS revealed
the formation of surface formate species at temperatures as low as
70 °C. DFT calculations of CO<sub>2</sub> hydrogenation on the
CuO surface suggested that hydrogenation reduced the CuO surface to
Cu<sub>2</sub>O, which facilitated the formation of formate. In situ
EXAFS supported a strong correlation between the Cu<sub>2</sub>O phase
fraction and the formate peak intensity, with the maxima corresponding
to where Cu<sub>2</sub>O was the only detectable phase at 170 °C,
before the onset of reduction to Cu at 190 °C. The concurrent
phase and crystallite size evolution were monitored by in situ XRD,
which suggested that the CuO NPs were stable in size before the onset
of reduction, with smaller Cu<sub>2</sub>O crystallites being observed
from 130 °C. Further reduction to Cu from 190 °C was followed
by a rapid decrease of surface formate and the detection of adsorbed
CO from 250 °C; these results are in agreement with heterogeneous
catalytic tests where surface CO was observed over the same temperature
range. Furthermore, CH<sub>4</sub> was detected in correspondence
with the decomposition of formate and formation of the Cu phase, with
a maximum conversion rate of 2.8% measured at 470 °C (on completely
reduced copper), supporting the indication of independent reaction
pathways for the conversion of CO<sub>2</sub> to CH<sub>4</sub> and
CO that was suggested by catalytic tests. The resulting Cu NPs had
a final crystallite size of ca. 44 nm at 500 °C and retained
a significantly active surface
Active Nature of Primary Amines during Thermal Decomposition of Nickel Dithiocarbamates to Nickel Sulfide Nanoparticles
Although [NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)<sub>2</sub>] is stable at high temperatures
in a range of solvents, solvothermal
decomposition occurs at 145 °C in oleylamine to give pure NiS
nanoparticles, while in <i>n</i>-hexylamine at 120 °C
a mixture of Ni<sub>3</sub>S<sub>4</sub> (polydymite) and NiS results.
A combined experimental and theoretical study gives mechanistic insight
into the decomposition process and can be used to account for the
observed differences. Upon dissolution in the primary amine, octahedral <i>trans-</i>[NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)<sub>2</sub>(RNH<sub>2</sub>)<sub>2</sub>] result as shown by <i>in situ</i> XANES and EXAFS and confirmed by DFT calculations.
Heating to 90–100 °C leads to changes consistent with
the formation of amide-exchange products, [NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)Â{S<sub>2</sub>CNÂ(H)ÂR}] and/or [NiÂ{S<sub>2</sub>CNÂ(H)ÂR}<sub>2</sub>]. DFT modeling shows that exchange occurs via
nucleophilic attack of the primary amine at the backbone carbon of
the dithiocarbamate ligand(s). With hexylamine, amide-exchange is
facile and significant amounts of [NiÂ{S<sub>2</sub>CNÂ(H)ÂHex}<sub>2</sub>] are formed prior to decomposition, but with oleylamine, exchange
is slower and [NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)Â{S<sub>2</sub>CNÂ(H)ÂOleyl}] is the active reaction component. The primary
amine dithiocarbamate complexes decompose rapidly at ca. 100 °C
to afford nickel sulfides, even in the absence of primary amine, as
shown from thermal decomposition studies of [NiÂ{S<sub>2</sub>CNÂ(H)ÂHex}<sub>2</sub>]. DFT modeling of [NiÂ{S<sub>2</sub>CNÂ(H)ÂR}<sub>2</sub>] shows
that proton migration from nitrogen to sulfur leads to formation of
a dithiocarbimate (S<sub>2</sub>Cî—»NR) which loses isothiocyanate
(RNCS) to give dimeric nickel thiolate complexes [NiÂ{S<sub>2</sub>CNÂ(H)ÂR}Â(μ-SH)]<sub>2</sub>. These intermediates can either
lose dithiocarbamate(s) or extrude further isothiocyanate to afford
(probably amine-stabilized) nickel thiolate building blocks, which
aggregate to give the observed nickel sulfide nanoparticles. Decomposition
of the single or double amide-exchange products can be differentiated,
and thus it is the different rates of amide-exchange that account
primarily for the formation of the observed nanoparticulate nickel
sulfides
Elucidating the Nature of Fe Species during Pyrolysis of the Fe-BTC MOF into Highly Active and Stable Fischer–Tropsch Catalysts
In
this combined <i>in situ</i> XAFS, DRIFTS, and Mössbauer
study, we elucidate the changes in structural, electronic, and local
environments of Fe during pyrolysis of the metal organic framework
Fe-BTC toward highly active and stable Fischer–Tropsch synthesis
(FTS) catalysts (Fe@C). Fe-BTC framework decomposition is characterized
by decarboxylation of its trimesic acid linker, generating a carbon
matrix around Fe nanoparticles. Pyrolysis of Fe-BTC at 400 °C
(Fe@C-400) favors the formation of highly dispersed epsilon carbides
(ε′-Fe<sub>2.2</sub>C, <i>d</i><sub>p</sub> = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly
Hägg carbides are formed (χ-Fe<sub>5</sub>C<sub>2</sub>, <i>d</i><sub>p</sub> = 6.0 nm). Extensive carburization
and sintering occur above these temperatures, as at 900 °C the
predominant phase is cementite (θ-Fe<sub>3</sub>C, <i>d</i><sub>p</sub> = 28.4 nm). Thus, the loading, average particle size,
and degree of carburization of Fe@C catalysts can be tuned by varying
the pyrolysis temperature. Performance testing in high-temperature
FTS (HT-FTS) showed that the initial turnover frequency (TOF) of Fe@C catalysts does not change significantly for
pyrolysis temperatures up to 600 °C. However, methane formation
is minimized when higher pyrolysis temperatures are applied. The material
pyrolyzed at 900 °C showed longer induction periods and did not
reach steady state conversion under the conditions studied. None of
the catalysts showed deactivation during 80 h time on stream, while
maintaining high Fe time yield (FTY) in the range of 0.19–0.38
mmol<sub>CO</sub> g<sub>Fe</sub><sup>–1</sup> s<sup>–1</sup>, confirming the outstanding activity and stability of this family
of Fe-based FTS catalysts