11 research outputs found
Phase control during the synthesis of nickel sulfide nanoparticles from dithiocarbamate precursors
Square-planar nickel bis(dithiocarbamate) complexes, [Ni(S2CNR2)2], have been prepared and utilised as single source precursors to nanoparticulate nickel sulfides. While they are stable in the solid-state to around 300 °C, heating in oleylamine at 230 °C, 5 mM solutions afford pure α-NiS, where the outcome is independent of the substituents. DFT calculations show an electronic effect rather than steric hindrance influences the resulting particle size. Decomposition of the iso-butyl derivative, [Ni(S2CNiBu2)2], has been studied in detail. There is a temperature-dependence of the phase of the nickel sulfide formed. At low temperatures (150 °C), pure α-NiS is formed. Upon raising the temperature, increasing amounts of β-NiS are produced and at 280 °C this is formed in pure form. A range of concentrations (from 5–50 mM) was also investigated at 180 °C and while in all cases pure α-NiS was formed, particle sizes varied significantly. Thus at low concentrations average particle sizes were ca. 100 nm, but at higher concentrations they increased to ca. 150 nm. The addition of two equivalents of tetra-iso-butyl thiuram disulfide, (iBu2NCS2)2, to the decomposition mixture was found to influence the material formed. At 230 °C and above, α-NiS was generated, in contrast to the results found without added thiuram disulfide, suggesting that addition of (iBu2NCS2)2 stabilises the metastable α-NiS phase. At low temperatures (150–180 °C) and concentrations (5 mM), mixtures of α-NiS and Ni3S4, result. A growing proportion of Ni3S4 is noted upon increasing precursor concentration to 10 mM. At 20 mM a metastable phase of nickel sulfide, NiS2 is formed and as the concentration is increased, α-NiS appears alongside NiS2. Reasons for these variations are discussed
Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO2 Hydrogenation
Highly monodispersed CuO nanoparticles (NPs) were synthesized via continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts for CO2 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 CO2/H2 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 CO2 hydrogenation on the CuO surface suggested that hydrogenation reduced the CuO surface to Cu2O, which facilitated the formation of formate. In situ EXAFS supported a strong correlation between the Cu2O phase fraction and the formate peak intensity, with the maxima corresponding to where Cu2O 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 Cu2O 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, CH4 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 CO2 to CH4 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
N-containing covalent organic frameworks as supports for rhodium as transition-metal catalysts in hydroformylation reactions
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
Active nature of primary amines during thermal decomposition of nickel dithiocarbamates to nickel sulfide nanoparticles
Although [Ni(S2CNBu2i)(2)] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 degrees C in oleylamine to give pure NiS nanoparticles, while in n-hexylamine at 120 degrees C a mixture of Ni3S4 (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 trans-[Ni(S2CNBu2i)(2)(RNH2)(2)] result as shown by in situ XANES and EXAFS and confirmed by DFT calculations. Heating to 90-100 degrees C leads to changes consistent with the formation of amide-exchange products, [Ni(S2CNBu(2)(i)){S2CN(H)R}] and/or [Ni{S2CN(H)R}(2)]. 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 [Ni{S2CN(H)Hex}(2)] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni(S2CNBu2i){S2CN(H)Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 degrees C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni{S2CN(H)Hex}(2)]. DFT modeling of [Ni{S2CN(H)R}(2)] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S2C-NR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni{S2CN(H)R}(mu-SH)](2). 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
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