In this combined in situ 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′-Fe2.2C, d p = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly Hägg carbides are formed (χ-Fe5C2, d p = 6.0 nm). Extensive carburization and sintering occur above these temperatures, as at 900 °C the predominant phase is cementite (θ-Fe3C, d p = 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 mmolCO gFe –1 s–1, confirming the outstanding activity and stability of this family of Fe-based FTS catalysts.

Although [Ni(S 2 CNBu i 2 ) 2 ] 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 n-hexylamine at 120°C a mixture of Ni 3 S 4 (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(S 2 CNBu i 2 ) 2 (RNH 2 ) 2 ] result as shown by in situ XANES and EXAFS and confirmed by DFT calculations. Heating to 90−100°C leads to changes consistent with the formation of amide-exchange products, [Ni(S 2 CNBu i 2 ){S 2 CN(H)R}] and/or [Ni{S 2 CN(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{S 2 CN(H)Hex} 2 ] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni(S 2 CNBu i 2 ){S 2 CN-(H)Oleyl}] 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 [Ni{S 2 CN(H)Hex} 2 ]. DFT modeling of [Ni{S 2 CN(H)R} 2 ]shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S 2 CNR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni{S 2 CN(H)R}(μ-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 amideexchange that account primarily for the formation of the observed nanoparticulate nickel sulfides. ■ INTRODUCTIONDithiocarbamate complexes 1 find extensive use as single-source precursors (SSPs) toward a range of metal sulfides 2 both as thin films and nanoparticles. 3−8 Their attractiveness for such applications stems from their ease of synthesis from cheap and readily available starting materials and the ability to tune the solubility, volatility, and decomposition properties of the complexes via simple changes to the amine substituents. The volatility and stability of side-products are also an advantage as they lead to their easy removal from the desired sulfide materials. Such syntheses generally involve decomposing dithiocarbamate complexes in high boiling primary amines, which can act as both the solvent and capping agent, among the most widely used being oleylamine. 9 While the solid-state structures of the dithiocarbamate precursors are easily established for example by single-crystal X-ray diffraction (XRD) studies, 1 in contrast little is known regarding their structure within the amine solution and the decomp...

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 x supported on TiO2. ReO x /TiO2 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 Re­(VII), Re­(IV), and Re(0) species at a ratio of 0.47:0.27:0.25, remaining comparatively stable during catalysis.

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(S2CN(i)Bu2)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, ((i)Bu2NCS2)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 ((i)Bu2NCS2)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.