A one-pot facile, impurity-free hydrothermal method to synthesize ultrathin α-FeOOH nanorods/graphene oxide (GO) composites is reported. It is directly synthesized from GO and iron acetate in water solution without inorganic or organic additives. XRD, Raman, FT-IR, XPS and TEM are used to characterize the samples. The nanorods in composites are single crystallite with an average diameter of 6 nm and an average length of 75 nm, which are significantly smaller than GO-free α-FeOOH nanorods. This can be attributed to the confinement effect and special electronic influence of GO. The influences of experimental conditions including reaction time and reactant concentration on the sizes of nanorods have been investigated. It reveals that the initial Fe2+ concentration and reaction time play an important role in the synthetic process. Furthermore, a possible nucleation-growth mechanism is proposed. As electrode materials for supercapacitors, the α-FeOOH nanorods/GO composite with 20% iron loading has the largest specific capacitance (127 F g−1 at 10 A g−1), excellent rate capability (100 F g−1 at 20 A g−1) and good cyclic performance (85% capacitance retention after 2000 cycles), which is much better than GO-free α-FeOOH nanorods. This unique structure results in rapid electrolyte ions diffusion, fast electron transport and high charging-discharging rate. In virtue of the superior electrochemical performance, the α-FeOOH nanorods/GO composite material has a promising application in high-performance supercapacitors.Download high-res image (263KB)Download full-size image

The effect of strontium as a chemical promoter on iron-based Fischer–Tropsch synthesis (FTS) catalysts was investigated and compared with that of potassium. Strontium was chosen for study because of its relationship to potassium through the Diagonal relationship in the Periodic Table. The catalysts were characterized by N2 physisorption, X-ray diffraction, laser Raman spectroscopy, Mössbauer effect spectroscopy, H2/CO temperature programmed reduction and temperature programmed hydrogenation. FTS reaction was tested in a fixed-bed reactor. It was found that strontium and potassium strengthened Fe-O bonds of iron oxides species, which is not favorable for the reduction of the catalysts in H2. Both of them enhanced the reduction and carbonization of the catalysts in CO and syngas atmosphere, suppressed the hydrogenation of surface carbon species, however, strontium is less effective than potassium. Besides, strontium improved the dispersion of iron oxide. Strontium did not significantly affect the activity of FTS and water gas shift (WGS), but facilitated the oxidation of iron carbides to Fe3O4 during FTS reaction process, while potassium significantly improved the FTS and WGS activity and inhibited the oxidation of iron carbide. Both of strontium and potassium decreased the selectivity of methane while facilitated the formation of heavy hydrocarbons and olefin, whatever, strontium exhibited a weaker effect compared to potassium.

A series of Pd–WOx/Al2O3 catalysts with different contents of WOx were prepared by stepwise incipient wetness impregnations. The influence of WOx on the physicochemical properties of Pd–WOx/Al2O3 catalysts, as well as their catalytic performance for the hydrogenolysis of glucose to 1,2-propanediol (1,2-PDO), was investigated. At low surface W density (0.3–2.1 W nm–2), distorted isolated WOx and oligomeric WOx are present on the Pd–WOx/Al2O3 catalysts. Furthermore, isolated WO4 are the dominating species on the Pd–WOx(5%)/Al2O3 catalyst. When the W density increased to 3.1 W nm–2, polymeric WOx species are dominant on the Pd–WOx(30%)/Al2O3 catalyst. The Pd surface area decreased while the acid amount increased with increasing W density. Furthermore, increased Lewis acid sites are provided by isolated WO4 and oligomeric WOx species whereas increased Brønsted acid sites exist on polymeric WOx species. Lewis acid sites promote glucose isomerization to fructose, which is an intermediate in glucose hydrogenolysis to 1,2-PDO. Metal sites catalyze C═O hydrogenation and C–C hydrogenolysis, which avoid the coke formation on catalysts. 1,2-PDO selectivity is dependent on the synergy of Lewis acid and metal sites; however, Brønsted acid sites have no contribution to the 1,2-PDO production. Typically, the Pd–WOx(5%)/Al2O3 catalyst possessing the optimal balance of Lewis acid and the metal site shows a 1,2-PDO selectivity of 60.8% at a glucose conversion of 92.2% and has a lifetime of over 200 h.Keywords: 1,2-propanediol; biomass; fructose; glucose; hydrogenolysis; isomerization; Pd−WOx/Al2O3; WOx

Effects of Mo addition on the Fischer–Tropsch synthesis (FTS) performance of precipitated Fe and FeCu catalysts were tested at 280 °C, 1.5 MPa, H2/CO = 2.0, 2,000 h−1. Reaction results in fixed-bed reactor indicate that Mo addition stabilizes the selectivity to higher molecular-weight hydrocarbons of Fe and FeCu catalysts. Carbonaceous species over the spent catalysts were studied by Mössbauer effect spectroscopy (MES) and temperature-programmed hydrogenation (TPH). It is found that Mo addition inhibits the carbon deposition on iron catalysts surface during the FTS reaction and thus probably protects the active sites for producing heavy hydrocarbons.

Effects of Mo addition on the performance of precipitated Fe catalysts for Fischer–Tropsch synthesis (FTS) were studied in a slurry reactor at 280 °C, 1.5 MPa, 2000 h−1, and syngas H2/CO = 1.2. The catalysts were characterized by N2 adsorption, H2 or CO temperature programmed reduction (TPR), NH3 temperature programmed desorption (TPD), X-ray diffraction (XRD), and Mössbauer effect spectroscopy (MES). It was found that there is a strong interaction between Fe and Mo, which inhibited the reduction and carburization of Fe catalyst. The surface acidity of the catalysts was also found to increase with addition of molybdenum. In the FTS process, Mo addition decreased the FTS activity of Fe catalyst, markedly enhanced C12+ hydrocarbon selectivity, while suppressed C2–C8 hydrocarbon selectivity as validated by non-ASF product distribution curves.Molybdenum modified participated Fe catalysts were prepared by two different ways. Mo addition markedly enhanced C12+ hydrocarbon selectivity, while suppressed C2–C8 hydrocarbon selectivity. The overall hydrocarbon product distribution apparently deviates from the Anderson–Schulz–Flory (ASF) kinetics, which is probably due to the acidic sites of Mo oxides promoting the olefin readsorption and secondary reaction.

The Fischer–Tropsch synthesis (FTS) performances of iron-based catalysts promoted with/without potassium compounds containing different acidic structural promoters (Al2O3, SiO2, and ZSM-5) were studied in this research. Characterization technologies of temperature-programmed reduction with CO (CO-TPR), powder X-ray diffraction (XRD) and Mössbauer effect spectroscopy (MES) were used to study the effect of K–structural promoter interactions on the carburization behaviors of catalysts. It showed that the addition states of potassium (K–Al2O3, K–SiO2, K–ZSM-5 and K-free) have a significant influence on the formation of iron carbides, which shows a following sequence in promotion of carburization: K–Al2O3 > K–SiO2 > K–ZSM-5 > K-free. The FTS reaction test was performed in a fixed bed reactor. It is found that Fe/K–Al2O3 catalyst leads to the highest CO conversion, Fe/K–ZSM-5 catalyst shows the highest H2 conversion, and Fe/K-free catalyst shows the lowest CO and H2 conversion. As for the hydrocarbon selectivity, Fe/K–SiO2 catalyst yields the lowest methane and the highest C5+ products, Fe/K–ZSM-5 catalyst yields higher methane and the highest liquid hydrocarbon product, whereas Fe/K-free catalyst yields the highest methane and the lowest C5+ products. These results can be explained from the interaction between potassium and structure promoters, and the spillover of reactants or intermediates from Fe sites to the surfaces of structural promoters.The Fischer–Tropsch synthesis (FTS) performances of iron-based catalysts promoted with/without potassium compounds containing different acidic structural promoters (Al2O3, SiO2, and ZSM-5) were studied in this research. The figure shows the effect of different potassium promoters on hydrocarbon selectivity and the results can be explained from the interaction between potassium and structure promoters, and the spillover of reactants or intermediates from Fe sites to the surfaces of structural promoters.

Fe/SiO2 catalysts with different Fe/Si molar ratios were used to investigate the effects of silica on chemical/structural properties and Fischer–Tropsch synthesis (FTS) performance of iron-based catalysts. In the chemical aspect, silica interacts with Fe species by the formation of FeOSi structure, which further transforms into an Fe2SiO4 phase during FTS reaction. The interaction largely disturbs the electronic structure of Fe atoms in iron oxide phases and in turn resists the reduction and activation of catalysts. In the structural aspect, silica increases the dispersion of Fe species and inhibits the aggregation of active iron particles. Addition of silica largely changes the adsorption sites of catalysts, i.e., decreases the number of weak H adsorption sites but improves the adsorption strengths of H, C, and O on reduced or carburized catalysts. With increasing amounts of silica, the chemical and structural effects cause the firstly decrease and then the increase of the initial FTS activity and the selectivities of heavy hydrocarbons and olefins during the Fischer–Tropsch synthesis. In addition, an important finding is that a proper amount of silica apparently suppresses the methane selectivity and stabilizes the iron carbide in the FTS reaction.Graphical abstractSilica affects iron Fischer–Tropsch catalysts in several ways. As a chemical promoter, it inhibits reduction and carburization of the iron, while in structural sense, it improves the iron dispersion and stabilizes the active phase. Finally, it suppresses the formation of methane. Download high-res image (157KB)Download full-size imageHighlights► Chemical and structural effects of silica on iron-based catalysts were investigated. ► Silica interacts with Fe to form FeOSi bond and disturbs the electron structures of Fe atoms. ► Silica largely decreases the iron particles and strengthens H, C, and O adsorptions on iron sites. ► High-silica-promotion largely suppresses the formation of methane.

FeMo/SiO2 catalysts were prepared by co-precipitation, sol–gel, or modified sol–gel (MSG) methods. These catalysts were characterized by N2 physisorption, H2 temperature-programmed reduction, X-ray diffraction, Mössbauer spectroscopy and transmission electron microscopy. Fischer–Tropsch synthesis (FTS) performances of catalysts were evaluated in a fixed-bed reactor at 300 °C, 1.5 MPa, 6 nL/(g of catalyst·h) and H2/CO molar ratio of 2.0. The MSG-derived FeMo/SiO2 catalyst possessed higher surface catalytic sites, smaller iron crystallites, weaker Fe–SiO2 interaction and easier reduction behavior relative to conventional method derived catalysts. The modified catalyst also exhibited higher activity, lower methane selectivity and better stability in FTS reaction.An FeMo/SiO2 catalysts prepared by a modified sol–gel method exhibited higher activity, lower methane selectivity and better stability in FTS reaction.Download full-size imageResearch Highlights►The modified sol–gel derived FeMo/SiO2 was different from the conventional sol–gel one in that Fe and Mo ions were incorporated into silica gel after the hydrolysis of tetraethoxysilane. ►The MSG-FeMo/SiO2 catalyst provided higher surface catalytic sites than the conventional prepared ones. ►The MSG-FeMo/SiO2 catalyst had high dispersion but weak Fe–SiO2 interaction. ►The MSG-FeMo/SiO2 catalyst exhibited high activity and low methane selectivity in FTS reaction.