Chenghua Zhang

Study of Manganese Promoter on a Precipitated Iron-Based Catalyst for Fischer-Tropsch Synthesis

Abstract The effects of Manganese (Mn) incorporation on a precipitated iron-based Fischer-Tropsch synthesis (FTS) catalyst were investigated using N2 physical adsorption, air differential thermal analysis (DTA), H2 temperature-programmed reduction (TPR), and Mossbauer spectroscopy. The FTS performances of the catalysts were tested in a slurry phase reactor. The characterization results indicated that Mn increased the surface area of the catalyst, and improved the dispersion of α-Fe2O3 and reduced its crystallite size as a result of the high dispersion effect of Mn and the Fe-Mn interaction. The Fe-Mn interaction also suppressed the reduction of α-Fe2O3 to Fe3O4, stabilized the FeO phase, and (or) decreased the carburization degree of the catalysts in the H2 and syngas reduction processes. In addition, incorporated Mn decreased the initial catalyst activity, but improved the catalyst stability because Mn restrained the reoxidation of iron carbides to Fe3O4, and improved further carburization of the catalysts. Manganese suppressed the formation of CH4 and increased the selectivity to light olefins (C=2–4), but it had little effect on the selectivities to heavy (C5+) hydrocarbons. All these results indicated that the strong Fe-Mn interaction suppressed the chemisorptive effect of the Mn as an electronic promoter, to some extent, in the precipitated iron-manganese catalyst system.

Facile synthesis of porous nitrogen-doped holey graphene as an efficient metal-free catalyst for the oxygen reduction reaction

Nitrogen-doped graphene is a promising candidate for the replacement of noble metal-based electrocatalysts for oxygen reduction reactions (ORRs). The addition of pores and holes into nitrogen-doped graphene enhances the ORR activity by introducing abundant exposed edges, accelerating mass transfer, and impeding aggregation of the graphene sheets. Herein, we present a straightforward but effective strategy for generating porous holey nitrogen-doped graphene (PHNG) via the pyrolysis of urea and magnesium acetate tetrahydrate. Due to the combined effects of the in situ generated gases and MgO nanoparticles, the synthesized PHNGs featured not only numerous out-of-plane pores among the crumpled graphene sheets, but also interpenetrated nanoscale (5–15 nm) holes in the assembled graphene. Moreover, the nitrogen doping configurations of PHNG were optimized by post-thermal treatments at different temperatures. It was found that the overall content of pyridinic and quaternary nitrogen positively correlates with the ORR activity; in particular, pyridinic nitrogen generates the most desirable characteristics for the ORR. This work reveals new routes for the synthesis of PHNG-based materials and elucidates the contributions of various nitrogen species to ORRs.

Facile synthesis of self-assembled ultrathin α-FeOOH nanorod/ graphene oxide composites for supercapacitors

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 6nm and an average length of 75nm, 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 (127Fg-1 at 10Ag-1), excellent rate capability (100Fg-1 at 20Ag-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.

Effect of Manganese Incorporation Manner on an Iron-Based Catalyst for Fischer-Tropsch Synthesis

Abstract A systematic study was undertaken to investigate the effects of the manganese incorporation manner on the textural properties, bulk and surface phase compositions, reduction/carburization behaviors, and surface basicity of an iron-based Fischer-Tropsch synthesis (FTS) catalyst. The catalyst samples were characterized by N2 physisorption, X-ray photoelectron spectroscopy (XPS), H2 (or CO) temperature-programmed reduction (TPR), CO2 temperature-programmed desorption (TPD), and Mossbauer spectroscopy. The FTS performance of the catalysts was studied in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that the manganese promoter incorporated by using the coprecipitation method could improve the dispersion of iron oxide, and decrease the size of the iron oxide crystallite. The manganese incorporated with the impregnation method is enriched on the catalysts surface. The manganese promoter added with the impregnation method suppresses the reduction and carburization of the catalyst in H2, CO, and syngas because of the excessive enrichment of manganese on the catalyst surface. The catalyst added manganese using the coprecipitation method has the highest CO conversion (51.9%) and the lowest selectivity for heavy hydrocarbons (C12+).

In Situ XRD Study on Promotional Effect of Potassium on Carburization of Spray-dried Precipitated Fe2O3 Catalysts

A study of the promotional effect of potassium on the carburization behavior of a series of spray‐dried precipitated Fe2O3 catalysts (100 Fe, 100 Fe/0.52 K, 100 Fe/1.54 K, and 100 Fe/2.4 K) has been performed by using in situ XRD. The average crystallite size evolution for species such as α‐Fe2O3, Fe3O4, χ‐Fe5C2, and θ‐Fe3C were followed. The potassium promoter clearly inhibits the reduction of α‐Fe2O3 to Fe3O4, where the decreasing binding energy in the Fe 2p3/2, O 1s and K 2p3/2 spectra from X‐ray photoelectron spectroscopy (XPS) suggests an electron density increase in Fe and O upon potassium promotion, leading to the enhanced covalency in the Fe−O bond. In terms of crystallite size during carburization, an optimum potassium loading exists in catalyst 100 Fe/0.52 K, which shows the fastest reduction to Fe3O4 with minimum crystallite sizes of around 7 to 15 nm. Potassium has no clear effect in determining the final crystallite size of χ‐Fe5C2. An arch‐shaped curve in the evolution of crystallite size of the Fe3O4 intermediate was observed, which can be explained by the activation energy difference between the bidirectional steps of the outward oxygen diffusion and the inward carbon diffusion.

Nonspherical hollow α-Fe2O3 structures synthesized by stepwise effect of fluoride and phosphate anions

Despite the significant progress in making hollow structures, it is still a challenge to synthesize some specialized hollow structures. In the present work, we obtained a new hollow hematite structure, tube-in-dodecahedron, by using the stepwise influences of fluoride and phosphate anions. Based on condition-dependent experiments, we proposed a “nucleation–aggregation–recrystallition and etching” mechanism, which also directed us to synthesize a series of hematite hollow structures, including hollow dodecahedron and hollow ellipsoid. The concentration of phosphate was found to play a decisive role in the control of these hollow structures. 0.08 mM is the critical point for keeping the top facets of dodecahedral hematite particles while 0.2 mM is the upper limit for keeping the lateral facets. The magnetic properties of these synthesized hollow hematite structures were found to be closely associated with the structures. The synthesized tube-in-dodecahedral hematite particles exhibited excellent photocatalytic reactivity toward organic dyes.

Comparative Study of Iron-Based Fischer–Tropsch Synthesis Catalysts Promoted with Strontium or Potassium

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.Graphical Abstract

Enhanced Fischer–Tropsch performances of graphene oxide-supported iron catalysts via argon pretreatment

Graphene oxide (GO)-supported iron oxide nanoparticles (NPs) have been successfully synthesized by a hydrothermal method and used as Fischer–Tropsch synthesis (FTS) catalysts. GO with oxygen-containing groups may have a strong interaction with iron oxide NPs, retarding the contact between active sites and syngas. Argon pretreatment is used to modify the physical and chemical properties of catalysts with the purpose of enhancing FTS performances. The catalysts are thoroughly characterized by N2 physisorption, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, temperature-programmed reduction, and Mossbauer spectroscopy. The pretreatment of catalysts in argon at 500 °C for 5 h can effectively increase the surface area and defects, weaken the Fe–GO interaction and expose more catalytically active sites. The Fe/GO-500 catalyst shows improved reduction and carburization behaviors, high CO conversion activity and C5+ selectivity, and excellent stability in the FTS reaction.

Precursor controlled synthesis of graphene oxide supported iron catalysts for Fischer–Tropsch synthesis

In this study, graphene oxide supported iron catalysts have been successfully synthesized by a hydrothermal method using three different iron precursors: ferrous acetate Fe(C2H3O2)2, ferric oxalate Fe2(C2O4)3 and ferric nitrate Fe(NO3)3. The Fe2(C2O4)3 derived catalysts (Fe/G-C) present a uniform dispersion with smaller sizes of iron nanoparticles (NPs). Density functional theory (DFT) calculations indicate the higher binding energy between ferric oxalate and graphene oxide (−1.53 eV) facilitates the formation of more seeds for nanoparticle growth, and therefore, smaller size and narrow size distribution of NPs are achieved. The Fe/G-C also exhibits easier reduction and carburization, resulting in high Fischer–Tropsch synthesis (FTS) activity and C5+ selectivity. Our thorough characterization studies enable us to conclude that the iron precursors significantly impact the structural properties of catalysts, which further affect their reduction/carburization abilities and FTS performances. This study affords us a rational design of high efficiency graphene oxide supported iron catalysts for FTS.

Synthesis of monodisperse iron oxide nanoparticles: effect of temperature, time, solvent and surfactant

ABSTRACT Iron oxide nanoparticles (NPs) were prepared by the thermal decomposition of iron-oleate. The effects of reaction temperature, time, solvent, and surfactant were tested. The synthesized NPs were characterized by X-ray diffraction, Transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis,, and Fourier transform infrared. The results show that reaction time has little effect on the sizes of synthesized NPs, while the solvent directly decides the sizes. There is a most appropriate temperature to fulfill perfect monodisperse iron oxide NPs. Moreover, different surfactants lead to different shapes of synthesized NPs.