Cobalt Time Yield Research Articles

Modulating Fischer-Tropsch synthesis performance on the Co3O4@SixAly catalysts by tuning metal-support interaction and acidity

ZIF-67 derived catalysts for Fischer-Tropsch synthesis have attracted much attention in recent years, while there is still a potential to improve their activity and selectivity. In this work, we prepared Si/Al co-immobilized Co 3 O 4 @Si x Al y catalysts by in-situ doping tetraethylorthosilicate and aluminum nitrate during the synthesis of ZIF-67. The effects of different Si/Al ratios on the metal-support interaction, acidity and FTS performance were explored. Results indicated that the Co 3 O 4 @Si 0 Al 4 catalyst exhibited the best FTS performance with the CO conversion as high as 79.9% and CTY (cobalt time yield) value of 19.5 × 10 −5 mol CO ·g Co −1 ·s −1 , which was ascribed to the moderate metal-support interaction and the most active Co sites. Meanwhile, the Co 3 O 4 @Si 3 Al 1 and Co 3 O 4 @Si 2 Al 2 catalysts exhibited higher iso-paraffin and olefin selectivity due to more acidic sites. The Co 3 O 4 @Si 0 Al 4 catalyst exhibited the highest FTS activity due to the moderate metal-support interaction (MSI), while the Co 3 O 4 @Si 3 Al 1 and Co 3 O 4 @Si 2 Al 2 catalysts showed certain hydrocracking and isomerization ability due to the formation of acidic sites. • Co 3 O 4 @Si x Al y catalysts were prepared by in-situ doping method based on ZIF-67. • The Co 3 O 4 @Si 0 Al 4 exhibited the highest Fischer-Tropsch synthesis (FTS) activity. • The high activity was due to moderate metal-support interaction and most active sites. • Acidic sites promoted the hydrocracking and isomerization of long chain hydrocarbons.

Numerical modeling and optimization of product selectivity and catalyst activity in Fischer-Tropsch synthesis via response surface methodology: Cobalt carbide particle size and [formula omitted] ratio effects

The effects of cobalt carbide (Co2C) catalyst particle size and usage ratio (H2/CO) on the Fischer-Tropsch selectivity, cobalt time yield and turnover frequency were investigated through statistical modeling by means of response surface methodology based on the data obtained over seven SiO2 supported cobalt catalysts at three different usage ratio levels. The effects of predictor variables on responses are illustrated in contour and surface plots and analyzed based on both model equation and plots. C5+ selectivity was found to increase with increasing Co2C particle size up to the range of 9.6–11.75 nm, after which decreases slightly. Furthermore, a linear relationship was detected between CH4 selectivity and C5+ selectivity for predicted data at all H2/CO ratios (0.5–2) in the Co2C particle size range of 5.3–9.4 nm. Multi-objective optimization conducted for the enhancement of C5+ selectivity predicted the particle size of 6.795 nm and usage ratio of 0.500 as optimum conditions.

Fischer-Tropsch synthesis by reduced graphene oxide nanosheets supported cobalt catalysts: Role of support and metal nanoparticle size on catalyst activity and products selectivity

In this paper, a series of cobalt catalysts supported on reduced graphene oxide (rGO) nanosheets with the loading of 5, 15 and 30 wt-% were provided by the impregnation method. The activity of the prepared catalysts is evaluated in the Fischer-Tropsch synthesis (FTS). The prepared catalysts were carefully characterized by nitrogen adsorption-desorption, hydrogen chemisorption, X-ray diffraction, Fourier transform infrared spectro-scopy, Raman spectroscopy, temperature programmed reduction, transmission electron microscopy, and field emission scanning electron microscopy techniques to confirm that cobalt particles were greatly dispersed on the rGO nanosheets. The results showed that with increasing the cobalt loading on the rGO support, the carbon defects are increased and as a consequence, the reduction of cobalt is decreased. The FTS activity results showed that the cobalt-time yield and turnover frequency passed from a maximum for catalyst with the Co0 average particle size of 15 nm due to the synergetic effect of cobalt reducibility and particle size. The products selectivity results indicated that the methane selectivity decreases, whereas the C5+ selectivity raises with the increasing of the cobalt particle size, which can be explained by chain propagation in the primary chain growth reactions.

Controllable synthesis of core-shell Co@C@SiO2 catalysts for enhancing product selectivity in Fischer-Tropsch synthesis by tuning the mass transfer resistance

Fischer-Tropsch synthesis (FTS) is the key step in converting syngas into clean fuels. Traditional supported catalysts for FTS are problematic because the active metal crystalline size is positively related to metal loading. Therefore, increasing active metal loading may reduce the cobalt time yield (CTY) since a high CTY is usually obtained when the Co size is 8 nm. Here, a ZIF-67 (Zeolitic imidazolate framework-67) with a MOF (Metal organic framework) structure is used as a precursor to prepare the [email protected] catalyst with not only high cobalt loading (55.6 wt%) but also with a small cobalt crystal size (as small as 8.6 nm). Core-shell [email protected]@SiO2-X catalysts with different SiO2 shell thicknesses were successfully prepared by coating different amounts of TEOS on the outer surface of [email protected] to modify product selectivity. Compared with 40 wt% Co/SiO2 catalyst, core-shell [email protected]@SiO2-X catalysts exhibited improved FTS performance. Furthermore, different gaseous hourly space velocities (GHSVs) were used to obtain CO conversion at similar levels to compare CTY and the turnover frequency (TOF). Among the catalysts, the [email protected]@SiO2-1 catalyst, with its better mass transfer ability and suitable hydrophilic property, presented the highest TOF (9.75 × 10−3 s−1) and lowest CH4 selectivity (9.75%). In addition, heavy hydrocarbons were effectively suppressed with the increase in shell thickness due to the increased mass transfer resistance.

Morphological and compositional changes of MFe2O4@Co3O4 (M = Ni, Zn) core-shell nanoparticles after mild reduction

NiFe2O4@Co3O4 and ZnFe2O4@Co3O4 core-shell nanoparticles were synthesized and reduced to be used as catalysts for the Fischer-Tropsch synthesis. The as-synthesized materials were determined to have an incomplete Co3O4 shell around the ferrite core with a maximum thickness of 3 nm. Using in-situ TEM, it was shown that reduction of these core-shell nanoparticles in pure hydrogen at 230 °C and 250 °C, respectively, resulted in the formation of small cobalt islands on the ferrite surface. Catalytic testing of the core-shell materials, NiFe2O4@Co3O4 and ZnFe2O4@Co3O4, after reduction showed a cobalt-time yield of 13.64 μmolCO·gCo−1·s−1 and 4.27 μmolCO·gCo−1·s−1 and a C5+ selectivity of 47 C-% and 68 C-%, respectively. The observed difference in cobalt-time yield and selectivity between NiFe2O4@Co3O4 and ZnFe2O4@Co3O4 core-shell nanoparticles was ascribed to a combination of effects that arose from the morphological and compositional changes that occurred after reduction and under Fischer-Tropsch synthesis conditions.

Surface modification of Co3O4 nanocubes with TEOS for an improved performance in the Fischer-Tropsch synthesis

ABSTRACT The surface of Co3O4 nano-cubes has been modified by adsorption of small amounts of tetraethoxysilane from a solution in n-hexane. The resulting silicate species on and near the Co3O4 surface have been characterized using XPS, FTIR and XANES, and may have formed neso or island silicates. The presence of these silicate species hinders the reduction of Co3O4. IR-visible bonds of some of the adsorbed species remain intact even after reduction in hydrogen at elevated temperatures. The intrinsic catalytic activity of the modified, reduced modified Co3O4 nano-cubes have been tested for their activity and selectivity in the Fischer-Tropsch synthesis under industrially relevant conditions. It is observed that the cobalt time yield increasing more than 7-fold by the presence of the silicate species on the surface. The increase in activity does not seem to correlate with a change in the resulting metal surface area.

The impact of support surface area on the SMSI decoration effect and catalytic performance for Fischer-Tropsch synthesis of Co-Ru/TiO2-anatase catalysts

A series of Co-Ru/TiO2 catalysts (10wt% Co, 0.5wt% Ru, nominal loadings) were prepared by impregnation of TiO2-anatase supports synthesized with different specific surface areas (Ti-L: 53m2/g, Ti-M: 117m2/g, and Ti-H: 148m2/g) by tuning the conditions of the hydrothermal synthesis and/or the calcination treatments. The most relevant physicochemical properties of supports and catalysts were determined by a set of techniques including ICP-OES, XRD, N2 physisorption, electron microscopy (FESEM, HAADF-STEM, HR-TEM), H2-TPR, H2 chemisorption, and IR-CO. Oxidized precursors were reduced in-reactor under flowing pure H2 at 400°C for 10h and evaluated for Fischer-Tropsch synthesis (FTS) in a fixed bed reactor at 220°C, 2.0MPa, and H2/CO molar ratio of 2. These catalysts exhibited the well-known strong metal-support interaction (SMSI) effect reported for TiO2 materials by which partially reduced TiOx species formed during the catalyst reduction step migrate and decorate the surface of the supported metal phases. The extent to which the SMSI effect occurred was found to increase with the surface area of the TiO2-anatase carrier, as supported by H2 chemisorption, TEM, and IR-CO surface titration experiments. As a consequence, the activity per total mass of cobalt or cobalt-time-yield (CTY) of the Co-Ru/TiO2 catalysts gradually declined with the increase in support surface area: Co-Ru/Ti-L>Co-Ru/Ti-M>Co-Ru/Ti-H. The catalysts, however, displayed similar initial TOFs, implying a negligible influence of the SMSI effect on the initial intrinsic activity of the surface Co0 sites. The high surface area Co-Ru/Ti-H catalyst exhibiting the most pronounced SMSI also presented the lowest C5+ selectivity. This behavior was explained by considering the contribution of two effects: the lower resistance to the intraparticle diffusion of α-olefins when increasing the support surface area, as inferred from the olefin-to-paraffin ratios and the values of the diffusion-related parameter χ, and the reduction in size of the cobalt ensembles on the terraces of Co0 nanoparticles, connected to the extent of SMSI, on which chain growth events are favored.

TiO2 polymorph dependent SMSI effect in Co-Ru/TiO2 catalysts and its relevance to Fischer-Tropsch synthesis

Pure anatase and rutile TiO2 samples were synthesized by thermal treatment of reverse microemulsions and applied as supports for preparing Ru-promoted Co catalysts (0.5wt% Ru, 10wt% Co). The catalysts were characterized by ICP-OES, XRD, Raman spectroscopy, N2 physisorption, H2-TPR, electron microscopy (FESEM, HAADF-STEM), H2 chemisorption, XPS, and in situ IR-CO after H2 reduction and reaction with syngas, and their catalytic performance for Fischer-Tropsch synthesis (FTS) studied at industrial conditions (220°C, 2.0MPa, H2/CO=2). The two catalysts exhibited comparable mean Co particle sizes (5–6nm) as well as high and alike degrees of cobalt reduction (ca. 90%). The SMSI decoration effect arising during H2 reduction was much more pronounced for the anatase-supported catalyst resulting in lower cobalt-time-yield (CTY) compared to that supported on TiO2-rutile. In situ IR-CO under syngas conversion conditions showed equivalent cobalt surface reconstruction and nature of the surface Co0 sites for both catalysts in their working state, and revealed a partial reversibility of the SMSI effect during FTS by which a significant fraction of the decorated Co0 centers in the anatase-based catalyst was uncovered and became available for reaction. The implication of this effect on TOFs is discussed. The C5+ selectivity was higher for the rutile-based catalyst, although a clear impact of the SMSI effect on selectivities was not inferred from our results.

Cobalt-Catalyzed Fischer–Tropsch Synthesis: Chemical Nature of the Oxide Support as a Performance Descriptor

The effect of the chemical nature of the oxide support on the performance of cobalt Fischer–Tropsch catalysts is investigated. A series of supports is synthesized via monolayer coverage of porous γ-Al2O3 with various oxides representative of a wide range of Lewis acid–base character, as quantified by UV–vis spectroscopy coupled to alizarin adsorption. Incorporation of cobalt (20 wt %) results in model catalysts with identical porosities and similar Co particle sizes (>10 nm), allowing the study of support effects without overlap from diffusional or particle size factors. Under realistic reaction conditions, the initial TOF scales with the acidity of the oxide support, whereas the cobalt time yield and selectivity to industrially relevant C13+ hydrocarbons show a volcano dependence, with a maximum at an intermediate acid–base character. As inferred from in situ CO-FTIR, “selective” blockage of a few cobalt sites, though crucial for CO hydrogenation, by atoms from basic oxides and “unselective” site blockag...

Titania-Decorated Silicon Carbide-Containing Cobalt Catalyst for Fischer–Tropsch Synthesis

The metal–support interactions of titanium dioxide decorated silicon carbide (β-SiC)-supported cobalt catalyst for Fischer–Tropsch synthesis (FTS) were explored by a combination of energy-filtered transmission electron microscopy (EFTEM), 59Co zero-field nuclear magnetic resonance (59Co NMR), and other conventional characterization techniques. From the 2D elemental maps deduced by 2D EFTEM and 59Co NMR analyses, it can be concluded that the nanoscale introduction of the TiO2 into the β-SiC matrix significantly enhances the formation of small and medium-sized cobalt particles. The results revealed that the proper metal–support interaction between cobalt nanoparticles and TiO2 led to the formation of smaller cobalt particles (<15 nm), which possess a large fraction of surface atoms and, thus, significantly contribute to the great enhancement of conversion and the reaction rate. The cobalt time yield of the catalyst after modification increased to 7.5 × 10–5 molCO gCo–1 s–1 at 230 °C, whereas the C5+ selecti...

Promotion Effects of Platinum and Ruthenium on Carbon Nanotube Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

Carbon nanotube supported nano-size monometallic and noble metal (Pt and Ru) promoted cobalt catalysts were prepared by incipient wetness impregnation (IWI) using solution of cobalt nitrate and characterized by nitrogen adsorption isotherm, X-ray diffraction (XRD), temperature programmed reduction, in situ magnetic method and TEM. Analysis of the magnetization and H2-TPR data suggested promotion with platinum and ruthenium significantly decreased the cobalt species reduction temperature. TEM and XRD results showed that the presence of noble metal promoters had no significant effect on the size of cobalt for carbon naotube as catalytic support. Promotion of cobalt carbon nanotube-supported catalysts with small amounts of Pt and Ru resulted in slight increase in Fischer–Tropsch cobalt time yield. The Pt and Ru promoted cobalt catalyst exhibited carbon monoxide conversion of 37.1 and 31.4, respectively. C5+ hydrocarbon selectivity was attained at 80.0%. The Pt promoted cobalt supported on carbon nanotube yielded better catalytic stability than that of the monometallic cobalt catalyst. Over carbon nanotube supported cobalt catalyst, the addition of noble promoter significantly decreased the cobalt reduction temperature, but had no significant influence on the cobalt particle size.

Influence of pore regularity on Fischer–Tropsch synthesis with Co/SiO2 Catalysts

Abstract The influence of pore regularity in Fischer–Tropsch synthesis over Co/SiO2 catalysts was investigated by using two series of silica with comparable pore sizes: mesoporous silica (SBA-15) and amorphous silica. It was found that cobalt dispersion was largely influenced by the support porous structure. The cobalt particles on mesoporous silica showed much more homogeneous size distribution and the increase of cobalt loadings did not result in a noticeable increase of Co3O4 particle size on SBA-15, while most of those on amorphous silica agglomerated into clusters. As a result, SBA-15 supported Co catalysts showed higher cobalt-time yield and C5+ selectivity in Fischer–Tropsch synthesis, particularly higher C12–18 fraction. However, the average particle size of Co3O4 supported on amorphous silica increased with the cobalt loadings, which led to lower cobalt-time yield and higher CH4 selectivity.

Fischer–Tropsch synthesis over un-promoted and Re-promoted γ-Al 2O 3 supported cobalt catalysts with different pore sizes

The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of γ-Al 2O 3 supported cobalt catalysts was investigated in fixed-bed reactors at T = 483 K, P = 20 bar, and H 2/CO = 2.0. Catalysts containing 20 wt.% cobalt and 0 or 0.5 wt.% rhenium were prepared by incipient wetness impregnation of different γ-Al 2O 3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re-promoted catalysts, also perrhenic acid. The γ-Al 2O 3 supports had very different pore characteristics. The post-calcination Co 3O 4 crystallite size was predominantly controlled by the γ-Al 2O 3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co 3O 4 was reduced in two steps to Co 0 with CoO as intermediate species. However, while reduction of Co 3O 4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co 0 was dependent on the catalyst properties. Large particles present in wide pores were easier to reduce than small particles located in narrow pores. In addition, Re promoted the reduction of CoO. The effect of rhenium as a reduction promoter was less pronounced at increasing pore size and particle size. Re also had a similar positive impact on the cobalt dispersion of the catalysts. Although Re significantly increased the Fischer–Tropsch synthesis cobalt-time yield, it did not modify the site-time yield. The deactivation rates of all the catalysts were also similar up to 100 h on stream. Positive correlations were found between the catalyst pore diameter and the C 5+ selectivity and between the cobalt particle size and the C 5+ selectivity. Re had a consistent positive, albeit small, effect on the C 5+ selectivity.

Cobalt species in promoted cobalt alumina-supported Fischer–Tropsch catalysts

The structure of cobalt species at different stages of the genesis of monometallic and Pt-promoted cobalt alumina-supported Fischer–Tropsch catalysts was studied using X-ray diffraction, UV–visible spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectron spectroscopy, and DSC–TGA thermal analysis. The catalysts were prepared by incipient wetness impregnation using solutions of cobalt nitrate and dihydrogen hexachloroplatinate. Both variation of catalyst calcination temperature between 473 and 773 K and promotion with 0.1 wt% of Pt had no significant affect on the size of supported Co 3O 4 crystallites. The size of cobalt oxide particles in the calcined catalysts seems to be influenced primarily by the pore diameter of the support. Cobalt reducibility was relatively low in monometallic cobalt alumina-supported catalysts and decreased as a function of catalyst calcination temperature. The effect was probably due to the formation of mixed surface compounds between Co 3O 4 and Al 2O 3 at higher calcination temperatures, which hinder cobalt reduction. Promotion with platinum spectacularly increased the rate of cobalt reduction; the promotion seemed to reduce the activation energy of the formation of cobalt metallic phases. Analysis of the magnetization data suggests that the presence of Pt led to the reduction of smaller cobalt oxide particles, which could not be reduced at the same conditions in the cobalt monometallic catalysts. Promotion of cobalt alumina-supported catalysts with small amounts of Pt resulted in a significant increase in Fischer–Tropsch cobalt time yield. The efficient control of cobalt reducibility through catalyst calcination and promotion seems to be one of the key issues in the design of efficient cobalt alumina-supported Fischer–Tropsch catalysts.

Investigations on the structural changes of two Co/SiO 2 catalysts by performing Fischer–Tropsch synthesis

The structural changes of Co/SiO 2 catalysts by performing FT synthesis were investigated, including the influences of cobalt particle sizes and of the reaction conditions, two Co/SiO 2 catalysts were prepared by impregnation method with cobalt nitrate as precursor: the one, Co/SiO 2(1), was prepared by direct reduction of SiO 2-supported nitrate precursor in H 2 at 350 °C using a slow temperature ramping protocol (0.2 °C/min), and the other, Co/SiO 2(2), was prepared by calcinations of the nitrate precursor in air at 450 °C, followed by H 2 reduction at 400 °C. The cobalt particles over these two catalysts are expected to be different in size. These two catalysts were used for FT synthesis for 60 h at 1.0 MPa and 200–240 °C. Both the reduced and used catalysts were characterized by XRD, extended X-ray absorption fine structure (EXAFS) and H 2 chemisorptions. The Co/SiO 2(1) catalyst exhibits obviously higher activity for FT synthesis than the Co/SiO 2(2) catalyst. However, on increasing reaction temperature from 200 to 240 °C, the ratio of cobalt-time yield (moles CO converted/total g-atom cobalt-s) on the Co/SiO 2(1) catalyst to that on the Co/SiO 2(2) catalyst decreases from 2.7 to 1.9. Co o crystallites formed on the Co/SiO 2(1) catalyst are significantly smaller than those on the Co/SiO 2(2) catalyst, and Co dispersion of the Co/SiO 2(1) catalyst is greatly higher than that of the Co/SiO 2(2) catalyst. During FT synthesis, the Co o crystallites only grow with short-range order, and no cobalt oxide is formed. But Co dispersions of the catalysts decrease significantly, which may result from the aggregation of the Co o crystallites. As the reaction temperature increases from 200 to 240 °C, the small cobalt particles on the Co/SiO 2(1) catalyst aggregate more significantly than the large cobalt particles on the Co/SiO 2(2) catalyst. This is the main reason responsible for the significant decrease in the ratio of the cobalt-time yield on the Co/SiO 2(1) catalyst to that on the Co/SiO 2(2) catalyst.

Synthetic Design of Cobalt Fischer-Tropsch Catalysts

ABSTRACTWe describe synthetic strategies for designing cobalt Fischer-Tropsch catalysts in order to maximize synthesis rates and C5+ selectivity. The support cobalt-precursor interaction must be balanced to maximize dispersion yet allow facile low temperature reduction. The cobalt time yield is independent of support and varies directly as the cobalt dispersion for dispersions in the 0.02 to 0.12 range [1].