Higher Cobalt Dispersion Research Articles

Manganese as a structural promoter in silica-supported cobalt Fischer-Tropsch catalysts under simulated high conversion conditions

Understanding the deactivation mechanism of cobalt-based Fischer-Tropsch catalysts is of significant practical importance. Herein, we explored the role of manganese as a structural promoter on silica-supported cobalt nanoparticles under simulated high CO conversion conditions, i.e., high relative humidity. The structural changes in cobalt dispersion and oxidation state were followed by in situ Mössbauer emission spectroscopy. Adding manganese oxide to silica-supported cobalt enhanced the dispersion of metallic cobalt in the reduced catalysts. This higher cobalt dispersion, however, led to a stronger tendency of cobalt silicate formation under humid conditions. Without manganese, the cobalt particles sintered, and the larger ones were prone to transformation into cobalt carbide under high conversion conditions. As such, silica is not preferred as a support for practical FTS.

One‐Step Synthesis of Core‐Shell‐Structured Mixed‐Metal CPO‐27(Cu,Co) and Investigations on Its Controlled Thermal Transformation

AbstractUsing the mixed‐metal approach, a direct synthesis route at ambient pressure was developed for a new type of bimetallic metal‐organic framework based on the CPO‐27 structure. The structural characterization of CPO‐27(Cu0.6−CS−Co0.4) using X‐ray diffraction, transmission electron microscopy, energy‐dispersive X‐ray mapping and X‐ray absorption spectroscopy revealed that the Cu2+ and Co2+ ions were exclusively incorporated at the metal positions of the CPO‐27 lattice, but with a core‐shell distribution within the crystallites. The parent framework material was then utilized as a precursor for the generation of novel bimetallic carbon‐supported materials using the controlled thermal decomposition in a reducing atmosphere. During this decomposition process, the distribution of the two metals remained the same, which resulted in unique needle‐shaped particles with a high dispersion of cobalt at the periphery of the amorphous carbon and agglomerated copper particles in the inside.

Significance of C3 Olefin to Paraffin Ratio in Cobalt Fischer–Tropsch Synthesis

The ratio between propene and propane (C3 o/p) during Fischer–Tropsch synthesis (FTS) has been analyzed based on both literature reports and experiments for five catalysts. The latter comprise four cobalt catalysts on γ-alumina with variations in pore sizes, and one catalyst on α-alumina. Overall variations include H2/CO feed ratio, residence time, water addition, transients between test conditions, CO conversion, cobalt particle size, promoter (Re), and support material. It was possible to rationalize all data based on secondary hydrogenation of olefins. In fact, it was deduced that olefins are dominating termination products in FTS, estimated to ca. 90% for C3, but that some paraffins most likely are also produced directly. Increased residence time and high H2/CO feed ratio favors olefin hydrogenation, while added water presumably displaces hydrogen on cobalt giving enhanced C3 o/p. High cobalt dispersion favors hydrogenation, as also promoted by Re. Effect of intraparticle diffusion is seen in transient periods; for example, as water is added or depleted. There is frequently positive correlation between C3 o/p and selectivity to longer chains; the latter expressed as C5+ selectivity, as both are sensitive to hydrogen activity. Some modifications, however, are needed due to the accepted volcano plot for C5+ selectivity with cobalt crystallite size. Titania as support shows unexpectedly low C3 o/p; probably due to SMSI (strong-metal-support-interaction).

Preparation of Highly Dispersed Nb2O5 Supported Cobalt-Based Catalysts for the Fischer–Tropsch Synthesis

In this work, niobia supported cobalt catalysts with both high cobalt dispersion and high cobalt loading (15 wt %) were prepared through the assistance of glow discharge plasma treatment, and their catalytic performance was tested in Fischer–Tropsch synthesis (FTS). Plasma treatment on support and especially on the decomposition of the catalyst precursor leads to a noticeable enhancement of cobalt dispersion. However, in addition to the cobalt dispersion improvement, plasma assisted decomposition of the cobalt precursor also led to the formation of CoNb2O5, which was inactive in FTS; moreover, the small cobalt particles underwent severe sintering during the reaction and caused significant and contineous deactivation. Plasma treatment on nobia could modify its porosity and hydrophilicity and generate surface functional groups; these features enabled an improved cobalt-niobia interaction and better cobalt dispersion on CoN(P)-C than on CoN-C. The enhanced FTS activity and best reaction stability on the CoN(P)-C catalyst were due to the possession of a suitable cobalt particle size and moderate cobalt-niobia interaction.

Toward a Full One-Pass Conversion for the Fischer–Tropsch Synthesis over a Highly Selective Cobalt Catalyst

The Fischer–Tropsch synthesis (FTS) has been studied for decades; however, it is very difficult to simultaneously obtain a high one-pass CO conversion and a high selectivity of long-chain (C5+) hydrocarbons. A layered CoMn/TiO2 catalyst is fabricated and used as an efficient catalyst in FTS. The unique layered structure of the CoMn/TiO2 catalyst effectively inhibits the agglomeration of Co nanoparticles and enhances the diffusion of the reactants and products in FTS. The catalyst exhibits superior activity, high C5+ selectivity, and excellent stability due to its unique multiple mesopores structure as well as high cobalt dispersion. Even at a nearly complete one-pass CO conversion (>95%), which is considered to be a very harsh condition in the FTS reaction, CoMn/TiO2 still exhibits a high C5+ hydrocarbon selectivity of around 93% and a very low selectivity of undesired C1 products (CH4 and CO2) of below 4.5%.

Catalytic hydropyrolysis of biomass using supported CoMo catalysts – Effect of metal loading and support acidity

Catalytic hydropyrolysis of biomass to green fuels was performed using supported, sulfided CoMo catalysts. With MgAl2O4 as support material the CoMo loading was varied between 4.1 and 12.0 wt% at constant Co/Mo atomic ratio of 0.3. Increasing the metal loading decreased the amount of oxygen in the condensed organic phase from 9.0 to 4.7 wt% on dry basis (db) and the condensable organic yield decreased from 25.2 to 22.7 wt% on dry ash free (daf) basis, corresponding to a decrease in the carbon recovery from 39 to 37%. Using zeolite H-ZSM-5 mixed with alumina as support with a CoMo loading of 4.1 wt%, the condensed organics contained only 5.2 to 6.1 wt% db oxygen. The condensable organic yield was between 23.9 and 24.4 wt% db, and the carbon recovery was 39–40%. Thus using an acidic support can remove the oxygen without decreasing the carbon recovery. The latter was ascribed to alkylation of the aromatics when the zeolite support was used.Elemental maps of the spent catalysts were obtained using STEM-EDS, showing that the CoMo phase was mainly located as monolayer MoS2 slab structures (>93%) on the support and indicated a high dispersion of cobalt, consistent with incorporation of Co into the MoS2 structure in the CoMoS phase. Potassium was observed on all the spent catalysts, indicating transfer of alkali metal from the biomass to the catalyst. Potassium may decrease the acidity of the catalyst over time, thus reducing the positive effect of using a more acidic support.

Properties of Carbon Xerogels Supported Cobalt‐Based Catalysts and Their Performance in CO Hydrogenation Reaction

AbstractCarbon xerogels (C−X), which synthesized by a convenient method with moderate porous texture and high surface area, was used as potential support for cobalt‐based catalysts, and the catalytic performance was studied by CO hydrogenation. The use of activated carbon (AC) and carbon nanotubes (CNTs) as supports was also investigated for comparison. The catalysts were prepared by incipient wetness impregnation method, and adjustment of cobalt dispersion on Co/C−X was achieved by variation of the decomposition approaches of cobalt nitrate. Characterization and CO hydrogenation results showed that C−X is a good candidate as catalyst support, it could homogeneously disperse both big cobalt particles and small ones. And the supported Co/C−X catalysts had either higher catalytic activity or better reaction stability, in comparison with CNTs and AC supported counterparts. Glow discharge plasma assisted decomposition of cobalt nitrate led to the formation of much smaller cobalt particles, and due to the inert properties of carbon materials, the Co/C−X‐P catalyst showed both higher cobalt dispersion and cobalt reducibility than the calcined counterpart, and thus presented a much higher CO hydrogenation activity with good reaction stability as well.

Preparation of mesoporous aluminosilicates with tunable morphologies and their effects on Fischer–Tropsch synthesis performance

ZSM-5 seeds grafted mesoporous aluminosilicates with different morphologies (SZM-1 (two-dimensional (2D) structure), MZM-2 (three-dimensional (3D) structure), MZM-1 (mixture of 2D and 3D structure)) were synthesized by varying benzene addition amount. These composites were used as supports for Co (20 wt%) catalysts, which were prepared by simple incipient wetness impregnation method. Their catalytic performance was evaluated by Fischer–Tropsch synthesis (FTS) reaction. The supports and corresponding catalysts were characterized by N2 adsorption–desorption, XRD, TEM, H2-TPR, NH3-TPD techniques. 20Co/SZM-1 catalyst showed a highest FTS activity and best reaction stability, due to its higher cobalt dispersion, good cobalt reducibility and the confinement effect of two-dimensional structure. Compared to 20Co/SZM-1, 20Co/MZM-2 catalyst exhibited much higher selectivities to short-chain hydrocarbons and iso-paraffins, which may be attributed to the combination effects of its three-dimensional open pore structure and stronger acidity, leading to less diffusion limitation of hydrocarbon products and thus presenting a smaller chain-growth probability.

Evolution of cobalt species in glow discharge plasma prepared CoRu/SiO2 catalysts with enhanced Fischer-Tropsch synthesis performance

Glow discharge plasma was applied to substitute thermal calcination during catalyst preparation. Evolution of cobalt particles in cobalt-ruthenium based catalysts at different stages (fresh prepared, reduced, after Fischer-Tropsch synthesis (FTS) reaction) were studied using a series of characterization techniques. Glow discharge plasma can be developed as an efficient technique to decompose metal salt. Cobalt dispersion was greatly enhanced by glow discharge plasma treatment, and higher initial cobalt dispersion was found on the one with lower treating intensity. However, cobalt crystallites generated by plasma treatment below certain intensity had poor thermal stability, they were tended to aggregate into large particles during reduction and reaction processes, and thus severe deactivations were observed during FTS tests. Besides cobalt sintering, mass transfer limitation at high FTS activity due to the pore plugging by long-chain hydrocarbons was also one of the important reasons for deactivation. The CRS-P140 catalyst with higher plasma treating intensity combined relatively high cobalt dispersion, high cobalt reducibility and relatively stable cobalt distribution during reduction or reaction, showed a high FTS activity with less activity loss.

Construction of three-dimensional nitrogen-doped graphene aerogel (NGA) supported cobalt catalysts for Fischer-Tropsch synthesis

Abstract Co/NGA catalysts were one-step hydrothermal synthesized during the construction of three-dimensional nitrogen-doped graphene aerogel (NGA). The structure and properties of the catalysts were characterized by TGA, Raman, FTIR, XRD,TEM, SEM, XPS and H2-TPR, and their catalytic performance was tested by Fischer-Tropsch synthesis (FTS) reaction. Characterization and reaction results showed that higher cobalt dispersion was found on the catalyst with higher cobalt loading, and majority of cobalt species in the freeze-dried samples was fcc phase Co°. Excessive cross linking originated by high Co2+ concentration during the construction of 3D NGA led to the restraint of mass transfer of reactants to cobalt active sites, and therefore, lower FTS activity was observed on the catalysts with higher cobalt loadings. The fcc phase Co° in fresh Co/NGA catalysts was transformed in to hcp phase Co° during FTS reaction, and as a result, catalytic activity was increased with increasing time on stream.

Effects of Functionalization of Carbon Nanotubes on Activity and Selectivity of Co/CNT Catalysts in Fischer-Tropsch Synthesis

The Fischer–Tropsch Synthesis (FTS) activities of cobalt-based catalysts supported on carbon nanotubes (CNTs) and functionalized carbon nanotubes (FCNTs) are investigated in this work. The cobalt-based catalysts are synthesized by the reverse micro-emulsion technique using a non-ionic surfactant, and characterized by the Brunauer-Emmett-Teller, X-ray diffraction, H2 chemisorption, temperature program reduction, and transmission electron microscopy techniques. The activities of the synthesized catalysts are evaluated in terms of the FTS production rate (g produced hydrocarbons /g.cat./h) and selectivity (percentage of the CO converted to hydrocarbon products). According to the TEM results, the synthesized cobalt nanoparticles have a narrow size distribution and are mostly confined inside the functionalized CNTs (FCNTs) . These nanoparticles are highly reducible as evidenced by the reduction peaks of the FCNT catalyst shifting to low temperatures. In comparison to non-functionalized CNT, FCNT increases the FTS rate and CH4 selectivity and decreases the C5+ selectivity as a catalytic support. In addition, the FCNT support preserves the high dispersion and reducibility of cobalt, which can be attributed to the hydrogen spill-over effect of the functional groups present on the CNT surface.

Plasma Assisted Preparation of CoPt/SiO2 Fischer‐Tropsch Catalysts: A Comparison of the Precursor Pre‐Thermal Treated Temperatures

AbstractA series of amorphous silica supported Co‐based catalysts were prepared using traditional incipient wetness impregnation methods followed by thermal treatment at different temperatures. Glow discharge plasma effects on cobalt species in the forms of cobalt nitrate, mixture of cobalt nitrate and Co3O4, and pure Co3O4 were compared and investigated by various characterizations and Fischer‐Tropsch synthesis (FTS) tests. Plasma enhancing metal dispersion effect was mostly happened during the decomposition of cobalt nitrate in plasma field; when cobalt species was already in the form of Co3O4, no apparent particle size change by plasma treatment was observed. Under the promotion of Pt, all the catalysts maintained at high reduction degree, thus the cobalt dispersion was almost linearly related to the number of active sites. The CoSi‐100D+P catalyst with both high cobalt dispersion and high cobalt reducibility, had the maximum number of active sites, and presented the highest FTS activity. Meanwhile, the product selectivity could also be affected by plasma treatment, hydrocarbons were shifted to lower carbon numbers. This effect could be observed even on the CoSi‐400C+P sample, in which the precursor was pre‐thermal treated at high temperature (400 °C), and cobalt species was already all transformed into Co3O4.

Optimization of solvent-free mechanochemical synthesis of Co/Al2O3 catalysts using low- and high-energy processes

In this work, alumina-supported cobalt (Co/Al2O3) catalysts were prepared using new solvent-free mechanochemical synthesis methods: low-energy vibratory ball milling (Fritsch, Pulverisette 0) and high-energy planetary ball milling (Retsch, PM 100). γ-Al2O3 supports and Co/Al2O3 catalysts after mechanochemical treatments were characterized using a combination of techniques. The study of solid particles revealed the abrasion and fragmentation phenomena of porous γ-Al2O3 particles and pore filling under milling. Functional cobalt particles introduced by the mechanochemical synthesis were observed to be preferentially localized on the outer surface of the alumina supports. High Fischer–Tropsch reaction rates were obtained with the catalysts prepared by optimized mechanochemical synthesis conditions. The enhanced catalytic performance can be attributed to the relatively high dispersion of cobalt and the absence of inert cobalt aluminates which are usually present in the catalysts synthesized by the conventional impregnation.

CO and CO2 Methanation Over Supported Cobalt Catalysts

CO and CO2 methanation was investigated over cobalt catalysts supported on different supports such as γ-Al2O3, SiO2, TiO2, CeO2, and ZrO2. Among them, the cobalt catalyst supported on the high-surface-area CeO2 was determined to be the most active for both CO and CO2 methanation. These catalytic activities increased with increasing surface area of CeO2. To increase the specific catalytic activity for CO and CO2 methanation, various Co–CeO2 catalysts with different cobalt contents were prepared with co-precipitation method. The optimum cobalt content was determined for both reactions. The prepared catalysts were characterized with N2 physisorption, temperature-programmed reduction with H2, pulsed-chemisorption of CO2, temperature-programmed desorption of CO2, and X-ray diffraction. The high cobalt dispersion and strong CO2 adsorption appeared to be responsible for the high catalytic activity for CO and CO2 methanation, respectively. This Co–CeO2 also showed the stable catalytic activity even after an exposure to high-temperature reaction conditions.

Precisely designing bimodal catalyst structure to trap cobalt nanoparticles inside mesopores and its application in Fischer-Tropsch synthesis

Bimodal catalyst support is a widely applied concept in solid catalysts but until now the supported metal on the bimodal support has no satisfied dispersion and the average metal particle size is generally larger than the pore size of small pores (mesopores) of the bimodal structure, lowering the power and significance of the bimodal structure. To overcome this problem, a novel ZrO2SiO2 bimodal pore support with enlarged depth of small pores, was successfully fabricated though self-assembly of nanosized ZrSiO4 inside pore of silica gel. Cobalt was finely dispersed on bimodal support by incipient-wetness impregnation. The bimodal support and catalyst were characterized by various techniques, such as BET, TEM, XRD, and TPR and H2-chemisorpion. The bimodal catalyst was applied in slurry-phase Fischer-Tropsch synthesis. Characterization results suggested that bimodal pore support processed larger BET surface than pristine silica gel, due to the newly formed small pores organized by packing of the ZrSiO4 nanoparticles. The enlarged BET surface area leaded to good dispersion of cobalt species. More importantly, it was observed that the deep small pores could serve as traps for Co nanoparticles, preventing sintering and agglomeration due to confinement effect. Co/30wt%ZrO2-Q-50 and Co/20wt%ZrO2-Q-100 bimodal catalyst with large BET surface and deep small pores exhibited excellent catalytic activity, and selectivity to heavy hydrocarbons due to high dispersion of cobalt and unique pore structure, while coexisting ZrO2 promoted the reduction of finely-dispersed cobalt oxide nanoparticles. It is interesting to find that different from the conventional theory where small metallic particle lowers chain-growth probability, the trapped smaller Co particles inside deep small pores achieved higher chain-growth probability due to the facilitated re-adsorption of olefinic intermediates happening inside deep small pores.

Biogas to syngas conversion without carbonaceous deposits via the dry reforming reaction using transition metal catalysts

This study investigates how morphology, active metal content and oxygen storage capacity of various bimetallic NiCo/CeZrO2 materials influence their catalytic activity and stability in the methane dry reforming reaction. Catalyst preparation procedure and chemical composition were steered to finally obtain materials, which do not accumulate carbon during the CH4/CO2 reforming reaction. Oxygen storage capacity of the CeZrO2 catalyst support was identified to play a vital role in retarding carbon accumulation over the tested NiCo/CeZrO2 materials. This property can be fully developed when a nanocrystalline solid solution of CeO2 and ZrO2 is formed. Secondly, a high dispersion of nickel and cobalt is crucial for two reasons: (i) catalysts which contain larger NiCo bimetallic particles (for example with 12–18wt.% active metal loading) exhibit a low metal-support interphase that results in enhanced coke formation rates; (ii) additionally, only a marginal gain in methane reforming rates are achieved at higher loadings, compared to catalysts with a 3–6wt.% active metal content. We demonstrated that a NiCo/CeZrO2 catalyst under relevant operating conditions after 400h TOS maintains 79 and 84% conversion of CH4 and CO2, with negligible coke accumulation.

Effect of preparation methods on activation of cobalt catalyst supported on silica fiber for Fischer–Tropsch synthesis

Silica fibers (SF) have been synthesized by facile combination of a sol–gel method and electrospinning technique and investigated as supports of cobalt catalysts for Fischer–Tropsch synthesis (FTS). Tetraethyl orthosilicate (TEOS) was used as precursors for the production of silica fibers. The cobalt supported on silica fibers (Co/SF) catalysts were prepared with the impregnation (IM), strong electrostatic adsorption (SEA), deposition precipitation (DP), and hydrothermal synthesis (HT) method and characterized by N2 adsorption, SEM, TEM, XRD, EDX, H2-TPR and H2-chemisorption. The results showed that the preparation methods significant affected the catalytic activity of Co/SF catalyst. The catalyst prepared by the SEA method showed the highest catalytic activity for FTS reaction, and their catalytic activities were varied in the order of SEA>DP>IM≫HT. The XRD analysis showed that the Co3O4 particles on the Co/SF catalyst prepared by SEA method (9nm) were smaller than those on catalyst prepared by IM method (30nm). The size of the Co3O4 particles was controlled by the preparation method, with small particles formed in catalyst prepared by SEA, DP and HT method. The excellent catalytic activity of Co/SF-SEA may be attributed to the smaller size of cobalt particles and higher cobalt dispersion on this catalyst.

Tuning interactions between zeolite and supported metal by physical-sputtering to achieve higher catalytic performances

To substitute for petroleum, Fischer-Tropsch synthesis (FTS) is an environmentally benign process to produce synthetic diesel (n-paraffin) from syngas. Industrially, the synthetic gasoline (iso-paraffin) can be produced with a FTS process followed by isomerization and hydrocracking processes over solid-acid catalysts. Herein, we demonstrate a cobalt nano-catalyst synthesized by physical-sputtering method that the metallic cobalt nano-particles homogeneously disperse on the H-ZSM5 zeolite support with weak Metal-Support Interactions (MSI). This catalyst performed the high gasoline-range iso-paraffin productivity through the combined FTS, isomerization and hydrocracking reactions. The weak MSI results in the easy reducibility of the cobalt nano-particles; the high cobalt dispersion accelerates n-paraffin diffusion to the neighboring acidic sites on the H-ZSM5 support for isomerization and hydrocracking. Both factors guarantee its high CO conversion and iso-paraffin selectivity. This physical-sputtering technique to synthesize the supported metallic nano-catalyst is a promising way to solve the critical problems caused by strong MSI for various processes.

Effect of Calcination Temperature and Pretreatment with Reaction Gas on Properties of Co/γ‐Al2O3 Catalysts for Partial Oxidation of Methane

The effects of calcination temperature and feedstock pretreatment on the catalytic performance of Co/γ-Al(2)O(3) catalysts were studied for partial oxidation of methane (POM) to synthesis gas, with emphasis on the role of feedstock pretreatment. The physicochemical properties of the catalysts were characterized by N(2) adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), H(2) temperature-programmed reduction (H(2) -TPR), and Raman spectroscopy. The results showed that the pretreatment of the catalyst by reaction gas significantly improved the catalytic activity and stability for the POM reaction. On the other hand, the effect of calcination temperature was less significant. Although the initial activity was increased by an increased calcination temperature, the catalyst without the feedstock pretreatment suffered a rapid deactivation. The reaction-atmosphere pretreatment was revealed as a process that mainly modified the surface structure of the catalyst. In that process, the formation of a CoAl(2)O(4) -like compound led to high Co metal dispersion after reduction, and the transformation of the carrier into α-Al(2)O(3) occurred over the catalyst surface. Both the high dispersion of cobalt and the presence of α-Al(2)O(3) surface phase were assumed as the important factors resulting in an excellent catalytic performance in terms of high activity and high stability.

Performance of Cobalt-Based Fischer-Tropsch Synthesis Catalysts Using Dielectric-Barrier Discharge Plasma as an Alternative to Thermal Calcination

Co-based catalysts were prepared by using dielectric-barrier discharge (DBD) plasma as an alternative method to conventional thermal calcination. The characterization results of N2-physisorption, temperature programmed reduction (TPR), transmission electron microscope (TEM), and X-ray diffraction (XRD) indicated that the catalysts prepared by DBD plasma had a higher specific surface area, lower reduction temperature, smaller particle size and higher cobalt dispersion as compared to calcined catalysts. The DBD plasma method can prevent the sintering and aggregation of active particles on the support due to the decreased treatment time (0.5 h) at lower temperature compared to the longer thermal calcination at higher temperature (at 500° C for 5 h). As a result, the catalytic performance of the Fischer-Tropsch synthesis on DBD plasma treated Co/SiO2 catalyst showed an enhanced activity, C5+ selectivity and catalytic stability as compared to the conventional thermal calcined Co/SiO2 catalyst.