Catalysts For Fischer-Tropsch Synthesis Research Articles

CO/CO2 hydrogenation for the production of lighter hydrocarbons over SAPO-34 modified hybrid FTS catalysts

Fischer-Tropsch synthesis (FTS) is known as the catalytic reactions which convert syngas (CO + H2) to value-added hydrocarbon products. GTL (gas to liquid)-FPSO (floating, production, storage, and offloading) process is one of the solutions for the monetization of CO2 rich natural gas or stranded natural gas in offshore. In this work, for the compact design of the GTL-FPSO process, the hybrid FTS catalyst was designed by the physical mixing of the Co/γ-alumina with the hydrocracking catalyst (γ-alumina and SAPO-34), and also Co/SAPO-34 catalyst was prepared without any γ-alumina. SAPO-34 was synthesized by the hydrothermal procedure, and Co/γ-alumina and Co/SAPO-34 catalysts were prepared by the impregnation method. The relationship of catalytic activity, methane selectivity, and C5+ selectivity with the acid sites were studied for hybrid FTS catalyst. It was found that by adding SAPO-34 to Co/γ-alumina catalyst, a lighter fuel can be produced. Also, the effect of CO2/(CO + CO2) ratio over the Co/SAPO-34 catalyst on the carbon conversion and product distribution was investigated for the first time. It was found that there is an optimum value for CO2/(CO + CO2) ratio in the feed that is around 0.25. At this ratio, the C5+ selectivities and carbon conversion are the highest, and the methane selectivity is the lowest. It was considered that the high amount of CO2 in the feed promotes the hydrocracking and the methanation reaction over Co/SAPO-34 catalyst.

Fischer-Tropsch synthesis over carbon-encapsulated cobalt and iron nanoparticles embedded in 3D-framework of carbon nanotubes

Development of alternative ways to produce clean fuels is a significant and strategically important challenge. One of the most effective approaches is the Fischer-Tropsch synthesis (FTS), which is realized over Co- or Fe-based catalysts. In this work spark plasma sintering was applied for the synthesis of bulk core-shell Co and Fe CNT-supported FTS catalysts. The effect of the sintering temperature on the structure, evolution, and catalytic performance of the samples was studied by combination of physicochemical methods. Catalysts showed high activities and TOFs without pre-reduction: 11.6 molCOgCo−1 s−1 and 0.1 s−1 for Co and 20.3 molCOgFe−1 s−1 and ~0.9 s−1 for Fe samples. The unusual dependence of C5+ selectivity on the Co particle size along with the presence of the amorphous carbide and amorphous carbon layers under graphitic shell in Fe catalysts point to the diffusion of reagents and products through shell defects and the catalytic reaction on the particle core surface.

Identify Zr Promotion Effects in Atomic Scale for Co-Based Catalysts in Fischer–Tropsch Synthesis

Introducing promoters on cobalt-based catalysts for Fischer-Tropsch synthesis (FTS) have been found efficient for adjusting their performance in converting syngas into long-chain hydrocarbons. Deta...

Fischer–Tropsch Synthesis: ZIF-8@ZIF-67-Derived Cobalt Nanoparticle-Embedded Nanocage Catalysts

The preparation of highly active and stable catalysts for syngas conversion is a major challenge for Fischer–Tropsch synthesis (FTS). Herein, we report a strategy to prepare a highly dispersed Co-embedded porous carbon nanocage (CoPCN) structure derived from a core–shell metal–organic framework (MOF) ZIF-8@ZIF-67 precursor. High Co loading (over wt 30%) is achieved while maintaining an optimal dispersion and particle size of the active Co phase when a ZIF-8@ZIF-67 is pyrolyzed at 920 °C. Besides, the porous channels and hollow structures of the CoPCN strengthen the diffusion of reactants and the hydrocarbon product, enhancing the C5+ selectivity and CO conversion. The CoPCN shows high stability in FTS with a CO conversion of 18.3%, 80.2% selectivity for long-chain hydrocarbons (C5+), and 8.9% selectivity for short-chain hydrocarbons (C2–C4) after 100 h time on stream. Compared with other MOF-derived FTS catalysts, CoPCN-920 can achieve higher C5+ selectivity at a lower reaction temperature. The present work uncovers the relationship between the porous structure and catalytic performance, providing an efficient method to prepare promising materials for enhanced FTS stability, activity, and selectivity.

The Effect of CO Partial Pressure on Important Kinetic Parameters of Methanation Reaction on Co-Based FTS Catalyst Studied by SSITKA-MS and Operando DRIFTS-MS Techniques

A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface.

Exfoliated graphite as a heat-conductive frame for a new pelletized Fischer–Tropsch synthesis catalyst

The introduction of exfoliated graphite into pelletized Co-based catalysts affects in the most positive way both catalytic and physico-chemical properties. Unlike other carbons, the exfoliated graphite allows combination of extended surface with high thermal conductivity. An open-ended system of slit-type pores is revealed by X-ray tomography. Electron microscopy shows graphitic platelets forming an all-penetrating heat-conductive frame. Element distribution analysis shows that this catalyst cannot be considered as a cobalt-on-carbon catalyst since the graphitic component carries insignificant amount of cobalt and serves as a heat-conductive frame only. Thermal conductivity of a catalyst with graphite frame is 2 times higher than that of its industrial predecessor with Al metal frame and 30 times higher than that of a catalyst without heat-conductive additive. Testing in exothermal Fischer–Tropsch synthesis shows a favorable influence of the graphitic additive, i.e. such catalyst shows productivity of 455 g/kg/h at GHSV of 3000 h−1.

High Selectivity to Aromatics by a Mg and Na Co-modified Catalyst in Direct Conversion of Syngas.

The demand for aromatics, especially benzene, toluene, and xylene, has been increased in recent years as the crucial feedstocks of coatings and pharmaceutical industry. In this work, a modified Fischer–Tropsch synthesis (FTS) catalyst FeNaMg was fabricated via a sol-precipitation method and integrated with an HZSM-5 aromatization catalyst for the aromatics synthesis from syngas by a one-step process. Syngas was first converted to lower olefins as intermediates on the active component of the FeNaMg catalyst followed by aromatization on zeolite. Different characterization approaches, such as BET, XRD, XPS, hydrogen temperature-programmed reduction, temperature-programmed desorption of CO, TG, and SEM, revealed that Mg efficiently optimized physicochemical properties of the Fe-based catalyst by generating a MgFe2O4 spinel structure. Further investigation demonstrated that the MgFe2O4 spinel structure could increase the syngas adsorption area, facilitating the reduction and carburization of the Fe phase, while Mg decreased CO2 selectivity (31.26 to21%) by restraining the water–gas shift reaction and improved the utilization efficiency of carbon. At the same time, alkali metal Na changed the surface electronic environment of the FTS catalyst to enhance CO adsorption as an electronic promoter, which suppressed methane formation by restraining over hydrogenation. Therefore, the synergism that existed between Mg and Na during the reaction escalated the CO conversion and aromatics selectivity to 96.19 and 51.38%, respectively.

Tunable Fe3O4 nanoparticles assembled porous microspheres as catalysts for Fischer-Tropsch synthesis to lower olefins

Fe catalyzed Fischer-Tropsch synthesis to lower olefins (FTO) has been recognized as a structure-sensitive reaction. Nanostructure Fe-based catalysts can expose more active surface. But bulk nanostructure Fe catalysts without support interaction show easily sintered and agglomerated, which makes hard to investigate size effect. Therefore, the effect of particle size on bulk Fe catalysts have rarely been reported. Herein, nanoparticles assembled Fe3O4 bulk catalysts were synthesized via a PAA-mediated solvothermal method to solely investigate the size effect of Fe phase. By tuning preparation parameters, a series of porous-Fe3O4 microspheres assembled by different nanoparticle size (8.5–16.5 nm) was obtained, maintaining constant microspheres size. When Fe3O4 nanoparticles are smaller than 10 nm, catalysts show similar catalytic activity (FTY). Beyond 10 nm, the FTY obviously decreases with increasing of particle size. Particularly, Fe3O4 with nanoparticle size of 9.9 nm performs the highest activity as well as C2-C4= selectivity and O/P ratio. The smaller particles attribute to C5+ formation, while inhibit CH4 selectivity. By CO-TPD, TPH-MS and XRD analysis, we discuss size effect on CO adsorption, surface carbon species and carburization degree, building relationship between particle size and catalytic activity. It is found that the carburization degree of iron phase correlates positively with catalytic activity. These results deepen understanding of the structure-performance relationship for bulk iron catalysts in FTO.

Fischer-Tropsch synthesis: Effect of nitric acid pretreatment on graphene-supported cobalt catalyst

As a support for cobalt Fischer-Tropsch synthesis catalyst, graphene needs to be pretreated to modify its surface properties for desired catalytic performance. Graphene was pretreated by thermal reflux using 20–60 % nitric acid. The pretreated graphene and catalysts were characterized with TPR, TEM, SEM, XRD, XPS and Raman spectroscopy to study the effect of treatment on microstructures of the materials. This work showed that cobalt metal uniformly dispersed on graphene, and with increasing pretreating acid concentration, surface defects increased and the cobalt particle size decreased, suggesting alleviated agglomeration of cobalt particles on graphene. Pretreating improved the reduction of the catalyst due to the increased defect sites, improved support-metal interaction and cobalt dispersion. Pretreated catalyst yielded better activity, higher C5+, but lower methane selectivity than unpretreated catalyst. Therefore, acid pretreatment of graphene can be an effective approach to modify the microstructure of the material, and can thus enhance the performance of the prepared catalyst.

Metal-organic framework (MOF)-derived catalysts for Fischer-Tropsch synthesis: Recent progress and future perspectives

Fischer-Tropsch Synthesis (FTS) is an important catalytic chemical reaction that converts a mixture of CO and H2 (syngas) derived from biomass, coal or natural gas into ultra-clean fuels or value-added chemicals. However, most traditional catalysts used in the Fischer-Tropsch process are faced with the problems of high deactivation rate triggered by sintering, phase changes and oxidation which hamper their catalytic performance. Metal-organic frameworks (MOFs)-derived materials have been a promising alternative in addressing the catalyst deactivation problems in FTS because of the encapsulation of their metal nanoparticles in carbon matrix and absence of large particle size, among other reasons. Therefore, this review emphasizes the most recent research headway in the investigation of MOFs as precursors to achieve high-performance FTS catalysts. Precisely, the design of iron and cobalt-based FTS catalysts from parent MOFs via MOF-mediated synthesis, the catalytic activity of the MOF-derived materials and the promoter effects under FTS operation were outlined and discussed. We have also evaluated the influence of MOF structures on the FTS performance and compared them with traditional/commercially available catalysts to show the importance of this approach. Finally, the challenges and opportunities to further expedite the extensive research efforts and promote their applications in material design and FT technology were mentioned.

Identification of efficient promoters and selectivity trends in high temperature Fischer-Tropsch synthesis over supported iron catalysts

In this work, 29 elements were evaluated as promoters for silica supported iron catalysts for high temperature Fischer-Tropsch synthesis using a high-throughput experimentation unit. The selected promoters include alkali/alkaline metals, transition metals, precious metals and lanthanides. Several general selectivity trends were observed and discussed. The selectivity enhancement to light olefins requires maintaining low selectivity to methane and light paraffins and at the same time, slowing the chain growth to the C5+ hydrocarbons.A major increase in Fischer-Tropsch rate principally due to higher intrinsic site activity, was observed over the catalysts promoted with metals with low melting points such as tin, antimony, bismuth and lead. These promoted catalysts also exhibited better stability. The effect of the promotion with tin and antimony on the olefin selectivity was not noticeable, while the presence of bismuth and lead results in the major enhancement of the selectivity to light olefins, lower methane and C2-C4 paraffin selectivities.

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.

Hydrothermal Stability of High-Surface-Area α-Al2O3 and Its Use as a Support for Hydrothermally Stable Fischer–Tropsch Synthesis Catalysts

Nanocrystalline corundum synthesized by ball milling of boehmite is found to be exceptionally robust toward chemical weathering, a common problem of transition aluminas in different applications, most notably in the case of supported catalysts, which are exposed to hydrothermal reaction environments. Detailed characterization and surface cation coordination analysis indicate that the absence of tetrahedral Al species on corundum makes it stable toward chemical weathering. A cobalt catalyst developed using nano-α-Al2O3 as the support showed Fischer–Tropsch synthesis activity and selectivity comparable to the benchmark Co/γ-Al2O3 and remained stable over 250 h on-stream.

Influence of amines in the synthesis and properties of mesostructured aluminas and assessment as supports for Co-based Fischer-Tropsch synthesis catalysts

The influence of addition of amines (hexylamine, tripropylamine, dodecylamine, and hexadecylamine) and dopants (Zr, Ce) on the properties of mesostructured aluminas (MAs) synthesized using either Tergitol 15-S-9 or Triton X-114 surfactants was investigated. Selected MAs were evaluated as supports for Co and Ru-Co catalysts and their Fischer-Tropsch synthesis (FTS) performance assessed at 493 K, 2.0 MPa, and H2/CO = 2. The materials were characterized by XRD, N2 sorption, TG-DTG, EA, XPS, H2-TPR, H2 chemisorption, and STEM. Regardless of the surfactant, all amines enlarged the pore size of undoped MAs by ca. 2–3 nm. The impact of amines on BET area and pore volume depended, however, on the surfactant-amine combination and on the presence and concentration of dopants. The Ru-promoted catalyst supported on the MA synthesized with Tergitol-tripropylamine doped with Zr displayed the highest FTS activity and C5+ selectivity due to both high cobalt reducibility and optimum particle size (8 nm).

INFLUENCE OF MANGANESE ON FORMATION OF ACTIVE PHASE OF COBALT CATALYST OF HYDROCARBON SYNTHESIS

The active phase and catalytic properties of cobalt catalysts of Fischer-Tropsch synthesis promoted with manganese and prepared by impregnation were studied. KSKG silica gel having a monodisperse structure with an average pore diameter of 12–16 nm was used as a support. The cobalt content in the obtained samples was 20.8-21.8 wt.%, manganese content was 0.5–2 wt.%. The structural characteristics of the catalysts were investigated by X-ray diffraction, transmission electron microscopy, temperature-programmed reduction and temperature-programmed hydrogen desorption. The catalytic activity and selectivity of the samples in the process of hydrocarbon synthesis were researched in the flow mode at a pressure of 0.1 MPa, at a Gas Hourly Space Velocity (GHSV) of 100 h–1, at the temperatures of 150–220 °C and a ratio of CO to hydrogen in synthesis gas of 2:1. Preliminary reduction of catalysts was carried out with hydrogen at a temperature of 400 °C. In the composition of the studied samples with manganese the formation of solid solutions of metals such as xCo3O4–(1–x)Mn3O4 was confirmed. It is shown that the reduction process of Co3O4 is determined by the composition and structure of solid solutions of components that are hard-to-recover compounds. The features of the promoting action of manganese depending on the concentration are determined. The introduction of 0.5–2 wt.% manganese allows to regulate the composition and microstructure of cobalt oxide compounds, the properties of metallic Co, increases the catalytic characteristics: activity, selectivity for C5+ and reduces the yield of methane. It is shown how, depending on the content of the metal additive, the manifestations of the structural (at a concentration of 0–1 wt.%) or electronic (at a concentration of 2 wt.%) promoting action of manganese increase. The optimum concentration of manganese is 1 wt.%. The catalyst is characterized by the minimum average size of crystallites, the highest value of the specific surface of cobalt metal and the maximum catalytic indicators.

Reduction and carburization of iron oxides for Fischer–Tropsch synthesis

Abstract The activation of iron oxide Fischer–Tropsch Synthesis (FTS) catalysts was investigated during pretreatment: reduction in hydrogen followed by carburization in either CO or syngas mixture, or simultaneously reduction and carburization in syngas. A combination of different complementary in situ techniques was used to gain insight into the behavior of Fe-based FTS catalysts during activation. In situ XRD was used to identify the crystalline structures present during both reduction in hydrogen and carburization. An increase in reduction rate was established when increasing the temperature. A complete reduction was demonstrated in the ETEM and a grain size dependency was proven, i.e. bigger grains need higher temperature in order to reduce. XPS and XAS both indicate the formation of a small amount of carbonaceous species at the surface of the bulk metallic iron during carburization.

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%.

Effect of start-up solvent on the performance of Co catalyst for Fischer-Tropsch synthesis in stirred-tank reactor

The starting solvent is essential for carrying out Fischer-Tropsch (FT) synthesis in a stirred-tank reactor. The most commonly used solvents are C28+ hydrocarbon oils, wax, heavier wax, and FT wax. In general, FT activity of Co-based catalysts was found to decrease for initial days, and one of the reasons may be related to the filling of catalyst pores with heavier wax leading to lower gas diffusions and thus lower CO conversion. In this work, therefore, solvents with varying molecular weight and polarity, including glycerol, 1-octanol, Polywax-655, and dimethyl phthalate were tested as a starting medium for 0.27%Ag-25%Co/Al2O3 catalyst. It was found that low molecular weight non-polar solvent, for example, Polywax 655 showed higher conversions. On the other hand, glycerol a high polar solvent tends to firmly bound with a catalyst surface and the associated water produced during FT synthesis could trigger catalyst deactivation from the oxidation of cobalt. This indicates the superiority of non-polar over polar solvents for FT synthesis. However, 1-octanol as a starting solvent showed the least catalytic deactivation rate and competitive FTS activity. Thus, 1-octanol has the potential as a start-up solvent for FT reaction in stirred tank reactors.

Hierarchical porous spinel MFe2O4 (M=Fe, Zn, Ni and Co) nanoparticles: Facile synthesis approach and their superb stability and catalytic performance in Fischer-Tropsch synthesis

The environmental pollution problem and the dwindling in the crude oil resources have been driving considerable researches into alternative routes for the production of clean and low cost liquid fuels. Fischer-Tropsch synthesis (FTS) is a key-technology and a clean alternative for the production of valuable organic chemicals and high standard liquid fuels through the reaction of syngas (CO/H2). In this study, one-pot hydrothermal reaction was adopted for the synthesis of robust MFe2O4 (M = Fe, Zn, Ni and Co) nanoparticles (NPs) with hierarchical porous-structured. The as-synthesized NPs and without the reported necessary pre-calcination step were appointed as robust catalysts in FTS at various reaction temperature (240–290 °C). The obtained catalytic results have demonstrated that CoFe2O4 NPs catalyst with its inversed spinel structure exhibited the optimum catalytic activity compared with the type of normal spinel structure of ZnFe2O4 catalyst. More precisely, CoFe2O4 NPs catalyst showed the best CO conversion ratio of 96% at 290 °C and the highest light-olefin (C2–C4) selectivity value of 63%. However, Fe3O4 NPs catalyst displayed the highest selectivity toward long chain hydrocarbon product (C5+) of 50 wt%, and the lowest CH4 selectivity value of 12% at 260 °C. Notably, the obtained results from X-ray diffraction and N2-physisorption characterization for the spent ferrite catalysts demonstrated that the co-existence of the active sites of Hägg iron carbide (χ-Fe5C2) and metallic Co phases and along with the high surface area and pore diameter plays the key role for the obtained high catalytic activity in CoFe2O4 NPs. Moreover, the produced catalysts in this study exhibited good stability performance within maintained high catalytic activity for a period of at least 700 h of industrial-relevant FTS conditions.

Removal of sodium compounds from Co/SBA-15 catalysts for Fischer-Tropsch Synthesis

In the present work, the synthesis of Co/SBA-15 catalysts intended for Fischer-Tropsch (FT) synthesis was performed. During the synthesis of catalysts there was contamination of samples with sodium nitrate due to the nature of the reducing agent used for the synthesis of metallic phase. This kind of impurity is not advantageous for the FT reaction. Attempts at removal of sodium compounds were carried out by means of simple leaching treatments, using ethanol and acid ethylenediaminetetraacetic (EDTA) as solvent and complexing agent, respectively, followed by heat treatment at temperatures of 150 or 300°C. It was possible to conclude that the treatment using EDTA was more effective in removing almost all the alkaline phase of samples, despite the occurrence of oxidation, agglomeration, and removal of the metal nanoparticles in this process. In addition, there were no significant differences in the product selectivity of FT synthesis of the catalysts after sodium removal, although the nanoparticles were larger than 10 nm.