Effect Of Cobalt Particle Size Research Articles

Synthesis of Nanostructured Catalytic Materials from Microemulsions

Since the 1980s [1,2], colloidal systems such as microemulsions (ME) have been widely investigated, especially for the synthesis of nanomaterials for various applications.[...]

Effect of Cobalt Particle Size on Acetone Steam Reforming

AbstractCarbon‐supported cobalt nanoparticles with different particle sizes were synthesized and characterized by complementary characterization techniques such as X‐ray diffraction, N2 sorption, acetone temperature‐programmed desorption, transmission electron microscopy, and CO chemisorption. Using acetone steam reforming reaction as a probe reaction, we revealed a volcano‐shape curve of the intrinsic activity (turnover frequency of acetone) and the CO2 selectivity as a function of the cobalt particle size with the highest activity and selectivity observed at a particle size of approximately 12.8 nm. Our results indicate that the overall performance of acetone steam reforming is related to a combination of particle‐size‐dependent acetone decomposition, water dissociation, and the oxidation state of the cobalt nanoparticles.

Impact of Hydrogenolysis on the Selectivity of the Fischer-Tropsch Synthesis: Diesel Fuel Production over Mesoporous Zeolite-Y-Supported Cobalt Nanoparticles.

Selectivity control is a challenging goal in Fischer-Tropsch (FT) synthesis. Hydrogenolysis is known to occur during FT synthesis, but its impact on product selectivity has been overlooked. Demonstrated herein is that effective control of hydrogenolysis by using mesoporous zeolite Y-supported cobalt nanoparticles can enhance the diesel fuel selectivity while keeping methane selectivity low. The sizes of the cobalt particles and mesopores are key factors which determine the selectivity both in FT synthesis and in hydrogenolysis of n-hexadecane, a model compound of heavier hydrocarbons. The diesel fuel selectivity in FT synthesis can reach 60 % with a CH4 selectivity of 5 % over a Na-type mesoporous Y-supported cobalt catalyst with medium mean sizes of 8.4 nm (Co particles) and 15 nm (mesopores). These findings offer a new strategy to tune the product selectivity and possible interpretations of the effect of cobalt particle size and the effect of support pore size in FT synthesis.

Impact of Hydrogenolysis on the Selectivity of the Fischer–Tropsch Synthesis: Diesel Fuel Production over Mesoporous Zeolite‐Y‐Supported Cobalt Nanoparticles

AbstractSelectivity control is a challenging goal in Fischer–Tropsch (FT) synthesis. Hydrogenolysis is known to occur during FT synthesis, but its impact on product selectivity has been overlooked. Demonstrated herein is that effective control of hydrogenolysis by using mesoporous zeolite Y‐supported cobalt nanoparticles can enhance the diesel fuel selectivity while keeping methane selectivity low. The sizes of the cobalt particles and mesopores are key factors which determine the selectivity both in FT synthesis and in hydrogenolysis of n‐hexadecane, a model compound of heavier hydrocarbons. The diesel fuel selectivity in FT synthesis can reach 60 % with a CH4 selectivity of 5 % over a Na‐type mesoporous Y‐supported cobalt catalyst with medium mean sizes of 8.4 nm (Co particles) and 15 nm (mesopores). These findings offer a new strategy to tune the product selectivity and possible interpretations of the effect of cobalt particle size and the effect of support pore size in FT synthesis.

Cobalt particle size effects on catalytic performance for ethanol steam reforming – Smaller is better

The effect of the cobalt particle size in the ethanol steam reforming reaction at 773K for hydrogen production was investigated using cobalt on carbon nanofiber catalysts. It was found that the turnover frequency increases with decreasing Co particle size, which was attributed to the increasing fraction of edge and corner surface sites with decreasing size. Regarding catalyst stability, a decrease in deactivation rate was observed with decreasing cobalt particle size. This was caused by a significantly lower amount of carbon deposition on the smallest Co particles than on larger ones, as concluded from transmission electron microscopy measurements. The reduced amount of carbon deposition is ascribed to a lower fraction of terrace atoms, proposed to be responsible for initiation of carbon deposition on catalysts with large (>10nm) Co particles. Therefore, it was concluded for this non-noble metal that the smallest particles perform best in catalysis of ethanol steam reforming.

Effect of Cobalt Particle Size on the Catalyst Intrinsic Activity for Fischer–Tropsch Synthesis

The active sites for Fischer–Tropsch synthesis are surface cobalt metal atoms. In order to obtain a high dispersion for reaction, ultra-small cobalt clusters (e.g., 1 nm) were supported on KL-zeolite using a chemical vapor deposition procedure. A Pt promoter was added to facilitate reduction of the cobalt oxide species. Upon activation in hydrogen, a high dispersion was obtained, but after exposure to realistic FT conditions in a pressurized continuously stirred tank reactor reactor, XANES and EXAFS results indicate that the fine cobalt clusters were susceptible to reoxidation, resulting in adverse activity and selectivity (30–37 % CH4 and 3.5–4.0 % CO2 on a C% basis).

Cobalt Particle Size Effects in the Fischer–Tropsch Synthesis and in the Hydrogenation of CO2 Studied with Nanoparticle Model Catalysts on Silica

We have investigated the effect of cobalt nanoparticle size in Fischer–Tropsch synthesis (CO/H2) and have compared it to data obtained for carbon dioxide hydrogenation (CO2/H2) using model catalysts produced by colloidal methods. Both reactions demonstrated size dependence, in which we observed an increase of the turnover frequency with increasing average particle size. In both case, a maximum activity was found for cobalt particles around 10–11 nm in size. Regarding the selectivity, no size-dependent effect has been observed for the CO2 hydrogenation, whereas CO hydrogenation selectivity depends both on the temperature and on the size of the particles. The hydrogenation of CO2 produces mainly methane and carbon monoxide for all sizes and temperatures. The Fischer–Tropsch reaction exhibited small changes in the selectivity at low temperature (below 250 °C) while at high temperatures we observed an increase in chain growth with the increase of the size of cobalt particles. At 250 °C, large crystallites exhibit a higher selectivity to olefin than to the paraffin equivalents, indicating a decrease in the hydrogenation activity.

Studies of Cobalt Particle Size Effects on Fischer–Tropsch Synthesis over Core–Shell‐Structured Catalysts

AbstractA series of core–shell‐structured catalysts that consist of different‐sized Co3O4 nano‐particles and silica shells were prepared by an in situ coating method. The reduced catalysts displayed uniform core sizes that ranged from 5.5–12.7 nm as ascertained by TEM, which concurred well with XRD analysis. The BET results revealed the highly mesoporous nature of the silica shell, which contributes to the facile access of the reactant gas to the active sites on the core particles. The degree of reduction of the calcined catalysts studied by H2 temperature‐programmed reduction was enhanced with increased Co particle size. In the Fischer–Tropsch synthesis, a volcano‐like curve was plotted as the CO conversion and Co‐time‐yield revealed a rapid growth if the particle size increased from 5.5 to 8.7 nm and then decreased with further increased particle size to 12.7 nm, which is an effect of the combination of Co dispersion and reducibility. However, the turnover frequency remained invariant for catalysts with particle sizes larger than 8.7 nm. If we consider the product selectivity, generally, larger particles led to a longer chain length of hydrocarbons with a larger chain‐growth probability. The selectivity towards methane decreased and the corresponding heavy hydrocarbons (C19+) increased continuously with the increase of particle size. The catalyst with a particle size of 8.7 nm exhibited the highest selectivity and the maximum space‐time‐yield towards middle distillates (C5–C18) because of the modest chain‐growth probability.

Understanding the Effect of Cobalt Particle Size on Fischer−Tropsch Synthesis: Surface Species and Mechanistic Studies by SSITKA and Kinetic Isotope Effect

Co/γ-Al(2)O(3) catalysts with particle sizes in the range of 4-15 nm were investigated by isothermal hydrogenation (IH), temperature programmed hydrogenation (TPH), and steady-state isotopic transient kinetic analysis (SSITKA). Kinetic isotope effect experiments were used to probe possible mechanisms on Co/γ-Al(2)O(3) with different particle size. It was found that CO dissociated on Co/γ-Al(2)O(3) catalysts at 210 °C. The total amount of CO(2) formed following the dissociation depends on the cobalt crystal size. O-Co binding energy was found to be highly dependent on the Co metal particle size, whereas similar C-Co binding energy was found on catalysts with different Co particle size. Very strongly bonded carbon and oxygen surface species increased with decreasing particle size and acted as site blocking species in the methanation reaction. SSITKA experiments showed that the intrinsic activity (1/τ(CH(x))) remained constant as the particle size increased from 4 to 15 nm. The number of surface intermediates (N(CH(x))) increased with increasing particle size. The apparent activation energies were found similar for these catalysts, about 85 kJ/mol. D(2)-H(2) switches further confirmed that the particle size did not change the kinetically relevant steps in the reaction. The reactivity of the active sites on the 4 nm particles was the same as those on the 8, 11, and 15 nm particles, and only the number of total available surface active sites was less on the 4 nm particles than on the others.

Cobalt particle size effects in Fischer–Tropsch synthesis: structural and in situ spectroscopic characterisation on reverse micelle-synthesised Co/ITQ-2 model catalysts

A series of 10%Co/ITQ-2 model catalysts have been prepared by combining a reverse micellar synthesis with a surface silylated ITQ-2 delaminated zeolite. The catalysts display rather uniform Co 0 particle size distributions in the 5–11 nm range as ascertained by XRD, H 2-chemisorption and (HR)TEM. Additionally, a low dispersed 30%Co/SiO 2 reference sample ( d(Co 0) = 141 nm) has been prepared by supporting a Co 3O 4 nanopowder on spherical SiO 2. H 2-TPR and DR UV–vis spectroscopy reveal that the preparative approach leads to highly reducible catalysts in the d(Co 0) range of 5.6–141 nm, while the activation energies for the stepwise Co 3O 4 → CoO → Co 0 reduction are found to be particle size dependent. Formation of barely reducible surface and bulk Co silicate species is observed for samples with d(Co 3O 4) ⩽ 5.9 nm. Under realistic Fischer–Tropsch synthesis conditions (493 K, 2.0 MPa) the TOF increases from 1.2 × 10 −3 to 8.6 × 10 −3 s −1 when d(Co 0) is increased from 5.6 to 10.4 nm, and then it remains constant up to a particle size of 141 nm. In situ and at work FTIR of adsorbed CO reveal a severe cobalt surface reconstruction towards more open crystal planes and/or defect sites (Co–carbonyl bands in the region of 2000–2025 cm −1) and suggest adsorbed C adatoms (surface carbidic species), derived from CO dissociation, as the true restructuring agent. Under FTS conditions, this Co surface reconstruction occurs similarly irrespective of the metal particle size. Moreover, an enhancement in the proportion of Co–SiO 2 interfacial Co δ+ sites (Co–CO band at 2060 cm −1) takes place particularly in small cobalt nanoparticles (5.6 nm) likely as a consequence of nanoparticle flattening, as suggested by TEM after catalysis. These Co–SiO 2 interfacial sites are tentatively proposed as responsible for the decreased TOF observed for d(Co 0) < 10 nm.

On the Origin of the Cobalt Particle Size Effects in Fischer−Tropsch Catalysis

The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.

Cobalt Particle Size Effects in the Fischer−Tropsch Reaction Studied with Carbon Nanofiber Supported Catalysts

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.