Small Catalyst Particles Research Articles

Carbon nanotube-supported catalysts prepared by a modified photo-Fenton process for Fischer–Tropsch synthesis

We demonstrate the use of a green catalyst precursor and oxidant in a modified photo-Fenton process for synthesis of efficient Fischer–Tropsch catalysts. The process yields small, stable, and well-dispersed catalyst nanoparticles (Fe or Co) on carbon nanotubes (CNTs) that do not require calcination due to the absence of foreign components (nitrate, chloride, or acetate) that must be removed during postsynthesis treatment. The synthesized catalysts were characterized by X-ray diffraction (XRD), N2 physisorption, H2 temperature-programmed reduction (H2 TPR), Raman spectroscopy, transmission electron microscopy (TEM), and inductively coupled plasma (ICP) spectroscopy. Results showed high catalyst dispersion and smaller catalyst particles with narrow particle-size distributions in comparison to catalysts synthesized by incipient wetness impregnation (IWI). Performance of CNT-supported catalysts (CO conversion, C5+ selectivity, and catalyst lifetime) prepared by the photo-Fenton process was evaluated during Fischer–Tropsch synthesis (FTS) and compared with that of CNT- and SiO2-supported catalysts synthesized via IWI. In general, a CNT-based Fe or Co catalyst, prepared by the photo-Fenton process, shows a higher performance at a lower FTS reaction temperature (200 °C for Co and 250 °C for Fe) and a decreased potential for catalyst sintering in comparison with that of conventional catalysts (synthesized via IWI). In particular, a Co catalyst obtained from the photo-Fenton process shows high CO conversion (∼80%) and outstanding selectivity for liquid hydrocarbons (C5+; ∼70%). The process demonstrates a new synthesis route for exploiting outstanding material properties of CNTs in catalysis.

Effects of particle size, pretreatment, and catalysis on microwave pyrolysis of corn stover

The effects of particle size, pretreatment (acid pretreatment and steam explosion), and catalysis of aluminum oxide (Al2O3) on heating efficiency, product distribution, and gaseous product composition of corn stover pyrolysis using microwave heating were investigated in this study. Both maximum temperature and heating rate increased with decreasing particle size of corn stover. The heating efficiency was also improved over Al2O3 and by applying both pretreatment methods. Adding 10-mesh Al2O3 increased the gaseous yield but decreased the liquid yield. However, this phenomenon did not exist when 50-mesh Al2O3 was used. This may be attributable to that small catalyst particles are encapsulated by other catalyst and biomass particles to reduce their catalytic activity. Applying acid pretreatment substantially decreased the gaseous yield but increased the liquid yield, whereas the effect of steam explosion was not significant. Approximately half of CO was produced during the first 5 min of experiment, but the yields of H2, CH4, and CO2 produced from microwave pyrolysis of corn stover pretreated by acid were relatively small or even none during this initial period. The reaction kinetics for microwave pyrolysis of corn stover was analyzed by using the first- and second-order reaction models.

Synergetic Effects of Molybdenum and Magnesium in Ni–Mo–Mg Catalysts on the One-Step Carbonization of Polystyrene into Carbon Nanotubes

The Ni, Ni–Mo, Ni–Mg, and Ni–Mo–Mg catalysts were systematically explored for the one-step conversion of polystyrene (PS) into carbon nanotubes (CNTs). The catalysts were characterized in the phase structure, morphology, reducibility, and carbonization performance. It was found that the Mo contributed to the small catalyst particle growth by the formation of NiMoO4 phase. The addition of Mg had no effect on catalyst particle sizes but effectively controlled the carbon solubility and led to the growth of CNTs by the generation of NiMgO2. Finally, we demonstrated that the Ni–Mo–Mg catalysts showed the most efficient for the conversion of PS into CNTs owing to the individually synergetic effect of Mo and Mg elements. Besides catalysts, the degradation additives and the carbonization temperature were also discussed for their effects on the carbonization of PS. It is believed that the work provides potential for the conversion of PS waste into CNTs.

Development of Carbon-Free Electrocatalysts for Water Electrolysis

Introduction Hydrogen production from water electrolysis and further electricity generation through fuel cells have been focused on as one of the efficient uses of renewable energy. Regarding to water electrolysis, the PEM type water electrolyzer is attractive in terms of reduced ohmic resistance and also a similar assembly to well-developed PEFCs. On the other hand, the relatively low active surface area of anode electrocatalysts is a problem since carbon supports are not usable in order to disperse small catalyst particles under the high potential condition over 1.5 V. So far, our group has been working on mesoporous materials for PEFCs. We have found that mesoporous materials can keep those porous structures even after being built into the device, and then, the porous structure can improve the mass transfer within catalyst layers. Based on the similar idea used in our PEFC studies, we propose development of carbon-free mesoporous metal catalysts with high active surface area to increase the efficiency of water electrolyzer. In this study, mesoporous Pt and Ir materials were particularly developed and further evaluated as anode electrocatalysts. Experimental method Similar to the synthesis of mesoporous carbon,1 a surfactant, Pluronic F127, was used as a template to form a porous structure. Pt(acac)2 and Ir(acac)3 were used as metal precursors. After mixing the surfactant and metal precursors, the resultant complex was slowly heated up in N2or Air atmosphere to make the mesoporous structure. After the basic material characterizations, their electrochemical measurements were performed using a common half-cell setup in the solution. Oxygen evolution reaction (OER) activity was evaluated as anode electrocatalysts. The durability against high potential like 1.8 V was also analyzed. Results and discussion The resulting Pt material was confirmed to have a porous structure constructed by aggregation of Pt nanoparticles as shown in Figure 1. Based on the nitrogen sorption measurement, the randomly oriented porous structure was also confirmed, and the pore diameter was found mostly smaller than 100 nm. With this porous Pt material, OER activity was evaluated and compared with a standard Pt/KB electrocatalyst, TEC10E50E. As shown in Figure 2, the initial activity resulted in the same even though ECSA was six times higher for Pt/KB. Then, the durability of porous Pt and Pt/KB was examined by applying continuous high potential of 1.8 V. Dot lines in Figure 2 shows the OER activity after 10 minutes of holding at 1.8 V．As expected, Pt/KB lost most of OER activity owing to carbon corrosion. Consequently, carbon-free porous Pt electrocatalyst was successfully prepared and its high activity and durability was confirmed. Similarly, porous Ir electrocatalyst is prepared and its OER activity will be discussed. Reference (1) A. Hayashi, et al., Electrochim. Acta, 53, 6117-6125 (2008). Figure 1

Mass Transfer Effect on the Oxidation of Alkenes and Phenol with Hydrogen Peroxide Using TS-1 Titanosilicate as a Catalyst

Liquid-phase epoxidation of alkenes and hydroxylation of phenol were carried out using TS-1 titanosilicate as a catalyst with the particle sizes ranging from 0.15 up to 25 µm. Experimental reaction rates as measured on the smallest catalyst particles were found to be free of mass transfer influence. This makes it possible to describe quantitatively the internal diffusion effects for large-size grains, so as to evaluate the values of reagent diffusion coefficients within the catalyst pores.

Comparison of conventional and spherical reactor for the industrial auto-thermal reforming of methane to maximize synthesis gas and minimize CO2

Auto-thermal reforming (ATR), a combination of exothermic partial oxidation and endothermic steam reforming of methane, is an important process to produce syngas for petrochemical industries. In a commercial ATR unit, tubular fixed bed reactors are typically used. Pressure drop across the tube, high manufacturing costs, and low production capacity are some disadvantages of these reactors. The main propose of this study is to offer an optimized radial flow, spherical packed bed reactor as a promising alternative for overcoming the drawbacks of conventional tubular reactors. In the current research, a one dimensional pseudo-homogeneous model based on mass, energy, and momentum balances is applied to simulate the performance of packed-bed reactors for the production of syngas in both tubular and spherical reactors. In the optimization section, the proposed work explores optimal values of various decision variables that simultaneously maximize outlet molar flow rate of H2, CO and minimize molar flow rate of CO2 from novel spherical reactor. The multi-objective model is transformed to a single objective optimization problem by weighted sum method and the single optimum point is found by using genetic algorithm. The optimization results show that the pressure drop in the spherical reactor is negligible in comparison to that of the conventional tubular reactor. Therefore, it is inferred that the spherical reactor can operate with much higher feed flow rate, more catalyst loading, and smaller catalyst particles.

Low temperature growth of single-walled carbon nanotubes from Ru catalysts by alcohol catalytic chemical vapor deposition

Among platinum-group elements, Ru is one of the most inexpensive metals, while its melting point (2334°C) is very high. In this study, we carried out single-walled carbon nanotube (SWCNT) growth by an alcohol catalytic chemical vapor deposition (CVD) in a high vacuum using Ru catalysts. By optimizing the ethanol pressure, we could grow SWCNTs below 700°C, and succeeded in growth at 500°C from Ru catalysts, for the first time. Raman measurements showed that, compared to the growth at 700°C, the SWCNTs grown at 500°C had smaller diameters (<1.26nm) and narrow diameter distribution. In addition, the diameters were smaller than those from the conventional catalysts for SWCNT growth, Fe. TEM observation showed that, due to the high-melting-point, the aggregation of Ru catalysts was suppressed and that the smaller catalyst particles were formed, resulting in the small-diameter SWCNTs.

Experimental proof of the existence of mass‐transfer resistance during early stages of ethylene polymerization with silica supported metallocene/MAO catalysts

The size of a silica supported metallocene/MAO (methylaluminoxane) catalyst plays an important role in determining its productivity during ethylene polymerization. From a chemical engineering point of view, this size dependency of catalytic activity of supported metallocenes is mathematically connected with the different levels of mass‐transfer resistance in big and small catalyst particles but no experimental evidence has been provided to date. The results of this systematic experimental study clearly demonstrate that the intraparticle monomer diffusion resistance is high in bigger catalyst particles during initial instants of ethylene polymerization and diminishes with the polymer particle growth. Two different silica supported metallocene/MAO catalysts provided the same results while highlighting the fact that catalyst chemistry should be carefully considered while studying complex chemical engineering problems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4476–4490, 2017

A facile method for preparation of efficient oxygen reduction catalyst for a microbial fuel cell cathode

A facile preparation method for efficient oxygen reduction catalyst was suggested by examining three different approaches, which are microwave heating of a precursor solution (MS), dried precursor powder (MD), and autoclave heating of precursor solution (AS). The highest reduction peak current observed in (−0.034 mA) in cyclic voltammogram was obtained from the MS catalyst. However, the reduction peak current of the AS and the MD catalysts were 94.12% and 50%, of the MS catalyst, respectively. The surface with the MS catalyst consisted of small catalyst particles embedded in MWCNT strands, and its oxygen content was 4.67%, which was considerably lower than those of the surfaces (25.62∼38.86%) with the MD- and the AS catalysts, wherein large lump aggregations of catalyst compounds were observed. The MFC with MS catalyst cathode had very low activation and ohmic resistances (7.7Ω, 5.0 Ω) at maximum power density, and its MFC with the MS catalyst cathode were as low as 7.7Ω, 5.0 Ω, respectively, at maximum power density and it’s limiting current (19.6 mA) was considerably higher than the MD catalyst (2.75 mA) and the AS catalyst (10.2 mA) cathodes. The maximum power density of the MFC with MS catalyst cathode was 1,425 mW/m2, which was quite higher than those of the MFCs with MD catalyst (270 mW/m2) and AS catalyst (833 mW/m2) cathodes. This result shows that microwave heating (MS) of the catalyst precursor solution is the most feasible method for preparation of an efficient oxygen reduction catalyst for MFCs.

A Comparative Study of Toluene Oxidation on Different Metal Oxides

This work reports the results of experimental and theoretical investigation of toluene oxidation on different metal oxide based catalysts (manganese oxide, MnOx, mixed manganese-iron oxide, MnFe, perovskite-type manganese oxide, LaMnO3 and commercial Pt-Al2O3 catalyst). Particular attention was devoted to single and mixed manganese based oxides and ceria based materials as alternatives to conventionally used noble metal containing catalysts. Toluene oxidation was performed under steady-state conditions in an integral fixed bed reactor operating over a wider range of reaction temperatures and at various space times. The influence of reaction variables on the rate of toluene oxidation was examined using the simple first-order kinetic model and the one-dimensional (1D) pseudo-homogeneous model to describe the reaction system. The proposed model was verified comparing the theoretical predictions with the experimental laboratory results. The results of catalytic tests indicated that the mixed manganese-iron oxide (MnFe) exhibited remarkable catalytic activity for the toluene oxidation, almost comparable with the activity of the commercial Pt-Al2O3. The reaction temperature T50 corresponding to 50% of the toluene conversion was observed at 419 K for the MnFe oxide and at 405 K for the Pt-Al2O3. A very good agreement of experimental data with the proposed 1D model was obtained. Based on the shape of the light-off curve and the values of the apparent activation energies, which decreased from 120.36 kJ/mol to 16.88 kJ/mol with reaction temperature increase, it was concluded that the reaction rate was probably limited by the mass transfer, no matter the relatively small catalyst particle size fraction employed in this study (315 - 400µm).

Sugar hydrogenation in continuous reactors: From catalyst particles towards structured catalysts

The density, viscosity and hydrogen solubility of selected sugars (l-arabinose, d-galactose, d-maltose and l-rhamnose) were determined at different temperatures (generally 60, 90 and 130°C). The role of internal diffusion resistance in porous catalyst layers for sugar hydrogenation was confirmed by numerical simulations based on kinetic data and physical properties. The simulations suggested the use of small catalyst particles or structured catalysts in continuous hydrogenation of the sugars to sugar alcohols. Continuous hydrogenation of l-arabinose was carried out in a laboratory-scale fixed bed reactor with ruthenium catalysts on three different supports (active carbon clothes, carbon nanotubes on sponge-like metallic structures, conventional active carbon catalyst particles). It was proved that continuous hydrogenation is a feasible alternative to batch technology for sugar hydrogenation over conventional catalyst particles and structured catalysts: l-arabinose was converted to arabitol with a very high selectivity.

Hierarchical chrysanthemum-flower-like carbon nanomaterials grown by chemical vapor deposition

Novel hierarchical chrysanthemum-flower-like carbon nanomaterials (CFL-CNMs) were synthesized by thermal chemical vapor deposition based on acetylene decomposition. A scanning electron microscope and a transmission electron microscope were employed to observe the morphology and structure of the unconventional nanostructures. It is found that the CFL-CNMs look like a blooming chrysanthemum with a stem rather than a spherical flower. The carbon flower has an average diameter of 5 μm, an average stem diameter of 150 nm, branch diameters ranging from 20 to 70 nm, and branch lengths ranging from 0.5 to 3 μm. The morphologies of the CFL-CNMs are unlike any of those previously reported. Fishbone-like carbon nanofibers with a spindle-shaped catalyst locating at the tip can also be found. Furthermore, the catalyst split was proposed to elucidate the formation mechanism of CFL-CNMs. A large and glomerate catalyst particle at the tip of the carbon nanofiber splits into smaller catalyst particles which are catalytic-active points for branch formation, resulting in the formation of CFL-CNMs.

On the Way to a More Open Porous Network of a Co–Re/Al2O3 Catalyst for Fischer–Tropsch Synthesis: Pore Size and Particle Size Effects on Its Performance

Five different γ-alumina supports were used, one commercial and four prepared in a macro-mesoporosity range of 7–1000 nm by different preparation methods. A simple method was used to obtain a macro-mesoporous alumina support by modification of an initial commercial mesoporous alumina with a pore generating agent. Supports were used to prepare Fischer–Tropsch synthesis (FTS) catalysts. Supports and catalysts were characterized by several techniques and tested in a lab-scale fixed-bed reaction unit in the FTS at 493 K and 2 MPa, with small (<63 μm) and large (500–710 μm) catalyst particle size (PS). Catalytic results showed that with small catalyst PS, the behavior is similar among them. With large catalyst PS, C5+ selectivity considerably decreased and CH4 selectivity increased for all catalysts due to diffusional restrictions. Nevertheless, the effect of diffusion limitations of reactants and products through catalyst pores were lower for catalysts with a higher mesoporosity (20–50 nm), and much lower for the catalyst obtained when the support was modified to add macroporosity between 100 and 1000 nm. These macropores improve the transport of reactants and heavy hydrocarbons produced between the gaseous phase and the active sites. As a consequence, a better performance of the catalyst is attained, reducing the non-desirable effects of a diffusion-limited regime.

Change in ORR Activity of Pd Core-Pt Shell Structured Catalyst with Different Pt Shell Coverage

Introduction Reduction of the amount of expensive Pt cathode catalyst in PEFCs is essential for their worldwide commercialization. Core-shell structured catalyst, in which Pt monolayer (PtML) shell is formed on non-Pt metal core surface, is one of the key technologies. Furthermore, it has been reported that ORR activity of the PtML shell is enhanced when Pd nanoparticles are used as core materials [1, 2]. Recently, we developed a new process for the Pt shell formation which needs no precise potential control and is suitable for scale-up synthesis of the core-shell structured catalyst (modified Cu-UPD/Pt replacement [3]). We further demonstrated that the ORR mass activity of a carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) synthesized by the new process is drastically enhanced with an accelerated durability test (ADT) performed at 80°C [3]. The ORR activity enhancement is considered to be due to a rearrangement of the surface Pt atoms and a decrease in number of low-coordinated Pt atoms associated with oxidative dissolution of Pd from the core nanoparticles. In this study, Pt/Pd/C catalysts with different Pt shell coverages are synthesized via the modified Cu-UPD/Pt replacement process and the influence of the Pt shell coverage on the ORR activity of the catalysts after ADT was investigated. Experimental Pt/Pd/C catalysts were synthesized with the modified Cu-UPD/Pt replacement process [3]. Carbon supported Pd core particles (Pd/C, Pd size: 4.2 nm, Pd loading: 30 wt.%, Ishifuku Metal Industry) were ultrasonically dispersed in 50 mM H2SO4 containing 10 mM CuSO4 and the solution was stirred with co-existence of a metallic Cu sheet at 5°C under Ar atmosphere. After stirred for 5 h, the Cu sheet was removed and K2PtCl4 with different concentrations was added to obtain the Pt/Pd/C catalysts with different Pt shell coverages. The Pt/Pd/C catalysts were characterized by TG, XRF, XRD, TEM, TEM-EDX, CV and XAFS. The ORR activity of the Pt/Pd/C catalysts was evaluated with the RDE technique in O2 saturated 0.1 M HClO4 at 25°C. The ADT was conducted using a rectangular wave potential cycling (0.6 V (3 s)-1.0 V (3 s) vs. RHE) in Ar saturated 0.1 M HClO4 at 80°C for 1,000 cycles. Results and discussion The compositions of the two Pt/Pd/C catalysts synthesized with different K2PtCl4 concentrations were Pt21Pd79 and Pt30Pd70 (atomic %) and their Pt shell thicknesses were calculated to be 0.6 and 1.0 ML, respectively (hereafter called Pt0.6 ML/Pd/C and Pt1.0 ML/Pd/C). CVs of the Pt/Pd/C catalysts are shown in Fig. 1. In the potential range of 0.05-0.1 V vs. RHE, hydrogen absorption/desorption peaks, which are characteristics of Pd, were observed and these peaks were stronger in the Pt0.6 ML/Pd/C catalyst, indicating that the Pt shell coverage is lower in the Pt0.6 ML/Pd/C catalyst. TEM images of the Pt0.6 ML/Pd/C catalysts are shown in Fig. 2. Small catalyst particles disappeared and the morphology changed into spherical shape after ADT. TEM-EDX analysis revealed that composition of the Pt0.6 ML/Pd/C and Pt1.0 ML/Pd/C catalysts changed to Pt48Pd52 and Pt50Pd50, respectively, after ADT. Suppose that the Pt shell does not dissolve during ADT, the Pd core dissolution in the Pt0.6 ML/Pd/C and Pt1.0 ML/Pd/C catalysts are estimated as 71% and 57%, respectively. Changes in ORR mass activity of the Pt/Pd/C catalysts after ADT are summarized in Fig. 3 together with a carbon supported Pt reference catalyst (Pt/C, Pt size: 2.8 nm, Pt loading: 46 wt.%, TEC10E50E, TKK). The ORR mass activities of both Pt/Pd/C catalysts were enhanced after ADT and the enhancement was higher for the Pt0.6 ML/Pd/C catalyst (2.6-fold of the Pt/C catalyst). As described above, the Pd core dissolution by the ADT is larger in the Pt0.6 ML/Pd/C catalyst (71%), which is due to the lower Pt shell coverage in the catalyst (Fig. 1). Since the ORR activity enhancement of the Pt/Pd/C catalyst during ADT arises from rearrangement of the Pt shell associated with the Pd core dissolution, it is considered that the lower Pt shell coverage in the Pt0.6 ML/Pd/C catalyst is, the more the Pd core dissolution and the rearrangement of the Pt shell is enhanced. The micro-structural change of the Pt/Pd/C catalysts after ADT will be presented at the meeting. Acknowledgement This work was supported by New Energy and Industrial Technology Development Organization (NEDO), Japan.

Systematic review of catalyst nanoparticles synthesized by solution process: towards efficient carbon nanotube growth

Nano-field research has been expanded rapidly since those tiny materials such as carbon nanotubes (CNTs), cobalt catalyst and iron catalyst can give huge impact to the application products with their extraordinary properties. The scientific discovery of these materials can be defined as a magic key to solve the raw materials shortage and unlock the limitation performance of the devices. CNTs have been found to be one of the new nanomaterials that can improve different kind of devices’ performance. CNT can be grown on the substrates with the presence of active metal catalysts. Since small metal catalyst particles (diameter <10 nm) are crucial in growing CNTs, the deposition method of metal catalyst on the substrates has been studied. The optional processes using solutions to produce catalyst nanoparticles will be discussed in this review. Sol–gel process along with spin coating is the most suitable deposition method with low cost of production and the easiness to control particle size deposited on the substrates.

Catalytic growth of multi-wall carbon nanotubes on carbon cloth using powdery ferrous sulfate heptahydrate as catalyst precursor

Multi-wall carbon nanotubes (MWCNTs) were synthesized on carbon fiber cloth by chemical vapor deposition using natural gas as the carbon source and FeSO 4 · 7H 2 O powder as the catalyst precursor. Scanning and transmission electron microscopy and Raman spectroscopy were used to characterize the product. Small catalyst particles were observed on the top of the tubes. The MWCNTs were dispersed uniformly and aligned perpendicular to the cloth. They had a high crystallinity, and few impurities and defects. The density and structure of the MWCNTs on the cloth can be controlled by deposition temperature and time. [New Carbon Materials 2013, 28(6): 421–427]

Effects of pH on the Properties of Mo-Modified Pt/C Synthesized by Microwave-Assisted Method as Catalyst for Electro-Oxidation of Ethanol

A series of Mo-modified Pt/C catalysts were synthesized, using a microwave-assisted technique, and the effects of pH value on the electrocatalytic performance of the resulted catalysts for ethanol oxidation were investigated. X-ray diffraction(XRD), transmission electron microscopy(TEM), and X-ray photoelectron spectroscopy(XPS) were employed to characterize the crystal structures, morphologies, particle sizes and the distributions, and electronic states of the catalysts. Cyclic voltammetry(CV), chronoamperometry(CA), and electrochemical impedance spectroscopy(EIS) were used to investigate the catalytic performances. The results revealed that alkaline environments are beneficial in the syntheses of catalysts with small and uniformly dispersed particles on the carbon support. The catalyst prepared at pH 14 exhibited the smallest catalyst particles, highest electrochemical surface area, best electrocatalytic activity, and durability for ethanol oxidation.

Catalytic growth of multi-wall carbon nanotubes on carbon cloth using powdery ferrous sulfate heptahydrate as catalyst precursor

Multi-wall carbon nanotubes (MWCNTs) were synthesized on carbon fiber cloth by chemical vapor deposition using natural gas as the carbon source and FeSO4·7H2O powder as the catalyst precursor. Scanning and transmission electron microscopy and Raman spectroscopy were used to characterize the product. Small catalyst particles were observed on the top of the tubes. The MWCNTs were dispersed uniformly and aligned perpendicular to the cloth. They had a high crystallinity, and few impurities and defects. The density and structure of the MWCNTs on the cloth can be controlled by deposition temperature and time.

Investigation on Fe-Sn-O Catalyst Activity for the Growth of Carbon Nanocoils

We have investigated the relationship between the catalyst and the growth of CNCs. It is found that with the increase of deposition time the yield of CNCs gradually increased, while their growth rate decreased exponentially. The short reaction time would lead to the growth of CNCs with smaller coil diameters, however, CNCs with larger coil diameters are increased gradually after a long-time reaction. The Fe-Sn-O catalyst particles are not uniform in sizes. It is considered that CNCs with smaller/larger coil diameters can be synthesized from the smaller/larger catalyst particles. A further fitting of the growth curve indicates that the as-prepared catalyst particles have different activities. It can be inferred that catalyst particles with different sizes have different catalytic activities. The smaller catalyst particles have a higher activity but a shorter lifetime, and the coil diameter of CNCs synthetized from it is smaller.

Catalyst Performance Testing in Multiphase Systems: Implications of Using Small Catalyst Particles in Hydrodesulfurization

Three different gas–liquid–solid reactor configurations have been used to investigate the performance of a P-doped NiMo/Al2O3 catalyst in the hydrodesulfurization of dibenzothiophene. The commonly used millipacked bed reactor with 250–500 μm catalyst particles diluted with 125 μm inert particles, a micropacked bed reactor with 55–90 μm catalyst particles, and a slurry reactor with 150–250 μm catalyst particles were used in the catalyst performance testing program. It appeared that the inherently small particle size in the packed beds causes the hydrodynamics to be dramatically different compared to the industrially applied trickle-bed reactors. For particles smaller than typically 2 mm the capillary forces predominate over the viscous and gravitational forces, in sharp contrast to large-scale industrial reactors. Since the gas flow follows preferential pathways through beds consisting of small particles, the poor radial dispersion of the gaseous components can cause mass-transport limitations, even for a rather slow reaction such as the hydrodesulfurization (HDS) of dibenzothiophene, as a result of the strong inhibition by the reaction product H2S. An adapted criterion is proposed for estimation of the contribution of poor radial dispersion in catalyst performance testing.