Spherical Catalyst Research Articles

Design a novel kind of open-ended carbon sphere for a highly effective counter electrode catalyst in dye-sensitized solar cells

A new kind of carbon sphere (denoted as OCS) with an open end on the surface has been synthesized successfully. The designed OCS shows higher electrocatalytic activity than the conventional solid and hollow carbon sphere (denoted as SCS and HCS) as counter electrode (CE) catalyst for the regeneration of both iodide (I−/I3−) and sulfide (T−/T2) redox shuttles in dye-sensitized solar cells (DSCs). For the iodide electrolyte, the DSCs using SCS, OCS, and HCS CEs yield high power conversion efficiency (PCE) values of 7.8, 8.7, and 8.1%, respectively, indicating the potential to replace expensive Pt CE. Most importantly, the carbon sphere catalysts exhibit obvious advantages when applied in sulfide redox shuttle. The OCS-CE based DSCs produces a high PCE of 6.4%, much higher than that for Pt-CE based DSCs (4.1%). The high catalytic activity of OCS benefits from the sufficient contact between the redox shuttle with the external and internal surfaces of OCS because an open end exists on the surface of OCS, which provide a diffusion channel for the electrolyte into the inner of OCS. This strategy for designing open-ended structure is highly important for the fields of drug delivery, nanodevice, and adsorption, but not confined to catalysis.

Synthesis of NiMo catalysts supported on mesoporous Al-SBA-15 with different morphologies and their catalytic performance of DBT HDS

Mesoporous SBA-15 materials with different morphologies were successfully synthesized through different methods by using pluronic P123 triblock copolymer (EO20PO70EO20) and cosurfactant (CTAB) as structure-directing agent. Moreover, highly ordered mesoporous Al-SBA-15 with a Si/Al molar ratio of 10 has been prepared with three different Al resources using post-synthesis methods. The characterization results of X-ray fluorescence spectroscopy, X-ray power diffraction, N2 adsorption–desorption and 27Al nuclear magnetic resonance spectroscopy revealed that the chemical grafting of aluminum chloride on the surface of SBA-15 was the best post-synthesis route for the preparation of Al-SBA-15 with different morphologies. The corresponding Al-SBA-15-supported NiMo hydrodesulfurization (HDS) catalysts with different morphologies were prepared and evaluated using DBT as a probe reactant. The sphere NiMo/AS-SP catalyst has higher dispersion degree of MoS2 slabs and more acid sites than other catalysts. The catalytic results showed that the morphologies and mesochannels of SBA-15 played an important role in the catalytic performance of DBT HDS over NiMo catalysts, especially in the diffusivity of DBT molecules in the catalysts, while they have little effect on the selectivity of DBT HDS. The kinetics investigations confirmed that the activity of DBT HDS over these catalysts followed the order: long-rod catalyst (NiMo/AS-LR)<hexagonal prism catalyst (NiMo/AS-HP)<short-rod catalyst (NiMo/AS-SR)<sphere catalyst (NiMo/AS-SP). The high activity of the last one was attributed to the superior diffusion of the supports and the better dispersion of the active components.

Butadiene production from bioethanol and acetaldehyde over tantalum oxide-supported spherical silica catalysts for circulating fluidized bed

The chemical 1,3-butadiene (BD) is usually produced from ethanol and acetaldehyde (ethanol to BD, ETB) over Ta2O5/SiO2 catalyst in fixed-bed system. However, the ETB process has a short catalyst-regeneration cycle due to rapid deactivation of the catalyst by coke. To overcome this problem, a circulating fluidized-bed (CFB) reactor could be used in the ETB reaction for continuous regeneration of deactivated catalyst. The catalyst supplied to a CFB reactor must be spherical, show good catalytic performance, and be mechanically strong. In this study, a series of samples of Ta2O5 catalyst on silica spheres with different pore and particle sizes, were prepared. The catalysts were examined for mechanical strength by an attrition test and in ETB reaction in the fixed-bed reactor. The Ta2O5/Q-6 catalyst, with particles size of 75–150μm, showed good mechanical strength and the best catalytic performance. This optimum spherical catalyst was tested to examine its hydrodynamic properties in a cold-bed CFB reactor.

Efficient removal of Cr(VI) from wastewater under sunlight by Fe(II)-doped TiO2 spherical shell

Fe(II)-doped TiO2 spherical shell catalyst was synthesized by one-pot hydrothermal method. The photocatalytic removal of Cr(VI) from plating wastewater under sunlight of the catalyst was demonstrated. It was found that the removal effectiveness of about 99.99% for initial Cr(VI) concentration of 102.3ppm and 99.01% for 153.4ppm under 3h sunlight irradiation is realized. The Fe(II) ions serve not only as reducing agents for reducing the Cr(VI) to Cr(III) but also as an intermedium of a two-step reduction, in which the TiO2 photoreduces the Fe(II) ions to Fe atoms firstly, and then the Fe atoms reduce the Cr(VI) to Cr(III). The improved photocatalytic activity of the catalyst is considered due to the synergistic effect of a multi reducing process by Fe(II) doping. The extended optical response and effectively utilization of sunlight of the special spherical-shell-like morphology also contribute to the enhanced photocatalytic activity.

Acetone hydrogenation in exothermic reactor of an isopropanol–acetone–hydrogen chemical heat pump: effect of intra-particle mass and heat transfer

Intra-particle mass and heat transfer plays an important role in performance of the exothermic fixed-bed reactor for an isopropanol–acetone–hydrogen chemical heat pump. In this work, an exothermic fixed-bed reactor model, taking into account the actual packing structure, is established in the commercial software Fluent. A 120° segment of a tube with tube-to-particle diameter ratio (n) of 4, where realistic particles are packed and set to porous media, is used to simulate the 3D external flow, concentration and temperature fields in the exothermic packed-bed reactor. The influence of catalyst particle diameter (d p) and micropore diameter (d 0) on the intra-particle temperature, species distribution, reaction rate and selectivity is discussed. The appropriate d p and d 0 are obtained. Simulation results showed that intra-particle temperature gradient is not obvious. Large d p and small d 0 lead to remarkable gradient of reaction rate inside the catalyst particle and the decrease in the catalyst efficiency and reduce the acetone conversion and the selectivity in isopropanol. The optimal results reveal that the spherical catalyst with d p of 1 mm and d pore of 10 nm is appropriate for high-temperature acetone hydrogenation.

On the Solution of Non-Isothermal Reaction-Diffusion Model Equations in a Spherical Catalyst by the Modified Adomian Method

We investigate the reactant diffusion within a porous catalytic pellet including the heat of reaction. The Lane–Emden boundary value problem with an Arrhenius reaction rate is used to model the reactant concentration. We combine the Volterra integral form with the Adomian decomposition method to solve the equivalent Fredholm–Volterra integral equation. We then estimate the reactant concentration at the center of the catalytic pellet and the iterated Shanks transform is used to improve the accuracy of the approximations. The objective error analysis formulas are used to demonstrate a high accuracy and rapid rate of convergence, which does not depend on a priori knowledge of the exact solution or comparison with an alternate approximation method. Thus low-stage approximations by the Adomian decomposition method are validated for parametric simulations of the reactant concentration profiles and the effectiveness factor profiles. Our approach demonstrates enhancements over previous investigations, and is readily extensible to more general diffusion reaction models in catalytic reactor engineering.

Gold on thiol-functionalized magnetic mesoporous silica sphere catalyst for the aerobic oxidation of olefins

We have synthesized a new catalyst based on thiol-functionalized silica-coated magnetic mesoporous nanocrystals as support and Au nanoparticles as active sites through a facile and environment-friendly approach and characterized by transmission electron microscopy (TEM), N2 adsorption–desorption, elemental analysis and inductive coupled plasma atomic emission spectrometer (ICP-AES), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). The synthesized catalyst exhibited high catalytic activity in the oxidations of cyclohexene and styrene by molecular oxygen at atmospheric pressure. Furthermore, this catalyst had an excellent recyclability, evidenced by being extensively reused for five times without any substantial loss of activity, which was mainly attributed to the enough strong combination between the Au nanoparticles and thiol groups onto the surface of support. Meanwhile, the catalyst could be easily separated from the reaction solution by applying an external magnetic field.

A numerical study of the effectiveness factors of nickel catalyst pellets used in steam methane reforming for residential fuel cell applications

A numerical study is performed to evaluate the effectiveness factors of commercial nickel catalyst pellets commonly used in small-scale steam methane reformers for residential fuel cell applications. Based on the intrinsic reaction kinetics of the steam reforming process, the standard composition of the partially reformed gas mixture is determined as a function of the methane conversion. The heterogeneous reforming reactions inside the spherical catalyst pellets are then modeled by considering the distributed reaction, multi-component diffusion and permeation, and conductive and convective heat transfer in the porous media. Various operating conditions, including the reforming temperature, steam-to-carbon (S/C) ratio, operating pressure, and geometrical parameters, such as the pellet diameter and mean pore size, are simulated. The effectiveness factors calculated for each condition are presented as a function of the methane conversion. Finally, simple correlations for the effectiveness factors are presented, and their accuracies are assessed.

Flame aerosol synthesis of carbon-supported Pt–Ru catalysts for a fuel cell electrode

This study describes how a flame spray pyrolysis method was successfully used to synthesize PtRu catalysts supported by carbon agglomerates. Nearly spherical catalysts composed of metallic Pt and Ru with molar ratio of 1:1 were produced in the flame and their size was about 1.9 nm. X-ray diffraction measurements revealed that amorphous-like Ru was well mixed into the Pt crystalline lattices. Through cyclic voltammetry for methanol oxidation reaction and CO stripping, it was found that the electrochemical activities of the catalysts produced from this process are comparable to or slightly better than those of an equivalent commercial sample with the same composition.

Influence of catalytic systems on process of model object hydrogenation

AbstractOn the basis of β-FeOOH, Fe(OA)3, Fe3O4 iron and spherical catalysts NiO/SiO2, Fe2O3/SiO2 derived from slag waste coals of heating electrical stations, the hydrogenation of model polycyclic hydrocarbon at presence of nanodimensioned catalysts antracene was studied. On the example of conversion of anthracene, it was shown that upon release of hydrogenation of the product yield and degradation of polycyclic hydrocarbons in the hydrogenation, the mentioned catalyst systems appeared to be in the following order: nanoparticles β-FeOOH, Fe(OA)3 and Fe3O4&gt;spherical catalysts NiO/SiO2, Fe2O3/SiO2&gt;commercial cobalt-molybdenum catalyst. The results showed that the catalysts studied are promising catalysts for the hydrogenation of polycyclic hydrocarbons and may be used for direct coal liquefaction.

Impact of Spherical Catalyst Particles on Gas‐Liquid Flow Regimes in Minichannels with Square Cross Section

AbstractThe design of process equipment involving minichannel structures necessitates detailed knowledge on the flow conditions inside the apparatus. Here, flow regimes in empty and sphere‐packed square channels were investigated by using optical flow visualization with a high‐speed camera. Flow maps as well as new correlations to predict the flow regime transitions based on the dimensionless gas and liquid Weber numbers are presented. Segmented flow regimes occurred in the studied empty and packed channels for very low gas and low liquid Weber numbers. Such a liquid flow in separate compartments may contribute to the design of highly efficient contacting apparatuses.

Effects of experimental conditions on the morphologies, structures and growth modes of pulsed laser-deposited CdS nanoneedles

CdS nanoneedles with different morphologies, structures, and growth modes have been grown on Ni-coated Si(100) surface under different experimental conditions by pulsed laser deposition method. The effects of catalyst layer, substrate temperature, and laser pulse energy on the growth of the CdS nanoneedles were studied in detail. It was confirmed that the formation of the molten catalyst spheres is the key to the nucleation of the CdS nanoneedles by observing the morphologies of the Ni catalyst thin films annealed at different substrate temperatures. Both the substrate temperature and laser pulse energy strongly affected the growth modes of the CdS nanoneedles. The secondary growth of the smaller nanoneedles on the top of the main nanoneedles was found at appropriate conditions. A group of more completed pictures of the growth modes of the CdS nanoneedles were presented.

Preparation of Ag‐Modified (B,P)‐Codoped TiO2 Hollow Spheres with Enhanced Photocatalytic Activity

AbstractA series of TiO2 hollow sphere catalysts with or without nonmetal (B,P) dopants and Ag‐modification were prepared through a sol–gel process with styrene–methyl methacrylate copolymer (PSMMA) microspheres as the template. The samples were characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), laser Raman spectroscopy (LRS), and UV/Vis diffuse reflectance spectroscopy (UV/Vis DRS), and their photocatalytic activities were evaluated by degradation of methylene blue (MB). Both B and P dopants could narrow the band gap of the TiO2 catalyst, and the loaded Ag only played the role of photoelectron transmitter. The two effects of the nonmetal dopants and the loaded Ag improved the photocatalytic activity of the TiO2 hollow spheres. Moreover, the hollow sphere structure and oxygen vacancies were beneficial to photocatalysis. Among all the catalysts prepared, Ag‐modified (B,P)‐codoped TiO2 hollow spheres exhibited the highest photocatalytic activity.

Stabilizing PdII on hollow magnetic mesoporous spheres: a highly active and recyclable catalyst for carbonylative cross-coupling and Suzuki coupling reactions

A hollow magnetic mesoporous silica sphere (HMMS) catalyst has been synthesized using polystyrene microspheres as a chemical template. The catalyst was characterized by TEM, XRD, XPS and vibrating sample magnetometry (VSM). The catalyst shows high activity for the carbonylative cross-coupling reaction of aryl iodides with arylboronic acids and Suzuki coupling reactions. The newly developed catalyst is easy to recover by magnetic separation from the liquid phase of the reaction and can be recycled. Importantly, the catalyst revealed high efficiency and high stability under the reaction conditions and during recycling stages.

Carbon materials as template for the preparation of mixed oxides with controlled morphology and porous structure

VPO bulk catalysts with spherical morphology and high surface areas have been prepared by the use of a carbon material as template. The carbon material is an activated carbon prepared by the hydrothermal method, using cellulose as starting material. Thus, the synthesis method described is simple and low-cost. The morphology and porous structure of these materials have been characterized by SEM, TEM, N2 and CO2 adsorption isotherms. The chemical composition was analyzed by XPS, XRD and Raman spectroscopy, showing the presence of V2O5 and VPO phases on the surface of these spherical catalysts. Methanol oxidation reaction was used to characterize the active centers on the surface of these materials and showed that both acid and redox sites are present. These materials were also tested during the propane ODH reaction and the results demonstrated that they are able to activate the hydrocarbon selectively to propylene. The characterization of these materials after reaction demonstrated that the morphology and the chemical composition are quite stable under the reaction conditions studied, since both VPO and V2O5 phases are detected. It was also observed that the biggest spheres are not stable during reaction conditions.

Visible light photocatalysis and electron emission from porous hollow spherical BiVO4 nanostructures synthesized by a novel route

Hollow spheres of BiVO4, made of pyramid like nanocrystals have been synthesized in an aqueous medium through a precursor solution mediated growth method with appropriate amount of polyvinyl alcohol (PVA). It was found that concentration of PVA in the precursor solution played a crucial role in determining the morphology of the product. Hollow spherical BiVO4 was formed with PVA concentration of 64mg/mL in the precursor solution while with 32mg/mL PVA concentration mixed morphology of hollow spherical and cubical structure was formed. Without adding PVA, only cubical (bulk like) BiVO4 powder was obtained. The structures have been characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Fourier-transformed infrared (FTIR) spectroscopy, UV–Vis spectroscopy, thermogravimetry analysis (TGA) and differential thermal analysis (DTA). As-prepared hollow spherical BiVO4 samples were having monoclinic structure with energy band gap 2.42eV. The hollow spherical BiVO4 exhibited excellent visible-light-driven photocatalytic efficiency for degrading N,N,N′,N′-tetraethylated rhodamine (RhB) organic dye, which was nearly 13 times efficient than that of the samples prepared by the same method but without PVA in the precursor solution. After five recycles, the hollow spherical BiVO4 catalyst did not exhibit any significant loss of photocatalytic activity, confirming the photocatalyst is essentially stable. The BiVO4 hollow spheres also exhibited electron field emission indicating that BiVO4 with tailored morphology can be used as a multi-functional material.

Investigation of a packed bed in a mini channel with a low channel-to-particle diameter ratio: Flow regimes and mass transfer in gas–liquid operation

This work analyzes flow regimes and mass transfer in a miniaturized packed bed reactor under reacting conditions. The reactor consisted of a cylindrical channel with an inner diameter of 2mm which was filled with spherical catalyst particles having an outer diameter of 0.8mm. The liquid phase hydrogenation of α-methylstyrene over a Pd/γ-Al2O3 catalyst was employed as reaction system. Based on experimental reaction rates, overall (gas–liquid–solid) volumetric mass transfer coefficients of hydrogen, which were between 0.5s−1 and 6.0s−1, were computed. Detailed discussions focus on the impact of the flow regime, the gas and liquid superficial velocities and, consequently, their ratio, as well as system pressure on overall mass transfer rates.

Numerical evaluation of the gas–liquid interfacial heat transfer in the trickle flow regime of packed beds at the micro and meso-scale

In the present work, two different models (micro-scale and meso-scale) were developed to investigate the heat transfer between the gas and liquid phases in the trickle flow regime of a packed bed reactor. In the micro-scale model, a simplified description of bed geometry, known as the double-slit model, was implemented to study the effects of different operating parameters, in terms of gas and liquid Reynolds, Prandtl and Eötvös numbers, on the interfacial Nusselt number. In the meso-scale model, the Volume-of-Fluid (VOF) approach was used to simulate trilobe, cylindrical and spherical catalyst shapes and accurately predict the effects of interface morphology and bed geometry on interfacial heat transfer. To validate the implemented methods, a simple packed bed reactor with spherical catalysts was developed to experimentally investigate the interfacial heat transfer of co-current, downward gas–liquid film flows. The results obtained from CFD simulations and experimental data were in agreement and accurately predicted bed reactor temperature profiles with a mean relative error of 2.15%. In both the micro- and meso-scale models, an increase in the Reynolds and Prandtl numbers increased the interfacial Nusselt number; whereas, an increase in dimensionless groups of the liquid phase or the Eötvös number caused the opposite effect. Finally, a new correlation was proposed that evaluated the gas–liquid Nusselt number in the trickle flow regime with a standard deviation of 7.19% compared to results acquired using the micro-scale model.

Novel aluminosilicate hollow sphere as a catalyst support for methane decomposition to COx-free hydrogen production

Novel Ni and Co supported on aluminosilicate hollow sphere catalysts were investigated to decompose methane into CO and CO2 free hydrogen and carbon nanotube. The hollow sphere structure was prepared by reverse microemulsion method and then Ni and Co were loaded by the impregnation method. The fresh catalysts and deposited carbon were characterized by several microscopic and spectroscopic techniques. The catalytic results showed that the Co based catalyst exhibited higher activity and durability at longer reaction time, due to the higher number and distribution of metal sites. The hollow sphere structure of the support could protect the metal particles against aggregation during the catalytic reactions. Accordingly, higher metal dispersion, stabilization and catalytic performance were achieved. The formation of nickel silicate is the main reason for the lower decomposition activity of Ni based catalyst at longer reaction time. TEM and Raman spectroscopic data revealed that the Co-based catalyst produced a relatively uniform diameter of MWCNTs with higher crystallinity and graphitization degree compared to the Ni-based catalyst.

Measurement of residence time distribution in a rotary calciner

Rotary calcination is widely used in catalyst manufacturing and many other industrial processes. In this article, the influence of operational variables and material properties on the mean residence time (MRT), hold up, and axial dispersion was investigated in a pilot plant rotary calciner. Residence time distributions (RTD) of spherical, cylindrical, and quadrilobe catalyst particles were measured and contrasted. The Saeman's model was successfully applied to predict the experimental bed depth and the MRT as particles flowed through the calciner. It was observed that increasing the feed rate did not significantly affect the MRT. Results for the different particles indicated that cylinders and quadrulobes exhibited less axial dispersion than spheres due to the decreased flowability. A reliable method was developed to provide a reasonable RTD prediction in rotary calcination systems. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4068–4076, 2013