Optimising the porous structure of heterogeneous reforming catalysts with a globular model of the grain

The theory of a globular grain model was used to construct an algorithm for analysing the problem of how the porous structure of heterogeneous catalysts affects their efficiency in model reforming reactions. Our analysis has substantiated the undeniable advantage of applying the bidispersive grain structure since its effectiveness is many times as high as the effectiveness of the monodispersive grain structure. Apparently, the method described in the present paper will be equally efficient when applied to heterogeneous catalysts made use of in other technological processes.     

High performance monolithic catalysts for hydrogenation reactions

The performances of two different monolithic catalysts are compared in the selective hydrogenation of fatty acid methyl esters (FAMEs). The monolith samples were a classical square channel cordierite and one modified with -Al₂O₃ blocking the macroporosity of the cordierite and rounding the channel cross section. On both samples a carbon layer was applied by carbonization of a polyfurfuryl alcohol coating obtained by dipcoating. The carbon was oxidized to create anchoring sites for ion exchange with a tetraamminepalladium(II) nitrate solution resulting in a carbon supported Pd catalyst. The rounded channels yielded an even carbon layer, whereas in the square channels an uneven carbon distribution resulted and penetration in the wall. The rounded channel shows a superior performance compared to the square channel sample in the partial hydrogenation of FAMEs, evidenced by the much lower formation of trans double bonds at similar hydrogenation levels of the double bonds. This is ascribed to the much shorter diffusion distances of the reactants and products.     

Control of transport phenomena in the interior of the reforming catalyst grain: A new approach to the optimisation of the reforming process

The aim of the study was to provide favourable conditions for the transport of the reagents by modifying the porous structure and shape of the reforming catalyst grain. The study entailed the analysis of the transport phenomena and reactions that occur in spherical, cylindrical and slab-shaped catalyst grains. To upgrade the efficiency of the catalyst, use was made of analytical equations that describe the changes in the concentrations of the reagents in the catalyst grain. The equations were derived from the mass balances and the kinetic scheme assumed for the reforming process. By virtue of the coefficients of reagent diffusion, the equations are a function of the porous structure parameters and can therefore define a more or less advantageous profile of concentrations. The modification of the profile by minimising the objective function proposed in the paper leads to the reduction in the influence of diffusion resistance on the course of the reforming process. The study demonstrates that after the porous structure of the experimental catalyst had been changed, it was possible to increase its specific surface area and mechanical strength and, at the same time, noticeably improve the conditions for the transport of reagents.     

Impedance model of electrolyte-insulator-semiconductor structure with porous silicon semiconductor

We present a generic impedance model for the porous silicon|electrolyte structure that is valid for a range of interfacial layers and bias in these structures. The model is validated using three widely different porous structures: short irregular silicon columns and pores, long cylindrical silicon columns and pores; and branched interconnected silicon microchannels and voids in a mesh structure. The model incorporates appropriate RC or constant phase elements for the different parts of the porous structure, namely, the top of the silicon columns (channels)|electrolyte, the column (channel) walls|electrolyte in the pores/channels, and the electrolyte|semiconductor interface at the base of the pores/channels. This physical model underscores the effects of column/channel depletion and accumulation, either due to applied bias or change of surface charge, to the impedance spectra of the device. The model helps to explain why the porosity needs to be optimized for specific applications and helps as a measurement tool for optimization.     

Li-S电池负极材料性能提升方法综述

硫和硫化物作为负极材料相较于商业化石墨电极具有更高理论比容量的优势,然而由于其"穿梭效应"和无限扩张的体积导致锂硫电池的性能低下。本文综述了目前一些研究方法,通过改变锂硫电池的形貌、多孔结构、催化剂等方向来提高锂硫电池的性能。     

An approximate model for diffusion and reaction in a porous pellet

A method of derivation of linear driving force approximation for diffusion and reaction processes in porous catalysts based on Laplace-Carson transform has been presented. The approximate model for any type of kinetic expression has been derived. Accuracy of developed model is good, especially in the range of small and intermediate Thiele modulus values that is used most often in cases of practice. This approximation yields a substantial simplification of analysis and computations and does not require any iterative and trial-and-error calculations.     

Effects of porous oxide layer on performance of Pd-based monolithic catalysts for 2-ethylanthraquinone hydrogenation

Pd/oxide/cordierite monolithic catalysts(oxide = Al_2O_3, SiO_2 and SiO_2\\Al_2O_3) were prepared by the impregnation method. The results of ICP, XRD, SEM–EDX, XPS and N_2 adsorption–desorption measurements revealed that the Pd penetration depth increased with increasing the thickness of oxide layer, and the catalysts with Al_2O_3 layers had the larger pore size than those with SiO_2 and SiO_2\\Al_2O_3 layers. Catalytic hydrogenation of 2-ethylanthraquinone(eA Q), a key step of the H_2O_2 production by the anthraquinone process, over the various monolithic catalysts(60 °C, atmosphere pressure) showed that the monolithic catalyst with the moderate thickness of Al_2O_3 layer(about 6 μm) exhibited the highest conversion of e AQ(99.1%) and hydrogenation efficiency(10.0 g·L~(-1)). This could be ascribed to the suitable Pd penetration depth and the larger pore size, which provides a balance between the distribution of Pd and accessibility of active sites by the reactants.     

Alumina foam catalyst supports for industrial steam reforming processes

The manufacture and the characterisation of alumina foams as alternative catalysts supports for industrial steam reforming processes are presented here. The possibility of use of alumina foams as catalysts supports in such processes is evaluated by studying their resistance toward mechanical and chemical stresses. The alumina foams produced are characterised owing to their processing parameters (slurry infiltration, sintering temperature, template pore size). Their ability to work in hydrothermal atmosphere is assessed by characterising the evolution of microstructures and mechanical strengths upon aging. Thermodynamic studies of the stability of alumina in industrial steam reforming working conditions are performed and correlated to the experiments to demonstrate the stability of such a system.     

Partial wetting in porous catalysts: wettability and wetting efficiency

Qualitative changes in the local (pore level) wetting efficiency of a porous catalyst as a function of catalyst wettability are proposed based on monitoring the 1-D motion of the solid-liquid-gas contact line. As catalyst wettability is increased, the resulting wetting efficiency is likely to show two distinct transitions viz., (1) a sudden decrease that is inspired by coalescence of neighboring rivulets and, (2) a sudden increase that is inspired by porous nature of the catalyst. As catalyst wettability decreases, the wetting efficiency is likely to decrease in the beginning but can then be held constant due to pinning or holding of the retracting contact line by liquid-filled pores. This will result in comparatively much higher wetting efficiencies. It is further proposed that under favorable circumstances this pinning can disappear and will, thus, allow the contact line to retract and wetting efficiency to decrease. In the end, the effect of catalyst geometry on the relation between wetting efficiency and wettability is presented.     

The effective diffusivities in porous media with and without nonlinear reactions

The question of whether effective diffusivities in porous materials under reactive and nonreactive conditions are equal is addressed. Previous studies had considered the problem with first-order reactions. We study the issue with two nonlinear reactions—a second-order reaction and one governed by the Michaelis-Menten kinetics. Pore network and continuum models of porous media are utilized to estimate the effective diffusivities under reactive and nonreactive conditions. We show that the two effective diffusivities are significantly different. The difference is due to the heterogeneities of the porous material, and the fluctuations that they cause in the spatially varying local concentrations and diffusivities, and can be as large as a few orders of magnitude. Theoretical analysis of diffusion and reactions in porous media is also presented that supports the results of the simulations. In particular, it is shown that the results of pore network simulations cannot be fitted to the classical continuum equation of diffusion and reaction, and that a more complex continuum equation should be used for this purpose.     

Corrosion under a porous layer: A porous electrode model and its implications for self-repair

A one-dimensional mathematical model for the corrosion of a metal surface under a porous layer (often an oxide layer) is presented using porous electrode theory. The model allows one to predict the rates of corrosion in a metallic system where oxygen reduction is the only cathodic reaction which occurs on the surfaces of both the metal and the porous oxide layer albeit at different rates. Thus the model simulates the scenario where the porous layer residing on the metal surface competes with or complements the oxygen reduction reaction happening on the metal surface. The study finds that a large thickness, low porosity, low oxide electrical conductivity, poor oxygen reduction on the porous layer, and high specific contact resistivity of the metal–oxide lead to poor spatial separation of the anodic and cathodic reactions and hence encourage self-repair. In particular the metal–oxide contact potential and the oxygen reduction rates on the oxide surface are shown to have profound implications for self-repair strategies for corrosion mitigation.     

Novel NMR techniques for porous media research

Recent years have seen a significant progress in the study of porous media of natural and industrial sources. This paper provides a brief outline of the recent technical development of NMR in this area. These progresses are relevant for NMR applications in material characterization.     

Effect of support on hydrogen production by auto-thermal reforming of ethanol over supported nickel catalysts

Nickel catalysts supported on various supports such as ZnO, MgO, ZrO₂, TiO₂, and Al₂O₃ were prepared by an impregnation method to investigate the effect of support on catalytic performance in hydrogen production by auto-thermal reforming of ethanol. Among the supported catalysts, the Ni/ZrO₂ and Ni/TiO₂ catalysts showed better catalytic performance than the other catalysts. The electronic structure of nickel species supported on ZrO₂ and TiO₂ was favorably modified for the reaction, and thus, the reducibility of nickel species supported on ZrO₂ and TiO₂ was increased due to the weak interaction between nickel and support. On the other hand, the Ni/MgO and Ni/ZnO catalysts exhibited poor catalytic performance in the auto-thermal reforming of ethanol due to the formation of a solid solution phase.     

Effect of silicates on the structure of Ni-containing catalysts obtained from hydrotalcite-type precursors

New nickel hydrotalcite-like compounds with silicates as interlayer anions used as catalyst precursors in the catalytic partial oxidation of methane were prepared by the coprecipitation method. The properties of these materials were compared with those of compounds obtained from carbonate-containing materials. The precursors and calcined samples were characterized by powder X-ray diffraction, FT-IR and Vis/UV/NIR spectroscopies, thermal analyses (DTA and TG), temperature programmed reduction (TPR) and N₂ adsorption/desorption at −196 °C. The results show that the incorporation of silicates in the lamellar compounds modifies the structural and textural properties of the precursors. After calcination, silicates – which are non-volatile anions – contribute to the final structure of the catalysts, which form a new forsterite-like phase, increasing their specific surface area but not altering the reducibility of the nickel species.     

Synthesis of carbon-supported nickel catalysts for the dry reforming of CH4

A series of carbon-based nickel (Ni) catalysts was prepared in order to investigate the effect of the preparation method on the dispersion of Ni and its final catalytic activity in the dry reforming of methane, i.e. CH₄ + CO₂ = 2H₂ + 2CO. Three parameters were studied: (i) the influence of the surface chemistry of the carbon used as support; (ii) the method of drying (conventional vs. microwave drying); and, (iii) the temperature of the reduction stage. In order to study the role of the surface chemistry of the commercial activated carbon used as support, the active carbon was tested as received and oxidized. Although a better Ni dispersion was achieved over the oxidized support, the conversions were much lower. It was also found that microwave drying offers various advantages over conventional drying, the main one being that less time is required to prepare the catalyst. Two reduction temperatures were used (300 and 500 °C), being found that it is necessary to adjust this parameter to prevent the Ni particles from sintering.     

Effect of starch on pore structure and thermal conductivity of diatomite-based porous ceramics

Considering the low-cost and environmental protection, the porous ceramics with high porosity using natural diatomite powder were successfully prepared by utilizing hot injection moulding and sacrificial fugitives. The impacts of different content of starch as a pore-forming agent on the phase composition, mechanical properties, thermal conductivity, and micro-structure of porous ceramics were investigated. The results demonstrate that starch content can significantly affect the mechanical properties and thermal conductivity of diatomite-based porous ceramics. When the starch content increased from 0 wt % to 50 wt %, the porosity increased from 61.2% to 80%, while the thermal conductivity decreases from 0.239 W/(m K) to 0.098 W/(m K). The low thermal conductivity of porous ceramics may be related to the macroporous–mesoporous composite structure. With the starch content increased, a greater chance of starch granule contact, higher internal pore sizes and a wider pore size distribution in the prepared samples, which resulting in lower mechanical strength, such as the three-point bending strength from 2.83 MPa to 0.46 MPa.     

Design of monolithic catalysts for multiphase reactions

Monolith reactors are emerging as an attractive alternative for gas-liquid-solid reactor applications. The use of monolithic catalysts in new reactors as well as in retrofit designs should be based on an optimal choice of monolith geometry and operating conditions.In this contribution, we illustrate through fundamental modeling of the transport-kinetic interactions in a monolith catalyst how such an optimal design may be evolved. We also highlight the potential benefits a monolith catalyst has as compared to a pellet-based trickle bed reactor.     

载体原料对Pt-Re催化剂正庚烷催化重整活性的影响

采用不同厂家生产的拟薄水铝石粉制得γ-Al_2O_3载体,通过共浸渍法制得一系列Pt-Re催化剂,并对其进行BET、XRD和NH_3-TPD表征。以正庚烷为原料,在反应温度500℃、反应压力1.0 MPa、空速2.0 h~(-1)和氢油体积比1 000∶1条件下对制备的Pt-Re催化剂进行活性评价,并采用红外硫碳分析仪测定反应后催化剂的积炭量。结果表明,制备的催化剂SBPR-2重整性能优于其他催化剂,且积炭较少,稳定性较佳,正庚烷转化率大于95%,芳烃选择性大于24%,积炭量为0.33%。     

Effect of pore structure on gas permeability constants of porous alumina

Porous alumina was fabricated using different particle size, sintering temperature, and particle size and content of poly (methyl-methacrylate) (PMMA) as pore former. The Forchheimer equation was used to investigate the relationship between porosity and average pore size, and obtain the permeability constants k₁ and k₂ (the viscous effect and the inertial effect, respectively). Compared to Darcy's law, the Forchheimer equation established a more realistic and reliable relationship between fluid pressure and fluid velocity. k₁ and k₂ were found to be more sensitive to the average pore size than to the porosity of alumina. Moreover, reliable relationships were confirmed between the average pore size and k₁, k₂, and their ratio (k₁/k₂).     

Wicke-Kallenbach and Graham's diffusion cells: Limits of application for low surface area porous solids

The influence of permeation flux induced by tiny pressure gradients on the counter-current gas diffusion in porous solids is discussed. Binary counter-current gas diffusion in porous samples with a broad range of pore radii (78 nm- was studied in arrangements of the Wicke-Kallenbach and Graham's diffusion cells. A small pressure difference between the cell compartments was observed both in the Wicke-Kallenbach cell (7 Pa) and Graham's cell (9 Pa). For samples with pore radii above significant deviations of diffusion fluxes were observed. Limits of pore sizes above which the additional permeation transport influences the diffusion results beyond acceptable level were obtained by simplified calculations. The distorted diffusion fluxes can be numerically freed from the contribution of permeation flux if permeation characteristics are available.