Further Validation of the Quartic Concentration Profile Approximation for Describing Intraparticle Transport in Cyclic Adsorption Processes

Models developed previously by the authors that describe nonlinear adsorption, and simultaneous pore and surface diffusion in a single particle, that are based on intraparticle quartic and parabolic concentration profile approximations, and that utilize the summation of the gas and adsorbed phases approach in the material balance formulations, were further validated under more diverse, yet more realistic, cycling conditions. Periodic square, sinusoidal and triangular wave functions were used to more accurately represent the periodic boundary conditions that the external surface of an adsorbent particle may be exposed to during repeated adsorption and desorption cycles in a fixed bed adsorber. Analytical solutions that describe the periodic uptake and release of the adsorbate by the adsorbent were obtained for all three periodic wave functions, and for both the quartic and parabolic profile approximations. By comparing the predictions obtained from both models with the exact numerical solution, the superiority of the quartic model over the parabolic model was clearly demonstrated for all wave functions, and for a wide range of adsorbate-adsorbent systems and bulk concentrations. Excellent agreement between the quartic and exact models was obtained in most cases. In general, the predictions improved as the wave function changed more gradually with time (triangular more gradual than sinusoidal and sinusoidal more gradual than square), as the degree of mathematical linearity of the adsorbate-adsorbent system increased, and as the maximum external surface concentration decreased (an isotherm nonlinearity effect). Subtle differences in the predictive ability of the new approximate models, stemming from the use of the different wave functions, were exposed. Overall, these results exemplify the importance of comparing the predictive ability of new approximate models that describe intraparticle transport under more diverse cycling conditions than are typically utilized in the literature, which has been dominated by the square wave function.     

Adsorption and order formation of colloidal nanoparticles on a substrate: a Brownian dynamics study

Adsorption process and order formation of electrostatically stabilized colloidal particles with a radius of 50 nm onto a planar surface with countercharge are examined. We perform Brownian dynamics simulations with a new three-dimensional cell model, in which the particle-particle and particle-substrate interactions are modeled based on the DLVO theory. The simulations yield the following results: (1) a larger bulk concentration would be required for larger kappaa to reach order formation to compensate for the decrease in the bulk potential; (2) the phase transition from a disordered to an ordered structure of the adsorbed particles on the substrate is considered to be of the Kirkwood-Alder type of transition through the examination of the two-dimensional pressure of the adsorbed particles; (3) the adsorbed particles are found to form a hexagonally ordered array, only if what we call "one-directional average force" acting on an adsorbed particle exceeds a critical value, which is independent of the ionic strength, or the interaction potentials. The critical value of the one-directional average force is interpreted as the force needed to keep an ordered structure by localizing adsorbed particles at fixed positions. In addition, the critical force is used to develop a new model to estimate the surface coverage at the order-disorder transition and it is demonstrated that the new model gives better estimation than other models previously reported.     

O2和CO表面催化反应的活性位分布模型

对于O_2和CO表面催化反应，建立了一个新的不可逆Monte Carlo模拟模型。在 二维格子中，引进了表面活性位和非活性位的要领。模型假设，一定浓度的活性位 随机分布在非活性位上，形成了活性位分布的二维格子模型反应器，并在ZGB模型 的基础上，考虑了氧原子和CO分子的表面扩散，特别是引进了吸附粒子的定向表面 扩散。其中，活性位和活性位最近邻是表面吸附物质反应的活性中心，而非活性位 的作用是通过表面扩散传质。当活性位浓度C_a = 1且考虑扩散时，模型还原为增 加了扩散的ZGB模型。当活性位浓度C_a = 1且只考虑氧的扩散时，模拟结果表明， 扩散几率达到某一数值（0.3）时，二级相变点完全消失。当活性位浓度C_a逐渐减 小时，单位活性位产生的CO_2的速率不断增大，这表明活性位的利用率提高了。     

Adsorption Calculations Using the Film Model Approximation for Intraparticle Mass Transfer

An approximate rate equation based on a film-model representation of diffusional mass transfer has been developed to describe the kinetics of multicomponent adsorption. The model describes mass transfer as a pseudo-steady state diffusion process through a flat film of thickness equal to one fifth of the particle radius. The flux relationships are integrated across the film yielding analytical expressions for the rate of mass transfer in a multicomponent adsorption system. The usefulness of the film model approximation is tested by carrying out calculations for three different practical adsorption systems: the adsorption of n-pentane and n-heptane mixtures on NaCaA zeolite discussed by Marutovsky and Bülow (1987); the adsorption of air in molecular sieve RS-10 discussed by Farooq et al. (1993); and the separation of air in a kinetically-controlled nitrogen PSA process discussed by Farooq and Ruthven (1990) and Sundaram and Yang (1998). In each case, the film model approximation predicts the expected trends accounting for the coupling of diffusion fluxes in the adsorbed phase.     

Water structure and dynamics at a silica surface: Pake doublets in 1H NMR spectra

Detailed knowledge about the dynamics and structure of liquids in the vicinity of a solid surface is important in several fields of research. In this study a homogeneous model system of colloidal and nonporous silica particles with a narrow particle size distribution was used to examine such properties of adsorbed water and 1-heptanol. Doublet (1)H water resonances ("Pake doublets") indicate a preferred spatial orientation for the water molecules, as well as a lower molecular density in the surface-induced water structures compared to bulk water. These surface-induced structures are found to extend at least 8 nm from the silica surface. T(1) relaxation measurements at several temperatures indicate weaker H-bonding in the adsorbed water compared to bulk water. T(2) relaxation measurements at several temperatures reveal the presence of two water phases and give quantitative information on the mobility of water molecules and proton exchange processes. The presence of 1-heptanol changes the water characteristics, primarily in the water phase closer to the surface, where water molecules experience decreased translational and increased rotational freedom. In the absence of water, adsorbed 1-heptanol forms surface aggregates encompassing several molecular layers, where the first adsorbed layer shows severe restrictions in mobility and subsequent layers are more mobile.     

研究表面扩散的双电解池方法及其在铂钌二元金属电催化研究中的应用

本文介绍一种由各自质子交换膜(作为电解质),参比电极和对电极在同一个工作电极上建立两个空间分离的、可独立控制的双电解池系统.它能够由第1个恒电位仪控制的第1电解池的工作电极产生某种吸附中间物,该中间物通过表面扩散到达第2个电解池的工作电极后,在第2个恒电位仪的控制下得到电化学检验.应用这一装置测量了铂电极上欠电势沉积的含氧吸附物种的表面扩散系数,并研究模拟铂钌电极电氧化有机小分子产生的毒性中间物与表面含氧吸附物种的相互作用.在质子交换膜燃料电池的燃料极的工作电势下,没有发现钌表面产生的含氧吸附物种扩散到铂的表面.作者据此假设Pt Ru协同催化作用的实现可能是由于铂上毒性中间物的表面扩散速率非常慢,限制了向钌表面的溢流速率.只有当Pt Ru边界足够大,中间物在铂表面扩散途径非常短时才能形成足够的流速,并在钌表面被表面含氧物种氧化成CO2,使铂表面被重新活化.     

Surface activity of solid particles with extremely rough surfaces

The solid particles are adsorbed at liquid-liquid interfaces and form self-assembled structures when the particles have suitable wettability to both liquids. Here, we show theoretically how the extreme roughness on the particle surface affects their adsorption properties. In our previous work, we discussed the adsorption behavior of the solid particles with microstructured surfaces using the so-called Wenzel model [Y. Nonomura et al., J. Phys. Chem. B 110 (2006) 13124]. In the present study, the wettability and the adsorbed position of the particles with extremely rough surfaces are studied based on the Cassie-Baxter model. We predict that the adsorbed position and the interfacial energy depend on the interfacial tensions between the solid and liquid phases, the radius of the particle, and the fraction of the particle surface area that is in contact with the external liquid phase. Interestingly, the initial state of the system governs whether the particle is adsorbed at the interface or not. The shape of the particle is also an important factor which governs the adsorbed position. The disk-shaped particle and the spherical particle which is partially covered with the extremely rough surface, i.e. Janus particle, are adsorbed at the liquid-liquid interface in an oriented state. We should consider not only the interfacial tensions, but also the surface structure and the particle shape to control the adsorption behavior of the particle.     

THE ADSORPTION OF LINEAR POLY(N-ISOPROPYLACRYLAMIDE)CHAINS ON SURFACTANT-FREE POLYSTYRENE NANOPARTICLES

The adsorption of linear poly(N-isopropylacrylamide) (PNIPAM) chains on surfactant-freepolystyrene (PS) nanoparticles was used as a model system to study the hydrophobic adsorption of polymeron the surface, because the hydrophobility of PNIPAM can be continuously varied by a small temperaturechange. The adsorption was investigated by a combination of static and dynamic laser light scattering (LLS)measurements, In static LLS, the absolute excess scattered light intensity led to the amount of PNIPAMadsorbed on the surface. In dynamic LLS, the hydrodynamic thickness of the adsorbed PNIPAM layer wasaccurately measured. For a given particle concentration, the adsorption increases as thc PNIPAMconcentration and the incubation temperature increase. The average density of the adsorbed PNIPAM layer isreciprocally proportional to the number of the PNIPAM chains on the surface, revealing a simple scaling ofthe chain density distribution. The adsorption follows the Langmuir's isotherm. The enthalpy changeestimated from the adsorption at 25℃and 30℃is slightly positive, indicating that the adsorption involvesthe coil-to-globule transition of the chains on the surface.     

An improved collision efficiency model for particle aggregation

A generalized geometric model is presented which describes the collision efficiency factor of aggregation (the probability of a binary particle or aggregate collision resulting in adhesion) for systems comprised of two oppositely charged species. Application of the general model to specific systems requires calculation of the area of each species available for collision with a second species. This is in contrast to previous models developed for polymer-particle flocculation that are based on the fractional surface coverage of adsorbed polymer. The difference between these approaches is suggested as an explanation for previously observed discrepancies between theory and observation. In the current work the specific case of oppositely charged nondeformable spherical particles (heteroaggregation) is quantitatively addressed. The optimum concentration of oppositely charged particles for rapid aggregation (maximum collision efficiency) as a function of relative particle size is calculated and an excellent correlation is found with data taken from literature.     

A general criterion based on the implicit function theorem to model aggregation and adsorption in colloidal dispersions

This paper, which may interest not only colloid scientists and physical chemists but also applied mathematicians, completes some previous results on aqueous silicon nitride dispersions. Experimental data on adsorption from liquid solution were first obtained by a titration method and then used to derive the number of solid particles from an equilibrium constraint. To discuss the complex mechanisms affecting simultaneous solid particle aggregation and small ion adsorption at the solid/liquid interface, the Dini implicit function theorem (DT) has been applied to the equilibrium condition for a former suspension Gibbs free energy. It was able to relate the average particle number to the ion concentration adsorbed, but not to unequivocally specify their dependence on the liquid phase pH. We attempt here to model aggregation both through bulk and interfacial quantities. The generalized DT-based criterion has first been formulated in all generality, and then adopted according to a wider investigation. The results obtained confirm the original guess, i.e., to regard solid aggregation as dominated by interfacial mechanisms.     

Solid phase adsorption of crystal violet lactone on silica nanoparticles to probe mechanochemical surface modification

The solid phase adsorption of crystal violet lactone (CVL) on five types of Stober silica nanopowders with BET specific surface areas in the range of 50-800 m2/g under dry milling conditions was described for the first time. The hydrogen bonding between surface silanol and the carboxylate of the ring-opened triphenylmethane dye (CVL+) led to the formation of monolayers of CVL+ in a flat-laid configuration. The lambda max of CVL+ in diffusive reflection visible spectra was influenced by the particle size of silica powders, suggesting that the microenvironmental polarity of adsorbed CVL+ is considerably reduced along with the decrease of the particle size. The solid phase adsorption of CVL obeyed Langmuir adsorption isotherms to give a saturated amount of CVL+ for every silica nanoparticle. The surface concentration of CVL+ on nanoparticles at the saturation was estimated to be 0.31 mg/m2 on average, disclosing that about 52% of the surface can be covered by CVL+ under the assumption that the BET-specific surface areas are equivalent to the real surfaces active for the CVL adsorption. The generation of the blue color of CVL provided a convenient means to estimate qualitative and quantitative analysis of the surface coverage with surface-active reagents, which conceal surface silanols. Subsequently, silica nanoparticles were milled with a surface modifier, followed by milling with CVL to observe the intensity of the blue color in order to disclose that the surface coverage with oligo- and polyethylene glycols as well as with nonionic surfactants by dry milling was specifically determined by the number of repeating oxyethylene units. Although the surface-active reagents were easily desorbed in water, the desorption was notably suppressed by milling with CVL, suggesting that the surface-modified particles with the surface-active reagents are covered with ultrathin films of CVL.     

The effect of the concentration of dispersed particles on the mechanisms of low-frequency dielectric dispersion (LFDD) in colloidal suspensions

There are two mechanisms which are currently used to explain the low-frequency (kHz range) dispersion of the dielectric permittivity of suspensions in electrolyte solutions (LFDD). The first, the surface diffusion mechanism (SDM), associates the LFDD with the diffusion of bound ions along the particle surface caused by the applied electric field. The second, the volume diffusion mechanism (VDM), follows from the generalization for alternating fields of the classical theory of the relaxation effect in electrophoresis and associates the LFDD with the diffusion of free ions in the diffuse double layer. It has been found that VDM is much more strongly dependent on particle concentration than SDM, opening new possibilities for the investigation of each of these two mechanisms separately. The reason is that when the concentration of particles in suspension increases, the characteristic length for the propagation of volume diffusion processes decreases together with the decrease of the free electrolyte volume, whereas the characteristic length for the surface diffusion remains constant. Correspondingly, when particle concentration is raised, the relaxation time of the VDM effect must decrease, whereas it must remain constant for the SDM mechanism. Thus, by varying the concentration of particles in suspension, one can separate the dispersion curves of SDM and VDM. A simple model is elaborated which can be useful to predict the volume fraction dependence of the parameters of LFDD; in particular, its amplitude and critical frequency. The results are compared with experimental data obtained with polymer latex dispersions of volume fractions ranging from 3 to 16%. It is found that the dielectric behaviour (the volume fraction dependence of both the amplitude and critical frequency of LFDD) of the dispersions is reasonably well explained with our model, thus demonstrating that VDM prevails in the systems studied. Experimental data previously found by other authors are also discussed in the light of the model presented.     

Parallel pore and surface diffusion of levulinic acid in basic polymeric adsorbents

The equilibrium and kinetics of levulinic acid (LA) adsorption on two basic polymeric adsorbents, 335 (highly porous gel) and D315 (macroreticular), were investigated. Experimental adsorption rates in batch stirred vessels under a variety of operating conditions were described successfully by the parallel pore and surface diffusion model taking into account external mass transfer and nonlinear Toth isotherm. The film-pore diffusion model was matched with the rate data and the resulting apparent pore diffusivities were strongly concentration-dependent and approached to a constant value for 335 adsorbent. Thus, the constant value was taken as the accurate pore diffusivity, while the pore diffusivity in D315 was estimated from the particle porosity. The surface diffusivities decreased with increasing initial bulk concentration for both adsorbents. The inverse concentration dependence was correlated reasonably well to the change of isosteric heat of adsorption as amount adsorbed.     

Enantiomer separation by strong anion-exchange capillary electrochromatography with dynamically modified sulfated beta-cyclodextrin

A novel mode of capillary electrochromatography (CEC) based on a dynamically modified stationary phase was presented for chiral separation. The capillary column was packed with strong anion-exchange (SAX) stationary phase packing; the sulfated beta-cyclodextrin (S-CD), which was added to the mobile phase, was dynamically adsorbed to the packing surface. Separation of enantiomers was achieved by their different abilities to form an inclusion complex with the adsorbed S-CD. The enantiomers of tryptophan, praziquantel, atropine, metoprolol, and verapamil were successfully separated in this system with a column efficiency of 36000-412000 plates/m. The resolution value obtained for atropine was as high as 11.23. The superiority of CEC with a dynamically modified stationary phase over that with a physically adsorbed stationary phase was demonstrated. The influence of ionic strength, S-CD concentration, and methanol content on separation was also studied.     

A Simple Two-Membered Model for Retention in RP-IPC with Hydrophobic Counterions

Abstract

The retention behavior of some organic anions in the presence of tetralkyl ammonium ions in the mobile phase is analized under conditions where these hydrophobic counterions do adsorb on the surface of a reversed-phase packing. The study comprises the effects of the counterion concentration in the mobile and in the stationary phase, the salt concentration in the eluent and the salt type. Experimental results are discussed in view of previously proposed mechanisms and 2 simple two-member expression relating corrected capacity factor to adsorbed counterion concentration is deduced. The retention model indicates that solute is present in the stationary phase at two different levels; first into a diffuse ionic cloud associated electrically to the ionic surface and, second, at the surface of the packing where it forms ion pairs with the adsorbed counterions.     

Search for a small hole in a cavity wall by intermittent bulk and surface diffusion

We study the search of a small round hole in the wall of a spherical cavity by a diffusing particle, which can reversibly bind to the cavity wall and diffuse on the surface being in the bound state. There are two channels for the particle first passage to the hole, through the bulk, and through the surface. We propose a coarse-grained model of the search process and use it to derive simple approximate formulas for the mean time required for the particle to reach the hole for the first time and for the probability of the first passage to the hole through the bulk channel. This is done for two distributions of the particle starting point: (1) Uniform distribution over the cavity volume and (2) uniform distribution over the cavity wall. We check the accuracy of the approximate formulas by comparing their predictions with the corresponding quantities found by solving the mixed bulk-surface diffusion problem numerically by the finite difference method. The comparison shows excellent agreement between the analytical and numerical results.     

不同粒径的Ni/SiO2催化剂上CH4和CO2吸附活化的漫反射傅里叶变换红外光谱研究

 采用原位漫反射傅里叶变换红外光谱研究了CH4和CO2在不同粒径的Ni/SiO2催化剂上的吸附及活化. 结果表明,在不同粒径的催化剂上,检测到有CH4解离生成的CHx(x=1～3)物种,以及催化剂表面吸附的CHx物种与表面-OH 作用生成的CHx-O物种. CH4的裂解强烈依赖于催化剂表面Ni颗粒的大小,在粒径8 nm左右的Ni颗粒上, CH4较易解离; CO2难以直接在Ni/SiO2催化剂表面发生解离吸附,但CH4解离生成的吸附H对CO2的解离吸附具有明显的促进作用; CH4与CO2共吸附时,较小粒径的Ni可以促进CO2与表面氧物种发生反应,生成单齿表面碳酸盐物种.     

Polarization-Dependent Sum-Frequency-Generation Spectroscopy for In Situ Tracking of Nanoparticle Morphology

The surface structure of oxide-supported metal nanoparticles can be determined via characteristic vibrations of adsorbed probe molecules such as CO. Usually, spectroscopic studies focus on peak position and intensity, which are related to binding geometries and number of adsorption sites, respectively. Employing two differently prepared model catalysts, it is demonstrated that polarization-dependent sum-frequency-generation (SFG) spectroscopy reveals the average surface structure and shape of the nanoparticles. SFG results for different particle sizes and morphologies are compared to direct real-space structure analysis by TEM and STM. The described feature of SFG could be used to monitor particle restructuring in situ and may be a valuable tool for operando catalysis.     

两段式气流床煤气化炉内气固流动数值模拟研究

建立了两段式气流床煤气化炉内气固两相流动的三维计算流体力学（CFD）模型,将气体视为连续介质,在Euler坐标系下考察气相的运动;将颗粒视为离散体系,在Lagrange坐标系下研究颗粒的运动。利用所建CFD模型对基本设计尺寸和操作条件下的两段式气流床煤气化炉内气固两相流动进行了模拟,给出了两段式气流床煤气化炉内的气固两相流动的规律和颗粒的分布规律。在此基础上,针对不同的结构(喉口直径变化)和不同的操作条件（两段气固进料量变化）进行了一系列的模拟比较。结果表明,喉口直径的变化对于炉内气固两相流动及颗粒分布有重要影响。随着喉口直径减小,喉口附近区域的气相回流增强,颗粒运动轨线变得更加曲折,颗粒分布发生明显变化。两段气固流量的改变可以明显改变炉内气固流动,随着一段反应区的气固流量增加和二段反应区气固流量减小,一段反应区内的气相回流更加显著, 二段反应区气相回流减弱,颗粒螺旋上升运动增强,反应器边壁处颗粒浓度增大,颗粒沉积现象减弱。     

Hydrophilicity of polymer effects on controlled particle coagulation in batch emulsion polymerization

The poly(methyl methacrylate-co-styrene) was prepared by batch emulsion polymerization to clarify the effect of characteristics of polymer on particle coagulation. Experimental results showed that the size of final latex particle increased with increasing methyl methacrylate in initial recipe, ranged from 84 to 193 nm, which was attributed to the particle coagulation. With the methyl methacrylate increased, the hydrophilicity of polymeric particle improved, thus led to the surfactant molecules packed loosely on the polymer surface, further, enhanced particle coagulation occurred. On the contrary, the surfactant molecules adsorbed on tightly the polymeric particle surface (methyl methacrylate content low) surface led to the electrostatic repulsion energy of polymer particle improved, and polymer particle stability was also improved. Thus, combined with the results previously reported by us (Colloid Polym Sci 291: 2385–2398, 2013 and Colloid Polym Sci 292: 519-525, 2014), the particle coagulation depended not only on the aqueous phase such as electrolyte concentration and methanol content, but also on the nature of polymer such as hydrophilicity.