Pore structure characterization of catalyst

Pore Structure Characterization of Catalyst Supports via Low-Field NMR Spectroscopy... 

R. Mann, A. Al-Lamy, and A. Holt, Visualized porosimetry for pore structure characterization of a Nickel/Alumina Reforming Catalyst, Trans, I, Chem, E. 73(A) 47 (1995). 

Several experimental methods are available to characterize catalyst pore structure. Some of them, useful in quantifying mass transfer of reactant and product inside the porous particle, will be only briefly discussed here. More details concerning methods for the physical characterization of porous substances are given by various authors [5,8,9],... 

Dimensional analysis of the coupled kinetic-transport equations shows that a Thiele modulus (4> ) and a Peclet number (Peo) completely characterize diffusion and convection effects, respectively, on reactive processes of a-olefins [Eqs. (8)-(14)]. The Thiele modulus [Eq. (15)] contains a term ( // ) that depends only on the properties of the diffusing molecule and a term ( -) that includes all relevant structural catalyst parameters. The first term introduces carbon number effects on selectivity, whereas the second introduces the effects of pellet size and pore structure and of metal dispersion and site density. The Peclet number accounts for the effects of bed residence time effects on secondary reactions of a-olefins and relates it to the corresponding contribution of pore residence time. 

A. Al-Lamy, Characterization of catalyst pore structure by image reconstruction from 3-D stochastic pore networks, Ph.D. dissertation, UMIST (1995). 

Characterization of the pore structure of amorphous adsorbents and disordered porous catalysts remains an important chemical engineering research problem. Pore structure characterization requires both an effective experimental probe of the porous solid and an appropriate theoretical or numerical model to interpret the experimental measurement. Gas adsorption porosimetry [1] is the principal experimental technique used to probe the structure of the porous material, although various experimental alternatives have been proposed including immersion calorimetry [2-4], positron... 

Pores, and especially mesopores (with sizes between 2 and 50 nm) and micropores (with sizes less than 2 nm), play an essential role in physical and chemical properties of industrially important materials like adsorbents, membranes, catalysts etc. In addition to pore structural characterization described above, the description of transport phenomena in porous materials has received attention due to its importance in many applications such as drying, moisture transport in building materials, filtration etc. Although widely different, these applications present many similarities since they all depend on the same type of transport phenomena occurring in a porous media environment. In particular, transport in mesoporous media and the associated phenomena of multilayer adsorption and capillary condensation have been investigated as a separation mechanism for gas mixtures [29]. 

It may be questioned how representative a pore really is for a catalyst particle. No doubt in the future, with growing attention for the more detailed characterization of catalyst particle structures, more elaborate representations of the particle will be used. Beeckman and Froment [1982] developed the theory for reaction and deactivation by coke formation in random networks of pores. The deactivation was considered to occur through site coverage and pore blockage. Diffusion limitations were also included, but only for deactivation by site coverage. 

For the detailed information, pore structure porosimetry techniques are used. These methods enable measurement of pore diameter, pore shape, pore volume, and pore distribution in the electrode catalyst and gas diffusion layers. However, for PEMFC, these layers have hydrophobic and hydrophilic pores and there is no suitable technique available for characterization of such complex pore structures. Combination of multiple porosimetry techniques are employed to characterize layers with both hydrophobic and hydrophilic pores. The pore structure characterization techniques include capillary flow porosimetry, water intrusion porosimetry, and mercury intrusion porosimetry (Jena and Gupta, 2002). In water... 

FTIR is especially useful for the characterization of surface species and the state of adsorbed molecules. A study of supported oxide catalysts involving FTIR and adsorption measurements is reported by Rives and his co-workers and the use of FTIR as an ancillary technique is referred to in several other papers. NMR measurements have been carried out in the context of image analysis (NMRI) of pore structures (Ewing et al), determination of dlffusivities of adsorbed species (Bahceli et al) and the pore structural analysis of wet materials (Smith and Davis). 

Improved characterization of the morphological/microstructural properties of porous solids, and the associated transport properties of fluids imbibed into these materials, is crucial to the development of new porous materials, such as ceramics. Of particular interest is the fabrication of so-called functionalized ceramics, which contain a pore structure tailored to a specific biomedical or industrial application (e.g., molecular filters, catalysts, gas storage cells, drug delivery devices, tissue scaffolds) [1-3]. Functionalization of ceramics can involve the use of graded or layered pore microstructure, morphology or chemical composition. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

The structure of the catalysts was characterized by X-ray diffraction, IR-spectroscopy and transmission electron microscopy, their thermal stability was followed by thermal analytical method. The specific surface area and pore size distribution of the samples were determined by nitrogen adsorption isotherms. 

The electron crystallography method (21) has been used to characterize three-dimensional structures of siliceous mesoporous catalyst materials, and the three-dimensional structural solutions of MCM-48 (mentioned above) and of SBA-1, -6, and -16. The method gives a unique structural solution through the Fourier sum of the three-dimensional structure factors, both amplitude and phases, obtained from Fourier analysis of a set of HRTEM images. The topological nature of the siliceous walls that define the pore structure of MCM-48 is shown in Fig. 28. 

The industrial view on catalyst characterization is different. Here the emphasis is mainly on developing an active, selective, stable and mechanically robust catalyst. In order to accomplish this, tools are needed which identify those structural properties that discriminate between efficient and less efficient catalysts. All spectroscopic information that helps to achieve this is welcome. Establishing empirical relationships between the factors that govern catalyst composition, particle size and shape and pore dimensions on the one hand and catalytic performance on the other are extremely useful in the process of catalyst development, although such relationships may not give much fundamental insight into how the catalyst operates in molecular detail. 

Here, following the works of J.H. De Boer (Delft, The Netherlands, see elsewhere [1,2]), by texture one means the individual geometrical structure of catalysts, supports, and other porous systems (PSs) at the level of pores, particles and their ensembles (i.e., on a supramolecular level scale of 1 nm and larger). In a more complete interpretation, texture includes morphology of porous space and the skeleton of a condensed (solid or sometimes liquid) phase, the shape, size, interconnectivity, and distribution of individual supramolecular elements of the system particles and pores (or voids) between particles, various phases, etc. In turn, texturology also involves general laws of texture formation and methods for its characterization [3],... 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

The isomer distribution in the hydroxylation of phenol, anisole and toluene or other aromatics on TS-1 is influenced by the reaction conditions, but is characterized by a tendency towards p-selectivity [105-106]. Furthermore secondary reactions leading to polynuclear aromatic byproducts are minimized. Both phenomena are ascribed to the pore structure of the catalyst, which is isostructural to ZSM-5 [96]. The selectivity for hydroxylation as well as the H2O2 efficiency decrease with increasing conversions as is shown in Figure 14 for the hydroxylation of phenol [106]. 

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