Mercury porosimetry catalyst characterization

The applied pressure is related to the desired pore size via the Washburn Equation [1] which implies a cylindrical pore shape assumption. Mercury porosimetry is widely applied for catalyst characterization in both QC and research applications for several reasons including rapid reproducible analysis, a wide pore size range ( 2 nm to >100 / m, depending on the pressure range of the instrument), and the ability to obtain specific surface area and pore size distribution information from the same measurement. Accuracy of the method suffers from several factors including contact angle and surface tension uncertainty, pore shape effects, and sample compression. However, the largest discrepancy between a mercury porosimetry-derived pore size distribution (PSD) and the actual PSD usually... 

Physico-chemical techniques are widely used for characterization of catalysts and porous materials in general. Well-known methods based on physical adsorption of inert gases (N2 and CO2) and penetration of mercury at elevated pressures provide information on the total surface area, pore volume, and pore size distribution (PSD) of the sample [1,2]. Gas adsorption and mercury porosimetry are often compared since they generate data of similar nature in the pore size range 4 - 100 nm. 

The precursor and the calcined catalyst were characterized by various techniques such as nitrogen adsorption, mercury porosimetry, X-ray diffraction (XRD), atomic emission spectrometry by inductively coupled plasma (ICP), thermogravimetric analysis, and temperature-programmed reduction (TPR). More details about the catalyst preparation and characterization can be found in a previous work (22). 

The aim of this work is to explore the applicability of the sol-gel method for the preparation of Ag/Si02 and Cu/Si02 catalysts and to see whether such a method can yield silver and copper species stabilized by the carrier. Characterization of the catalyst structure by several physical and chemical techniques, including N2 adsorption-desorption isotherms, mercury porosimetry measurements, X-ray diffraction and transmission electron microscopy, has been used to correlate the microstructure of Ag/Si02 and Cu/Si02 catalysts with their catalytic performance. 

The composition, the textural and surface properties of the catalysts were studied by AAS, mercury porosimetry, XRD, XPS and TG-DTA [16,17]. The amount of metallic copper on the surface of catalyst was determined by titration with N2O [18]. The characterization of bulk and surface properties of the catalysts is given elsewhere [14,16,17]. 

Several authors have investigated the differences in deactivation for small or for large pores within the catalysts. In general, the researchers have employed mercury porosimetry for the characterization of the actual catalysts and they have employed mostly models of parallel pores of differing dimensions (as contrasted with an interconnected network) in their simulations. 

Leonard C. Drake—His Contributions to the Development of Mercury Porosimetry for Catalyst Characterization... 

Preparation of controlled porosity materials becomes an increasingly important function in catalyst research and preparation. Mercury porosimetry is a unique characterization technique since it permits measurement of the full range of pore diameters. The contributions that Dr. Leonard C. Drake made to... 

Mercury porosimetry is advantageously used for characterizing various shaped industrial catalysts in which diffusion processes play a role. The macropore distribution is of major importance for the turnover and lifetime of industrial catalysts and is decisively influenced by the production conditions. 

The model equations for the catalyst pellet also contain the tortuosities td, tk or a combination of both t and es. Their determination requires specific equipment. Well instrumented catalyst characterization equipment, including computerized data treatment, is commercially available, in particular for mercury porosimetry and nitrogen-sorption and -desorption. 

The objective of this smdy is to be able to estimate the effective transport resistance of a porous medium by characterizing its void morphology by mercury porosimetry. A series of porous catalyst solids were obtained differing only in void morphology, overall porosity and pore sizes. We cahnilated the tortuosity by a dynamic experiment employing solid-gas chromato phy, SGC. Tortuosities of aU solids were very si ar, in the range of 5-25. Transport resistance is more easily related to overall volume porosity rather than specific network architectu features observable by porosimetry. 

The textural characterization of the supports and catalysts pore size distribution, pore volume, and surface areas were determined by use of mercury intrusion porosimetry using a Micromeritics Poresizer 9320 and nitrogen gas adsorption/desorption isotherms carried out on a Micromeretics ASAP 2000 respectively. For the porosimetiy analysis a contact angle of 140° and surface tension of480mNm for mercury were assumed. 

In Table 1 the results obtained from the textural characterization of the supports and catalysts by nitrogen adsorption and mercury intrusion porosimetry are presented. In the table the values of surface area obtained from the gas adsorption results, using the BET method for which the linear portion was usually located in the relative pressure range of 0.05 to 0.3 Sbet [9], and those from the intrusion curve of the porosimetry analysis, using a nonintersecting cylindrical pore model Sng [10], are shown. The pore volume Vp is that recorded at the liighest intrusion pressure reached during the porosimetry analysis, and as such represents the pore volume of pores between ca. SOpm to 3mn pore radius. The pore radii were taken from the maxima of the curves of pore size distribution. 

In this work the merits of the use of a natural fibrous mineral, sepiolite, as a binder to produce titania based monoliths of high mechanical strength and abrasion resistance is discussed. The monoliths of square channels were conformed with an initial 7.5 channels cm and 1 mm wall thickness. TTie textural characterization was made by mercury intrusion porosimetry (MIP), nitrogen adsorption/desorption (BET), and X-ray diffraction (XRD). The mechanical resistance, dimensional changes and weight losses al each stage of heat treatment were also determined. The thermal expansion coefficients (TEC) of the monoliths were determined between 200 and 400 C, since in practice the usual working temperature of DENOX catalysts lies between 250°-350 C. 

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... 

Out of other physical methods used for porosity analysis, mercury intrusion porosimetry is worth mentioning. This method can be tried at all levels of electrode development, i.e. from catalyst to matrix stage. Testing of a matrix is done at low pressures (upto 400 kPa) so that the soft structure of the matrix is not deformed. Fig. 11 shows a typical result of PAFC cathode developed by the author s laboratory. The matrix is also characterized by other simple tests, like water loading, rate of water migration when dry matrix is dipped in one end etc. (Caires et al., 1997). 

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