Instrumentation, mercury porosimetry

Nitrogen sorption measurements were performed on a Quantachrome Autosorb 6B (Quantachrome Corporation, Boynton Beach, FL, USA). All samples were degassed at 423 K before measurement for at least 12 hours at 1 O 5 Pa. Mercury-porosimetrie has been measured on a Porosimeter 2000 (Carlo Erba Instruments) Scanning electron micrographs were recorded using a Zeiss DSM 962 (Zeiss, Oberkochen, Germany). The samples were deposited on a sample holder with an adhesive carbon foil and sputtered with gold. 

The author thanks Roberto Cordero (TTC Analytical Service Corp., Caguas, PR), Michael L. Strickland (Micromeritics Instrument Corporation), and Micromeritics Analytical Services for the mercury porosimetry intrusion study of the poly-[HIPE] sample. 

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... 

Nitrogen adsorption/condensation is used for the determination of specific surface areas (relative pressure < 0.3) and pore size distributions in the pore size range of 1 to 100 nm (relative pressure > 0.3). As with mercury porosimetry, surface area and PSD information are obtained from the same instrument. Typically, the desorption branch of the isotherm is used (which corresponds to the porosimetry intrusion curve). However, if the isotherm does not plateau at high relative pressure, the calculated PSD will be in error. For PSD s, nitrogen condensation suffers from many of the same disadvantages as porosimetry such as network/percolation effects and pore shape effects. In addition, adsorption/condensation analysis can be quite time consuming with analysis times greater than 1 day for PSD s with reasonable resolution. 

Nitrogen sorption measurements were performed by use of a Sorptomatic 1900 Turbo apparatus by Carlo Erba Instruments. All samples were degassed at 393 K before measurement for at least 24 hours at 10 mbar. The mercury porosimetry measurements were carried out on a Porosimeter 2000 apparatus by Carlo Erba Instruments. A contact angle of 141.3° for Hg was used. The samples were degassed at 393 K before measurement for 24 h. SEM of the porous glass membranes was carried out on a Phillips ESEM XL 30 PEG microscope. 

In Table 1 manufacturers of carrier gas and volumetric adsorption measuring instruments are compiled. Most of these companies offer also mercury porosimeter and gas pycnometer. Mercury porosimetry extends the measuring range of the sorption method towards larger pores. Table 2 gives a survey on the commercial offer of gravimetric apparatus. 

Mercury porosimetry is performed nearly exclusively on automatic commercial instruments that differ mainly in the highest operative pressure, which determines the size of smallest attainable pores. The highest pressure is limited by the uncertainty about the validity of the Washburn equation, which forms the basis of data evaluation. In pores with sizes similar to the mercury atom the assumption that physical properties of liquid mercury (surface tension, contact angle) are equal to bulk properties is, probably, not fully substantiated. For this reason the up-to-date instruments work with pressures up to 2000 - 4000 atm, only. 

Porosimeter An instrument for the determination of pore size distribution by measuring the pressure needed to force liquid into a porous medium and applying the Young-Laplace equation. If the surface tension and contact angle appropriate to the injected liquid are known, pore dimensions can be calculated. A common liquid for this purpose is mercury hence, the term mercury porosimetry. 

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. 

Nitrogen adsorption-desorption isotherms measured at 77 K with a Micromeritics ASAP 2000 instrument were used to obtain values of the specific surface area, Sbet, estimated from a linear section of adsorption isotherm, taking five points from 0.05-0.2 p/p range, and pore volume VpN2, determined from the amount adsorbed at p/po of about 0.98. Bulk density, pb, was determined from mercury porosimetry data afforded by Micromeritics Auto Pore 9220. The later quantity was used to calculate the total pore volume = VpN2 + Vmacropores using the expression Vt = l/pb-l/ps [7], where ps is skeletal density, taken equal to 2.2 g cm", i.e. density of amorphous silica obtained by the sol-gel method [8]. 

In current mercury porosimetry instrumentation the process of intrusion into a porous network can be achieved by two distinct methods. One relies upon an equilibrium pressure step, which waits for a series of voids/pores to be filled, the time allocated for this "pseudo-equilibrium" step being of the order of seconds. Since the nature of porous materials is not uniform the time necessary for pressure equilibrium to occur varies from pressure step to pressure step. Care must therefore be taken to ensure pressure equilibration has occurred prior to a subsequent pressure increase. A second and alternate method for mercury intrusion is one in which pores/voids are filled by the steady and continuous application of pressure to cause mercury to flow into a porous network. 

An instrument based on this design has been examined by Henrion [58], who states that equation (1.69) breaks down for non-random voidage. A classic case is with porous material where diffusion through the wide voids between particles completely swamps the diffusion through the narrow voids within particles. In cases such as this there is good agreement between diffusion and mercury porosimetry. 

In this study mercury intrusion porosimetry (MIP) analyses were employed to determine the pore size distribution and pore volume over the range of approximately 100 pm down to 7.5 nm diameter, utilising CE Instruments Pascal 140/240 apparatus, on samples previously dried overnight at 150°C. The pressure/volume data were analysed by use of the Washburn Equation [14] assuming a cylindrical nonintersecting pore model and taking the mercury contact angle as 141° and surface tension as 484 mN m [10]. For the monolith... 

The primary pore size distributions of the cases A and B in Table 3.5 are experimentally determined by mercury injection porosimetry (MIP) (Fermeglia and Pricl, 2009) on the instrument of PoreMaster GT 60. The MIP enables the measurements of both the pressure required to force mercury into the pores of CLs and the intruded Hg volume at each pressure. The employed equipment operates from 13 kPa to a pressure of 410 MPa, equivalent to the pores with the diameters, d, ranging from 100 [xm to 0.0036 xm. On the other hand, the 3 V method provides a tool to theoretically calculate the agglomerate volume depending on the probe radius. In analogy to the... 

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