Pore analysis by adsorption

Equation (8.7) is the working equation for pore size analysis by adsorption unless more specific information is available regarding pore geometry and the wetting angle. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

Besides the molecular probe method using gas adsorption,107 162 recently, the TEM image analysis method163"167 has been applied to evaluate the surface fractal dimension of porous materials. The most attractive fact in this method is that the pores in different size ranges can be extracted from the TEM images which include contributions from many different pore sizes by the inverse fast Fourier transform (FFT) operation by selecting the specific frequency range.165 167... 

If information on total metals is required, the sample must be acidified before analysis. If information on dissolved metals only is required, the sample may be filtered (using a specified pore size) before analysis. Losses may occur however, by adsorption during filtration. 

Specific surface ranges from 1 to 1000 square meters/gram. It is most often measured by adsorption of nitrogen at Its atmospheric saturation pressure (-195.8 C), with analysis of the data by the BET adsorption equation (problem P6.01.02). Pore diameters of common catalysts range from 10 to 200 Angstroms (10-8 cm) problem P6.01.01 discusses such data. Porosity of a bed of... 

Obviously liquid residence time is not an appropriate parameter to describe pore diffusion effects in fluidized bed adsorption. This may be elucidated by assessing particle side transport by a dimensionless analysis. Hall et al. [73] described pore diffusion during adsorption by a dimensionless transport number Np according to Eq. (17), De denoting the effective pore diffusion coefficient in case of hindered transport in the adsorbent pores and Ue the... 

Table 1 Proportions of total amino groups and p-CD (determined by elemental analysis), accessible amines (titration by conductimetry), specific surface areas, pore radii and adsorption capacities of lead cation and p-nitrophenol for the different samples...

The importance of the pore size in Prussian blue analogues is supported by differential pair distribution function analysis of X-ray and neutron scattering data of hydrogen- and deuterium-loaded Mn3[Co(CN)6]2 [119]. This shows that no evidence for adsorption interactions with unsaturated metal sites exists and that the hydrogen molecules are disordered about the center of the pores defined by the cubic framework. In conclusion, experimental results indicate that optimum pore dimensions in Prussian blue analogues are predominantly responsible for the heat of adsorption at low loadings rather than the polarizing effect of open metal sites. 

A novel approach is reported for the accurate evaluation of pore size distributions for mesoporous and microporous silicas from nitrogen adsorption data. The model used is a hybrid combination of statistical mechanical calculations and experimental observations for macroporous silicas and for MCM-41 ordered mesoporous silicas, which are regarded as the best model mesoporous solids currently available. Thus, an accurate reference isotherm has been developed from extensive experimental observations and surface heterogeneity analysis by density functional theory the critical pore filling pressures have been determined as a function of the pore size from adsorption isotherms on MCM-41 materials well characterized by independent X-ray techniques and finally, the important variation of the pore fluid density with pressure and pore size has been accounted for by density functional theory calculations. The pore size distribution for an unknown sample is extracted from its experimental nitrogen isotherm by inversion of the integral equation of adsorption using the hybrid models as the kernel matrix. The approach reported in the current study opens new opportunities in characterization of mesoporous and microporous-mesoporous materials. 

The aim of this study is to eompare pore structure characteristics of two industrial catalysts determined by standard methods of textural analysis (physical adsorption of nitrogen and mercury porosimetry) and selected methods for obtaining parameters relevant to transport processes (multicomponent diffusion and permeation of gases). 

J. Hagymassy, Jr., I. Odler, M. Yudenfreund, J. Skalny and S. Brunauer, Pore structure analysis by water vapour adsorption 111. Analysis of hydrated calcium silicates and Portland cement. /. Colloid Interface Sci., 38 (1972) 20. 

Cluster-assembled carbon film are very porous with a pore diameter peaked at 3-4 nm, as shown by adsorption/desorption isotherm analysis [32]. The gases used in our experiments have a molecular or atomic size much smaller than the average pore size, so it is reasonable to assume that they equally diffuse in the mesoporous film network and interact with the same amount of linear carbon structures. This explains the similar Rq value observed for the three gases. The time required to reach the asymptotic value of is affected by the momentum transferred from the molecule to the film network during collisions, hence by the mass of the gas molecule. We note that H2 does not seem to chemically interact with sp structures. 

The other posibility for characterizing structural properties of inorganic sorbents deals with the thermogravimetric analysis. By means of this analysis the pore size distribution, pore volume and specific surface area of adsorbents and catalysts which are fundamental adsorption characteristics may be obtained. 

---