Surface area values

The macroporous particles prepared by using only linear polystyrene as diluent yielded lower pore volume and specific surface area values. 

The presence of closed pores was demonstrated by Kozawa [22] by measuring the BET surface area of EMD samples of various particle sizes. Kozawa s new method for the determination of the closed pore is based on the relationship of the BET surface area and the particle size, by extrapolating the surface area value to zero particle size (Fig. 17). Table 8 shows the percentage of closed pores of various EMD samples. 

The BET surface area values are also reported with the distribution of porosity between microporosity (pore diameter <1.8 nm) deduced from N2 adsorption isotherms (t-curves) and mesoporosity (pore diameter > 1.8 nm). The following trend is observed for high atomic M/HPA ratio used for the precipitation, the precipitates exhibited high surface area mainly due to microporosity. However, depending on the nature of the coxmter cation and also of the previous ratio values, the textural characteristics were not similar. In particular, it is interesting to note the presence of mesopores for (NH4)2.4P, CS2.9P, CS2.7P and Cs2.4Si samples. 

The reference Pt-Ba/y-Al203 (1/20/100 w/w) catalyst shows surface area values in the range 140-160 m2/g, a pore volume of 0.7-0.8cc/g and an average pore radius close to 100 A (measured by N2 adsorption-desorption at 77 K by using a Micromeritics TriStar 3000 instrument). Slight differences in the characterization data are associated to various batches of the ternary catalyst [24,25],... 

The smaller surface area values of the catalysts indicate that the palladium blocks some part of the surface of this mesoporous carbon. The decreased amount of the CO-yielding complexes on the catalyst surface compared to that of the carbon support indicates that the palladium is attached to CO generating groups. The increased concentration of C02 containing complexes on the catalysts surfaces can be due to the steps of the preparation. To clarify this it needs further experiments. 

The chemical composition of the samples was determined using an inductively Coupled plasma atomic emission spectrometer (ICP-AES) JY 38 from Jobin Yvon. Specific surface area values were determined by BET method using a Micromeritics Instrument Corp. FlowSorb 2300. The basicity of the materials was studied by temperature programmed desorption (TPD) of C02 used as a probe molecule. The equipment was described in a previous work [7]. FTIR spectra of pellets pressed at 2.5xl08 Pa were recorded with a Vector 22 spectrometer from Brucker. The samples were diluted with KBr (lOOmg KBr - 1.5mg of the sample). 

Table 1. Studied samples and thermal treatment temperatures, Nd203 content determined by ICP analysis, specific surface area values measured by BET method and Tmax of the desorption peaks in C02 TPD.

It appeared that the chemical composition was close to the theoretical one. Surface area values decreased from Mg to Sr samples, which are related to the calcination temperatures necessary to eliminate the nitrate anions and organic compounds. Moreover, the SG samples presented higher surface area than the evaporated ones, although the calcination temperatures were higher (Table 1). 

Fumed silica is prepared by burning volatile silicon compounds such as silicon tetrachloride. This type of silica contains less than 2% combined water and generally no free water. It reacts readily with hydroxyl groups. The particle size is in the region 5-10 nm. Fumed silicas are not generally used in conventional rubber compounding but find application with silicone rubber. The recognised surface area values for best reinforcement of silicone rubber by an amorphous silica lies between 150-400 m2/g. 

The theory for single-point BET measurements is similar however, an assumption is made that the intercept of the PIV(P0-P) versus PIPQ plot equals zero. This assumption is not always valid and can result in surface area values that differ from multipoint values. When the assumption is valid, the single-point method is simpler and faster, and it results in accurate surface area values. 

Methods for synthesizing highly porous microspheres were investigated, and surface area measurements were used to confirm the porous nature of the samples [19]. A high surface area was measured and was compared with the calculated surface area value. The measured value was 35 times that of a nonporous particle, indicating the extensive porosity of the spheres. The surface area was also used to explain the drug release mechanisms in the pores of these systems. 

Polyacrylic nanoparticles have been prepared to investigate their use as colloidal drug carriers, and surface area measurements were used for characterization [14], When the experimental surface area values were compared with calculated values, it was found that the measured surface area value was 10 times smaller than the calculated value. The discrepancy was explained by the surfactant used in the nanoparticle preparation. The surfactant appears to coat the particles when it is not completely removed, resulting in the low surface areas observed for the particles. Particles prepared without surfactant showed good agreement with the calculated values. 

The extent to which measurement results can be distorted by additives is particularly severe in the case of disazo yellow pigments, whose preparation involves fatty amines (Sec. 1.8.1.2,2.4.1.1). Measurements are commonly carried out under conditions that give rise to the cleavage of volatile amines or ammonia, resulting in seemingly even negative specific surface area values. 

It Is Important to use highly accurate surface area values of a crystal particle In growth rate equations, when the equations Include a term of the surface area. In practice, however. It Is difficult to measure the surface area unless the crystal possesses a simple geometry (e.g., sphere, cube, octahedron, etc.), or unless the B.E.T. method of measuring surface area Is applied. Therefore, a characteristic diameter Is usually defined, and the area calculated from the diameter Is used. 

XPS has been used to characterize the three mixtures containing respectively 7,25, and 50 weight % of Bi2Mo30i2 (Table II samples J,K and L). These samples have been characterized before and after catalytic reaction (table III). Bi, Mo, Fe, Co and O have been analyzed. The Mo/0 ratio remains equal to 0.25 for all the samples, before and after catalysis which confirms that no new phase was formed since the molybdates suspected to have formed, have a much lower Mo/0 ratio (0.17 for Bi2Mo06 and Bi3FeMo20i2). Concerning the Bi/(Fe+Co) ratio, it can first be observed that before catalysis this ratio was always lower than that calculated from chemical analysis. This can be explained by the difference between the particles size of the bismuth molybdate and the iron and cobalt molybdates which is in a ratio of more than 30 as calculated from differences in surface area values, 0.3 and 9 to 22 m. g Secondly the Bi/(Fe+Co) ratio increased systematically after catalysis which could be explained by the decrease in size of the bismuth molybdate particles or by the covering of the iron and cobalt molybdate particles by the bismuth molybdate or by both effects. 

The area A represents the total surface area of all the channels in the powder bed which offer resistance to flow. This excludes the surface area generated by close-ended or blind pores and crevices since they make no contribution to the frictional area. It follows then that equation (7.26) will give surface area values always less than BET or other adsorption techniques. 

Attention has also been given to possible sources of error in specific surface area analysis. Lobenstein (5) has noted the problems associated with the lack of additivity in BET theory as applied to mixtures and the "interaction" between two or more components of dental materials. He has also derived equations to estimate the extent of the "interaction" as well as to correct for it to give a more realistic surface area value. 

Table 13.1 Experimental preparation conditions of Y-M02N type samples and resulting surface area values...

Table 13.6 Surface area values of an aged Mo2N-A powder in function of outgasing temperature and time...

The retention volume computed by Equation 2, V, can be related to the adsorption coefficient and the surface area value of the sorbent bed by ... 

The obtained gels show very low porosity and surface area values (table 8.8). Therefore this procedure is only used for dedicated applications, such as the production of ceramics. 

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