Molecular probe adsorption

Characterization of IOM-CMS Materials by Molecular Probe Adsorption. Since carbon molecular sieves, and the IOM-CMS materials prepared herein, are both polycrystalline and amorphous, we have characterized their sieving properties phenomenologically by molecular probe adsorption. The probe... 

The nitrogen isotherm and molecular probe adsorption data both suggest that the carbon must reach some minimum value, set by the physical and chemical properties of the oxide, before it converts the composite into a molecular sieve. 

B. Alonso, I. Klur, D. Massiot, Studies of surfaces through molecular probe adsorption and solid-state NMR, Chem. Commun. 8 (2002) 804—805. 

These procedures proposed by Dubinin and by Stoeckli arc, as yet, in the pioneer stage. Before they can be regarded as established as a means of evaluating pore size distribution, a wide-ranging study is needed, involving model micropore systems contained in a variety of chemical substances. The relationship between the structural constant B and the actual dimensions of the micropores, together with their distribution, would have to be demonstrated. The micropore volume would need to be evaluated independently from the known structure of the solid, or by the nonane pre-adsorption method, or with the aid of a range of molecular probes. 

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

Coluccia, S., Marchese, L. and Martra, G. (1999) Characterisation of microporous and mesoporous materials by the adsorption of molecular probes FTIR and UV-Vis studies, Microporous Mesoporous Mater., 30, 43. 

Liquid-phase breakthrough experiments were also developed in order to characterize mesopores. The principle of the methodology relied on the analysis of the diffusion and adsorption of molecular probes with various molecular dimensions and adsorption strength. The relative proportion of occluded and accessible mesopores in the studied dealuminated Y zeolite could then be estimated. To allow this estimation, it is necessary to use molecular probes that can or cannot penetrate into the microporosity of the Y zeolite (see Figure 2). 

Since Avnir and Pfeifer s pioneer works83"86 regarding the characterization of the surface irregularity at the molecular level by applying the fractal theory of surface science, the molecular probe method using gas adsorption has played an important role in the determination of surface fractal dimension of the porous and particulate materials. 

The other molecular probe method is the single-probe method (SP method), which is separately proposed by Avnir and Jaroniec,93 and Pfeifer et al.108-112 In the SP method, a single adsorption isotherm is analyzed using a modified FHH theory. The FHH model was developed independently by Frenkel,113 Halsey,114 and Hill,115 and describes the multilayer adsorption coverage. Since the SP method uses only one probe molecule, this method is more convenient than the MP method. However, there are many theoretical limitations in applying the SP method to determination of the surface fractal dimension. Therefore, it is really necessary to discuss whether the SP method is an adequate tool to investigate the surface fractal dimension or not before applying the SP method to certain system. 

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... 

Coluccia, S., Marchese, k., and Martra, G. (2000) Molecular probes for the characterization of adsorption sites in micro-and mesoporous materials, in Photofunctional Zeolites (ed. M. Anpo), NOVA Science, New York, pp. 39-74. 

Xenon is a very useful molecular probe for adsorption studies. 129Xe is a spin nucleus of 26.44% natural abundance and a very wide range of chemical shifts (334). The shielding of the xenon atom with respect to the bare nucleus has been estimated to be 5642 ppm (335), and the 129Xe chemical shift is extremely sensitive to physical environment as shown by its strong dependence on density in the pure phases the liquid at 224 K resonates 161 ppm downfield from the gas at zero density, whereas the solid at 161 K has its resonance at — 274 ppm. The atomic diameter of xenon is 4.6 A, i.e., comparable to the size of zeolitic channels. 

Experimental. PRODAN was purchased from Molecular Probes and the purity checked by reverse phase HPLC. There were no detectable impurities. Stock solutions (1 mM) were prepared in absolute ethanol and stored in the freezer. CF3H was purchased from Matheson and passed through a single adsorptive Q trap (Matheson) prior to entering the pumping system. According to the manufacturer this gives an 02 level < 5 ppm. 

The structure of this review is as follows in Section II, the activation and templating methods for preparing the porous carbons are briefly summarized. Section III surveys the structural characteristics of the porous carbons by using gas adsorption method. In Section IV, the molecular probe method and the image analysis method for quantitative characterization of the pore surface irregularity and the size distribution irregularity based upon the fractal theory are discussed in detail. Section V is devoted to... 

Recently, Pcs organized on surface have been used as molecular probes for the determination of quantum confined effects [207], CoPc molecules form ordered self-assembled monolayers (SAM) on the top of Pb(lll) thin films grown on a Si( 111) substrate with the Pc units lying flat on the surface, as revealed by atomically resolved STM. A close analysis of the STM data revealed that the Pc molecules adsorb and self-assemble on the surface following a thickness-dependent adsorption pattern, which is ultimately related to the quantum size effects of the metal surface. 

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