Characterization physisorption

The physisorption bond, being relatively weak, is even more difficult to characterize than the chemisorption one. Some aspects of this are covered in Section XVII-10. 

In this work, the catalytic reforming of CH4 by CO2 over Ni based catalysts was investigated to develop a high performance anode catalyst for application in an internal reforming SOFC system. The prepared catalysts were characterized by N2 physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). 

Physisorption is a weak interaction characterized by the lack of a true chemical bond between adsorbate and surface, i.e. no electrons are shared. The physisorption interaction is conveniently divided into to parts A strongly repulsive part at close distances and Van der Waals interactions at medium distances of a few A. 

Two characterization techniques will be discussed in this chapter, viz. physisorption and chemisorption. Physisorption mainly yields information on catalyst texture and morphology, whereas chemisorption studies potentially give information regarding the active catalyst sites. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

Preparation and physisorption characterization of D-glucose-templated mesoporous silica sol-gel materials. Chemistry of Materials, 11, 2023-2029. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

For this purpose, all three catalyst supports were initially synthesized by a chemical vapor deposition (CVD) process and thereafter, using a wet impregnation method, loaded with cobalt as the active component for FTS. The as-synthesized Co/nanocatalysts were then characterized by applying electron microscopic analysis as well as temperature-programmed desorption, chemi- and physisorption measurements, thermogravimetric analysis, and inductively coupled plasma... 

Results of Carbon Nanomaterial Catalyst Characterization ICP, Chemisorption, Physisorption, Thermogravimetric Analysis (TG)... 

Surface Acoustic Waves (SA Ws). The basic idea of this technique is to use the dependence of the frequency and propagation of surface acoustic waves on mass loading in a film. The porous film has to be deposited on a piezoelectric substrate (quartz), which is then placed into a physisorption setup to condense nitrogen at 77 K. Adsorption and condensation of N2 result in a shift of the oscillation frequency, and thus measurements of the oscillation frequency as a function of N2 partial pressure provide an adsorption-desorption isotherm.30 Although the technique has proven to provide a concise characterization of porous films,29,30 the requirement for the deposition directly onto the SAW piezoelectric substrate represents a certain restriction. 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

Another important field of the application of fractal approach to texturology is related to surface roughness. Anvir and Pfeifer [212,213] proposed characterization of surface irregularities by adsorption and established two methods, based on Mandelbrot s fundamental equations of type 9.69. According to the first method of Dt calculation, one uses the relations that interrelate a number of molecules in a complete monolayer during physisorption, nm, or an accessible surface area, A, with a cross-sectional area, w, which correspond to one molecule in a monolayer ... 

Suitable characterization techniques for surface functional groups are temperature-programmed desorption (TPD), acid/base titration [29], infrared spectroscopy, or X-ray photoemission spectroscopy, whereas structural properties are typically monitored by nitrogen physisorption, electron microscopy, or Raman spectroscopy. The application of these methods in the field of nanocarbon research is reviewed elsewhere [5,32]. 

TPRS = temperature-programmed reaction spectroscopy XRD = X-ray diffraction BET = Brunauer-Emmett-Teller method (specific BET surface area) and BJH = Barrett-Joyner-Halenda method (determination of pore volume and diameter), both determined by nitrogen physisorption NMR= characterization by solid-state NMR. 

Overlap between p orbitals leads to cohesive energies of typically less than 0.4 eV molec The much stronger ionic and covalent bonding have binding energies of 10 and 3 eV atom respectively. Finally, physisorption is the weakest form of absorption to a solid surface characterized by a lack of a true chemical bond (chemisorption) between substrate and adsorbate and will be discussed in Chapter 4 (see e.g., Zangwill, 1988). 

The fresh and spent catalysts were characterized with the physisorption/chemisorption instrument Sorptometer 1900 (Carlo Erba instruments) in order to detect loss of surface area and pore volume. The specific surface area was calculated based on Dubinin-Radushkevich equation. Furthermore thermogravimetric analysis (TGA) of the fresh and used catalysts were performed with a Mettler Toledo TGA/SDTA 851e instrument in synthetic air. The mean particle size and the metal dispersion was measured with a Malvern 2600 particle size analyzer and Autochem 2910 apparatus (by a CO chemisorption technique), respectively. 

The Ru/C catalyst was provided by PMC, Inc. and consists of 5.0 wt% Ru on activated carbon support. The Ni/Re/C cataiyst comprises 2.5 wt% Ni and 2.5 wt% Re on activated carbon. Both cataiysts were characterized via N2 physisorption and H2 chemisorption using a Micromeritics ASAP 2010 apparatus the resuits are presented in Table 1. 

All gases adsorb to solid surfaces below a critical temperature because of the van der Waals attraction. In general, when adsorption is dominated by physical interaction rather than chemical bonding, we talk about physisorption. Physisorption is characterized by several features ... 

Physisorption is characterized by a relatively weak binding with adsorption energies of a few 10 kJ/mol. In chemisorption a chemical bond is established between the adsorbate and the adsorbent desorption energies are in the range of 100-400 kJ/mol. 

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