Temperature-programmed desorption method

Two mesoporous silica molecular sieves synthesized by using n-octadecyl-ammonium bromide and n-dodecylammonium bromide as a templates were characterized for their pore size distribution by temperature programmed desorption method and low temperature nitrogen adsorption method. The pore size distributions and total pore volumes determined by the two methods agree quite well and are within experimental error. 

Clays have both Brpnsted and Lewis acid sites, the amount and strength of which can be substantially modified by acid treatments and/or ion exchange [31-34], The strength of the Brpnsted sites can be determined in different ways Hammett indicators, butylamine titration, IR spectroscopy using probe molecules, microcalorimetric or temperature programmed desorption methods [7,9,34-39], There is a direct correlation between acid strength and composition (Table 2),... 

Various techniques are used to obtain information on the active centers of catalysts, such as selective poisoning, measurement of the catalyst acidity and its strength, field electron and ion microscopy, infrared spectroscopy, fiash-filament desorption, differential isotopic method, etc. A temperature-programmed desorption method, which will be described and discussed in the present article, is in principle similar to the fiash-filament desorption method, reviewed recently by Ehrlich (1). It differs, however, from it in several respects. Modifications have been necessary in order to make the construction and operation of the apparatus easier and to adapt it to studies of materials other than metals, for example the conventional oxide catalysts. The conditions employed are much more similar to those ordinarily used in catalytic reactions than is the case with the fiash-filament method. An additional important feature of the modified technique is that it permits in some cases simultaneous study of a chemisorption process and the surface reaction which accompanies it. At the same time the modifications made have sacrificed some of the simplicity of the flash-filament method. For example, an obvious complication may arise from the porous structure of the conventional catalytic materials, in contrast to the relatively smooth surfaces of metal filaments. The potential presence of this and other complications requires extension of the relatively simple theoretical treatment of flash-filament desorption to more complicated cases. 

Yoshimoto R., Hara K.,Okumura K, Katada N., M. Niwa M., (2007) Analysis of Toluene Adsorption on Na-Form Zeolite with a Temperature-Programmed Desorption Method. /, Phys. Chem. Ill, 1474-1479. 

The temperature programmed desorption method originating from GC is most commonly used in determining the surface acidity of catalysts. 

The opposite of adsorption, desorption, represents the end of the catalytic cycle. It is also the basis of temperature-programmed desorption (TPD), an important method of studying the heats of adsorption and reactions on a surface, so that the activation... 

Desorption is important both because it represents the last step in a catalytic cycle and because it is also the basis of temperature-programmed desorption (TPD), a powerful tool used to investigate the adsorption, decomposition and reaction of species on surfaces. This method is also called thermal desorption spectroscopy (TDS), or sometimes temperature programmed reaction spectroscopy, TPRS (although strictly speaking the method has nothing to do with spectroscopy). 

Elementary steps in which a bond is broken form a particularly important class of reactions in catalysis. The essence of catalytic action is often that the catalyst activates a strong bond that cannot be broken in a direct reaction, but which is effectively weakened in the interaction with the surface, as we explained in Chapter 6. To monitor a dissociation reaction we need special techniques. Temperature-programmed desorption is an excellent tool for monitoring reactions in which products desorb. However, when the reaction products remain on the surface, one needs to employ different methods such as infrared spectroscopy or secondary-ion mass spectrometry (SIMS). 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

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). 

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... 

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]. 

Complementary techniques in catalytic chemistry involve temperature programmed (TP) methods where a reaction is investigated by subjecting the catalyst immersed in a reactant, to a temperature ramp. Rates of both reduction and oxidation can be studied. The extent of the catalytic reaction is then plotted as a function of temperature. In TP-desorption (TPD), the desorbed material is detected and plotted against temperature. 

The interpretation of the spectra of surface-adsorbed species, on singlecrystal surfaces in particular, is helped by complementary evidence derived from diffraction methods (LEED, PED) and from other nonvibrational spectroscopies (UPES, XPES, NEXAFS, SIMS, etc.). In particular, temperature-programmed desorption (TPD) is often measured in parallel with... 

With the aim of suppressing the concurrent condensation reactions a number of doped MgO catalysts was prepared and their catalytic activity investigated (Table 2). Surface basicity of these catalysts was measured by means of temperature programmed desorption of irreversibly adsorbed C02 (see methods section). Desorption peaks in the TPD experiments are considered to appear at higher temperatures as the basic sites on the surface... 

FIGURE 20 Amount of NH3 adsorbed on acid sites measured by temperature-programmed desorption. The three curves show different acid site distributions for the same catalyst composition prepared by different methods. 

Finally, new methods of analysis have recently been developed that may allow characterization of single atoms on surfaces such as atomic force microscopy.9 In certain cases, in situ experiments can be done such as the study of electrodes, enzymes, minerals and biomolecules. It has even been shown that one atom from a tip can be selectively placed on a desired surface.10 Such processes may one day be used to prepare catalysts that may enhance selectivity. Other methods that show promise as regards detection of surface catalytic intermediates are temperature programmed desorption techniques.11 Selective poisoning of some surface intermediates with monitoring via temperature programming methods may also allow the preparation of more selective catalysts. 

In contrast to the acetaldehyde decarbonylation, reactions with ethanol over Rh (111) did not lead to formation of methane but rather to an oxametallocycle via methyl hydrogen abstraction. These data suggest that ethanol formed over supported rhodium catalysts may not be due to hydrogenation of acetaldehyde. This study shows how surface science studies of model catalysts and surfaces can be used to extract information about reaction mechanisms since the nature of surface intermediates can often be identified by methods such as temperature programmed desorption and high resolution electron energy loss spectroscopy. 

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