Temperature-programmed desorption-mass methods

Interaction of nanomaterials with native cells is an important problem in modem life science. Recent progress in mass spectrometry provides a vital tool to study this problem. Advances in applications of mass spectrometry for investigating the interaction of nanoparticles with cell membranes and biomacromolecules are based on at least two methods. The first is matrix-assisted laser desorption ionization (MALDI),1 and the second is temperature-programmed desorption mass spectrometry (TPD MS), newly developed to study the interaction of nanoparticles with a cell surface.2 ... 

After the chemisorption step the oxygen was swept by flowing Ar, and temperature-programmed desorption -TPD method- was performed in the TGA by heating the sample at 15°C min from room temperature to 1000 °C. The desorbed gases (CO and CO2) were followed by means of a mass spectrometer (MS). The optimisation of the coupling system and the parameters used have been described elsewhere [6]. 

Our purpose in this text is principally to present temperature scanning methods. These generally involve multiple rampings as one seeks to delineate the kinetics of a system over a wide range of conditions. However, there is a well known and established technique for the semi-quantitative study of desorption phenomena, the Temperature Programmed Desorption (TPD) method. The equations developed below are also applicable to results that can be obtained using some of the versions of the traditional TPD apparata. In such cases they can be used to quantify the TPD results to yield the kinetics of the process and/or to check for extraneous influences that can result in anomalous results, effects such as mass diffusion, heat diffusion, or purely kinetic effects. 

Concerning the activation by ethylene monomer, Liu et al. reported an extensive investigation on reduction of the hexavalent chromate by means of XPS, temperature-programmed desorption-mass spectrometry (TPD-MS) methods and found that surface chromium species might exist in three oxidation states (1) surface chromate Cr(VI)Ox,surf species, (2) surface-stabihzed trivalent Cr(III) species, and (3) surface-stabilized Cr(II) species (Liu et al., 2002). Some short alkenes including propylene and butylene as well as the reduction by-product of formaldehyde were confirmed based on TPD-MS characterizations (Liu et al., 2003). 

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). 

Heats of adsorption can be experimentally measured by calorimetry, temperature programmed desorption (TPD), and adsorption isotherms taken at different temperatures. Calorimetry involves the direct measmement of temperature rise caused by the adsorption of a known amount of gas on to a well-characterized surface. TPD is the most coimnon method of determining the heats of adsorption. In this procedure, molecules are adsorbed on to a clean well-characterized substrate at a fixed temperature. The sample is then heated in a linear fashion while the pressure of the desorbing species is monitored with a mass spectrometer. The desorption rate E (t ) is given by... 

Dealuminated M-Y zeolites (Si/Al = 4.22 M NH4, Li, Na, K, Cs) were prepared using the dealumination method developed by Skeels and Breck and the conventional ion exchange technique. These materials were characterised by infrared spectroscopy (IR) with and without pyridine adsorption, temperature-programmed desorption (t.p.d.) of ammonia. X-ray difiracto-metry (XRD) and differential thermoanalysis (DTA). They were used for encapsulation of Mo(CO)5. Subsequent decarbonylation and ammonia decomposition was monitored by mass spectrometry (MS) as a function of temperature. The oxidation numbers of entrapped molybdenum as well as the ability for ammonia decomposition were correlated to the overall acidity of the materials. It was found that the oxidation number decreased with the overall acidity (density and/or strength of Bronsted and Lewis acidity). Reduced acidity facilitated ammonia decomposition. 

The Kiselev-Zhuravlev constant aoH value of 4.6 was obtained with a deuterium-exchange method that distinguished between surface and bulk OH and with a mass spectrometric thermal analysis (MTA) method in conjunction with temperature-programmed desorption (TPD) (66). 

A new method is described in [91] for the stndy of the nonnniformity of AC. It is based on the mass-spectrometry control of temperature-programmed desorption of prodncts, from the catalyst surface, at the initial stage of the gas-phase polymerisation of olefins. Polymerisation conditions have been selected in a way to favonr the formation of low MW products (up to 14 monomer links in a chain). The anthors report two definite maximums in the areas of 180-210 and 280-320 °C in the process of desorption from the Si02/TiCl4-Al(C2H5)2Cl surface. Therefore, the catalyst contains at least two types of AC with different activation energies of thermal destruction of Ti-C bonds. This publication also contains calculations of the activation energy distribution of thermal Ti-C bond destruction for various types of AC. 

Water and CO desorption from oxides, carbons, or polymers was studied at 300-900 K by the one-pass temperature-programmed desorption (TPD) with mass-spectrometry control (OPTPD-MS) method (chamber pressure 10" Torr, sample weight 5-7 mg, heating rate 2K/s, with a short distance [ 0.5cm] between sample and MS detector) with a MSC-3 ( Electron, Sumy, Ukraine) time-of-flight mass spectrometer (sensitivity 2.2x10 A/Torr, accelerating voltage 0.5 kV, pulse frequency 3 kHz). 

The characterization of active sites of solid catalysts includes the determination of active sites nature, the estimation of their density (or population, i.e. the number of active sites per unit of mass or per unit of surface area), their strength and strength distribution. Active sites can be acidic, basic and, in certain cases, amphoteric. All mentioned characteristics are very important for catalysts functionality therefore, many experimental techniques are invented and adapted for their investigation. Among others, mainly spectroscopic methods (like NMR, IR, XPS, XRF...), temperature-programmed desorption is particularly important because it can be useful in the characterization of all mentioned features. 

In the direct calorimetric determination (-id/f rta)r), the amount adsorbed (%) is calculated either from the variations of the gas pressure in a known volume (volumetric determination) or from variations of the mass of the catalyst sample in a static or continuous-flow apparatus (gravimetric determination). In a static adsorption system, the gas is brought into contact with the catalyst sample in successive doses, whereas in a dynamic apparatus the catalyst is swept by a continuous flow. Comparative calorimetric studies of the acidity of zeolites by measuring ammonia adsorption and desorption using static (calorimetry linked to volumetry) and temperature-programmed (DSC linked to TG) methods can be found in the literature [17],... 

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