Thermal desorption, surface acidity

Formic acid decomposes generally on metal surfaces to carbon dioxide and hydrogen. A thermal desorption study on Cu(l 10) showed that the reaction... 

Thermal desorption techniques have gained great popularity owing to their comparatively easy operation. Various different mathematical analyses of the thermodesorption profiles have been proposed by different authors to provide kinetic information about the distribution of the surface acid strength [22]. A simple approach is based on the analysis of thermodesorption curves collected at different heating rates ( 3). The shift of the temperature of the maximum rate of desorption (Tmax) as a function of P can be exploited to derive activation parameters (i.e. TJ... 

Chemical methods of determining the nature and surface concentrations of active components have also been attempted Mo oxide by butene adsorption and, in reduced catalysts, by oxygen chemisorptionM0S2 and/or active site concentrations by pyridine adsorption and thiophen adsorption and temperature-programmed desorption of thiopen cf. p. 200), surface acidity by NH3 adsorption cf. p. 199). Sulphides in a Ni-Mo/ AI2O3 catalyst have been characterized by differential thermal analysis of the catalyst in oxygen. ... 

This chapter describes the results of the acidity characterization of a selected silica surface with VT-DRIFT spectroscopy. Examples of the capabilities of the method are demonstrated by the qualitative determination of the adsorption and thermal desorption characteristics of pyridine on amorphous, porous silica gel. Procedures for the determination of isothermal desorption rate constants and activation energy of desorption are presented and discussed as a means of assessing acid site strength. 

The formation of carbon surface oxides, phenols, quinones, lactones, and car-boxyhc acids upon the electrooxidation of carbon has been detected by physical methods such as infrared spectroscopy [262], ellipsometry [263], x-ray photoelectron spectroscopy [262,264,265], thermal desorption, and electrochemistry (see refs. [8, 96, 248, and 261] and references therein). Cyclic voltammograms of oxidized carbons exhibit increased charge in the potential interval from 0.4 to... 

Surface areas of some representative clays, namely Zn(ll), Fe(lll), Ti(IV), Zr(IV) and Ce(lll), were calculated from BET isotherms determined at 77 K. In all cases the values obtained fall within the range of 220-240 m /g, with the exception Zr-clay whose values are around 190 m2/g. The number of acid sites was determined by stepwise thermal desorption of ammonia at 373 K, then swept by a flow of dry nitrogen while the temperature was raised by steps of 50 K. The amount of ammonia evolved from the solid was monitored by conductimetry. X-ray diffraction patterns were recorded on a Phillips computer-driven X-ray diffractometer using CuKai radiation. 

In general, when results obtained from titration methods are compared with the catalytic activity, the correlation is not necessarily good. One of the reasons is that only a small fraction of the total acid sites measured by titration are active for a given catalytic reaction. The low temperature at which amine adsorption is measured with the use of color indicators favors adsorption on aU sites, including the weakest ones. Thermal desorption methods may discriminate between sites of different strength but are unable to distinguish between Lewis and Bronsted sites. When coupled with vibrational spectroscopy data, the TPD technique may, indeed, be adequate for analysis of surface acidity [136]. However, it is almost... 

The Rollgen s group [9] reports the thermal desorption of [M + Alkah]" quasi-molecular ions from a electrically heated metal smface (wires or ribbons) for so-dimn alkah salts of carboxylic acids and mixtures of alkali halides with a crown ether, glucose and adenosine. With benzo-15-crown-5 the desorption of [M + Na]" ions takes place even below the threshold temperature for thermionic emission of alkah ions. Alkali ion attachment has also been performed by thermal desorption of thermally labile analytes such as saccharides, pharmaceuticals, peptides, steroids and their mixtirres. Bombick and Alhson [8] describe a desorption/ioniza-tion method where samples are deposited directly on thermionic emission materials (lK20 lAl203 2Si02) and heated within the mass spectrometer s source. Ions representative of the sample are formed. The proposed mechanism involves the gas-phase addition of emitted potassium ions to neutrals desorbed from the surface. 

Much of the effort on the electrooxidation of ethanol has been devoted mainly to identifying the adsorbed intermediates on the electrode and elucidating the reaction mechanism by means of various techniques, as differential electrochemical mass spectrometry, in situ Fourier transform infrared spectroscopy, and electrochemical thermal desorption mass spectroscopy. The established major products include CO2, acetaldehyde, and acetic acid, and it has been reported that methane and ethane have also been detected. Surface-adsorbed CO is still identified as the leading intermediate in ethanol electrooxidation, as it is in the methanol electrooxidation. Other surface intermediates include various Ci and C2 compounds such as ethoxy and acetyl [102]. There is general agreement that ethanol electrooxidation proceeds via a complex multi-step mechanism, which involves a number of adsorbed intermediates and also leads to different byproducts for incomplete ethanol oxidation, as shown in Figure 1.22. 

A parallel set of changes occurs on thermal desorption of py. It is possible therefore to correlate the relative strengths of the Lewis acid surface sites, as determined from the thermal behavior of the py 8a vibrational modes, with the temperature-dependent changes observed in the -OH stretching region. The correspondences are shown in Table 7.2. 

When a strongly basic probe, like 2-phenylethylamine (PEA), in the decane solvent is used for titrating the NBO and NBP surfaces (Fig. 8.14), the information obtained on the two acid surfaces support the conclusions obtained from the more classical determinations of acidity using gas-solid calorimetric adsorption and thermal desorption approaches [110]. The calorimetric curve of acid-base titration in decane for NBP always lies above that of NBO, and the difference between the two curves increases as the titration progresses further. This seems confirming that the main difference between the NBO and NBP surfaces has to be ascribed to the medium and weak strength acid sites. 

Princeton University Press, Princeton, 222 pp Bowker M, Madix RJ (1981a) XPS, VPS, and thermal desorption studies of the reactions of formaldehyde and formic acid with the Cu(llO) surface. Surface Sci 102 542-565 Bowker M, Madix RJ (1981b) The adsorption and oxidation of acetic acid and acetaldehyde on Cu(llO). Applications Surface Sci 8 299-317 Bowker M, Houghton H, Waugh KC (1983) The interaction of acetaldehyde and acetic acid with the ZnO surface. J Catal 79 431-444 Brown MA (1951) The mechanism of thermal decarboxylation. Q Rev Chem Soc Lond 5 131-146... 

The surface acidity of the ceria-doped SBA-15 samples was studied by a temperature-programmed desorption of ammonia (NH3-TPD). The measiuements were performed with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD) and a mass quadmpole spectrometer (Thermostar, Balzers). Prior to the ammonia sorption, the samples ( 100 mg) were outgassed in a flow of O2 (5% in He) at 500°C for Ih, then, cooled to room temperature under He and saturated in a flow of NH3 (5% in He, 30 mL/min) for Ih. Subsequently, the catalysts were purged in a He flow at lOO C for Ih until a constant baseline level was reached. The ammonia desorption was carried out with a linear heating rate (10°C/min) up to 1050°C under a flow of He (30 mL/min). Cahbration of the TCD were carried out in order to evaluate the ammonia desorption peaks. 

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