Desorbed product detection

Controlled electrochemical experiments are designed to probe select aspects of the formic acid electrooxidation reaction as a function of material selection and/or experimental conditions. Unfortunately, the selected experimental technique employed imposes deviations from a complex three-dimensional catalyst layer used in an operational DFAFC and thus results in inconsistencies between techniques. Assuming the current-potential relationship is always directly correlated to Faraday s law for charge and CO2 production, the assessment techniques can be broken down into three general categories (1) indirect correlation, (2) desorbed product detection, and (3) direct catalyst surface analysis. 

Although TPD is a versatile and useful technique widely available within the surface-science community, it does have some limitations. For one, because the experiments are carried out under vacuum, they can only probe irreversible reactions no readsorption of the desorbing products is possible. In addition, as the temperature is ramped during detection, the surface temperature and the reaction rates become coupled in a way difficult to separate or control. Of particular importance here is the fact that as the reactions proceed and the products desorb, the surface coverages decrease, so the rates at higher temperatures correspond to the new lower surface concentrations. In order to overcome this problem, isothermal kinetic experiments have been carried out using molecular beams [22,23],... 

In principle, the neutral desorbed products of dissociation can be detected and mass analyzed, if ionized prior to their introduction into the mass spectrometer. However, such experiments are difficult due to low ejfective ionization efficiencies for desorbed neutrals. Nevertheless, a number of systems have been studied in the groups of Wurm et al. [45], Kimmel et al. [46,47], and Harries et al. [48], for example. In our laboratory, studies of neutral particle desorption have been concentrated on self-assembled monolayer targets at room temperature [27,28]. Under certain circumstances, neutrals desorbed in electronically excited metastable states of sufficient energy can be detected by their de-excitation at the surface of a large-area microchannel plate/detector assembly [49]. Separation of the BSD signal of metastables from UV luminescence can be effected by time of flight analysis [49] however, when the photon signal is small relative to the metastable yield, such discrimination is unnecessary and only the total yield of neutral particles (NP) needs to be measured. 

First we want to know the interaction with the bare substrate does the molecule become physisorbed, chemisorbed, dissociate or react with the surface atoms, what is the interaction time To answer these questions we have to prepare a clean MgO surface, which is obtained by cleavage under UHV (the cleanliness and the perfect surface order are checked by diffraction of He atoms and AES [23]). Then, a pulsed beam of NO is directed to the MgO clean surface and the (possible) desorbed products (NO, N2, N20, N02, 02) are simultaneously detected by the mass spectrometer. 

Adsorbed acetaldehyde on Pt(lll) at small coverage decomposed during heating in TPD [61]. Desorbed products were CH CHO (from the recombination of co-adsorbed H adatoms and acetyl, t) (C)-CH3CO, groups), CO, and CH, and AES detected carbon (0 0.1) left on the surface following TPD. 

Reaction temperature, ca 310 K sample, 0.2 g CH4, 200 pmol irradiation time, 3 h. H2 was analyzed by GC TCD (detection limit, 0.04 pmol), hydrocarbons were analyzed by GC FID (experimental error, < 4%). Based on the initial amount of CH4. Mainly CjHg. The products were obtained by the desorption procedure at 573 K for 15 minutes, mainly C2H4. Calculated from the total yield of gaseous products and thermally desorbed products. Radiation time was 12 h. Irradiation time was 24 h. The sample was pretreated at 773 K before photoreaction, tr. = trace, n.d. = not detectable. 

An advantage of OPTPD-TOFMS is detection of desorbed products in their one pass from a sample to a mass-spectrometer detector through the ionization region with no possibility for other particles (desorbed but not detected at once) to return here. A regime of desorption and observation was realized in the installation of a chamber of black type ( black is due to specific conditions allowing to detect practically only particles desorbed from sample in the one-pass regime, that is, other processes do not distort the signal) [25]. 

Some examples of carbene dimer formation resulting from diazoalkane decomposition on transition-metal surfaces have been reported. Diazomethane is decomposed to give ethylene and N2 upon passage over a C0O/M0O3 catalyst as well as on Ni, Pd, Fe, Co, Ru and Cu surfaces 367). Similarly, 2-diazopropane is readily decomposed on Raney nickel 368). At room temperature, propene and N2 were the only detectable products, but above 50 °C, the carbene dimer 2,3-dimethyl-2-butene started to appear which reached its maximum yield at 100 °C, where approximately 40 % of the carbene fragments dimerized. It is assumed 367,368), that surface carbenes are formed as intermediates from both diazomethane and 2-diazopropane which either dimerize or desorb by migration of a P-hydrogren, if available (Scheme 40). 

Fig. 1.97.1. Schema of the Coulometer MeBzelle DL 36 for measurement of residual moisture content (RM) after Karl Fischer. In the titration cell (1) iodine is electrolytically produced (3) from an iodine-containing analyt (2). Water in the titration cell reacts with the iodine. When the water is used up, a small excess of iodine is produced, which is detected by special electrodes, which leads to iodine production being stopped. The amount of water in the cell can be calculated from the reading of the coulometer, and the amount of electrical charge needed. The solids are introduced into the cell either by a lock, or the water is desorbed in an oven and carried by a gas stream into the cell. 10 pg in a sample can be detected with an accuracy of reading of 0.1 pg (KF Coulometer DL36, Mettler-Toledo AG, CH-8603 Schwerzenbach, Switzerland).

These experiments also show the value of NEXAFS as a technique for following the kinetics of surface processes. We have shown that experiments can be tailored so a specific reaction can be studied, even if gas evolution is not involved. This represents an advantage over thermal desorption experiments, where several steps may be required in order to desorb the products to be detected. Another advantage of NEXAFS is that rates are measured isothermally, so the kinetic parameters can be determined with accuracy. Finally, NEXAFS is not a destructive technique, so we need not to worry about modifying the surface compounds while probing the system, as would be the case with other techniques such as Auger electron spectroscopy. 

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