Electrochemical thermal desorption

Electrochemical Thermal Desorption Mass Spectroscopy (ECTDMS). 132... 

Fig. 1.3. Experimental setup for electrochemical thermal desorption mass spectroscopy (ECTDMS). C = electrochemical cell, W = working electrode, El = electrolyte inlet, EO = electrolyte outlet, EH = electrode holder, V = valve, TP = turbo pump, VC = vacuum chamber, L = light source, W = window, P = protective jacket, A = aperture to analysis chamber, GI = grid ion source, S = SEM detector.

Different experimental approaches were applied in the past [6, 45] and in recent years [23, 46] to study the nature of the organic residue. But the results or their interpretation have been contradictory. Even at present, the application of modem analytical techniques and optimized electrochemical instruments have led to different results and all three particles given above, namely HCO, COH and CO, have been recently discussed as possible methanol intermediates [14,15,23,46,47]. We shall present here the results of recent investigations on the electrochemical oxidation of methanol by application of electrochemical thermal desorption mass spectroscopy (ECTDMS) on-line mass spectroscopy, and Fourier Transform IR-reflection-absorption spectroscopy (SNIFTIRS). 

Iwasita et used the similar techniques and confirmed that the adsorbate contains a proton atom but concluded that the adsorbate is C-OHad ie same group executed electrochemical thermal desorption mass spectroscopy, in which the methanol absorbing electrode was washed by the supporting electrolyte, transferred to the UHV environment, heated to desorb the adsorbates to analyze them by mass spectroscopy. They found hydrogen molecules in the desorbed gas as well as CO and the ratio of hydrogen to CO decreased as the concentration of methanol increased. 

ECTDMS Electrochemical thermal desorption mass spectrometry... 

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. 

Figure 4.43. Thermal desorption spectra after gaseous oxygen adsorption on a Pt film deposited on YSZ at 673 K and an 02 pressure of 4x 10"6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (I=+15 pA) for various time periods.29-30 Reprinted from ref. 30 with permission from Academic Press.

Figure 4.45. Thermal desorption spectra (bottom) and corresponding catalyst potential variation (top) after electrochemical O2 supply to Ag/YSZ at 260-320°C at various initial potentials Uwr Each curve corresponds to different adsorption temperature and current, thus different values of Uwr, in order to achieve nearly constant initial oxygen coverage.31 Reprinted with permission from Academic Press.

Figure 5.3. Oxygen thermal desorption spectra after electrochemical O2 supply to Pt/YSZ at 673 K (I = +12 pA for 1800 s) followed by isothermal desorption at the same temperature at various times as indicated on each curve.4,7 Reprinted from ref. 7 with permission from Academic Press.

The good agreement between electrochemical and UHV data, documented in Figure 4, is a very important result, because it proves for the first time that the microscopic information which one obtains with surface science techniques in the simulation studies is indeed very relevant to interfacial electrochemistry. As an example of such microscopic information, Figure 5 shows a structural model of the inner layer for bromide specific adsorption at a halide coverage of 0.25 on Ag 110 which has been deduced from thermal desorption and low energy electron diffraction measurements /12/. Qualitatively similar models have been obtained for H2O / Br / Cu( 110) /18/and also for H2O/CI /Ag 110. ... 

The region of the cyclic voltammogram, corresponding to anodic removal of Hathermal desorption spectra of platinum catalysts. However, unlikely the thermal desorption spectra, the cyclic-voltammetric profiles for H chemisorbed on Pt are usually free of kinetic effects. In addition, the electrochemical techniques offer the possibility of cleaning eventual impurities from the platinum surface through a combined anodic oxidation-cathodic reduction pretreatment. Comparative gas-phase and electrochemical measurements, performed for dispersed platinum catalysts, have previously demonstrated similar hydrogen and carbon monoxide chemisorption stoichiometries at both the liquid and gas-phase interfaces (14). 

Electrochemists often use mass spectrometry as a tool for the identification of electrolysis products ex situ, but the approach is conventional and requires no amplification here. Mass spectrometry (MS) can also be used to sample volatile species produced at a porous electrode connected directly to a mass spectrometer. Alternatively, the solution in the electrochemical cell can be introduced into the mass spectrometer inlet by a thermospray or electrospray approach. Moreover, electrodes can be removed from the cell and introduced into a UHV chamber and their surface examined by MS using conventional desorption techniques, such as laser or thermal desorption, or bombardment of the surface with an ion beam (secondary-ion mass spectrometry or SIMS) to produce the ions that are mass-analyzed. 

Various mass spectroscopies are applicable ex situ in order to obtain molecular information. Secondary ion mass spectroscopy (SIMS) can be used [124, 125]. Thermal desorption mass spectroscopy is a viable alternative as a less intrusive and more surface sensitive tool [126, 127]. So far, the latter method has been applied exclusively to adsorbed hydrogen and carbon monoxide formed in an electrochemical reaction of organic CHO-compounds. 

The stability of a monolayer can be dramatically increased by cementing the film with lateral chemical bonds. One example includes incorporation in the monolayers of the amide group, capable of intermolecular H-bonding with the adjacent molecules . The monolayers of amide thiol RNHCOCH2SH are 10 times more stable towards UV oxidation than the layers of the corresponding alkanethiol lacking an amide group. Enhanced electrochemical stabihty and suppressed thermal desorption of these monolayers was also reported. 

A range of methods including electrochemical techniques, autoradiography, and thermal desorption have been used to study hydrogen trapping. Mechanical relaxation methods have also found wide use and are described in several reviews [87, 88]. 

There are a number of techniques that can be used in the field. These include electrochemical sensors for gases such as O2 and SO2 and diffusive samplers containing immobilized reagents that produce a visible color change with visual detection on exposure to a specific chemical. Passive diffusion tubes can also be used for analyte preconcentration. Subsequent laboratory analysis is usually undertaken by thermal desorption coupled with GC. This approach is particularly useful for trace organic compounds such as polyaromatic hydrocarbons (PAHs) and VOCs. Spec-trometric techniques such as Fourier transform infrared (FTIR) spectrometry, correlation spectrometry, and the laser based LIDAR (light detection... 

All in all, the simulations reviewed provide a consistent picture of the surface dynamics. With increasing field, the surface becomes more mobile, which entails larger step fluctuations and a decrease of the step stiffness. At the same time, the island shapes become more rounded and the coarsening faster. The same effects occur with increasing temperature. It has often been observed that in certain electrochemical experiments, the potential plays a similar role to that of the temperature in UHV. Thus, electrochemical desorption spectra obtained by a potential sweep bear a certain similarity to thermal desorption spectra in UHV. 

Smith GJ (2009) Coupled electrokinetic-thermal desorption. In Reddy KR, Cameselle C (eds) Electrochemical remediation technologies for polluted soils, sediments and groundwater. Wiley, Hoboken... 

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