Differential Electrochemical DEMS

Recently the proposed Oz evolution mechanism was supported by the results of a DEMS (Differential Electrochemical Mass Spectrometry) study performed by Wohlfahrt-Mehrens and Heitbaum [71] on Ru electrodes. Using this mass spectroscopic technique and lsO labeling for the determination of reaction products during 02 evolution, it could be verified that the oxygen of the oxide formed on Ru takes part in the 02 evolution process. The same observation was made for Ru02 electrodes when using labeled H2lsO. 

Mass spectrometry (MS) is an extremely powerful method of chemical analysis and the possibility of measuring electrochemical reaction products via MS was first suggested by Grambow and Bruckenstein (1977). The technique of differential electrochemical mass spectroscopy (DEMS) was later perfected and pioneered by Wolter and Heitbaum (1984). 

Figure 2.115 Experimental system for differential electrochemical mass spectroscopic (DEMS) measurements with automatic data acquisition. TP = turbo pump, IC = inlet chamber, A = analysis chamber, S — screw mechanism to control aperture between both chambers. After Iwasita and...

A long disputed issue of the nature of strongly bound species in this reaction has been recently revived with the vibrational spectroscopy studies of Bewick et al. (30) using EMIRS technique and of Kunimatsu and Kita (31) using polarization modulation IR-reflection-absorption technique. These data indicated the only CO is a strongly bound intermediate. Heitbaum et al. (32) on the other hand advocate COH, and most recently HCO (33), as the poisoning species on the basis of differential electrochemical mass spectroscopy (DEMS). 

A series of pubKcations was devoted to the electrocatalytic reduction of nitrate by the Eindhoven group [50-54]. On the basis of these works, a comparative study was performed to determine the reactivity of nitrate ions in 0.1 mol dm concentration on eight different polycrystaUine electrodes (platinum, palladium, rhodium, ruthenium, iridium, copper, silver, and gold) in acidic solution using cyclic voltammetry, chronoamperometry, and differential electrochemical mass spectroscopy (DEMS) [50]. 

The mechanisms of the oxidation of solvents such as THF and PC were studied by several groups, utilizing FTIR and XPS spectroscopy [107-109] and on-line mass spectrometry (DEMS-differential, electrochemical mass spectroscopy [110-112]). For example, using ex situ FTIR spectroscopy, Lacaze et al. [46] showed that THF in FiC104 solutions are polymerized on electrodes biased to high potentials. The proposed mechanism involves oxidation of C104 as an initial step, as shown in Scheme 7 [46,102], ESR measurements also support such a mechanism. However, there are also suggestions for possible direct oxidation... 

Although the data of Herrero et al. [34] were interpreted in terms of a parallel reaction scheme model, such a model is certainly not established by their treatment, and Vielstich and Xia [36] have criticised such a model on the basis of their Differential Electrochemical Mass Spectroscopy (DEMS) data [37]. At least below a potential of 420 mV, the very sensitive DEMS technique detects no C02 evolved from a polycrystalline particulate Pt electrode surface on chemisorption of methanol indeed, the only product detected other than adsorbed CO, in very small yield (one or two orders of magnitude smaller), is methyl formate from the intermediate oxidation product HCOOH. This is graphically illustrated in Fig. 18.2 in which the clean electrode is maintained at 50 mV, a 0.2M methanol/O.lM HCIO4 electrolyte introduced, and the electrode swept at 10 mV s I anod-... 

We will first describe briefly the main experimental techniques coupled with electrochemical methods Infrared Reflectance Spectroscopy (IRS), Electrochemical Quartz Crystal Microbalance (EQCM), Differential Electrochemical Mass Spectrometry (DEMS), Chemical Radiotracers and High Performance Liquid Chromatography (HPLC). 

A quantitative study of the electro-oxidation of ethanol on carbon-supported Pt, PtRu and PtsSn catalysts was made by Behm and co-woikers " nsing combined voltarmnetric and on-line Differential Electrochemical Mass Spectrometry measnrements (DEMS). 

Whether the oxidative or single nonoxidative desorption of the carbon monoxide occurs from the polycrystalline nickel surface was studied by differential electrochemical mass spectrometry (DEMS) [172] ... 

As a result, Wolter and Heitbaum obtained a mass signal proportional to the rate of the electrochemical reaction, i.e. to the current flowing through the porous electrode. Because of the applied pumping technique and because of the proportionality between electrochemical current (i.e. derivative of consumed charge) and mass signal, the method was called differential electrochemical mass spectrometry (DEMS)." ... 

M. FujUiara, T. Noguchi, A novel differential electrochemical mass spectrometer (DEMS) with a stationary gas-permeable electrode in a rotational flow produced by a rotating rod. J. Electroanal. Chem. 347, 457-463 (1993)... 

P. Bogdanoff, N. Alonso-Vante, on-line determination via differential electrochemical mass spectroscopy (DEMS) of chemical products formed in photoelectrocatalytical systems. Berichte der Bunsengellschaft fur physikalische Chem. 97, 940-943 (1993)... 

Itmumerable mechanistic studies of alcohol oxidation on Pt-based electrocatalysts in acidic media have been published over the last few years. Methanol, " ethanol ° and ethylene glycol have been the most studied substrates and their oxidation paths on Pt or Pt alloys have been substantiated using a variety of in situ, extra situ and operando techniques as well as quantum mechanical calculations. The experimental techniques include reflection IR spectroscopy (IR), surface enhanced IR asbsorption spectroscopy (SEIRAS), " attemrated total reflection-IR absorption spectroscopy (ATR-IRAS), differential electrochemical mass spectroscopy (DEMS), single potential alteration IR spectroscopy... 

In one of the most relevant papers in this field, Dima et al. [13] studied the electrocatalytic behavior of different polycrystalline metals such as Ru, Rh, Ir, Pd, Pt, Cu, Ag and Au for nitrate (100 mM) reduction in 0.5 M H2SO4. On the basis of the peak current density related to nitrate reduction on cyclic voltammograms, the activities of each electrode were compared. It was determined that rhodium is the most active catalyst among the noble metals for the reduction of nitrate, with the activity decreasing in the order Rh, Ru, Ir, Pt, Pd and Cu, Ag, Au for transition metals. The high electrocatalytic performance of Rh for nitrate reduction was also observed by Brylev et al. [19]. By using Differential Electrochemical Mass Spectrometry (DEMS), a reduction mechanism for nitrate reduction has been determined for transition metals (Fig. 2). 

Electrochemical measurements can also be coupled with mass spectrometry. Figure 2.17 shows a schematic diagram of the apparatus for differential electrochemical mass spectrometry (DEMS). Here the chamber connected directly to the electrochemical cell and the mass spectrometer (MS) is pumped differentially by turbo pumps PA and PB. Electrolysis products are passed into the ionization chamber (i), analyzed in the quadrapole mass filter (ii), and detected with either a Faraday cup (iii) or electron multiplier (iv). Such DEMS measurements can be used in situ to identily electrolysis products. This may lead to an understanding of the electron-transfer reaction mechanism and optimization of the reaction process. 

Strategies for the development of novel catalytic materials and the design of highly active catalysts for DLFC applications largely depend on a detailed understanding of the reaction mechanism and, in particular, of the rate-limiting step(s) during the electrooxidation under continuous reaction conditions. The most commonly used technique in the electrochemical studies of fuel cell reaction mechanisms has been voltammetry, chronoamperometry (chronopotentiometry), in situ spectroscopic techniques, e.g., electrochemically modulated infrared spectroscopy (EMIRS) and infrared reflection-absorption spectroscopy (IRRAS), differential electrochemical mass spectroscopy (DEMS) and ex-situ techniques, e.g.. X-ray photoelectron spectroscopy (XPS) [92]. 

Lanz and Novak" studied gas evolution at thick graphite electrodes in y-butyrolactone EC DMC electrolyte by Differential Electrochemical Mass Spectrometry (DEMS). TIMREX SPG 6, SPG 15 and SPG 44 carbons were tested. They found that SEI formation on these thick electrodes was not yet complete after the first charge/discharge cycle. The amount of ethylene and hydrogen gas evolved decreases with increasing percentages of GBL in an EC/DMC electrolyte, indicating that the SEI layer is built up from GBL rather than from EC decomposition products. 

Abstract In this chapter, we present new insights in direct alcohol fuel cell-related anode electrocatalysis based on quantitative differential electrochemical mass spectrometry (DEMS) studies. First, we review the history and development of the DEMS technique, as well as the calibration method for quantification. We then discuss some contributions of quantitative DEMS to the study of the mechanism of methanol electrooxidation on Pt and PtRu model catalysts. We also discuss quantitative DEMS studies of the mechanism of dissociative adsorption and electrooxidation of ethanol and acetaldehyde at Pt, Pt3Sn, PtRu, and PtRh nanoparticle catalysts. Finally, the mechanism of dissociative adsorption and electrooxidation of ethylene glycol and its oxidative derivatives on carbon-supported Pt, Pt3Sn, and PtRu nanoparticle catalysts are discussed, based on quantitative DEMS results. 

KMC simulations have also been combined with differential electrochemical mass spectrometry (DEMS) to give new insights into the mechanisms and kinetics of adsorbed CO electro-oxidation on a platinum electrode. On the basis of DEMS experimental observations, the authors proposed a... 

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