Differential electrochemical mass spectroscopy DEMS

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). 

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]. 

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-... 

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... 

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]. 

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... 

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). 

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