Differential electrochemical mass spectroscopy

At present, most workers hold a more realistic view of the promises and difficulties of work in electrocatalysis. Starting in the 1980s, new lines of research into the state of catalyst surfaces and into the adsorption of reactants and foreign species on these surfaces have been developed. Techniques have been developed that can be used for studies at the atomic and molecular level. These techniques include the tunneling microscope, versions of Fourier transform infrared spectroscopy and of photoelectron spectroscopy, differential electrochemical mass spectroscopy, and others. The broad application of these techniques has considerably improved our understanding of the mechanism of catalytic effects in electrochemical reactions. 

K. and Enyo, M. (1989) Surface species produced on Pt electrodes during HCHO oxidation in sulfuric add solution as studied by infrared reflection-absorption spectroscopy (IRRAS) and differential electrochemical mass spectroscopy (OEMS)./. Electroanal. Chem., 258, 219-225. 

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). 

Study of fhe mechanism of MeOH oxidation over Pt and PtRu surfaces has recenfly been given new insights using a combination of experimental and theoretical approaches. The use of electrochemically linked mass spectroscopy techniques (e.g., differential electrochemical mass spectroscopy— DBMS) has allowed the quantification of the MeOH oxidation reaction in terms of comparing CO2 yields with electrons passed. In addition, detection and quantification of reaction intermediates has also been demonstrated. In addition, use of theorefical fechniques such as DFT has allowed calculation of adsorbafe energies, probing reaction pathways, and activation of H2O to provide active OH species. 

Bagdonoff P, Friebe P, Alonso-Vante N (1998) A new inlet system for differential electrochemical mass spectroscopy applied to the photocorrosion of p-InP (111) single crystals. J Electrochem Soc 145 576-582... 

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]. 

Similar studies on palladium/copper electrodes was carried out using differential electrochemical mass spectroscopy (DBMS), rotating ring-disk electrodes and EQCM [144]. In acidic electrolytes, the activity increased linearly with Cu coverage in alkaline electrolytes, a different dependence on coverage was observed. 

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-... 

SDEMS Scanning differential electrochemical mass spectroscopy... 

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... 

Several research groups have used differential electrochemical mass spectroscopy (DBMS) to monitor product conversion during formic acid electrooxidation [2, 21, 37, 86-88]. In Fig. 3.2, the origins of the CO2 product formation pathway is investigated by using isotopically labeled formic acid [37]. 

Pt-Sn/C catalysts with 10-20 at. % of Sn exhibited best activity at low potentials. Results from Jiang et al. [68] showed that the ft-SnOx/C catalyst with 30 at.% of Sn was the most active among four different Pt/Sn ratios. An integrated surface science and electrochemistry study of the SnOx/Pt(lll) model catalysts indicate a volcano dependence of the EOR activity on the surface composition, with the maximum at the SnOx coverage of 37 % [69]. Despite the improved overall EOR activity of optimized Pt- n system, on-line differential electrochemical mass spectroscopy (OEMS) studies have shown that acetic acid and acetaldehyde represent the dominant products with CO2 formation contributing only 1-3 % [68]. 

Wolter O, Heitbaum J (1984) Differential electrochemical mass spectroscopy (DBMS) - a new method for the study of electrode processes. Ber Bunsenges Phys Chem 88 2-6... 

BaltruschatH,SchmiemannU(1993)TheadsOTptionof unsaturated organic species at single crystal electrodes studied by Differential Electrochemical Mass Spectroscopy. Ber Bunsenges Phys Chem 97(3) 452-460... 

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]. 

---