On-line mass spectroscopy

Iwasita T, Vielstich W. 1986. On-line mass spectroscopy of volatile products during methanol oxidation at platinum in acid solutions. J Electroanal Chem 201 403-408. 

Investigation of Methanol Adsorbate by On-Line Mass Spectroscopy and... 

On-Line Mass Spectroscopy as a Tool for the Study of Exchange Reactions... 

On line mass spectroscopy is a very sensitive method capable of giving on-line responses in less than 0.2 s [11,12],... 

Fig. 1.2. Flow cell for adsorption measurements and on-line mass spectroscopy. The working electrode (w.e.) at the bottom of the cell is conneced directly to the MS c.e. = counter electrode, r.e. = reference electrode.

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). 

Using on-line mass spectroscopy [65] carbon dioxide and formic acid were demonstrated as soluble products of methanol oxidation. The former gives the most intense MS signal according to the fact that it is the main product. There are two main problems to detect formic acid as such. In the presence of carbon dioxide most of the m/e signals of HCOOH overlap with signals of the major product. Besides this, in the presence of methanol, formic acid reacts to form the methyl ester ... 

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

The use of conventional electrochemical methods to study the effect of metal adatoms on the electrochemical oxidation of an organic adsorbate may be in some cases of limited value. Very often, in the potential region of interest the current due to the oxidation of an organic residue is masked by faradaic or capacitive responses of the cocatalyst itself. The use of on-line mass spectroscopy overcomes this problem by allowing the observation of the mass signal-potential response for the C02 produced during the oxidation of the adsorbed organic residue. 

On-line mass spectroscopy has been proved to be a powerful tool to analyze volatile species frnm electrochemical reactions. ... 

Similar studies were carried out with benzoic acid on porous palladium electrodes [150]. The objective of this work was to investigate the adsorption processes and the reactivity of benzoic acid on different noble metals, in order to compare these results with those obtained for related aromatic compounds. On-line mass spectroscopy analysis of volatile products revealed that the adsorption of benzoic acid is irreversible at platinum while it is mainly reversible on palladium. Accordingly, different catalytic activity of platinum and palladium was found in the electrooxidation. 

Other electrochemical processes of organic compounds on Pb electrodes or electrodes with UPD Pb have been studied - formaldehyde [323], oxalic acid [386], trichloro- and trifluoroethane [387], 1-phenylethylamine [388], 3-hydroxychi-nuclidine [388], dichlorodifluoromethane [389], polychlorobenzenes [390], 1-propa-nol [391], pyrrole polymerization [392], and inorganic compounds - phosphine [388] and sulfate(IV) ions [393]. Simultaneous catalytic or inhibiting influence of organic solvents - acetonitrile, dimethyl-sulfoxide, and Pb + presence on electrooxidation of small organic molecules on Pt electrodes has been studied using on-line mass spectroscopy [394],... 

FIGURE 4.24 First cycle of CV (bottom) with scan rate of 0.5 mV s and simultaneous gas evolution as detected by on-line mass spectroscopy. (Reprinted from Electrochemistry Communications, 1, Hahn, M. et al, Gas evolution in activated carbon/propylene carbonate based double-layer capacitors, 925-930, Copyright 2005, with permission from Elsevier.)... 

An astonishing aspect of formic acid oxidation on palladium is the formation of CO2 at potentials of hydrogen oxidation and oxygen evolution. On-line mass spectroscopy analysis of volatile products reveals the production of CO2 not only in the double-layer region but also near 0.25 and 1.75 V [101]. The fact that the current and the CO mass signal during a potential scan do not follow the same pattern indicates that HCOOH oxidation proceeds through parallel mechanisms. 

Solis V, Iwasita T, Pavese A et al (1988) Investigation of formic acid oxidation on palladium in acidic solutions by on-line mass spectroscopy. J Electroanal Chem Interfacial Electrochem 255 155-162... 

Pavese A, Solis V (1991) Comparative investigation of formic acid and formaldehyde oxidation on palladium by a rotating ring-disc electrode and on-line mass spectroscopy in acidic solutions. J Electroanal Chem Interfacial Electrochem 301 117-127... 

Regarding the intrinsic kinetic effect of Sn, Xia investigated PiSn d (obtained by underpotential deposition) employing on-line mass spectroscopy in conjunction with electrochemical techniques [166]. The maximum electrocatalytic activity in 0.01 M HCOOH - 0.1 M HCIO4 was observed for a Sn d surface coverage of-0.70... 

Tops0e, N.-Y., Topspe, H., and Dumesic, J.A. Vanadia/titania catalysts for selective catalytic reduction (SCR) of nitric oxide by ammonia. I. Combined temperature programmed in situ FTIR and on-line mass spectroscopy studies. J. Catal. 1995,151, 226-240. 

By working at higher temperatures of 170 °C, Arico et al. reported a CO2 efficiency of ca. 95% in a DEFC with PtRu anode electrode [116]. The relative product distribution for the electrocatalytic oxidation of ethanol in a DEFC with Pt and Pt-Ru anode catalysts has been also studied by Wang et al. [117]. They used on-line mass spectroscopy in the temperature range of 150-190 °C. Acetaldehyde was reported as the main reaction product and CO2 was detected as a minor product in the studied catalysts. Remarkably, the formation of CO2 increased with the H20/ethanol mole ratio used as feed. Fujiwara and coworkers [118] measured the carbon dioxide and acetaldehyde selectivity in ethanol electrooxidation by DBMS on electrodeposited Pt and PtRu and reported that Ru addition improves slightly CO2 selectivity, however, carbon dioxide remains as a minority product. 

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