Electrocatalyst mass spectrometry

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

Reaction or exchange with stable isotopic tracers and quantitative identification of all products by mass spectrometry provides indications for molecular interactions on the surface. Reactions can be studied at steady state or by following the transient distribution of isotopic products. Langer and co-workers (25,26) presented the first steady-state mechanistic analysis for the electrocatalytic hydrogenation of ethylene on Pt in deuterated electrolytes. Proton abstraction in electroorganic synthesis has also been verified using deuterated solvents (374, 375). On-line mass spectrometry permitted indirect identification of adsorbed radicals in benzene and propylene fuel cell reactions (755,795,194). Isotopic radiotracers provided some notion on adsorption isotherms (376, 377) and surface species on electrocatalysts (208, 378, 379). 

The products of the aqueous catalytic reduction, as determined by tandem bulk electrolysis and mass spectrometry, were mainly NH2OH, NH3, and N2O with very minor amounts of N2. Meyer attributed the N2O formation as evidence of a bimolecular dimerization between reduced nitroxyl adducts(Fe -NO ) as shown in Eqs. (4.9)-(4.10) and Figure 4.7. Su and coworkers utilized Co tetra(Af-methyl-4-pyridyl)porphyrin (TMPyP) as an electrocatalyst to examine the pH dependence for nitrite reduction in aqueous solution. At low pH, both hydroxy-lamine and ammonia are formed at high pH only hydroxylamine is detected with no observable N-N coupled products. 

Particle size effects of highly dispersed supported catalysts (Pt) on the hydrogen oxidation reaction were evaluated by the same authors using this technique (97). Moreover, electrocatalysts for oxidation of methanol were screened using a technique called scanning differential electrochemical mass spectrometry (98, 99). This method uses a capillary probe scanned over the array that allows the intake and detection by mass spectrometry of products generated locally on each electrode. 

Smolinka, T, Heinen, M., Chen, Y. X., Jusys, Z., Lehnert, W., and Behm, R. J. (2(X)5). COj reduction on Pt electrocatalysts and its impact on Hj oxidation in COj containing fuel cell feed gas-A combined in situ infrared spectroscopy, mass spectrometry and fuel ceU performance study. Electrochim. Acta 50(25-26) 5189-5199... 

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