SNIFTIRS approach

The SNIFTIRS approach. The acronym SNIFTIRS means Subtractively Normalized Interfacial Fourier Transform Infrared Spectroscopy. The basic concept of this method involves the fact that the raw data obtained directly from the Fourier Transform process contain components which are undesirable. Firstly, there is material in the solution which may have affected the spectrum. Secondly, unwanted information on certain material on the electrode (adsorbed water, for example) is best eliminated. 

The SNIFTIRS approach is clearly related in some ways to the EMIRS technique, in that it does involve potential modulation, (but not lock-in detection), albeit at a much lower frequency ca. 0.01-to 0.02 Hz [83, 85,137], than the ca. 10 Hz typically employed in EMIRS experiments. Consequently, SNIFTIRS is also restricted to electrochemical systems that are essentially reversible over the timescale of the poterttial modulation, but has proved extremely sensitive, and is generally reported as being surface specific, only detecting potential-induced changes in adsorbed species [119, 138]. One specific exception to this generalization is where 1 and 2 are chosen such that the species of interest are fiiUy adsorbed at one potential, and fiiUy desorbed at the other potential [16, 85]. This led Weaver and Corrigan [139] to coin the general acronym PD IRS, for those approaches that involve multiple reference spectra as well as multiple sample spectra. Thus, Fig. 11(d) is an example of the PDIRS approach, as is Fig. 11(e), in which the sample potential is sequentially decreased... 

HER as a function of overpotential clearly fit models [5, 150] in which the coverage of the intermediate is low at potentials above H2 evolution, and increases only slowly as the potential is moved into the hydrogen evolution region. This led to a search for the intermediate, the identity of which was first directly addressed using in situ FTIR again by Bewick [151] and then more recently, by Kitamura, Tokuda and coworkers [152]. Both groups started from a model in which the HER intermediate was beheved to be hydrogen atoms adsorbed on an on-top site on Pt, and employed the SNIFTIRS approach, but different data collection times, and obtained somewhat different results. 

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). 

Infrared methods have been used to study adsorbed species (reactants, intermediates, products), to examine species produced in the thin layer of solution between electrode and window, and to probe the electrical double layer. These approaches have been especially useful with species that have a high infrared absorption coefficients, like CO and CN . In favorable cases, information about the orientation of an adsorbed molecule and the potential dependence of adsorption can be obtained. For example, the SNIFTIRS spectrum obtained with a 0.5 mM aqueous solution of p-difluorobenzene in 0.1 M HCIO4 at a Pt electrode is shown in Figure 17.2.6 (62). The spectrum results from both dissolved (positive AR/R-values) and surface adsorbed (negative AR/R-values) p-difluorobenzene. 

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