the basics of in-situ electrochemcial ftir spectroscopy
TRANSCRIPT
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The basics of in-situ Electrochemcial FTIR spectroscopy
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•Molecular composition and symmetry
•Bond lengths and force constants
•Identity and orientation of adsorbed intermediates/poisons/products
•Mechanism
Actually-most important by far is simply the ability to identify adsorbed poisons and solution and adsorbed products and intermediates and so elucidate mechanism………
Why (in-situ) infra red spectroscopy
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Problems with the application of IR Spectroscopy in-situ to the study of the electrode/electrolyte
interface
1. All common solvents, and especially water absorb IR radiation very strongly. Water, 1640 = 20 mol-1 dm3 cm-1; to keep water absorption in this region down to 0.6, optical pathlength must be 5 m, and preferably
c. 2 m.
2. High sensitivity and stability are required to be able to ‘pick out’ the very weak absorptions of the near electrode species, (M – mM, and/or monolayer), from the intense background absorptions and noise.
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These two problems are addressed thus:
1 In external reflectance spectroscopy by trapping a thin layer ( a few microns) of solution between the reflective WE and the cell window. This minimises the solvent absorption which is then anulled using differential data collection methodology.
2 Early in-situ IR systems used lock-in detection techniques which suffer from a number of problems including: complicated hardware and data collection, long measurement times, complicated data collection protocols. The advent of Fourier Transform InfraRed (FTIR) spectrometers rendered such techniques obselete.
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A area above spectrometer, S spectrometer sample compartment. (a) IR beam in, (b) UV laser beam, (c) cooling/heating water in, (d) IR-transparent prism, (e) glass cell body, (f) water jacket, (g) Teflon cap and cell body, (h) thermo-couple leads, (i) contact wire, (j) electrolyte, (k) reflective working electrode.
The thin-layer configuration:External reflectance in-situ FTIRS
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The in-situ FTIR cell
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Data manipulation
The reference spectrum, (R0, 8 cm-1 resolution, 16 or 100 co-added and averaged scans, 3s or 16s, respectively, per scanset), was collected at 1100 mV. The potential of the electrode was stepped up from -200mV and a series of spectra, Rn, collected as as a function of potential, or the potential was stepped and held, and spectra, Rn, collected as a function of time.
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The spectra are presented as:
(Rn/R0) vs /cm-1
Peaks pointing up, to +(Rn/R0), arise from the gain of absorbing species in Rn with respect to R0, and peaks pointing down, to -(Rn/R0), to the loss of absorbing species.
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(1) The in-situ electrochemical FTIR cell; (2) BioRad FTS 6000 Spectrometer; [(3) 325nm
Laser]; (4) Heated block controller; (5) Control PC; (6) Heating/ cooling unit; (7)
Cooling/heating tubes to cell.