In situ FTIR studies of 4-cyanopyridine adsorption at the Au(111) electrode

Abstract

Abstract:In situ electromodulated reflectance Fourier transform infrared (FTIR) spectroscopy has been employed to study the adsorption of 4-cyanopyridine (4-CNPy) at an Au(1 I I) electrode surface. The vibrational spectra have been used to study (i) the dependence of the band intensity on the surface coverage, (ii) the character of surface coordination, and (iii) the stability of adsorbed 4-CNPy molecules. It has been observed that the vibrational bands in the spectra acquired in the electroreflection experiment are significantly broader than the corresponding spectra acquired in a transmission cell. Some weaker bands seen in the spectra recorded in transmission were not observed in the electroreflectance experiment. The electroreflectance spectra were dominated by the two ring deformation bands observed at 1416 cm-' and 1554 cm-I. The intensities of these bands correlated well with the surface concentrations of 4-CNPy molecules determined from independent electrochemical studies. The integrated absorption intensities of the bands recorded in the electroreflection experiments were larger by a factor of five than the absorption intensities measured in the transmission cell. This indicates that the electric field of the photon acting on a molecule, present in front of the gold electrode, is significantly enhanced by reflection from the electrode. The infrared experiments suggest that at positive potentials the 4-CNPy molecules are coordinated to the metal surface through the nitrogen atom of the aromatic ring. The 4-CNPy molecules are oxidized at the Au electrode at potentials higher than 0.6 V (SCE) and are reduced to form (4-CNPy)-' ion at potentials lower than -1 .I V (SCE).

Study of the Adsorption of 4-Cyanopyridine on Au(111) Using Sum Frequency Generation Nonlinear Spectroscopy

“…The (b)-type peak at 905 cm ± ±1 corresponds to out of plane vibration modes and can be seen only when the molecule is in flat configuration on the surface, which is the case at intermediate potentials. This flat configuration is suggested by many authors [3,5,6]. Moreover, the assignment of the modes is supported by ab initio calculations [13].…”

“…A couple of years ago, it was already mentioned that 4-CP undergoes a change of its adsorption geometry when the potential is modified [2]. Since then, various studies have been performed on this molecule adsorbed either on gold [3] or silver [4 to 6] electrodes. The second reason for taking interest in 4-CP is that we have studied the adsorption of cyanide [7] and pyridine [8] on noble metal electrodes and have developed efficient tools to probe alternately both cyanide and pyridine groups.…”

“…Optical techniques are useful to go further in the understanding of these processes and to assign properly the different peaks to molecular processes occurring on the metal electrode. For example, the adsorption of 4-cyanopyridine has been extensively investigated by Furukawa et al [4], Rubim [6] and Shi et al [5] using Surface Enhanced Raman Spectroscopy and by Chen et al [3], with in situ electromodulated reflectance Fourier Transform Infrared Spectroscopy (FTIR).…”

SFG and DFG investigation of Au(111), Au(210), polycrystalline Au, Au–Cu and Au–Ag–Cu electrodes in contact with aqueous solutions containing KCN and 4-cyanopyridine

“…The electrochemical behaviour of 4-cyanopyridine (4CP) has been intensively studied in a close-knit series of papers, which have addressed the issues of adsorption, reorientation and reactivity [1][2][3][4][5][6][7][8][9][10][11][12][13]. The pioneering voltammetric study of 4CP at Au(111) [1] essentially disclosed the potential range of adsorption and highlighted a potentialdependent reorientation process.…”

“…The electrodic study of 4CP has been furthered by spectroelectrochemical methods. FTIR studies of 4CP on Au(111) [2,3] have disclosed: (i) the formation of 4-cyanopyridinium cations at cathodic potentials exceeding -1 V SCE ; (ii) the hydrolysation of 4CP to isonicotinamide at potentials more anodic than 50 mV SCE ; (iii) the existence of an oxidation reaction at anodic potentials exceeding 0.6 V SCE . Studies on Au(111) and Au(100) [4,5] have shown that the reactivity of 4CP can be rationalised in terms of successive reactions which are driven by a sequence of polarisation steps: 4CP adsorbed vertically through the heteroaromatic N can undergo protonation at high cathodic potentials, if the electrode with an adsorbed protonated species is polarised to more anodic values, the adsorbate can hydrolyse and eventually transform into isonicotinamide via a tautomeric equilibrium.…”