Katsuaki Shimazu

In situ measurements of water adsorption on a platinum electrode by an electrochemical quartz crystal microbalance

Adsorption of water at solid electrode-aqueous electrolyte solution interfaces is well known. According to the double-layer model proposed by Bockris and Reddy [l], the first layer closest to the electrode surface is composed only of water molecules when no specific adsorption takes place. In the case where a specifically adsorptive species is introduced into the solution, this species replaces the first-layer water molecules. Some of the adsorbed water molecules remain in the first layer and can interact laterally with specifically adsorbed species. In order to understand the adsorption and electron transfer processes at the molecular level, therefore, it is essential to examine the details of water adsorption, particularly to quantify the number of adsorbed water molecules and to determine the orientation and structure of the adsorbed water layer. Besides electrochemical measurements, adsorption of water at solid electrodeaqueous solution interfaces has been examined by in situ techniques such as the radiotracer method [2-61, surface-enhanced Raman scattering (SERSI [7-lo], UV reflectance spectroscopy [l l] and electrochemically modulated IR reflectance spectroscopy (EMIRS) [12,13]. The recently developed electrochemical quartz crystal microbalance (EQCM) [14] is a useful instrument for examining adsorption at the monolayer level because of its high sensitivity. However, EQCM studies of phenomena at the monolayer level are rather limited, probably owing to the difficulty of constructing a stable EQCM in solution. So far the EQCM has been used for studying the underpotential deposition of metals [15-181, the electrosorption of anions [19], adsorbed oxygen monolayers [20] and self-assembled monolayers of viologen derivatives [21] and ferrocene derivatives [22-251. Among these

Electrochemical oxidation of CO on Pt in acidic and alkaline solutions: Part I. voltammetric study on the adsorbed species and effects of aging and Sn(IV) pretreatment

Abstract CO oxidation on a Pt electrode was studied in terms of voltammograms, Tafel plots, surface species and the effects of pH (1–13), Sn(IV) treatment, aging, surface history and CO-adsorption potential. The electrochemical behaviour changes critically above or below a solution pH of ca. 11. The oxidation rate in acids is one to two orders of magnitude smaller than in alkaline solutions and its Tafel plot reveals a characteristic retardation region over the potential range from 0.5 to 0.8 V. The voltammetric oxidation of the surface species gives two peaks, I and II (I is minor), in acidic solution and only one (corresponding to I) in alkaline solution. Sn(IV) pretreatment excludes II in acidic solution and enhances CO oxidation by one to two orders of magnitude, resulting in a Tafel plot similar to that in alkaline solution. The aging in the hydrogen region, however, was found to have the same effect. CO adsorption on the aged surface in the hydrogen region or just after removal of the adsorbed hydrogen in acidic solution gives only I and a higher oxidation rate, whereas CO adsorption in the double-layer region after the reduction of surface oxygen gives I and II and a lower oxidation rate. Thus, the surface history and the CO-adsorption potential are the essentially important factors in controlling CO oxidation.

Electrochemical quartz crystal microbalance studies of self-assembled monolayers of 11-ferrocenyl-1-undecanethiol: Structure-dependent ion-pairing and solvent uptake

Abstract Mass transport accompanying the redox process of ferrocenyle-undecanethiol monolayers has been examined by an electrochemical quartz crystal microbalance (EQCM) for both compact and loosely packed layers with different surface coverages prepared by self-assembly method. Ferricinium cations formed upon the oxidation of ferrocene group associate with anions in solution to form ion pairs. The anion association was not accompanied by water incorporation into the layers independent of the surface coverage, unless the layers were pretreated with water before the measurements were made. Immersion of the thiol-modified electrode in water caused a drastic increase in interfacial mass transport upon oxidation, particularly at a loosely packed layer. It is concluded that this increase is the result of the incorporation of water molecules into the space between the alkyl chains of the oriented ferrocenylundecanethiol. The space seems to be preoccupied by hexane during the self-assembly process.

An In Situ Electrochemical Quartz Crystal Microbalance Study of the Dissolution Process of a Gold Electrode in Perchloric Acid Solution Containing Chloride Ion

The anodic dissolution process of a Au(111) oriented electrode in HClO 4 solution containing various concentrations of chloride ion was investigated using an electrochemical quartz crystal microbalance (EQCM) under potential sweep and potentiostatic conditions. The in situ EQCM results clearly demonstrated that gold dissolves through a 3e - oxidation process in the anodic potential region. No evidence for the 1e - dissolution was obtained in the present study.

Layer-by-layer self-assembly of composite films of CdS nanoparticle and alkanedithiol on gold: An X-ray photoelectron spectroscopic characterization

Abstract Layer-by-layer self-assembly of composite thin films of cadmium sulfide (CdS) nanoparticle and alkanedithiol was achieved on a gold substrate by an alternate immersion into solutions of dithiols (1,6-hexanedithiol and 1,10-decanedithiol) and solution containing CdS nanoparticles (ca. 3 nm in diameter). The layer-by-layer structure was confirmed by angle-resolved X-ray photoelectron spectroscopy (XPS) at each composite-film preparation step. The proposed structure and mechanism of self-assembly were in agreement with our previous results obtained by Fourier transform infrared reflection-absorption spectroscopy.

Coverage dependent behavior of redox reaction induced structure change and mass transport at an 11-ferrocenyl-1-undecanethiol self-assembled monolayer on a gold electrode studied by an in situ IRRAS–EQCM combined system

The potential dependent structure change and mass transport at an 11-ferrocenyl-1-undecanethiol (FcC11SH) self-assembled monolayer on a gold electrode with various coverages were investigated by simultaneous Fourier transform infrared reflection absorption spectroscopy (IRRAS) and electrochemical quartz crystal microbalance (EQCM) measurements in 0.1 M HClO4 solution. As soon as the terminal ferrocene group was oxidized to a ferricenium cation (Fc+), a number of upward and downward IRRAS bands were observed and the surface mass was increased. The intensity of the IRRAS bands corresponded well with the mass change. This behavior is attributed to the redox reaction induced orientation change in the FcC11SH monolayer and ion pair formation between the Fc+ cation and the perchlorate anion in solution. The FcC11SH monolayer with a lower coverage was less stable and was decomposed at a less positive potential than that with a high coverage. Models are proposed to explain the coverage dependent behavior of the FcC11SH monolayer induced by the redox reaction of the terminal ferrocene moiety.

Formation and characterization of thiol-derivatized zinc (II) porphyrin monolayers on gold

Abstract Self-assembled monolayers of two kinds of thiol-derivatized porphyrins, which differ in the number of alkanethiol side chain, have been constructed on Au. X-ray photoelectron, visible and Fourier transform infrared spectra confirmed that surface structure was approximately the same as anticipated; porphyrin molecules having a single chain are somewhat tilted against surface normal, and porphyrins with four chains are oriented coplanar.

Preparation of binary metal electrocatalysts by self-assembly of precursor ionic species on gold and reduction of nitrate ions

Pt/Sn binary metal electrodes were prepared by the successive adsorption of Sn2+ and PtCl62− ions onto a gold substrate followed by the reduction of the adsorbed PtCl62−. The atomic ratios of Sn and Pt to the surface Au were determined to be approximately 1 and 0.3, respectively, using a quartz crystal microbalance (QCM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. Therefore, binary metal layers at a monolayer level were formed. These Pt/Sn binary metal electrodes exhibited a very high electrocatalytic activity for the reduction of nitrate ions with the predominant formation of ammonium ions.

Electrochemical oxidation of CO on Pt in acidic and base solutions: Part II. Structural change of the Pt surface and mechanism of electrooxidation of CO studied by in-situ infrared reflection spectroscopy

Abstract The effect of surface roughening of Pt by potential cycling and of the initial adsorption potential on the CO oxidation mechanism was studied by monitoring the potential dependence of the infrared spectra of the adsorbed CO in the course of the electrooxidation. The most interesting phenomenon has been observed in the course of oxidation of CO adsorbed in the hydrogen region on moderately roughened Pt: the infrared spectra split into two bands, of which the one at the lower wavenumber disappears first at less positive potentials than the other band, at the higher wavenumber, with a concomitant large band shift to lower wavenumber, while the other band shows no such band shift when it disappears at more positive potentials. It is concluded that two oxidation mechanisms are operating on Pt at the same time in such a case: an island mechanism, in which the oxidation proceeds from the edges of the CO islands, and random oxidation which proceeds homogeneously in the CO layer. This implies that there are two CO domains on Pt. Irrespective of the degree of surface roughening, an adsorbed CO layer formed in the hydrogen region is oxidized around 0.5 V/RHE, while CO adsorbed in the double layer region, 0.4 V, is oxidized around 0.6 V/RHE, i.e., the former is more electroactive. However, even for the same adsorption potential, 0.4 V, the previous history of the Pt (i.e., whether the Pt has experienced a hydrogen region or an oxygen region before 0.4 V) plays an important role in determining the reactivity of the adsorbed CO layer.

Acid-base and catalytic properties of Al2O3ZnO

Alumina-zinc oxide catalysts of various compositions were prepared from their nitrates by coprecipitation and the acidic and basic properties were measured by titrations with n-butylamine and benzoic acid using various indicators. The acidities (the numbers of acid sites) at H0 = −3.0 and H0 = −5.6 of Al2O3ZnO catalysts prepared from their nitrates, were found to be lower than those of Al2O3ZnO catalysts prepared from their chlorides by the same method. With the increase in ZnO content, the acidities per unit surface area decreased monotonously, whereas a maximum basicity was observed on Al2O3ZnO (1:1), the highest base strength (pKa) being 12.2. The activity change for alkylation of phenol with methanol was correlated well with the acidity change and the highest selectivity (almost 100%) for formations of ortho products, o-cresol and 2,6-xylenol, was observed on Al2O3ZnO (1:9). Isomerization of 1-butene was suggested to proceed on basic sites on Al2O3ZnO catalysts. In isomerization of 3-carene (I), the selectivity for formation of 2-carene (II) was more than 90% over Al2O3ZnO (1:9). From poisoning experiments with CO2 and pyridine, the active sites for formation of 2-carene were concluded to be both acidic and basic sites.