Ken Yao

Mass transport on an anionic clay-modified electrode as studied by a quartz crystal microbalance

Abstract A quartz crystal microbalance (QCM) has been used to investigate mass transfer coupled with electron transfer through a hydrotalcite clay film. The electrode was prepared by casting an aqueous suspension of hydrotalcite (denoted as Mg–Al–HT) or hydrotalcite ion-exchanged with [Fe(CN) 6 ] 3− (denoted as Mg–Al–HT–[Fe(CN) 6 ]) 3− . Adsorption studies using QCM and UV-visible spectroscopic measurements revealed that [Fe(CN) 6 ] 3− was incorporated as an ion-paired species with a counter cation. The adsorbed species on an electrode in contact with a 0.1 M NaCl solution was identified as a divalent ion-pair (Na[Fe(CN) 6 ] 2− ) while the adsorbed species in contact with a 0.1 M Na 2 SO 4 or Na 2 CO 3 solution was a monovalent ion-triplet (Na 2 [Fe(CN) 6 ] − ). Electrochemical QCM measurements on an electrode modified with Mg–Al–HT–[Fe(CN) 6 ] 3− revealed that one Na[Fe(CN) 6 ] 2− or Na 2 [Fe(CN) 6 ] − ion was leached from a film when one or two [Fe(CN) 6 ] 3− ions were reduced, respectively. The leaching of these anions took place in order to compensate the excess negative charge generated in a film.

Electrochemical STM observation of [Fe(CN)6]3− ions adsorbed on a hydrotalcite crystal surface

Abstract The electrochemical STM method has been applied to a hydrotalcite clay-modified electrode with the aim of obtaining evidence for the formation of a surface layer of the [Fe(CN) 6 ] 3−/4− couple. As a result, ordered surface layers of [Fe(CN) 6 ] 3−/4− ions on a hydrotalcite-modified electrode have been observed with an in-situ electrochemical STM instrument: the initial lattice of [Fe(CN) 6 ] 3− with a =1.40±0.04 nm, b =1.85±0.04 nm and α =63±3° changed to an expanded lattice of [Fe(CN) 6 ] 4− with a =1.94±0.04 nm, b =1.90±0.04 nm and α =60±3° due to the electrochemical reduction.

Clay-modified electrodes as studied by the quartz crystal microbalance: adsorption of ruthenium complexes

Abstract A quartz crystal microbalance (QCM) has been applied to study the adsorption of [Ru(bpy)3]2+ (bpy=2,2′–bipyridyl) and [Ru(NH3)6]3+ by a clay-modified electrode. An electrode was prepared by depositing an aqueous suspension of synthetic sodium saponite onto a gold-coated quartz crystal and drying it under air. After the electrode was swelled for 20 to 120 h in an aqueous 0.01 M Na2SO4 solution, a metal complex was injected into the electrolyte solution. The resistance of the electrode did not change during the adsorption process so that the mass change of the electrode was monitored in terms of the change of resonance frequency. In case of [Ru(bpy)3]2+, the saturated adsorption of racemic [Ru(bpy)3]2+ caused the mass increase of 0.49±0.03 g per 1 g of clay, while that of enantiomeric [Ru(bpy)3]2+ caused the mass increase of 0.29±0.02 g per 1 g of clay. On the basis of these values, it was concluded that racemic [Ru(bpy)3]2+ was exchanged with hydrated sodium ion ([Na(H2O)4]+) in a clay film as a monovalent ion-paired species ([Ru(bpy)3]2+·1/2SO42−), while enantiomeric [Ru(bpy)3]2+ was exchanged with [Na(H2O)4]+ partly as a divalent ion and partly as a monovalent ion-paired species. Under the similar conditions, the saturated adsorption of [Ru(NH3)6]3+ caused the mass increase of 0.17±0.01 g per 1 g clay. The results indicated that [Ru(NH3)6]3+ was replaced with [Na(H2O)4]+ in a clay film as a monovalent ion-paired species ([Ru(NH3)6]3+·SO42−). These results were compared with the adsorption experiments of the same complexes by colloidally dispersed sodium saponite.

Clay-modified electrodes as studied by the quartz crystal microbalance: Redox processes of ruthenium and iron complexes

Abstract The electrochemical quartz crystal microbalance (EQCM) has been employed to investigate the mass transport processes on a clay-modified electrode. The systems investigated were the redox couples of [Ru(bpy) 3 ] 2 − 6 (bpy = 2,2′-bipyridine), [Ru(NH 3 )] 6 2+/3+ and [Fe(CN) 6 ] 4 −/3− . A clay-modified electrode was prepared by depositing synthetic saponite onto a gold coated quartz crystal. An electrode was allowed to swell for more than 50 h in 0.01 M Na 2 SO 4 or NaCl or NaClO 4 prior to the adsorption and electrochemical measurements. The EQCM results revealed that the charge balancing during a redox reaction was accomplished by leaching or incorporating mobile ions in the clay film. For the [Ru(bpy) 3 ] 2+/3 couple, one [Ru(bpy) 3 ] 3− molecule was eliminated from the clay film when three [Ru(bpy) 3 ] 2+ ions were oxidized. For the [Ru(NH 3 ) 6 ] 2+ 3+ couple, one SO 4 2 ion, which was co-adsorbed with the ruthenium complex, was removed from the clay film when two [Ru(NH 3 ) 6 ] 3+ molecules were reduced. For the [Fe(CN) 6 ] 4−/3− couple, a part of the excess charge generated by the initial oxidation of [Fe(CN) 4 ] 4− was canceled out by the elimination of a sodium ion bound by a clay layer. No mass transfer was detected during the oxidation at the later stage. This was probably because sodium ions in the aqueous medium within a clay film carried excess charge. The results are discussed in relation to the reported electrochemical behavior of these metal complexes.

Unusual catalytic effect of the two-dimensional molecular space with regular triphenylphosphine groups.

A novel organic-inorganic hybrid 2D molecular space with regular triphenylphosphine groups (triphenylphosphineamidephenylsilica, PPh(3)APhS) was successfully synthesized through grafting triphenylphosphine groups in the 2D structure of layered aminophenylsilica dodecyl sulfate (APhTMS-DS), which was developed in our previous research, with regular ammonium groups. The 2D structures were kept after the grafting reaction of triphenylphosphine groups in PPh(3)APhS. The catalytic potentials of 2D molecular space with regular triphenylphosphine groups were investigated. An unusual catalytic effect was found in a carbon-phosphorus ylide reaction. The PPh(3)-catalyzed reaction of modified allylic compounds, including bromides and chlorides with tropone yielded a [3 + 6] annulation product. However, an unusual [8 + 3] cycloadduct was obtained in the reaction of modified allylic compounds, including bromides and chlorides with tropone catalyzed by PPh(3)APhS. Otherwise, the stable catalytic intermediate was successfully separated, and the reaction activity of the catalytic intermediate was confirmed in the reaction of modified allylic compounds with tropone catalyzed by PPh(3)APhS. This research is the first successful example of directly influencing catalytic reaction processes and product structures by utilizing the chemical and geometrical limits of 2D molecular spaces with regular catalyst molecules and affords a novel method for controlling catalytic reaction processes and catalyst design.