Development of a spectro-electrochemical cell for soft X-ray photon-in photon-out spectroscopy

We developed a spectro-electrochemical cell for X-ray absorption and X-ray emission spectroscopy, which are element-specific methods to study local electronic structures in the soft X-ray region. In the usual electrochemical measurement setup, the electrode is placed in solution, and the surface/interface region of the electrode is not normally accessible by soft X-rays that have low penetration depth in liquids. To realize soft X-ray observation of electrochemical reactions, a 15-nm-thick Pt layer was deposited on a 150-nm-thick film window with an adhesive 3-nm-thick Ti layer for use as both the working electrode and the separator window between vacuum and a sample liquid under atmospheric pressure. The designed three-electrode electrochemical cell consists of a Pt film on a SiC window, a platinized Pt wire, and a commercial Ag|AgCl electrode as the working, counter, and reference electrodes, respectively. The functionality of the cell was tested by cyclic voltammetry and X-ray absorption and emission spectroscopy. As a demonstration, the electroplating of Pb on the Pt/SiC membrane window was measured by X-ray absorption and real-time monitoring of fluorescence intensity at the O 1s excitation

Electrocatalytic activity and volatile product selectivity for nitrate reduction at tin-modified Pt(100), Pd(100) and Pd–Pt(100) single crystal electrodes in acidic media

We prepared Sn-modified Pt(100), Pd(100) and Pd–Pt(100) single crystal electrodes and investigated the nitrate reduction reaction (NO3RR) activity and the product selectivity for them using online electrochemical mass spectroscopy (OLEMS), also known as differential electrochemical mass spectroscopy (DEMS). OLEMS measurements allowed us to quantify volatile products of N2, N2O and NO and confirm the production of N2 at Sn/Pd(100) but not at Sn/Pt(100). Pd-doping to Pt(100) with a 3 atomic % increased the product selectivity for the NO3RR to N2. These results indicate that the presence of Pd in the (100) surface is the key to produce N2, which seems to be related to the hydrogen adsorption energy to the metal surface. The suppression of hydrogenation of intermediate species at the electrode surface could lead to the production of N2. This work will guide us to understand N2 production mechanism for the NO3RR and develop highly selective electrocatalysts for denitrification

Excitation Wavelength Dependent Three-Wave Mixing at a CO-Covered Platinum Electrode

The interfacial electronic structure of a CO-covered polycrystalline platinum electrode has been studied by using the optical second harmonic generation (SHG) and sum frequency generation (SFG) techniques with various excitation wavelengths. Although the nonlinear optical (NLO) signal was enhanced by the absorption of CO at all excitation wavelengths employed in this study, the potential dependent behaviors of the NLO signal were different between the near-infrared excitation and the visible excitation. The difference was attributed to the different mechanisms for the enhancement of the NLO signal. While the electron transition from the Fermi level of Pt to the unoccupied 2πa* orbital of adsorbed CO was considered to be coupled with the NLO photon in the case of visible excitation, the dc field induced SHG regime was applied to the result obtained by the near-infrared excitation

In Situ Optical Second Harmonic Rotational Anisotropy Measurements of an Au(111) Electrode during Electrochemical Deposition of Tellurium

The electrodeposition of Te on a single-crystalline Au(111) electrode was studied with 1064-nm-excited SH rotational anisotropy measurements. The SH rotational anisotropy was significantly changed with the first underpotential deposition (upd) of Te, and the bulk Te deposition attenuated the anisotropic character of the overall surface symmetry. The change in the SH rotational anisotropy during the first upd of Te was examined using two different models. The first model considered only the contribution of the Au(111) surface to the SHG response, while the second one took into account the contributions of both the Au(111) substrate and the adsorbed Te layer. In the former case, the change in the SH rotational anisotropy can be explained by considering the quenching of the nonlinear susceptibility of the Au(111) surface. The analysis based on the latter model resulted in a rotation angle of 608 for the adsorbed Te layer against the the Au(111) lattice. This value was not consistent with that expected from the adsorbate structure of the first upd layer of Te, i.e., (√3 x √3)R30°. Thus, the former model seems to be more appropriate to explain the present results. The SH rotational anisotropy measurement also suggests that the morphology became more isotropic after the bulk Te deposition

Electrodeposition of Flattened Cu Nanoclusters on a p-GaAs(001) Electrode Monitored by in situ Optical Second Harmonic Generation

In situ optical second harmonic generation (SHG) technique was employed to investigate the shape and density of Cu nanoclusters, which were electrochemically formed on p-GaAs(001) electrode surfaces. Since GaAs is not a centrosymmetric medium, a significant portion of SHG signal arises from the bulk dipole susceptibility, but it was possible to separate a surface-induced signal from a bulk-induced signal by choosing an appropriate experimental geometry and appropriate data processing. The rotational anisotropy (RA) pattern of the SHG signal from a p-GaAs(001) electrode changed in both shape and magnitude during potential cycling in an electrolyte solution containing Cu2+. The surface plasmon-induced SHG signal from Cu nanoclusters deposited on GaAs was attributed to the modulation source for the RA-SHG pattern. More detailed study was carried out with both in situ SHG and ex situ AFM measurements for Cu nanoclusters deposited by potential step. The results showed that the SHG signal at the present optical geometry was sensitive to the number of oblate or flattened Cu nanoclusters with lateral diameter larger than 30 nm and that the SHG enhancement occurred because of resonant coupling between the surface plasmon induced in the flattened Cu nanoclusters and the near-infrared fundamental light

Electrocatalytic nitrate reduction on well-defined surfaces of tin-modified platinum, palladium and platinum-palladium single crystalline electrodes in acidic and neutral media

Nitrate anion is one of the main nitrogen-containing-pollutants in groundwater and can be removed using denitrification systems including electrocatalytic systems. Herein we report on electrocatalytic nitrate reduction catalyzed on tin-modified single crystalline electrodes of palladium, platinum and palladium-platinum alloy in acidic and neutral media. We have prepared electrodes with the (111) surface or the (100) surface and modified their surface with tin. Cyclic voltammetry of the electrodes has revealed that the tin-modified alloy (trimetallic) electrodes show higher electrocatalytic activity than the tin-modified platinum or palladium (bimetallic) electrodes, and the catalytic reaction is more efficiently catalyzed on the (100) surface rather than the (111) surface. The tin-modified PdPt(100) electrode shows the highest catalytic activity in acidic media as well as in neutral media. X-ray photoelectron spectroscopy suggests that metallic tin forms on the (100) surface, but divalent tin species on the (111) surface, indicating that a surface alloy of tin may form on the (100) surface, resulting in enhancement of the electrocatalytic activity. Our findings suggest that design and preparation of ternary metallic electrodes with the (100) surface will pave the way to the development of practical systems on electrocatalytic denitrification

Host-guest chemistry between cyclodextrin and a hydrogen evolution catalyst cobaloxime

We report the host-guest chemistry between cyclodextrin and a bisdimethylglyoximato cobalt complex, cobaloxime. Cobaloxime forms a 1 : 1 host-guest assembly with beta- or gamma-cyclodextrin but not with alpha-cyclodextrin. The assembly of cobaloxime with gamma-cyclodextrin enhances the photocatalytic hydrogen evolution activity of a homogeneous system that contains an organic dye eosin Y in a neutral aqueous solution under visible-light irradiation

Oxygen Reduction Reaction Catalyzed by Self-Assembled Monolayers of Copper-Based Electrocatalysts on a Polycrystalline Gold Surface

We report two-dimensional model systems to study the electrocatalytic activities of dinuclear copper complexes for various electrocatalytic reactions including the oxygen reduction reaction (ORR), where we can use electrochemical techniques as well as surface-sensitive techniques such as X-ray photoelectron spectroscopy and vibrational sum frequency generation spectroscopy. Heteroaromatic thiols, including four triazoles and a thiadiazole, are used as metal ligands as well as anchors to a polycrystalline gold electrode. The thiols are self-assembled on the polycrystalline gold electrode and then react with copper(II) ions to give monolayers of copper-based ORR catalysts on the surface. The dinuclear copper complexes of 1,2,4-triazole-3-thiol and 3-amino-1,2,4-triazole-5-thiol show ORR activity and pH-dependent catalytic behavior similar to that of counterparts supported on carbon black, suggesting that our two-dimensional systems can serve as model catalysts for carbon-supported molecular catalysts. We have also self-assembled dinuclear copper complexes with long alkyl or perfluoroalkyl chains on the surface and studied their orientation on the surface and oxygen transport

Electrocatalytic Oxygen Reduction at Multinuclear Metal Active Sites Inspired by Metalloenzymes

Polymer electrolyte fuel cells (PEFCs) are a clean, sustainable device to convert chemical energy to electricity and can provide power for automobiles, trains, and ships. In PEFCs, the oxygen reduction reaction (ORR) occurs at the cathode and is catalyzed at electrocatalysts. The activity of ORR electrocatalysts is known to limit the overall performance of PEFCs because the ORR is more sluggish than the hydrogen oxidation reaction at the anode. In the state-of-the-art PEFC, platinum group metal (PGM)-based ORR electrocatalysts are used. Since PGMs are rare and expensive, highly active and durable non-PGM ORR electrocatalysts are required for widespread applications of PEFCs. In nature, metalloenzymes such as cytochrome c oxidase and multicopper oxidases efficiently catalyze the ORR and utilize multinuclear iron and/or copper complexes as active sites. The structure of these active sites and enzyme reaction mechanisms would give us design concepts of artificial non-PGM electrocatalysts for the ORR, possibly leading us to develop next-generation non-PGM electrocatalysts. Herein, recent research progress on understanding enzymatic ORR reaction mechanisms and developing non-PGM ORR electrocatalysts is reviewed from the viewpoint of bio-inspired approaches