Nagahiro Hoshi

Electrochemical reduction of CO2 on single crystal electrodes of silver Ag(111), Ag(100) and Ag(110)

Abstract Structural effects on the rates of CO 2 reduction were studied on Ag(111), Ag(100) electrodes in 0.1 M KHCO 3 using macroelectrolysis. The partial current density of each reduction product (CO, HCOO − , H 2 ) was measured at various potentials. All the Ag single crystal electrodes mainly gave CO at any potential. The partial current density of CO on the atomatically stepped Ag(110) was remarkably higher than those on the flat Ag(111) and Ag(100),is the case with Pt group metals. The partial current density of HCOO − gave a smaller orientation dependence.

Electrochemical reduction of CO2 at copper single crystal Cu(S)-[n(111)×(111)] and Cu(S)-[n(110)×(100)] electrodes

Electrochemical reduction of CO2 was studied using two series of single-crystal electrodes, Cu(S)-[n(111)×(111)] and Cu(S)-[n(110)×(100)] at a constant current density of 5 mA cm−2 in 0.1 M KHCO3 aqueous solution. Copper single crystals were grown from 99.999% copper metal in a graphite crucible, and the crystal orientation was determined by the X-ray back reflection method. The surface treatment of the copper single crystal electrodes was studied in detail, and the reproducibility of the CO2 reduction was greatly improved. The product distribution of the CO2 reduction varies greatly with the crystal orientation. CO2 reduction at the Cu(110) (=Cu(S)-[2(111)×(111)]) electrode gives a current yield of 20% of CH3COOH; the formation of CH3COOH in CO2 reduction has not been reported previously. The yield of CH4 was very low (6%) at the Cu(110) electrode. The formation of CH4 and CH3COOH changes significantly with the crystal orientation. A decrease of the step atom density in the Cu(S)-[n(111)×(111)] series reduces the yield of CH3COOH and enhances that of CH4. Introduction of the (100) step to the Cu(110) basal plane, leading to the Cu(S)-[n(110)×(100)] series with kink sites, diminishes the feature of the Cu(110). The Cu(210) (=Cu(S)-[2(110)×(100)]), which has the highest number of dangling bonds of fcc metals, gives a high yield of CH4 with a product distribution similar to that of Cu(111) which has the lowest density of dangling bonds.

Voltammograms of the single-crystal electrodes of palladium in aqueous sulfuric acid electrolyte: Pd(S)-[n(111)×(111)] and Pd(S)-[n(100)×(111)]

Abstract A series of Pd(S)-[ n (111)×(111)] and Pd(S)-[ n (100)×(111)] electrodes give voltammograms in 0.5 M H 2 SO 4 , which depend remarkably on the surface structure. Pd surfaces with more than three atomic rows of terrace show redox peaks due to the desorption and adsorption of HSO 4 − /SO 4 2− between 0.25 and 0.33 V (RHE). In the oxide film formation region, Pd(S)-[ n (111)×(111)] electrodes give two anodic peaks at 0.95 and 1.1 V (RHE): the charge of the former peak increases with the increase of the step atom density, whereas that of the latter depends linearly on the terrace atom density. Pd(S)-[ n (100)×(111)] electrodes provide an anodic peak at 0.93 V (RHE). The charge correlates with the terrace width. These electrodes give no peak for which the charge depends on the step density.

Quantitating the lattice strain dependence of monolayer Pt shell activity toward oxygen reduction.

Lattice strain of Pt-based catalysts reflecting d-band status is the decisive factor of their catalytic activity toward oxygen reduction reaction (ORR). For the newly arisen monolayer Pt system, however, no general strategy to isolate the lattice strain has been achieved due to the short-range ordering structure of monolayer Pt shells on different facets of core nanoparticles. Herein, based on the extended X-ray absorption fine structure of monolayer Pt atoms on various single crystal facets, we propose an effective methodology for evaluating the lattice strain of monolayer Pt shells on core nanoparticles. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity.

Electrochemical reduction of carbon dioxide at a series of platinum single crystal electrodes

Electrocatalytic activity in the reduction of CO2 to adsorbed CO was studied systematically on a series of Pt single crystal electrodes using voltammetry. The single crystal electrodes examined were stepped surfaces (Pt(S)-[n(111)×(111)]), Pt(S)-[n(111)×(100)], Pt(S)-[n(100)×(111)]), and kinked step surfaces (Pt(S)-[n(110)×(100)] and Pt(S)-[n(100)×(110)]). Atomically flat surfaces, Pt(111) and Pt(100), show poor activity for CO2 reduction. Introduction of step sites to (111) or (100) surface significantly enhances the electrocatalytic activity in CO2 reduction. The rate increases proportionally with the step atom density. The order of the activity series is obtained for the stepped surfaces: Pt(111)<Pt(100)<Pt(S)-[n(111)×(100)]<Pt(S)-[n(111)×(100)]<Pt(S)-[n(111)×(111)]<Pt(110). The most active site in the stepped surface is derived from the psudo-4-fold bridged site in Pt(S)-[n(111)×(111)]. Kinked step surfaces show higher activity than stepped surfaces: the reduction rate per kink atom is more than twice as high as the value per step atom. Densely packed kink atoms along the step line greatly promote the reduction of CO2.

Voltammograms of stepped and kinked stepped surfaces of palladium: Pd(S)-[n(111)×(100)] and Pd(S)-[n(100)×(110)]

Abstract Voltammograms of stepped and kinked stepped surfaces of Pd (Pd(S)-[ n (111)×(100)] and Pd(S)-[ n (100)×(110)]) were measured in 0.5 M H 2 SO 4 . Both series gave redox peaks around 0.25 V (RHE) for which the intensity decreases with the decrease of the terrace width in 0.5 M H 2 SO 4 . No peak was observed around 0.25 V in 0.1 M HClO 4 . In the oxide film formation region, Pd(S)-[ n (111)×(100)] and Pd(S)-[ n (100)×(110)] electrodes gave single anodic peaks at 1.1 and 0.9 V (RHE), respectively. The peak intensity diminished with the decrease of the terrace width. Both series presented no peak for which the intensity was enhanced with the increase of step atom density. Charges of the anodic peaks equaled those calculated on the assumption that a monolayer of the oxide film (Pd–OH or Pd 2 O) covers the entire terrace.

Structural effect on the rate of CO2 reduction on single crystal electrodes of palladium

The structural effect on the rate of CO2 reduction was studied with voltammetry on single crystals of Pd (Pd(111), Pd(100), Pd(110)) in 0.1 M HClO4 saturated with CO2. CO2 was reduced to an adsorbed product at potentials more negative than −0.1 V vs. RHE. The rate of CO2 reduction depends remarkably on the crystal orientation: Pd(100) < Pd(111) < Pd(110). The Pd(111) surface shows uniquely high activity, whereas (111) is the surface of lowest activity in CO2 reduction for other Pt group metals (Pt, Rh, Ir). The rate of CO2 reduction at −0.5 V vs. RHE on Pd(110) is two orders of magnitude higher than that of Pt(110).

Step density dependence of co2 reduction rate on Pt(S)-[n(111) × (111)] single crystal electrodes

CO2 reduction was studied by voltammetry with Pt single crystal electrodes of Pt(S)-[n(111) × (111)] at constant potentials. The rate of CO formation depends on crystal orientation as below; Pt(110) > Pt(331) > Pt(221) > Pt(332) > Pt(997) > Pt(111). The initial rates of the CO formation plotted against the amount of adsorbed hydrogen (QH0) gave a maximum of around QH0 = 40 μCcm− 2 on the electrodes with high step density, Pt(110), Pt(331) and Pt(221), whereas the electrodes with low step density, Pt(332), Pt(997) and Pt(111), showed monotonous increase with QH0. The activity series shown above was well correlated with the step density of crystal orientations except Pt(110)(1 × 2). The correlation and the anomalously high activity of Pt(110) are discussed with regard to the adsorption sites for hydrogen atoms with reference to UHV works in literatures. Reduction of CO2 proceeds with adsorbed hydrogen regenerated on the adsorption sites.

Electrochemical reduction of carbon dioxide on kinked stepped surfaces of platinum inside the stereographic triangle

Structural effects on the rate of CO2 reduction are studied on kinked stepped surfaces of Pt inside the stereographic triangle. The surfaces examined are Pt(431), Pt(531) and Pt(532), which consist of two or three atomic rows of (111) terraces and (111) or (100) steps incorporated with kink atoms. The rate of CO2 reduction is enhanced with the increase of the kink atom density. The kinked stepped surfaces have a higher activity for CO2 reduction than the analogous stepped surfaces that have similar terrace and step structures without kink sites. Introduction of kink atoms to Pt(S)-[3(111)/(100)] surface changes the potential dependence of the reduction rates significantly. # 2002 Elsevier Science B.V. All rights reserved.

Significant enhancement of the electrochemical reduction of CO2 at the kink sites on Pt(S)-[n(110)×(100)] and Pt(S)-[n(100)×(110)]☆

Abstract The surface structures giving high activity in CO 2 reduction were studied using voltammetry with kinked step surfaces of platinum: Pt(S)-[ n (110)×(100)] ( n= 2, 9), Pt(S)-[ n (100)×(110)] ( n= 2, 3, 9). The reduction rates of CO 2 on Pt(S)-[ n (110)×(100)] electrodes were higher than those on Pt(110) which gave the highest activity among the stepped surfaces reported previously. The rates on Pt(S)-[ n (100)×(110)] were over twice as high as those on Pt(S)-[ n (100)×(111)]. Pt(210)=Pt(S)-[2(100)×(110)] gave the highest rate of CO 2 reduction. This remarkably high activity in CO 2 reduction may be derived from the kink sites characteristic of Pt(S)-[ n (110)×(100)] and Pt(S)-[ n (100)×(110)].