By the use of in situ scanning tunneling microscopy and surface X-ray scattering techniques, we have clarified the surface structure and the layer-by-layer compositions of a Pt skin/Pt 3 Co(111) single-crystal electrode, which exhibited extremely high activity for the oxygen reduction reaction. The topmost layer was found to be an atomically flat Pt skin with (1 × 1) structure. Cobalt was enriched in the second layer up to 98 atom %, whereas the Co content in the third and fourth layers was slightly smaller than that in the bulk. By X-ray photoelectron spectroscopy, the Co in the subsurface layers was found to be positively charged, which is consistent with an electronic modification of the Pt skin. The extremely high activity at the Pt skin/Pt 3 Co(111) single crystal is correlated with this specific surface structure.

No abstract

When the activity of a catalyst was estimated by a rotating ring disk electrode (RRDE) method, the modified glassy carbon (GC) electrode was generally fabricated by casting catalyst suspension prepared by various methods, followed by drying under various conditions. There are many preparation methods for catalyst suspension. And it is known that the dispersivity of the catalyst on the GC electrode, which affects collection efficiency of H2O2as well as current density, depends on the above methods and conditions. Since an optimum method and condition often varies with the kinds of catalysts, researchers have to find them. However, it is difficult to evaluate the dispersivity on the electrode and reproducibility quantitatively. In this study, the dispersivity of multi wall carbon nanotube supported Pt catalyst (Pt/MWCNT) on the modified electrode prepared by various methods was estimated by digitizing height data of the electrode using a laser surface roughness meter. 30 wt% Pt/MWCNT (Pt particle size: 4.26 ± 0.09 nm) catalyst was synthesized by a microwave polyol process. 2 mg/mL of 30 wt% Pt/MWCNT-0.1 wt% various alcohol (MeOH, EtOH, and 2-PrOH) suspensions were prepared by ultrasonication for 10 min. 10 μL of the suspensions cast on a GC electrode, and the electrode allowed to dry by three method: 1) in air for 1 h (abbreviated as Air), 2) in saturated ethanol vapor pressure for 1 h (EV), and 3) in air with infrared irradiation for 10 min at 60 °C (IR). The samples were named as [solvent] – [drying method]. The modified electrodes were observed with a digital microscope (MSP-3080, PANRICO). And the surface height data of the modified electrodes were digitized with a laser surface roughness meter (SV-C4500 CNC, Mitutoyo), and the data were analyzed by a software "Roughness Analyzer" developed by our laboratory. The software detects secondary particles of the catalyst (clusters) on the electrode and the user can obtain the volumes, the average height, and the area of the clusters as well as the distribution of the catalysts from the center of the electrode. Fig. 1 (a) shows cumulative cluster volume distribution for MeOH–Air, 2-PrOH-Air and EtOH–EV. The electrode surface prepared by the EtOH-EV method had the clusters up to 1.9×10-3 mm3, whereas those prepared by MeOH-Air and EtOH-EV had the clusters up to 6.7×10-4 mm3 and 1.9×10-3 mm3, respectively, suggesting that the EtOH-EV method tend to be aggregated to produce large secondary particles. Fig. 1 (b) shows the average cluster heights of the samples. The standard deviation of the average cluster height for 2-PrOH-Air was the smallest of the three methods, indicating that the 2-PrOH-Air method is the most reproducible casting method of the three methods. Some quantification methods will be reported in the meeting. Figure 1

Although ammonia is expected as the next generation of energy medium, even platinum, which is a relatively active ammonia oxidation (AO) catalyst, have an insufficient performance for direct ammonia fuel cells (DAmFCs) operating below 100°C. Highly active AO catalysts at low temperatures needs to be developed in order to use ammonia as a fuel for DAmFCs. AO mechanism on platinum is reported as follows: After NH3 is adsorbed on the platinum surface, the AO proceeds via two competitive paths. One is the formation of hydrazine analogues, followed by the oxidation of the analogues to release N2 molecules. The other is the oxidation of an ammonia molecule to form nitrogen atoms adsorbed onto the surface of platinum (Nads) known as poisoning species. However, after the surface of the platinum is covered with Nads, the elimination process of Nads is not known. In this study, the elimination process was studied by a normal pulse voltammetry. 33.6 wt% Pt/MWCNT (average particle size: 5.57 ± 0.11 nm) catalyst was synthesized by a microwave polyol method. A 2 mg of Pt/MWCNT was suspended in 1 mL of Pt/MWCNT-0.1 wt% Nafion-MeOH by ultrasonication for 10 min. 10 μL of the suspension was cast onto a glassy carbon (GC) disk electrode (6 mm φ), followed by drying for 1 h in air. Electrochemical measurements were performed with a conventional three electrode system equipped with the modified GC electrode, an Au wire counter electrode and a reversible hydrogen electrode (RHE) in 0.1 M KOH or 0.1 M NH3-0.1 M KOH. The normal pulse voltammetry was performed as follows: After the Nads was formed by applying 0.6 V for 180 s, E recover (V) was applied for various times (1 s，5 s，10 s，20 s，60 s, and 180 s) to eliminate Nads. Then, the AO current at 0.6 V after 1 s was used as an indicator for the amount of Nads poisoning. Fig. 1 shows the dependence of AO current at 0.6 V after 1 s on recovery potential. The AO current gradually recovered by maintaining the potential below 0.4 V vs. RHE, indicating that the adsorbed N atoms were eliminated from the catalyst surface. It took a few minutes to recover the AO current. In addition, the decrease in potential accelerated the recovery of the AO current, especially below 0.3 V vs. RHE. The final percentage of the recovery increased with a decrease in the potential. The recovery rates below 0.25 V called hydrogen adsorption/desorption region increased with decreasing potential, probably due to the reduction of Nads. The time dependence of the recovery rate was analyzed by the sum of two exponentials, suggesting that the fast and slow reactions exists in the recovery process. Figure 1