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Interlayer and surface Ir-modified Pt/Pd(111) model catalyst surfaces [Pt/Ir/Pd(111) and Ir/Pt/Pd(111)] were synthesized as surface structural models for third element-modified core–shell-type Pd@P...

Extended X-ray absorption fine structure (EXAFS) is a powerful tool to determine the local structure in Pt nanoparticles (NP) on carbon supports, active catalysts for fuel cells. Highly oriented pyrolytic graphite (HOPG) covered with Pt NP gives samples with flat surfaces that allow application of surface science techniques. However, the low concentration of Pt makes it difficult to obtain good quality EXAFS data. We have performed in situ highly sensitive BCLA-empowered Back Illuminated EXAFS (BCLA + BI-EXAFS) measurements on Pt alloy nanoparticles. We obtained high quality Pt L3-edge data. We have devised a novel analytical method (model building analysis) to determine the structure of multi-component nanoparticles from just a single absorption edge. The generation of large numbers of structural models and their comparison with EXAFS fits allows us to determine the structures of Pt-containing nanoparticles, catalysts for the oxygen reduction reaction. Our results show that PtCo, PtCoN and AuPtCoN form a Pt-shell during electrochemical dealloying and that the ORR activity is directly proportional to the Pt–Pt bond length.

Highly active and inexpensive anode materials are required for large-scale hydrogen production using alkaline water electrolysis (AWE). Here, heterolayered nanostructures of Ni-Fe hydroxides/oxides with high activity for the oxygen evolution reaction (OER) were synthesized on a 316 stainless steel (SS) substrate through constant current density electrolysis. The thicknesses, morphologies, and compositions of the nanostructures, generated through dealloying and surface oxidation of the SS elements with severe oxygen microbubble evolution, were dependent on the electrolysis time. Nanostructural analyses showed that the heterolayered Ni-Fe hydroxide/oxide nanostructures were generated during the initial stage of electrolysis, growing nanofiberlike Ni-Fe hydroxide layers with increasing electrolysis time of up to 5 h. The prolonged electrolysis resulted in densification of the nanofiber structures. The OER overpotential at 10 mA/cm

We investigated the microstructures of Au-deposited PtCo@Pt core-shell nanoparticles (NPs) and discussed enhancement of the oxygen reduction reaction (ORR) properties by Au termination of low-coordination sites of the Pt-shell. The Au-deposited PtCo@Pt NPs showed an improved electrochemical structural stability, together with slight increment in increased initial, pristine area-specific activity relative to the non-Au-deposited PtCo@Pt NPs. Atomic-level microstructural characterization was performed by a back-side illumination fluorescence X-ray absorption fine structure (BI-FXAFS) method and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS). The BI-FXAFS results indicated that compressive lattice strain in the Pt-shells of the PtCo@Pt NPs was almost unchanged by the subsequent Au deposition. Furthermore, STEM-EDS mapping of Pt, Co, and Au clearly showed that the deposited Au tended to localize at low-coordination sites of the Pt-shell surface, e.g., edges and corners. The atomic-level microstructural characterization conducted in this study demonstrated that effective Au surface terminations of the Pt-shell enhance the ORR durability of Pt-based core-shell type NP catalysts.

Oxygen reduction reaction (ORR) activity was investigated for nano-flake-like carbon-modified Pt75Ni25(111) surfaces. Surface cleaning through Ar+-sputtering and thermal annealing in an ultra-high vacuum (~ 10−8 Pa) resulted in a Pt-enriched topmost surface, i.e., a Pt(111)-skin on Pt75Ni25(111). Arc plasma deposition (APD) of graphite under 0.08 Pa N2 and in vacuum (~ 10−6 Pa) generated nitrogen-doped and non-doped nano-flake-like carbon on the Pt(111)-skin surfaces, respectively. For the latter, non-doped carbon-modified Pt(111)-skin, the area-specific initial ORR activity estimated in O2-saturated 0.1 M HClO4 decreased with increasing thickness of the deposited carbon. In contrast, the former, nitrogen-doped carbon with 2 and 6 A mass-thickness enhanced the ORR activity. The Pt 4f band energies for the nitrogen-doped Pt(111)-skin were measured by X-ray photoelectron spectroscopy (XPS) and showed the chemical shift to higher biding energy (~ 0.2 eV) compared with the corresponding bands for the non-doped and Pt(111)-skin surfaces. As for the electrochemical structural stability, a specific amount of the non-doped carbon species tends to suppress the degradation of the Pt(111)-skin under applying potential cycles. The results indicate that the surface modifications by the carbon hexagonal networks of the nano-flakes could be applicable to improve ORR performance of the practical Pt-M alloy catalysts.

The oxygen reduction reaction (ORR) has been studied on single crystals of the ternary alloys Pt45Pd45Co10 and Pt77Pd8Co15 in O2-saturated 0.1 M HClO4 using a hanging meniscus rotating disk electrode. Specific activity for the ORR depends markedly on the composition and surface structure of the single crystal electrodes. The activity on Pt45Pd45Co10(hkl) increases in the order Pt45Pd45Co10(100) ≈ Pt45Pd45Co10(111)

We investigated electrochemical stability of the NiFe-hydroxide/oxide-catalyst-layer covered stainless-steel(SS)-anode (NiFe-HyOx/SS) for alkaline water electrolysis. The NiFe-HyOx catalyst layer was synthesized through constant current density electrolysis of 30 mA/cm2 in 1 M KOH solution of a 316 SS plate at 75 ℃ for 5 h. The initial overpotential of oxygen evolution reaction (OER) was estimated to be ca. 270 mV at 100 mA/cm2 and the potential kept almost constant during applying the 20,000 potential cycles (PCs) of 0.5 and 1.8 V vs. reversible hydrogen electrode in 7 M KOH at 20 ℃. Scanning transmission electron microscopic observations conducted before and after the PCs loading revealed that the 50 nm-thick, nanofiber-like NiFe-HyOx catalyst layer remained unchanged in structure and in thickness, while the ca. 850 nm-thick, relatively dense NiFe-(hydro)oxide interlayer was generated under the catalyst layer. The results suggest that the superior OER property of the NiFe-HyOx/SS anode is originated from the surface catalyst layer.

We evaluated the electrochemical CO2 reduction reaction (ECR) on low-index Au single crystal surfaces (Au(111), (100), and (110); Au(hkl)) and discussed the surface-atomic-arrangement-dependence of...

The oxygen reduction reaction (ORR) activity and electrochemical stability of well-defined n monolayer (ML)-Pt/PtxNi100−x(111) (n = 2 and 4; x = 75, 50, and 25) model electrocatalyst surfaces were investigated in this study. The initial activity of the as-prepared two-monolayered Pt-covered PtxNi100−x(111) substrates (2ML-Pt/PtxNi100−x(111)) increased with increasing Ni composition in the PtxNi100−x(111) substrate. In particular, 2ML-Pt/Pt25Ni75(111) showed the initial activity that was 25 times higher than that of clean Pt(111) although the higher Ni composition resulted in destabilization of the catalyst upon the application of potential cycles (PCs). As for 4ML-Pt/PtxNi100−x(111), activity enhancements were insensitive to alloy composition and thicker Pt shell layers stabilized the catalyst against PC applications. In particular, the activities of 4ML-Pt/Pt50Ni50(111) and 4ML-Pt/Pt25Ni75(111) gradually increased during 1000 PCs probably because of the PC-induced mono-atomic heights and nanometer-size islands that had (110) and (100) steps introduced into the topmost (111) terraces. Thus, the simultaneous tuning of core–alloy composition and Pt shell thickness is vital for developing practical, highly efficient Pt-based alloy ORR electrocatalysts.

Voltammetric dealloying is a typical method to synthesize Pt-shell/less-noble metal (M) alloy core nanoparticles (NPs) toward the oxygen reduction reaction (ORR). The pristine nanostructures of the Pt–M alloy NPs should determine the ORR activity of the dealloyed NPs. In this study, we investigated the voltammetric dealloying behavior of the Pt–Co and nitrogen-introduced Pt–Co alloy NPs generated by synchronous arc-plasma deposition of Pt and Co. The results showed that the dealloying behavior is sensitive to cobalt nitride in the pristine NPs, leading to the preferential generation of a Pt-rich shell/Pt–Co alloy core architecture having enhanced ORR activity.

We demonstrate highly enhanced ORR activity and electrochemical stability of Pt/Ir(111) model core-shell catalysts prepared by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns for the surfaces show that Pt grew epitaxially on the clean Ir(111) substrate and the corresponding scanning tunneling microscope images collected in UHV reveal atomically flat terraces with 50–80 nm widths at a substrate temperature of 673 K. In contrast, the corresponding surfaces prepared at a substrate temperature of 303 K show island-like topmost surface structures. The two-monolayer (ML)-thick Pt grown on Ir(111) (Pt2ML/Ir(111)) surfaces, prepared at substrate temperatures of 303 K and 673 K, show ca. 6 and 24 times higher ORR activities than clean Pt(111), respectively. The anomalous activity enhancement for the latter surface prepared at 673 K is probably caused by homogeneous surface strain acting on the Pt shells that is derived from the 2.2% lattice mismatch between the Pt and Ir. The preparation-temperature–dependent ORR activity suggests that the activity can be dominated by the topmost surface and interface structures of the Pt shell–Ir(111) bimetallic system. Furthermore, while the initial ORR activity of pristine surfaces decreases with increasing Pt shell thickness, the stability during room temperature potential cycling between 0.6 and 1.0 V in a 0.1 M HClO4 solution was greatly enhanced above three ML thickness; the Pt4ML/Ir(111) surface prepared at 673 K retained 6.5 times higher ORR activity than Pt(111), even after 5000 potential cycles. The ORR activity and electrochemical stabilities for the Pt/Ir(111) bimetallic surfaces are the highest among the MBE-prepared Pt/M(111) (M = Ir, Pd, Au) systems reported to date. The results obtained in this study show that Pt/Ir core–shell nanostructures are potential candidates for highly active and durable ORR catalysts.

We investigated oxygen reduction reaction (ORR) activity and electrochemical stability of Pt/Pd(111) model electrocatalysts having well-defined topmost and sub-surface structures. The Pt/Pd(111) bimetallic surfaces were prepared by vacuum-deposition of Pt on clean Pd(111) at substrate temperatures of x ( x -Pt y nm /Pd(111), where x = 573 K or 673 K, y = 0.6 or 1.2) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns and UHV scanning tunneling microscopy images revealed that Pt grew epitaxially on the Pd(111) substrate under the aforementioned Pt deposition conditions. High substrate temperatures resulted in thermal diffusion of deposited Pt with substrate Pd atoms. The Pt atomic ratio at the topmost surface of 573K-Pt 0.6nm /Pd(111) is estimated to be 95% using He-ion scattering spectroscopy (ISS), which is 8% greater than that of 673K-Pt 0.6nm /Pd(111). ORR activities of the 573K-Pt 0.6nm /Pd(111) and 673K-Pt 0.6nm /Pd(111) surfaces were 6.3 and 3.6 times higher than that of clean Pt(111), respectively, indicating that the activity is sensitive to the topmost surface Pt atomic ratios. Moreover, electrochemical stability of 573K-Pt 0.6nm /Pd(111) evaluated under potential cycle loadings (0.6 V–1.0 V) is better than that of 673K-Pt 0.6nm /Pd(111). Depth profiles of the surfaces judged by corresponding ISS spectra suggest that the stability stems not only from the topmost Pt atomic ratios but also effective Pt-shell thickness determined by thermal diffusion of Pt and Pd. Furthermore, an increase in Pt-thickness from 0.6 nm to 1.2 nm improved the electrochemical stability: 573K-Pt 1.2nm /Pd(111) retained 5 times more activity vs. clean Pt(111) even after 2000 potential cycles, at which the activity of 573K-Pt 0.6nm /Pd(111) was the same as that for initial activity of clean Pt(111). The results obtained in this study demonstrate that atomic-level structures Pt-Pd bimetallic alloy surfaces determine the ORR activity and electrochemical stability of practical Pd@Pt core–shell catalysts.

Surface strain and electronic interactions (i.e., strain and ligand effects) play key roles in enhancing the oxygen reduction reaction (ORR) catalytic activity of Pt-based alloy catalysts. Herein, we evaluate the ORR activity enhancement factors for Pt(111)-shell layers on Pt25Ni75(111) single-crystal surfaces prepared by molecular beam epitaxy under ultrahigh vacuum (UHV). Scanning tunneling microscopy images of the pristine surfaces collected under UHV revealed periodic surface modulations, known as Moire patterns, suggesting that the topmost Pt(111)-shell layers are compressively strained by the influence of the underlying Ni atoms. The correlation between the ORR activities and estimated strains for 3-ML- and 4-ML-thick Pt shells (where ML represents monolayer), each having −1.7% and −1.2% strained Pt-shells, correspond well to the strain-based theory predictions. On the other hand, a 2-ML-thick Pt shell, with −2.8% strain, exhibits a remarkable ORR activity enhancement, i.e., 25 times higher than the...

Single-nanometer-scale Pt Si fine particles (Pt Si NPs) were synthesized via the arc-plasma deposition of Si on a highly oriented pyrolytic graphite at a fixed substrate temperature of 600 °C, followed by electron-beam deposition of Pt at the temperatures between 100 and 600 °C in ultra-high vacuum. X-ray diffraction patterns of the vacuum-synthesized Pt Si NPs showed that a solid solution of Pt Si was the major component of the particles. Minor diffraction peaks, due to the intermetallic compounds (Pt3Si1 and Pt12Si5), at the Pt-deposition temperature (Tsub-Pt) of 500 °C, were also observed. Scanning tunneling microscopic images exhibited that Pt Si NPs with an average diameter less than 10 nm were dispersed in the substrate at Tsub-Pt temperatures up to 500 °C. The Pt Si NPs synthesized at a Tsub-Pt of 300 and 450 °C showed 1.7 times higher initial mass activity for oxygen reduction reaction (ORR) compared to commercial carbon-supported Pt NPs catalysts and showed better electrochemical stability than pure Pt NPs. These results demonstrate that the arc-plasma deposition of Si NPs, followed by e-beam deposition of metal elements (Pt) in ultra-high vacuum is a new class dry-synthesis for the single-nanometer-scale fine particles of Pt Si for electrocatalysis, e.g. ORR.

The electroreduction of CO2 on well-defined M/Cu(111) (M = Ni and Pt) bimetallic surface systems fabricated using molecular beam epitaxy was studied. The total faradic efficiency for CO2 reduction using one-monolayer (ML)-thick Pt epitaxially grown on a Cu(111) substrate (1-ML Pt/Cu(111)) was nearly the same as that for clean Pt(111). In contrast, the 1-ML-thick Ni/Cu(111) system exhibited increased selectivity for CH4 production compared with that of clean Ni(111), which may stem from the geometric tensile strain induced by the underlying Cu(111) substrate. Notably, bimetallic surfaces consisting of 0.1-ML-thick Ni or Pt grown on Cu(111) exhibited significantly different reduction behaviors compared with those of Cu because of the presence of the a small amount of epitaxially grown metal. For the 0.1-ML-thick Ni/Cu(111) system, the total faradaic efficiency for CO2 reduction and the production rate for CO were enhanced compared with those for clean Cu(111), whereas the production of CH4 decreased. In contrast, the total faradaic efficiency was significantly suppressed for the 0.1-ML-thick Pt/Cu(111) bimetallic substrate, with only a very small amount of CH4 production. The difference in the catalytic properties is attributed to the difference in the adsorption energies for CO, which is an intermediate in the electrochemical production of CH4 and C2H4.

We investigated the electrochemical stability of a Pt/Pd(111) model core–shell structure for oxygen reduction reaction catalysts in 0.1 M HClO4 at 80°C by performing potential cycles (PCs) between 0.6 and x V vs. RHE (x = 0.8–1.0; xV-PCs). Pristine Pt/Pd(111) shows an ORR activity four times higher than that of Pt(111), which considerably decreased after 5000 cycles of 0.9- and 1.0 V-PCs and remained nearly unchanged for the 0.8- and 0.85 V-PCs samples. These results suggest that the atomic-scale structural changes at the Pt/Pd interface that depend on PC windows determine the durability of Pd–Pt core–shell structures.

In this study, we perform ultrahigh vacuum (UHV) and arc-plasma synthesis of strain-controlled Pt(111) model shells on Pt-Co(111) layers with various atomic ratios of Pt/Co and an oxygen reduction reaction (ORR) activity enhancement trend against the surface strain induced by lattice mismatch between the Pt shell and Pt-Co alloy-core interface structures was observed. The results showed that the Pt(111)-shell with 2.0% compressive surface strain vs intrinsic Pt(111) lattice gave rise to a maximum activity enhancement, ca. 13-fold higher activity than that of clean Pt(111). This study clearly demonstrates that the UHV-synthesized, strain-controlled Pt shells furnish useful surface templates for electrocatalysis.