Pt-enriched Layer Research Articles

Channel-flow triple electrode for simultaneous in situ detection of platinum and copper dissolution

• In situ method for monitoring dissolution of Pt–Cu alloys was developed. • Established method enables Pt and Cu detection with high time resolution. • Pt and Cu dissolved independently from Pt–75 at% Cu alloy under potential cycling. • The dissolution behavior of Pt–75 at% Cu was unaffected by the scan rate. A channel-flow triple electrode (CFTE) was constructed for in situ monitoring of the dissolution of Pt–Cu alloys during electrochemical measurements. In the CFTE, Cu 2+ ions dissolved from upstream Cu or Pt–Cu alloy electrodes were quantitatively detected by electrochemical reduction of Cu 2+ to Cu on the downstream detection electrode. The dissolution behaviors of Cu-rich Pt–Cu alloys in 0.5 M H 2 SO 4 during anodic polarization and potential cycling were investigated using CFTE. The level of Pt dissolution was below the detection limit, and Pt could not be detected during the anodic polarization of Pt–Cu alloys. However, the dissolution of both Pt and Cu was detected under potential cycling for a Pt–75 at% Cu alloy (Pt 25 Cu 75 ). The dissolution of Cu as Cu 2+ from Pt 25 Cu 75 occurred continuously at the pit bottoms through the thick Pt-enriched layer at potentials above 0.7 V vs the standard hydrogen electrode during potential cycling. In contrast, Pt dissolved mainly from the Pt-enriched layer. The potential cycling scan rate did not affect the mechanism of Cu dissolution from Pt 25 Cu 75 . The high time-resolution of detection by the CFTE enables dissolution peaks to be distinguished under potential cycling at a high scan rate. The CFTE therefore provides an excellent method for monitoring the dissolution of Pt–Cu alloys.

Selective Dissolution of Palladium and Consequent Enrichment of Platinum in Pt-Pd Alloys

Introduction Polymer electrolyte fuel cells (PEFCs) are expected to be widespread as a clean energy source, and platinum (Pt) alloys are used as a cathode catalyst for low-temperature operation. Pt-palladium (Pd) alloy catalysts show higher oxygen reduction reaction activity than Pt catalysts.1 Furthermore, Pd exhibits a higher corrosion resistance than less noble metals such as cobalt, nickel, and cupper under PEFCs operating conditions. Thus, the durability of Pt-Pd alloy catalysts is expected to be greater than that of the other alloys. In this study, dissolution behavior of Pt-Pd alloys under potential cycling simulating PEFCs operating conditions was investigated. Experimental Single-phase and polycrystalline Pt-50at.% Pd (Pt-50Pd) and Pt-75Pd were prepared by arc-melting. A total of 1,000 cycles of potential cycling test (CV) was conducted with the potential range of 0.05-1.4 V vs. SHE, simulating start-up/shut-down cycles. The test solution (0.5 M H2SO4) was sampled and replaced every 100 cycles, and the amount of dissolved Pd and Pt from the alloys was quantified using an inductively coupled plasma-mass spectrometer (ICP-MS). After the CV, the outermost surface of the alloys was analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Results and Discussion The CV of Pt-50Pd consists of hydrogen (H) adsorption/desorption peaks, oxide formation/reduction peaks. These features are similar to the CV features of pure Pt, indicating that the Pt-enriched layer formed on the alloy surface after the 1,000th cycles of CV. On the other hand, the peak positions of the oxide formation/reduction of Pt-75Pd are in good agreement with those of pure Pt. However, H absorption peak was observed in the CV of Pt-75Pd. Thus, Pd atoms were partially exposed to the surface after the CV.The amount of dissolved Pt from Pt-50Pd and Pt-75Pd was almost similar that from pure Pt. In contrast, the amount of dissolved Pd from Pt-50Pd and Pt-75Pd was about 80-90% smaller than that from pure Pd. When the amount of dissolved Pt was compared with that of Pd of Pt-50Pd and Pt-75Pd during the last 100 cycles during CV, the ratio of dissolved Pt to Pd was 1:1.7 and 1:4.6, respectively. Hence, Pd selectively dissolved from Pt-Pd alloy, which confirmed that the Pt-enriched layer formation on the surfaces. The outermost surface analysis by TOF-SIMS also supported the Pt-enrichment. The dissolution rate of Cu and Pd from Pt-75Cu and Pt-75Pd during 100 cycles of CV were 3300 and 3.6 μg cm- 2, respectively, indicating that the selective dissolution rate of Pd is extremely small when the Pd concentration in the alloy is above parting limit.2 Based on these experimental results, the structures of Pt-enriched layer formed on Pt-50Pd and Pt-75Pd as a result of the selective dissolution of Pd might be different. References J. Zhang, M.B. Vukmirovic, Y. Xu, M. Mavrikakis, R.R Adzic, Angew. Chem. Int. Ed., 44(14) 2132 (2005)M. Artymowicz, J. Erlebacher, R.C. Newman, Philos. Mag., 89 1663 (2009).

Dissolution and the Consequent Surface Enrichment of Platinum in Pt–Cu Binary Alloys under Potential Cycling

In this study, electrochemistry was combined with mass spectroscopy and surface analysis to investigate the selective dissolution of Cu and the consequent surface enrichment of Pt in Pt–Cu binary alloys. Two different compositions of Pt–50at% Cu (Pt–50Cu) and Pt–75at% Cu (Pt–75Cu) underwent potential cycling. Cu from both alloys was selectively dissolved, although trivial Pt dissolution occurred, causing the subsequent formation of a Pt-enriched layer on the alloys’ surfaces. The morphology of the layer was quite different between the two alloys. The Pt-enriched layer formed on the Pt–50Cu surface was extremely thin and compact, further suppressing Cu dissolution from the alloy in the later stage of potential cycling. The Pt-enriched layer formed on the Pt–75Cu surface contained numerous pits, mainly because a lot of Cu was dissolved from Pt–75Cu during potential cycling. Thus, Cu continued to dissolve from the bottom of the pits at the later stage, causing the thickness of the Pt-enriched layer to gradually grow with an increase in the cycle number. The correlation between Pt and Cu dissolution and surface morphological changes of Pt–Cu alloys was discussed.

Dissolution Behavior and Changes in the Surface Morphology of Pt-Cu Nanoparticles

Platinum (Pt) nanoparticles is used as cathode catalyst in polymer electrolyte fuel cell (PEFC) to enhance oxygen reduction reaction, however high cost of Pt is preventing popularization of PEFC. Therefore, Pt-transition metal (M) alloy is expected to be used as cathode catalyst instead of Pt in order to lower the amount of Pt usage in PEFC. Both Pt and M dissolution under PEFC operating condition is one of the biggest problem, because their dissolutions lead to performance loss of PEFC. In this study, we selected Cu as additive elements, and the dissolutions and the consequent surface morphological changes of Pt-Cu nanoparticles were investigated by inductively coupled plasma mass spectroscopy (ICP-MS), and field emission-scanning electron microscopy (FE-SEM). Plating bath was prepared by mixing 6 mM K2PtCl4, 6 mM CuSO4·5H2O, and 0.5 M Na2SO4, and pH of the bath was adjusted to 2.0 by adding H2SO4. Pt-Cu nanoparticles were pulse-electroplated on glassy carbon substrate in this bath. Electroplated nanoparticles were fully dissolved by aqua regia, and then tested aqua regia was analyzed with ICP-MS for determining chemical composition of the nanoparticles. Chemical composition was also investigated by using FE-SEM-energy dispersive X-ray spectroscopy, and was determined to be Pt-35 at. % Cu (Pt-35Cu). Pt-35Cu was subjected to potential cycling in 0.5 M H2SO4 at 298 K. A total of 5,000 cycles of potential cycling were applied between 0.05 V and 1.4 V vs. standard hydrogen electrode at 100 mV s-1. Potential cycling was interrupted at the 50th, 200th, and 500th cycles, and every 500 cycles thereafter, and the test solutions were sampled for quantitative ICP-MS analysis of both Pt and Cu ions dissolved from Pt-35Cu. Furthermore, surface morphologies before and after potential cycling were observed by FE-SEM at identical location. Before potential cycling, electroplated Pt-35Cu was composed of numerous Pt-Cu nuclei (less than 3 nm in diameter) that agglomerated to form larger secondary particles (30–50 nm). The surfaces of Pt-35Cu were rapidly flattened with increasing cycle number. After 1,000 cycles of potential cycling, the fine structure of primary nuclei completely vanished and reconstructed to form flattened surface. During this potential cycling, both Pt and Cu dissolved from Pt-35Cu, and the amount of dissolved Cu ions was much larger than the amount of dissolved Pt ions. Especially during the initial potential cycling, Cu was selectively dissolved from Pt-35Cu, and Pt-enriched layer was consequently formed on its surface. Cu dissolution was thereafter suppressed by Pt-enriched layer formation. Even the large Cu dissolution in the initial stage doesn’t affect to surface morphology change. Accordingly, surface morphology change during potential cycling is not likely due to Cu dissolution, but is likely due to Pt surface diffusion and/or Pt dissolution and re-deposition. The authors acknowledge Ookayama Materials Analysis Division, Tokyo Institute of Technology, for assistance with the FE-SEM, and ICP-MS analysis.

Formation of Pt Skin Layer on Ordered and Disordered Pt-Co Alloys and Corrosion Resistance in Sulfuric Acid

The formation of the Pt-enriched layer (Pt skin layer) formed at the surface of the ordered and disordered Pt-Co alloy specimens by a dealloying treatment and its corrosion resistance under potential cycling in sulfuric acid were examined to clarify the dissolution behavior of the Pt skin layer in polymer electrolyte fuel cell-operating conditions. The ordered and disordered Pt-Co specimens were obtained by heat treatment at 1073 and 1473 K, respectively. Co at the alloy surfaces dramatically dissolved in an early phase of the dealloying treatment in naturally aerated 0.5 M H2SO4 at 298 K, and Pt skin layers were formed. Pt skin layers ca. 2 monolayer in thickness were formed on the Pt-Co alloy specimens by the dealloying treatment in 0.5 M H2SO4. The cyclic voltammetry measurements of the Pt-Co specimens showed the existence of Pt skin layers after the dealloying treatment and the inhibition of the Pt oxide formation on the Pt skin layers. The Pt oxide formation for ordered Pt-Co was more suppressed than that for the disordered Pt-Co. The Pt skin layers on the Pt-Co specimens exhibited a higher corrosion resistance than pure Pt, and the dissolution of the Pt skin layer for ordered Pt-Co was more inhibited than that for disordered Pt-Co under potential cycling in the potential range of 0.6–1.4 V in 0.5 M H2SO4 (dissolution test). A thin, continuous Pt skin layer remained at the surface of the ordered Pt-Co specimen after the dissolution test. The formation of a uniform Pt skin layer seems to provide high oxidation resistance to the surface, leading to high corrosion resistance.Graphical The dissolution of Pt skin layer for the ordered Pt-Co was highly suppressed compared with that for the disordered Pt-Co and pure Pt. The suppression of Pt-O formation seems to be the main reason for the high corrosion resistance. The formation of a thin and continuous Pt skin layer is important to prevent the dissolution of Pt.

First-principles calculations on Fe-Pt nanoclusters of various morphologies

Bimetallic FePt nanoparticles with L10 structure are attracting a lot of attention due to their high magnetocrystalline anisotropy and high coercivity what makes them potential material for storage of ultra-high density magnetic data. FePt nanoclusters are considered also as nanocatalysts for growth of carbon nanotubes of different chiralities. Using the DFT-LCAO CRYSTAL14 code, we have performed large-scale spin-polarized calculations on 19 different polyhedral structures of FePt nanoparticles in order to estimate which icosahedral or hcp-structured morphology is the energetically more preferable. Surface energy calculations of all aforementioned nanoparticles indicate that the global minimum corresponds to the nanocluster possessing the icosahedron “onion-like” structure and Fe43Pt104 morphology where the outer layer consists of Pt atoms. The presence of the Pt-enriched layer around FePt core explains high oxidation resistance and environmental stability, both observed experimentally.

Dissolution and the Consequent Surface Morphology of Pt-Cu Binary Alloys Under Potential Cycling

Polymer electrolyte fuel cell (PEFC) is regarded as clean energy conversion system, however its high cost prohibits the progress of PEFC. Thus, a reduction in the amount of Pt used as cathode catalyst is required. In order to overcome this problem, Pt transition-metal (Pt-M) alloy catalysts have been developed instead of Pt catalysts, however a deterioration of Pt-M alloy catalysts is concerned because both Pt and M are dissolved from Pt-M alloy catalysts under PEFC operating conditions. In this study, potential cycling tests and inductively coupled plasma mass spectroscopy (ICP-MS), field emission-scanning electron microscopy (FE-SEM), and field emission auger electron spectroscopy (FE-AES) were combined, and dissolution behavior of Pt-Cu alloys has been investigated under potential cycling by using these techniques. Pt50-Cu50 (Pt-50 at% Cu) and Pt25-Cu75 were fabricated by arc-melting under an Ar atmosphere. These alloys were subjected to potential cycling tests at 298 K in 0.5 M H2SO4 solution with a two-compartment stationary cell. A double junction silver / silver-chloride electrode was used as reference electrode, and Au coil was used as the counter electrode. A total of 1,000 cycles of potential cycling tests were employed between 0.05 V and 1.4 V vs. SHE at 100 mV s-1. Tests was interrupted every 100 cycles, and the test solution was sampled for quantitative ICP-MS analysis of both Pt and Cu ions dissolved in it. In addition, surface structure and depth profile after potential cycling tests were evaluated by FE-SEM and FE-AES, respectively. FE-AES depth profiles before and after 100 cycle potential cycling confirmed that Pt-enriched layer formed on Pt25-Cu75 surface after potential cycling. Moreover, the amount of dissolved Pt and Cu ions during 1st sequential order of 100-cycles-potential cycling were 1 monolayer (ML) and 2370 ML, respectively. Hence, Pt-enriched layer formed by the large amount of Cu selectively dissolved from Pt25-Cu75. FE-SEM image of Pt25-Cu75 after 100 cycle potential cycling showed numerous pits (d pit = 2 – 5 nm) formed on its surface, and the large amount of Cu dissolution occurs from 2nd to 10th sequential order of 100-cycles-potential cycling. Thus, selective dissolution of Cu seems not to be suppressed in the latter cycles because Cu dissolution continuously occurs through the pits. On the other hand, FE-SEM image of Pt50-Cu50 after 100 cycle potential cycling was extremely flat. Accordingly, dense Pt-enriched layer formed on its surface, and it suppressed further Cu dissolution from the alloy. The authors acknowledge Ookayama Materials Analysis Division, Tokyo Institute of Technology, for assistance with the ICP-MS analysis.

Study on Dissolution Behavior of Pt-M Binary Alloys Under Potential Cycling By Channel Flow Multi Electrodes

Introduction Pt-M alloys catalysts have attracted attention over the years and expected to be used in proton exchange membrane fuel cells (PEMFCs) cathode catalysts due to their high oxygen reduction activity than pure Pt and reduction of Pt usage in PEMFCs [1]. However, Pt-M catalysts is easily degraded by both Pt and M dissolution from Pt-M alloys during on-off cycle and load cycle, which leads the performance loss of PEMFCs [2]. Thus, to understand the nature of dissolution mechanism of both Pt and M from Pt–M alloy catalysts is very significant to advance durability of PEMFCs under its operating conditions. In this study, we investigated that the dissolution mechanism of Pt-50 at% Fe alloy (Pt50-Fe50) and Pt50-Cu50 under potential cycling with a channel flow multi electrodes (CFME), and elucidate the effect of additive element to the dissolution behavior of Pt50-M50. Experimental Pt50-Fe50 and Pt50-Cu50 were subjected to potential cycling tests at 298 K using Ar-purged 0.5 M H2SO4 solution with a CFME. A double junction KCl-saturated Ag / Ag-Cl electrode was used as reference electrode, and Au coil was used as the counter electrode. Flow rate of the electrolyte was 10 cm s-1 in order to establish laminar flow condition. Potential cycling tests were employed between 0.05 V and 1.4 V vs. SHE at 20 mV s-1. Before potential cycling, Pt50-M50 was set at 0.45 V vs. SHE in order to remove Pt oxide layer formed on Pt–M alloy surface in ambient air and to measure baseline currents of collector electrodes (CEs). Reactions and potentials for detecting the dissolved Fe2+, Fe3+, and Cu2+ on Au-CEs are as follows, Fe2+ → Fe3+ + e- (E c = 1.0 V vs. SHE) (1) Fe3+ + e-→ Fe2+ (E c = 0 V vs. SHE) (2) Cu2+ + 2e-→ Cu (E c = 0 V vs. SHE) (3) Dissolved Fe2+, Fe3+, and Cu2+ from Pt50-M50 are monitored by the current change of CEs. Results and discussion Figure 1 shows M detection current with a CFME during potential cycling, and red and blue line show the results of Pt50-Fe50 and Pt50-Cu50, respectively. As shown in inset, Fe dissolution from Pt50-Fe50 starts from around 0.3 V in anodic scan. Then Fe dissolution is clearly enhanced between 0.6 and 1.4 V in both anodic and cathodic scan, and finally terminate at 0.3 V in cathodic scan. Wang et al reported that Pt dissolution occur above approximately 0.6 V [3], accordingly major Fe dissolution is enhanced by Pt dissolution. On the contrary, minor Fe dissolution between 0.3 and 0.6 V in both anodic and cathodic scan is contributed by Pt surface diffusion, because Pt atoms are easily diffuse on Pt-M alloy surface in this potential range due to no adsorbed species on its surface [4]. Cu dissolution from Pt50-Cu50 only appears above 0.6 V, whereas Fe dissolution from Pt50-Fe50 starts from 0.3 V. Moreover, the amount of Cu dissolved from Pt50-Cu50 is smaller than that from Pt50-Fe50, which indicate that Cu is more durable elements than Fe. This is explained by the difference of standard electrode potentials between Cu and Fe. Pt-enriched layer formed on Pt50-Cu50 is denser and flatter than that on Pt50-Fe50, because the amount of dissolved Cu is small due to more noble standard electrode potential than Fe. Flat Pt-enriched layer inhibits Pt surface diffusion between 0.3 and 0.6 V, and Pt dissolution above 0.6 V. Reference [1] V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer, C. A.Lucas, G. Wang, P. N. Ross, and N. M. Markovic: Nat. Mater. 6 (2007) 241–247. [2] H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner: Appl. Catal. B: Environ. 56 (2005) 9–35. [3] Z. Wang, E. Tada, and A. Nishikata: J. Electrochem. Soc. 161 (4) (2014) F380–F385 [4] Q. Xu, E. Kreidler, and T. He: Electrochim. Acta 55 (2010) 7551–7557. Figure 1

Effect of Potential on Dissolution of Pt-Fe Alloys Studied By Channel Flow Multi Electrode

Introduction Core/shell structure Pt-M alloys catalysts are anticipated to be applied in PEMFCs cathode catalyst layer owing to their high catalytic activity than pure Pt [1]. However, dissolution of Pt and M is a significant problem, which causes the performance loss of PEMFCs [2], when the potential loads such as on-off cycle and load cycle were applied to the cathode catalysts. Therefore, understanding the detailed dissolution mechanism of Pt and M from Pt–M alloy catalysts is very important to improve durability of PEMFCs under its operating conditions. In this study, we investigated that the dissolution behavior of Pt-50 at% Fe alloy (Pt50-Fe50) under potential cycling with channel flow multi electrodes (CFME), and clarify the effect of potential range to the dissolution behavior of Pt50-Fe50. Experimental Pt50-Fe50 (1 mm x 5 mm) used as working electrode (WE), and two Au plates (1 mm x 2 mm) were used as collector electrodes (CEs). These electrodes were embedded into CFME cell by epoxy resin, and kept in parallel. The gap distance between WE and CEs was 100 μm, and the gap between two CEs was 1 mm. Potential cycling tests were employed at 298 K using Ar-purged 0.5 M H2SO4 solution. A double junction KCl-saturated Ag / Ag-Cl electrode was used as reference electrode, and Au coil was used as the counter electrode. In order to satisfy laminar flow condition, flow rate of the test solution was 10 cm s-1. In potential cycling tests, lower potential limit set at 0.05 V vs. SHE, and upper potential limit (E upper) varied from 0.6 to 1.4 V vs. SHE, and scan rate was 20 mV s-1. Before the potential cycling tests, the W.E. was kept at 0.45 V vs. SHE in order to eliminate Pt-oxide layer formed on alloy surface in ambient air. A reaction and potential for detecting the dissolved Fe2+ on Au-CE is as follows, Fe2+→Fe3++e- (E c = 1.0 V vs. SHE) (1) Dissolved Fe2+ from Pt50-Fe50 are monitored by the current of the CE. Results and discussion Fig. 1 shows Fe2+detection current (i Fe(II)) change in anodic scan during potential cycling, and black, red, and blue lines represent results of E upper = 1.4 V, 1.0 V, and 0.6 V, respectively. In case of E upper = 1.4 V, Fe2+ detection peak was observed between 0.3 – 0.7 V. Similarly, Fe2+ detection peak was appeared in same region when E upper set at 1.0 V, however, the peak height was much smaller compared with the peak observed when E upper =1.4 V. From these results, Fe dissolution as Fe2+ was not suppressed during potential cycling when E upper was 1.0 and 1.4 V. On the contrary, i Fe(II) kept constant value between 0.05 – 0.6 V when E upper set at 0.6 V, and it means that Fe dissolution as Fe2+ was suppressed during potential cycling. The similar trend was observed in the cathodic scan. The dissolution behavior of Pt50-Fe50 strongly depended on E upper of potential cycling. This may be attributed to the structure of Pt-enriched layer formed on alloy surface. In case of E upper = 0.6 V, potential cycling doesn’t induce Pt dissolution [3], thus dense Pt-enriched layer formed on alloy surface and Fe dissolution is suppressed by Pt-enriched layer. While, in case of higher E upper like 1.0 and 1.4 V, Pt-enriched layer seems to be less effective. Figure 1

Selective Dissolution Behavior of Fe from Pt-Fe Binary Alloy in Sulfuric Acid by CFDE

Introduction Proton exchange membrane fuel cells (PEMFCs) is developed as a clean energy conversion device to overcome recent environmental problems. Core-shell structured Pt-M alloys are expected to be used in PEMFCs cathode due to their high catalytic activity than pure Pt1). However, many researchers reported performance loss of catalysts after PEMFCs long-run operation2), and selective dissolution of less noble metal (M) is one of the major issue of it3). Selective dissolution is inevitable for Pt-M alloy catalysts, thus the perception about dissolution rate of M is significantly important for catalyst application to PEMFCs. Hence, in this study, we investigated the dissolution behavior of Fe by in-situ channel flow double electrode (CFDE) system. Experimental Pt-50 at% Fe alloy (Pt50-Fe50) was made by arc-melting under Ar atmosphere. The specimen was vacuum-encapsulated in a silica glass tube at 2.0 x 10-5 torr, and homogenized at 1473 K for 1 week, and cooled in furnace. After heat treatment, Pt50-Fe50was cut into 1 mm x 5 mm, and used as working electrode (W.E.). Two Au plates whose geometric area are 1 mm x 2 mm was used as collector electrodes (C.E.). These electrodes are embedded into epoxy resin, and kept in parallel. The gap between W.E. and C.E. is 100 mm, and between two C.E. is 1 mm.Potentiostatic polarization measurements were employed at 298 K using Ar-purged 0.5 M H2SO4 solution. A double junction KCl-saturated Ag / Ag-Cl electrode was used as reference electrode and Au wire was used as the counter electrode. Flow rate of the test solution was 10 cm s-1. In potentiostatic polarization, W.E. (E w) was polarized at 1.0, 1.2, and 1.4 V vs. SHE. Before the polarization, the W.E. was kept at 0.45 V vs. SHE in order to eliminate Pt-oxide layer on alloy surface. A reaction and potential for detecting the dissolved Fe2+ and Fe3+on Au C.E. are as follows,Fe2+→Fe3++e- (E c= 1.0 V vs. SHE) (1)Fe3++e-→Fe2+ (E c= 0 V vs. SHE) (2)Dissolved Fe2+ and Fe3+ from Pt50-Fe50are monitored by the current of the C.E. Results and discussion Fig. 1 shows the changes of the W.E. current i w and C.E. current i c at E c = 1.4 and 0 V when the W.E was polarized at E w = 1.4 V. The i w immediately increased by changing W.E. potential from 0.45 V to 1.4 V, and then gradually deceased to a constant value. The i c at E c = 1.0 V for detecting the dissolution of Fe2+ does not respond to the i w change on W.E. On the other hand, the i c at E c = 0 V for Fe3+ shows similar trend to the i w change. As can be seen in Fig. 1(c), the selective dissolution of Fe3+ occurs just after the polarization and afterwards approaches the base current (approximately 200 nA ). This means that the surface of the Pt50-Fe50is quickly covered with a Pt-enriched layer. Selective dissolution mechanism of Fe from Pt-Fe alloy will be further discussed by using these CFDE results in the session. Reference [1] T. Toda, H. Igarashi, H. Uchida, and M. Watanabe, J. Electrochem. Soc. 146(1999) 3750.[2] H.A. Gasteiger, S.S. Kocha, B. Sompalli, and F.T.Wagner, Appl. Catal. B. Environ. 56(2005) 9.[3] S. Mukerjee, and S. Srinivasan, Handbook of Fuel Cells, vol2 (2003) 502.

Selective Dissolution of Pt–Co Binary Alloys and Surface Enrichment of Platinum in Sulfuric Acid Solution

The selective dissolution of cobalt and the consequent surface enrichment of platinum in Pt­Co binary alloys immersed in 0.5mmol·m13 H2SO4 at 298K were investigated. Four different Pt­Co alloy thin films with various Pt contents, deposited on glassy carbon sheets by physical vapor deposition, were investigated: 51 at% Pt (Pt51­Co49), 43 at% Pt (Pt43­Co57), 28 at% Pt (Pt28­Co72), and 24 at% Pt (Pt24­Co76). When the thin films were immersed for 24 h in a 0.5mmol·m13 H2SO4 solution, a Pt-enriched layer was formed on the surface of all the alloys due to Coselective dissolution. The enriched layer formed in the higher-Pt alloys (Pt51­Co49 and Pt43­Co57) significantly suppressed further dissolution, whereas the lower-Pt alloys (Pt28­Co72 and Pt24­Co76) exhibited more extensive selective dissolution and roughened surfaces. In this study, the selective dissolution and surface morphology of the Pt­Co alloys are discussed on the basis of these experimental results. [doi:10.2320/matertrans.M2014107]

Selective dissolution of binary Pt–Co alloys of different compositions in sulphuric acid solution

The dissolution of Pt–Co alloys in a 0.5M H2SO4 solution has been studied under immersion and potential cycling conditions. Under immersion conditions, the Pt-enriched layer formed on Pt50–Co50 and Pt40–Co60 alloys suppresses further dissolution of Co. Potential cycling between 0 and 1.4V vs. standard hydrogen electrode enhances the selective dissolution, but is suppressed in the initial stage. In contrast, the amount of selective dissolution of Co from Pt30–Co70 increases linearly with time during the immersion. Potential cycling significantly accelerates the selective dissolution of Pt30–Co70. The selective dissolution mechanism of Pt–Co alloys is discussed in this paper.

Effect of potential cycling on dissolution of equimolar Pt–M (M: Co, Ni, Fe) alloys in sulfuric acid solution

Dissolution of equimolar Pt–M (M: Co, Ni, Fe) alloys covered with a Pt-enriched layer on the surface has been studied under potential cycling in 0.5M H2SO4 solution at 25°C. The Pt-enriched layer was formed on the surface of Pt–M alloys by a selective dissolution treatment of M under immersion condition in the solution. The amounts of Pt and M dissolved under potential cycling with various combinations of upper potential limit Eupper and lower potential limit Elower were determined with an inductively coupled plasma mass spectrometry (ICP-MS). During the selective dissolution treatment, rest potential Erest of the Pt–M alloys approaches to that of pure Pt, indicating the enrichment of Pt on the surface. The Pt-enriched layer strongly suppresses further selective dissolution of M under the immersion condition. However, when the Pt–M alloys are exposed under potential cycling in the potential ranges of Eupper>Erest (≈1.0V vs. SHE) and Elower<0.5V, the selective dissolution of M takes place again, although it is suppressed at the initial stage of the potential cycling by formation of the thicker Pt-enriched layer.

Dissolution of Pt–M (M: Cu, Co, Ni, Fe) binary alloys in sulfuric acid solution

The dissolution behavior of equimolar Pt–M (M: Cu, Co, Ni, Fe) alloys has been studied under conditions of immersion, potentiostatic polarization, and potential cycling in 0.5 M H 2SO 4 solution at 25 °C. The quantity of dissolved ions under these conditions has been determined with inductively coupled plasma mass spectrometry (ICP-MS). In 3-h immersion tests, selective dissolution of M atoms occurs immediately after immersion and is quickly suppressed. The rest potential shifts rapidly in the noble direction and approaches that for pure Pt. The general features of cyclic voltammograms of the alloys are very similar to those for pure Pt, although the current is considerably larger for the Pt–Fe alloy. These results indicate that the surfaces of these alloys are covered with a Pt-enriched layer because of preferential dissolution of M atoms. The resulting suppression of dissolution is reversed by potentiostatic polarization at 1.4 V. The enhancement at 1.4 V is more remarkable under the condition of potential cycling.

Changes of the Pt 3Co(100) surface stoichiometry with NO adsorption

An ordered alloy surface of Pt 3Co(100) has been studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and temperature programmed reaction (TPR). The clean surface exhibited a c(2 × 2) LEED structure, when the sample was carefully annealed at temperatures lower than the critical temperature for order-disorder transition of the bulk. With NO adsorption, the surface structure was reconstructed to a (1 × 1) LEED structure and was changed back to the c(2 × 2) structure by reaction with hydrogen at 500 K. From the results of LEED and AES studies combined with the H 2 + D 2 → 2HD reaction, it was deduced that the reconstructed (1 × 1) surface is terminated by a Pt-enriched layer, and that the clean c(2 × 2) surface reveals an ordered PtCo outmost layer. Furthermore, compared with the reaction of N 2O and the adsorption of nitrogen activated by high frequency discharge, it was shown that the surface reconstruction originated from adsorbed surface nitrogen and not from surface oxygen.

Surface enrichment of Pt 10Rh 90(111) : I. Annealing in vacuum and low pressure environments

The variation in the surface composition of a model catalyst surface, a Pt 10Rh 90(111) single crystal, has been studied as a function of temperature and gas phase composition using ion scattering spectroscopy (ISS) with Ne 20, Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). As the single crystal is heated in vacuum, the surface composition of the first layer of this alloy becomes enriched in Pt beginning at 600°C, reaching a surface composition of 30% Pt at 1000°C, in general agreement with previously reported findings. The equilibrated surface compositions measured by ISS and AES were very similar. Depth profiling shows the Pt-enriched layer is of atomic thickness on the surface. A comparison of the ISS and AES data suggests that the sub-surface layer is enriched in Rh, in agreement with the oscillatory behavior in the composition of near-surface layers reported by other researchers. In low pressure reducing conditions (1 × 10 −6 Torr H 2), Pt enrichment measured by ISS on the surface approaches 40%. In low pressure oxidizing conditions (1 × 10 −6 Torr O 2), the surface layer equilibrates near, but above the bulk composition in Pt concentration, but no oxidation of the metal was observed.

The segregation in single crystal Ni−Pt alloys: A model for multilayer segregation

Equilibrated (111) surfaces of Ni−Pt single crystal alloys terminate with a Pt-enriched layer. A multilayer segregation model including a Pt−Pt bond strengthening accounts well for the observed results.