Platinum-Enriched Ni/Pt(111) Surfaces Prepared by Molecular Beam Epitaxy: Oxygen Reduction Reaction Activity and Stability

Abstract

Oxygen reduction reaction (ORR) activities and electrochemical stabilities were evaluated for Ni/Pt(111) model electrode-catalysts fabricated by molecular beam epitaxy (MBE). Exposure of clean Pt(111) to 1.0-Langmuir (i.e., fractional coverage, ª = 1.0 in the Langmuir isotherm) CO at 300 K produced linear-bonded and bridge-bonded COPt IR bands at 2093 and 1858 cm ¹1. In contrast, 3.0-nm-thick Ni deposition onto Pt(111) at 823 K (823 K-Ni 3.0nm /Pt(111)) showed broad IR bands for adsorbed CO at around 2064 cm ¹1 ; the separation of reflection high-energy electron diffraction (RHEED) streaks of the 823 K-Ni 3.0nm /Pt(111) is greater than for clean Pt(111). In contrast, 923 KNi 3.0nm /Pt(111) yielded a single sharp IR band because of linear-bonded CO at 2080 cm ¹1, and the separation of the RHEED streaks is almost the same as that for the Pt(111). The results suggest that a Pt-enriched topmost surface is generated above 923 K through surface segregation of the substrate Pt atoms during the Ni deposition. After transferring the sample from ultra-high vacuum to an electrochemical system, without being exposed to air, changes in ORR activities of the MBE-prepared 923 K-Ni 3.0nm /Pt(111) were evaluated in a 0.1-mol L ¹1 HClO 4 aqueous solution under applied potential cycles of between 0.6 and 1.0 V vs. RHE. The ORR activity enhancement factor vs. clean Pt(111) for the asprepared 923 K-Ni 3.0nm /Pt(111) was estimated to be twelve, and this factor was reduced to four after the application of 1000 potential cycles. However, a 943 K-Ni 3.0nm /Pt(111) sample, which showed a relatively low ORR activity (an enhancement factor of eight), had the activity enhancement factor of six even after the application of 1000 potential cycles. These results reveal that the sub-surface structures and composition of Pt-based alloys determine not only initial ORR activity but also electrochemical durability of the ORR catalysts.

Electrochemical Properties of Pt Epitaxial Layers Formed on Pd(111) in Ultra-High Vacuum

“…Considering the CV and LSV results obtained in this study (Pt/Pd(111)), the shrinkage in Q H and a positive shift in the onset potential of hydroxyl species are common CV characteristics for highly active Pt-M(111) bimetallic surfaces. Because lattice mismatch between Pt and Pd is less than 1%, the much smaller changes in Q H and the positive shift in the hydroxyl species of Pt 1.2nm /Pd(111) relative to those of Ni or Co 26,29,31,32,50 might result from the relatively low compressive strains of the topmost Pt(111) lattice.…”

“…z E-mail: n-todoroki@material.tohoku.ac.jp EC conditions were discussed for the Ni/Pt(111), [27][28][29][30][31] Co/Pt(hkl), 32,33 and Pt/Au(hkl) bimetallic surfaces. 34,35 In this study, we focused on the ORR activities of the Pt/Pd(111) bimetallic surfaces prepared through vacuum depositions of Pt on a Pd(111) substrate in UHV.…”

Microscopic surface structures and ORR activities for vacuum-deposited Pt/Ni/Pt(111) and Pt/Ni/Pt(110) sandwich structures

“…Briefly, the UHVprepared Pt/Ni/Pt atomic sandwich structures were transferred from UHV into a N 2 -purged stainless-steel glove box without air exposure [41]. EC measurements were conducted under ambient N 2 (1 atm) using a glass cell that included a platinum counter electrode and a reversible hydrogen electrode (RHE).…”

“…As an example, potential sweeps up to the standard electrode potentials of the less-noble metal M and potential cycling across the standard electrode potentials of M atomically modify the topmost bimetallic surface structures and thereby provide information related to the durability of Pt-M catalysts. From these viewpoints, we have effectively used molecular beam epitaxy (MBE) and ultra-high-vacuum (UHV) techniques to prepare model electrocatalyst surfaces for ORR and have investigated the initial ORR activities of the prepared bimetallic surfaces of Ni/Pt(1 1 1) [39][40][41][42], Co/Pt(h k l) [43], and Pt/ Au(hkl) [44], and their ORR responses under specific EC conditions.…”

(Invited) Electrochemical Properties for UHV-Prepared Pt/M Well-Defined Single Crystal Surfaces

“…From these points of view, we have demonstrated the usefulness of molecular beam epitaxy (MBE) techniques for preparing well-defined Pt/M model catalyst surfaces. We have reported not only the initial ORR activities of the surfaces but also the EC response of the Pt/Ni, (18)(19)(20)(21) Pt/Co, (22,23) and Pt/Au (24,25) bimetallic alloy surfaces under specific conditions. The UHV-prepared model Pt shells were transferred to a N 2 -purged glove box without air exposure.…”