On-Line Mass Spectrometry for Ethanol Oxidation on Well-Defined Sn/Pt(hkl) Electrode Surfaces

Abstract

Introduction Direct ethanol fuel cells (DEFCs) have attracted attention for their practical high energy density and low environmental burden. To generate electricity with high efficiency, ethanol needs to be completely oxidized to CO2. To date, Xia et al. have reported that the ethanol oxidation reaction (EOR) is sensitive to the topmost surface atomic arrangements of Pt [1]. Furthermore, Pt-based alloy catalysts have been studied extensively for EOR with low overpotentials. Therefore, it is important to clarify relations between EOR activity and topmost surface structures of Pt-based alloys to develop practical Pt-based alloy catalysts for DEFCs. In this study, we focused on Sn/Pt catalysts which have been studied well as alloy catalysts for EOR. On-line electrochemical mass spectrometry (OLEMS) should be effective for in-situ analysis of gaseous reaction products. Therefore, the relations of EOR onset potentials, potential-dependent gaseous products, and catalyst surface structures are investigated for well-defined Sn/Pt(hkl) surfaces prepared by vacuum depositions of Sn on Pt(hkl) single crystal substrates. Experimental Sample fabrication processes of the well-defined Sn/Pt(hkl) were conducted in ultra-high vacuum (UHV). Pt single crystal substrates (Pt(hkl): Pt(111), (110) and (100)) were cleaned by Ar+ sputtering and subsequent annealing at 1273 K in UHV. 0.1 nm-thick Sn was deposited onto the cleaned Pt(hkl) by an electron-beam evaporation method at room temperature, followed by annealing at 1000 K to flatten the topmost surfaces. The UHV-prepared Pt(hkl) and Sn/Pt(hkl) were transferred from UHV chambers to the electrochemical system set in a N2-purged glove box without being exposed to air. Cyclic voltammetry (CV) of the surfaces was conducted in N2-purged 0.1 M HClO4. Subsequently, ethanol was added to the solution to prepare a 2 M solution of ethanol in 0.1 M HClO4, and then CV and OLEMS measurements were performed. Results and Discussion The anodic currents of EOR for the Pt(hkl) and Sn/Pt(hkl) surfaces recorded in ethanol added HClO4 solutions are presented in Fig.1 (a). EOR onset potentials of the Sn/Pt(hkl) surfaces (solid curves) shift negatively in comparison to the corresponding Pt(hkl) surfaces (dashed curves). The negative potential shifts suggest that the Sn/Pt(hkl) surfaces exhibit higher EOR activity than the corresponding Pt(hkl) surfaces, probably because of the ensemble effect of the surface Pt and Sn atoms [2]. Fig.1 (b) shows the OLEMS results of the Sn/Pt(hkl) surfaces. The onset potentials of CO2 evolution are summarized in Table.1. Fig.1 (b) and Table.1 suggest that potential sweeps in the anodic direction (0.2 V to 0.9 V) cause CO2 evolution, and that the EOR onset potentials depend on the substrate atomic arrangements of Pt. In particular, the Sn/Pt(110) shows the lowest EOR onset potentials, as judged by EOR currents (Fig.1 (a)) and CO2 evolution (Fig.1 (b)). The results can be attributed to the effectiveness of Pt(110) steps against ethanol C-C bond cleavage [3] and the ensemble effect of the surface Pt and Sn atoms against CO poisoning of the Pt atoms [2]. Acknowledgement This work was partly supported by a Grant-in-Aid for scientific research (A) from the Japan society for the promotion of science (T. W.).