Abstract:We prepared monodisperse Pt 2 Ru 3 nanoparticles supported on carbon black and Sb-doped SnO 2 (denoted as Pt 2 Ru 3 /CB and Pt 2 Ru 3 /Sb-SnO 2 ) with identical alloy composition and particle size distribution by the nanocapsule method. The activities for the hydrogen oxidation reaction (HOR) of these anode catalysts were examined in H 2 -saturated 0.1 M HClO 4 solution in both the presence and absence of carbon monoxide by use of a channel flow electrode at 70 • C. It was found that the CO-tolerant HOR mass activity at 0.02 V versus a reversible hydrogen electrode (RHE) on the Pt 2 Ru 3 /Sb-SnO 2 electrode was higher than that at the Pt 2 Ru 3 /CB electrode in 0.1 M HClO 4 solution saturated with 1000 ppm CO (H 2 -balance). The CO tolerance mechanism of these catalysts was investigated by in situ attenuated total reflection Fourier transform infrared reflection-adsorption spectroscopy (ATR-FTIRAS) in 1% CO/H 2 -saturated 0.1 M HClO 4 solution at 60 • C. It was found, for the Pt 2 Ru 3 /Sb-SnO 2 catalyst, that the band intensity of CO linearly adsorbed (CO L ) at step/edge sites was suppressed, together with a blueshift of the CO L peak at terrace sites. On this surface, the HOR active sites were concluded to be more available than those on the CB-supported catalyst surface. The observed changes in the adsorption states of CO can be ascribed to an electronic modification effect by the Sb-SnO 2 support.
Abstract:The CO-tolerance mechanism of a carbon-supported Pt-Fe alloy catalyst with two atomic layers of stabilized Pt-skin (Pt 2AL -PtFe/C) was investigated, in comparison with commercial Pt 2 Ru 3 /C (c-Pt 2 Ru 3 /C), by in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy in 0.1 M HClO 4 solution at 60 • C. When 1% CO (H 2 -balance) was bubbled continuously in the solution, the hydrogen oxidation reaction (HOR) activities of both catalysts decreased severely because the active sites were blocked by CO ad , reaching the coverage θ CO ≈ 0.99. The bands in the IR spectra observed on both catalysts were successfully assigned to linearly adsorbed CO (CO L ) and bridged CO (CO B ), both of which consisted of multiple components (CO L or CO B at terraces and step/edge sites). The Pt 2AL -PtFe/C catalyst lost 99% of its initial mass activity (MA) for the HOR after 30 min, whereas about 10% of the initial MA was maintained on c-Pt 2 Ru 3 /C after 2 h, which can be ascribed to a suppression of linearly adsorbed CO at terrace sites (CO L, terrace ). In contrast, the HOR activities of both catalysts with pre-adsorbed CO recovered appreciably after bubbling with CO-free pure H 2 . We clarify, for the first time, that such a recovery of activity can be ascribed to an increased number of active sites by a transfer of CO L, terrace to CO L, step/edge , without removal of CO ad from the surface. The Pt 2AL -PtFe/C catalyst showed a larger decrease in the band intensity of CO L, terrace . A possible mechanism for the CO-tolerant HOR is also discussed.
Carbon monoxide is a common contaminant in hydrogen produced by the reformation of natural gas and acts as a poison for platinum-based catalysts in hydrogen anodes in polymer electrolyte fuel cells (PEFCs). CO poisoning can be alleviated through the use of Pt skin-covered Pt alloys including first row transition metals, e.g., Pt-Fe, Pt-Co and Pt-Ni, as well as second row transition metals, e.g., Pt-Ru, on which CO is known to adsorb more weakly than on pure Pt.1 Thus, hydrogen would be able to compete more successfully for adsorption sites. The weakened CO adsorption has usually been explained on the basis of the d-band center being lowered and antibonding orbitals becoming populated. However, our recent density functional theory (DFT) results, together with a range of experimental results, as reported by our group (H. Yano, et al.) separately at this Meeting, suggest that the situation is somewhat more complicated.2-5 In addition, little attention has been paid to the hydrogen oxidation reaction (HOR) itself, which we have found to be enhanced on these surfaces in parallel with experimental work.2,3 Our calculations indicate that the adsorption of H atoms is also weakened, so that the latter can become more mobile on the surface. The largest degree of weakening was found for the Pt-Fe alloy.3 Making use of realistic models for the surfaces, with (110) steps and (111) terraces, we found evidence for a new type of hydrogen spillover mechanism, in which H2 adsorbs at steps, dissociates, and the dissociated H atoms diffuse to (111) terraces.3,4 Finally, the calculations indicate that the Pt skin/Pt alloy structure is more rigid than that of pure Pt, which might be a factor in the enhanced durability of the catalyst under fuel cell operating conditions.5 This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References H. Igarashi, T. Fujino, Y. Zhu, H. Uchida, and M. Watanabe, P hys. Chem. Chem. Phys. 3, 306 (2001). G. Y. Shi, H. Yano, D. A. Tryk, M. Watanabe, A. Iiyama, and H. Uchida, Nanoscale 8, 13893 (2016). G. Y. Shi, H. Yano, D. A. Tryk, A. Iiyama, and H. Uchida, ACS Catal. 7, 267 (2017). Y. Ogihara, H. Yano, T. Matsumoto, D. A. Tryk, A. Iiyama, and H. Uchida, Catalysts 7, 8 (2017). G. Y. Shi, et al., to be submitted. Fig. 1. Adsorption energies for (A) H2 or 2H, (B) CO, and (C) H2O at step edges (triangles) and terraces (squares). In (A), the solid symbols denote dissociated 2H, while the open symbols denote undissociated H2. In all cases except for PtRu, undissociated H2 does not adsorb on the surface, either at step edges or terraces. On pure Pt(221), H2 dissociates spontaneously when close to the surface, while on Pt1AL–PtFe, Pt1AL–PtCo and Pt1AL–PtNi, it “floats” away from the surface.3 Figure 1
For residential polymer electrolyte fuel cells (PEFCs) operated with H2-rich (reformate) fuel gas, the hydrogen oxidation reaction (HOR) at Pt catalysts is severely poisoned even by trace amounts of CO contained in the reformate due to a strong blocking of active sites. So far, Pt-Ru alloy catalysts have been used to mitigate such CO poisoning. However, the HOR mass activity at Pt-Ru alloys is not sufficiently high, so that a large amount of precious metals (Pt and Ru) must be used. In addition, since Ru is not stable at high potentials (≥ 0.8 V), leaching into the acidic electrolyte membrane, the anode must be protected so as not to be oxidized by air. Recently, we have developed a Ru-free hydrogen anode catalyst by forming a stabilized Pt-skin (one to two atomic layers: xAL) on PtM-alloy (M = Fe, Co, and Ni) nanoparticles supported on carbon black (PtxAL‒PtM/C).1,2 These new catalysts exhibited a high HOR activity, in both the presence and absence of CO, as well as a robustness.3,4 The apparent values of mass activity (MA app) for the HOR on PtxAL‒PtFe/C at 20 mV vs. RHE in H2-saturated 0.1 M HClO4 solution were 2‒3 times larger than those for a commercial Pt2Ru3/C (c-Pt2Ru3/C) catalyst in the absence and presence of adsorbed CO (COad) at 70 °C as shown in Fig. 1. The retention of MA app on these PtxAL-PtM/C catalysts after an accelerated durability test (ADT) simulating daily start-stop cycles repeated exposure to the reformate gas and air (2,500 potential cycles between 0.02 V and 0.95 V) in N2 purged-0.1 M HClO4 solution at 70 oC was as high as ca. 80 % as shown in Fig. 2. The apparent area-specific activity values at the PtxAL‒PtM/C remained unchanged during ADT. This suggests that the dealloying of the M component in the PtxAL‒PtM/C was suppressed by the stabilized Pt-skin layer. To clarify the mechanism of such an enhancement of HOR activity and CO-tolerance for the PtxAL‒PtM/C, we performed multilateral-analyses, by in-situ attenuated total reflection Fourier transform infrared reflection-adsorption spectroscopy (ATR-FTIRAS), density functional theory (DFT) calculations, and in-situ X-ray absorption spectroscopy (XAS) techniques. It was found that the adsorption energy of CO on terrace sites on the PtxAL skin covering the PtM alloy was weakened by the modified electronic structure. The HOR activity of PtxAL‒PtFe/C with pre-adsorbed CO was found to recover appreciably after bubbling with CO-free pure H2. Such a recovery can be ascribed to an increase in the number of active sites by a transfer of linearly adsorbed COad at terrace sites (COL, terrace) to step/edge sites (COL, step/edge) on the surface of the Pt-skin layer, without removal of COad from the surface.4,5 We will also discuss possible mechanisms for the CO-tolerant HOR. This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The synchrotron radiation experiments were performed at BL14B2 (Proposal Nos. 2015B3388 and 2016A1805) and BL16B2 (2016A5390 and 2016B5390) of SPring-8 in cooperation with the Device-Functional Analysis Department of NISSAN ARC Ltd. Figure 1
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