Article pubs.acs.org/JPCC
Electrochemical Quartz Crystal Microbalance Analysis of Nitrogen Oxide-Promoted Platinum Dissolution in HClO4 Takafumi Morita, Hironori Kuroe, Akira Eguchi, and Shin-ichiro Imabayashi* Department of Applied Chemistry, Faculty of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, 135-8548 Tokyo, Japan ABSTRACT: To elucidate the mechanism of NaNO2-promoted Pt dissolution, the weight loss of Pt black-deposited Au quartz resonators was measured in 0.1 mol dm−3 HClO4 containing NaNO2 under potential cycling and constant potential conditions. A continuous weight loss of ca. 50−60 ng cm−2 cycle−1 was detected in the presence of 10 mmol dm−3 NaNO2 during potential cycling above a high potential limit of 1.2 V vs RHE (VRHE) and below a low potential limit (EL) of 0.9 VRHE. A weight loss was observed at constant potentials (0.6− 0.8 VRHE) when oxides were initially present; the net weight loss (Δw) magnitude increased as the amount of the initially formed Pt oxide increased. These results indicate that Pt dissolution occurred when Pt oxides were reduced in the presence of NaNO2. The good agreement between the Δw per potential cycle and the Δw at a constant potential (EL of the potential cycling = constant potential) revealed that the formation of Pt oxides in the positive sweep was required for the continuous weight loss under potential cycling. The Δw value increased for higher concentrations of NaNO2 under both conditions. The products of NaNO2 reduction were thought to participate in Pt dissolution.
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INTRODUCTION The trace amount of contaminants in the air, such as SO2, H2S, NO2, NH3, NaCl, and volatile organic compounds, has recently been identified as one of the significant factors contributing to the degradation of catalysts in polymer electrolyte fuel cells (PEMFCs).1−3 Sulfur compounds (SO2, H2S) are the most serious contaminants and are irreversibly adsorbed on the reactive sites of the catalyst surface, thereby reducing the electrochemical active surface area (ECSA) of platinum (Pt) catalysts.4−19 Volatile organic compounds including benzene, toluene, and ethylene also block the reactive sites of Pt catalysts through reversible adsorption and reduce the ECSA.20,21 Cl− poisoning, on the other hand, mainly results in Pt dissolution through the formation of platinum chlorides (PtCl42−, PtCl62−) with a consequent decrease of the ECSA;22−25 however, blocking of the Pt active sites by chloride adsorption also takes place. Nitrogen oxides and NH3 reduce the ECSA owing to adsorption on the reactive sites of the catalyst surface and also decrease the proton conductivity of the membrane owing to the replacement of H+ with NH4+.5,8,11,26−32 Nitrogen dioxide (NO2) is a major contaminant in the air that is derived from automotive vehicle exhaust and industrial manufacturing processes, and has a rapid detrimental impact on fuel cell performance.5,8,11,29,31 NO2 has been reported to decrease the ECSA of Pt catalysts via adsorption on the catalyst surface.29,31 Recently, we found that the weight of Pt black electrodes decreased continuously under successive potential cycling in 0.1 mol dm−3 HClO4 containing NaNO2 at the millimolar level.33 To the best of our knowledge, Pt dissolution induced by NO2− or NO2 poisoning has not been reported. © 2014 American Chemical Society
In the present work, we investigated the dissolution of Pt in 0.1 mol dm−3 HClO4 containing NaNO2 in further detail by using the electrochemical quartz crystal microbalance (EQCM) method.33 The EQCM method has been applied to study the formation of Pt oxides34−36 and the dissolution of Pt.22,23,37−41 The electrochemical evaluation of the influence of NO2 on Pt dissolution is inherently complicated owing to the complex electrochemical behavior of nitrite. In 0.1 mol dm−3 HClO4, NaNO2 dissociates into Na+ and NO2−, and most NO2− ions exist as HNO2 (log [CNO2−/CHNO2] = −3.35 + pH). Table 1 lists the electrochemical reactions of HNO2 as well as its reduction intermediates and the corresponding redox potentials (reproduced with permission from ref 42. Copyright 1974 National Association of Corrosion Engineers). The electrochemical reduction of HNO2 in acidic media is complex. Based on the redox potential values under 10−2−10−3 mol dm−3 NaNO2, HNO2 (oxidation state: +3) can be oxidized to NO2 (+4) and NO3− (+5) and reduced to NO (+2), N2O (+1), N2 (0), and NH4+ (−3) in the potential range (0.6−1.4 VRHE) employed in the present work. However, previous works indicate that the electrochemical reduction of nitrite does not necessarily occur stepwise in the order of the oxidation number, and its mechanism depends on the electrode material and measurement conditions.43,44 The EQCM method facilitates the evaluation of the effect of NO2 on Pt dissolution, because it enables the detection of changes in the mass of the Pt electrode while controlling the electrode potential. The current ex situ examination of Pt dissolution induced by the addition of Received: February 7, 2014 Revised: June 25, 2014 Published: July 8, 2014 15114
dx.doi.org/10.1021/jp5013494 | J. Phys. Chem. C 2014, 118, 15114−15121
The Journal of Physical Chemistry C
Article
The ECSA values of all the Ptb-deposited Au electrodes used in the EQCM measurements were maintained in the range of 3.3−4.6 cm2, where the surface morphology did not change significantly and the amount of Pt dissolution was independent of ECSA. The increase in ECSA caused by the deposition of Ptb improved not only the detection limit but also the S/N ratio. Quartz crystal microbalance (QCM) and cyclic voltammetric measurements were carried out in a thermostated chamber at 37 °C using a QCA922 QCM (Seiko EG&G) and a Potentiostat/Galvanostat Model 263A (Princeton Applied Research) instrument. A three-electrode glass cell equipped with a Ptb-deposited Au working electrode, a Ptb wire counter electrode, and reversible hydrogen electrode (RHE) was used for all measurements. All chemicals used in the present work were of reagent grade and were used without further purification. Water was deionized and purified to give a specific resistance of >18.2 MΩ cm and total organic carbon of 0.8 VRHE) is reflective of the more facile reduction of HNO2. The products of the reduction of HNO2 might contribute to Pt dissolution. Although no net weight decrease took place at 0.9 VRHE, the amount of weight loss at 0.9 VRHE was larger in the presence of NaNO2 than in the absence of NaNO2, implying that the reduction of Pt oxide or Pt dissolution was enhanced by NaNO2. Figure 6 summarizes the net weight loss (Δw) during a single potential cycle and while holding a constant potential for 1 h as a function of the EH or EL value of the potential cycle and the constant potential (Econst) in the presence of 10 mmol dm−3 of NaNO2. The Δw value was higher when a lower EL or a higher EH value of potential cycling was employed. The dependence of Δw on the EH and EL values indicates that Pt dissolution took place during the reduction of Pt oxides in the presence of NaNO2 or its reduction products. The magnitude of Δw at Econst (●) was also greater at a more negative potential but less dependent on the potential compared to the magnitude of Δw under potential cycling conditions (■). Consequently, the difference between the Δw values obtained under potential cycling and constant potential conditions became larger at a more positive potential. This fact can be explained by the length of time for which the system remained in the potential
became larger (Figure 4c,e), with a concomitant increase in the magnitude of the weight loss of the electrode (Figure 4d,f). In the case of EL = 0.6 VRHE, the weight of the electrode decreased by ca. 80 ng cm−2 in the potential range of 0.9−0.6 VRHE and increased by ca. 25 ng cm−2 owing to the formation of Pt oxide at potentials above 1.2 VRHE, resulting in a net weight loss of ca. 55 ng cm−2 cycle−1, which is greater than the amount of the initially formed oxide. The weight loss in the first cycle was greater than that in the following cycles, which can be explained by the fact that the amount of Pt oxides initially formed at 1.4 VRHE (37 ng cm−2) was larger than that formed in the potential sweep (25 ng cm−2). In the absence of NaNO2, on the other hand, the net weight loss was 0.5 ng cm−2 cycle−1 in the case of EL = 0.6 VRHE (data not shown). Change in the Electrode Weight at a Constant Potential. In the previous section, it was elucidated that the electrode weight decreased when the Pt oxides were reduced in the potential range of 0.9−0.6 VRHE for each negative potential sweep under potential cycling conditions. The dependence of the extent of Pt dissolution on the electrode potential was evaluated by monitoring the changes in the weight of and current density at the Ptb-deposited Au electrode in 0.1 mol dm−3 HClO4 as a function of the holding time at the constant potential of 0.6, 0.7, 0.8, or 0.9 VRHE (Figure 5). Similar to the measurements in Figure 4, Pt oxides were initially formed at 1.4 VRHE in NaNO2−free HClO4. The horizontal dotted lines in Figure 5a,c indicate the average amount of initially formed Pt oxides (37 ng cm−2). The electrode potential was held at 1.0 VRHE for 10 s and stepped to the potential of 0.9, 0.8, 0.7, or 0.6 VRHE. In the absence of NaNO2, there was no net weight loss; the reduction of the initially formed oxides took place at a potential-dependent rate, and the reduction was greater at more negative potential. In the presence of NaNO2, on the other 15117
dx.doi.org/10.1021/jp5013494 | J. Phys. Chem. C 2014, 118, 15114−15121
The Journal of Physical Chemistry C
Article
from 0.4−1.3 in the potential range of 1.0−1.6 VRHE. Bucur reported that the monolayer of OH−species formed on Pt black electrodes in 1 mol dm−3 HClO4 at the rest potential (1.04 VRHE) was converted to PtO during the anodic polarization to 1.6 VRHE.36 On carbon-supported Pt nanoparticles, the OH species were gradually transformed into Pt− O species between 1.08 and 1.35 VRHE, and the coverage of Pt− O reached unity at ∼1.2−1.3 VRHE.47,48 X-ray photoelectron spectroscopy analysis indicated that a bilayer structure consisting of an inner layer of Pt(II) oxide and an outer layer of Pt(IV) oxide is formed at potentials greater than 1.3 VSHE.49 From these previous reports, it was evident that PtO was the main component, whereas a small amount of PtO2 was generated in the potential range of 1.0−1.6 VRHE. The weight loss of the electrode at 0.6 VRHE in the absence of NaNO2 (○) was almost equivalent to the weight of the initially formed Pt oxides (⧫), supporting the postulate that only the initially formed oxides were reduced, and no net weight loss took place without NaNO2. The addition of 10 mmol dm−3 NaNO2 brought about a net weight loss; the weight loss measured at 0.6 VRHE increased from 5 to 92 ng cm−2 as the potential of oxide formation increased from 1.0 to 1.6 VRHE, as shown in Figure 7 (●). In the absence of the initial oxide formation, no net weight loss was detected despite the presence of 10 mmol dm−3 NaNO2 (data not shown). The presence of oxides is deemed necessary for Pt dissolution on the basis of the data. A net weight loss of 60−70 ng cm−2 was obtained at 0.6 VRHE when the oxides were initially formed at 1.4 VRHE. Assuming that the net weight loss was determined by the weight of Pt dissolved during the reduction of PtO, the value of 60−70 ng cm−2 means that ca. 15% of Pt atoms forming oxides were dissolved. This percentage term increased from 10% at 1.2 VRHE to 17% at 1.6 VRHE, indicating that Pt dissolution occurred more readily for Pt oxides formed at a more positive potential. Pt dissolution behavior similar to that observed herein, in which Pt dissolution occurs during the oxide reduction, has been observed under the cathodic atmosphere of polymer electrolyte fuel cells (in the absence of contaminants).50−52 Topalov and colleagues investigated the dissolution of polycrystalline Pt in acidic media and revealed that the Pt dissolution mainly occurred when the Pt oxides were reduced.52 During oxide formation, adsorbed oxygen atoms penetrate the Pt lattice via place exchange at potentials above 1.2 VRHE, which weakens the Pt−Pt bonds and causes a distortion of the surface lattice.53 This is thought to induce Pt dissolution when the potential sweep includes the potential range higher than 1.2 VRHE in acidic media. Similar potential-dependent changes in the oxide structure seem to be reflected in the dependence of the Pt dissolution on the potential of oxide formation in this work, although the Pt dissolution occurs at a much higher rate in the presence of NaNO2. Dependence of the Weight Loss on the Concentration of NaNO2. The magnitude of the net weight loss depends on the concentration of NaNO2 as shown in Figure 8. The evaluation of the dependence of the net weight loss per potential cycle (Δw) on EL demonstrated a similar trend in the presence of 10 and 5 mmol dm−3 of NaNO2 except for the difference in the weight (from 57 to 40 ng cm−2) at 0.6 VRHE. Thus, the NaNO2 concentration-dependent change in Δw became more significant at lower potential. The mechanism underlying the enhanced Pt dissolution in the presence of NaNO2 is not completely understood. It was reported that nitrogen oxides such as NO, NO2, and N2Osome of them are
Figure 6. Mass change of Ptb-deposited Au working electrode during single potential cycling (■) and at constant potentials (●, ○), measured in 0.1 mmol dm−3 HClO4 containing 10 mmol dm−3 NaNO2, as a function of the EH, EL, and Econst values. The Δw values at 100 s (○) and 3600 s (●) after applying the potential of Econst are plotted.
range where the reduction of Pt oxides takes place (
