Enhanced Supply of Hydroxyl Species in CeO2-Modified Platinum

Subscriber access provided by Flinders University Library

Article 2

Enhanced Supply of Hydroxyl Species in CeO-Modified Platinum Catalyst Studied by in situ ATR-FTIR Spectroscopy Yu Katayama, Takeou Okanishi, Hiroki Muroyama, Toshiaki Matsui, and Koichi Eguchi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00108 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Enhanced Supply of Hydroxyl Species in CeO2Modified Platinum Catalyst Studied by in situ ATRFTIR Spectroscopy

Yu Katayama, Takeou Okanishi, Hiroki Muroyama, Toshiaki Matsui, Koichi Eguchi*

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan

ACS Paragon Plus Environment

1

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 40

ABSTRACT

The recent development of anion exchange membranes (AEMs) has increased the potential of anion exchange membrane fuel cells (AEMFCs). Although highly active electrocatalysts for specific reactions have been successfully developed by placing the most importance on the fuel species, only few studies have focused on OHad (hydroxyl adsorbed species), which is known to be a common reactive species in alkaline environments. In this study, highly oxophilic CeO2 was selected as a surface modifier for a Pt electrode. We first applied in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to ionomer-coated Pt and CeO2modified Pt surfaces for clarifying the adsorption behavior of OHad. As a result, a distinct change in adsorption behavior of OHad was confirmed in blank KOH solution. These peculiar characteristics were applicable to various electrochemical oxidation reactions. During the ammonia oxidation reaction, the acceleration of the formation of NOad species was observed in CeO2-modified Pt, suggesting the enhancement of OH adsorption. Furthermore, the degree of activity enhancement by CeO2 addition was investigated for the CO oxidation reaction, methanol oxidation reaction, and ethanol oxidation reaction. Under basic conditions, each of these reactions exhibited distinct activity enhancement. In contrast, under acidic conditions, the promoting effect on these reactions was not observed. These results strongly indicate the potential of our catalyst design strategy and the importance of OHad species as reactive species in alkaline environments.

KEYWORDS: in situ ATR-FTIR, Hydroxyl adsorption, Electrocatalysis in alkaline solution, Cerium oxide, Platinum


2

Page 3 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

TEXT 1. Introduction Anion exchange membrane fuel cells (AEMFCs) have gained extensive attention because of their ability to operate with electrocatalysts composed of low-cost materials in comparison with their acid counterparts. For example, the electrocatalysts in proton exchange membrane fuel cells (PEMFCs) require a considerable amount of Pt to achieve decent performance1 2. This requirement with respect to the electrocatalysts is a critical obstacle limiting the widespread application of PEMFCs. In fuel cells with alkaline electrolytes, however, the oxygen reduction reaction (ORR), which is known as a cathode reaction in fuel cell systems, can proceed over much less expensive nonprecious electrocatalysts, including Ag and other transition metals3 4 5 6. In addition, it has recently been emphasized that AEMFCs have potential advantages in the reaction rate of ORR in comparison with acid-type fuel cells2. A major limiting factor for the commercialization of AEMFCs has thus been the development of anion exchange membranes, toward which some significant progress is being made7. Although hydrogen is now considered the main fuel for fuel cells, some characteristics stemming from its physical properties, such as difficulty in liquefaction, have limited the utilization of fuel cells in many cases8. To overcome this problem, hydrogen carriers such as methanol, ethanol, and ammonia have been proposed as alternative fuels for fuel cell systems9 10 11 12 13

. In the case of alkaline environments, the adsorbed OH species (hereafter denoted as

OHad) acts as the reactive species in many related electrochemical reactions, including the oxidation reactions of the aforementioned hydrogen carriers. Thus, in alkaline environments, promoting the adsorption of fuel species as well as OHad species is important for achieving high electrocatalytic activity14

15

. In fact, some researchers recently reported the development of


3

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 40

highly active electrocatalysts of multimetallic alloys by tuning the OH adsorption behavior of the active metal sites6

16 17 18 19

. At present, fine-tuning of the electronic state of metal surfaces by

alloying is mainly used to alter the adsorption behavior of electrocatalysts. Although this strategy has achieved success in the development of catalysts for methanol electro-oxidation in alkaline environments20, it is not always appropriate for other electrochemical reactions in alkaline media. In the case of the ammonia oxidation reaction, for instance, the fully dehydrogenated adsorbate of atomic nitrogen (Nad) is well known to serve as a poisoning species on Pt electrodes. The trend in affinity of Nad species for the metal surface resembles that in affinity of OHad species21. Because the bimetallic alloy catalyst cannot selectively adsorb OHad without promoting the adsorption of poisonous Nad species as a result of this analogous adsorption behavior, a highly active catalyst for the ammonia oxidation reaction cannot be designed using the conventional alloying method. Promoting the selective adsorption of OHad species is important in the development of more widely applicable electrocatalysts for use in alkaline environments. Here we present a new approach that can be applied for the rational design of electrocatalysts for electrochemical reactions that involve OHad as reactive species. In our previous report, we revealed that the activity of the ammonia oxidation reaction was closely related to the amount of OHad species adsorbed onto the electrode surface22. In this study, we aimed to enhance the OH adsorption without changing the electronic state of the active metal species in order to suppress the adsorption of spectator species. Accordingly, we designed the catalyst surface to have two different functional fields: (i) a selective OH adsorption field and (ii) a highly active field for the oxidation reaction. More specifically, cerium oxide (CeO2) was added to the Pt catalyst surface to serve as a reservoir of OHad23

24

. We expected CeO2, which is oxophilic, to continuously


4

Page 5 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

supply OHad to active metal sites where the electrochemical oxidation reaction occurs. Using this approach, the OH adsorption property can be specifically enhanced without changing the electronic state of the active metal species. In this study, our objective was to demonstrate the effectiveness of this concept. First, the electrode surface species over both a CeO2-modified Pt catalyst and an ionomer-coated Pt catalyst were investigated using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to clarify the change in OH adsorption behavior during the electrochemical reaction. To establish the validity of our catalyst design strategy, we then conducted electrochemical measurements using various fuels under both alkaline and acid conditions.

2. Experimental 2.1. Additive preparation An aqueous dispersion of CeO2-containing solution was prepared by adding as-received CeO2 (6.75 mg, Wako Pure Chemical) to ultrapure water (Millipore Milli-Q) and then ultrasonically dispersing the mixture for 2 h. The anion exchange ionomer solution (AS-4, Tokuyama Corp., diluted to 1 wt% solution with ethanol, 0.7 mL) was then added to the resultant dispersion liquid (0.75 mL) and ultrasonically dispersed for 1 h to obtain the CeO2-containing ionomer solution.

2.2. Electrocatalyst preparation


5

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 40

The Pt working electrode was a thin film (ca. 50 nm) formed on the total reflecting plane of a hemispherical Si ATR prism (radius 22 mm) by the electroless deposition technique25. First, to fabricate a Pt nanofilm, the base plane of the Si prism was subjected to a hydrophilic treatment with 40% NH4F solution for 1 min. Palladium was then deposited onto the base plane with 1% HF–1 mM PdCl2 for 5 min at room temperature. After the palladium-coated base plane was rinsed with water, platinum deposition was performed by immersion in the Pt plating solution at 50°C for 12 min. This Pt plating solution was prepared by mixing LECTROLESS Pt 100 basic solution (30 mL, Electroplating Engineering of Japan Ltd.), LECTROLESS Pt 100 reducing solution (0.6 mL), 28% NH3 solution, and ultrapure water. Osawa et al.26 reported the structure of a chemically deposited Pt electrode fabricated in a similar manner. To prepare the ionomercoated and CeO2-modified Pt prisms, the ionomer solution (AS-4, Tokuyama Corp., diluted to 0.5 wt% solution with ethanol and ultrapure water, 50 µL) and the prepared CeO2-containing ionomer solution (50 µL) were dropped onto the Pt/Si prism, respectively. The surface of the Pt/Si prism was observed by scanning electron microscopy (SEM, Nvision 40, Carl Zeiss-SIINT) before and after the electrochemical measurement.

2.3. In situ ATR-FTIR measurements The details of ATR-FTIR spectroscopy have been described elsewhere27 28 29 30 31. For in situ ATR-FTIR measurements, a Pt prism, ionomer-coated Pt prism, and CeO2-modified Pt prism were used. Each prism was mounted in a spectro-electrochemical cell equipped with an Ag/AgCl reference electrode and a platinum wire counter electrode (Figure 1). The cell was then placed in a homemade reflection optics system at an incidence angle of 70°, as described previously25. An


6

Page 7 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

FTIR spectrometer equipped with an MCT detector (NICOLET8700, Thermo Fisher Scientific) was used for the in situ ATR-FTIR measurements. The optical path was purged with N2 gas. The electrolyte solutions were prepared by mixing 28 wt.% NH3 solution (Wako Pure Chemical) with KOH (Sigma–Aldrich, >85 wt.%) and ultrapure water. After deoxygenating the electrolyte solution by purging with Ar, the prism surface was pre-treated by cycling the potential between 0.05 and 0.90 V vs. reversible hydrogen electrode (RHE). For electrochemical measurements, cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) were conducted at room temperature using a HSV-100 (Hokuto Denko) electrochemical analyzer. All potentials in this study were converted to the corresponding potentials vs. RHE. The electrochemical surface area (ECSA) for each electrocatalyst was calculated from cyclic voltammograms (CVs) recorded in Ar-purged electrolyte by integrating the charge in the hydrogen adsorption/desorption region (0.05–0.45 V vs. RHE). The current density expressed was normalized by ECSA. All spectra are shown in the absorbance units defined as log(I0/I), where I0 and I represent the intensity of the spectra under reference conditions and under each experimental condition, respectively. The reference condition was set to the potential at 0.05 V vs. RHE in blank KOH solution. Therefore, the background spectrum was collected with the potential maintained at 0.05 V vs. RHE in 1 M KOH solution. The IR spectra in the wavenumber region below 1050 cm−1 had a poor S/N ratio because of the strong IR absorption by the Si prism; thus, only the results in the region above 1050 cm−1 are reported.

2.4 Electrochemical measurements


7

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 40

For electrochemical measurements, an ionomer-coated Pt disk electrode and a CeO2-modified Pt disk electrode were used. Linear sweep voltammograms for each electrode in 1 M KOH, 1 M KOH–0.1 M NH3, 1 M KOH–0.1 M ethanol, 1 M KOH–0.1 M methanol, 1 M HClO4, 1 M HClO4–0.1 M ethanol, and 1 M HClO4–0.1 M methanol solutions were recorded. The ionomercoated and CeO2-modified Pt disk electrodes were prepared by dropping 6.0 µL of the dispersion liquid containing AS-4 and CeO2, respectively, onto the surface of Pt disk electrodes (Hokuto Denko, 0.196 cm2). Each measurement was conducted using a conventional three-electrode cell; platinum wire and RHE were used as counter and reference electrodes, respectively. The electrolyte solutions were prepared by mixing NH3 solution, ethanol (99.5%, Wako Pure Chemical), or methanol (99.9%, Wako Pure Chemical) with KOH or HClO4 (70%, Wako Pure Chemical) and ultrapure water. After deoxygenation of the electrolyte solution by purging Ar, linear sweep voltammetry was conducted at 25°C and at a scanning rate of 20 mV s–1 using HSV-110 (Hokuto Denko). ECSA for each electrode was calculated in the same manner as that described in section 2.3. The current density expressed was normalized by ECSA.

3. Results and discussion 3.1. Characterization of electrocatalysts Figure 2 shows CVs of the disk electrodes in 1 M KOH at 25°C. As reported previously32, for the ionomer-coated Pt surface, hydrogen desorption was observed in the potential range from 0.05 to 0.45 V vs. RHE, followed by the double-layer capacitance in the potential range from 0.45 to 0.70 V. Subsequently, a pseudocapacitance corresponding to OH adsorption was observed at 0.70–0.85 V. Finally, the adsorption of oxygen-containing species, that is, oxide


8

Page 9 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

formation, proceeded at potentials above 0.85 V. In the case of the CeO2-modified Pt disk electrode, the voltammogram was approximately the same as that in case of the ionomer-coated Pt disk despite the addition of CeO2 to the Pt-disk electrode, indicating that the bond strength and/or adsorption enthalpy of Pt–Had and Pt–OHad did not change. The surface morphologies of the Pt/Si prism, ionomer coated Pt/Si prism, and CeO2modified Pt/Si prism were observed by SEM (Figure 3). The SEM image in Figure 3 (a) shows the characteristic surface morphology of the Pt particles deposited by the electroless deposition method. In contrast, distinguishing the boundaries among particles for a Pt/Si prism after pretreatment consisting of potential cycling between 0.05 and 0.90 V for 15 times was difficult (Figure 3 (b)). This difficulty is a result of the rearrangement of Pt atoms during the potential cycling, resulting in a change in the structure of the particles. Importantly, even after more than 50 voltammetry cycles between 0.05 and 0.90 V, the surface morphology of the Pt/Si prism remained unchanged from that shown in Figure 3 (b), indicating that, in this potential range, no further microstructural change occurred. Figures 3 (c) and (d) show SEM images of the ionomercoated Pt/Si prism and CeO2-modified Pt/Si prism before measurement, respectively. As evident in Figure 3 (d), the CeO2 particles used in this study had a mean particle diameter of ca. 0.3 µm. Furthermore, the surface out of contact with CeO2 was approximately the same as that of the ionomer-coated Pt/Si prism. Figure 4 shows CVs of ionomer-coated and CeO2-modified Pt/Si prisms measured in the spectro-electrochemical cell. The shape of the voltammogram for the ionomer-coated Pt/Si prism was similar to that of the Pt disk electrode, indicating that electrochemically active Pt was successfully deposited onto the Si surface. Because all the electrochemical behavior observed in Figure 4 was identical to the typical response of Pt, the electrochemical property of the deposited


9

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 40

Pt in each prism was not affected by the ionomer solution or the CeO2-containing ionomer solution on the Pt/Si prism surface. We subsequently investigated the ionomer-coated Pt/Si prism and CeO2-modified Pt/Si prism using in situ ATR-FTIR spectroscopy.

3.2. In situ observation of OHad species in blank KOH solution Figure 5 shows the representative time-resolved ATR-FTIR spectra of the ionomer-coated Pt surface simultaneously acquired with the linear sweep voltammogram in 1 M KOH. The IR spectra of the ionomer-coated Pt/Si prism closely resembled those of the CeO2-modified Pt/Si prism (data not shown). In Figure 5, two clear potential-dependent peaks are observed at 1226 cm−1 and 1616 cm−1. The band at 1226 cm−1 is attributed to the Si–O–Si stretching band related to the Si prism33

34

. Because Si reacts with the KOH solution, the intensity of this band varies

among measurements. The OH bending mode appeared at 1616 cm−1 as a negative-going band above 0.3 V. The intensity of this band increased to the negative side with increasing potential, indicating that the interfacial water initially adsorbed onto the electrode surface was gradually replaced by other absorbed species. In this case, this absorbed species should be OHad35. According to previous studies, the small band at ca. 1130 cm−1 is attributable to the librational mode of OHad species on the Pt surface (hereafter denoted as Pt–OHad)36

35 37 38

. As shown in

Figure 6, this band clearly emerged at potentials above 0.7 V, where the adsorption of OH onto the Pt surface began. Because this band directly reflects the amount of OHad species on the Pt surface, in this study, the area of the 1130 cm−1 band was selected as an indicator of the OHad amount on the Pt surface.


10

Page 11 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

To clarify the effect of the CeO2 additive on the OH adsorption behavior, we investigated the potential dependency of the Pt–OHad band area. We define the Pt–OHad band area as the integrated value of absorbance for the 1130 cm−1 band. The potential dependency of the calculated Pt–OHad band area was closely related to the CV response (Figure 7 (a)). This result also supports the hypothesis that the 1130 cm−1 band area can be used as an index of the amount of OHad species on the Pt surface. In the case of the CeO2-modified Pt/Si prism, however, the tendency was substantially different. As shown in Figure 7 (b), the Pt–OHad band area was independent of the CV response but simply responded linearly to the applied potential. This result clearly shows that the CeO2 additive affected the OH adsorption behavior of Pt. One possible mechanism for this change in OH adsorption behavior induced by the CeO2 additive is the interaction between the interfacial water and OHad on the CeO2 surface. In the wavenumber range from 3600 to 3800 cm−1, sharp peaks are distinctly observed only in the spectrum of the CeO2-modified Pt/Si prism (Figure 8). It has been shown that these bands can be assigned to OH bridging species adsorbed onto Ce atoms of various valence39. These OHad species on the CeO2 additive possibly interact with interfacial water through hydrogen bonding, leading to a change in the structure of the interfacial water. In fact, as shown in Figure S5, the potential dependency of the band area of interfacial water (OH bending mode, 1616 cm−1) was strongly affected by CeO2 modification. This change in structure of interfacial water may cause the OH− species in the vicinity of the electrode surface to behave as if they are concentrated, resulting in an enhancement of OHad supplied to the Pt surface. Such a unique behavior indicates that the CeO2 additive acts as an OH reservoir.

3.3. In situ observation of adsorbates during the ammonia oxidation reaction


11

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 40

To further investigate the effect of the OH adsorption behavior in the actual reaction, we conducted in situ ATR-FTIR measurements during the ammonia oxidation reaction. The most widely accepted reaction mechanism for ammonia oxidation reaction over Pt electrode was proposed by Gerischer and Mauerer 40;

NH3 ( aq ) → NH3,ad

(1)

NH3,ad + OHδad− → NH2,ad + H2O + δ e−

(2)

NH2,ad + OHδad− → NHad + H2O + δ e−

(3)

NH x ,ad + NH y , ad → N 2 H x + y , ad

N2 H x+ y ,ad + ( x + y)OHδad− → N2 + ( x + y ) H2O + ( x + y ) δ e−

(4) (5)

where x = 1 or 2 and y = 1 or 2. Gerischer et al. also suggested the production of atomic nitrogen adspecies (Nad) via dehydrogenation of NHad species. In addition, the formation of NO and NO2 species were observed in several studies

41 42

. Although methanol and ethanol were alternative analysis

subjects, measurements could not be conducted with these compounds because of the overlapping of emergent peaks related to the reaction intermediates with the Pt–OHad band at approximately 1130 cm−1 43 44. Figure 9(a) shows the typical time-resolved ATR-FTIR spectra of the ionomer-coated Pt surface simultaneously acquired with LSV in the presence of ammonia. Matsui et al. recently reported the assignment of bands observed during the ammonia oxidation reaction 45. According to them, the bands at 1269 cm−1, 1504 cm−1, and 1665 cm−1 can be assigned to N2H4,ad, bridged NOad, and NH3,ad, respectively. In addition, a relatively weak band was observed at ca. 1300


12

Page 13 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

cm−1 when the potential exceeded 0.9 V. This band, which was not reported by Matsui et al., is related to the adsorption species of NO2. In fact, the wavenumber of this band was almost identical to that of the symmetric stretching of nitro-form NO2

46

. In the case of the CeO2-

modified Pt surface, the same number of bands was observed as in the case of the ionomercoated Pt surface, but their relative intensity was slightly changed (Figure 9 (b)). Furthermore, some other differences were observed in the spectrum shape between ionomer-coated Pt and CeO2-modified Pt surfaces. The most notable difference was the band intensity observed at ca. 1504 cm−1, which originates from the bridged NOad. In addition, the NO2,ad band at ca. 1300 cm−1 became obvious in the spectrum of the CeO2-modified Pt surface. The Pt–OHad band was also observable in the presence of NH3, indicating the change in its adsorption behavior by CeO2 addition (Figure 10). To clarify the effect of CeO2 on the OH adsorption behavior during the oxidation reaction, we calculated the potential dependency of each band in the same manner as that in Figure 7. Note that, in this case, each calculated band area was normalized via the following procedure. In general, the band areas measured with different prisms cannot be compared because the band area depends not only on the amount of adsorbed species but also on the degree of surface enhancement effect of each prism. In this study, these two factors were considered to normalize the band areas of two prisms. We first calculated the amount of COad over each prism using CO stripping voltammetry. According to previous studies, the band area and surface coverage exhibit a linear relationship47 48. Therefore, by combining the calculated COad amount and the COad band area of the CO stripping voltammogram collected simultaneously, we obtained a coefficient reflecting the degree of surface enhancement effect of each prism. The band area data shown in Figure 7 were normalized using the resulting coefficient. Although this calculation is a simple


13

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 40

approach, it is sufficient to clarify the overall trend of these prisms, as confirmed by comparing the initial band areas for NH3,ad in each prism. The amount of NH3,ad species is known to be closely related to the amount of Had species in KOH solution, which is correlated with ECSA49. The ratio of ECSA for CeO2-modified Pt/Si to ionomer-coated Pt/Si was 17.26 cm2/11.75 cm2 = 1.47, which well matched the ratio of the normalized band area of NH3,ad for CeO2-modified Pt/Si to ionomer-coated Pt/Si, 1.53/1.05 = 1.46. In Figure 11 (a), the potential dependencies of N2H4,ad, NOad, and NH3,ad in ionomer-coated Pt/Si were consistent with those reported in a previous study45. The potential dependence of the Pt-OHad band was also reasonable, which corresponds well with the results in Figure 7. In the case of the CeO2-modified Pt/Si prism, the band areas of the N2H4,ad, NOad, and Pt-OHad species behaved somewhat differently from those of the ionomer coated Pt/Si prism. First, the band area of N2H4,ad species became larger in the CeO2-modified Pt/Si prism. Because N2H4,ad is known to be the intermediate species in the ammonia oxidation reaction, an increase in the amount of N2H4,ad is related to a higher NH3 oxidation current density observed in the CeO2-modified Pt/Si prism. According to previous studies, N2H4,ad is formed by the dimerization of NHx,ad species (x = 1 or 2, dehydrogenated form of NH3,ad), which means that the amount of NHx,ad species is closely related to the amount of N2H4,ad species. Moreover, the OHad species is widely acknowledged to participate in the formation of NHx,ad by assisting the dehydrogenation of NH3,ad. On the basis of these considerations, we concluded that the overall trend in the N2H4,ad amount indirectly reflects the OHad supply capacity of an electrode surface. Therefore, the observed increase in the N2H4,ad band area suggests an enhancement of OH adsorption by the CeO2 additive.


14

Page 15 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The behavior of the NOad band area also reflected the enhancement of OH adsorption. The NOad species, which was recently revealed as the byproduct in the ammonia oxidation reaction, was generated from the oxidation of monomer N-containing species (probably dehydrogenated NHx species) by OHad species50. Many theoretical studies have suggested that the rate-determining step of the ammonia oxidation reaction is the catalytic dimerization of NHx,ad to form N2Hy,ad (2 ≤ y ≤ 4)51 52. This slow N2Hy,ad formation step was not included in the reaction pathway of NOad formation. Therefore, the formation of NOad species will be preferentially promoted when the amount of OH in the vicinity of the electrode surface increases. For the CeO2-modified Pt/Si prism, the amount of formed NOad species was more than three times greater than that for the ionomer-coated Pt/Si prism. According to the previous discussion, an increase in the NOad band area strongly suggests that the CeO2 additive accelerated the supply of OH species. The most interesting behavior was observed for the potential dependence of the Pt–OHad band area in the case of the CeO2-modified Pt surface. As observed in Figure 7 (b), the Pt–OHad band area linearly responded to the applied potential in 1 M KOH. In the presence of NH3, however, the Pt–OHad band area showed a plateau region between 0.3 V and 0.9 V, where the oxidation of NH3 proceeded. This change in potential dependence is likely due to the continuous consumption of the Pt–OHad species during the oxidation reaction. The important consideration is that, in the case of the CeO2-modified Pt/Si prism, a sufficient amount of Pt–OHad species remained on the electrode and prevented the deterioration of the reaction rate during the reaction. For the NH3 oxidation reaction, other authors have suggested that 6 moles of OHad species are consumed to completely oxidize 1 mole of NH3 to N2 40. Under such conditions, the rate of the NH3 oxidation reaction is reasonably hypothesized to be strongly affected by the supply rate of OHad species. In fact, in the case of the ionomer-coated Pt/Si prism, almost no OHad species remained on the Pt


15

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

surface, resulting in a lower peak current density. Moreover, as mentioned previously, the bond strength and/or adsorption enthalpy of Pt–OHad was not affected by CeO2 modification. Therefore, this enhancement in electrocatalytic activity by CeO2 modification is presumed to be mainly due to the enhancement of the rate of OH supply to the reaction sites. That is, the CeO2 additive contributes to the increase in OH concentration in the vicinity of the electrode surface rather than to the surface coverage of OHad. On the basis of the results in Figure 11, the enhancement of OHad supplied by the CeO2 additive was validated. Furthermore, this additional Pt–OHad species that arose from CeO2 addition can participate in the reaction and successfully promote the reaction. Such a phenomenon is expected to positively affect the reactivity in various electrochemical reactions in an alkaline environment that consumes OHad species upon reaction.

3.4. Applicability to electrode reaction in alkaline media The results presented thus far clearly show the enhancement of OH adsorption by CeO2 addition. Many previous studies have identified OHad species act as reactive species in various reactions under alkaline environments14

15

. Therefore, to clarify the influence of this change in

OH adsorption behavior on the electrocatalytic activity, we obtained linear sweep voltammograms in various cardinal reactions such as the CO oxidation reaction, methanol oxidation reaction, and ethanol oxidation reaction (Figures S8–S11). The mechanisms of these reactions are well established, and the OHad species has been clarified to play an important role, particularly in an alkaline environment. In this section, the activity of Pt- and CeO2-modified Pt


16

Page 17 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

electrocatalysts under both alkaline and acid conditions is compared to further elucidate the effect of CeO2 modification. Notably, the stability of CeO2 under acidic conditions has been well studied and CeO2 is known to be chemically stable under such conditions53 54. The CO oxidation reaction is the simplest reaction among those studied; it proceeds via the Langmuir–Hinshelwood (L–H) reaction of CO with adsorbed OH species55. For Pt bimetallic systems such as PtSn, these electrocatalysts have been reported to be bi-functional in nature, which facilitates the oxidation of CO; CO exclusively adsorbed onto the Pt sites, whereas OHad species adsorbed onto the more oxophilic Sn sites56

32 20

. In the case of the CeO2-modified Pt

disk electrode, however, the OHad species adsorbed onto the CeO2 surface cannot directly participate in the reaction but can interact with the interfacial water, thereby likely accelerating the OHad supply to the Pt surface. As shown in Figure 12, the oxidation peak current density for the CeO2-modified Pt disk electrode was approximately 15% higher than that of the Pt disk electrode under alkaline conditions. In contrast, under acidic conditions, the CeO2 additive cannot act as an OH adsorption field and the oxidation peak current density of the CeO2modified Pt disk electrode decreased by more than 45% in comparison with that of the Pt disk electrode. The methanol and ethanol oxidation reactions are much more complex than the CO oxidation reaction because of the formation of numerous reaction intermediates. To date, many indexes have been used to express the catalytic activities of these reactions, and the amount of OHad species is a commonly used one57 58. Many researchers achieved a higher oxidation current using bimetallic alloy catalysts such as PtRu, PtNi, and oxide-containing Pd4 59 60. The proposed mechanisms for the promotion of methanol and ethanol oxidation reactions are similar to that of the CO oxidation reaction, that is, the oxophilic sites provide OHad species and accelerate further


17

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

oxidation of alcohol decomposition fragments. For methanol and ethanol oxidation reactions in an alkaline environment, the oxidation peak current densities of the CeO2-modified Pt disk electrode were approximately 25% and 30% higher than those of the Pt disk electrode, respectively (Figure 12). However, as in the case of the CO oxidation reaction, the observed oxidation peak current density of the CeO2-modified Pt disk electrode in 1 M HClO4 was smaller than that of the Pt disk electrode, indicating that the CeO2 additive blocked the reaction sites under acidic conditions. The trend in Figure 12 suggests that the CeO2 additive enhances the adsorption of OHad species and facilitates the oxidation reaction involving the OHad species in an alkaline environment. Because the addition of CeO2 does not change the bond strength and/or adsorption enthalpy of Pt–Xad (which is a reasonable assumption because the Pt–Had energy is not affected by the addition of CeO2), CeO2 addition is the one of the simplest and promising ways to enhance the OHad supply capacity of electrocatalysts. 4. Conclusion In this study, the effect of CeO2 as an additive to a Pt electrocatalyst was investigated in detail. The results of in situ ATR-FTIR measurements collected in blank KOH solution suggested that the CeO2 additive affected the OH adsorption behavior of Pt without changing the bond strength and/or adsorption enthalpy of Pt–Had. This change in OH adsorption behavior was also observed during the ammonia oxidation reaction. Interestingly, the band area of NOad was relatively larger in the spectrum of the CeO2-modified Pt/Si prism than in that of the normal Pt/Si prism, indicating the promotion of OH adsorption by the CeO2 additive. This enhancement of OH adsorption by the CeO2 additive positively affected the catalytic activity toward CO oxidation,


18

Page 19 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

methanol oxidation, and ethanol oxidation reactions in alkaline environments. Surprisingly, in comparison with the Pt electrocatalytst, the obtained oxidation current was increased by as much as 30% when the dispersion liquid of CeO2 was simply dropped onto the Pt electrode surface. These results clearly indicate that our catalyst design strategy is valid and that OHad is an important reactive species in alkaline environments. Furthermore, this method is by far the simplest way to enhance the OH supply capacity of electrocatalysts without changing the electronic structure of the metal species. Thus, this method has the potential to further improve the activity of existing electrocatalysts for various reactions in alkaline environments.

FIGURE CAPTIONS Figure 1 Schematic of the spectro-electrochemical cell for in situ ATR-FTIR measurements. Figure 2 Cyclic voltammograms of ionomer-coated Pt disk and CeO2-modified Pt disk electrodes in 1 M KOH at 25°C with a scanning rate of 20 mV s−1. Figure 3 SEM images of the (a)–(b) bare Pt/Si prism, (c) ionomer-coated Pt/Si prism, and (d) CeO2-modified Pt/Si prism. Note that (a), (c), and (d) represent as-prepared surfaces and (b) represents the surface after a pre-treatment comprising 15 cycles of potential between 0.05 and 0.90 V.


19

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

Figure 4 Cyclic voltammograms of ionomer-coated Pt/Si prism (black line) and CeO2-modified Pt/Si prism (blue line) in 1 M KOH at 25°C with a scanning rate of 20 mV s−1. Figure 5 Time-resolved IR spectra of the ionomer-coated Pt surface simultaneously acquired with the linear sweep voltammogram in 1 M KOH at 25°C with a scanning rate of 20 mV s−1. Figure 6 Time-resolved IR spectra of the ionomer-coated Pt surface simultaneously acquired with the linear sweep voltammogram in 1 M KOH at 25°C with a scanning rate of 20 mV s−1. Figure 7 Potential dependency of the band area at 1130 cm−1 for (a) the ionomer-coated Pt/Si prism and (b) the CeO2-modified Pt/Si prism in 1 M KOH. Data were collected from timeresolved IR spectra simultaneously acquired with the cyclic voltammogram shown as a broken line. Figure 8 Time-resolved IR spectra of (a) the ionomer-coated Pt and (b) the CeO2-modified Pt surface simultaneously acquired with the linear sweep voltammogram in 1 M KOH at 25°C with a scanning rate of 20 mV s−1. Figure 9 Time-resolved IR spectra of (a) the ionomer-coated Pt and (b) the CeO2-modified Pt surface simultaneously acquired with the linear sweep voltammogram in 1 M KOH–0.1 M NH3 at 25°C with a scanning rate of 20 mV s−1. Figure 10 Time-resolved IR spectra of the (a) ionomer-coated Pt and (b) the CeO2-modified Pt surface simultaneously acquired with the linear sweep voltammogram in 1 M KOH–0.1 M NH3 at 25°C with a scanning rate of 20 mV s−1.


20

Page 21 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 11 Potential dependency of the current and the normalized band area of Pt–OHad (blue line), NH3,ad (red line), N2H4,ad (orange line), and NOad (black line) of (a) ionomer-coated Pt/Si and (b) CeO2-modified Pt/Si electrodes. Figure 12 Trend in variation of the oxidation peak current density for the ammonia oxidation reaction, ethanol oxidation reaction, methanol oxidation reaction, and CO oxidation reaction in 1 M KOH and in 1 M HClO4.

ASSOCIATED CONTENT Supporting Information Available: List of the ECSA values for each electrode used in this study, additional results for the in situ ATR-IR spectra for bare and ionomer-coated Pt/Si prisms, the effect of substitution of hydrogen isotopes on the 1130 cm−1 band, the potential dependence of the peak wavenumber for the Pt–OHad band, the potential dependence of the IR band area related to the interfacial adsorbed water, the time dependence of the IR band area during potentiostatic operation, and the linear sweep voltammograms used to obtain Figure 12. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel.: +81-75-383-2519; fax: +81-75-383-2520 E-mail address: [email protected] (K. Eguchi). Author Contributions


21

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This work was supported by the Council for Science, Technology and Innovation (CSTI), Crossministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). We thank Tokuyama Corporation for supplying the anion exchange ionomer (AS-4).

REFERENCES 1.

Dresselhause, M. S.; Thomas, I. L., Nature, 2001, 332-337.

2.

Steele, B. C. H.; Heinzel, A., Nature, 2001, 354-352.

3.

McLean, G. F.; Niet, T.; Prince-Richard, S.; Djilali, N., Int. J. Hydrogen Energy, 2002,

27, 507-526. 4.

Spendelow, J. S.; Wieckowski, A., Phys. Chem. Chem. Phys., 2007, 9, 2654-2675.

5.

Holewinski, A.; Idrobo, J. C.; Linic, S., Nat. Chem., 2014, 6, 828-834.

6.

Markovic, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N., Fuel Cells, 2001, 105-

116. 7.

Merle, G.; Wessling, M.; Nijmeijer, K., J. Membrane Sci., 2011, 377, 1-35.

8.

Schlapbach, L.; Zuttel, A., Nature, 2001, 353-358.

9.

Kordesch, K.; Hacker, V.; Gsellmann, J.; Cifrain, M.; Faleschini, G.; Enzinger, P.;

Fankhauser, R.; Ortner, M.; Muhr, M.; Aronson, R. R., J. Power Sources, 2000, 86, 162-165. 10.

Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P., J. Power Sources,

2006, 155, 95-110.


22

Page 23 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

11.

Cairns, E. J.; Simons, E. L.; Tevebaugh, A. D., Nature, 1968, 217, 780-781.

12.

Christensen, C. H.; Johannessen, T.; Sorensen, R. Z.; Norskov, J. K., Catal. Today, 2006,

111, 140-144. 13.

Antolini, E.; Gonzalez, E. R., J. Power Sources, 2010, 195, 3431-3450.

14.

Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.;

Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M., Nat. Mater., 2012, 11, 550-557. 15.

Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der Vliet,

D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M., Nat. Chem., 2013, 5, 300-306. 16.

Subbaraman, R.; Danilovic, N.; Lopes, P. P.; Tripkovic, D.; Strmcnik, D.; Stamenkovic,

V. R.; Markovic, N. M., J. Phys. Chem. C, 2012, 116, 22231-22237. 17.

Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., J. Phys. Chem., 1995, 8290-8301.

18.

Gasteiger, H. A.; Markovic, N. M.; Ross, P. N.; Cairns, E. J., J. Phys. Chem., 1994, 617-

625. 19.

Strmcnik, D. S.; Tripkovic, D. V.; Vliet, D. v. d.; Chang, K.-C.; Komanicky, V.; You, H.;

Karapetrov, G.; Greeley, J. P.; Stamenkovic, V. R.; Markovic, N. M., J. Am. Chem. Soc., 2008, 15332-15339. 20.

Permyakova, A. A.; Han, B.; Jensen, J. O.; Bjerrum, N. J.; Shao-Horn, Y., J. Phys. Chem.

C, 2015, 119, 8023-8031. 21.

Masel, R. I., Principles of Adsorption and Reaction on Solid Surfaces; John Wiley &

Sons, Inc.: New York, US, 1996, 137-139. 22.

Katayama, Y.; Okanishi, T.; Muroyama, H.; Matsui, T.; Eguchi, K., J. Phys. Chem. C,

2015, 9134-9141. 23.

Morterra, C.; Bolis, V.; Magnacca, G., J. Chem. Soc., Faraday Trans., 1996, 1991-1999.


23

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

24.

Boehm, H. P., Discuss. Faraday Soc., 1971, 264-275.

25.

Miki, A.; Ye, S.; Osawa, M., Chem. Comm., 2002, 1500-1501.

26.

Miki, A.; Ye, S.; Senzaki, T.; Osawa, M., J. Electroanal. Chem., 2004, 563, 23-31.

27.

Osawa, M.; Yoshii, K.; Ataka, K.-i.; Yotsuyanagi, T., Langmuir 1994, 640-642.

28.

Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M., J. Am. Chem. Soc., 2003, 3680-

3681. 29.

Kunimatsu, K.; Senzaki, T.; Tsushima, M.; Osawa, M., Chem. Phys. Lett., 2005, 401,

451-454. 30.

Ataka, K.; Yotsuyanagi, T.; Osawa, M., J. Phys. Chem., 1996, 10664-10672.

31.

Osawa, M.; Ataka, K.-i.; Yoshii, K.; Yotsuyanagi, T., J. electron spectrosc. relat.

phenom., 1993, 371-379. 32.

Markovic, N. M.; Ross, P. N., Surf. Sci. Rep., 2002, 45, 117-229.

33.

Müller, C. M.; Molinelli, A.; Karlowatz, M.; Aleksandrov, A.; Orlando, T.; Mizaikoff,

B., J. Phys. Chem. C, 2012, 116, 37-43. 34.

Miyazaki, S.; Nishimura, H.; Fukuda, M.; Ley, L.; Ristein, J., Appl. Surf. Sci., 1997, 113-

114, 585-589. 35.

Nakamura, M.; Nakajima, Y.; Hoshi, N.; Tajiri, H.; Sakata, O., Chemphyschem, 2013, 14,

2426-31. 36.

Fisher, G. B.; Sexton, B. A., Physical Review Letters, 1980, 44, 683-686.

37.

Nakamura, M.; Nakajima, Y.; Kato, K.; Sakata, O.; Hoshi, N., J. Phys. Chem. C, 2015,

119, 23586-23591. 38.

Tanaka, H.; Sugawara, S.; Shinohara, K.; Ueno, T.; Suzuki, S.; Hoshi, N.; Nakamura, M.,

Electrocatalysis, 2014, 6, 295-299.


24

Page 25 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

39.

Bazin, P.; Saur, O.; Lavalley, J. C.; Daturi, M.; Blanchard, G., Phys. Chem. Chem. Phys.,

2005, 7, 187. 40.

Gerischer, H.; Mauerer, A., J. Electroanal. Chem. , 1970, 421-433.

41.

Gootzen, J. F. E.; Eonders, A. H.; Visscher, W.; van Santen, R. A.; van Veen, J. A. R.,

Electrochim. Acta, 1998, 43. 42.

Vidal-Iglesias, F. J.; Solla-Gullón, J.; Feliu, J. M.; Baltruschat, H.; Aldaz, A., J.

Electroanal. Chem., 2006, 588, 331-338. 43.

Yang, Y.-Y.; Ren, J.; Li, Q.-X.; Zhou, Z.-Y.; Sun, S.-G.; Cai, W.-B., ACS Catal., 2014,

4, 798-803. 44.

Zhou, Z.-Y.; Wang, Q.; Lin, J.-L.; Tian, N.; Sun, S.-G., Electrochim. Acta, 2010, 55,

7995-7999. 45.

Matsui, T.; Suzuki, S.; Katayama, Y.; Yamauchi, K.; Okanishi, T.; Muroyama, H.;

Eguchi, K., Langmuir, 2015, 31, 11717-11723. 46.

Rima, F. R.; Nakata, K.; Shimazu, K.; Osawa, M., J. Phys. Chem. C, 2010, 114, 6011-

6018. 47.

Kunimatsu, K., J. Electroanal. Chem., 1986, 213, 149-157.

48.

Leung, L. W. H.; Wieckowski, A.; Weaver, M. J., J. Phys. Chem., 1988, 92, 6985-6990.

49.

de Vooys, A. C. A.; Koper, M. T. M.; van Santen, R. A.; van Veen, J. A. R., J.

Electroanal. Chem., 2001, 506, 127-137. 50.

Finkelstein, D. A.; Bertin, E.; Garbarino, S.; Guay, D., J. Phys. Chem. C, 2015, 119,

9860-9878. 51.

Skachkov, D.; Venkateswara Rao, C.; Ishikawa, Y., J. Phys. Chem. C, 2013, 117, 25451-

25466.


25

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

52.

Diaz, L. A.; Botte, G. G., Electrochimica Acta, 2015, 179, 519-528.

53.

Masuda, T.; Fukumitsu, H.; Fugane, K.; Togasaki, H.; Matsumura, D.; Tamura, K.;

Nishihata, Y.; Yoshikawa, H.; Kobayashi, K.; Mori, T.; Uosaki, K., J. Phys. Chem. C, 2012, 116, 10098-10102. 54.

Fugane, K.; Mori, T.; Ou, D. R.; Yan, P.; Ye, F.; Yoshikawa, H.; Drennan, J., Langmuir,

2012, 28, 16692-16700. 55.

Spendelow, J. S.; Goodpaster, J. D.; Kenis, P. J. A.; Wieckowski, A., J. Phys. Chem. B,

2006, 9545-9555. 56.

Switzer, E. E.; Olson, T. S.; Datye, A. K.; Atanassov, P.; Hibbs, M. R.; Cornelius, C. J.,

Electrochim. Acta, 2009, 54, 989-995. 57.

Lai, S. C.; Koper, M. T., Phys. Chem. Chem. Phys., 2009, 11, 10446-10456.

58.

Tripković, A. V.; Popović, K. D.; Momčilović, J. D.; Draić, D. M., J. Electroanal.

Chem., 1996, 418, 9-20. 59.

Tripković, A. V.; Štrbac, S.; Popović, K. D., Electrochem. Comm., 2003, 5, 484-490.

60.

Xu, C.; Tian, Z.; Shen, P.; Jiang, S. P., Electrochim. Acta, 2008, 53, 2610-2618.


26

Page 27 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Thermometer CO or Ar

Reference electrode Counter electrode

Working electrode

Figure 1 Y. Katayama et al. 1


ACS Catalysis

40

Current density / A cm-2 ECSA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

CeO2-modified Pt

30

Ionomer coated Pt

20 10 0 -10 -20 -30 -40 0.0

0.2

0.4 0.6 Potential / V vs. RHE

0.8

1.0



Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(a) Before pre-treatment

(b) After pre-treatment

前

後 200 nm

200 nm (c) Ionomer coated Pt/Si

(d) CeO2-modified Pt/Si CeO2

CeO2

200 nm

CeO2

200 nm

Figure 3 Y. Katayama et al.


ACS Catalysis

40

Current density/ A cm-2 ECSA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

30 20 10 0 -10 -20 Ionomer coated Pt/Si prism CeO2-modified Pt/Si prism

-30 -40 0.0

0.2

0.4 0.6 Potential / V vs. RHE

0.8

1.0



Page 31 of 40

E / V vs. RHE

1616

Si-O-Si

cm-1

0.9 0.8 0.7 Absorbance / %

Potential scanning direction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.6 0.5 0.4 0.3 0.2 0.05 0.0015 abs.

1800 1700 1600 1500 1400 1300 1200 Wavenumber / cm-1



ACS Catalysis

Si-O-Si

E / V vs. RHE

0.0005 abs.

Pt-OHad

1.2 1.1 1.0 0.9

Absorbance / %


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.05

1350

1300

1250

1200

1150

1100

1050

-1

Wavenumber / cm



(a) Ionomer coated Pt/Si prism

40 30

Band area / a. u.

20 10 0 -10 -20 -30 -40

0.2

0.4 0.6 0.8 1.0 Potential / V vs. RHE

1.2

(b) CeO2-modified Pt/Si prism

40 30 20 10 0 -10 -20 -30


0.0

Band area / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis


Page 33 of 40

-40 0.0

0.2

0.4 0.6 0.8 1.0 Potential / V vs. RHE

1.2



ACS Catalysis



H2O 0.001 abs.

OHad on CeO2 E / V vs. RHE

0.9

H2O

0.8 Potential scanning direction

0.7 Absorbance / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 40

0.6 0.5

0.4

0.3 0.2 0.05

3800

3600

3400

3200 -1

Wavenumber / cm

3000 3800

3600

3400

3200

3000

-1

Wavenumber / cm

Figure 8 Y. Katayama et al. ACS Paragon Plus Environment

Page 35 of 40

(a) Ionomer coated Pt/Si prism NOad

NO2,ad

E / V vs. RHE 1.2

Si-O-Si H2Oad

1.1 1.0 0.9 0.8 Absorbance / %


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.7 0.6 0.5 0.4 N2H4,ad

0.3

0.2 0.05

1800

NH3,ad

1700

1600

0.002 abs.

1500

1400

1300

1200

Wavenumber / cm-1 Figure 9 Y. Katayama et al.


ACS Catalysis


Si-O-Si

NOad

E / V vs. RHE

NO2,ad

1.2

H2Oad

1.1 1.0 0.9 Absorbance / %


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

0.8 0.7 0.6 0.5 0.4 0.3 N2H4,ad

0.2 0.05

1800

NH3,ad

1700

1600

0.002 abs.

1500

1400

1300

1200

-1

Wavenumber / cm



Page 37 of 40



Si-O-Si

Si-O-Si 0.001 abs.

Pt-OHad

Pt-OHad

E / V vs. RHE

1.2 1.1

N2H4,ad

1.0 0.9 N2H4,ad

Absorbance / %


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.05

1300

1200

1100

Wavenumber / cm-1

1300

1200

1100

Wavenumber / cm-1 Figure 10 Y. Katayama et al.


ACS Catalysis

6


200

Normalized band area

5

NOad NH3,ad

150

Pt-OHad

4 3

100

2 50

1 0 0.0

6

0.2

0.4 0.6 0.8 Potential / V vs. RHE

1.0



N2H4,ad

0 1.2

200

NOad

5

NH3,ad

150

Pt-OHad

4 3

100

2 50

1 0 0.0

0.2

0.4 0.6 0.8 Potential / V vs. RHE

1.0


N2H4,ad

Normalized band area

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

0 1.2



Page 39 of 40

50

1 M KOH 1 M HClO4

40 30 CeO2-Pt Pt Pt 100(iPeak ｰ iPeak )/iPeak

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

20 10 0 -10 -20 -30 -40 -50



ACS Catalysis

OH Red

Alkaline electrolyte

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

OHｰ

Ox Ox

CeO2

H2O

Pt Red

OHｰ

TOC image


Enhanced Supply of Hydroxyl Species in CeO2-Modified Platinum

Recommend Documents