Index Planes Of Pt Research Articles

Correlation of the Adsorption Geometry of Melamine and the ORR Activity on the Low Index Planes of Pt

Development of electrocatalysts with higher activity for the oxygen reduction reaction (ORR) is important to reduce the amount of Pt loading in the electrocatalyst. Controlling the surface structure of Pt electrodes enhances the ORR activity [1-3]. The ORR activity of the high-index planes of Pt decreases with the increase of the IR band intensity of PtOH, showing that PtOH is the inhibition species of the ORR [4]. The density functional theory (DFT) calculation predicts the enhancement mechanism of the activation of the ORR: terrace edge destructs the structure of the adsorbed water on (111) terrace to destabilize the PtOH [5]. Modification of Pt electrodes with hydrophobic species also changes the structure of adsorbed water and enhances the ORR activity [6-13]. Especially, melamine enhances the ORR activity of all the n(111)-(111) series of Pt [10] as well as Pt nanoparticles [9]. The enhancement of the activity is due to a decrease of Pt oxides [9,10], however, the mechanism of the decrease of Pt oxides has not been shown experimentally. In this study, we have studied the adsorption geometries of melamine and the adsorbed water structure on Pt(111) and Pt(100), of which increase ratios of the ORR activity are the highest and lowest respectively, using infrared reflection absorption spectroscopy (IRAS). The electrolytic solution was 0.1 M HClO4 / D2O containing 1×10-6 M melamine. Reference spectra of the IRAS were collected at 0.10 V, and sample spectra were measured from 0.20 to 1.0 V at a resolution of 4 cm−1. The spectra were averaged over 1280 scans. All the potentials were referred to the RHE. A vibrational mode with a dipole moment parallel to the surface cancels out with the mirror image dipole moment; IRAS spectra cannot be observed, whereas a vibrational mode of which the dipole moment is not parallel to the surface can be observed with IRAS (surface selection rule). The following 4 bands were observed and assigned using DFT calculation: νC=N (Ring) (1463 cm-1), δC-N (1573 cm-1), δN-H (1604 cm-1) and νC-N (1706 cm-1). All the dipole moments of these modes are parallel to the molecular plane of melamine. Thus, melamine is adsorbed on the surface with its molecular plane perpendicular or tilted to the surface. Figure 1 shows the adsorption geometries of melamine on Pt(111) and Pt(100). Melamine is adsorbed on the surface with NH2 groups due to the steric hindrance. Purple arrows show the directions of the dipole moments of νC=N (Ring) and δN-H, and red ones present those of δC-N and νC-N. IRAS band of the δN-H (purple arrow) on Pt(111) was larger than that on Pt(100). This result shows that the adsorption geometry of melamine on Pt(111) is more perpendicular to the surface than that on Pt(100). The band of the ice-like water was found on both Pt(111) and Pt(100) without melamine. The band disappeared on Pt(111) after the melamine modification, but melamine didn’t affect the ice-like water on Pt(100). The charge of Pt oxide formation decreased after melamine modification on Pt(111), whereas it did not change on Pt(100). More vertical geometry of melamine can break the hydrogen bonds of the ice-like water and destabilize PtOH on Pt(111), resulting in the higher increase ratio of the ORR.Acknowledgment This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO).

Nanoisland Formation on the Low Index Planes of Pt By Repeated Electro-Oxidation and-Reduction

Pt-based electro-catalysts are the most common cathode material in proton exchange membrane fuel cells given their high oxygen reduction reaction activity and stability. However, practical applications are limited by catalyst performance loss and degeneration. A main cause for this is nanoscale Pt surface restructuring, which occurs upon formation and reduction of a Pt surface oxide. During oxidation, a fraction of the Pt surface atoms are extracted in a process known as “place-exchange” from the electrode and are bound at specific sites in an oxide about 1 – 4 Å above the surface [1]. Upon subsequent oxide reduction, these atoms as well as the vacancies from which they originate can agglomerate, leading to the nucleation of adatom and vacancy islands. If this process is repeated, nanoislands with a well-defined lateral size of about 20 – 60 Å are grown [2-6]. Detailed studies of this growth process have been performed on the Pt(111) electrode [2-6], revealing two distinct growth regions which are characterized by different vertical scaling exponents [3].For the development of catalysts with superior long-term stability, a more detailed knowledge of the restructuring of the other low index planes of Pt is required. In our recent study of the Pt oxides on Pt(111) and Pt(100) we already showed a strong dependence of the structural stability of the surface on differences in the atomic-scale oxide structure formed on those two surface orientations [1]. However, the influence of surface structure on the nanoisland growth after repeated oxidation and reduction is largely unclear.We present a comparative operando grazing incidence small angle X-ray scattering (GISAXS) study of the nanoisland formation on Pt(111), Pt(100) and Pt(110) in acidic electrolyte. The GISAXS measurements of the nanoisland growth were performed at a high photon energy of >70 keV, which lowered background scattering from the electrolyte, enabling us to resolve the islands after the very first ox./red. cycle. Differences in island shape and growth behaviour on the low index planes will be discussed.We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497, MA 1618/23-1 and the BMBF via 05K19FK3.References Fuchs et al., Nat. Catal. 3, 754–761 (2020)Sugawara and K. Itaya, J. Chem. Soc., Faraday Trans. 185, 1351-1356 (1989)Jacobse et al., Nat. Mater. 17 (3), 277-282 (2018)J. Rost, et al., Nat. Commun. 10, 5233 (2019)Ruge et al., J. Am. Chem. Soc. 139, 4532 (2017)Jacobse, et al. ACS Cent. Sci. 5 (12), 1920–1928 (2019)

Structural effects on water molecules on the low index planes of Pt modified with alkyl amines and the correlation with the activity of the oxygen reduction reaction

Abstract Our previous study shows that the activity of the cathodic reaction (oxygen reduction reaction: ORR) of fuel cells is increased markedly by the adsorption of alkyl amines on a well-defined Pt(111) electrode. On Pt(100) and Pt(110) electrodes, however, the adsorption of alkyl amines deactivates the ORR significantly. We have studied the possible factors of the activation and deactivation by infrared (IR) reflection absorption spectroscopy. An IR band of icelike water is found on all the electrodes after the modification with alkyl amines. The band of the icelike water on Pt(111) is located at a higher wavenumber than those on Pt(100) and Pt(110). These findings indicate that the cluster size of icelike water on Pt(111) is smaller than those on Pt(100) and Pt(110). The cluster size of the ice correlates with the ORR activity on Pt electrodes.

In situ observation of Pt oxides on the low index planes of Pt using surface enhanced Raman spectroscopy

In situ vibrational spectra of Pt oxides that cannot be measured with IR spectroscopy have been studied on the low index planes of Pt using surface enhanced Raman spectroscopy with bare Au nanoparticles (NPSERS). Two bands appear around 570 and 340 cm-1 at higher potentials in 0.1 M HClO4 saturated with Ar, which are assigned to the stretching vibration of Pt-O(H) and the libration vibration of Pt-O, respectively. NPSERS spectra are measured in O2 saturated solution for the first time. The band intensities of Pt-O(H) and Pt-O in O2 saturated solution are enhanced significantly compared with those in Ar saturated solution. The onset potentials of Pt-O and Pt-O(H) formation are 1.15 V(RHE) on Pt(100) and 1.2 V(RHE) on Pt(111) and Pt(110). The onset potential of Pt-O and Pt-O(H) and band shape differ from the results obtained using shell isolated surface enhanced Raman spectroscopy (SHINERS). The Pt-O and Pt-O(H) band intensities are normalized using COad as an internal standard. The Pt-O(H) band intensity depends on surface structures as Pt(110) < Pt(111) ≪ Pt(100), whereas the Pt-O band gives a different intensity order for Pt(111) and Pt(110) as Pt(111) ≤ Pt(110) ≪ Pt(100) in O2 saturated solution.

Infrared spectroscopy of adsorbed OH on n(111)–(100) and n(111)–(111) series of Pt electrode

Abstract Infrared spectroscopy of adsorbed hydroxide (OHad) has been performed on the high index planes of Pt: n(111)–(100) and n(111)–(111) series. The bending mode of OHad (δPtOH) appears on these surfaces around 1080 cm− 1, which is similar to the result on the low index planes of Pt. The band intensity of δPtOH depends on surface structure and electrode potential. At 0.9 V vs RHE, the band intensity of δPtOH increases with the decrease of the step atom density: the intensity of δPtOH is higher on the surfaces with lower activity of the oxygen reduction reaction (ORR). These results suggest that the adsorption of OHad is site specific and the deactivating species of the ORR.

Infrared Reflection Absorption Spectroscopy of OH Adsorption on the Low Index Planes of Pt

The adsorption of hydroxide (OHad) on Pt has been studied on the low index planes of Pt using infrared reflection absorption spectroscopy (IRAS) in electrochemical environments. We discuss the correlation between the integrated band intensity of the bending mode of OH of Pt–OH (δ PtOH) and the charge density of the oxide formation of Pt. The band of δ PtOH is observed around 1100 cm−1, and the onset potential depends on the surface structure. The onset potential of δ PtOH on Pt(110) and Pt(100) overlaps with the hydrogen adsorption/desorption potential region. The order of the integrated band intensity of δ PtOH at 0.9 V vs RHE is opposite to the order of the oxygen reduction reaction (ORR) activity. This finding supports that the OHad is one of the species deactivating the ORR.

The Influence of Pt Oxide Film on the Activity for the Oxygen Reduction Reaction on Pt Single Crystal Electrodes

Correlation between Pt oxide and the activity for the oxygen reduction reaction (ORR) has been investigated on the low index planes of Pt (Pt(111), Pt(100), and Pt(110)) using voltammogram and rotating disk electrode (RDE). Pt oxide is formed by holding the potential at 1.0 V vs. RHE. The ORR activity decreases with the increase of the time of Pt oxide formation. The order of the ORR activity is Pt(100) < Pt(111) < Pt(110) in 0.1 M HClO4 after the formation of Pt oxide. This order is identical with that without Pt oxides. Formation of hardly reducible Pt oxide deactivates the ORR activity on Pt(111) remarkably. The amount of Pt oxides (PtOH, PtO and hardly reducible Pt oxide) increases as Pt(100) < Pt(111) < Pt(110) at 1.0 V.

Active sites for the oxygen reduction reaction on the high index planes of Pt

Abstract Structural effects on the oxygen reduction reaction (ORR) have been studied on the high index planes of Pt ( n (1 1 1)–(1 1 1), n (1 1 1)–(1 0 0), n (1 0 0)–(1 1 1) and n (1 0 0)–(1 1 0)) using rotating disk electrode (RDE) in 0.1 M HClO 4 . The activity for the ORR increases with the increase of the step atom density on the surfaces with (1 1 1) terrace from n = ∞ to n = 5 ( n is the number of terrace atomic rows), whereas the surfaces with (1 0 0) terrace do not show structural effects on the ORR activity. The activity of the surfaces with (1 1 1) terrace is higher than that on the surfaces with (1 0 0) terrace. The active sites for the ORR are located between the (1 1 1) terrace edge and the (1 1 1) terrace atomic row neighboring to the edge on Pt electrodes.

Structural dependence of intermediate species for the hydrogen evolution reaction on single crystal electrodes of Pt

Abstract Adsorbed hydrogen and water were measured during the hydrogen evolution reaction (HER) on the low and high index planes of Pt in 0.5 M H2SO4 using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at the atop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridge H) is found on Pt(100). The band intensity of the adsorbed hydrogen depends on temperature, indicating that the bands are due to the intermediate species for the HER. The band of the atop H appears on stepped surfaces with (110) step, whereas the asymmetric bridge H is observed on Pt(211) = 3(111)–(100) and Pt(311) = 2(111)–(100) that have (100) step. The absence of the atop H on Pt(100), Pt(211), and Pt(311) can be attributed to the relative stability of the bridge site.

Active sites for the hydrogen oxidation and the hydrogen evolution reactions on the high index planes of Pt

Abstract Structural effects on the hydrogen oxidation reactions (HOR) and the hydrogen evolution reactions (HER) have been studied on (n−1)(1 1 1)−(1 1 0), n(1 0 0)−(1 1 1), n(1 0 0)−(1 1 0) and n(1 1 1)−(1 0 0) series of Pt using rotating disk electrode (RDE) in 0.1 M HClO4 saturated with H2. The value of the exchange current density j0 increases linearly with the increase of the step atom density dS from n = ∞ to n = 9, where n denotes the number of terrace atomic rows, on (n−1)(1 1 1)−(1 1 0), n(1 0 0)−(1 1 1), and n(1 0 0)−(1 1 0) series. This indicates that the activity for the HOR/HER of the step is higher than that of the terrace. The values of j0 of (n−1)(1 1 1)−(1 1 0), n(1 0 0)−(1 1 1) and n(1 0 0)−(1 1 0) series become constant on the surfaces with n ⩽ 9, whereas those of n(1 1 1)−(1 0 0) series increase with the increase of dS. The activity for the HOR/HER per step atom increases in the sequence: n(1 1 1)−(1 0 0)

Structural Effects on the Fuel Cell Reactions on the Stepped Surfaces of Platinum and Palladium

Structural effects on the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) have been studied on the high index planes of Pt and Pd in 0.1 M HClO4. The exchange current density jo of the HOR increases with the increase of the step atom density dS from n = ∞ to 9 on(n−1)(111)-(110), n(111)-(100), n(100)-(111) and n(100)-(110) surfaces of Pt, where n shows the number of the terrace atomic rows. On the surfaces with n ≤ 9, however, the values of j0 do not depend on dS. These facts support that the step is the active site for the HOR on Pt electrodes, however, only part of the step atoms contributes to the HOR. Pt electrodes of (n−1)(111)-(110) series have the highest activity for the HOR. The activity for the ORR increases with the increase of the terrace atom density dT on Pd electrodes. The step atoms do not affect the ORR. Pd(100) has the highest activity for the ORR. The specific activity of Pd(100) is 4.2 times as high as that of the standard Pt catalyst (TEC10E50E). The mass activity of Pd surfaces covered with monolayer of Pt (Pt/Pd(hkl)) is 7 times as high as that of TEC10E50E.

Oxygen reduction reaction on the low index planes of palladium electrodes modified with a monolayer of platinum film

Abstract Oxygen reduction reaction (ORR) has been studied on the low index planes of Pd modified with a monolayer of Pt (Pt/Pd( hkl )) in 0.1 M HClO 4 with the use of hanging meniscus rotating disk electrode. The activity for ORR on bare Pd( hkl ) electrode depends on the surface structure strongly, however, voltammograms of ORR on Pt/Pd( hkl ) electrodes do not depend on the crystal orientation. The specific activities of Pt/Pd( hkl ) electrodes at 0.90 V (RHE) are higher than that on Pt(1 1 0) which has the highest activity for ORR in the low index planes of Pt. The mass activity on Pt/Pd( hkl ) electrode is 7 times as high as a commercial Pt/C catalyst.

Structural Effects of Electrochemical Oxidation of Formic Acid on Single Crystal Electrodes of Palladium

Structural effects on the rates of formic acid oxidation have been studied on Pd(111), Pd(100), Pd(110), and Pd(S)-[n(100) x (111)] (n = 2-9) electrodes in 0.1 M HClO4 containing 0.1 M formic acid with use of voltammetry. On the low index planes of Pd, the maximum current density of formic acid oxidation (jP) increases in the positive scan as follows: Pd(110) < Pd(111) < Pd(100). This order differs from that on the low index planes of Pt: Pt(111) < Pt(100) < Pt(110). Pd(S)-[n(100) x (111)] electrodes with terrace atomic rows n > or = 3 have almost the same jP as Pd(100), except Pd(911) n = 5. The value of jP on Pd(911) n = 5 is 20% higher than those of the other surfaces. Pd(311) n = 2, of which the first layer is composed of only step atoms, has the lowest jP in the Pd(S)-[n(100) x (111)] series. The adsorption geometry of the reaction intermediate (formate ion) is optimized by using density functional theory.