Correlation of the Adsorption Geometry of Melamine and the ORR Activity on the Low Index Planes of Pt
Authors: Katagiri Yukiya , Nakamura Masashi , Hoshi Nagahiro 
Published: December 2024

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).

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Figure 1