Graphene cover-promoted metal-catalyzed reactions

Abstract

Graphitic overlayers on metals have commonly been considered as inhibitors for surface reactions due to their chemical inertness and physical blockage of surface active sites. In this work, however, we find that surface reactions, for instance, CO adsorption/desorption and CO oxidation, can take place on Pt(111) surface covered by monolayer graphene sheets. Surface science measurements combined with density functional calculations show that the graphene overlayer weakens the strong interaction between CO and Pt and, consequently, facilitates the CO oxidation with lower apparent activation energy. These results suggest that interfaces between graphitic overlayers and metal surfaces act as 2D confined nanoreactors, in which catalytic reactions are promoted. The finding contrasts with the conventional knowledge that graphitic carbon poisons a catalyst surface but opens up an avenue to enhance catalytic performance through coating of metal catalysts with controlled graphitic covers.

Keywords: CO oxidation; confinement effect; graphene; interface catalysis; platinum.

Fig. 1.

CO PM-IRRAS (A) and CO-TPD (B) spectra acquired from the 0.5 ML Gr/Pt(111) surface exposed to CO at various pressures (Torr) at room temperature. Each IR and TPD spectrum was recorded after 10 min of CO exposure at the indicated pressure with subsequent evacuation to UHV. (C) Electron density difference of CO adsorbed at the Gr/Pt(111) interface. Black balls: C; red balls: O. Blue and yellow surfaces in the electron density difference are for electron depletion and electron accumulation, respectively. (D) CO PM-IRRAS from Pt(111), 0.5 ML Gr/Pt(111), 0.8 ML Gr/Pt(111), and 1 ML Gr/Pt(111) surfaces exposed to 1 × 10⁻⁶ Torr CO for 10 min at room temperature.

Fig. 2.

CO intercalation under the full monolayer graphene overlayer. (A) In situ CO PM-IRRAS study in CO adsorption on the 1 ML Gr/Pt(111) surface at various CO pressures. (B) Dependence of the on-top CO adsorption intensity as a function of CO pressure. In situ ambient pressure XPS O 1s (C) and Pt 4f_7/2 (D) spectra from the 1 ML Gr/Pt(111) surface exposed to UHV, 1 × 10⁻⁶, 0.001, 0.01, and 0.1 Torr CO, respectively. The Pt 4f_7/2 peaks can be deconvoluted into surface component, bulk component, and Pt species bonded with CO, which are all marked by dashed lines. (E) I–V curves and (F) LEEM image of the CO-intercalated Gr/Pt(111) surface. The 1 ML Gr/Pt(111) surface was exposed to CO atmosphere close to 1 bar in a high-pressure cell attached to the UHV-LEEM system, and then transferred to the UHV chamber for LEEM imaging.

Fig. 3.

CO oxidation at Pt(111) and Gr/Pt(111) surfaces. (A) Arrhenius plots of CO₂ formation rate on the Pt(111) surface and the Gr/Pt(111) surfaces in the temperature range of 525–625 K. The reaction gas consists of 20 Torr CO and 10 Torr O₂. The Inset schematically illustrates the simultaneous intercalation of CO and O₂ molecules underneath graphene flakes through the domain boundaries and the release of CO₂ from the interface. In situ CO PM-IRRAS spectra acquired from the Pt(111) (B), 0.5 ML Gr/Pt(111) (C), and 1 ML Gr/Pt(111) (D) surfaces in 30 Torr CO/O₂ (2:1) at the indicated reaction temperatures.

Fig. 4.

DFT calculations of CO oxidation on Pt(111) with or without graphene. (A) Calculated reaction barriers for oxidation between CO and O on Pt(111) and Gr/Pt(111), respectively. Values in the brackets are in unit of electron volt. (B) Electron density difference of the transition state at the Gr/Pt(111) interface. Black balls: carbon; red balls: oxygen. Blue and yellow surfaces in the electron density difference are for electron depletion and electron accumulation, respectively.