Effects of molecular adsorbates on the binding energy of Pt dimers supported on graphene

Abstract

Introduction

Reducing the use of expensive and rare precious metals is essential for the cost reduction and widespread use of various catalyst systems. One of the simple ways to reduce the amount of catalyst metal is by highly dispersing ultrafine catalysts to achieve high specific surfaces. It is well known that catalytic activities of highly dispersing ultrafine catalysts strongly depend on their sizes.[1] Therefore, controlling and maintaining the size of highly dispersing ultrafine catalysts are essential for practical applications. It has been reported that the dynamic behaviors of agglomeration and redispersion of sub-nanometric Pt catalysts depend on atmospheres.[2] Reductive atmospheres enhance catalyst agglomeration, while catalyst redispersion occurs in oxidative atmospheres. In this study, we investigated molecular adsorbate dependence of the stability of sub-nanometric Pt catalysts. We calculated the Pt-Pt binding energy in Pt dimers with various molecular adsorbates using first-principles calculations based on density functional theory.

Methods

We performed the first-principles calculations, implemented in the plane-wave and projector-augmented wave method code, the Vienna Ab-initio Simulation Package (VASP). We considered the van der Waals interaction with the non-local correlation functional rev-vdW-DF2. We adopted graphene as catalyst support materials. We considered H₂, O₂, H₂O, OH, CO, CO₂, N₂, and NO as molecular adsorbates.

Results

In the case of Pt dimers on pristine graphene, the calculated Pt-Pt binding energy is 0.60 eV/atom, which is significantly smaller than the atomization energy of bulk Pt of 5.51 eV/atom.[3] We revealed that H₂, OH, and NO adsorption on Pt dimers does not affect the Pt-Pt binding energy. The Pt-Pt binding energy changes with these molecular adsorbates are less than 0.05 eV. On the other hand, O₂, H₂O, CO, and N₂ adsorption decreases the Pt-Pt binding energy. The corresponding Pt-Pt binding energy with O₂, H₂O, CO, and N₂ are 0.41, 0.36, 0.41, and 0.37 eV/atom, respectively. In the case of O₂ and H₂O adsorption, the most stable dissociation structures are those in which the adsorbed molecule dissociates as the Pt dimer dissociates. For CO₂ adsorption, the Pt-Pt binding energy increases to 1.15 eV/atom. This is because spontaneous CO₂ bending can be observed only on the Pt dimers, resulting in stable adsorption. It has been reported that such CO₂-bending adsorption structures are also stable on other transition metal dimers.[4]

References

[1] Y. Watanabe, X. Wu, H. Hirata, and N. Isomura, Catal. Sci. Technol. 1, 1490 (2011). [2] L. Liu, D. N. Zakharov, R. Arenal, P. Concepcion, E. A. Stach, and A. Corma, Nat. Commun. 9, 574 (2018). [3] L. Schimka, R. Gaudoin, J. Klimeš, M. Marsman, and G. Kresse, Phys. Rev B 87, 214102 (2013). [4] Y. Pan, C. Liu, T. S. Wiltowski, Q. Ge, Catal. Today, 147, 68 (2009).