Graphdiyne-supported single-atom Sc and Ti catalysts for high-efficient CO oxidation

Abstract

Single-layer graphdiyne was proposed as substrate for single-atom Sc and Ti catalysts with much larger binding energy and higher thermal migration barrier than graphene. By density functional theory calculations, the electronic properties, thermal stabilities and catalytic abilities of Sc and Ti adatoms on single-layer graphdiyne were theoretically investigated. The results indicate that the C sites on graphdiyne surface are the most stable binding sites for Sc and Ti adatoms. The high migration barrier prevents the aggregation of Sc and Ti adatoms on graphdiyne surface. On graphdiyne-Sc or graphdiyne-Ti, CO could be catalytically oxidated by a four-step reaction. The reaction is both energetically and kinetically favorable with low potential barriers. Overall, Sc and Ti adatoms on single-layer graphdiyne would be excellent catalysts for CO oxidation.

Introduction

Substrate-supported noble metal nanoparticles are widely used in heterogeneous catalysis. For a long time, people have insisted in minimizing the size of noble metal nanoparticles to enhance the performance of catalysts. Since low-coordinated metal atoms often act as catalytic active sites, the catalytic activity usually increases with decreasing size of metal nanoparticles. The ultimate size limit for metal particles is the single-atom catalyst, which contains isolated metal atoms scattered on supports. Single-atom metal catalysts anchored to substrate maximize the efficiency of metal atom use and exhibit more superior catalytic activity than conventional metal nanoparticles in many important chemical reactions [1], [2], [3], [4]. In recent years, people have focused on searching for anti-CO catalysts or effective techniques to transform CO into other molecules [5], [6], [7], [8], [9], [10], [11], [12]. As separate and highly active centers, single-atom catalysts have the potential to directly transform CO into other molecules. For example, single-atom Pt₁/FeO_x catalyst shows a high catalytic efficiency to CO oxidation and preferential oxidation of CO in H₂ [1], and single-atom Ir₁/FeO_x catalyst shows remarkable performance in the water gas shift reaction (CO + H₂O→CO₂ + H₂) [3]. Single Au atom embedded in graphene monovacancy was also theoretically predicted as highly efficient catalyst for CO oxidation [13]. The success in single-atom catalysts open a new way to efficiently removing the CO contamination. Meanwhile, the corresponding theoretical research on understanding the catalytic mechanism could provide guidance for future applications of single-atom catalysts.

The high surface energy promotes the aggregation of metal nanoparticles or single-atom catalysts. Appropriate substrates that strongly interact with single-atom catalysts could prevent the aggregation. On the other hand, to enhance the catalytic efficiency, it is beneficial to spread single-atom catalysts on substrates with large specific surface area. In recent years, the rapid development on graphene [14], [15], [16], [17], [18], molybdenum disulfide [19], [20], [21] and other two-dimensional materials provides more choices of substrates for stabilizing single-atom catalysts. With large specific surface area, two-dimensional materials are advantageous to use them as the substrates. However, people found that the interactions between graphene and metal atoms are rather weak because the strong π bonds in pristine graphene are rather chemically inert. Theoretical calculation showed that the adsorption energies of transition adatoms on graphene are about 1eV [22], [23], [24], [25], and the migration barriers of transition adatoms on graphene are about 0.2–0.8 eV, indicating that the transition adatoms would migrate on graphene surface at room temperature [26]. Graphyne and its family, a series of hypothetical two-dimensional carbon allotropes that were predicted about twenty years ago [27], [28], [29], have been considered as possible substrates with large binding energies to single-atom catalysts. The graphyne sheets are composed of sp²-hybridized hexagonal C rings and sp-hybridized CC linkages, in which the additional p_x-p_y π/π* states in the CC bonds could rotate towards any direction perpendicular to the bonds. This makes it possible for the π/π* states to all point towards the single-atom catalyst and lead to large binding energy to metal atoms. In 2010, graphdiyne has been successfully synthesized on the surface of copper via a cross-coupling reaction using hexaethynylbenzene [30]. Recent theoretical studies mainly concentrated on the basic electronic properties of graphdiyne [31], [32], [33] and other hypothetical graphyne-like structures [34], [35], [36], [37]. A recent report [38] has suggested graphdiyne as catalyst for CO oxidation. Meanwhile, our previous works have suggested the large binding energy of single-layer graphdiyne sheet to single metal atoms [39] and small Pt nanoparticles [40]. Thus single-atom catalysts anchored to single-layer graphdiyne sheet would be promising thermally stable catalysts.

In this work, density functional theory (DFT) calculations were employed to investigate the thermal stabilities and catalytic abilities of Sc and Ti adatoms on single-layer graphdiyne. The binding ability of single-layer graphdiyne to Sc and Ti adatoms was found stronger than graphene. The migration barriers of Sc and Ti adatoms on graphdiyne are high enough for preventing the aggregation of these adatoms. Then, minimum-energy path (MEP) calculations were performed to investigate the catalytic CO oxidation by Sc and Ti adatom. According to the results, in O₂ ambient CO could be catalytically oxidated on Sc or Ti adatoms via a four-step reaction. The reaction was found both thermodynamically and kinetically favorable, with stable reaction intermediates and low potential barrier. Overall, with both high thermal stability and catalytic ability, Sc and Ti adatoms on single-layer graphdiyne should be excellent catalysts for removing the CO contamination.