Elsevier

Catalysis Today

Volume 368, 15 May 2021, Pages 148-160
Catalysis Today

Single Cu atom supported on modified h-BN monolayer as n-p codoped catalyst for CO oxidation: A computational study

https://doi.org/10.1016/j.cattod.2020.03.058Get rights and content

Highlights

  • 2D h-BN containing C dopants or B vacancies could stabilize single Cu atom.

  • CO oxidation follows termolecular Eley-Rideal mechanism on the Cu/modified h-BN.

  • Cu/Vb-BN shows superior catalytic performance for low temperature CO oxidation.

  • Cu/CBN behaves better for CO oxidation at elevated temperature.

Abstract

Single atom catalysts (SACs) have aroused keen interests recently because of their superior catalytic performance and high atomic utilization. In this work, CO oxidation over the single Cu atom supported on two kinds of p-type doped (hole-doped and C-doped) hexagonal boron nitride (called Cu/Vb-BN and Cu/CBN hereafter) monolayer was systemically studied by DFT calculations. The good stability of the two SACs derives from obvious electrons transfer and orbital hybridization between Cu atom and the support. Both of the SACs follow the termolecular Eley-Rideal (TER) mechanism with OCOOCO* as intermediate. The barrier of rate-limited step is only 0.45 eV for Cu/Vb-BN, meaning its potential applications for low-temperature CO oxidation, whereas 0.90 eV for Cu/CBN, indicating it more suitable for higher temperature reaction because of enhanced adsorption strength of reactant and structural stability. The results of this work provide a theoretical evidence that the low-cost Cu supported on p-type h-BN monolayer catalyst could be a promising candidate for oxidation reactions.

Introduction

Compared with nanoparticles and cluster, the so-called single-atom catalysts (SACs) have special electronic and geometric structures [1]. After the terminology was first defined in 2011 to describe the high CO oxidation activity of single Pt atoms supported on FeOx [2], SACs have been widely studied in many reactions, such as WGS [3], CO2 hydrogenation [4], ammonia synthesis [5], HER [6,7] et al. due to their superior catalytic performance and 100 % atomic utilization. The activities and stabilities of SACs were closely related to the support, which was a key issue during for SACs applications [8]. In general, there are two ways to stabilize the single metal atom. One is the metal atom form the coordinate bond with the non-metallic atom of the support, such as O atoms on the oxide support [9,10] and N, S, P atoms on the C-rich support [11]. The other is the metal atom bond with the metal atom on the support, for example, Pt atom on the Pt-Cu single-atom alloy [12]. The interaction also has a strong impact on the nature of the single metal atom and the following reaction performance.

The transition metal oxides (TMOs) and zeolites are the most common support for SACs [9,10,13]. Until now, FeOx [2,14], CeO2 [9,15], TiO2 [16] and MnOx [17] was reported to be capable of anchoring single metal atom to prepare SACs by using improved wet chemistry method, the mass-selected soft-landing technique or atomic layer deposition (ALD) method [2,18,19]. In addition, two-dimensional (2D) materials such as graphene [20,21], graphyne [22], MoS2 [23] and the hexagonal boron nitride (h-BN) [24] could also be the suitable support for SACs because of their large specific surface area and excellent thermal stability. Very recently, Wang et al. reported the epitaxial growth of a large-sized single-crystal h-BN monolayer, overcoming excessive nucleation and twin boundaries problems [25]. Though it has similar planar structure like graphene, the h-BN has shown perfect thermal stability which could be stable under high temperature up to 1000 K [26,27]. Furthermore, Jin et al. [28] found that the lattice defects could be generated by the controlled energetic electron beam irradiation in which the boron vacancies are the dominant ones, in line with the computational result by Du et al. [29]. These boron vacancies (VB) could provide the adsorption sites for the metal atom such as Co, Ru, Fe and Cu to prepare SACs for the catalytic reaction [30].

Because most of the doped transition metal atom often showed n-type character, transforming h-BN into p-type semiconductor could be an idea to stabilize the SACs as a result of the strongly electrostatic interaction between n-type and p-type dopants, known as the n–p co-doped method [31]. Recently David et al. pointed out that the h-BN with the presence of B vacancies show the strong p-type character [32]. Besides, replacing N with low-valence element could reach this goal as well [33]. So the C doped h-BN was chosen as the alternative support to make a comparison with the support with B vacancies as mentioned above.

Considering its irreplaceable role in industrial and academic applications, CO oxidation is regarded as the representative model reaction for the investigating the performance of chosen catalysts. Though various catalysts have been confirmed to be efficient in CO oxidation, the typical ones usually contain noble metals such as Pt and Pd [[34], [35], [36]]. It’s meaningful to downsize the catalysts to the atomistic level or develop base metal-based ones in order to reduce the high cost of noble metal, while the SACs could meet the requirement. Copper, as one of the cheapest metal, was expected to be accessible for large-scale application and Cu/h-BN catalysts has been investigated for CO oxidation in several studies. For example, Lin et al. [30] paid attention to the adsorption of CO and O2 on Cu/h-BN and Liu et al. [57] investigated two possible pathways for CO oxidation on Cu/h-BN, implying the reaction proceeding through LH mechanism. However, the new mechanism such as ‘TER’ still needs to be discussed in details. Furthermore, the comparison between single Cu atom supported on different kinds of modified h-BN for oxidation reaction haven’t been reported to our knowledge.

In this work, we present an extensive theoretical investigation of the electronic structure and stability of single Cu atom on two different p-type (hole-doped and C-coped) h-BN monolayer by computational method. In order to understand their catalytic performance in a deeper way, CO oxidation was used as a probe reaction. Three possible reaction pathways were studied based on the calculated activation barrier. Our studies shed light on developing Cu/h-BN and Cu/CBN catalysts for oxidation reactions.

Section snippets

Theoretical methods

The spin-polarized calculations were performed with the Perdew–Burke–Ernzerhof (PBE) [37] functional within the generalized gradient approximation (GGA) [38,39]. The projector-augmented wave (PAW) method was used to treat the core–valence electron interaction [40,41]. The valence electronic states were expanded in plane wave basis sets with a cutoff energy of 400 eV. The long-range interaction has an obviously influence on the adsorption nature of the reactant species on the 2D materials and

Structural stability of single Cu atom supported on p-type doped h-BN

Before paying attention to CO oxidation, the stability of Cu atom on the h-BN should be checked firstly. The optimized structures of Cu atom located on different supports were summarized in Fig. 2. For the pristine h-BN monolayer, the most stable configuration of Cu adsorption is on the top of N site with 2.175 Å and corresponding adsorption energy of 0.40 eV, similar with the Ag atom binding with stoichiometric h-BN [24]. The charge transfer from the Cu atom to the neighboring N atom is 0.01

Conclusion

In this work, single Cu atom supported on the two kinds of p-type doped (hole-doped and C-doped) h-BN monolayer as n–p codoped SACs was systematically investigated by DFT calculations. From the calculation, both of SACs show good anti-sintering ability while the Cu supported on the h-BN with B vacancies seems to be more stable than the C-doped one. To explore their catalytic application, CO oxidation was used as a probe reaction. The adsorption of CO and O2 species is stronger on the Cu/CBN

CRediT authorship contribution statement

Minhua Zhang: Conceptualization, Methodology, Supervision. Jingbo Du: Data curation, Writing - original draft. Yifei Chen: Visualization, Investigation, Validation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (No. 21406159)

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