Elsevier

Applied Surface Science

Volume 537, 30 January 2021, 147881
Applied Surface Science

Pd-doped C3N monolayer: A promising low-temperature and high-activity single-atom catalyst for CO oxidation

https://doi.org/10.1016/j.apsusc.2020.147881Get rights and content

Highlights

  • Pd-doping behavior on the N-vacancy C3N monolayer is analyzed.

  • First time Pd-doped C3N monolayer used for CO oxidation.

  • Both ER and LH mechanism has admirable energy barrier with previous reports.

Abstract

Using the first-principles theory, we theoretically investigated the CO/O2 adsorption and CO oxidation on the Pd-doped C3N (Pd-C3N) monolayer which we proposed as a low-temperature and high-activity catalyst for CO oxidation. Pd dopant is stably anchored on the N vacancy of C3N monolayer, forming the large binding energy of −4.00 eV without the cluster possibility. Given the larger adsorption energy of Pd-C3N monolayer upon O2 compared to CO molecule, we assume that Eley-Rideal (ER) mechanism is the preferred pathway for CO oxidation. For comparison, we also implemented the Langmuir-Hinshelwood (LH) mechanism to comprehensive understand the CO oxidation processes. Our calculations indicated that the energy barriers for ER and LH in the first step are 0.64 and 0.72 eV, with quite large energy drop of 2.99 and 2.0 eV to release a CO2 molecule, respectively. That means, the CO oxidation using Pd-C3N monolayer is fully-energetically favorable even at room temperature, which could give a novel insight into developing novel single-atom catalyst (SAC) based on C3N monolayer with high-efficiency and low-temperature.

Introduction

As a crucial prototype reaction, CO oxidation not merely provides many beneficial information about the mechanistic steps including the adsorption of reactants, the dissociation of O2 and the desorption of CO2 [1], [2], but also stimulates the exploration of innovative atomic-scale catalysts helping alleviate the CO emission from vehicles and industries, or remove the CO toxicant from H2 fuel cells [3], [4], [5], [6], [7]. Previous studies have indicated that some noble metals, such as Pt [8], [9], Au [10], [11], Pd [9], [12], [13], Rh [9], [14] and Ir [9], exhibit excellent performance for CO oxidation, due to their high catalytic activity. Increasingly, single-atom catalyst (SAC) is introduced with the purpose of cutting down the consumption of high-priced metal-based catalysts to realize the above reaction [15], [16], [17], [18], [19]. On the other hand, two-dimension (2D) nanomaterials, such as carbon-based materials [20], [21], hexagonal boron (indium) nitride [22], [23], [24], and carbon nitrides [25], [26], [27], become the most popular substrates to anchor the metal dopants, given their large specific area and high electron mobility for surface interactions [28], [29], [30]. In that case, the combined benefits of metal with good catalytic and 2D nano-support with superior chemical activity make these doped-media as novel-emerging candidates for CO oxidation.

However, the stability for a metal atom on certain surface should be emphasized due to the weak interaction between two analytes. To overcome such problem, defects are designed in 2D materials to enhance their binding force with certain metal adatom, making the trapped dopant more stable on the related nano-support. For example, in the graphene system, the divacancy is created to stably embed a metal atom, and the embedded-systems are demonstrated behaving strong catalytic activity for CO oxidation [31], [32]. Beyond that, in the h-BN system, the introduction of single B-vacancy or N-edge vacancy effectively guarantees the thermal stability of metal-doped configurations, which even provide the inert material with admirable catalytic behavior compared with bulk metal [33], [34].

Recently, the successful synthesis of hole-free C3N monolayer arouses considerable attention in the research community [35]. It is reported that the intrinsic C3N monolayer is an indirect bandgap semiconductor with graphene-like structure, tunable bandgap, notable carrier migration capability and outstanding electrochemical performance [36], [37], [38], which allows its promising applications in electronics and related fields. Nowadays, most of the research focus on the optical and sensing properties of C3N monolayer [39], [40], but its catalytic behavior remains unexplored. In this work, we using first-principles theory investigated the Pd-doping behavior on the single N-vacancy C3N monolayer, and the catalytic performance of Pd-doped C3N (Pd-C3N) monolayer upon CO oxidation. It is found that, the single Pd atom preferred to be trapped at the N-vacancy of the C3N monolayer with a quite high energy barrier for Pd adatom diffusing to the neighboring site, which supports its potential for SAC application. With respect to the catalytic activity, Pd-C3N monolayer behaves comparable performance for CO oxidation with those of traditional noble metals, with lower energy barrier and working temperature as well as high activity. We are hopeful this work can provide some guidance to explore novel C3N-based materials as SAC.

Section snippets

Computational details

We implemented all the calculations spin-polarized within the Dmol3 package [41] where Perdew-Burke-Ernzerhof (PBE) function with generalized gradient approximation (GGA) was employed to treat the electron exchange and correlation terms [42]. The DFT-D method by Grimme was adopted [43] to better understand the Van der Waals force and Long-range interactions. A 2 × 2 C3N supercell with a vacuum region of 15 Å was established to perform the whole simulation reported below, wherein the defined

Pd-doping behavior on single N-vacancy C3N monolayer

The single N-vacancy C3N monolayer is formed by removing one N atom from the optimized C3N supercell, which has the lattice constant of 4.92 Å, in agreement with other theoretical results (4.90 Å [49], [50]). The geometric and electronic structures of N-vacancy C3N monolayer is shown in Fig. 1(a1)–(a2). The distances of the neighboring C atoms around the N-vacancy are all measured to be 2.52 Å, a little longer than those in intrinsic C3N monolayer (2.46 Å) probably due to the repulsive

Conclusions

In this paper, we investigated the CO/O2 adsorption and CO oxidation on the Pd-C3N monolayer using the first-principle theory. Firstly, we analyzed the Pd-doping and Pd-clustering behaviors on the N-vacancy C3N monolayer, which indicates that Pd dopant can be stably anchored on the N vacancy site of C3N monolayer, forming the large binding energy of −4.00 eV without the cluster possibility. The comparable adsorption for CO and O2 showed that Eley-Rideal (ER) mechanism is the preferred pathway

Author contributions

Hao Cui performed the research and wrote this manuscript, and Zhongqi Liu and Pengfei Jia helped analyze the results.

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.

Acknowledgments

This work is supported by Natural Science Foundation of Chongqing (No. cstc2020jcyj-msxmX0500), Fundamental Research Funds for the Central Universities (Grant No. SWU120001), and National Natural Science Foundation of China (Grant No. 61906160).

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