Hydrogenation of CO₂ to formic acid on the single atom catalysis Cu/C₂N: A first principles study

Highlights

• The single atom catalysis Cu/C₂N is stable thermodynamically.

• The CO₂ hydrogenation to formic acid catalyzed by Cu/C₂N has two possible mechanisms.

• The reaction can be realized on the Cu/C₂N at the low-temperature due to the low barrier (0.53 eV) of the dominant path.

• This work extends the new application of M/C₂N.

Abstract

C₂N monolayer has been proved to catalyze some important reaction, such as CO oxidation and N₂ reduction reaction efficiently due to the periodic porous structure and the electron enrichment on nitrogen atoms. However the catalytic performance of C₂N in CO₂ reduction still needs comprehensive investigation. In this work, the potential of Cu atom embedded C₂N monolayer (Cu/C₂N), as a single-atom catalyst (SACs) for hydrogenation of CO₂ to formic acid, has been evaluated by the first-principles calculations. The computational results show that the reaction can proceed via two feasible mechanisms, named as path I and path II, which start from the initial co-adsorption of H₂ and CO₂ on Cu/C₂N (H₂ + CO₂@Cu/C₂N) and H₂ adsorption on Cu/C₂N (H₂@Cu/C₂N), respectively. Path II exhibits the obvious superiority due to the low barrier all through the whole channel. The highest energy barrier in path II is only 0.53 eV, which means that CO₂ hydrogenation to formic could be realized on the Cu/C₂N at the room temperature. The high activity of the single atom catalyst Cu/C₂N implies the potential application in the industrial CO₂ hydrogenation. This study also promotes a new path to design catalysts for the reduction of CO₂ and further broadens the range of applications for C₂N-based materials.

Introduction

With the development of industry and economy, more and more fossil fuels have been consumed, which causes the continuous rise of CO₂ concentration in the global atmosphere. As is known to all, carbon dioxide is a major greenhouse gas and the massive emission leads to severe climate deterioration. From another point of view, CO₂ is considered as a huge carbon source due to the high concentration. If CO₂ can be effectively used, not only the greenhouse effect can be alleviated, but also the problem of energy shortage can be solved to some extent. Therefore, more and more researchers are working on CO₂ reduction and transformation to the carbon containing inorganic or organic fuels [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]].

The CO₂ resource utilization, such as conversion into the high value-added chemical products through chemical catalysis or biological immobilization has become a research hot spot in recent years [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]]. The CO₂ can be chemically converted into methanol, methane, formic acid, dimethyl carbonate and other chemicals by hydrogenation, photocatalytic, electrocatalytic, biocatalytic reduction. In these conversions, due to the excellent stability, high catalytic activity and significant atomic efficiency, single atom catalysts have attracted special attention [[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. In 2011, a single atomic platinum catalyst was synthesized for the first time, and the single atom catalyst showed excellent catalytic performance in CO oxidation reaction, and on this basis the concept of “single atom catalysis” was proposed [31]. Single atomic catalyst plays a vital role in CO₂ conversion, which can catalyze the hydrogenation of CO₂ to methanol, methane, formic acid and so on. Liu et al. theoretically studied the methanol synthesis from CO₂ and H₂ on single metal-modified model Mo₆S₈ catalyst using density functional theory (DFT) [32]. Their computations showed that atomic metal improve the activity of Mo₆S₈ during the methanol production. Liu's group also summarized the activity and reaction mechanisms of the oxide-supported metal catalysts, which are selective to produce CO, CH₃OH, and CH₄. In Sirijaraensre's report, the Cu-embedded graphene (Cu/dG) shows promising catalytic activity for the direct transformation of formic acid from carbon dioxide and hydrogen molecules [33].

Due to the important role of the catalysts in the CO₂ hydrogenation, design and synthesis of the more efficient catalyst are of scientific and practical significance. Without doubt, Single atom catalyst is the most competitive for the hydrogenation of CO₂ among all kinds of catalysts. Various substrate materials have been used as the support of single atom catalysts, including metal oxides, FeO_x, CeO₂, Al₂O₃; two-dimensional (2D) materials, graphene, graphyne, graphitic carbon nitride (g-C₃N₄) [42,43], hexagonal BN (h-BN), and 2D polymeric metal-phthalocyanine sheet due to their large specific surface areas, good electronic and thermal properties. Recently, a new two-dimensional layered material C₂N monolayer with high thermal and structural stability was obtained experimentally [34], which is easy and economic to synthesize. The special N₆ cavities uniformly distributed in the C₂N monolayer serve as the optimal site for capturing individual metal atom, and therefore the C₂N monolayer can be used as a single atom supporter like graphene and g-C₃N₄ [35]. Recent reports showed that inserting a single metal atom (M) into C₂N monolayer (M/C₂N) can catalyze CO oxidation at low temperature with a remarkable catalytic effect [[44], [45], [46]]. Besides, some studies have also reported that a single metal atom anchored on a C₂N monolayer can act as a catalyst for N₂ reduction reaction, oxygen reduction reaction and hydrogen evolution reaction [[47], [48], [49], [50], [51], [52], [53]], predicting that M/C₂N may become a new star in the single atom catalyst family. Up to our best knowledge, the catalytic hydrogenation of CO₂ on M/C₂N has not been reported yet. Therefore, the performances of C₂N-supported single atom catalysis in the hydrogenation of CO₂ still remain to be investigated.

In this work, CO₂ hydrogenation reaction catalyzed by single atomic catalyst Cu/C₂N was investigated using density functional theory. There are two reasons for using copper atom as the embedded metal. On one hand, the first sub-group (Cu, Ag, Au) is found to be highly active in the hydrogenation of carbon dioxide [33,[54], [55], [56], [57], [58]]. On the other hand, copper is readily available, inexpensive, environmentally friendly and abundant in storage. Our calculated results indicated that the Cu/C₂N is a promising single atomic catalyst for CO₂ hydrogenation.