Full length articleHydrogenation of CO2 to formic acid on the single atom catalysis Cu/C2N: A first principles study
Graphical abstract
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, FeOx, 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.
Section snippets
Computational details
Spin-unrestricted density functional theory (DFT) calculations are carried out with the Dmol3 package [59]. The generalized gradient approximation (GGA) method with Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation energy is used [60,61]. DFT semi-core pseudopotentials (DSPPs) core treatment is implemented for relativistic effects, which replaces core electrons by a single effective potential [62]. In addition, double numerical plus polarization (DNP) is chosen as the basis set and the
Adsorption of Cu atom on the C2N monolayer
Various configurations with Cu embedded in the different position of the N6 cavity were considered in the computations (Supporting information, Fig. S1). Among all the active sites, the single metal prefer to be embedded in the cavity of C2N monolayer by bonding with N(1) and N(2) atoms, which is consistent with the previous studies [[44], [45], [46]]. The optimized configuration of the most stable Cu/C2N monolayer is shown in Fig. 1(a). The Cu atom carries 0.485 e charge on the surface of the
Conclusions
The hydrogenation of CO2 to formic acid on the single atom Cu embedded in C2N monolayer was investigated using the first-principles calculation. The reaction can undergo two mechanisms starting from the different initial states, i.e. CO2 and H2 co-adsorption on Cu/C2N (IS1a) and H2 adsorption on Cu/C2N (IS1b). Since CO2@Cu/C2N adsorption system is unstable with the unfavorable interaction energy of −0.18 eV, it was not considered as a reactant for the subsequent reaction. Because the highest
Acknowledgements
This work is supported by National Natural Science Foundation of China (No. 21647007) and the Science and Technology Research Program of Education Department of Jilin Province (No. [2014]B044).
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