Pseudo-copper Ni–Zn alloy catalysts for carbon dioxide reduction to C₂ products

Abstract

Electrocatalytic CO₂ reduction reaction (CO₂RR) to obtain C₂ products has drawn widespread attentions. Copper-based materials are the most reported catalysts for CO₂ reduction to C₂ products. Design of high-efficiency pseudo-copper catalysts according to the key characteristics of copper (Cu) is an important strategy to understand the reaction mechanism of C₂ products. In this work, density function theory (DFT) calculations are used to predict nickel–zinc (NiZn) alloy catalysts with the criteria similar structure and intermediate adsorption property to Cu catalyst. The calculated tops of 3d states of NiZn₃(001) catalysts are the same as Cu(100), which is the key parameter affecting the adsorption of intermediate products. As a result, NiZn₃(001) exhibits similar adsorption properties with Cu(100) on the crucial intermediates *CO₂, *CO and *H. Moreover, we further studied CO formation, CO hydrogenation and C–C coupling process on Ni–Zn alloys. The free energy profile of C₂ products formation shows that the energy barrier of C₂ products formation on NiZn₃(001) is even lower than Cu(100). These results indicate that NiZn₃ alloy as pseudo-copper catalyst can exhibit a higher catalytic activity and selectivity of C₂ products during CO₂RR. This work proposes a feasible pseudo-copper catalyst and provides guidance to design high-efficiency catalysts for CO₂RR to C₂ or multi-carbon products.

Graphical abstract

Keywords

pseudo-copper catalysts / surface and electronic structure / adsorption abilities / Ni–Zn alloys / CO₂RR C₂ products / DFT

1 Introduction

The excessive consumption of fossil fuels dramatically increases CO₂ concentration in the atmosphere and results in global warming and energy crisis. Developing and utilizing renewable energy sources to optimize the energy structure have received widespread attention [1–6]. Electrocatalytic or photocatalytic CO₂ reduction reactions (CO₂RR) can convert intermittent electricity or solar energy into available chemical energy [7–11], and accelerate the carbon cycle. CO₂RR to obtain C₂ products (ethylene [12–14] and ethanol [15–17], etc.) with higher energy density than C₁ products has become the focus in the energy catalysis field [18, 19]

Copper-based (Cu-based) materials are the most widely reported CO₂ reduction catalysts to generate C₂ products [19–21], due to its unique surface structure [22–24] electronic structure [25–29] and intermediate adsorption ability [30–33]. Many researches focus on improving the Faradic efficiency (FE) of C₂ products by forming Cu-based alloys, such as Cu–Ag [34, 35] and Cu–Zn [36, 37] alloys. In addition, other non-copperbased alloys (Ni–Al [38, 39] Ni-Ga [39, 40] and Pd–Au [41] alloy) have been reported to produce low yield of C₂ products, which indicates the possibility of non-copper-based catalysts producing C₂ products. According to the key characteristics of copper-based catalysts for C₂ products to design high-efficiency pseudo-copper catalysts is an important strategy to enhance the activity and selectivity of C₂ products. There are two criteria for evaluating pseudo-copper catalysts to generate C₂ products: (i) Copper-like surface structure and electronic structure. Cu(100) with unique surface structure sensitivity [23, 24, 42] and electronic structure [25–29] can control the activity and selectivity toward C₂ products. (ii) Copper-like adsorption abilities of *CO₂, *CO and *H. These adsorption abilities are the essential descriptors for CO₂ activation [43, 44] CO hydrogenation, C–C coupling [31, 32], and hydrogen evolution reaction (HER) [30, 33]. Based on above criteria, design of pseudo-copper catalysts with high-efficiency C₂ products is promising and significant.

Alloying is a feasible strategy for obtaining pseudo-copper catalysts [45], because the alloying effect is an effective method to regulate the adsorption abilities, surface and electronic structure of metal-based catalysts [38, 46]. The surface and electronic structure are primary consideration of component metals in pseudo-copper alloy catalysts. Nickel (Ni) and Zinc (Zn) are adjacent to Cu in the periodic table of elements. Ni has the face-centered cubic (fcc) lattice similar with Cu and its 3d states is adjustable [29]. Zn exhibits excellent potential in adjusting electronic structure of alloy catalysts [47]. Ni–Zn alloy can exhibit a similar electron/atom (e/a) ratio with Cu, which indicates that Ni–Zn alloy will have a stable copper-like lattice structure [48]. In addition, the electronic structure and intermediate adsorption ability of Ni–Zn alloys can be tuned by interaction between Ni and Zn. Ni–Zn alloys with copper-like surface and electronic structure, and intermediate adsorption ability are potential to obtain C₂ products in CO₂RR.

In this work, NiZn(110) and NiZn₃(001) are selected as the active facets due to their similar square geometric site structure to Cu(100). The CO₂ formation and C–C coupling on NiZn(110) and NiZn₃(001) are further investigated to analyze the feasibility of C₂ products. The free energy profiles of C₂ products formation are used to study the reaction mechanism of NiZn(110) and NiZn₃(001). The calculated results indicate that the NiZn and NiZn₃ alloys exhibit copper-like properties with these critical criteria. The energy barrier of C₂ products formation on NiZn₃(001) is even lower than Cu(100). Therefore, the NiZn and NiZn₃ alloys are potential pseudo-copper catalysts for CO₂ reduction to C₂ products. This work proposes a feasible pseudo-copper catalyst and provides guidance to design high-efficiency catalysts for CO₂RR to C₂ or multi-carbon products.

2 Computational methods

The Vienna Ab initio Simulation Package (VASP) was used to perform spin-polarized DFT [49–51]. The projector augmented wave (PAW) method was applied to describe the effective potential between the ion core and the electrons. The generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional was adopted to calculate the exchange-correlation energy [52]. We set the kinetic cutoff energy of the plane wave as 450 eV. The convergence criteria for force and energy were set as 0.02 eV·Å⁻¹ and 10⁻⁵ eV per atom, respectively [53]. We built the (3 × 3 × 4) surface supercells for all slab models, including NiZn(110), NiZn(101), NiZn₃(001), NiZn₃(100), Cu(100) Cu(111), Ni(100), Ni(111), and Zn(001). The corresponding structures are shown in Fig. S1. For the Brillouin zone integration, we used a Monkhorst–Pack mesh with 3 × 3 × 1 K-points. The computational details of the adsorption energy and Gibbs free energy are shown in the Supporting Information.

3 Results and discussion

3.1 Surface and electronic structures

Cu(100) is proven to be the most favorable surface orientation for C–C coupling [22], which can be attributed to its square ensemble sites [23, 24]. Like Cu, NiZn and NiZn₃ alloys also have a face-centered tetragonal (fct) structure. As shown in Fig. S1, the NiZn(110), NiZn(101), NiZn₃(001), NiZn₃(100), Ni(100) and Zn(001) exhibit the similar square geometric site structure to Cu(100). Furthermore, Table S1 compares the calculated lattice constants of these facets. The lattice constants of Cu(100) were calculated to be a = b = 11.04 Å, while the Ni(100) and Zn(001) were a = b = 10.53 Å and a = b = 11.25 Å, respectively. This structural difference makes a big difference in catalytic performance. Interestingly, the lattice constants of NiZn(110) (a = 9.80 Å, b = 11.59 Å) and NiZn₃(001) (a = b = 11.01 Å) are more closer to Cu(100) [Fig. 1(a)] due to the structure rearrangement of Zn on Ni in alloy phases. The basic site configuration of NiZn(110) and NiZn₃(001) can be same as that of Cu(100), which has a great influence on the whole catalytic C₂ formation reaction process.

As shown in Fig. 1(b), the d-band of Cu(100) mainly distributes below the Fermi level (E_F), and its top of the 3d states are located around –1.8 eV. The anti-bonding states are filled for Cu(100), which leads to Cu(100) has favorable adsorption for key intermediates [28]. As for Ni(100), the d-band cross the E_F and part of the antibonding states is empty for Ni(100), which will lead to strong adsorption abilities of intermediates and not conducive to further activation of adsorbate [29]. The d-band of Zn(001) mainly distributes at –8 eV, which results in too weak adsorption of intermediates. Both NiZn(110) and NiZn₃(001) exhibit copper-like electronic structure in the density of states (DOS) calculations. Moreover, the top of the 3d states of NiZn₃(001) is almost the same to Cu(100). These results suggest that the formation of the alloy with Zn efficiently bring the apparent depression of the 3d states of Ni near E_F, which makes the DOS of NiZn alloys similar to that of Cu.

The d-band position is closely related to the binding strength between intermediates and catalysts. In detail, the electronic interaction between the adsorbate and the 3d states lead to the formation of bonding and antibonding states [29, 54]. The lower position (relative to the Fermi level) of the d-band, the weaker binding strength, due to the occupancy of anti-bonding states. Compared with Ni(100), the adjusted d-band positions of NiZn alloys indicate suitable adsorption properties of intermediates, similar to Cu(100), which is favorable to the further activation of intermediates (CO₂, CO), and avoiding CO poisoning or strong hydrogen evolution reactions (HER).

3.2 Adsorption abilities of *CO₂, *CO and *H

Copper-like adsorption abilities of *CO₂, *CO and *H are regarded as the key criterion to evaluate the potential catalytic performance of pseudo-copper catalysts. The adsorption abilities of *CO₂, *CO and *H directly influence the CO formation [43, 44] and C–C coupling [31, 32, 55, 56], and HER [30, 33], respectively. The adsorption energies of *CO₂, *CO and *H on different facets are shown in Table S2.

The CO₂ reduction process is accompanied by the competitive HER and hydrogenation process. The relationship between the adsorption energy of *H (∆E_H) and reactants {}^{*}{\text{CO}}_2(\text{Δ}{E}_{{\text{CO}}_2}) and the relationship between ∆E_H and crucial intermediate *CO (∆E_CO) both are very important. As shown in Fig. 2(a), the upper right region indicates that both the \text{Δ}{E}_{{\text{CO}}_2} and ∆E_H are weak, not conducive to the CO₂ reduction reaction. The lower left region indicates that ∆E_H is much stronger than \text{Δ}{E}_{{\text{CO}}_2} , which will lead to the active site being occupied by *H, and is not conducive to CO₂ activation. In the circle area, NiZn(110) and NiZn₃(001) exhibit copper-like *CO₂ and *H adsorption energies. Proper \text{Δ}{E}_{{\text{CO}}_2} and ∆E_H indicate the suitable coordination of CO₂ adsorption and CO₂ hydrogenation process, which provides the possibility for the generation of intermediate CO. The adsorption abilities of *CO and *H are shown in Fig. 2(b). Similarly, the upper right region indicates that both the ∆E_CO and ∆E_H are weak, not conducive to the CO hydrogenation and C–C coupling reaction. The lower left region indicates that both ∆E_CO and ∆E_H are strong, which will lead to CO poisoning and severe competing HER. In the circle area, NiZn(110) and NiZn₃(001) exhibit copper-like *CO and *H adsorption energies. Proper ∆E_CO and ∆E_H indicate the suitable coordination of CO hydrogenation and C–C coupling process, which provides the possibility for the generation of C₂ products.

The Cu-like adsorption abilities of NiZn(110) and NiZn₃(001) are mainly due to the fact that Zn adjusts the surface and electronic structure of Ni. By screening and evaluation, NiZn(110) and NiZn₃(001) are the potential pseudo-copper catalyst to generation C₂ products during CO₂RR.

3.3 Reaction pathway of *CO formation

CO is the key intermediate for the formation of C₂ products, and the first intermediate for CO formation from CO₂ is *COOH [44, 46] The reaction pathways of CO formation are analyzed on NiZn(110) and NiZn₃(001) [Fig. 3(a)]. The formation energy barriers of *CO (∆G_∗CO) on NiZn(110) and NiZn₃(001) are 0.09 eV and 0.21 eV, respectively, which both are much lower than Cu(100) (0.53 eV) (Table S3). The adsorptions of *COOH intermediate on NiZn(110) and NiZn₃(001) are much stronger than on Cu(100), which is the key factor of lower formation energy barriers of *CO on NiZn(110) and NiZn₃(001).

In addition, the competitive reaction of formate (*HCOOH) formation is also considered on NiZn(110) and NiZn₃(001) [Fig. 3(b)]. The formation energy barriers of formate (∆G_∗HCOOH) on NiZn(110) and NiZn₃(001) are 0.62 eV and 0.53 eV (Table S4), which is higher than formation energy barriers of *CO. This result indicates that the formation of *CO on NiZn(110) and NiZn₃(001) is easier than *HCOOH. As for Cu(100), the formation energy barriers of formate (∆G_∗HCOOH) is 0.45 eV, which is slightly lower than ∆G_∗CO of 0.53 eV. It suggests that NiZn(110) and NiZn₃(001) have higher selectivity to *CO than Cu(100).

3.4 Reaction pathway of C–C coupling

C–C coupling is reported to be the crucial reaction for the C₂ formation [57]. There are three main pathways for the C–C coupling reaction [32] [Fig. 4(a)]: (i) *CO–CO intermediate formed via *CO coupling with CO_(g); (ii) *CHO is formed via *CO hydrogenation, and coupling with CO_(g) to form *CHO–CO intermediates; (iii) *CO hydrogenation to form *COH, then *COH hydrogenation to form *CH₂ intermediate, and the C–C bond formed via *CH₂ dimerization.

The formation energy barriers of three intermediates (*CHO, *COH and *CO–CO) during C–C coupling are calculated to study the C₂ product evolution. As shown in Table S5, both the free energy barriers of *COH and *CO–CO are much higher than *CHO. Therefore, *CHO–CO is considered as the main intermediates for C–C coupling reaction on NiZn(110), NiZn₃(001), and Cu(100)

In the *CHO–CO pathway, NiZn(110) and NiZn₃(001) show similar reaction trends to Cu(100) [Fig. 4(b)]. Compared with Cu(100) and NiZn(110), NiZn₃(001) has the lowest free energy barrier (0.60 eV) (Table S6) which can be attributed to the optimal Cu-like surface/electronic structure and intermediate adsorption abilities of NiZn₃(001). For NiZn(110), although it has the lowest free energy barrier of *CO→*CHO (0.57 eV), the energy barrier of *CHO→*CHO–CO is higher due to the strong *CO adsorption abilities on NiZn(110), which is detrimental to the C–C bond formation.

3.5 Reaction mechanism of C₂ products formation

To understand the reaction mechanism of C₂ products formation on NiZn(110) and NiZn₃(001), the C₂ intermediates in the reaction pathway from *CHO–CO to *CHO–CH are investigated. *CHO–CH is the crucial precursor of the C₂ product, and the *CHO–CH₂ intermediate formed by its hydrogenation is considered to be the bifurcation of the pathway for C₂H₄ and C₂H₅OH formation [32, 58].

Three possible pathways of *CHO–CO hydrogenation on NiZn(110), NiZn₃(001) and Cu(100) are considered: alternate (*CHO–CHO) (Fig. S2 and Table S7), hybrid (*CHO–COH) (Fig. S3 and Table S8) and distal (*CHOH-CO) (Fig. S4 and Table S9) pathways. The lowest free energies of *CHO–CH formation on NiZn₃(001) and Cu(100) both are alternate pathway[Fig. 5(b) and Fig. S5], while that on NiZn(110) is hybrid pathways [Fig. 5(a)].

The main formation mechanism of C₂H₄ and C₂H₅OH are formed via the deoxygenation and hydrogenation of *CHO–CH₂ intermediate, respectively. Figure 5(c) compares the free energies of the reaction pathway from CO₂ to *CHO–CH₂ on NiZn(110) (Fig. S6), NiZn₃(001) (Fig. S7), and Cu(100) (Fig. S8). In detail, the step of *CO hydrogenation is the rate determination step (RDS) on NiZn₃(001) and Cu(100), but the RDS on NiZn(110) is the *CHO–CO formation via *CHO and *CO coupling. Moreover, NiZn₃(001) shows the lowest energy barrier (0.60 eV) for CO₂RR to *CHO–CH₂, compare with Cu(100) (0.70 eV) and NiZn(110) (0.82 eV). This calculation result is consistent with the screening criteria. NiZn₃(001) is an ideal pseudo-copper catalyst for CO₂ reduction to C₂ products.

4 Conclusion

In summary, we designed the Ni–Zn alloys as the promising pseudo-copper catalysts and predicated reaction mechanism for CO₂RR to C₂. DFT calculations revealed that NiZn and NiZn₃ had the two key criteria of pseudo-copper alloys: copper-like surface and electronic structure and intermediate adsorption abilities. NiZn₃(001) alloy had lower free energy barrier (0.60 eV) than Cu(100) (0.70 eV) and NiZn(110) (0.82 eV) in the reaction from CO₂ to *CHO–CH₂, and the C–C bond was formed through the *CHO and CO coupling. The *CHO–CH₂ intermediate was preferred formed along the alternate hydrogenation pathway. Importantly, the copper-like nature of NiZn and NiZn₃ alloys were derived from the interaction of Ni and Zn, which tuned their lattice structure and 3d state toward Cu. Here, two critical criteria of the pseudo-copper catalyst are established to form C₂ products from CO₂, and provides guidance to design high-efficiency catalysts for CO₂RR to C₂ or multi-carbon products.

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