Han
Xiao
ab,
Yihong
Lian
a,
Shiduo
Zhang
a,
Minyi
Zhang
*ab,
Jiye
Zhang
*c and
Chunsen
Li
*abde
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. E-mail: myzhang@fjirsm.ac.cn; Chunsen.li@fjirsm.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cSchool of Materials Science and Engineering, Shanghai University, Shanghai 200444, China. E-mail: jychang@shu.edu.cn
dFujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen University, Xiamen, Fujian 361005, China
eFujian College, University of Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
First published on 21st April 2023
The photocatalytic hydrogenation of CO2 by Cu-deposited ZnO (Cu/ZnO) polar surfaces is investigated through density functional theory (DFT) calculations combined with experimental work. The DFT results demonstrate that, without Cu-loading, CO2 and H2 present weak physisorption on the clean ZnO polar surface, except that H2 undergoes strong chemisorption on the ZnO(000) surface. Cu deposition on the ZnO polar surface could remarkably enhance the CO2 chemisorption ability, due to the induced charge redistribution on the interface of the Cu/ZnO polar surface systems. Additionally, a Cu-nanoisland, which was simulated using a Cu(111) slab model, exhibited strong ability to chemically adsorb H2. Thus, H2 may act as an adsorption competitor to CO2 on the Cu/ZnO(000
), while, in contrast, CO2 and H2 (syngas) may have more opportunity to simultaneously adsorb on Cu/ZnO(0001) to promote the CO2 hydrogenation. These facet-dependent properties lead us to assume that Cu/ZnO(0001) should be a favorable photocatalyst for CO2 hydrogenation. This assumption is further verified by our photocatalysis experiment based on a ZnO single crystal. According to the theoretical and experimental results, the optimal HCOO* reaction pathway for the photocatalytic hydrogenation of CO2 on Cu/ZnO(0001) is proposed. In this optimal HCOO* path, the hydrogenation of CO2* step and hydrogenation of HCOO* step could be promoted by the coupling of a photo-generated spillover proton and a photoelectron on the interface of Cu/ZnO(0001). This research demonstrates the feasibility of the photocatalytic reduction of CO2 on Cu/ZnO(0001), and will help to develop related high-efficiency catalysts.
On the other hand, many studies have also focused on Cu-decorated ZnO, which generally exhibits reduced optical band-gap energy, increased optical absorption ability, and improved photocatalytic properties compared with bare ZnO.22–25 Pawar et al. recently reported that Cu-doped ZnO microstructures showed enhanced performance in the photocatalytic degradation of methylene blue and rhodamine B dyes due to the existence of a large amount of (001) polar surfaces.26 Moreover, Tsang et al. found that the exposed polar face (002) of Cu-deposited ZnO (Cu/ZnO) showed higher selectivity towards the hydrogenation of CO2 to methanol compared with other crystal facets,27 and thus demonstrated the important role of facet type in regulating catalytic reactivity.28 This evokes speculation that the ZnO polar surface in synergy with copper might also exhibit good photocatalytic CO2 hydrogenation ability.
It has been found that the interaction of the deposited Cu and ZnO has a significant impact on the activation and hydrogenation of CO2.5,6,29 However, little is known about the structure–activity relationship between deposited Cu and the polar surfaces of ZnO, such as the active sites, and the mechanism of CO2 activation on polar surfaces. Moreover, no consensus has been reached on the adsorption behavior of CO2 on ZnO(0001) and ZnO(000) polar surfaces in previous experimental and theoretical work.26,27,30 Therefore, a detailed study of how CO2 interacts with clean ZnO polar surfaces as well as Cu/ZnO polar surfaces will help to provide an understanding of the intrinsic mechanism of the photocatalytic hydrogenation of CO2 by Cu/ZnO photocatalyst.
In the present paper, our study focuses on exploring the mechanism of the photocatalytic hydrogenation of CO2 on the Cu-deposited ZnO polar surfaces, ZnO(0001) and ZnO(000). By performing theoretical simulations, we investigated the adsorption behaviors of CO2 and H2 on the polar surface of clean and Cu-deposited ZnO polar surfaces, respectively. The potential active polar surface, Cu/ZnO(0001), is proposed based on the elucidation of adsorption sites, relaxation geometries, and electrical properties, as we shall show. Then, DFT calculations were performed to investigate the reaction mechanism of CO2 hydrogenation on the Cu/ZnO(0001) surface and the favorable reaction pathway in terms of energy was identified. Furthermore, illustrative experiments were done accordingly, and the results were in good agreement with the theoretical predictions. Finally, based on the combined theoretical and experimental works, we propose a possible reaction route for the direct photocatalytic hydrogenation of CO2 on the Cu/ZnO surface.
For the adsorption of Cu atoms and adsorbate molecules (CO2 or H2) on a clean ZnO polar surface, we simulated four symmetry adsorption sites, namely a top-site, bridge-site, fcc-site, and hcp-site (Fig. 1(b)). For the adsorption of CO2 or H2 on the Cu/ZnO system, four symmetry adsorption sites are also considered. The 1st–3rd positions are the edge site, the site close to the Cu atom, and the site far from the Cu atom, respectively (see Fig. 1(c)). The 4th adsorption site is a metallic site located far from the ZnO support. For simplified calculation, an ideal Cu(111) surface with a six-layer slab and a p(4 × 4) unit cell was used. In this model, only the top three layers were relaxed, and k-point meshes of 4 × 4 × 1 were used. The energy of adsorption/binding Ead is defined as:
Ead = Eads/sub − Eads − Esub |
Moreover, we constructed the Cu9 cluster loading on the ZnO(0001) surface with a size of (4a × 4b) and (2 × 2 × 1) k point meshes to explore the CO2 hydrogenation pathways (Fig. 1(d) and (e)). In this model, only the top two Zn–O layers were relaxed, while the bottom four Zn–O layers were kept on their previously optimized crystal positions. The starting geometry of the Cu9/ZnO(0001) surface was first determined by running ab initio molecular dynamics (AIMD) using a Nose thermostat at a temperature of 300 K for 12 ps. The subsequent energy minimized snapshots extracted from the AIMD trajectories were then singled out and optimized.
Further, we investigated CO2 and H2 adsorption on the clean and Cu-deposited ZnO polar surfaces (see Fig. S1 and S2 in ESI†). Based on our calculations, only weak physisorption of CO2 was found on the two clean polar surfaces (see Table S1† and the stable adsorption states of CO2 on the two clean surfaces, Fig. 2(c) and (e)), which is in line with previous theoretical studies.40 In contrast, H2 adsorption on the ZnO polar surfaces showed facet-dependent properties. For the sites of ZnO(0001), H2 adsorption only resulted from the physisorption of H2. However, for the ZnO(000) surface, H2 adsorption on the bridge-site and fcc-site could lead to strong chemisorption of H2. In these cases, the H–H bond of H2 is broken, and two dissociated H atoms are separately bound to two adjacent surface O atoms of the ZnO(000
) surface.
The characteristics of CO2 and H2 adsorption on the Cu/ZnO polar surfaces were investigated by using the most stable Cu/ZnO polar surface with Cu deposited on the fcc-hollow site of the ZnO polar surface (see Fig. S2 in ESI†). The calculated adsorption energies with different deposition sites are listed in Table 1. In addition, in order to simulate the top of the deposited Cu nano-islands on the ZnO polar surfaces in the experiments, both the metallic sites of Cu/ZnO(0001) and Cu/ZnO(000) were simplified and simulated using the Cu(111) slab models (see Fig. 2(k) and (l)).
Cu/ZnO system | Edge | Close | Far | Metallic |
---|---|---|---|---|
(0001)/CO2 | −0.89 | −0.06 | −0.05 | −0.03 |
(0001)/H2 | −0.05 | −0.02 | −0.02 | −0.41 |
(000![]() |
−0.66 | −0.58 | −0.08 | −0.03 |
(000![]() |
−3.57 | −4.45 | −4.67 | −0.41 |
For the Cu/ZnO(0001) surface, CO2 adsorption on the edge site has the lowest adsorption energy of −0.89 eV (see Table 1 and Fig. 2(g)). In this case, the linear CO2 is bent to a bidentate carbonate with an internal angle O–C–O of 126.8°. Its C atom bonds to the Cu atom with C–Cu bond length of 1.98 Å, and one of its O atoms bonds to a surface Zn atom with an O–Zn bond length of 1.96 Å (see Fig. 2(g)). In the case of the Cu/ZnO(000) surface, CO2 adsorption on the edge-site of Cu/ZnO(000
) also leads to the lowest adsorption energy of −0.66 eV (see Table 1 and Fig. 2(i)). In this structure, the C atom of CO2 binds with the O-terminated facet with a C–O bond length of 1.41 Å, and one of its O atoms bonds to the Cu atom with the Cu–O bond length of 1.88 Å. The internal O–C–O angle of CO2 is bent to 126.3°. Thus, Cu deposition on ZnO polar surfaces would highly activate the CO2 and promote the chemisorption of CO2. Moreover, the chemisorption of CO2 occurs only in close proximity to the Cu deposition sites of Cu/ZnO(0001) and Cu/ZnO(000
) surfaces. For the far-sites and metallic sites of Cu/ZnO(0001) and Cu/ZnO(000
), the adsorption of CO2 results from only weak physisorption. This feature demonstrates that Cu deposition on ZnO polar surfaces could play an important role in the chemisorption and activation of CO2.
On the other hand, similar to the case of the clean ZnO(0001) surface, the adsorption of H2 on the Cu/ZnO(0001) surface was unfeasible (see Table 1), except for the metallic sites. H2 adsorption on the Cu(111) surface leads to strong chemisorption of H2 with an adsorption energy of −0.41 eV (see Table 1). Therefore, H2 adsorption on any of the sites of Cu/ZnO(000) could result in the strong chemisorption of H2. Similar to the cases of H2 adsorption on the clean ZnO(000
) surface and Cu/ZnO(000
), the chemisorption of H2 on the Cu(111) surface results in the dissociation of H2. As shown in Fig. 2(l), the two dissociated H atoms are loaded on two neighbouring Cu atoms of the Cu(111) surface (dH–Cu = 1.75 Å). The dissociated H atom might further evolve to the spillover proton among the Cu nano-islands by the photo-generated holes.41
Thus, the above results show that Cu deposition on ZnO polar surfaces would promote CO2 chemisorption on the edge sites of Cu/ZnO(0001) and Cu/ZnO(000). These chemisorbed states rarely occur on the clean ZnO polar surfaces. Moreover, Cu deposition on ZnO(0001) could also promote the chemisorption H2 by inducing the Cu nano-islands on the Cu/ZnO(0001) surface. However, Cu deposition on ZnO(000
) has a minor effect on the H2 adsorption, since H2 adsorption on any of the sites of Cu/ZnO(000
) could result in the strong chemisorption of H2. Therefore, when Cu/ZnO(0001) and Cu/ZnO(000
) are exposed to CO2 and H2, the Cu/ZnO(000
) system could be saturated and poisoned by the strong chemisorption of H2. In contrast, the adsorption competitors CO2 and H2 could be simultaneously chemically adsorbed and activated on Cu/ZnO(0001) (on which the H2 undergoes chemisorption on metallic sites and CO2 chemisorption occurs on edge sites). Thus, we propose that Cu/ZnO(0001) is an ideal surface for the hydrogenation of CO2.
In the chemisorbed CO2 states of the Cu/ZnO polar surfaces, the 2p orbitals of the O and C atoms from CO2 could introduce some states located in the fundamental gaps of these two systems (Fig. 3(a) and (b)). For the chemisorbed CO2 state of Cu/ZnO(0001), the O(CO2) 2p orbital hybridizes remarkably with the valence orbitals of Cu in the top of the valence band. The C(CO2) 2p orbitals contribute to the conduction band, which is mainly dominated by the 4s orbitals of the Zn atoms. In the case of the chemisorbed CO2 state of the Cu/ZnO(000) surface, the O(CO2) 2p orbital hybridizes with the Cu 3d orbital in the valence band and the surface O 2p in the top of the valence band (ranging from −4.6 to 0.2 eV), while the C(CO2) 2p contributes to the conduction band in the high-energy-level region (i.e., from 5 to 6 eV). The PDOS results illustrate that there are substantial covalent interactions between CO2 and the Cu/ZnO polar surface systems, which clearly confirm the formation of chemical bonds between the adsorbed CO2 and the Cu/ZnO polar surface systems.
Analysis of the electronic density difference (see the contour diagrams in Fig. 3(c) and (d)) again confirms the covalent contribution of the interaction between CO2 and the Cu/ZnO polar surface system. For the edge sites of Cu/Zn(0001), the charge density changes mainly emerge on the 2p orbitals of the C and O of CO2, and the Cu atom, as well as the relevant surface Zn atom. Similar charge density changes have been found in the chemisorbed CO2 states of the Cu/ZnO(000) surface, with the charge density changes mainly emerging on the 2p orbitals of C and O of CO2, the 3d orbitals of the Cu atom, and the 2p orbitals of the linked surface O. The charge density changes of the two Cu/ZnO systems indicate the covalent interaction between CO2 and the deposited Cu atom. Moreover, the Cu atom donates electrons to CO2, and the C–O double bonds in CO2 are greatly weakened and elongated. The depleted charge density of Cu and the accumulated charge density on CO2 imply that Cu deposition could result in charge redistribution and improve the electron donation ability of ZnO polar surfaces. As a result, the chemisorption of CO2 is remarkably promoted on the edge-sites of Cu/ZnO polar surfaces.
Bader charge analyses44,45 were applied to further identify the charge states of CO2 adsorption on Cu/ZnO polar surfaces (see Table 2). Without CO2 adsorption, there is significant net negative charge distributed on the Cu atom of Cu/ZnO(0001), indicating that there is electron transfer from ZnO to Cu. This is consistent with our XPS results for Cu/ZnO(0001) (see section 3.4). When CO2 is adsorbed on the Cu/ZnO(0001), CO2 gains electrons with a partially negative charge of −0.9|e|, which is donated from Cu/ZnO(0001). The Cu atom and the surface Zn atoms of Cu/ZnO(0001) inject electrons to CO2 with 0.34|e| and 0.32|e|, respectively. In the case of Cu/ZnO(000) system, Cu is still the primary electron donor, contributing 0.10|e|; however, surface O atoms prefer to accept the negative charges of −0.03|e|, leading to much less negative partial charges of −0.04|e| in CO2. This difference in charge transfer in between Cu/ZnO(0001) and Cu/ZnO(000
) is mainly attributed to their different surface atoms, i.e., the surface Zn atoms of Cu/ZnO(0001) and surface O atoms of Cu/ZnO(000
), which are characterized by electrophobic and electrophilic properties, respectively, as mentioned in ref. 46.
CO2 chemisorbed on Cu/ZnO(0001) | CO2 chemisorbed on Cu/ZnO(000![]() |
|||||||
---|---|---|---|---|---|---|---|---|
Cu | Znb | Surface Znc | CO2 | Cu | Ob | Surface Oc | CO2 | |
a Negative and positive values of ΔQ indicate electron gain and donation, respectively. b For the surface atom involved in the interaction between the adsorbate and substrate. c Sum of the charges of all surface atoms in a unit cell except the atom bonding to the CO2. | ||||||||
Q Cu/ZnO | −0.31 | 0.89 | 6.98 | 0.00 | 0.92 | −1.07 | −7.49 | 0.00 |
Q CO2−Cu/ZnO | 0.03 | 1.21 | 7.08 | −0.90 | 1.02 | −1.11 | −7.52 | −0.04 |
ΔQ | 0.34 | 0.32 | 0.10 | −0.90 | 0.10 | −0.04 | −0.03 | −0.04 |
As a result, Cu deposition on the ZnO polar surface systems would encourage CO2 activation through its electron donor ability. Moreover, Cu/ZnO(0001) possesses surface Zn atoms with electrophobic properties, which would provide more electron transfer to CO2 chemisorption than Cu/ZnO(000). Thus, Cu/ZnO(0001) could induce more electron transfer to promote the formation of chemisorbed CO2 species on its edge site than Cu/ZnO(000
).
Fig. 4(c) shows the Gibbs energy profile and intermediate structures of CO2 hydrogenation by Cu9/ZnO(0001). The initial step corresponds to the chemisorption of CO2 on the interface of Cu9/ZnO(0001). It is energetically favourable to form the exothermic intermediate CO2* with an energy of −0.3 eV. Here, molecules that have been activated are denoted with a star. Subsequently, the hydrogenation of CO2* step could generate two different intermediates, HOCO* and HCOO*, due to the fact that the target atom attacked by H can be either the O or C atom of CO2*. Our calculation results reveal that the formation of both HCOO* and HOCO* are endothermic processes. However, the formation of the intermediate HCOO* requires a smaller activation energy than that of HOCO*. Moreover, since the C atom of CO2* is combined with the interfacial Cu atom, the C atom could be activated by the greatly accumulated charge around the interface of Cu9/ZnO(0001). Particularly, in the HCOO* path, the spillover proton on the Cu nano-islands could couple with the photoelectron to attack the C atom of CO2*, which could greatly promote the formation of the intermediate HCOO*. The intermediate HCOO* is metastable and, subsequently, HCOO* would undergo structural rearrangement to form the bi-HCOO* intermediate with an energetic exothermicity of 0.55 eV. In this structural rearrangement step, the upper O atom of HCOO* would shift to the interface of Cu9/ZnO(0001) and combine with the interfacial Cu and Zn atoms to form the intermediate bi-HCOO*. In our calculations, bi-HCOO* should overcome a large activation energy to accept another H to form the final product trans-HCOOH*. However, the interfacial O atom of bi-HCOO* could be further attacked by the second coupling of a spillover proton and photoelectron, which could promote the process of bi-HCOO* evolving to the final product trans-HCOOH*.29 In view of the HCOO* path, the hydrogenation of CO2* step and hydrogenation of bi-HCOO* step are both feasible to be assisted by the coupling of a spillover proton and a photoelectron. By contrast, in the hydrogenation of CO2* step in the HOCO* path, the upper O of CO2*, as the target of spillover proton attack in the cis-HOCO* formation step, is far from the interface of Cu9/ZnO(0001). Thus, the formation of the intermediate HOCO* is less assisted by the coupling spillover proton and photoelectron near the interface. Therefore, based on our calculations, the HCOO* path is the optimal reaction pathway for the photocatalytic hydrogenation of CO2 by Cu/ZnO(0001).
![]() | ||
Fig. 5 (a) XPS Cu 2p3/2 and (b) O 1s core level spectra; (c) IR spectrum data of the Cu/ZnO(0001) catalyst after the photocatalytic reaction. |
Combining the theoretical and experimental works, we propose the optimal reaction route of the direct photocatalytic hydrogenation of CO2 by Cu/ZnO(0001) as illustrated in Fig. 6. In the oxidation half-reaction, an H2 molecule is adsorbed by Cu nanoislands and then is bound to a Cu atom after disassociation. These two disassociated H atoms are further oxidized to two H+ by the photo-generated holes. In the reduction half-reaction, CO2 is absorbed at the edge site of Cu/ZnO(0001) and becomes a bidentate carbonate structure (intermediate CO2*, structure (1) in Fig. 6). The intermediate CO2* is then attacked by H+ coupling with a photoelectron on the interface, via simultaneous C–Cu bond cleavage and C–H bond formation steps, resulting in the HCOO* intermediate (structure (2) in Fig. 6). The intermediate HCOO* is metastable, and it is energetic favourable for it to undergo structural rearrangement to form the intermediate bi-HCOO* (structure (3) in Fig. 6). Finally, the secondly spillover proton coupled with the photoelectron would attack the interfacial O of the bi-HCOO* intermediate, leading to the final trans-HCOOH* product (formic acid).
![]() | ||
Fig. 6 The proposed schematic illustration of the reaction route of the direct photocatalytic hydrogenation of CO2 on the Cu/ZnO(0001) surface. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr01001e |
This journal is © The Royal Society of Chemistry 2023 |