Nanosheet Supported Single-Metal Atom Bifunctional Catalyst for

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Nanosheet Supported Single-Metal Atom Bifunctional Catalyst for Overall Water Splitting Chongyi Ling, Li Shi, Yixin Ouyang, Xiao Cheng Zeng, and Jinlan Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02518 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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Nanosheet Supported Single-Metal Atom Bifunctional Catalyst for Overall Water Splitting Chongyi Ling,1 Li Shi,1 Yixin Ouyang,1 Xiao Cheng Zeng,2,3* Jinlan Wang1,4* 1

School of Physics, Southeast University, Nanjing 211189, China

2

Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of

Nebraska, Lincoln, NE 68588, USA 3

Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and

Technology of China, Hefei, Anhui 230026, China 4

Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal

University, Changsha 410081, China

KEYWORDS: Boron monolayer, Single atom catalyst, Bifunctional catalyst, Electrochemical water splitting

ABSTRACT: Nanosheet supported single-atom catalysts (SACs) can make full use of metal atoms and yet entail high selectivity and activity, while bifunctional catalysts can enable higher performance while lowering the cost than two separated unifunctional catalysts. Supported single-atom bifunctional catalysts are therefore of great economic interest and scientific importance. Here, based on first-principles computations, we report a design of the first single-

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atom bifunctional eletrocatalyst, namely, isolated nickel atom supported on β12 boron monolayer (Ni1/β12-BM), to achieve overall water splitting. This nanosheet supported SAC exhibits remarkable electrocatalytic performance with the computed overpotential for oxygen/hydrogen evolution reaction being just 0.40/0.06 V. Ab initio molecular dynamics simulation shows that the SAC can survive up to 800 K elevated temperature, while enacts a high energy barrier of 1.68 eV to prevent isolated Ni atoms from clustering. A viable experimental route for the synthesis of Ni1/β12-BM SAC is demonstrated from computer simulation. The desired nanosheet supported single-atom bifunctional catalysts not only show great potential for achieving overall water splitting, but also offers cost-effective opportunities for advancing clean energy technology.

Introduction Miniaturization of the metal catalysts can be a cost-effective way to boost their catalytic performance by enhancing the surface area/volume ratio, strengthening the selectivity toward a special product, as well as improving their intrinsic catalytic activity.1-8 Nanosheet supported SACs are the ultimate low-end limit for metal particles. Over the past few years, SACs have attracted enormous attention owning to their distinct advantages in full use of all metal atoms, their homogenous active sites with distinct selectivity, as well as their much improved catalytic activity over conventional metal nanoparticles.9-12 However, reduced metal-particle size can also result in increased surface free energy, thereby making the SACs more prone to aggregation into metal clusters.10 A strong support that can anchor the individual metal atom firmly to prevent metal atoms from aggregation is crucial to maintain high performance of SACs. To date, various


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metal oxides (such as FeOx), crystalline metal surfaces, graphene sheets, metal-organic frameworks, among others, have been investigated as the support for SACs.9, 12-16 For overall water splitting, bifunctional catalysts always show higher performance than two separate unifunctional catalysts, as the best working conditions for the two unifunctional catalysts are generally not the same.17, 18 Moreover, bifunctional catalysts can lower the product cost due to the less usage of equipment and less preparation and optimization procedures compared with two separate unifunctional catalysts. Hence, the development of bifunctional catalysts for the overall water splitting has gained increasing attention. Several candidates have been proposed in the past few years.17, 19-26 However, to our knowledge, nanosheet supported single-atom bifunctional catalysts have not been reported in the literature. In this letter, we present a computer-aided design of the first nanosheet supported bifunctional single-atom catalyst, namely, β12-boron monolayer (β12-BM) supported nickel atom, to achieve the overall electrochemical water splitting. To this end, we have examined six possible SAC systems, TM1/β12-BM (TM = Ti, V, Mn, Fe, Co, and Ni), out of 18 candidates. Among the six systems, the Ni1/β12-BM exhibits high oxygen evolution reaction (OER) activity coupled with low overpotential of 0.40 V, and an excellent hydrogen evolution reaction (HER) performance with the Gibbs free-energy change of hydrogen adsorption being 0.06 eV. Ab initio molecular dynamics simulation indicates that the β12-boron monolayer supported bifunctional SAC presents high stability up to 800 K. Specifically, the diffusion barrier of Ni atom on β12-BM amounts to 1.68 eV, high enough to prevent Ni atoms from clustering. A possible experimental route to synthesize the Ni1/β12-BM SAC is elucidated through computer simulation, with using the NiCl2 as a precursor. Computational details


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The first-principles computations were performed using the projector augmented wave (PAW) method27 as implemented in the Vienna ab initio simulation package (VASP 5.4).28, 29 The generalized gradient approximation in the Perdew-Burke-Ernzerhof (PBE) form30, 31 and a cut-off energy of 600 eV for plane-wave basis set were adopted. The convergence threshold was 10-5 eV and 0.03 eV/Å for energy and force, respectively. A vacuum space of at least 15 Å was used so that the interaction between two periodic units can be neglected. Supercells consisting of 4 × 3 × 1 unit cells were used and the Brillouin zones were sampled by Monkhorst-Pack k-point meshes. Both thermal and dynamical stabilities of the catalyst were examined by using ab initio molecular dynamics (AIMD) simulation and climbing nudged elastic band method32,

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,

respectively. The HER and OER performances were evaluated by computing the reaction free energy (∆G) based on spin-polarized calculation for each step via the equation ∆G = ∆E + ∆EZPE – T∆S

(1)

where the ∆E is the adsorption energy of a given group, ∆EZPE and ∆S are the difference in the zero-point energy and the difference in entropy, respectively, between the adsorbed state and the corresponding free-standing state. The calculated EZPE and S are presented in Table S3. More computational details are given in the Supporting Information. Results and Discussion Boron monolayers have been recently synthesized in the laboratory, and become new members of two-dimensional atomic-layered materials.34, 35 Typically, BMs are composed of electron-deficient hexagonal holes and electron-rich trigonal holes

36

. The hexagonal-hole

regions are expected to bind metal atoms tightly, whereas the trigonal-hole regions cannot. Such special geometric structures enable BMs as a unique support for SACs. Indeed, our recent work has shown that BMs with hexagonal holes, such as β12, χ3, α1 and β1 BMs, exhibit high intrinsic


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activity for HER.37 Thus, if a single metal atom anchored on BMs also presents high OER performance, the SAC would be able to serve as a bifunctional electrocatalyst for the overall water splitting (Figure 1a).

Figure 1. (a) Schematic of the single-atom, bifunctional catalyst for overall water splitting. The hexagonal holes of BMs are electron-deficient and are used as supports to anchor metal atoms. If a SAC can present high activity for OER, such a SAC is a “single atom, bifunctional” catalyst for overall water splitting as the surface B atoms are highly active sites for HER. (b) Elementary reactions of OER and HER and the structures of the adsorbed states for each species, including *H, *OH, *O and *OOH.

Although tens of BM structures have been predicted theoretically,38 today, only three polymorphs of BM (trigonal, β12 and χ3) have been realized experimentally.34,

35

Because

trigonal BM was found to be electron-rich,36 only β12 and χ3 BMs that contain hexagonal holes are examined as potential support for SAC in this study. First, we compute the adsorption energies for all first-row transition metal atoms on the hexagonal hole of β12 and χ3 BMs whose structures are presented in Figure S1 in the Supporting Information (SI). As shown in Table S1, all the TM atoms present rather negative adsorption energies on both BMs, indicating strong interaction between the TM atoms and BMs. Moreover, the adsorption energy of β12 BM is


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always more negative than that of χ3 BM, suggesting that β12 BM is likely a better support for anchoring TM atoms.

Figure 2. Free energy diagram for (a) Ti1/β12-BM, (b) V1/β12-BM, (c) Mn1/β12-BM, (d) Fe1/β12-BM, (e) Co1/β12-BM and (f) Ni1/β12-BM for OER at zero potential (U = 0), where the elementary reaction with ∆G in red represents the potential-determining step.

For SACs, one major issue is that isolated metal atoms tend to aggregate into clusters due to the high surface free energy.10 To assess the stability of SACs, we compute the adsorption energy of TM atom on BM (Eb), the cohesive energy of TM atoms in their metal crystals (Ecoh), as well as the energy difference ∆Eb = Eb - Ecoh (see Table S1). The negative ∆Eb means that the TM atom on the BM surface is energetically more favorable than in the bulk form. As the TM clusters are intrinsically less energetically favorable than the metal crystals,39 the single TM


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atoms are unlikely to aggregate into clusters even when ∆Eb is close to zero. We set a criterion for judging the stability of a system under consideration, that is, ∆Eb < 0.15 eV. Only 6 TM1/β12BM systems, i.e., TM = Ti, V, Mn, Fe, Co and Ni, meet the criterion (see Figure S2). Next, we investigate the OER performance for the 6 potential TM1/β12-BM catalysts. The whole OER entails four elementary steps: i) a H2O molecule dissociates into an OH group, which is adsorbed on the catalyst surface (*OH); ii) the *OH further dissociates into O group (*O); iii) the *O reacts with another H2O molecule and produces an OOH group (*OOH); iv) the final product, O2, forms and is then released. In every elementary step, the release of an H+ cation and an electron always occurs (Figure 1b). The overpotential of OER can be obtained via calculating the reaction free energy of each elementary step (see details in SI). As shown in Figure 2a and 2b, the calculated ∆GOH are rather negative, -1.74 and -1.73 eV for Ti1/β12-BM and V1/β12-BM, respectively, indicating the strong binding between OH and the TM atom. The final step, where the gas-phase O2 produced desorbs from the catalyst surface, is the potential-determining step because it is very difficult to proceed. The calculated overpotential (η) is 2.12 and 2.16 V for Ti1/β12-BM and V1/β12-BM catalyst, respectively, both being too high to catalyze the OER efficiently. In addition, the high binding capacities would make the catalysts easily poisoned. Hence, Ti1/β12-BM and V1/β12-BM are ruled out as good catalysts for OER. For Mn1/β12-BM, Fe1/β12-BM and Co1/β12-BM catalysts, the third step, where an adsorbed O atom reacts with a H2O molecule to form a *OOH, is the potential-determining step (Figure 2c to 2e). However, the calculated η is still high: 1.92, 1.31 and 0.85 V for Mn1/β12-BM, Fe1/β12-BM and Co1/β12-BM, respectively. Thus, these three systems are not ideal as catalysts for OER either. Remarkably, only Ni1/β12-BM is predicted to possess favorable overpotential of 0.40 eV for OER, and the second step, where an adsorbed OH group dissociates into *O, is the potential-determining step


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(Figure 2f). To evaluate the accuracy of our results, revised-PBE (RPBE) functional which describes the binding energies with sufficient accuracy is also employed.40 As shown in Figure S3, the RPBE results are consistent with those PBE values (the calculated overpotentials are 0.41 and 0.40 V, respectively), indicative of the reliability of PBE for current systems. Besides, the calculated ∆GOH is 0.55 eV, implying that the catalyst is hard to get poisoned. As a further support, Gibbs free energy of the reaction for the Ni oxide formation on Ni1/β12-BM 1

1

2

4

(Ni1 /β12 -BM(s) + O2 (g) = β12 -BM(s) +

Ni4 O4 (s)) is calculated as a function of O2 partial

pressure (pO ) under 298.15 K. Ni4O4 cluster is used as the prototype, where the structure is 2

constructed based on its bulk form.41 As shown in Figure S4, the formation of Ni4O4 cluster would occur if pO exceeds 2.8 × 1027 Pa. This high pressure indicates that oxidation of Ni on 2

β12-BM is extremely difficult, suggesting long time stability of Ni1/β12-BM under operation. We therefore conclude that Ni1/β12-BM is a highly promising catalyst for OER. Insights into distinct OER performance for different catalysts can guide us to improve the catalytic performance and design better electrocatalysts. As presented in Section 1 in SI, the ∆G for each step is actually determined by the adsorption energy of *OH, *O and *OOH. That is to say, the distinct OER performance stems from the different binding strength of the three adsorption species on different catalysts. The d band center (εd), which has been widely used to describe the binding strength of adsorbates on different surfaces,42-47 is thus computed. As shown in Figure 3a, a clear shift of εd to low-energy level is seen with the increase of the atomic number of TM atoms (from Ti to Ni), implying that the binding strength of *OH, *O and *OOH will decrease accordingly. This trend is consistent with our calculation results shown in Figure 3b, where a negative correlation between ∆G and εd is observed. More interestingly, ∆G and the


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corresponding εd for each adsorption group presents a linear relation (Figure 3b), indicating that the OER performance can be described by the trend of εd quite well.

Figure 3. (a) Calculated PDOS of d band of the TM atoms in TM1/β12-BM, TM=Ti, V, Mn, Fe, Co and Ni. The d band center is marked by the red dash line and the Fermi level is set as zero. (b) Free energy of each species (OH, O and OOH) and (c) the potential of each reaction as a function of the d band center, where the shadow area is the theoretical overpotential under different energy level of d band center.

The correlation between the potential of each elementary step and εd is further shown in Figure 3c. For metals with a relatively high εd (> -1.68 eV), such as Ti and V, the binding strength between the reactant species and the TM atoms can be rather strong. As a result, the reactant tends to be trapped on the surface. So the desorption of O2 would be very difficult and the last step of OER becomes the potential-determining step. On the other hand, when the εd is


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relatively low (< -3.19 eV), the interaction between the reactants and catalysts would be too weak to carry out the subsequent reactions. In this case, the first step of OER, where a H2O molecule is adsorbed on the catalyst and dissociates into *OH, H+ and e-, is the potentialdetermining step. A high overpotential is inevitably needed under both circumstances. When εd is within a moderate range (from -1.68 to -3.19 eV), the binding strength of reactants would be neither too strong nor too weak. As a result, the second or the third step will be the potentialdetermining step. Meanwhile, the sum of potential of the second and third step is close to a constant (~3.20 V), leading to the lowest overpotential when the potential of the second and third step is equal. As seen from Figure 3c, the blue line (second step) and the red line (third step) intersect at εd = -2.82 eV, at which the catalyst would have the lowest theoretical overpotential for OER. The detailed scheme discussed above offers a strategy for improving the OER performance of SAC, that is, tuning εd to be as close as possible to -2.82 eV. As a comparison, we also evaluate the OER performance for the pure β12-BM. As shown in Figure S5a, all 10 possible sites, including 3 top sites (T1, T2 and T3), 5 bridge sites (B1, B2, B3, B4 and B5) and 2 hollow sites (H1 and H2), are taken into account. After full structural relaxation, the OH group on B3 and B4 sites moves to the neighboring top sites; OH on H1 and H2 sites drifts to the adjacent bridge sites; OOH on T1, B1 and B2 sites dissociates into *OH and *O, which are respectively adsorbed on two different sites. Hence, only B5, T2 and T3 sites can possibly serve as the sites for OER. However, these three sites present rather poor OER activity as the corresponding calculated overpotentials are 2.08, 1.27 and 1.00 V for B5, T2 and T3 sites, respectively (Figure S5b to S5d). So pure β12-BM is not a good catalyst for OER.


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Figure 4. (a) T1, T2 sites on the top of B atoms and TNi site on the top of Ni atom in Ni1/β12-BM and (b) corresponding ∆GH as compared with that of T1 and T2 sites on pure β12-BM.

Our recent work has shown that the basal plane of pristine β12-BM is highly active for HER.37 Will the HER performance of β12-BM be retained or even improved upon the adsorption of Ni atom? To address this question, we select the top sites of B atoms which are directly connected to Ni atom [T1(Ni1/β12-BM) and T2(Ni1/β12-BM) sites in Figure 4a], and the top site of Ni (TNi) to evaluate the catalytic activity for HER. As displayed in Figure 4b, the calculated ∆GH of TNi site is quite positive (0.53 eV), suggesting that the proton-electron-transfer process is difficult to proceed. So the TNi site is not an active site for HER. For T1(Ni1/β12-BM) and T2(Ni1/β12-BM)

sites, the calculated ∆GH are 0.06 and 0.20 eV, respectively, indicating that both sites

can give rise to high catalytic activity for HER. As a comparison, ∆GH of T1 and T2 sites in pure β12-BM are also included in the figure. For T1 site, the ∆GH of Ni1/β12-BM is much smaller than that of pure β12-BM, while T2 site gives similar HER activity upon the adsorption of Ni atom. As a whole, the Ni1/β12-BM shows improved catalytic activity for HER since the solid lines (Ni1/β12BM) are closer to zero than the dashed lines (pure β12-BM). The high HER performance illustrated here and the excellent OER activity proposed above indicate that the Ni1/β12-BM is a


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promising bifunctional catalyst for the overall water splitting. It is worth noting that most of recent produced bifuncational electrocatalysts for the overall water splitting are also Ni-based materials (nickel phosphides, nitrides, sulfides and oxides),17-19, 22, 48-50 in line with our results that Ni1/β12-BM is active for both HER and OER, whereas other β12-BM based SACs are not. We conclude that the Ni1/β12-BM is a promising bifunctional SAC. The influence of Ni adsorption on the HER activity of β12-BM can be ascribed in two aspects. First, the adsorption of Ni atom can make the flat BM surface curved and the surface of Ni1/β12-BM around the Ni atom is actually under tensile strain (Figure 4a and Table S2), resulting in decreased ∆GH according to previous study.51 Second, the adsorption of Ni atom can induce redistribution of the charge, where the positive/negative charge transfer leads to the increase/decrease of ∆GH.51, 52 For the T1 site, the charge decreases from 3.36 to 3.20 e with the adsorption of Ni atom. Therefore, the combined effects of tensile strain and electron loss render the T1 site in Ni1/β12-BM with a much lower ∆GH than that of pure β12-BM (0.06 eV vs 0.23 eV). On the contrary, the adsorption of Ni atom can make the T2 site gain electrons, from 2.81 to 2.96 e. Thus, the competitive effect of tensile strain and electron gain results in the ∆GH of T2(Ni1/β12-BM) site almost unchanged, compared with that of pure β12-BM (0.20 eV vs 0.19 eV). Besides the predicted high catalytic activity, the likelihood of experimental realization is also a crucial issue. For the synthesis of SACs, wet chemistry method is a suitable way to achieve highly dispersed single atoms.9, 10, 53 In such a process, an appropriate precursor and support are the two key factors. To examine the possibility of experimental realization of Ni1/β12BM, we select NiCl2, an important Ni source in industry, as the metal precursor to simulate the synthetic route for Ni1/β12-BM as follows (Figure 5a):


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NiCl2 + * → *NiCl2

(2)

*NiCl2 + 2H3O+ + 2e- → *Ni(HCl)2 + 2H2O

(3)

*Ni(HCl)2 → *Ni + 2HCl

(4)

where the * donates a hexagonal hole of β12-BM.

Figure 5. (a) Top and side view of designed synthetic route for Ni1/β12-BM. (b) Snapshot of atomic configuration of ab initio molecular dynamics simulations for the synthetic process of Ni1/β12-BM. The upper panel is the initial structure and the lower is final structure. The model we used here includes a β12-BM supercell consisting of 4 × 3 × 1 unit cells, 3 NiCl2 molecules, and 28 H2O molecules. The simulation is run for 2 ps, where the temperature increases from 0 to 350 K within the first 1.4 ps and then keeps at 350 K for 0.6 ps. (c) Minimum-energy pathway of the Ni atoms between two hexagonal holes on the surface of β12-BM. The pink, blue, green, white and red lines or balls represent the B, Ni, Cl, H and O atoms respectively.


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The first step is the spontaneous adsorption of a free NiCl2 on the β12-BM with the adsorption energy of -1.44 eV. Next, the Cl- ions in the adsorbed NiCl2/β12-BM interact with H+ ions in the solution, forming two H-Cl bonds. Our calculations show that when approaching to NiCl2/β12-BM, a H3O+ ion will dissociate into a H2O molecule and an H+ ion while forming a bond with Cl- ion without encountering any energy barrier. The adsorption of H+ on NiCl2/β12BM results in the Cl-Ni bond-length increase from 2.18 to 2.49 Å, an indication of weakening of the binding strength of Cl-Ni bond. The HCl groups actually bind very weakly to the Ni atom, with a binding energy of -0.12 eV, suggesting that the HCl groups can be easily removed from the surface. Thus, the Ni1/β12-BM could be obtained. We further perform ab initio molecular dynamics simulations as illustrated in Figure 5b. It is observed that NiCl2 in the solution will adsorb on the surface of β12-BM, followed by desorption of Cl- ions, and the ultimate formation of Ni1/β12-BM system within 2 ps at the temperature of 350 K. These simulations are consistent with our design described above, suggesting that the synthesis of Ni1/β12-BM through such an approach is likely realized in the laboratory. Once dispersed single atoms are realized on the surface of BM, the BM support will anchor the single atoms and prevent the aggregation of the TM atoms from forming clusters due to the favorable ∆Eb value of 0.10 eV discussed previously. We have also computed the energy difference between the formations of clusters and dispersed single atoms on β12-BM with 3 and 4 Ni atoms. As shown in Figure S6, the formation of dispersed single atoms is energetically favorable, by 0.7 and 0.8 eV per Ni atom, compared to the formation of a cluster of 3 and 4 Ni atoms on β12-BM, respectively. In other words, the probability of cluster formation is on the order of 10-12/10-14 for the systems of β12-BM with 3/4 Ni atoms, based on the Arrhenius formula. Furthermore, AIMD simulations were performed to show that an adsorbed Ni clusters


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on β12-BM can rapidly disperse into isolated Ni atoms very quickly (within 1 and 2.5 ps under 800 K for Ni3 and Ni4, respectively; see Figure S7). Moreover, the dispersed single atoms are tightly anchored on the hexagonal holes instead of aggregating into clusters even the simulation time is as long as 10 ps. The minimum-energy pathway for the Ni atom moving between two hexagonal holes is also computed by using the climbing nudged elastic band method

32, 33

. As

shown in Figure 5c, relocation of the Ni atom needs to overcome an energy barrier of 1.68 eV. All these computational evidences show that the adsorbed Ni atom can hardly diffuse to form Ni clusters on β12-BM. The Ni1/β12-BM is, indeed, a highly robust single-atom catalyst. Conclusion In summary, we have designed a nanosheet supported single-atom catalyst, Ni1/β12-BM, with excellent catalytic activity for both HER and OER. The computed overpotential of Ni1/β12BM for OER is only 0.40 V. A principle for improving the OER performance of SACs is provided, that is, by adjusting the εd to be close to -2.82 eV. Moreover, the ∆GH for Ni1/β12-BM can reach to 0.06 eV, showing higher activity and performance than bare β12-BM for HER. The enhanced HER activity of Ni1/β12-BM is attributed to the cooperative or competitive effect of the tensile strain, and charge transfer due to the Ni adsorption. The strong binding strength, high diffusion barrier, and excellent thermal stability suggest that Ni1/β12-BM is a very stable singleatom catalyst. Such a catalyst may be synthesized by using NiCl2 as precursor. Overall, its high stability, excellent OER and HER performance endow Ni1/β12-BM a highly promising SAC for cost-effective water splitting, thereby offers high possibility for clean energy production. ASSOCIATED CONTENT


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Supporting Information. The details about the calculations of HER and OER overpotentials; structures of single metal atom supported on β12- and χ3-BMs; screening of promising SACs; OER activity of Ni1/β12-BM calculated by using RPBE functional; Gibbs free energy of the formation of Ni4O4 on Ni1/β12-BM as a function of pO ; OER performance of pure β12-BM; 2

structures and adsorption energies of clusters as well as dispersed single atoms; AIMD simulation of β12-BM with the adsorption of Ni clusters are included in the Supporting Information. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (J.W.); [email protected] (X.C.Z.) Note The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Ministry of Science and Technology (2017YFA0204800), the National Science Fund (21525311, 21373045), Jiangsu 333 project (BRA2016353), the Fundamental Research Funds for the Central Universities and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1670) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin. XCZ was supported by a State Key R&D Fund of China (2016YFA0200604) to USTC.

REFERENCES


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Nano Letters

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A nanosheet supported single atom, bifunctional catalyst, Ni1/β12¬-BM is designed for the first time, which show excellent HER and OER activities with overpotentials of just 0.06 and 0.40 V.


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Nanosheet Supported Single-Metal Atom Bifunctional Catalyst for

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