Strain and Ligand Effects on CO2

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Strain and ligand effects on CO2 reduction reactions over Cu-metal heterostructure catalysts Fuzhu Liu, Chao Wu, and Shengchun Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07081 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Strain and Ligand Effects on CO2 Reduction Reactions over Cu-Metal Heterostructure Catalysts Fuzhu Liu, † Chao Wu,*, ‡ Shengchun Yang*,†, †

⊥

School of Science, Key Laboratory of Shaanxi for Advanced Materials and

Mesoscopic Physics, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi’an, 710049, People's Republic of China. ‡

Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi’an, 710054

⊥

Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou

Academy of Xi’an Jiaotong University, 215000, Suzhou, People's Republic of China.

ACS Paragon Plus Environment


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ABSTRACT The strain and ligand effects on the adsorption energies of key intermediates (*COOH, *CO, *CHO and *COH) in CO2 reduction reactions on the Cu-M(111) (M = Ni, Co, Cu, Rh, Ir, Pd, Pt) hetero-layered catalysts have been quantitatively separated using the first-principles calculations. Opposite to the common belief that strain is always the leading factor influencing catalytic performance of the core-shell type heterostructure catalysts, the ligand effect due to the underneath hetero elements should not be ignored and may become dominant for strain-insensitive adsorbates (*CO and *COH). Moreover, the models of Cu(2ML/3ML)-M(111) (M=Ir, Rh, Pt, Pd) have been shown to be better catalysts for CO2 reduction, as they require lower overpotential to drive the reaction than the Cu(111) slab. Particularly, the overpotential is predicted to be lowered by 0.17 V for Cu(3ML)-Ir(111) model catalyst. Thus, both effects should be considered in heterostructure catalyst design.


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1. Introduction Heterostructure catalysts, such like core-shell or alloy nanoparticles, have demonstrated better catalytic performance and economical utilization of precious metals in a number of reactions compared with their single component counterparts.1-5 For example, (metal or alloy) core-(Pt) shell catalysts show enhanced catalytic activities for oxygen reduction reactions (ORR) than pure Pt catalysts.6-10

The

reason has been ascribed to weaker adsorption of reacting species caused by the compressive or tensile surface strain, a result of the smaller or larger core lattice compared to the shell lattice, e.g., Pd@Pt nanoparticle and PtPb@Pt nanoplate,11-12 leaving the contributions from the underneath element via its different electronic structure unresolved.13 Nevertheless, the surface strain should be viewed as an efficient way to tune the catalytic activity of heterostructure catalysts rather than being a dominant factor determining their reactivity. Similarly, catalysts with few atomic layers of Cu deposited on other metal substrates have been used to improve CO2 reduction reaction (CO2RR) over pure Cu catalysts,14 but analysis on the electronic contribution from the substrate metal is missing.15-16 Moreover, alloy nanoparticles like icosahedral Pt3Ni and Au75Pd25 have exhibited superior performance in ORR and in the oxidation of cyclohexane, respectively.17-18 Again, in these cases, the tensile strain induced by the shape of nanoparticles acting along with the changed electronic structure due to the alloying element, concertedly influence their catalytic activity, yet the detailed mechanism is left unexplained.19-20 Evidently, the roles of strain and ligand effects in heterostructure catalysts need to



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be clarified, as the latter can lead to electronic charge transfer among different elements, which is crucial in adsorbate-surface bond formation.21,22 However, in experiments, the decoupling of the two effects is complicated by the wide distribution of shape and size of the core-shell nanoparticles.18 For alloys, the problem is further aggravated by the huge configuration space of the compositing elements. On the other hand, from the theoretical perspective, we can build simple slab catalyst models with layers of different elements to simulate core-shell type heterostructure catalysts. This will leave out the unnecessary complexity and still contain both the essential effects of strain and charge transfer among different elements. With these simple models, we can separate the two effects.23-24 In order to construct meaningful and realistic slab models, we first need to choose a proper reaction. And CO2RR was selected as the representative reaction, because it involves a number of commonly encountered reaction intermediates and it is very important to achieving sustainable carbon utilization.25 As electrocatalysts, most transition metals (e.g., Au, Ag, Zn, Cu, Ni, Pt, Fe) are capable of reducing CO2 to methane, methanol, and CO via tuning the applied potential.26 Only Cu is identified to possess high selectivity for hydrocarbon production, but the major obstacle at present is the large overpotential of ~1 V needed to drive the reaction.27 Encouragingly, a recent theoretical investigation suggests that the overpotential of penta-twinned Cu nanowires can be lowered to 0.5 V under a 8% tensile strain.28 Accordingly, catalysts consisting of hetero element layers may provide more room for improvement by incorporating the electronic regulation on top of the strain control.14-16 Here, we build


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a series of heterostructure slab models, each containing one, two, or three monolayers (ML) of Cu atoms over one of six metal substrates (Ni, Co, Rh, Pd, Ir and Pt). Due to the lattice mismatch between the substrate metal and Cu, different strain is introduced into the deposited Cu layer (Table 1). At the same time, the ligand effect from the substrate metal also affects the properties of the surface Cu with different thickness of the Cu overlayer. According to calculations based on the computational hydrogen electrode model, the CO2RR overpotential can be linearly correlated to the relative adsorption strength of the intermediates (*COOH, *CHO, *CH2O, *CO, etc.) generated during the hydrogenation process.29 The first-principles calculations were used to study the adsorption energy of these key intermediates over the models. More importantly, we quantitatively extract the contributions from the strain and ligand effects to the adsorption energies, respectively. In addition, how the strain and ligand effects can be employed for future catalyst design was also discussed.

2. Computational Methods The all electron spin unrestricted DFT calculations were performed using the Dmol3 program package.30-31 The electron exchange and correlation effects were described by the generalized gradient approximation (GGA) with the PBE functional.32-33 The valence electron wave functions were expanded into a set of atomic orbitals composed of the double numerical plus p-functions (DNP) basis set.34 Seven (111) surfaces of Ni, Co, Cu, Rh, Pd, Ir and Pt were modeled as substrates using experimental lattice constants (for a detailed discussion on the appropriateness of using experimental lattice parameters, see Supporting Information including Table S1).35 A 3×3 five-layer



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slab supercell with bottom two layers frozen was used to simulate adsorption (Supporting Information Figure S1). In hetero layered slabs, the top one, two, and three layers are substituted by Cu and labeled as Cu(1ML), Cu(2ML) and Cu(3ML). Each slab was separated by a vacuum of 15 Å to minimize the interactions between images. Brillouin-zone integrations were performed on a grid of 3×3×1 Monkhorst−Pack k-point mesh. The width of the Fermi smearing of the Kohn−Sham states was set to kBT = 0.005 Hartree. The force and energy were converged to 0.002 Hartree/Å and 10−5 Hartree, respectively. The adsorption energies of adsorbates (*COOH, *CO, *CHO, and *COH) are referenced to the energy of C atom in graphene, H atom in H2, and O atom according to E(H2O) - E(H2).29, 36 Table 1. Lattice mismatch in Cu-metal hetero layered catalysts. Metal Lattice mismatch % of Cu's lattice Ni -2.5 Co -2.0 Cu 0.0 Rh 5.2 Ir 6.2 Pd 7.6 Pt 8.6 3. Results and Discussion The experimental lattice constants of the substrate metals (Table 1) are applied to the pure Cu(111) slab models to produce reference states, which will be compared to the Cu-M(111) hetero-layered models later. To make the strain sampling more evenly distributed, strain of -4.0%, 2.0% and 4.0% was additionally considered and applied to the slabs, respectively. Then the adsorption energies of the key intermediates


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(*COOH, *CO, *CHO, and *COH) on the pure Cu slabs are computed and found to response differently to strain (Figure 1). For *COOH and *CHO, their adsorption strengths linearly increase by about 0.3 eV from -4.0% to 8.6% strain, showing a fairly wide range of adsorption energy change. While for *CO and *COH, the variation is small (< 0.1 eV), and eventually, the adsorptions are stabilized at tensile strain over 4%, suggesting their strain-insensitivity. The different behaviors can be explained by examining their adsorption configurations. *COOH and *CHO are tilted on the surface with both the C and O atoms bonded to two neighboring surface Cu atoms, whereas *CO and *COH vertically adsorb at a fcc site through the C atom (Figure 1b, insets).37 Naturally, a greater dependence of adsorption on strain can be expected for *COOH and *CHO, as the distance between the two tethering Cu atoms directly affects the C-O bonds in the adsorbed species (Figure 1c). When applied strain is over 4%, the flat response of adsorption to strain of *CO and *COH results from that the adsorption-induced local contraction counteracts the applied strain. For instance, the distance between any two Cu atoms of the three Cu atoms composing a fcc site, after adsorbing CO, shrink by about 0.02Å. Nevertheless, the non-linearity of adsorption energies with respect to the larger strain might stem from the difference in the s- and p-electron density of the adsorbates, which regulates their interaction with the surface metal. The corresponding systematic study is current undergoing in our group.



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Figure 1. Adsorption energies of key intermediates (*COOH, *CO, *CHO, and *COH) in CO2RR on the strained Cu(111) surface. (a) Adsorption energy versus strain. Smaller values of E*X correspond to stronger adsorption and for the definition see Computational method. (b) Adsorption energy change versus strain. Note that the negative and positive values represent the strengthening and weakening of adsorption with respect to the unstrained case, respectively. Insets: Configurations of the key intermediates on the 0% strained Cu(111) surface (top view). Red, white, gray and orange balls represent O, H, C and Cu atoms, respectively. (c) Configurations of *COOH, *CO, *CHO, and *COH on the 2% tensile strained and unstrained Cu(111) surfaces. It has been shown in the literature that the reaction direction, i.e., the selectivity of products of CO2RR was conveniently described by the adsorption strength of the


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commonly encountered CO.36 Therefore, it is necessary to investigate the adsorption of the reaction intermediates in CO2RR on the Cu-M(111) (M = Ni, Co, Rh, Pd, Ir and Pt) hetero-layered models defined above and check whether the linear scaling relations between the adsorption energies of CO and other intermediates still hold in these hetero-structure slabs. Moreover, the effect of the overlayer's thickness was taken into consideration in our models with the Cu overlayer of three thicknesses, which was labeled as Cu(n ML) (n = 1, 2, 3). The adsorption energies of CO versus other intermediates are presented in Figure 2a, and each data point corresponds to the adsorption energy of one adsorbate over one Cu-M(111) model. Evidently, a good linear relation between the adsorption energies of *COH and *CO can be observed, agreeing well with the previous reports.36, 38 However, the much more scattered points in *CHO- and *COOH-related data suggest that their binding strengths deviate from linear scaling with respect to that of *CO, thus may provide unconventional catalytic property as new CO2RR catalysts.39 The variations of adsorption of *COOH and *CHO exhibit similar patterns, and most of them present enhanced adsorption relative to the pure Cu slabs (symbols below the dashed lines, Figure 2a). Both the difference and similarity in the three correlations (*COH, *CHO, and *COOH vs. *CO) are rooted in the binding configurations of the intermediates (Figure 1c), thus good linear scaling of adsorption energies is likely to only exist among adsorbates of similar binding configurations. In order to obtain desired CO2RR products at a lower overpotential, the adsorption strengths of the intermediates involved in the potential-limiting step should



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be altered in favor of generating the product-related intermediates.29, 40-41 For Au and Ag catalysts, obtaining CO at a lower overpotential requires a much strengthened adsorption of *COOH, as it is the hard-to-get intermediate generated during the potential-determining step of CO2 reduction. In other words, if one wants to keep the following *CO formation step (*COOH + H+ + e- → *CO + H2O) moving forward, the enhancement of the *COOH adsorption should not be much larger than that of the *CO adsorption.42 In contrast, for the Cu and Cu-M catalysts, the protonation of *CO to form *CHO is always potential-limiting (*CO + H+ + e- → *CHO), which means that *CHO is more difficult to form than *COOH. It has been shown that there is a scaling relation between *CHO and *CO on metal catalysts.36 To make the further conversion of *CHO to CH4 and CH3OH (e.g. *CHO + 3H+ + 3e- → *CH3OH) at a lower overpotential, the scaling relation of *CHO and *CO needs to broken. To be more specific, the adsorption of *CHO should be strengthened more than that of *CO,36, 43-45 For the Cu(2ML/3ML)-M (M = Ir, Rh, Pt, Pd) models, the CHO-related points are at the lower right corner of Figure 2b correspond to the adsorption strength enhancing of *CHO and the adsorption weakening of *CO, which suggests that the reaction to afford *CHO from *CO experiences a smaller free energy rise (less endergonic) and thus CO2RR will take place at a reduced overpotential. Although the strain

and

ligand

effects

of

the

Cu-M

models

do

not

change

the

overpotential-determining step,28 the selectivity of products (CH4 or CH3OH) depends on the relative strengths of the surface-OCH3 interaction and the O-C bond of the adsorbed *OCH3.43 Moreover, the competing hydrogen evolution reaction (HER) also


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needs to be considered to ensure the selectivity. The competing side reaction HER is suppressed by other reaction species over Cu-based surfaces. Calculations have shown that the stronger adsorptions of *O (-4.29 eV) and *OH (-3.18 eV) on Cu(111) surface than *H (-2.45 eV) result in little opportunity to produce H2 (Eb= E*X+slab - Eslab - EX(g)).46-48 Moreover, a report by Peterson group indicated that the CO2 reduction environment can suppress the HER on Cu surface due to the present of the impurities such as CO.49 Although tensile strain can make the adsorption of these adsorbates stronger, the adsorption strength order should retain. Thus we expect that the HER will not affect the CO2 electroreduction on Cu-based surfaces.



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Figure 2. (a) Adsorption energies of CO versus other intermediates. Hollow, ×, and

＋ represent the values of models with 1, 2 and 3 ML of Cu overlayer, respectively. Smaller values of E*X and E*CO correspond to stronger adsorption, however, the two adsorption energies are referenced to different species (see Computational method). Each symbol represents the adsorption energy of one adsorbate over one Cu-M model. The dashed lines represent the adsorption energies (E*X) of the corresponding intermediates on the unstrained pure Cu(111) surface. (b) Energy changes of the adsorbates referenced to the pure Cu(111) surface (dashed lines). For each strain value applied to the hetero-layered slabs, the adsorption energies on the pure Cu slabs with the same strain (Figure 3a, solid symbols) serve as the references. Evidently, for each adsorbate, the Cu(nML)-M models present different interaction strengths relative to the pure Cu slabs, which was affected by both the thickness of the Cu overlayer and the sublayer metal. With the increase of the thickness of the overlayer, the adsorptions of all intermediates on all models become weaker. The adsorptions on the 3ML and 2ML Cu overlayer models (Figure 3a, cross symbols) are weaker than that on the pure Cu slabs. For models with 1ML Cu overlayer (Figure 3a, open symbols), the variation of the adsorption strengths depends both on the sublayer metal and the adsorbate. The adsorption of all intermediates is significantly strengthened on the Cu(1ML)-Rh/Ir models, while it varies little on the Cu(1ML)-Pd/Pt models. For example, the *CO adsorption changes little for Pt and Pd (< 0.05 eV) and varies evidently for Rh and Ir (> 0.10 eV, Figure 3a). Consequently, the surfaces of the latter two Cu(1ML)-M models are more likely to be occupied by


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CO and the subsequent reactions are thus inhibited.50 For Ni and Co substrates (i.e., the compressed Cu overlayer), the adsorbates present similar variations except for *COH. *COH exhibits much weaker binding at the Cu(1ML)-Ni/Co models versus the compressed pure Cu slab models, while other adsorbates have similar or stronger binding at the Cu(1ML)-Ni/Co models in comparison with the compressed pure Cu slab models. In short, the variation in the thickness of the overlayer combined with the variation of the sublayer metal present a much wider range for tuning reactions like CO2RR.

Figure 3. Adsorption of key intermediates (*COOH, *CO, *CHO, and *COH) on the Cu(n ML)-M (M = Ni, Co, Rh, Ir, Pd and Pt) models. (a) Adsorption energy. Solid



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symbols represent the values on the pure Cu slab models. (b) Energy contribution for ligand effect. Hollow, ×, and ＋ represent the values on the 1ML, 2ML, and 3ML of Cu-covered surfaces, correspondingly. The dashed lines represent the adsorption energies (E*X) of the corresponding intermediates on the pure unstrained Cu(111) surface. Furthermore, the sublayer metal influences the overlayer and the adsorption in two aspects: strain and ligand effects. Here, we extract the energy contributions of ligand effect (Figure 3b) to the adsorption energy (Figure 3a) by subtracting the strain contribution (Figure 1b) from the total adsorption energy. For example, the Cu-M models with Co atoms as the sublayers correspond to a compressive strain of 2.5% applied to the Cu overlayer. Thus, the adsorption energy of an adsorbate on such models should contain a 2.5% compressive strain contribution, which can be located according to the strain-adsorption energy relation (Figure 1b). By taking out the strain contribution, the ligand effect is quantitatively obtained. The variation of the ligand effect on adsorption demonstrates similar trends for all adsorbates and the ligand effect from substrate metals follows the order of Ir > Co > Pt > Rh ≈ Pd > Ni. The electronic contribution from Cu(1ML)-substrates trends to strengthen the adsorption of intermediates (open symbols below the dashed lines, Figure 3b). While for the Cu(2ML/3ML)-M models, the ligand effect prefers to reduce the adsorption strength of adsorbates (cross symbols above the dashed lines). For example, the ligand effect on tuning the *CO and *COH adsorption is prominent on Cu(3ML)-Rh/Ir surface than that of the *COOH and *CHO adsorption. In brief, the ligand effect is caused by the


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sublayer metal and influenced by the thickness of overlayer. To compare the strain and ligand effects, we plot their contributions on the adsorption energy of intermediates during CO2RR in Figure 4. For *COOH and *CHO (Figure 4a and 4c), the strain effect (-0.15 ~ -0.25 eV) on tuning the adsorption energy is stronger than that of the ligand effect (-0.10 ~ 0.12 eV) over the Cu-M (M = Rh, Ir, Pd, Pt) hetero-layered models. For example, the adsorption strengths of *COOH and *CHO on the Cu(3ML)/Ir surface are the combined result of the ligand effect (0.11 eV for *COOH and 0.10 eV for *CHO) and the strain effect (-0.18 eV for *COOH and -0.16 eV for *CHO). As the latter is greater and stabilizing, strengthened adsorptions occurred. Additionally, over the Cu-Co and Cu-Ni models, both the two effects are of similar small values (about 0.05 eV). In contrast, for the adsorption of *CO and *COH over the Cu-M (M =Co, Ni, Rh, Ir, Pd, Pt) models, the ligand effect (0.05 ~ 0.2 eV) is stronger than the strain effect (< 0.05 eV), with the exceptions of the Cu(2ML)-Pt and Cu(1ML)-M (M = Co, Pd, Pt) models (Figure 4b and 4d). For instance, when CO is adsorbed on the Cu(3ML)-Ir surface, the ligand effect (0.15 eV, destabilizing) is stronger than the strain effect (-0.03 eV, stabilizing), resulting in the weakened adsorption of *CO. It is worth noting that the strain effect from substrates on the adsorption of *COOH and *CHO follows the order of Pt > Pd > Ir > Rh and a completely opposite sequence is observed for *CO and *COH. In brief, the strain and ligand effects are adsorbate (configuration)-dependent in addition to the substrate dependency (Supporting Information Figure S2). These results clearly show that surface strain does not always dominate the catalytic reactions for the core-shell



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heterostructures. A recent experiment supports our observation by showing that the electrochemical properties of the Pt(2ML) overlayer are dominated by the direct electronic contribution from the substrate Ni atoms.51

Figure 4. Strain effect versus ligand effect on tuning the adsorption energy of key intermediates during CO2RR. The energy contribution for adsorption of *COOH, *CO, *CHO and *COH are shown in (a), (b), (c) and (d), with different Cu cover-layer over Ni, Co, Rh, Ir, Pd, Pt substrates, individually. The data points linked by the same vertical dashed line are results for the same substrate metal with different Cu overlayers. The key intermediates during CO2RR on the Cu overlayer exhibit strain-sensitivity (e.g., *COOH and *CHO) and strain-insensitivity (e.g., *CO and *COH) due to their


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different binding configurations (Figure 1c). The adsorption of strain-sensitive adsorbates can be strain-tuned in a wider range (about 0.35 eV) than that of strain-insensitive adsorbates (about 0.05 eV). In contrast, the latter ones respond more strongly to the ligand effect (about 0.20 eV) than the former ones (about 0.12 eV). Rather than individually affecting the reactions, these two effects may work together. To exemplify the synergistic effects, we test the Cu(1ML)-Ir and Cu(3ML)-Ir models as potential catalysts for CO2RR. As shown in Figure 5, the adsorption of strain-sensitive *COOH on the Cu(1ML)-Ir and Cu(3ML)-Ir surfaces is stronger (by 0.25 eV and 0.08 eV, respectively) than over the Cu(111) surface. However, the adsorption of strain-insensitive *CO intermediate is strengthened by 0.16 eV on the Cu(1ML)-Ir model and weakened by 0.11 eV on the Cu(3ML)-Ir surface, relative to the Cu(111) surface, for which the ligand effect dominates over the strain effect. Furthermore, the adsorption of *CHO on Cu(1ML)-Ir and Cu(3ML)-Ir surfaces is strengthened (by 0.26 eV and 0.06 eV, respectively) than that of the Cu(111) surface. If we follow the literature to assume the hydrogenation of *CO to *CHO as the rate-determining step of CO2RR,36 the calculated hydrogenation energies indicate that the Cu(1ML)-Ir and Cu(3ML)-Ir catalysts require a lower overpotential by 0.10 V and 0.17 V to drive the reaction than the Cu(111) surface, respectively. Strain has little influence on the *H adsorption strength. The *H adsorption energies on the unstrained and 6.2% tensile strained Cu(111) surfaces are -0.315 eV and -0.305 eV, respectively. In addition, compared to the Cu(111) surface, the *H adsorption is strengthened by 0.175 eV on the Cu(1ML)-Ir(111) model and is weakened by 0.072 eV on the



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Cu(3ML)-Ir(111) model (Figure 5b). Thus, for the Cu(3ML)-Ir(111) model, it is apparent that the side reaction of HER is suppressed by

the ligand effect rather than

the strain effect. Therefore, the separation of the strain and ligand effects provides a way to understand the variation of the adsorption of key adsorbates. Moreover, the proper combination of the two effects is more efficient in adjusting the catalytic performance of the core-shell type catalysts.52

Figure 5. (a) Key reaction energetics of the CO2 reduction reaction and (b) hydrogen adsorption energy on the Cu, Cu(1ML)-Ir and Cu(3ML)-Ir surfaces. 4. Conclusions We have investigated the roles of the strain and ligand effects on tuning the


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adsorption energies of key intermediates generated (including *COOH, *CO, *CHO, and *COH) during CO2 reduction reactions over core-shell type heterostructure catalysts by means of the first-principles calculations. The strain-induced variation of adsorption energy is adsorbate-dependent, i.e., the adsorption of *CO and *COH are much less strain-sensitive than *COOH and *CHO on the Cu(111) surface, which can be explained from their different binding configurations (Figure 1). The adsorption strengths of the intermediates on the Cu-M (M = Ni, Co, Cu, Rh, Ir, Pd, Pt) hetero-layered slab models suggest that both the strain and ligand effects concertedly make some of the catalyst models promising candidates for the electroreduction of CO2 with lower overpotential, though their long term stability and durability need to be investigated.53-54 Furthermore, we quantitatively separate the contributions of the strain and ligand effects on the adsorption energy of the key intermediates over these hetero-layered catalysts. In contrast to the traditional viewpoint that the strain effect is always the dominant factor influences the reactivity of the core-shell catalysts, we identify that for strain-insensitive adsorbates, e.g., *CO and *COH, the ligand effect is more important. Therefore, our study provides a new understanding of the two effects and may benefit the future design of catalysts fully utilizing both the strain and ligand effects. Additionally, the Cu-M(111) models serve as one example to show the role of strain and ligand effects. Other surfaces like (211) can be also analyzed following the similar way and they may produce different results and trends from the (111) model. The surface-dependence of the strain and ligand effects may provide new opportunity for catalyst design.



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ASSOCIATED CONTENT Supporting Information Test results for the selection of core-treatment method and the number of total layers with respect to the number of the fixed bottom layers. Reaction energy of the key reaction steps for CO2 reduction reaction on Cu, Cu(1ML)/Ir and Cu(3ML)/Ir surfaces. Direct comparison of the energy contributions from strain and electronic effects. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work is supported by National Natural Science Foundation of China (No. 21477096, 51320105014, and 51271135), the Fundamental Research Funds for the Central Universities, and the Natural Science Foundation of Shaanxi Province (No. 2015JM5166). We acknowledge National Supercomputing Center in Shenzhen for providing the computational resources and materials studio (MS6.1-Dmol3) and Xi'an Jiaotong University High Performance Computing Center.

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Strain and Ligand Effects on CO2

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