Enhancing CO2 Electroreduction by Tailoring Strain and Ligand

Subscriber access provided by Fudan University

Article 2

Enhancing CO Electroreduction by Tailoring Strain and Ligand Effects in Bimetallic Cu-Rh and Cu-Ni Heterostructures Tuhina Adit Maark, and Birabar Ranjit Kumar Nanda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00940 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Enhancing CO2 Electroreduction by Tailoring Strain and Ligand Effects in Bimetallic Cu-Rh and Cu-Ni Heterostructures

Tuhina Adit Maark* and B. R. K. Nanda

Condensed Matter Theory and Computational Lab, Department of Physics, IIT Madras, Chennai 600036, India.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

ABSTRACT. We show how epitaxially grown Cu-Rh and Cu-Ni heterostructures exploit the strain effect, due to the lattice mismatch, and the ligand effect, arising from the electronic interaction between the heterolayers, to achieve improved CO2 electroreduction. In this study we have performed density functional calculations on Cui/Mj/Cu(211) sandwiched surfaces and Cui/M(211) overlayers (where M = Rh or Ni, varied i and j monolayers). We examined the free energy profiles of the reaction mechanisms for CO2 reduction to CO and CH4. We find that in Cu1/M1/Cu(211), in which the Cu monolayer experiences only a pure ligand effect, the influence of Ni is weaker than Rh and it decreases the overpotential for CO2 reduction by ~10-20 mV. A larger decrease (33-64 mV) in the overpotential is predicted for other sandwiched surfaces: Cu1/Ni2/Cu(211), Cu2/Rh1/Cu(211), and Cu2/Rh2/Cu(211) in which the ligand effect is weaker. In the Cu1/M(211) overlayer, Cu is affected by both the strain and ligand effects, of which the latter dominates. As the number of Cu monolayer increases from one to three, the strain effect becomes dominant in the Cu overlayers. We demonstrate that the tensile strain on Cu in Cu23/Rh(211)

overlayers causes a significant decrease (by 86 mV) in the overpotential for CO2

electroreduction, while the compressive strain in Cu2-3/Ni(211) overlayers has an opposite effect. Furthermore, Cu2/Rh2/Cu(211) and Cu2-3/Rh(211) will also exhibit an increase in exchange current density, i.e., electrocatalytic activity for CO2 reduction. This is accompanied by a retention of selectivity for CO and CH4 over hydrogen evolution.

KEYWORDS. electrochemical reduction, surfaces, heterogeneous catalysis, overpotential, DFT


2

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Electrochemical reduction of CO2 to a reusable fuel is complicated because of the possibility of formation of several products (e.g., CO, CH4, HCOOH, CH3OH)1, differences in product selectivity over different electrocatalysts2, and competition from the hydrogen evolution reaction (HER)3. Even Cu, which is the best known transition metal catalyst for this reaction, is associated with a large overpotential (η) of ~1 V.4 Epitaxial growth of a thin catalytic layer on a metallic support with a different lattice constant is a well-known approach to engineer strain for modifying its surface reactivity. Kibler et al. showed that the M(111) support on which a pseudomorphic monolayer (ML) of Pd was deposited influenced its catalytic activity for HER and formic acid electrooxidation based on whether the Pd ML was compressed or expanded due to the lattice mismatch.5 Recently, several Pt-bimetallic catalysts have been developed via dealloying, which exhibit increased reactivity for the oxygen reduction reaction.6-9 Utilizing the same methodology of electrochemical dealloying on PdCu3 nanoparticles has also resulted in attaining HER activity as high as that of Pt.10 Currently, strategies similar to the above are being considered for CO2 electroreduction. Cu overlayers on Pt(111) and Pt(211) surfaces have been found to have a lower selectivity for hydrocarbon generation and more preference for HER than polycrystalline Cu.11 In comparison, a higher activity for CO2 electroreduction has been experimentally observed for Pd overlayers on Au(111) and Cu on Pd/Au(111) than bare Pd layers.12 In another study Monźo et al. synthesized Au@Cu core@shell nanoparticles (NPs) with well-defined structures.13 They demonstrated that the CO2 reduction catalytic activity and product selectivity varied with the Cu shell thickness.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

For instance, Au cubic NPs with 7-8 MLs of Cu preferably produced H2 and ethylene, but when the number of Cu MLs was increased to 14 the catalyst was selective towards formation of H2 and CH4. Recently, it has been reported that Au island deposited on Cu is more effective than an Au-Cu alloy for improving CO2 electroreduction.14 Recently, Sarfraz et al. have reported a Cu surface decorated with Sn to exhibit an efficient and elective reduction of CO2 to CO.15 The presence of Sn atoms disallows the adsorption of H and thereby HER, while leaving CO production unaffected. It is important to note that in these bimetallic heterointerfaces the electrocatalytic layer is not influenced by the 'strain effect', but is also affected by the 'ligand effect' arising from its electronic interaction with the underlying substrate. Adit Maark and Peterson have illustrated through density functional theory (DFT) calculations that in regard to HER, the strain and ligand effects in Pd pseudomorphic overlayers are additive.16 Thus, if the individual contributions of these two effects on CO2 reduction could be understood, then we would be able to better optimize the type of bimetallic heterstructures and the thickness of electrocatalytic layer most suitable for enhancing the catalytic reactivity. In this work, we tailor the strain and ligand effects in Cu-Rh and Cu-Ni bimetallic heterostructures as catalysts for the electrochemical reduction of CO2 to CO and CH4. To the best of our knowledge, this is the first study in which the individual and combined influence of the two effects are understood and approaches for enhancing the electrocatalytic properties of Cu are proposed as well as tested via density functional calculations. Examination of Cu1/M1/Cu4(211) sandwiched surfaces and Cui/Mj(211) overlayers (where subscripts represent number of MLs) allows modelling of the pure ligand effect and the combined ligand and strain effect on Cu, respectively. The choice of Ni and Rh as supports in the metal overlayers is two-fold. Firstly, we


4

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


get to span two different strain regimes: compression and tension. Secondly, these are comparatively less expensive than the typically used Au and Pt substrates.11-13,17,18 We show that ligand effect of Ni in Cu1/M1/Cu4(211) marginally decreases the η for CO2 reduction to CO and CH4, while that of Rh increases it compared to Cu(211). We anticipate a greater decrease in η on surfaces with reduced ligand effect and find this to be the case for Cu1/Ni2/Cu3(211), Cu2/Rh1/Cu3(211), and Cu2/Rh2/Cu2(211). Furthermore, based on our study of Cui/Mj(211) overlayers (i = 1-3, i + j = 6) we illustrate that application of tensile strain and not compressive strain will cause an even more significant decrease of η by ~86 mV. Interestingly, this shift is even greater than that achieved experimentally with a Cu ML deposited on (a more expensive) polycrystalline Pt.17 The decrease in η will be accompanied with a simultaneous increase in exchange current density of the reduction products, corresponding to a higher electrocatalytic activity of these surfaces for CO2 reduction than Cu electrodes.

RESULTS AND DISCUSSION The reaction mechanisms for CO2 reduction to CO and CH4 considered in this work are depicted in Scheme 1. In the scheme, * represents an active site on the surface. The free energy profiles at U = 0 V vs. the reversible hydrogen electrode (RHE) on Cu(211) (hereafter referred as Cu) derived from our previous study19 for these reaction mechanisms are displayed together in Fig. 1. Except for the CO desorption step, all other steps involve a (H+/e-) transfer. The figure reveals that CO2 protonation to COOH*, CO* protonation to CHO*, CO desorption, and OH* protonation to H2O are associated with free energy changes (∆Greac) that are large and positive. Therefore, in this study we choose to consider the adsorption of only the intermediates participating in these steps, namely, COOH, CO, CHO, and OH. Furthermore, η, which is the


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

Scheme 1. Reaction mechanisms of CO2 reduction to CO and CH4 considered in this work. In the figure * represents an active site on the surface.

Figure 1. Free energy profile of CO2 reduction to CO and CH4 on Cu (211) at U = 0 V vs. RHE. Except CO desorption, all steps involve a (H+/e-) transfer. The major uphill steps are highlighted in red. difference between the equilibrium potential (U0) and the onset potential for a particular reduction product, can be estimated theoretically as U0 + (∆Gmax/e) in our case, where ∆Gmax is the maximum positive free energy change in the free energy profile of the reaction mechanism. Thus, our objective is to decrease ∆Gmax in order to decrease η. This in turn can be achieved by


6

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


modifying the binding energies (BEs) of the adsorbates participating in the corresponding step. Therefore, in case of CO2 reduction to CO our interest is in strengthening adsorption of COOH and weakening that of CO on the surface. In regard to CO2 reduction to CH4, we would benefit from weakening adsorption of CO and OH, while strengthening adsorption of COOH and CHO on Cu. In other words, relative BEs (with Cu as reference) of COOH and CHO should be negative and of CO and OH positive on the catalyst surface for it to show improved CO2 electroreduction. Ligand versus strain effect Figure 2 displays the models of the bare Cu1/M1/Cu4 sandwiched surface and Cui/Mj overlayers (i = 1-3, i + j = 6). It is to be noted that in the former, M adopts the lattice parameters of Cu and therefore, the top Cu ML is affected only by the ligand effect. In the Cui/Mj overlayers both the strain and ligand effect act on the top Cu layer. Variation of the relative BEs of the adsorbates, calculated with Cu as the reference, are also shown in Fig. 2. Therefore, in the plots, positive and negative values imply weakening and strengthening, respectively of the adsorbate binding on the surface with respect to Cu. The following trends are common to all the four adsorbates: (i) the pure ligand effect of Rh and Ni in Cu1/M1/Cu4 strengthens the BEs, (ii) Ni has a weaker ligand effect than Rh, (iii) BEs are also strengthened on Cu1/M5 by a magnitude similar to the corresponding Cu1/M1/Cu4 surface, implying that the ligand effect dominates in these cases, and (iv) as the number of Cu MLs increases in the Cui/Mj overlayers, the BEs relatively weaken due to reduction of the ligand effect and at i = 3 the strain effect dominates. Kitchin et al. have shown that the interaction between Pt and 3d transition metal (TM) layers in Pt1/TM1/Pt(111) sandwiched surfaces is linearly correlated to the d-band width.20


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

Figure 2. (Top panel) Side views of Cu1/M1/Cu4 (A1), Cu1/M5 (A2), Cu2/M4 (A3), and Cu3/M3 (A4) heterostructures, where M = Ni or Rh. Brown and silver spheres are Cu and M atoms, respectively. (Bottom panel) Relative binding energies (BEs) (with Cu as reference) of key intermediates on A1-A4 surfaces (M = Ni or Rh). Negative and positive values indicate strengthening and weakening of binding, respectively.

Further, as the d-band width increases, the d-band center, which is the central moment of the dband, decreases. Ruban et al. have calculated the shift in the d-band center, which is the central moment of the d band, in Cu/TM(111) overlayers relative to pure Cu(111) to be -0.27 and 0.18 eV for TM = Rh and Ni, respectively.21 As seen from our results, such overlayers are similar in behavior to the type of sandwiched surfaces studied in this section due to the strong dominance of the ligand effect. Thus, we can expect the d-band center of Cu1/Rh1/Cu4 to be < Cu1/Ni1/Cu4 and consequently, the interaction between Cu and Rh in Cu1/Rh1/Cu4 to be > that between Cu


8

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Ni in Cu1/Ni1/Cu4. In this view, it is understandable why the ligand effect of Rh is stronger than that of Ni. Cu in Cui/Nij overlayers experiences a compressive strain (-4.4%) and in Cui/Rhi is under a tensile strain (+5.1%). Based on the d-band theory21,22, BE of an adsorbate is increased on compression and decreased on expansion. Therefore, we anticipate the relative BEs to be positive on Cu3/Ni3 and negative on Cu3/Rh3. It can be seen from Fig. 2 that the relative BEs of COOH and CHO exhibit the expected strain trend, i.e., they are negative when M = Rh and positive when M = Ni. In comparison, relative BEs of OH and CO show an opposite strain effect: for M = Ni, the relative BEs of CO and OH are ≤ 0 and when M = Rh relative BE of CO > 0. Recalling that negative relative BEs (with Cu as reference) of COOH and CHO and/or positive of CO and OH will result in decrease of the η, we expect Cu2/Rh2 and Cu3/Rh3 to show enhanced CO2 reduction properties. Figures 3(a) and (b) display large and positive ∆Greac at U = 0 V vs. RHE in the CO2 to CO reaction mechanism on Cu, Cui/Mj, and Cu1/M1/Cu4 for M = Rh and Ni, respectively. On Cu ∆Gmax is 0.546 eV and it corresponds to CO2 protonation to COOH*. It can be seen from the figures that on Cu1/M1/Cu4 ∆Greac for CO2 protonation to COOH* decreases and for CO desorption increases relative to Cu. On Cui/Mj surfaces as i increases from one to three, ∆Greac for COOH* formation increases and for CO desorption simultaneously decreases. Overall, compared to Cu ∆Gmax is significantly less on Cu2/Rh4 and Cu3/Rh3. This is in agreement with our expectations based on the BE trends. The large and positive ∆Greac in the CO2 to CH4 reaction mechanism are shown in Figs. 3(c) and (d) for M = Rh and Ni, respectively. For CH4 generation ∆Gmax is calculated as 0.68 eV


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

Figure 3. Large and positive ∆Greac at U = 0 V vs. RHE on Cu1/M1/Cu4 (A1), Cu1/M5 (A2), Cu2/M4 (A3), and Cu3/M3 (A4) in the CO2 reduction to CO reaction mechanism for (a) M = Rh and (b) M = Ni and in the CO2 reduction to CH4 reaction mechanism for (c) M = Rh and (d) M = Ni.

on Cu, corresponding to CO* to CHO* step. This is equivalent to an onset potential of -0.68 V, which is in reasonable agreement with the experimentally reported values on a Cu electrode (-0.8 V1, -0.79 V17). The stronger BE changes when M = Rh versus when M = Ni, shift the step associated with ∆Gmax to OH* protonation on Cu1/Rh1/Cu4 and Cui/Rhj (i = 1-2), such that, ∆Gmax on Cu2/Rh2 < Cu. Although, on Cu3/Rh3 CO* protonoation to CHO* is the step with ∆Gmax, the corresponding value is much less than on Cu due to the weaker BE of CO. Therefore, consistent with our predictions according to changes in BEs, Cu2/Rh2 and Cu3/Rh3 will have enhanced CO2 reduction to CH4 electrocatalytic properties.


10

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reducing ligand effect Our results discussed in the previous subsection illustrated that the ligand effect of Rh and Ni on Cu in the Cu1/M1/Cu4 sandwiched surfaces is such that a noteworthy decrease in the η for both CO and CH4 production will not occur. Thus, if the strength of this ligand effect could be modified, then improved CO2 reduction characteristics may be achieved. To this end, we tailored the number of Cu and M layers by the following three approaches: (a) sandwiching two MLs of M directly below the top Cu ML, (b) pushing the single M ML further away from the top Cu ML, and (c) combining approaches (a) and (b). To examine these methodologies, we studied the following model surfaces: Cu1/M2/Cu3, Cu2/M1/Cu3, and Cu2/M2/Cu2 surfaces, respectively. Figure 4 shows the relative BEs of COOH, CHO, CO, and OH on the abovementioned surfaces relative with Cu as reference. It is noticeable that in general the relative BEs on these surfaces increase compared to those on the analogous Cu1/M1/Cu4 surface, indicating a reduction of the pure ligand effect on the top Cu layer. Furthermore, the relative BEs are mostly negative and tend to approach 0 on moving from Cu1/M1/Cu4 to Cu2/M2/Cu2 (except in case of OH). We, therefore, expect that in CO2 reduction to CO, COOH* protonation will be the step with ∆Gmax on Cu2/M1/Cu3 and Cu2/M2/Cu2. In this case the relatively stronger stabilization of COOH than CO when M = Rh will decrease the η. With regard to CO2 reduction to CH4, the η will reduce on Cu2/Rh1/Cu3 and Cu2/Rh2/Cu2 if CHO* to CO* remains the step with ∆Gmax. Due to the comparatively less variation of the BEs in the M = Ni series, a decrease in the η may not occur. Figures 5(a)-(d) display the positive and large ∆Greac in the reaction mechanisms of CO2 reduction to CO and CH4 on all the sandwiched surfaces. The dashed lines in the figures define the position of the ∆Gmax on Cu in each case. Similar to the trend in Figs. 3(a)-(d), as ∆Greac for


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

Figure 4. Relative binding energies (BEs) (with Cu as reference) of key intermediates on Cu1/M1/Cu4, Cu1/M2/Cu3, Cu2/M1/Cu3, and Cu2/M2/Cu2 sandwiched surfaces. Negative and positive values indicate strengthening and weakening of binding, respectively.

COOH* formation increases, that for CO desorption decreases. In accordance with our predictions based on the relative BEs, we find that ∆Gmax shows little to no reduction on Cu2/Ni1/Cu3 and Cu2/Ni2/Cu2 for both CO and CH4 formation, but a noticeable decrease is observed on Cu2/Rh1/Cu3 and Cu2/Rh2/Cu2 compared to Cu. Interestingly, ∆Gmax for CO2 reduction to CO also decreases substantially on Cu1/Rh2/Cu3 because even though the relative BE of CO is negative, the ∆Greac for CO* desorption on this surface is less than that for COOH* formation on Cu. Understanding strain effect


12

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Large and positive ∆Greac at U = 0 V vs. RHE on Cu1/M1/Cu4, Cu1/M2/Cu3, Cu2/M1/Cu3, and Cu2/M2/Cu2 sandwiched surfaces in the CO2 reduction to CO reaction mechanism for (a) M = Rh and (b) M = Ni and in the CO2 reduction to CH4 reaction mechanism for (c) M = Rh and (d) M = Ni. The blue dashed line in (a) and (b) depicts the ∆Gmax on Cu(211) corresponding to COOH* formation in the CO2 reduction to CO reaction mechanism. The orange dashed line in (c) and (d) denotes the ∆Gmax on Cu(211) corresponding to CHO* to CO* step in the CO2 reduction to CH4 reaction mechanism.

Our calculations on Cui/Mj (i = 2 or 3) overlayers presented in subsection titled Ligand versus strain effect showed that noticeable decrease of the η will be achieved with Rh and not Ni as the underlying support. We believe that this is a direct consequence of the tensile strain acting on Cu and an opposite strain trend exhibited by BE of CO. In order to ascertain this, we constructed hypothetical overlayers in which Cu MLs were deposited on a compressed Rh(211) and an


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

expanded Ni(211) surface. Compressed Rh(211) was generated using the lattice parameters of Ni(211), while expanded Ni(211) was constructed using those of Rh(211).

Figure 6. Relative binding energies (BEs) (with reference to Cu(211)) of key intermediates on Cui/Rhj, compressed-Cui/Rhj (comp-Rh), Cui/Nij, and expanded-Cui/Nij (exp-Ni) overlayers (i = 1-3, i + j = 6). Negative and positive values indicate strengthening and weakening of binding, respectively.

Relative BEs (with reference to Cu) of key adsorbates on Cui/Rhj, compressed-Cui/Rhj, Cui/Nij, and expanded-Cui/Nij are illustrated in Fig. 6. Relative BEs of COOH and CHO on compressed-Cui/Rhj are shifted up compared to (unstrained) Cui/Rhj, while the same on expanded-Cui/Nij overlayers are shifted down compared to (unstrained) Cui/Nij. This is qualitatively in agreement with the d-band theory21,22, according to which expansion strengthens


14

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BEs and compression weakens them. In comparison, the relative BEs of CO and OH show an opposite strain effect. These deviations from the expected strain induced trend are also observed for i = 3 overlayers, in which the ligand effect is reduced, therefore, they may be due to different responses of the adsorbates to strain. Recalling that negative relative BEs (with Cu as reference) of COOH and CHO and positive relative BEs of CO and OH positive will lead to enhanced CO2 reduction, based on the comparative shifts of BEs of COOH and CO we expect expanded-Cu1/Ni5 to yield a decreased η for CO2 reduction to CO similar to (unstrained) Cui/Rhj (i = 2 or 3) overlayers provided COOH* formation is the most uphill step in free energy on these strained overlayers. The strong weakening of BE of CO observed on expanded-Cui/Nij (i = 2 and 3) will prove to be more advantageous for CO2 reduction to CH4. In comparison, we suspect compressed-Cui/Rhj to behave similarly to the (unstrained) Cui/Nij and be unsuitable for CO2 reduction. Figure 7 compares the large and positive ∆Greac in the CO2 reduction to CO and CH4 reaction mechanisms on Cui/Rhj with expanded-Cui/Nij and on Cui/Nij with compressed-Cui/Rhj overlayers. It can be clearly seen that ∆Greacs on the latter two surfaces are similar and the ∆Gmaxs are either > or similar to on Cu. This confirms that compression of Cu will not be beneficial for its electrocatalytic properties for CO2 reduction. ∆Gmax on expanded-Cu1/Ni5 for CO2 reduction to CO and ∆Gmax on expanded-Cui/Nij (i = 1-3) for CO2 reduction to CH4 is less than on Cu, similar to the behavior of Cu2/Rh4 and Cu3/Rh3. Thus, it can be inferred that application of a pure expansive strain on enhanced its electrocatalytic activity for reduction of CO2 to CH4.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

Figure 7. Large and positive ∆Greac at U = 0 V vs. RHE in the CO2 reduction to CO reaction mechanism on (a) Cui/Rhj and expanded (exp)-Cui/Nij and (b) Cui/Nij and compressed (comp)-Cui/Rhj overlayers. Large and positive ∆Greac at U = 0 V vs. RHE in the CO2 reduction to CH4 reaction mechanism on (c) Cui/Rhj and expanded (exp)-Cui/Nij and (d) Cui/Nij and compressed (comp)-Cui/Rhj overlayers. The blue dashed line in (a) and (b) depicts the ∆Gmax on Cu(211) corresponding to COOH* formation in the CO2 reduction to CO reaction mechanism. The orange dashed line in (c) and (d) represents the ∆Gmax on Cu(211) corresponding to CHO* to CO* step in the CO2 reduction to CH4 reaction mechanism. Comparing best surfaces A decrease in η can be estimated by the decrease in ∆Gmax with Cu as reference as η = U0 + (∆Gmax/e), where U0 is the equilibrium potential. Table 1 lists the negative shifts η that are predicted for CO2 reduction to CO and CH4 on some of the surfaces studied herein. For CO2 reduction to CO, most noticeable decrease in η (~ -85 mV) occurs on Cu2/Rh4, Cu2/Rh2/Cu2, and expanded-Cu1/Ni5. In case of CO2 reduction to CH4, maximum reduction in η (-86 to -92 mV) is


16

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


calculated for Cu3/Rh3 and expanded-Cui/Nij (n = 1 – 3). Thus, depositing 0.54-0.81 nm of Cu on a Rh support or sandwiching 0.54 nm of Rh below a Cu layer of similar thickness will enhance the CO2 reduction to CO and CH4 generation. Furthermore, these shifts are even greater than that observed experimentally (-80 mV) on a Cu ML deposited on polycrystalline Pt17 for CO2 reduction to CH4. Thus, utilizing the abovementioned Cu-Rh catalysts will also lead to a reduction in material costs to the requirement of thinner Rh layer in these surfaces and relative inexpensiveness of Rh compared to Pt. Table 1 also presents ∆Gmax calculated for the competing HER. We considered the following reaction mechanism for studying HER: * + (H+/e-) → H* (1) H* + (H+/e-) → H2 + * (2) where * is an active site of the surface. On each of the surfaces in the table, the H2 release step (eq. (2)) is the most uphill in free energy. The corresponding ∆Gmax for HER on Cu2/Rh2/Cu2, Cu2/Rh4, and Cu3/Rh3, is in the range of 0.051 to 0.090 eV, which is similar to that for Cu (0.052 eV). This suggests that these particular surfaces will retain the higher selectivity for CO or CH4 production versus H2 release, just like pure Cu electrodes. Figure 8 displays the CO2 to CO reduction activity on (211) surfaces of transition metals as a function of BEs of COOH and CO. This activity plot, which is adapted from Ref.

23

,

resembles a volcano with the top (innermost) region having maximum CO2 reduction activity, which decreases as one moves outward. In Fig. 8 the position of Cu is taken from Ref.

23

BEs

calculated with the RPBE density functional, while Cu2/Rh4 and Cu3/Rh3 are placed based on the BE shifts calculated in this work using the PBE density functional. The straight green line represents the “linear scaling relationship” between BEs of COOH and CO on transition metals.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

Table 1. Decrease in overpotential (∆η) (with Cu as reference) predicted for CO2 reduction to CO and CH4 and maximum free energy change (∆Gmax) for the hydrogen evolution reaction (HER) on selected surfaces. Surface

∆η(CO2 to CO)

∆η(CO2 to CH4)

∆Gmax(HER)

(mV)

(mV)

(eV)

0

0

0.052

Cu2/Rh4

-85

-72

0.067

Cu3/Rh3

-48

-86

0.051

Cu1/Ni1/Cu4

-

-10

0.084

Cu2/Rh1/Cu3

-46

-33

0.100

Cu2/Rh2/Cu2

-86

-63

0.080

Cu1/Ni2/Cu3

-63

-33

0.059

Cu2/Ni1/Cu3

-24

-

0.059

Cu2/Ni2/Cu2

-15

-12

0.049

expanded-Cu1/Ni5

-86

-92

0.090

expanded-Cu2/Ni4

-

-90

0.030

expanded-Cu3/Ni3

-

-90

0.020

Cu

Due to this relationship any change in BE of COOH is likely to be accompanied by a similar change in sign and magnitude in BE of CO. However, on Cu2/Rh4 and Cu3/Rh3 the BEs of COOH and CO shift in opposite direction and this relationship is broken. As a result, the positions of these catalysts move inward relative to Cu in the activity plot, implying an increase in exchange current density of CO during CO2 reduction. Thus, utilizing these Cu-Rh bimetallic


18

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Kinetic activity ‘volcano’ contour plot at overpotential = 0.35 V for CO2 reduction to CO on (211) transition metal surfaces adapted from Ref.

23

. The BEs include solvation

effects. The innermost region encapsulated by the red lines is the top of the activity volcano and corresponds to maximum current density. As one moves outward from the red contour to the dark blue contour, the activity decreases. The position of Cu is taken from Ref.

23

employing the RPBE density functional, while Cu2/Rh4 and Cu3/Rh3 are placed based on BE shifts obtained from our calculations using the PBE density functional. The straight green line depicts the linear relationship between BEs of COOH and CO on transition metals.

surfaces as electrocatalysts allow a decrease in overpotential with a simultaneous increase in exchange current density of the reduction products and retention of product selectivity for CO and CH4 formation over H2 generation. In addition, the material cost will also be lower compared to bimetallic heterostructures employing Au or Pt supports as Rh is relatively inexpensive.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

SUMMARY AND CONCLUSIONS Summarizing, in this study we used DFT calculations and examined two types of bimetallic (211) hetero-surfaces, namely, Cui/Mj/Cuk sandwiched surfaces to model the pure ligand effect on Cu and Cui/Mj overlayers to analyze the combined influence of ligand and strain effects on Cu. We decreased the ligand effect independently by shifting the M layer deeper in the Cu lattice, placing two MLs of M (vs. the standard 1 ML) directly below the top Cu layer, and combining the first two methodos in the sandwiched surfaces. We also filtered out the ligand effect in the overlayers by increasing the number of Cu MLs from one to three, such that finally only the strain effect persists. Further, we studied hypothetical expanded-Cui/Nij and compressed-Cui/Rhj overlayers in order to confirm that the electrocatalytic properties in the Cu3/M3 overlayers are purely a result of the strain effect. We correlated the binding energy changes to the variation in the maximum positive free energy change in the reaction mechanisms, which in turn led to the shifts in the η associated with CO2 reduction to CO and CH4 on Cu. We conclude that the ligand effect in Cu1/M1/Cu4 and Cu1/M5 strengthens the binding energyies of all adsorbates, such that the overpotential increases for M = Rh or marginally decreases by 10-20 mV for M = Ni. Decreasing the ligand effect in both the sandwiches surfaces and overlayers is key to decreasing the overpotential compared to Cu(211). In general, Ni layers in the Cu lattice and as the underlying support for Cu is unsuitable for CO2 electroreduction. In comparison, Cu2/Rh2/Cu2, Cu2/Rh4, and Cu3/Rh3 yield the most decrease (of ~ -86 mV) in overpotential for CO2 reduction CO or CH4. Interestingly, the calculated shift in the overpotential for these surfaces is comparable with that reported using the expensive polycrystalline Pt as the substrate. These Cu-Rh bimetallic heterostructures are promising


20

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrocatalysts for CO2 reduction due to the simultaneous decrease in overpotential, increase in exchange current density that is a measure of activity, and retention of selectivity for CO and CH4 production over H2 generation. To the best of our knowledge these electrocatalytic properties have not been reported to be satisfied together for transition metals based catalysts for CO2 reduction.

THEORETICAL CALCULATIONS All calculations were performed with Quantum Espresso24 taking into account spin polarization and employing the PBE exchange-correlation functional and ultrasoft pseudopotentials. All the surfaces were generated as six layer slabs of (3 × 3) unit cells and 20 Å vacuum using PBEcalculated lattice constants of 3.672, 3.509 and 3.860 Å for Cu, Ni, and Rh, respectively. Geometry optimization of each surface was carried out at 4 × 8 × 1 k-points, planewave cutoff of 33 Ry, and convergence threshold for forces of 0.004 Ry/Bohr. Often in theoretical calculations for studying electroreduction of CO2 a choice of 3-4 layers is considered standard for generating a transition metal (211) surfaces.25-27 This lends confidence that the total number of six layers used to build each model surface in this study will be able to also capture the bulk properties. Adsorption of COOH, CO, CHO, OH, and H was considered at the ontop, bridge, ontop, bridge, and three-fold bridge site, respectively on all the surfaces, based on our previously reported work19. Binding energies (BEs) of the adsorbates were computed using total energies of ½ H2 molecule, ½ graphene, and (H2O-H2) to reference each H, C, and O atom, respectively. The total energies of H2 and H2O were obtained by placing the molecules in a 15 × 15 × 15 Å supercell and employing 1 × 1 × 1 k-point. The unit cell of graphene contained basis of two C atoms and


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

the lattice parameter was a = b = 2.456 Å, where C-C bond distance was 1.42 Å. The free energy change ∆Greac associated with a step in the reaction mechanism was obtained using the computational hydrogen electrode28 as: ∆Greac = ∆Ereac + ∆EZPE + ∆∫CPdT - T∆S + ∆GU (3) This model allows each (H+/e-) pair to be referenced as ½ H2 (g). In eq. (3) (i) ∆Ereac is the reaction energy including the solvation effects and (ii) ∆EZPE, ∫CPdT, and T∆S are the net zeropoint energy, heat capacity and entropic corrections, respectively taken from Ref.27. The term ∆GU = neU introduces the dependence on the electrical potential, U with respect to the reversible hydrogen electrode (RHE) and in this work n = 1 and U = 0 V.

AUTHOR INFORMATION Corresponding Author *Dr. Tuhina Adit Maark Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT TAM would like to acknowledge IIT Madras for her Institute Postdoctoral Fellowship. The authors acknowledge the Computer Center, IIT Madras for providing computational facilities.


22

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1. Hori, Y.; Murata, A.; Takahashi, R. Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution. J. Chem. Soc. Faraday Trans. 1 1989, 85, 2309-2236. 2. Hori, Y.; Wakebe, H.; Tsukamato, T.; Koga, O. Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueous Media. Electrochim. Acta 1994, 39, 1833-1839. 3. Marković, N. M.; Ross, P. N. Surface Science Studies of Model Fuel Cell Electrocatalysts. Surf. Sci. Rep. 2002, 45, 117-229. 4. Hori, Y. Electrochemical CO2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry; Springer: New York, 2008; Vol. 42, Chapter 3, pp 89−189. 5. Kibler, L. A.; El‐Aziz, A. M.; Hoyer, R.; Kolb, D. M. Tuning Reaction Rates by Lateral Strain in a Palladium Monolayer. Angew. Chem. Intl. Ed. 2005, 44, 2080-2084. 6. Koh, S.; Strasser, P. Electrocatalysis on Bimetallic Surfaces: Modifying Catalytic Reactivity for Oxygen Reduction by Voltammetric Surface Dealloying. J. Am. Chem. Soc. 2007, 129, 12624–12625. 7. Mani, P.; Srivastava, R.; Strasser, P. Dealloyed Pt−Cu Core−Shell Nanoparticle Electrocatalysts for Use in PEM Fuel Cell Cathodes. J. Phys. Chem. C 2008, 112, 2770– 2778.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

8. Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Core–Shell Compositional Fine Structures of Dealloyed PtxNi1–x Nanoparticles and Their Impact on Oxygen Reduction Catalysis. Nano Lett. 2012, 12, 5423–5430. 9. Sethuraman, V. A.; Vairavapandian, D.; Lafouresse, M. C.; Adit Maark, T.; Karan, N.; Sun, S.; Bertocci, U.; Peterson, A. A.; Stafford, G. R.; Guduru, P. R. Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt. J. Phys. Chem. C 2015, 119, 19042–19052. 10. Jana, R.; Bhim, A.; Bothra, P., Pati, S. K.; Peter, S. C. Electrochemical Dealloying of PdCu3 Nanoparticles to Achieve Pt-like Activity for the Hydrogen Evolution Reaction. ChemSusChem 2016 (DOI: 10.1002/cssc.201601081). 11. Varela, A. S.; Schlaup, C.; Jovanov, Z. P.; Malacrida, P.; Horch, S.; Stephens, I. E. L.; Chorkendorff, I. CO2 Electroreduction on Well-Defined Bimetallic Surfaces: Cu Overlayers on Pt(111) and Pt(211). J. Phys. Chem. C 2013, 117, 20500–20508. 12. Januszewska, A.; Jurczakowski, R.; Kulesza, P. J. CO2 Electroreduction at Bare and CuDecorated Pd Pseudomorphic Layers: Catalyst Tuning by Controlled and Indirect Supporting onto Au(111) Langmuir, 2014, 30, 14314–14321. 13. Monzó, J.; Malewski, Y.; Kortlever, R.; Vidal-Iglesias, F. J.; Solla-Gullón, J.; Koper, M. T. M.; Rodriguez, P.. Enhanced Electrocatalytic Activity of Au@Cu Core@Shell Nanoparticles Towards CO2 Reduction. J. Mater. Chem. A 2015, 3, 23690-23698. 14. Back, S.; Kim, J.-H.; Kim, Y.-T.; Jung, Y. Bifunctional Interface of Au and Cu for Improved CO2 Electroreduction. ACS Appl. Mater. Interfaces 2016, 8, 23022-23027.


24

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Sarfraz, S.; Garcia-Esparza, A. T.; Jedidi, A.; Cavallo, L.; Takanabe, K. Cu–Sn Bimetallic Catalyst for Selective Aqueous Electroreduction of CO2 to CO. ACS Catal. 2016, 6, 2842–2851. 16. Adit Maark, T.; Peterson, A. A. Understanding Strain and Ligand Effects in Hydrogen Evolution over Pd(111) Surfaces. J. Phys. Chem. C 2014, 118, 4275–4281. 17. Reske, R.; Duca, M.; Oezaslan, M.; Schouten, K. J.; Koper, M. T.; Strasser, P. Controlling Catalytic Selectivities During CO2 Electroreduction on Thin Cu Metal Overlayers. J. Phys. Chem. Lett. 2013, 4, 2410-2413. 18. Friebel, D.; Mbuga, F.; Rajasekaran, S.; Miller, D. J.; Ogasawara, H.; Alonso-Mori, R.; Sokaras, D.; Nordlund, D.; Weng, T. C.; Nilsson, A. Structure, Redox Chemistry, and Interfacial Alloy Formation in Monolayer and Multilayer Cu/Au (111) Model Catalysts for CO2 Electroreduction. J. Phys. Chem. C 2014, 118, 7954-7961. 19. Adit Maark, T.; Nanda, B. R. K. CO and CO2 Electrochemical Reduction to Methane on Cu, Ni, and Cu3Ni (211) Surfaces. J. Phys. Chem. C 2016, 120, 8781-8789. 20. Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Modification of the Surface Electronic and Chemical properties of Pt(111) by Subsurface 3d Transition Metals. J. Chem. Phys. 2004, 120, 10240-10246. 21. Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K., Surface Electronic Structure and Reactivity of Transition and Noble Metals. J. Mol. Catal. A 1997, 115, 421429.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

22. Hammer, B.; Nørskov, J. K. Electronic Factors Determining the Reactivity of Metal Surfaces. Surf. Sci. 1995, 343, 211-220. 23. Handen, H. A.; Varley, J. B.; Peterson, A. A.; Nørskov, J. K. Understanding Trends in the Electrocatalytic Activity of Metals and Enzymes for CO2 Reduction to CO. J. Phys. Chem. Lett. 2013, 4, 388-392. 24. Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A. QUANTUM ESPRESSO: A Modular and Open-source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. 25. Rasul, S.; Anjum, D. H.; Jedidi, A.; Minenkov, Y.; Cavallo, L.; Takanbe, K. A Highly Selective Copper-Indium Bimetallic Electrocatalyst for the Electrochemical Reduction of Aqueous CO2 to CO. Angew. Chem. Int. Ed. 2015, 54, 2146-2150. 26. Peterson, A. A.; Nørskov, J. K. Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3, 251-258. 27. Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. How Copper Catalyzes the Electroreduction of Carbon Dioxide into Hydrocarbon Fuels. Energy Environ. Sci. 2010, 3, 1311-1315. 28. Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. R.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-cell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892.


26

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Insert Table of Contents Graphic


27

Enhancing CO2 Electroreduction by Tailoring Strain and Ligand

Recommend Documents