Computational Screening of Near-Surface Alloys for CO2

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES 2

Computational Screening of Near-Surface Alloys for CO Electroreduction Zhonglong Zhao, and Gang Lu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03705 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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 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 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.

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 39 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

ACS Catalysis

Computational Screening of Near-Surface Alloys for CO2 Electroreduction Zhonglong Zhao and Gang Lu* Department of Physics and Astronomy, California State University Northridge, California 91330, United States

1 ACS Paragon Plus Environment

ACS Catalysis 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 39

ABSTRACT: Electrochemical conversion of carbon dioxide (CO2) into chemical feedstocks provides an attractive solution to our pressing energy and environment problems. Here, we report that transition metal near-surface alloys (NSAs) are promising catalysts for CO2 electroreduction. Based on first-principles calculations on 190 candidates, we propose a number of NSAs which show promise of highly active and selective catalysts for formic acid, carbon monoxide, methanol, and ethylene production, while simultaneously suppress competing hydrogen evolution reaction (HER). We predict that Pd/W, Au/Hf, and Au/Zr NSAs are more active than most known electrodes for formic acid formation with overpotentials significantly lower than that of HER. Ag/Hf and Ag/Zr are revealed as superior catalysts for the production of carbon monoxide with overpotentials of 0.77 V lower than that on pure Ag electrode. We find that methanol and ethylene can be produced on Ag/Ta and Ag/Nb NSAs whose overpotentials are ~15% lower than that on Cu (211) surface. On the other hand, their overpotentials for HER are six times more negative than that on Cu (211). The work demonstrates the great potential of transition metal catalysts by modulating their near surface properties.

KEYWORDS: CO2 electroreduction, near-surface alloys, formic acid, CO, methanol, ethylene, DFT calculation


Page 3 of 39 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

ACS Catalysis

1. INTRODUCTION The electrochemical conversion of CO2 into value-added chemicals and fuels, such as formic acid (HCOOH), carbon monoxide (CO), methane (CH4), methanol (CH3OH), and ethylene (C2H4) using renewable energy sources (i.e., solar, hydro, and wind) represents one of the most attractive means to mitigate our pressing energy and environmental threats.1-3 Since the first report of Hori et al.4 that CH4 and C2H4 can be formed as main products from CO2 reduction reaction (CO2RR) on Cu electrode, many transition metals (e.g., Au, Ag, Zn, Pt, Pd, Ni, Fe, etc.) and their alloys (Au-Cu, Pd-Pt, Ga-Pd, Ni-Ga, Cu-In, etc.) have been found active for CO2RR.511

However, most of the metal catalysts suffer from sluggish kinetics, low efficiency, and poor

product selectivity. For example, on Cu and its alloys, overpotential (UOP) for CH4 formation is quite high, at –1 V vs. reversible hydrogen electrode (RHE), and at least 16 different products are formed.6-7, 12-13 Au and Ag have been demonstrated as the most active pure metal catalysts in producing CO. However, UOP of –0.9 V and –1.2 V, respectively, is required to reach a high (> 80%) CO Faradaic efficiency.14-16 Moreover, as a common side reaction on metal electrodes, hydrogen evolution reaction (HER) consumes protons and electrons at low voltages and can significantly reduce the Faradaic efficiency for CO2RR.7, 17-18 For instance, it is reported that the Faradaic efficiency for HER (at –1 V vs. RHE) is ~10% on Au and Ag, ~20% on Cu, and ~90 % on Pt, Ni, and Fe surfaces, respectively.6 It is generally believed that the catalytic performance of a metal catalyst is determined by the chemical properties of its near-surface region.19 Thus, near-surface alloys (NSAs) in which a secondary alloying metal (solute) is introduced near the surface of the host metal, could serve as model systems to provide crucial insights necessary for improving the performance and for rational design of metal catalysts for CO2RR. Although only a minute amount of solute is present


ACS Catalysis 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 39

in surface region, NSAs are known to possess unusual catalytic properties, departing drastically from those of the host metals.20-22 For example, by electro-deposition of monolayer Cu on Pt (111), Bandarenka et al.23 revealed that Pt/Cu NSAs could activate CO oxidation reaction that is inert on Pt. By vapor deposition of V at elevated temperatures, Klötzer et al.24 found that Pd/V and Rh/V subsurface NSAs could enhance CO hydrogenation by a factor of 4 as compared to pure Pd. By evaporating monolayer Ni onto Pt (111), Hwu et al.25 showed that Ni/Pt NSA is active for hydrogenation of cyclohexene, although Pt and Ni individually are not active for this reaction. On the theory side, using density functional theory (DFT) calculations,22, 26 Greeley and Mavrikakis were among the first to systematically explore the potentials of NSAs for hydrogen related reactions, which motivated the present work. Despite the advances in NSA catalysts, experimental and/or computational screening of a large number of NSAs continues to pose significant challenges, especially for reactions such as CO2RR, involving multiple intermediates. In this work, we attempt to address the challenges by computational screening of a large number (~190) of transition metal NSAs with significantly improved performance on CO2RR. Here we focus on simple but common products on metal electrodes,6, 12 including HCOOH, CO, CH4, CH3OH, and C2H4 to illustrate general trends and shed lights on underlying chemistry. We choose Ag, Au, Cu, Pd, and Pt as host metals since they are representative of weak (Ag, Au), intermediate (Cu), and strong (Pd, Pt) binding to CO2RR intermediates.6 By alloying the five hosts with 20 solute metals (Ti, V, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au), we obtain a broad spectrum of free energies for reaction intermediates, which enables us to identify promising NSAs for specific products. As will be shown in this paper, a number of NSAs are predicted to exhibit exceptional performance for CO2RR. 2. COMPUTATIONAL DETAILS


Page 5 of 39 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

ACS Catalysis

The close-packed (111) surface of the host metals is selected to construct NSAs in this work owing to its stability. The equilibrium lattice parameters of Ag, Au, Cu, Pd, and Pt are calculated as 4.162, 4.172, 3.636, 3.952, and 3.976 Å, respectively. The surface slab model consists of four atomic layers with a 3 × 3 in-plane supercell. The adjacent slabs are separated by 17 Å vacuum in the normal direction of the surface. To determine the atomic structure of a NSA, the bottom layer of the slab model is fixed at its equilibrium bulk geometry while the top three layers are allowed to relax. Following Greeley and Mavrikakis,22, 26 two idealized classes of NSAs with the total solute coverage of one monolayer on the top layer (overlayer) or in the second layer (subsurface) of the host metal are considered in the present work. The overlayer configuration has often been termed as “skin” owing to the full monolayer at the surface. We use the notation A*/B to denote the overlayer configuration, and B/A to represent the subsurface NSAs hereafter. In both cases, “A” refers to the solute, and “B” denotes the host metal (Figure 1). The relative stability between the overlayer and subsurface configurations can be assessed by calculating the segregation energy (Eseg), defined as the energy required to move a solute from the bulk to the top surface of the host.22 If Eseg is negative, the overlayer configuration (A*/B) is more stable, and conversely, the subsurface configuration (B/A) is more stable.22 In this way, we can estimate the relative stability of 190 NSAs, and find 95 of them stable. Figure 1 displays color-coded matrix of Eseg for the NSAs; the corresponding numerical values can be found in Table S1. The “-” (or “+”) sign indicates an energetically stable overlayer (or subsurface) configuration while “0” denotes a pure metal. Since previous experiments found that the strong interaction between CO* (* indicates adsorbed species) and Pt could destabilize Pt/Cu NSAs,23 we further examine the stability of the 95 NSAs in the presence of key reaction intermediates on the surfaces. By fully relaxing the atomic geometries of the NSAs with intermediates (such as


ACS Catalysis 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 39

COOH*, HCOO*, and CO*) on the surfaces, we have identified 59 stable NSAs which show no substantial surface reconstruction. Thus, they will be the subjects of following free energy calculations. To further examine the stability of proposed NSAs, we have also calculated the migration energy (Emig), which is defined as the energy required to move a solute atom from the second layer to the surface (for subsurface NSAs) or from the surface to the second layer (for overlayer NSAs).22 A positive value of Emig indicates that the migration is energetically unfavorable and thus the NSA is stable. As shown in Table S2, the proposed seven NSAs (which will be discussed later in the paper) are all stable. However, possible segregation of solutes deeply into the host is not considered here as it is known that the deep segregation of solutes (i.e., to the third layer) does not affect substantially the trends in the adsorbate behavior.22 Finally, ab initio molecular dynamics (MD) simulations are also performed at 300 K to examine the thermodynamic stability of the predicted NSAs with key reaction intermediates on the surface. The proposed NSAs are found to be stable during the MD simulations, as evidenced by negligible changes in the radial distribution functions (Figure S1).


Page 7 of 39 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

ACS Catalysis

Figure 1. Color-coded table of the surface segregation energies (Eseg) for 190 NSAs. A nearsurface alloy consists of a host element, “B” in rows and a solute element, “A” in columns, forming either the overlayer (A*/B) or the subsurface (B/A) configuration. The DFT calculations were performed using Vienna ab initio simulation package (VASP).27 The exchange-correlation potential in the revised Perdew-Burke-Ernzerhof (RPBE) functional form was used28-29 with a plane-wave energy cutoff of 400 eV. For the surface calculations, the Brillouin-zone is sampled with a 3 × 3 × 1 k-mesh according to the Monkhorst-Pack scheme.30 Ab initio MD simulations were performed using NVT (constant number of particles, volume, temperature) ensemble31-33 and the Brillouin zone integration was restricted to the Γ point. A four-layer 6 × 6 in-plane supercell (144 metal atoms) was used in the MD simulations. The energy barriers of forming OCHCHO* and OCCHO* from CO* and CHO* were calculated by using Climbing-Image-Nudged Elastic Band (CI-NEB) method34, and the transition state was determined with five images and confirmed with the frequency analysis. The binding energies of the reaction intermediates were calculated based on the usual definition: EB[CxHyOz] = E[CxHyOz] – Eslab – xEC – yEH – zEO, where E[CxHyOz] and Eslab denote the energy of the total system and the clean slab, respectively. EC, EH, and EO are referenced to the energy of graphene, gaseous hydrogen (H2), and the difference between H2O and H2, respectively. The free energies of the reaction intermediates were derived from the corresponding binding energies by adding the zero-point energy (ZPE), heat capacity (CP), and entropy (-TS) corrections, which were calculated based on the molecular vibration analysis.35 The approximate solvation corrections to the reaction intermediates due to Peterson et al.13 were employed in the present work, avoiding the highly expensive treatment of explicit solvation effect. The computational hydrogen electrode (CHE) model36 was used to estimate the free energy change (∆G) at each intermediate


ACS Catalysis 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 39

step. In the CHE model, the chemical potential of a proton-electron pair, µ(H+ + e–), is assumed in equilibrium with half of that of the gaseous H2 at 101325 Pa and any pH values. When an external potential U is applied, µ(H+ + e–) is shifted by –eU, i.e., µ(H+ + e–) = 0.5 µH2 – eU. This model allows us to compute ∆G at each intermediate step as a function of the external potential U. The limiting potential (UL) for each intermediate step is thus defined as UL = –∆GU = 0 V/e, and UOP for the entire reduction reaction is the least negative UL that guarantees all intermediate steps exergonic. In this study, we assume that the proton-electron transfer barrier in each reaction step is small and can be overcome at room temperature. This is a common assumption used in many theoretical studies which generally have demonstrated satisfactory agreements to experimental results.6, 13, 37-39

Figure 2. Reaction pathways for CO2RR in the first hydrogenation step (top) and HER (bottom). CxHyOz represents hydrocarbons reduced from CO2. H+ + e– reactants are omitted. 3. RESULTS AND DISCUSSION 3.1. CO2RR vs. HER in the First Hydrogenation Step. As mentioned above, hydrogen is the main byproduct in CO2RR on most metal catalysts. This is because HER often has much less negative UOP compared to CO2RR, and as a result metals that are active for CO2RR are also active for HER. For example, UOP for HCOOH, CO, and CH4 production on Cu (211) are 8 ACS Paragon Plus Environment

Page 9 of 39 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

ACS Catalysis

calculated as –0.41, –0.41, and –0.74 V (vs. RHE), respectively, while UOP for HER is only -0.03 V.13 Therefore, HER poses a serious problem for CO2RR on Cu. It is generally believed that the competition between CO2RR and HER is determined primarily by the relative stability of the first hydrogenation intermediates, i.e., COOH* or HCOO* in CO2RR vs. H* in HER (Figure 2).40-41 If COOH* and/or HCOO* are more stable than H*, they would occupy active surface sites, and CO2RR would proceed as the primary reaction. Conversely, H* occupies the active sites and HER would dominate. Up to now, the search of catalysts in favor of the first intermediates of CO2RR over HER has proved infertile on pure metals, intermetallic alloys,40 and single-atom catalysts.41 Therefore, in this work, we examine the first reaction steps of CO2RR and HER on NSAs. Specifically, we compute the free energy changes for the reduction of CO2 to the first intermediate (CO2 → COOH*/HCOO*) as well as those for the formation of H* (* → H*, here * represents the clean slab) on the five host metals and 59 NSAs, and the results are summarized in Figure 3. We find that almost all NSAs favor HER over CO2RR with COOH* as the product in the first hydrogenation step (Figure 3a) because the free energy changes for reaction step CO2 → COOH* are greater than those for the formation of adsorbed H*, i.e., ∆G[CO2 → COOH*] > ∆G[* → H*]. In contrast, with HCOO* as the product (Figure 3b), 26 NSAs are found to favor CO2RR over HER, i.e., ∆G[CO2 → HCOO*] < ∆G[* → H*]. Note that HCOO* is more stable than COOH* on these 26 NSAs (Table S4), suggesting that CO2 will be preferentially reduced to HCOO* in the first step. Among the 26 NSAs, Ag- and Au-based NSAs are predicted to suppress HER the most, being deviated from the iso-energy line (∆G[CO2 → HCOO*] = ∆G[* → H*]) the farthest (Figure 3b). This is understandable since Ag and Au are among the poorest catalysts for HER.6 It is also clear that on the pure metals (Ag, Au, Cu, Pd, and Pt), HER is always favored over CO2RR. In the remaining of the paper, we will


ACS Catalysis 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 39

address two issues: (1) continuing the pathway of HCOO* formation, can we discover NSAs which could completely suppress HER? (2) Continuing the pathway of COOH* formation, can we discover NSAs producing hydrocarbons (CxHyOz) with much lower overpotentials than Cu despite the presence of HER? It’s worth noting that the competition between the first hydrogenation intermediates, COOH* and HCOO*, leads to different products. For example, CO, which is a key intermediate for more reduced products, such as CH4 and C2H4,5-6, 37 cannot be formed via the HCOO* pathway.

Figure 3. Changes in free energy for the formation of (a) COOH* and (b) HCOO* intermediates against changes in free energy for the formation of adsorbed H* on proposed pure transition metals (TMs) and NSAs. Above the dashed (iso-energy) line, HER is energetically favorable and below it, COOH* or HCOO* is more favorable.


Page 11 of 39 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

ACS Catalysis

Figure 4. (a) Changes in free energy for the formation of formic acid (HCOOH) against changes in free energy for the formation of adsorbed H* on the proposed 26 NSAs. (b) The overpotential volcano plot for formic acid production as a function of G[HCOO]. 3.2. Production of formic acid. Formic acid (HCOOH) is one of the simplest products of CO2RR, involving only two proton-electron pairs in the reaction. Despite its simplicity, formic acid has found applications in fuel cells, hydrogen storage, and chemical synthesis.42-45 As shown above, both COOH* and HCOO* can be reduced to formic acid, leading to two competing pathways, i.e., COOH* pathway (CO2 → COOH* → HCOOH) and HCOO* pathway (CO2 → HCOO* → HCOOH).13 Therefore, the 26 NSAs identified above which favor the formation of HCOO* over H*, are promising catalysts for selective production of formic acid via the HCOO* pathway. Next, we aim to find catalysts among the 26 NSAs which can produce formic acid while suppress HER. To this end, it is necessary that the overpotential for the formation of formic acid be less negative than that for HER.13 Hence, the free energy change for the second hydrogenation step via the HCOO* pathway (HCOO* → HCOOH) should be lower than that for the formation of H* (* → H*), the latter being the overpotential determining step for HER in the 11 ACS Paragon Plus Environment

ACS Catalysis 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 39

case of weak H* binding. We find that 20 of the 26 NSAs could satisfy the condition: ∆G[HCOO* → HCOOH] < ∆G[* → H*], as illustrated in Figure 4a. In addition, the competing COOH* pathway on these 20 NSAs is found to be unfavorable relative to the HCOO* pathway (Table S5). The volcano plot for formic acid production on the 20 NSAs is shown in Figure 4b, and three NSAs (Pd/W, Au/Hf, and Au/Zr) near the top of the volcano are identified as the most promising catalysts for formic acid production.

Figure 5. Free energy diagram for (a) formic acid production and (b) HER on Au, Au/Hf, Au/Zr, Pd, and Pd/W at 0 V vs. RHE. The adsorption geometries are shown. Next, we compare the free energy diagrams between formic acid production and HER on Pd/W, Au/Hf, and Au/Zr as well as pure Pd and Au. As shown in Figure 5, the overpotential determining step on Pd/W and Au/Hf is HCOO* → HCOOH while on Au/Zr is CO2 → HCOO*, consistent with the volcano plot. We find that UOP for the production of formic acid on Au/Hf and Au/Zr as –0.16 and –0.17 V vs. RHE, respectively, which are ~87% less negative than that on the pure Au (–1.27 V). UOP for the production of formic acid on Pd/W is –0.23 V vs. RHE, 54% less negative than that on the pure Pd (–0.50 V). In fact, the predicted overpotentials on


Page 13 of 39 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

ACS Catalysis

Pd/W, Au/Hf, and Au/Zr are comparable to those found in the most active formic acid catalysts, including the partially oxidized atomic Co layers (–0.24 V),46 Fe carbonyl cluster (–0.23 ~ –0.44 V),47 and Pd nanoparticles dispersed on a carbon support (–0.20 V).48 More importantly, UOP for the production of formic acid on Pd/W, Au/Hf, and Au/Zr is 39%, 74%, and 71% less negative than that for HER (–0.38, –0.61, and –0.59 V) as shown in Figure 5b, which suggests that HER is suppressed on these NSAs, leading to highly selective CO2RR. Finally, we note that the reduction of HCOO* to intermediate H2COO* is unfavorable13 on Pd/W, Au/Hf, and Au/Zr with calculated UL of –1.68, –1.98, and –2.00 V vs. RHE, respectively. 3.3. Production of CO. CO is a valuable product widely used in chemical industry, via for example, Monsanto process and Fischer-Tropsch process.49 The reaction pathway to CO is more complex than to formic acid and involves two hydrogenation steps plus one nonelectrochemical CO desorption step, i.e., CO2 → COOH* → CO* → CO.13 Since CO can be formed only via the COOH* pathway, HER is unavoidable. Figure 6a depicts the free energy diagram for CO production on the five host metals. Here, we use ∆G1, ∆G2, and ∆G3 to represent the free energy change over the three consecutive reaction steps. It is known that the adsorption strength of CO* determines the selectivity of the reaction.5-6 As shown in Figure 6a, the adsorption of CO* on Ag and Au is relatively weak, with its free energy higher than that of the desorbed species (∆G3< 0). Thus, CO would escape from the surface as the final product.4, 6 In contrast, the adsorption of CO* is relatively strong on Cu, Pd, and Pt (∆G3 > 0) and CO could stay on the surface. If the adsorption is not overly strong as on Cu, CO* can be further reduced to hydrocarbons (e.g., CH4 and C2H4). Otherwise, CO* may poison the surface as on Pd and Pt.48, 50-51

Therefore, a useful guideline for devising CO catalyst is that the surface should bind CO*

weakly enough to guarantee its desorption (∆G3 < 0). At the same time, it should bind COOH*


ACS Catalysis 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 39

appropriately to minimize ∆G1 and ∆G2, i.e., to facilitate the activation of CO2. On Ag and Au, although the desorption of CO is favorable (∆G3 < 0), the adsorption of COOH* is too weak. As a result, UOP for CO production (CO2→ COOH*) is more negative than –1 V vs. RHE (Figure 6a). Thus, our goal here is to search for NSAs that bind CO* and COOH* appropriately so as to produce CO efficiently. According to Figure 6a, the minimal UOP (target) for CO production is reached when ∆G1 = ∆G2 and ∆G3= 0. Based on the conditions, we determine that the minimal UOP = –0.14 V vs. RHE (cyan line in Figure 6a). In Figure 6b, we present UOP as a function of G[COOH] and G[CO], i.e., the free energy of COOH* and CO*, respectively on a contour plot. The minimal UOP corresponds to G[COOH] = 1.235 eV and G[CO] = 1.31 eV, as indicated in Figure 6b. The contour plot is divided by ∆G3 = 0 line. On the left (grey color) of the line, CO adsorption is favored; and on the right, CO desorption is favored (∆G3 < 0). In the following, we focus on the (right) region where CO can desorb from the surface so as to produce CO. To facilitate the discussion, this region is color-coded according to UOP. The dashed line corresponding to ∆G1 = ∆G2 further divides the region into two sub-regions, with ∆G1 > ∆G2 above the line and ∆G1 < ∆G2 below the line. The dashed line represents an overpotential “trough” and the minimal UOP (target) is located at the intersection between the dashed line (∆G1 = ∆G2) and the vertical ∆G3 = 0 line.


Page 15 of 39 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

ACS Catalysis

Figure 6. (a) Free energy diagram for CO production on pure metal (111) surfaces at 0 V vs. RHE. The cyan dashed lines represent the free energies of the ideal target material. (b) UOP contour map for CO production in terms of the free energies of COOH* and CO*. (c) The ratio between UOP[HER] and UOP[CO2 → CO] and (d) the ratio between UOP[CO2 → HCOOH] and UOP[CO2 → CO] as a function of the overpotential for CO production on the proposed NSAs. The NSAs in the up-right corner (the shadow area) are predicted as superior CO catalysts. In search of the best NSAs for CO production, we place all stable NSAs (59 of them) on the contour map. Firstly, we notice the approximate scaling relations between G[COOH] and G[CO] for all NSAs; there are five straight lines, one for each group of NSAs that share the same host (Ag, Au, Cu, Pd, or Pt), as shown in Figure 6b. Similar scaling relation has been observed in 15 ACS Paragon Plus Environment

ACS Catalysis 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 39

Ag-based single-atom catalysts previously.52 Secondly, we find that 21 NSAs are located in the region of ∆G3 < 0, most of them based on Ag and Au (Table S6); they are candidates for CO formation. Thirdly, all the straight lines are approximately parallel to ∆G1 = ∆G2 line, implying that none of the NSAs examined here would hit the target. On the other hand, we can rank the NSAs according to their slopes. For example, Ag- and Au-based NSAs have larger slopes (1.54 and 0.99) than Pt- (0.72), Pd- (0.76), and Cu-based (0.88) NSAs, thus they would approach the overpotential “trough” faster. Among the 21 candidates, Ag/Hf, Ag/Zr, Zn*/Ag, Au/Hf, and Au/Zr NSAs are identified as the most active catalysts for CO production. There are two competing reactions to CO production, HER and formic acid production. We next examine the side or competing reactions on the 21 candidates. In the following, we plot the ratio between the overpotential for HER and the overpotential for CO production (in Figure 6c), and the ratio between the overpotential for HCOOH production and the overpotential for CO production (in Figure 6d), as a function of the overpotential for CO production. The NSAs at the top-right corners are predicted to possess relatively higher overpotentials for the side reactions (HER and HCOOH production), and thus are good candidates as CO catalysts. Combining the results from Figure 6c and 6d, we conclude that Ag/Hf and Ag/Zr are the most promising CO catalysts among the 21 NSAs. The free energy diagrams for CO production and HER on Ag/Hf, Ag/Zr, and Ag (111) are shown in Figure 7. The UOP for CO formation on Ag/Hf and Ag/Zr is calculated as –0.50 and –0.47 V vs. RHE, respectively, which is 61% less negative than that on pure Ag (–1.25 V). Note that the calculated UOP on Ag matches very well to previous theoretical and experimental values.14, 53 The predicted UOP values for CO production are comparable to those on the most active transition metal catalysts, such as molecular functionalized Au nanoparticles (NPs),54 Pd


Page 17 of 39 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

ACS Catalysis

NPs,55 and SnO2/Cu core/shell NPs.56 As to HER, UOP on Ag/Hf and Ag/Zr is calculated as – 0.33 and –0.32 V, respectively, which is ~33% less negative than that for CO production, thus HER is present in CO2RR to CO. However, Zhang et al.18 have discovered that a high coverage of CO2RR intermediates could weaken H binding on transition metal surfaces. Thus, for metals on the right-leg of the HER volcano, such as Ag/Hf and Ag/Zr, the presence of the intermediates could actually suppress HER. The observation that the metals on the right-leg of the HER volcano can suppress HER may serve as a design principle for CO2RR.

Figure 7. Free energy diagram for (a) CO production and (b) HER on Ag, Ag/Hf, and Ag/Zr at 0 V vs. RHE. The adsorption geometries are shown in the insets. It is evident from 7a that the higher activity of Ag/Hf and Ag/Zr relative to Ag stems from a sharp decrease in the intermediate free energies, corresponding to a stronger adsorption on the surfaces. This enhanced adsorption can be understood from the ligand effect of the NSAs. For example, we find that there is significant electron transfer (0.58 e per Hf atom) from the solute Hf to Ag host in Ag/Hf. The transferred electrons are found to fill the bonding states of localized d orbitals (dz2 component) below the Fermi level of the surface atoms (Figure S2),


ACS Catalysis 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 39

which enhances the adsorption of the intermediate COOH* and lowers its free energy. The lowered free energy of the intermediate (COOH*) in turns reduces the overpotential as shown in Figure 7a. In fact, we find that the electron transfer between a solute and host is also responsible for enhanced catalytic activities on other NSAs, such as Au/Hf and Ag/Ta discussed below. This observation is not surprising since charge transfer is the only mechanism at play in a NSA to modify its chemistry. In fact, we have observed a correlation between the differential electronegativity and the overpotential for CO production on 21 NSAs (Figure S3). Thus, a design principle for superior NSA catalysts is to choose metal pairs with a greater differential electronegativity. 3.4. Production of CH3OH and C2H4. In this section, we examine possible reduction of CO2 to more reduced products, including CH4, CH3OH, and C2H4, which involves multiple reaction steps and intermediates.13 It has been found experimentally and computationally that CO is a key intermediate whose adsorption strength determines the reaction activities of these more reduced products.5-6,

37

If CO binds too strongly, it may poison the surface; conversely, CO

would desorb from the surface as the primary product. Among metal catalysts, Cu is known to possess an intermediate adsorption strength of CO and as a result, capable of reducing CO2 to CH4 and C2H4.4 Thus, we can use the free energy of CO as a descriptor to identify promising candidates for CO2RR. In particular, the NSAs on which CO* adsorption is favored, i.e., ∆G3 > 0 should be the focus of the screening, which is opposite to the case of CO production. Based on Figure 6b, we propose Ag/Ta, Ag/Nb, and Cu/Ti as three promising catalysts to form more reduced products for two reasons: (1) they have intermediate CO binding energies, similar to that of Cu (the vertical dashed line in Figure 6b); (2) they have relatively low G[COOH], hence the first hydrogenation step of CO2RR is expected to be efficient.


Page 19 of 39 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

ACS Catalysis

Figure 8. Reaction pathways examined for CO2RR to CH4 and CH3OH. Two major pathways are shown by red and blue arrows and the branched pathways are indicated by grey arrows. H+ + e– reactants are omitted. To examine the subsequent reduction steps, we focus on two major pathways of CH4 production proposed by Peterson et al.13 and Nie et al.57 on low-index Cu surfaces. As shown in Figure 8, the first pathway is denoted by red arrow and the second one by blue arrow. These two pathways primarily differ in the reduction intermediates of CO*, i.e., CHO* or COH*. Branched pathways to CHOH*, CH2OH*, and CH3OH along these two pathways are also examined, indicated by grey arrows. Figure 9a shows the free energy diagrams of the lowest free energy pathways on Ag/Ta and Cu (211) surface. In addition, we have determined the free energy diagrams of CO2RR to CH4 and CH3OH on the three NSAs, including all relevant branches and the results are summarized in Figure S4. We find that instead of forming CH4, Ag/Ta and Ag/Nb can produce CH3OH, following the pathway (CO2 → COOH* → CO* → CHO* → CH2O* → OCH3* → CH3OH). Note that this pathway is similar to that of CH4 formation on Cu,13 but deviates from it at the intermediate of OCH3* (Figure 9a); the free energy gain for forming CH3OH against O* + CH4 is 0.37 and 0.31 eV, respectively, on Ag/Ta and Ag/Nb. In contrast, 19 ACS Paragon Plus Environment

ACS Catalysis 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 39

the free energy gain on Cu (211) for the formation of O* + CH4 as opposed to CH3OH is 0.43 eV. Since the energy gain on Ag/Ta and Ag/Nb is similar to that on Cu, which is known to produce CH4 as the major product,4 we predict that CH3OH could also be the major product on Ag/Ta and Ag/Nb. We find that the protonation of CO* to CHO* is the overpotential determining step on Ag/Ta and Ag/Nb, and UOP is calculated as –0.67 and –0.68 V vs. RHE, respectively, ~15% less negative than that on Cu (211) (–0.79 V). In addition, we calculate UOP for HER on Ag/Ta and Ag/Nb as –0.25 V (Figure 9b and Figure S5), which is six times more negative than that on Cu (211) (–0.04 V). These results suggest that Ag/Ta and Ag/Nb are not only active for CH3OH production, but also less prone to HER as compared to Cu. In contrast, UOP of HER on Cu/Ti is estimated as –0.01 V (Figure S5). Our prediction of NSA catalysts to produce liquid fuel (CH3OH) represents an important contribution to an active research direction, which consists of theoretical discoveries of transition metal-based catalysts,58 atomic Ni and Pt supported on defective graphene,59 Ni-Cu dimer supported on defective graphene,60 and grain boundary sites on Au (110)61 for CO2RR to CH3OH. The reduction of CO2 to C2H4 is more complicated because it involves C-C coupling reactions. Such coupling reactions can be accomplished through different intermediates, such as CO*, CHO*, CH2*, etc. Previous studies suggested that increasing the degree of hydrogenation of the C1 intermediates could facilitate the coupling kinetics,62 and a pathway via CHO*-CHO* coupling is favored over those involving other intermediates.63 Therefore, in this work, we focus on CHO*-CHO* coupling and the subsequent hydrogenation steps as a pathway to C2H4 (Figure S6). Figure 9c and Figure S7 show the lowest free energy reaction pathway for C2H4 production on Ag/Ta and Ag/Nb. We determine the energy barrier of CHO* dimerization for the formation of OCHCHO* on Ag/Ta and Ag/Nb as 0.26 eV and 0.30 eV, respectively (Figure 9c). We have


Page 21 of 39 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

ACS Catalysis

also considered the coupling of CO*-CO* and CO*-CHO* on Ag/Ta and Ag/Nb,62, 64 but found very high energy barriers for the couplings (1.29 and 1.28 eV for CO*-CO* coupling and 0.43 and 0.85 eV for CO-CHO* coupling). Thus, they are excluded in the plausible reaction pathway. The transition-state configuration for the proposed CHO*-CHO* coupling with a C-C distance of 2.31 Å is shown in the inset of Figure 9c. Along this pathway, all endothermic steps after the formation of OCHCHO* require a lower bias than the protonation of CO* to CHO*, thus UOP for C2H4 production on these two NSAs is identical to that of CH3OH production. Note that the same dimerization reaction is responsible for C2H4 production on Cu (100), whose energy barrier is 0.22 eV.63 The dimerization barriers on Ag/Ta and Ag/Nb are similar to that on Cu where C2H4 is a major product,4, 6 thus we expect C2H4 could also be a major product on Ag/Ta and Ag/Nb. Finally, we compare the reaction pathways for C2H4 formation on Ag/Ta and Cu (100) in Figure 9c. We find that the two materials share much of the same pathway until the last two intermediates.


ACS Catalysis 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 39

Figure 9. Free energy diagram for (a) CO2RR to CH4 and CH3OH on Cu (211) and Ag/Ta; (b) HER on Cu (211) and Ag/Ta; and (c) The formation C2H4 on Cu (100) and Ag/Ta at 0 V vs. RHE. The reaction pathway after CHO* on Cu (100) is taken from ref. 63. The optimized structure of the transition-state (TS) in CHO*-CHO* coupling on Ag/Ta is shown in inset. The adsorption geometry of the reaction intermediates can be found in Figure S8. The present work is focused on theoretical predictions of transition metal-based NSAs as promising catalysts for CO2RR. This type of NSAs could be synthesized by using electro- or vapor-deposition techniques. For example, ideal monolayer bimetallic NSAs, such as Pt/Cu,23 Pd/V,24 Rh/V,24 and Ni/Pt25 have been synthesized successfully in experiments. However, some of the proposed NSAs may be challenging to fabricate, owing to possible oxidization of the alloys. Given the impressive progress in advanced nanofabrication capabilities, we remain optimistic that precise controls of surface structures could be accomplished to synthesize the challenging NSAs. Future first-principles studies focusing on Pourbaix diagrams are needed to assess the stability of NSAs as a function of pH and applied potentials. 4. CONCLUSIONS In summary, we have systematically studied electrochemical reduction of CO2 into useful products including HCOOH, CO, CH4, CH3OH, and C2H4 on 190 monolayer NSAs based on DFT calculations. We have identified 26 NSAs that may favor the formation of HCOO* over HER in the first hydrogenation step of CO2. Among them, 20 NSAs are predicted to be selective for HCOOH production. In particular, Pd/W, Au/Hf, and Au/Zr are the most active catalysts with UOP calculated as –0.23, –0.16, and –0.17 V vs. RHE, respectively, outperforming most known electrodes. We predict Ag- and Au-based NSAs as promising CO catalysts with Ag/Hf and Ag/Zr being the most active whose overpotentials are ~0.77 V less than that on pure Ag. Ag/Ta 22 ACS Paragon Plus Environment

Page 23 of 39 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

ACS Catalysis

and Ag/Nb are revealed as superior catalysts for CO2RR to CH3OH and C2H4 with UOP calculated as –0.67 and –0.68 V vs. RHE, respectively, which are ~15% less negative than that of Cu (211) for CH4 and C2H4 production (–0.79 V). Furthermore, UOP for HER on Ag/Ta and Ag/Nb is six times more negative than that on Cu (211) surface. Our work provides crucial insights to the design of superior transition metal catalysts for CO2RR and hopefully will inspire experimental effort in this vibrant research area. ASSOCIATED CONTENT Supporting Information. Surface segregation and migration energies of the considered NSAs; free energies and free energy corrections of the key reaction intermediates; reaction UOP for different products; radial distribution functions of the proposed NSAs; electronic density states of the proposed NSAs; reaction UOP as a function of differential electronegativity; free energy diagrams for CH3OH, C2H4 production, and HER on the proposed NSAs; reaction pathways examined for C2H4 production; adsorption geometries for the reaction intermediates (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


ACS Catalysis 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 39

This work was supported by the Office of Naval Research (N00014-15-1-2092) and National Science Foundation (DMR-1205734). REFERENCES (1)

Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy

Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729-15735. (2)

Olah, G. A.; Prakash, G. K. S.; Goeppert, A. Anthropogenic Chemical Carbon Cycle for

a Sustainable Future. J. Am. Chem. Soc. 2011, 133, 12881-12898. (3)

Jhong, H.-R. M.; Ma, S.; Kenis, P. J. A. Electrochemical Conversion of CO2 to Useful

Chemicals: Current Status, Remaining Challenges, and Future Opportunities. Curr. Opin. Chem. Eng. 2013, 2, 191-199. (4)

Hori, Y.; Kikuchi, K.; Suzuki, S. Production of CO and CH4 in Electrochemical

Reduction of CO2 at Metal Electrodes in Aqueous Hydrogencarbonate Solution. Chem. Lett. 1985, 14, 1695-1698. (5)

Gattrell, M.; Gupta, N.; Co, A. A Review of the Aqueous Electrochemical Reduction of

CO2 to Hydrocarbons at Copper. J. Electroanal. Chem. 2006, 594, 1-19. (6)

Kuhl, K. P.; Hatsukade, T.; Cave, E. R.; Abram, D. N.; Kibsgaard, J.; Jaramillo, T. F.

Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces. J. Am. Chem. Soc. 2014, 136, 14107-14113. (7)

Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. Synergistic Geometric and

Electronic Effects for Electrochemical Reduction of Carbon Dioxide Using Gold–Copper Bimetallic Nanoparticles. Nat. Commun. 2014, 5, 4948. (8)

Kortlever, R.; Peters, I.; Koper, S.; Koper, M. T. M. Electrochemical CO2 Reduction to

Formic Acid at Low Overpotential and with High Faradaic Efficiency on Carbon-Supported Bimetallic Pd–Pt Nanoparticles. ACS Catal. 2015, 5, 3916-3923. (9)

Fiordaliso, E. M.; Sharafutdinov, I.; Carvalho, H. W. P.; Grunwaldt, J.-D.; Hansen, T.

W.; Chorkendorff, I.; Wagner, J. B.; Damsgaard, C. D. Intermetallic GaPd2 Nanoparticles on SiO2 for Low-Pressure CO2 Hydrogenation to Methanol: Catalytic Performance and In Situ Characterization. ACS Catal. 2015, 5, 5827-5836.


Page 25 of 39 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

ACS Catalysis

(10)

Studt, F.; Sharafutdinov, I.; Abild-Pedersen, F.; Elkjær, C. F.; Hummelshøj, J. S.; Dahl,

S.; Chorkendorff, I.; Nørskov, J. K. Discovery of a Ni-Ga Catalyst for Carbon Dioxide Reduction to Methanol. Nat. Chem. 2014, 6, 320-324. (11)

Rasul, S.; Anjum, D. H.; Jedidi, A.; Minenkov, Y.; Cavallo, L.; Takanabe, 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. (12)

Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New Insights into the

Electrochemical Reduction of Carbon Dioxide on Metallic Copper Surfaces. Energy Environ. Sci. 2012, 5, 7050-7059. (13)

Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Norskov, J. K. How Copper

Catalyzes the Electroreduction of Carbon Dioxide into Hydrocarbon Fuels. Energy Environ. Sci. 2010, 3, 1311-1315. (14)

Wu, J.; Yadav, R. M.; Liu, M.; Sharma, P. P.; Tiwary, C. S.; Ma, L.; Zou, X.; Zhou, X.-

D.; Yakobson, B. I.; Lou, J.; Ajayan, P. M. Achieving Highly Efficient, Selective, and Stable CO2 Reduction on Nitrogen-Doped Carbon Nanotubes. ACS Nano 2015, 9, 5364-5371. (15)

Zhu, W.; Michalsky, R.; Metin, Ö.; Lv, H.; Guo, S.; Wright, C. J.; Sun, X.; Peterson, A.

A.; Sun, S. Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO2 to CO. J. Am. Chem. Soc. 2013, 135, 16833-16836. (16)

Mistry, H.; Reske, R.; Zeng, Z.; Zhao, Z.-J.; Greeley, J.; Strasser, P.; Cuenya, B. R.

Exceptional Size-Dependent Activity Enhancement in the Electroreduction of CO2 over Au Nanoparticles. J. Am. Chem. Soc. 2014, 136, 16473-16476. (17)

Elgrishi, N.; Chambers, M. B.; Fontecave, M. Turning It Off! Disfavouring Hydrogen

Evolution to Enhance Selectivity for CO Production During Homogeneous CO2 Reduction by Cobalt-Terpyridine Complexes. Chem. Sci. 2015, 6, 2522-2531. (18)

Zhang, Y.-J.; Sethuraman, V.; Michalsky, R.; Peterson, A. A. Competition between CO2

Reduction and H2 Evolution on Transition-Metal Electrocatalysts. ACS Catal. 2014, 4, 37423748. (19)

Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A. M.;

Nørskov, J. K.; Stensgaard, I. Design of a Surface Alloy Catalyst for Steam Reforming. Science 1998, 279, 1913-1915.


ACS Catalysis 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

(20)

Page 26 of 39

Rodriguez, J. A.; Goodman, D. W. The Nature of the Metal-Metal Bond in Bimetallic

Surfaces. Science 1992, 257, 897-903. (21)

Pleth Nielsen, L.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E.; Engdahl, C.; Stoltze,

P.; Jacobsen, K. W.; Nørskov, J. K. Initial Growth of Au on Ni(110): Surface Alloying of Immiscible Metals. Phys. Rev. Lett. 1993, 71, 754-757. (22)

Greeley, J.; Mavrikakis, M. Alloy Catalysts Designed from First Principles. Nat. Mater.

2004, 3, 810-815. (23)

Bandarenka, A. S.; Varela, A. S.; Karamad, M.; Calle-Vallejo, F.; Bech, L.; Perez-

Alonso, F. J.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I. Design of an Active Site towards Optimal Electrocatalysis: Overlayers, Surface Alloys and Near-Surface Alloys of Cu/Pt(111). Angew. Chem., Int. Ed. 2012, 51, 11845-11848. (24)

Klötzer, B.; Unterberger, W.; Hayek, K. Adsorption and Hydrogenation of CO on Pd(1 1

1) and Rh(1 1 1) Modified by Subsurface Vanadium. Surf. Sci. 2003, 532–535, 142-147. (25)

Hwu, H. H.; Eng, J.; Chen, J. G. Ni/Pt(111) Bimetallic Surfaces: Unique Chemistry at

Monolayer Ni Coverage. J. Am. Chem. Soc. 2002, 124, 702-709. (26)

Greeley, J.; Mavrikakis, M. Near-Surface Alloys for Hydrogen Fuel Cell Applications.

Catal. Today 2006, 111, 52-58. (27)

Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy

Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (28)

Hammer, B.; Hansen, L. B.; Nørskov, J. K. Improved Adsorption Energetics within

Density-Functional Theory using Revised Perdew-Burke-Ernzerhof Functionals. Phys. Rev. B 1999, 59, 7413-7421. (29)

Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made

Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (30)

Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B

1976, 13, 5188-5192. (31)

Nosé, S. A Unified Formulation of the Constant Temperature Molecular Dynamics

Methods. J. Chem. Phys. 1984, 81, 511-519. (32)

Shuichi, N. Constant Temperature Molecular Dynamics Methods. Prog. Theor. Phys.

1991, 103, 1-46.


Page 27 of 39 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

ACS Catalysis

(33)

Bylander, D. M.; Kleinman, L. Energy Fluctuations Induced by the Nose Thermostat.

Phys. Rev. B 1992, 46, 13756-13761. (34)

Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band

Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 99789985. (35)

Cramer, C. J. In Essentials of Computational Chemistry: Theories and Models. 2nd ed.;

Wiley & Sons: Chichester, U. K., 2004. (36)

Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.;

Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892. (37)

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. (38)

Shi, C.; Hansen, H. A.; Lausche, A. C.; Norskov, J. K. Trends in Electrochemical CO2

Reduction Activity for Open and Close-Packed Metal Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 4720-4727. (39)

Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.;

Fan, F.; Cao, C.; de Arquer, F. P. G.; Safaei, T. S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.; Kelley, S. O.; Sargent, E. H. Enhanced Electrocatalytic CO2 Reduction via Field-Induced Reagent Concentration. Nature 2016, 537, 382-386. (40)

Karamad, M.; Tripkovic, V.; Rossmeisl, J. Intermetallic Alloys as CO Electroreduction

Catalysts—Role of Isolated Active Sites. ACS Catal. 2014, 4, 2268-2273. (41)

Cheng, M.-J.; Clark, E. L.; Pham, H. H.; Bell, A. T.; Head-Gordon, M. Quantum

Mechanical Screening of Single-Atom Bimetallic Alloys for the Selective Reduction of CO2 to C1 Hydrocarbons. ACS Catal. 2016, 6, 7769-7777. (42)

Yu, X.; Pickup, P. G. Recent Advances in Direct Formic Acid Fuel Cells (DFAFC). J.

Power Sources 2008, 182, 124-132. (43)

Mura, M. G.; Luca, L. D.; Giacomelli, G.; Porcheddu, A. Formic Acid: A Promising Bio-

Renewable Feedstock for Fine Chemicals. Adv. Synth. Catal. 2012, 354, 3180-3186. (44)

Hietala, J.; Vuori, A.; Johnsson, P.; Pollari, I.; Reutemann, W.; Kieczka, H. In Formic

Acid. Ullmann's Encyclopedia of Industrial Chemistry, Wiley: New York, 2016; pp 1-22.


ACS Catalysis 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

(45)

Page 28 of 39

Grasemann, M.; Laurenczy, G. Formic Acid as a Hydrogen Source - Recent

Developments and Future Trends. Energy Environ. Sci. 2012, 5, 8171-8181. (46)

Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; Li, D.; Yang, J.; Xie, Y. Partially

Oxidized Atomic Cobalt Layers for Carbon Dioxide Electroreduction to Liquid Fuel. Nature 2016, 529, 68-71. (47)

Taheri, A.; Thompson, E. J.; Fettinger, J. C.; Berben, L. A. An Iron Electrocatalyst for

Selective Reduction of CO2 to Formate in Water: Including Thermochemical Insights. ACS Catal. 2015, 5, 7140-7151. (48)

Min, X.; Kanan, M. W. Pd-Catalyzed Electrohydrogenation of Carbon Dioxide to

Formate: High Mass Activity at Low Overpotential and Identification of the Deactivation Pathway. J. Am. Chem. Soc. 2015, 137, 4701-4708. (49)

Höök, M.; Fantazzini, D.; Angelantoni, A.; Snowden, S. Hydrocarbon Liquefaction:

Viability as a Peak Oil Mitigation Strategy. Phil. Trans. R. Soc. A 2014, 372, 20120319. (50)

Akhade, S. A.; Luo, W.; Nie, X.; Bernstein, N. J.; Asthagiri, A.; Janik, M. J. Poisoning

Effect of Adsorbed CO During CO2 Electroreduction on Late Transition Metals. Phys. Chem. Chem. Phys. 2014, 16, 20429-20435. (51)

Klinkova, A.; Luna, P. D.; Dinh, C.-T.; Voznyy, O.; Larin, E. M.; Kumacheva, E.;

Sargent, E. H. Rational Design of Efficient Palladium Catalysts for Electroreduction of Carbon Dioxide to Formate. ACS Catal. 2016, 6, 8115-8120. (52)

Lim, H.-K.; Shin, H.; Goddard, W. A.; Hwang, Y. J.; Min, B. K.; Kim, H. Embedding

Covalency into Metal Catalysts for Efficient Electrochemical Conversion of CO2. J. Am. Chem. Soc. 2014, 136, 11355-11361. (53)

Back, S.; Yeom, M. S.; Jung, Y. Active Sites of Au and Ag Nanoparticle Catalysts for

CO2 Electroreduction to CO. ACS Catal. 2015, 5, 5089-5096. (54)

Cao, Z.; Kim, D.; Hong, D.; Yu, Y.; Xu, J.; Lin, S.; Wen, X.; Nichols, E. M.; Jeong, K.;

Reimer, J. A.; Yang, P.; Chang, C. J. A Molecular Surface Functionalization Approach to Tuning Nanoparticle Electrocatalysts for Carbon Dioxide Reduction. J. Am. Chem. Soc. 2016, 138, 8120-8125. (55)

Gao, D.; Zhou, H.; Wang, J.; Miao, S.; Yang, F.; Wang, G.; Wang, J.; Bao, X. Size-

Dependent Electrocatalytic Reduction of CO2 over Pd Nanoparticles. J. Am. Chem. Soc. 2015, 137, 4288-4291. 28 ACS Paragon Plus Environment

Page 29 of 39 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

ACS Catalysis

(56)

Li, Q.; Fu, J.; Zhu, W.; Chen, Z.; Shen, B.; Wu, L.; Xi, Z.; Wang, T.; Lu, G.; Zhu, J.-J.;

Sun, S. Tuning Sn-Catalysis for Electrochemical Reduction of CO2 to CO via the Core/Shell Cu/SnO2 Structure. J. Am. Chem. Soc. 2017, 139, 4290-4293. (57)

Nie, X.; Esopi, M. R.; Janik, M. J.; Asthagiri, A. Selectivity of CO2 Reduction on Copper

Electrodes: The Role of the Kinetics of Elementary Steps. Angew. Chem., Int. Ed. 2013, 52, 2459-2462. (58)

Back, S.; Kim, H.; Jung, Y. Selective Heterogeneous CO2 Electroreduction to Methanol.

ACS Catal. 2015, 5, 965-971. (59)

Back, S.; Lim, J.; Kim, N.-Y.; Kim, Y.-H.; Jung, Y. Single-Atom Catalysts for CO2

Electroreduction with Significant Activity and Selectivity Improvements. Chem. Sci. 2017, 8, 1090-1096. (60)

Li, Y.; Su, H.; Chan, S. H.; Sun, Q. CO2 Electroreduction Performance of Transition

Metal Dimers Supported on Graphene: A Theoretical Study. ACS Catal. 2015, 5, 6658-6664. (61)

Dong, C.; Fu, J.; Liu, H.; Ling, T.; Yang, J.; Qiao, S. Z.; Du, X.-W. Tuning the

Selectivity and Activity of Au Catalysts for Carbon Dioxide Electroreduction via Grain Boundary Engineering: A DFT Study. J. Mater. Chem. A 2017, 5, 7184-7190. (62)

Montoya, J. H.; Peterson, A. A.; Nørskov, J. K. Insights into C-C Coupling in CO2

Electroreduction on Copper Electrodes. ChemCatChem 2013, 5, 737-742. (63)

Luo, W.; Nie, X.; Janik, M. J.; Asthagiri, A. Facet Dependence of CO2 Reduction Paths

on Cu Electrodes. ACS Catal. 2016, 6, 219-229. (64)

Huang, Y.; Handoko, A. D.; Hirunsit, P.; Yeo, B. S. Electrochemical Reduction of CO2

Using Copper Single-Crystal Surfaces: Effects of CO* Coverage on the Selective Formation of Ethylene. ACS Catal. 2017, 7, 1749-1756.


ACS Catalysis 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 30 of 39

TOC graphic


Page 31 of 39 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

ACS Catalysis

Figure 1. Color-coded table of the surface segregation energies (Eseg) for 190 NSAs. A near-surface alloy consists of a host element, “B” in rows and a solute element, “A” in columns, forming either the overlayer (A*/B) or the subsurface (B/A) configuration. 70x32mm (300 x 300 DPI)

ACS Paragon Plus Environment

ACS Catalysis 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 2. Reaction pathways for CO2RR in the first hydrogenation step (top) and HER (bottom). CxHyOz represents hydrocarbons reduced from CO2. H+ + e– reactants are omitted. 62x64mm (300 x 300 DPI)


Page 32 of 39

Page 33 of 39 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

ACS Catalysis

Figure 3. Changes in free energy for the formation of (a) COOH* and (b) HCOO* intermediates against changes in free energy for the formation of adsorbed H* on proposed pure transition metals (TMs) and NSAs. Above the dashed (iso-energy) line, HER is energetically favorable and below it, COOH* or HCOO* is more favorable. 71x34mm (300 x 300 DPI)


ACS Catalysis 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 4. (a) Changes in free energy for the formation of formic acid (HCOOH) against changes in free energy for the formation of adsorbed H* on the proposed 26 NSAs. (b) The overpotential volcano plot for formic acid production as a function of G[HCOO]. 73x35mm (300 x 300 DPI)


Page 34 of 39

Page 35 of 39 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

ACS Catalysis

Figure 5. Free energy diagram for (a) formic acid production and (b) HER on Au, Au/Hf, Au/Zr, Pd, and Pd/W at 0 V vs. RHE. The adsorption geometries are shown. 66x29mm (300 x 300 DPI)


ACS Catalysis 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 6. (a) Free energy diagram for CO production on pure metal (111) surfaces at 0 V vs. RHE. The cyan dashed lines represent the free energies of the ideal target material. (b) UOP contour map for CO production in terms of the free energies of COOH* and CO*. (c) The ratio between UOP[HER] and UOP[CO2 → CO] and (d) the ratio between UOP[CO2 → HCOOH] and UOP[CO2 → CO] as a function of the overpotential for CO production on the proposed NSAs. The NSAs in the up-right corner (the shadow area) are predicted as superior CO catalysts. 110x81mm (300 x 300 DPI)


Page 36 of 39

Page 37 of 39 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

ACS Catalysis

Figure 7. Free energy diagram for (a) CO production and (b) HER on Ag, Ag/Hf, and Ag/Zr at 0 V vs. RHE. The adsorption geometries are shown in the insets. 66x29mm (300 x 300 DPI)


ACS Catalysis 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. Reaction pathways examined for CO2RR to CH4 and CH3OH. Two major pathways are shown by red and blue arrows and the branched pathways are indicated by grey arrows. H+ + e– reactants are omitted. 41x20mm (300 x 300 DPI)


Page 38 of 39

Page 39 of 39 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

ACS Catalysis

Figure 9. Free energy diagram for (a) CO2RR to CH4 and CH3OH on Cu (211) and Ag/Ta; (b) HER on Cu (211) and Ag/Ta; and (c) The formation C2H4 on Cu (100) and Ag/Ta at 0 V vs. RHE. The reaction pathway after CHO* on Cu (100) is taken from ref. 63. The optimized structure of the transition-state (TS) in CHO*CHO* coupling on Ag/Ta is shown in inset. The adsorption geometry of the reaction intermediates can be found in Figure S7. 100x67mm (300 x 300 DPI)


Computational Screening of Near-Surface Alloys for CO2

Recommend Documents