Barriers of Electrochemical CO2 Reduction on Transition Metals

Subscriber access provided by the University of Exeter

Full Paper 2

Barriers of Electrochemical CO Reduction on Transition Metals Chuan Shi, Karen Chan, Jong Suk Yoo, and Jens K. Norskov Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00103 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016

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.

Organic Process Research & Development 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 20

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

Organic Process Research & Development

Barriers of Electrochemical CO2 Reduction on Transition Metals Chuan Shi,† Karen Chan,†,‡ Jong Suk Yoo,† and Jens K. Nørskov∗,†,‡ †Department of Chemical Engineering, Stanford University, Stanford, CA, USA ‡SLAC National Accelerator Laboratory, Menlo Park, CA, USA E-mail: [email protected]

1

ACS Paragon Plus Environment


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

Abstract Theoretical investigations of electrochemical CO2 reduction have received increasing interest due its potential impact on renewable energy storage. We use density functional theory with an explicit solvent model of the electrochemical interface to calculate activation energy barriers for various proton-electron transfer elementary reactions steps for CO2 reduction on Au, Cu, and Pt surfaces. We find the protonation of unhydrogenated oxygen to be trivial compared to the protonation of carbon and R-OH species, which induces C-O scission. Our revised free energy diagram for the reduction of CO2 to methane on Cu(211) includes these observations and suggests that the dominant pathway includes *CHOH and *CH as intermediates rather than *OCH3 .

Graphical TOC Entry

CO2 reduction barriers on transition metal surfaces

2


Page 2 of 20

Page 3 of 20

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


Electroreduction of CO2 has received considerable interest recently, as it could be a means to store energy from intermittent renewable sources and at the same time mitigate the rising levels of anthropomorphic CO2 emissions. Using electricity from renewable sources, one could use water and CO2 to generate hydrocarbons and alcohols for use as fuels or commodity chemicals. The products of CO2 reduction could be used as fuel or feedstock for the existing infrastructure that uses fuels from fossil sources. Substantial improvements in activity and selectivity of electrocatalysts are required for such a process to become viable. 1–3 Polycrystalline copper is the most widely-studied heterogeneous catalyst for CO2 reduction, 4–9 since it is the only pure metal catalyst is selective towards hydrocarbons, although at a prohibitively high overpotential of around 1 V. 4,10 Recent work on nanostructured Cu has reported onset potentials for C-C coupled products from CO reduction at overpotentials of only 0.3V, but the specific current densities remain low. 11 Transition metals such as platinum and nickel are poisoned by CO and are primarily selective for hydrogen evolution rather than CO2 reduction in typical bicarbonate electrolytes and have received less attention. 12,13 The other coinage metals such as gold and silver produce primarily CO from CO2 reduction, while the p-block metals tend to produce primarily formate. 13,14 This makes copper unique as the only transition metal catalyst that produces a significant amount of methane and C-C coupled products. 13,14 Previous theoretical analyses have usually applied a thermochemical approach, which allows for the determination of the reaction free energies for the elementary steps without explicit consideration of solvated protons or the potential at the interface. 15–22 Calculated limiting potentials, defined as the potential at which a the reaction free energy becomes exergonic, have been shown to correlate well with experimental onset potentials. 13,23,24 Scaling relations between intermediates of many of the late transition metals have also established catalyst screening criteria. 25 Surmountable reaction free energies, 23–25 however, are a necessary but not sufficient condition for facile kinetics, nor can they predict selectivities towards the myriad of possible

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

(a)

(b)

Page 4 of 20

(c)

Figure 1: Barriers for proton-electron transfers (a) to *CHOH to form *CH and H2 O on Cu(211) (b) to *CO to form *CHO on Cu(111) (c) to *OCH3 to form CH3 OH on Au(111). All barriers were calculated at constant charge and the corresponding work function is shown in red. Lines were drawn to guide the eye. reduced products. Recent works have have taken initial steps beyond the thermochemical approach. Nie et al. considered proton transfer barriers on Cu(111) using surface hydrogenation barriers as an approximation to proton transfer ones. 26,27 Xiao et al. used an implicit solvation method to maintain a constant potential during proton-electron transfers to calculate barriers on Cu(111). 28 Cheng et al. calculated both proton and surface hydrogenation barriers on Cu(100) surface using quantum molecular dynamics using a simple implicit solvation model to describe the electrochemical interface. 29 Hussain et al. use a unit cell extrapolation scheme and explicit solvation to calculate barriers on a variety of FCC (111) transition metals. 30 These works show differences in pathways on stepped and flat Cu from those that were predicted from a solely thermochemical analysis. 23,24 In this work, we have calculated relevant proton-transfer reaction barriers in the pathway from CO2 to CH4 and CH3 OH on 4 representative transition metal surfaces: Pt(111), Cu(111), Cu(211), and Au(111). CO binds strongly on Pt, moderately on Cu, and weakly on Au. 25 We have used an explicit solvent approach to the metal|solution interface and applied 4


Page 5 of 20

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


an extrapolation scheme to obtain estimates of the barrier at constant potential. 31,32 At reducing potentials, we expect the surface of many transition metal catalysts to favor adsorbed hydrogen over species such as *O or *OH, opening the way for surface hydrogenations as an alternate pathway to direct protonation. 25,33 We also calculate and compile barriers for the surface hydrogenation versions of many relevant barriers for CO2 reduction and discuss how they compare to their electrochemical counterparts. We propose a generic reaction network on transition metals based on the trends in barriers considered, and propose a revised reaction pathway on Cu(211) All atomistic calculations were performed using the DACAPO code using with the revised Perdew-Burke-Ernzerhof (RPBE) functional, a plane-wave basis set, and ultrasoft pseudopotentials. 34–38 All calculations used periodic boundary conditions, a planewave cutoff of 340 eV, and a density cutoff of 500 eV. Computational cell sizes used in this work were 3 × 2, 3 × 3, and 3 × 4, with 3–4 layers of metal atoms, of which two layers were fixed. These cells used, respectively, 4 × 6 × 1, 4 × 4 × 1, and 4 × 3 × 1 Monkhorst-Pack 39 grids of k-points to sample the first Brillouin zone. All binding energies in this work were calculated using optimized geometries relaxed to 0.03 eV ˚ A−1 using the BFGS linesearch method, implemented within the Atomic Simulation Environment (ASE). 40 The climbing-image nudged elastic band (NEB) 41,42 method was used to find transition states, with forces minimized to 0.05 eV ˚ A−1 . Vibrational frequencies were calculated within the harmonic approximation 43 using the finite difference method with displacements of ±0.01˚ A. Surface hydrogenation barriers were calculated in vacuum. Proton–electron transfer barriers were calculated using an explicit solvent model of the electrochemical interface, which includes the metal slab, adsorbates, and a water layer with a solvated proton. 31 The electronic structure calculation spontaneously separates an electron from the extra hydrogen in the water layer to form a double layer capacitor. The water layer is positively charged and the surface layer negatively charged. By adjusting the number of extra hydrogen atoms in

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 20

the water layer, it is possible to adjust the charge and potential of the surface. 31 Water structures were determined on Cu(211) using a minima hopping algorithm, which seeks global minima for atomic geometries. The algorithm uses a combination of constrained molecular dynamics and geometry optimization steps. 44 We selected the water structure by considering various water densities and computational cell sizes, and selected the water structure with the lowest binding energy per water molecule. (111) water structures were assumed to follow a hexagonal ice-like structure as determined in UHV experiments on Pt(111). 45 Fig. 1 shows three examples of the barriers calculated in this work. Fig. 1a shows the potential energy diagram for the proton-electron transfer to *CHOH to form *CH and H2 O on Cu(211). In the initial state, a relaxed water structure on Cu(211) is perturbed by the inclusion of an additional hydrogen atom. In the final state, the proton has been transferred to the oxygen end of *CHOH to form *CH and H2 O, discharging the capacitor. Figs. 1b and 1c show examples of barriers for proton-electron transfers to *CO to form *CHO on Cu(111) and to *OCH3 to form CH3 OH and H2 O on Au(111), respectively. The potential at the interface was calculated by relating the work function to the experimental work function of the standard hydrogen electrode via

USHE =

Φ − ΦSHE . e

(1)

ΦSHE for which we use the experimentally-determined value of ∼ 4.4eV. 46 The calculations performed using this model of the electrochemical interface are done at constant charge rather than constant potential. The number of electrons in the computational cell is constant for each image along the reaction coordinate, but due to the the transfer of charge across the interface, the work function (and thus the electrochemical potential represented by the calculation) of the system may differ. The bottom panels of Fig. 1 show the corresponding shifts in work function along the reaction paths; shifts of 1.5-3eV are

6


Page 7 of 20

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


observed for all 3 barriers, corresponding to 1.5-3V shifts in potential. In this work, we apply a recently developed method to circumvent this issue using a capacitor model, 32,47 which allows us to extrapolate from the constant charge to constant potential limit using the change in interfacial charge. The basis of this method is that the electrostatic contributions to the energy can be approximated by a capacitor model, which allows us to determine the potential dependence of a given barrier using a single constant charge calculation. The method was benchmarked to calculations that were explicitly extrapolated to constant potential using a series of increasing cell sizes. There are several caveats with this approach. Firstly, there is an additional contribution to the potential when adsorbate dipoles re-orient which is not accounted for in the current version of the model. Secondly, density functional theory gives a charge delocalization error that can affect the charge-extrapolated energetics. 48–52 Finally, the dipole orientation of water has been shown to shift the theoretical NHE reference work function. 53 Ongoing work seeks to resolve these issues. The focus of the analysis here is on the overall trends in the data. Key proton-transfer reaction barriers for CO2 reduction to CH4 and CH3 OH on Pt(111), Cu(111), Cu(211), and Au(111) have been calculated and their extrapolated values at 0V vs. SHE are shown in Table 1. Both ∆E and Ea are electronic energies that have been corrected for potential using the extrapolation scheme of Ref. 32,47 The values for the original electronic energies can be found in the Supporting Information. Where reaction energies ∆E and Ea are equal, there is no additional activation barrier to the thermochemical reaction energy. We have also listed cases where the calculation lead to a spontaneous H transfer to the surface, which suggests surface hydrogenation pathway to be more favorable. We have broadly classified the barriers into four groups: 1. R-O → R-OH 2. R-OH → R + H2 O 3. CHx → CHx+1 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

4. CO → CHO Table 1: Table of charge extrapolated proton electron transfer barriers Reaction *CO2 → *COOH *CO2 → *COOH *CO → *COH CO → *COH CO → *COH *CHO → *CHOH *OCH3 →CH3 OH *COOH → *CO *COH → *C *COH → *C *COH → *C *COH → *C *CH2 OH → *CH2 *CHOH → *CH *CHOH → *CH *CHOH → *CH *CHOH → *CH *CO → *CHO *CO → *CHO *CO → *CHO *CO → *CHO *C → *CH *CH → *CH2 *CH2 → *CH3 *CH3 →CH4 *CH2 → *CH3 *CH3 → CH4 *C → *CH *OCH3 →CH4 + *O *OCH3 →CH4 + *O *OCH3 →CH4 + *O

Classification R-O → R-OH R-O → R-OH R-O → R-OH R-O → R-OH R-O → R-OH R-O → R-OH R-O → R-OH R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O R-OH → R + H2 O Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C Transfer to C

Surface Pt(111) Cu(211) Pt(111) Pt(111) (H*) Cu(111) Cu(211) Au(111) Pt(111) Pt(111) Pt(111) (H*) Cu(111) Cu(211) Pt(111) Au(111) Cu(211) Cu(111) Pt(111) Au(111) Cu(111) Cu(211) Pt(111) Cu(211) Cu(211) Cu(211) Cu(211) Pt(111) Pt(111) Pt(111) Pt(111) Cu(211) Au(111)

∆E at Φ = 4.4eV Ea at Φ = 4.4eV -0.51 0 -0.05 0.13 0.64 0.64 0.75 0.75 0.84 0.84 0.06 0.06 -1.95 0.03 -0.56 0.66 0 0.68 -0.63 1.14 0.49 0.99 -0.63 0.84 -0.33 0.34 -0.48 0.58 -0.37 0.47 -0.58 1.42 -0.99 0.38 -0.09 0.54 0.57 1.11 0.43 0.97 1.43 1.91 Spontaneous H transfer to surface Spontaneous H transfer to surface Spontaneous H transfer to surface Spontaneous H transfer to surface 1.25 1.25 0.91 0.91 0.16 0.32 Unable to converge, unfeasible barrier Unable to converge, unfeasible barrier Unable to converge, unfeasible barrier

Figure 2 presents the data for these 4 groups in the form of Brønsted-Evans-Polanyi (BEP) plots, i.e. Ea vs. ∆E, at 0VSHE . Certain proton-electron transfers are indicated with symbols i.e. *CO → *CHO (inverted triangles), *COH → *C (squares), and *CHOH → *CH (pentagons). The shaded blue region indicates the range of Ea = 0.55eV to 0.95eV. A barrier of 0.75eV would give a turnover frequency of approximately ∼1/s−1 at room temperature with a typical prefactor. The shaded range allows for the inaccuracies in the extrapolation method discussed above, as well as uncertainty in DFT, 54 coverage effects, 24 and free energy corrections. 23 This rate of ∼1/s−1 is considered to be the threshold for facile kinetics. 55 At 8


Page 8 of 20

Page 9 of 20

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


∆E > 0, the dashed blue line corresponds to Ea = ∆E , i.e. the case where there is no additional activation energy on top of the reaction energy for the proton-electron transfer step. At ∆E < 0 it corresponds to Ea = 0.

Figure 2: Brønsted-Evans-Polyani (BEP) relationships for 4 classes of elementary steps relevant for CO2 electroreduction plotted as Ea vs. ∆E extrapolated to 0 V vs. SHE. The blue dashed line represents a reaction with no additional barrier above its reaction energy. Inverted triangles: *CO → *CHO, squares: *COH → *C, pentagons: *CHOH → *CH, and circles showing all other reactions. In general, we find no cases where the R-O → R-OH transfers are kinetically limited when the thermodynamic energies are surmountable. On the other hand, R-OH → R + H2 O and transfers to C may be kinetically limited even if the thermodynamics are favorable. We rationalize these trends with a simple electronegativity argument. Transfers to O are more facile than C due to O having a larger electronegativity. Transfers to R-OH → R + H2 O involve a proton transfer to O as well, but the overall barrier also depends on breaking the R-O bond, which is not generally facile. CO → CHO shows linear scaling between activation and reaction energy. We also calculated relevant surface hydrogenation barriers, listed in the Supporting Information and Fig. 3, shown as Brønsted-Evans-Polyani (BEP) relations. This data set includes our own calculations along with data found in CatApp. 56 In all cases, the barriers for the surface hydrogenation barriers are calculate at low adsorbate coverage in vacuum.

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

Figure 3: Brønsted-Evans-Polanyi relationships for surface hydrogenations relevant to CO2 reduction. Data taken primarily form CatApp supplemented by our own calculations. See Supporting Information for table of values. Colors indicate the metal/facet and follow the same legend as in Fig. 2 Additionally, green: Rh(111), cyan: Pd(111), magenta: Ag(111). In general, surface hydrogenations on transition metals of CO2 reduction intermediates are feasible with the exception of certain reactions on strong-binding transition metals (Pt, Pd, and Rh) which in CO2 reducing conditions would be subject to adsorbate-adsorbate interactions, and CO → COH, which has a low electrochemical barrier. Surface hydrogenation may be the predominant way in which CHx are hydrogenated, since attemps to determine proton-electron transfer barriers yielded spontaneous H adsorption on the surface. Based on the collection of proton transfer and surface hydrogenation barriers, we can classify transfers to R-O as generally facile, while R-OH → R + H2 O and transfers to C are not. We attempted to determine the barriers for proton transfer to *OCH3 to form CH4 on several surfaces, but the calculations invariably led to O-C scission prior to proton transfer and the creation of a CH3 radical with an insurmountable >2eV barrier. The corresponding surface hydrogenation barrier is also insurmountable at > 2eV. Therefore the path from *OCH3 to form CH4 is not expected to ever be viable. A purely thermodynamic analysis like that performed by Ref. 23 predicts that this elementary step to be the most favorable. Our finding that this step should be kinetically infeasible is in agreement with 10


Page 10 of 20

Page 11 of 20

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


Ref., 27 who have estimated proton transfer barriers on Cu(111) by approximating them by surface-hydrogenation ones. This is also consistent with the analysis on Cu(100) 29 done using quantum molecular dynamics. Based on this classification, we have generated a generic reaction network for the pathway from CO2 to CH4 and CH3 OH, valid for at least the 4 transition metals studied, shown in Fig. 4. As Au , Cu and Pt span a wide range of *CO binding energies, 25 we expect this result to hold for other transition metals as well. Reactants and products are indicated in green. Facile barriers were represented by solid lines, possibly difficult ones by dashed lines; dotted lines represent the insurmountable methoxy pathway. Fig. 5 shows an updated approximate free energy diagram for Cu(211) including the barriers for the predicted pathway, which in contrast to the thermochemical analysis 23 shows a pathway with neither *OCH3 nor CH2 O. We have found, in agreement with other calculations in the literature, 27 that *OCH3 tends to form methanol over methane. We find the protonation of *CHO to *CHOH to be facile, which leaves the only feasible C-O scission step to be *CHOH to *CH. Therefore, the selectivity towards methane vs. methanol tis likely determined by the reaction energetics towards *CHOH vs. CH2 O. This is consistent with an experimental study showing formaldehyde reduction favors methanol over methane. 9 We have not included any electrochemical CO → COH barriers for Cu(211) due to the large (∼ 0.8eV) difference in adsorption energy between COH and CHO, which is larger than the barrier at 0V vs. SHE. Dashed lines represent steps that likely involve a proton-electron transfer, and as solid lines represent steps likely to be surface hydrogenations. For simplicity the transfer coefficient β in this diagram was assumed to be 0.5. Our calculations prefer the *CO → *CHO pathway on Cu(211) over *CO → *COH as proposed by Refs 26,29 primarily because of thermodynamics rather than kinetics. We agree with other calculations with the conclusion that the protonation of the oxygen-end of *CO is kinetically easier than the carbon end, but on Cu(211) in particular, the thermodynamic difference between the stability of *COH and *CHO is much larger than on the other facets

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

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

Figure 4: Generic reaction network based on calculated barriers. Green circles indicate gas phase products and reactant, and black circles the adsorbed reaction intermediates. Black solid lines indicate facile barriers, and dashed ones barriers that can be difficult. We have also included a dotted line from OCH3 to CH4 , which is not expected to ever be surmountable. (approximately 0.8 eV on Cu(211) versus 0.5 eV on Cu(111) and 0.4 eV on Cu(100)). In all cases, *CHO is the thermodynamically-favored intermediate, but the exact surface, kinetics, solvation, and coverage effects can play a role in which pathway is presented as the ”correct” one. In summary, we have calculated relevant proton-electron transfer barriers for CO2 reduction on 4 representative transition metal surfaces: Pt(111), Cu(111), Cu(211), and Au(111). We have classified barriers into 4 groups: R-O to R-OH, R-OH to R + H2 O, CHx → CHx+1 , 12


Page 12 of 20

Page 13 of 20

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: Free energy diagram for the most likely pathway for CO2 electroreduction to CH4 on Cu(211) at two different potentials. Dashed lines represent steps that likely involve a proton-electron transfer as opposed to solid lines which represent steps that are facile as surface hydrogenations. An electrochemical transfer coefficient of 0.5 was assumed for the proton-electron transfers. and CO → CHO. In general the first group is facile, whereas the others are not generally facile. We attribute these trends to the electronegativity of O vs. C, and also the variation in R-OH bond strengths that contributes to the R-OH to R + H2 O barrier. Based on the barriers determined, we have proposed a revised CO2 to CH4 pathway which goes through a *CHOH, *CH, *CH2 , *CH3 pathway to CH4 . Future work will address barriers for CC coupling and subsequent products, as well as integrating the reaction energetics into a microkinetic model for CO2 reduction

Acknowledgement This material is based on work supported by the Air Force Office of Scientific Research through the MURI program under AFOSR Award No. FA9550-10-1-0572. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

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

Supporting Information Supporting Information for this article contains full tables of the surface hydrogenation barriers used for Figure 3. Additionally, we have included the calculated energies, charges, and work functions necessary for the extrapolation to constant potential used in this work.

References (1) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and homogeneous approaches to conversion of CO2 to liquid fuels. Chem. Soc. Rev. 2009, 38, 89–99. (2) Whipple, D. T.; Kenis, P. J. a. Prospects of CO 2 Utilization via Direct Heterogeneous Electrochemical Reduction. J. Phys. Chem. Lett. 2010, 1, 3451–3458. (3) Finn, C.; Schnittger, S.; Yellowlees, L. J.; Love, J. B. Molecular approaches to the electrochemical reduction of carbon dioxide. Chem. Commun. (Camb). 2012, 48, 1392– 9. (4) 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. (5) Takahashi, I.; Koga, O.; Hoshi, N.; Hori, Y. Electrochemical reduction of CO2 at copper single crystal Cu(S)-[n(111)(111)] and Cu(S)-[n(110)(100)] electrodes. J. Electroanal. Chem. 2002, 533, 135–143. (6) Kim, J.; Summers, D.; Frese, K. Reduction of CO2 and CO to methane on Cu foil electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1988, 245, 223–244. (7) Hori, Y.; Takahashi, I.; Koga, O.; Hoshi, N. Electrochemical reduction of carbon dioxide

14


Page 14 of 20

Page 15 of 20

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


at various series of copper single crystal electrodes. J. Mol. Catal. A Chem. 2003, 199, 39–47. (8) Li, C. W.; Kanan, M. W. CO2 reduction at low overpotential on Cu electrodes resulting from the reduction of thick Cu2O films. J. Am. Chem. Soc. 2012, 134, 7231–4. (9) Schouten, K. J. P.; Kwon, Y.; van der Ham, C. J. M.; Qin, Z.; Koper, M. T. M. A new mechanism for the selectivity to C1 and C2 species in the electrochemical reduction of carbon dioxide on copper electrodes. Chem. Sci. 2011, 2, 1902. (10) Azuma, M. Electrochemical Reduction of Carbon Dioxide on Various Metal Electrodes in Low-Temperature Aqueous KHCO[sub 3] Media. J. Electrochem. Soc. 1990, 137, 1772. (11) Li, C. W.; Ciston, J.; Kanan, M. W. Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper. Nature 2014, 508, 504–507. (12) Hori, Y.; Wakebe, H.; Tsukamoto, 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. (13) Kuhl, K. P.; Hatsukade, T.; Cave, E. R.; Abram, D. N.; Kibsgaard, J.; Thomas, F. Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces. 2014, (14) Hori, Y. Mod. Asp. Electrochem.; Springer New York: New York, NY, 2008; pp 89–189. (15) Nø rskov, 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. (16) Calle-Vallejo, F.; Koper, M. Theoretical considerations on the electroreduction of CO to C2 species on Cu (100) electrodes. Angewandte Chemie 2013, 125, 7423–7426. 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

(17) Hirunsit, P. Electroreduction of carbon dioxide to methane on copper, copper–silver, and copper–gold catalysts: a DFT study. The Journal of Physical Chemistry C 2013, 117, 8262–8268. (18) Lim, H.-K.; Shin, H.; Goddard III, W. A.; Hwang, Y. J.; Min, B. K.; Kim, H. Embedding covalency into metal catalysts for efficient electrochemical conversion of CO2. Journal of the American Chemical Society 2014, 136, 11355–11361. (19) Tripkovic, V.; Vanin, M.; Karamad, M.; Bjrketun, M. E.; Jacobsen, K. W.; Thygesen, K. S.; Rossmeisl, J. Electrochemical CO2 and CO Reduction on MetalFunctionalized Porphyrin-like Graphene. The Journal of Physical Chemistry C 2013, 117, 9187–9195. (20) Karamad, M.; Tripkovic, V.; Rossmeisl, J. Intermetallic Alloys as CO Electroreduction Catalysts—Role of Isolated Active Sites. ACS Catal. 2014, 4, 2268–2273. (21) Chen, Z.; Zhang, X.; Lu, G. Overpotential for CO2 electroreduction lowered on strained penta-twinned Cu nanowires. Chemical Science 2015, 6, 6829–6835. (22) Li, Y.; Chan, S. H.; Sun, Q. Heterogeneous catalytic conversion of CO2: a comprehensive theoretical review. Nanoscale 2015, 7, 8663–8683. (23) Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nø rskov, J. K. How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels. Energy Environ. Sci. 2010, 3, 1311. (24) Shi, C.; Hansen, H. a.; Lausche, A. C.; Nø rskov, J. K. Trends in electrochemical CO2 reduction activity for open and close-packed metal surfaces. Phys. Chem. Chem. Phys. 2014, 16, 4720–7. (25) Peterson, A. A.; Nø rskov, J. K. Activity descriptors for CO 2 electroreduction to methane on transition-metal catalysts. J. Phys. Chem. Lett. 2012, 3, 251–258. 16


Page 16 of 20

Page 17 of 20

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


(26) Nie, X.; Esopi, M. R.; Janik, M. J.; Asthagiri, A. Selectivity of CO(2) Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps. Angew. Chem. Int. Ed. Engl. 2013, 52, 2459–62. (27) Nie, X.; Luo, W.; Janik, M. J.; Asthagiri, A. Reaction mechanisms of CO2 electrochemical reduction on Cu(111) determined with density functional theory. J. Catal. 2014, 312, 108–122. (28) Xiao, H.; Cheng, T.; III, W. A. G.; Sundararaman, R. Mechanistic Explanation of the pH Dependence and Onset Potentials for Hydrocarbon Products from Electrochemical Reduction of CO on Cu (111). Journal of the American Chemical Society 2016, 138, 483–486, PMID: 26716884. (29) Cheng, T.; Xiao, H.; Goddard III, W. A. The Free Energy Barriers and Reaction Mechanisms for the Electrochemical Reduction of CO on the Cu(100) Surface Including Multiple Layers of Explicit Solvent at pH 0. J. Phys. Chem. Lett. 2015, (30) Koziel, S.; Leifsson, L.; Lees, M.; V.Krzhizhanovskaya, V.; Dongarra, J.; Sloot, P. M.; Hussain, J.; Sklason, E.; Jnsson, H. International Conference On Computational Science, ICCS 2015 Computational Study of Electrochemical CO2 Reduction at Transition Metal Electrodes. Procedia Computer Science 2015, 51, 1865 – 1871. (31) Rossmeisl, J.; Sk´ ulason, E.; Björketun, M. r. E.; Tripkovic, V.; Nø rskov, J. K. Modeling the electrified solid–liquid interface. Chem. Phys. Lett. 2008, 466, 68–71. (32) Chan, K.; Nø rskov, J. K. Electrochemical Barriers Made Simple. J. Phys. Chem. Lett. 2015, 2663–2668. (33) Sk´ ulason, E.; Tripkovic, V.; Björketun, M. r. E.; Gudmundsdóttir, S.; Karlberg, G.; Rossmeisl, J.; Bligaard, T.; Jónsson, H.; Nø rskov, J. K. Modeling the Electrochemical Hydrogen Oxidation and Evolution Reactions on the Basis of Density Functional Theory Calculations. J. Phys. Chem. C 2010, 114, 18182–18197. 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

(34) Hammer, B.; Hansen, L.; Nø rskov, J. Improved adsorption energetics within densityfunctional theory using revised Perdew-Burke-Ernzerhof functionals. Phys. Rev. B 1999, 59, 7413–7421. (35) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. Iterative Minimization Techniques for Abinitio Total-Energy Calculations - Molecular-Dynamics and Conjugate Gradients. Rev. Mod. Phys. 1992, 64, 1045–1097. (36) Kresse, G. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15–50. (37) Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 1990, 41, 7892–7895. (38) Dacapo pseudopotential code. https://wiki.fysik.dtu.dk/dacapo. (39) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188–5192. (40) Bahn, S.; Jacobsen, K. An object-oriented scripting interface to a legacy electronic structure code. Comput. Sci. Eng. 2002, 4, 56–66. (41) Jónsson, H.; Mills, G.; Jacobsen, K. W. In Class. Quantum Dyn. Condens. Phase Simulations; Berne, B., Ciccotti, G., Coker, D. F., Eds.; World Scientific, 1998; Chapter 16, p 385. (42) 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, 9978. (43) Cramer, C. Essentials of Computational Chemistry, second edi ed.; Wiley, 2004. (44) Peterson, A. A. Global optimization of adsorbate-surface structures while preserving molecular identity. Top. Catal. 2014, 57, 40–53. 18


Page 18 of 20

Page 19 of 20

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) Ogasawara, H.; Brena, B.; Nordlund, D.; Nyberg, M.; Pelmenschikov, A.; Pettersson, L. G. M.; Nilsson, A. Structure and Bonding of Water on Pt(111). Phys. Rev. Lett. 2002, 89, 276102. (46) Trasatti, S. The Absolute Electrode Potential: an Explanatory Note (Recommendations 1986). Pure Appl. Chem. 1986, 58 . (47) Chan, K.; Nørskov, J. K. Potential Dependence of Electrochemical Barriers from ab Initio Calculations. J. Phys. Chem. Lett. 2016, 1686–1690. (48) Perdew, J. P.; Zunger, A. Self-interaction correction to density-functional approximations for many-electron systems. Physical Review B 1981, 23, 5048. (49) Perdew, J. P.; Parr, R. G.; Levy, M.; Balduz Jr, J. L. Density-functional theory for fractional particle number: derivative discontinuities of the energy. Physical Review Letters 1982, 49, 1691. (50) Mori-Sánchez, P.; Cohen, A. J.; Yang, W. Many-electron self-interaction error in approximate density functionals. The Journal of chemical physics 2006, 125, 201102. (51) Ruzsinszky, A.; Perdew, J. P.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E. Density functionals that are one-and two-are not always many-electron self-interaction-free, as shown for H. The Journal of chemical physics 2007, 126, 104102. (52) Cohen, A. J.; Mori-Sánchez, P.; Yang, W. Insights into Current Limitations of Density Functional Theory. Science 2008, 321, 792–794. (53) Tripkovic, V.; Björketun, M. r.; Sk´ ulason, E.; Rossmeisl, J. Standard hydrogen electrode and potential of zero charge in density functional calculations. Phys. Rev. B 2011, 84, 1–11.

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

(54) Medford, A. J.; Wellendorff, J.; Vojvodic, A.; Studt, F.; Abild-Pedersen, F.; Jacobsen, K. W.; Bligaard, T.; Nørskov, J. K. Assessing the reliability of calculated catalytic ammonia synthesis rates. Science 2014, 345, 197–200. (55) Nørskov, J.; Studt, F.; Abild-Pedersen, F.; Bligaard, T. Fundamental Concepts in Heterogeneous Catalysis; Wiley, 2014. (56) Hummelshøj, J. S.; Abild-Pedersen, F.; Studt, F.; Bligaard, T.; Nørskov, J. K. CatApp: A Web Application for Surface Chemistry and Heterogeneous Catalysis. Angew. Chemie Int. Ed. 2012, 51, 272–274.

20


Page 20 of 20

Barriers of Electrochemical CO2 Reduction on Transition Metals

Recommend Documents