Why and How Carbon Dioxide Conversion to Methanol Happens on

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Why and how carbon dioxide conversion to methanol happens on functionalized semiconductor photoelectrodes Shenzhen Xu, Lesheng Li, and Emily A. Carter J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09946 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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Journal of the American Chemical Society

Why and How Carbon Dioxide Conversion to Methanol Happens on Functionalized Semiconductor Photoelectrodes Shenzhen Xu,1 Lesheng Li,1 and Emily A. Carter2* 1Department

of Mechanical and Aerospace Engineering, and 2School of Engineering and

Applied Science, Princeton University, Princeton, New Jersey 08544-5263, United States

* Corresponding author, email: [email protected]

ACS Paragon Plus Environment

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Abstract Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH–*) has been postulated to enable selective CO2 photo-electro-reduction to CH3OH. This hypothesis is supported by recent estimates of sufficient 2-PyH–* lifetimes and low barriers for hydride transfer (HT) to CO2. However, the complete mechanism for reducing CO2 to CH3OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO2 reduction intermediates. Predicted barriers suggest that HT to HCOOH requires adsorbed HCOOH* reacting with 2-PyH–*, a new catalytic role for the surface. HT to HCOOH* produces CH2(OH)2 but subsequent HT to CH2(OH)2 forming CH3OH is hindered. However, CH2O, dehydrated CH2(OH)2, easily reacts with 2-PyH–* producing CH3OH. Further reduction of CH3OH to CH4 via HT from 2-PyH–* encounters a high barrier, consistent with experiment. Our finding that the GaP surface enables HT to HCOOH* explains why the primary CO2 reduction product over CdTe photoelectrodes is HCOOH rather than methanol, as HCOOH does not adsorb on CdTe and so the reaction terminates. The stability of 2-PyH–* (vs. its protonation product DHP*), the relative dominance of CH2(OH)2 over CH2O, and the required desorption of CH2(OH)2* are the most likely limiting factors, explaining the low yield of CH3OH observed experimentally.


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Introduction The world’s ever-expanding energy demand coupled with excessive carbon dioxide (CO2) emissions that are causing accelerated climate change1 prompts development of sustainable energy technologies that can hasten the retirement of fossil fuels. Using captured CO2 as a feedstock to generate carbon-based fuels could reduce fossil-fuel extraction and use, stabilizing emissions. Photoelectrochemical CO2 reduction also could be a vehicle to convert and store excess intermittent, renewable energy (from solar to chemical energy). The inert nature of CO2, however, poses fundamental challenges to achieving efficient, highly selective photoelectrochemical CO2 reduction. Chief among these challenges are the lack of optimal photoelectrodes that can both convert sunlight efficiently and enable efficient, selective catalytic reduction. Gallium phosphide (GaP), which has been studied extensively as a light-harvesting material2,3 (see, e.g., its surface modification,4 polymer functionalization,5,6 dye/quantum dot sensitization,7–9 and its nanowire morphology,10 for promising photoelectro-properties), early on was shown to photo-reduce CO2 with high selectivity in the absence of co-catalysts.11,12 However, large overpotentials were required to obtain products of CO2 reduction. Decades later, a p-GaP electrode under illumination, coupled with a pyridine (Py) co-catalyst in acidic water solution was demonstrated to exhibit high selectivity toward methanol (CH3OH) (almost 100% faradaic efficiency) at underpotentials.13 Despite its high selectivity and lack of overpotential, the overall yield of methanol was very low: the methanol quantum efficiency MeOH ((mol methanol  6)/(mol incident photon)13) was only ~10% at 0.3 V underpotential, illuminated by a single-wavelength (365 nm) light source. To make further progress, the atomic-scale origins of this system’s promising features and its disappointingly low yield must be found. By elucidating the complete CO2 reduction mechanism, new strategies may emerge to improve overall performance.


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In brief, a wide range of experimental13–18 and theoretical evidence19–27 suggests that CO2 is reduced via a heterogeneous process on the electrode surface and that the catalytic intermediate is a Py-derived species. After systematic elimination of other candidates, Carter and coworkers predicted that a Py reduction product, adsorbed 2-pyridinide (2-PyH–*; * indicates an adsorbed species) facilitates hydride transfer (HT) to CO2 on all three semiconductors where this chemistry has been observed.25,27 Moreover, they predicted strong thermodynamic driving forces and low barriers to form 2-PyH–* photoelectrochemically.25,27 However, the lifetime of 2-PyH–* is of concern, since this transient species could be protonated to form adsorbed dihydropyridine (DHP*), an adsorbate predicted to be relatively inactive for CO2 reduction.25,26 Very recent work suggests that 2-PyH–* situated next to OH-* (present at the GaP/water interface)23,28,29 should exist long enough for HT to CO2 to occur.30 These same models predict high barriers to hydrogen evolution via either 2-PyH–* or H-*,30 consistent with the observed high selectivity toward CO2 reduction,11,13 further validating the 2-PyH–* mechanistic hypothesis. Up to now, mechanistic work has focused on understanding the first CO2 reduction step, namely to make formate (HCOO-). However, rate-limiting factors along the entire CO2 reduction pathway all the way to CH3OH remain to be identified. The work presented herein achieves exactly that, assuming the catalysis is carried out by the one proposed intermediate left standing, namely 2-PyH–*. Thereafter, we propose strategies to overcome these rate-limiting factors to improve the performance of 2-PyH–* co-catalyzed CO2 reduction on semiconductor photoelectrodes.

Results We begin by considering what happens after formate (HCOO–) is produced via the first HT from 2-PyH–* to CO2.25,27 The measured pKa of formic acid (HCOOH) is 3.831 while the optimal


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pH of the aqueous electrolyte is 5.2.13 A small fraction (~4%) of the HCOO– thus will be protonated to form HCOOH under these conditions. Because 2-PyH–* and HCOO– are both negatively charged, HT to HCOO– will not occur because of Coulomb repulsion. The neutral species HCOOH formed by protonation of formate therefore must be the hydride acceptor instead. An analogous argument holds for HT to the subsequent reduction intermediates, so we only consider HT to neutral species and not to their deprotonated conjugates (HCOOH vs. HCOO-, CH2(OH)2 vs. CH2(OH)O-, CH3OH vs. CH3O-). Unfortunately, lowering the pH to form more HCOOH deactivates 2-PyH-*,30 which explains why the optimal pH is higher than the pKa of HCOOH. Both HCOOH and formaldehyde (CH2O) were detected during CO2 reduction on illuminated p-GaP (without Py)11 and on Pt (with Py).32 Thus, another CO2 reduction intermediate could be CH2O. However, no HCOOH or CH2O were detected during CO2 reduction at an illuminated p-GaP electrode with Py.13 The presence of Py clearly changes the mechanism (as already indicated by the large change in overpotential), again providing evidence for a Py-derived species driving the chemistry. If in the presence of Py the barriers to further reduction of HCOOH and CH2O are small, these species simply may have short lifetimes. The concentrations of the transient intermediates HCOOH and CH2O also could be extremely low and essentially undetectable due to the low overall yield.13 Although HT to HCOOH first produces deprotonated methanediol CH2(OH)O–, at pH = 5.2 it will protonate promptly to form CH2(OH)2, because of the significantly higher pKa of CH2(OH)2 (13.3).33 Subsequent dehydration of CH2(OH)2 produces CH2O until an equilibrium is reached; the dehydration equilibrium constant is 7.87 x 10-4,34 and hence the amount of CH2O will be relatively small (perhaps another reason for the low overall yield, vide infra). Thus both CH2(OH)2 and CH2O are CO2 reduction intermediates to consider.


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Both the minority species CH2O and the majority species CH2(OH)2 may further reduce to CH3OH via HT from 2-PyH–*. HT to CH2(OH)2 directly produces methanol via an SN2 reaction in which OH- departs as H- arrives. However, HT to CH2O first yields methoxide CH3O– ; as the pKa of CH3OH is 15.5,35 at pH = 5.2 CH3O– will protonate promptly to form CH3OH. In principle, the possibility of CH3OH being further reduced to methane (CH4) exists, as well. Our work below explains why reduction stops at CH3OH.

CH2O H

O

HT

HT

HT

C

CH 3OH

HCOOH

CH4

CH 2(OH)2

Figure 1. CO2 reduction intermediates and sequential hydride transfer (HT) reaction pathway. The CO2 reduction intermediates we investigate are HCOOH, CH2(OH)2/CH2O, CH3OH, and CH4 (Figure 1). We assume that the catalytic intermediate 2-PyH–* is the hydride donor and its possible reduction reactions of the above molecules and calculated thermodynamic driving forces (room temperature free energy changes at pH = 5.2) for those reactions are given in Table 1. The free energies of the CO2 reduction intermediates were obtained by modeling the isolated molecules dissolved in water (described by continuum solvation, the implicit Solvation Model based on solute electron Density (SMD)36). An empirical value was used for the free energy of a dissolved proton (pH = 5.2).37 All of the reactions are thermodynamically favorable, including for CH4 formation. Given that the latter does not form experimentally, we conclude that the reactions are under kinetic control; we therefore evaluate kinetic barriers to each reaction next.


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Table 1. Free energy changes (eV) at pH = 5.2 and room temperature for HCOOH, CH2(OH)2, CH2O, and CH3OH reacting with 2-PyH–*, forming CH2(OH)2, CH3OH, CH3OH, and CH4, respectively. Free energies for all of the above molecules are obtained by modeling isolated molecules dissolved in water (described by SMD). An empirical value is used for the free energy of a dissolved proton at pH = 5.2 in water.37

Because of the large pKas of the products above, proton transfer (PT) steps are unlikely to be rate limiting; we therefore only evaluate HT kinetics, maintaining consistency with our previous models for examining HT kinetics from 2-PyH–* to CO2.25,27 We also did not consider concerted HT-PT steps. At the weakly acidic pH = 5.2, the probability that a CO2 reduction intermediate reacting with 2-PyH–* also has a solvated proton nearby will be low. For CO2 electro-reduction on Cu,38 PT to CO2 reduction intermediates was predicted to involve a hydrogen bond (HB) network. Even at pH = 0, the configuration modeling PT utilized a HB chain of four water molecules connecting H3O+ to the CO2 reduction intermediate.38 Constructing a similar HB network to reasonably represent the configuration of a concerted HT-PT to CO2 reduction intermediates at pH = 5.2 is not feasible computationally, given the five orders of magnitude decrease in proton concentration. Admittedly, a concerted HT-PT may lower the activation energy compared to a HT


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followed by a separate PT. Thus, the model assumed here presumably leads to upper bounds on barriers and lower bounds on rates. The HT reactions investigated here are: HCOOH+ 2-PyH–*  CH2(OH)O– + Py*

(1)

CH2(OH)2 + 2-PyH–*  CH3OH + Py* + OH–

(2)

CH2O + 2-PyH–*  CH3O– + Py*

(3)

CH3OH + 2-PyH–*  CH4 + Py* + OH–

(4)

In each HT reaction (1) – (4), the CO2 reduction intermediates can be either in aqueous solution or adsorbed on the semiconductor surface. For example, CH3OH in (4) can be either CH3OHsol or CH3OH* (the subscript “sol” means “in solution”). The corresponding atomic structures of CH3OHsol + 2-PyH–* and CH3OH* + 2-PyH–* (serving as illustrative cases) are shown in Figure 2. Our computed adsorption free energies on GaP(110) – the most active facet experimentally17 – suggest the existence of adsorbed intermediates HCOOH* (Gads = 0.07 eV),39 CH2(OH)2* (Gads = -0.4 eV), CH2O* (Gads = -0.08 eV), and CH3OH* (Gads = -0.23 eV). Translational, rotational, and vibrational entropies of each isolated species (HCOOH, CH2(OH)2, CH2O, and CH3OH) in solution are included in the adsorption-energy calculations using the implicit SMD solvation model. The Ga adsite is assumed to have no other adsorbate bound prior to binding the adsorbate of interest. This assumption is valid because earlier work showed that the H2O adsorption free energy Gads is only -0.048 eV,39 indicating that at room temperature there should be a dynamic equilibrium between Ga sites with weakly bound water and those with no water that could readily adsorb the adsorbate under consideration. For all adsorbed CO2 reduction intermediates reacting with 2-PyH–*, the half-dissociated H2O monolayer on the GaP surface predicted by theory and seen experimentally23,28,29 was


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modeled as usual by explicit H2O molecules;23–25,27,30 long-range solvent polarization was modeled by SMD (as described in the Method Section, vide infra). For CO2 reduction intermediates in solution reacting with 2-PyH–*, in addition to the explicit H2O adlayer on GaP(110), more explicit H2O molecules were added to HB with the CO2 reduction intermediates in order to more accurately approximate the solvent environment. We added one H2O molecule to HB with each O atom and each acidic H atom in the CO2 reduction intermediates, with long-range polarization modeled as usual with SMD. Thus, three, four, one, and two explicit H2O molecules are hydrogen-bonded to HCOOH, CH2(OH)2, CH2O, and CH3OH in the solvent region, respectively. The atomic structure of an illustrative case CH3OHsol + 2-PyH–* is shown in Figure 2B, where two H2O molecules are hydrogen-bonded with CH3OH in the solvent region. This approach was validated in earlier work on the acidities of various molecules in aqueous solution.39 Thus, we have four HT reactions, each with two options: reduction intermediates in solution or adsorbed on GaP. We therefore computed eight kinetic barriers, shown in Table 2. CH 3O H sol + 2-PyH -*

CH 3O H * + 2-PyH -*

CH 3OHsol 2-PyH -*

2-PyH -*

CH3OH*

H2O

H2O

Ga

P

O

C

(A )

N

H

(B)

Figure 2. Atomic geometries of (A) CH3OH* + 2-PyH–* and (B) CH3OHsol + 2-PyH–* to illustrate the structural setup of adsorbed and solvated CO2 reduction intermediates reacting with 2-PyH–*.


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The black arrow represents the direction of the HT process, pointing from the hydride moiety in 2-PyH–* to the carbon atom in CH3OH. Table 2. Barriers for (1), (2), (3), (4) correspond to HTs in Eqs. (1), (2), (3), and (4), respectively. Each HT has two possibilities, Msol + 2-PyH–* or M* + 2-PyH–*, where M represents the CO2 reduction intermediates HCOOH, CH2(OH)2, CH2O, and CH3OH. Grey boxes show HT barriers of Eq. (2) where CH2(OH)2 is the reactant. Green barriers show the most kinetically favorable HTs.

HT (1) HCOOH  CH2(OH)O -

in solution

1.04 eV

adsorbed on surface

0.23 eV

HT (2) CH2(OH)2  CH3OH + OHHT (3) CH2O  CH3O -

2.3 eV 0.21 eV 1.13 eV ~0 eV

HT (4) CH3OH  CH4 + OH-

1.8 eV

1.3 eV

Based on the predicted HT barriers in Table 2, CO2 reduction should proceed as follows: a. HCOOH* + 2-PyH–*  CH2(OH)O–* + Py*, followed by protonation to form CH2(OH)2. The free energy diagram and critical structures (Figure 3) illustrate the importance of HCOOH and CH2(OH)O- adsorption on surface Ga sites. The relative stabilization of the product (strong dative bonding of the anion to the positively charged Ga) decreases the barrier via the reaction’s significant exoergicity (-1.02 eV; Figure 3). This value is much larger than the exoergicity (-0.07 eV; Figure S1) of the analogous reaction with formic acid in solution: HCOOHsol + 2-PyH–*  CH2(OH)O–sol + Py*. Thus, without surface stabilization, HCOOH reduction will not occur at room temperature. b. Note that CH2(OH)2  CH3OH is kinetically forbidden at room temperature (even via CH2(OH)2*), while CH2O  CH3OH will be rapid for CH2Osol and CH2O*. A recent report on HT to CH2(OH)2/CH2O for CH3OH production catalyzed by homogeneous molecular


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complexes reached similar conclusions.40 Thus, dehydration of CH2(OH)2 to CH2O must occur prior to further reduction. The measured activation energy for CH2(OH)2 dehydration is 0.58 eV,41 a surmountable barrier at room temperature. Once CH2O is present, the HT step CH2O* + 2-PyH–*  CH3O–* + Py* or CH2Osol + 2-PyH–*  CH3O–sol + Py* will occur, yielding CH3OH upon protonation (Figure 4). c. The next HT reaction CH3OH  CH4 is kinetically forbidden at room temperature (even via CH3OH*), which explains why the CO2 reduction stops at CH3OH. Free energy diagrams for all kinetically forbidden HT steps appear in Section 1 of the Supporting Information (SI).

HCOOH* + 2-PyH -*  CH 2(OH)O -* + Py* 0.6

Fr ee Ener gy (eV)

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


0.23 eV

0.3 0 -0.3 -0.6 -0.9 -1.2

IS

TS

IS

Py*

CH2(OH)O -*

2-PyH -*

HCOOH*

FS

TS


FS

11


Figure 3. Free energy (T = 298.15 K) of critical points for HCOOH* + 2-PyH–*  CH2(OH)O–* + Py*. Atomic structures of initial state (IS), transition state (TS), and final state (FS) are at bottom. Same convention appears in all similar figures in this paper.

CH 2O sol + 2-PyH -*  CH 3O -sol + Py*

(A ) Fr ee Ener gy (eV)

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

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0.4 0 -0.2 -0.4 -0.6 -0.8

IS

(B)

0.21 eV

0.2

IS

TS

FS

TS

FS

CH 2O* + 2-PyH -*  CH 3O -* + Py* Structure relaxation

Kinetic bar r ier ~0 eV

Figure 4. (A) Free energy (T = 298.15 K) of critical points for CH2Osol + 2-PyH–*  CH3O–sol + Py*. (B) Schematic of the structure relaxation of CH2O* +2-PyH–*  CH3O–* + Py*. As the geometry of CH2O* +2-PyH–* spontaneously relaxes to the geometry of CH3O–* + Py*, the activation energy is zero.


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Based on the results in Table 2, the HT barrier decreases significantly when the CO2 reduction intermediate adsorbs on a surface Ga site. Adsorption on this site promotes HT kinetics because it draws electrons away from the CO2 reduction intermediates via donor-acceptor interaction between an O lone pair from the adsorbate and an empty Ga 4p orbital. The Ga atom will change from trigonal planar geometry (sp2 hybridization) to tetrahedral (sp3 hybridization) upon accepting the lone pair of electrons from an adsorbate. The reduced electron density around the molecule facilitates attack of the negatively charged hydride, reducing the HT barrier. Thus another catalytic role for the electrode surface, in addition to facilitating 2-PyH–* formation, is to enable lower kinetic barriers for HT. Our calculations indicate that HT to HCOOH can only occur at the electrode surface and not in solution, with adsorption of HCOOH therefore required to form CH3OH. As HCOOH adsorption on GaP(110) is essentially thermoneutral (Gads = 0.07 eV), we expect nearly half the formic acid produced will adsorb, consistent with detection of CH3OH as the final product. However, experiments on CO2 reduction at CdTe photoelectrodes show that CO2 reduction stops at HCOOH.18 Cluster model calculations (following our earlier work on CdTe23,25) exploring HCOOH adsorption behavior on CdTe show that HCOOH does not adsorb on this material (Section 2 of the SI). This finding, coupled with the high barrier found for HT to HCOOH in solution, thus explains why CO2 reduction at CdTe photoelectrodes only produces HCOOH.18 All of the above evidence and arguments further validate the heterogeneous nature of CO2 reduction reactions at various photoelectrodes, and should put to rest previous homogeneous mechanistic hypotheses.32,42,43 The overall reaction pathway for 2-PyH–*-catalyzed CO2 reduction to CH3OH on the GaP(110) surface is summarized in Figure 5. To verify the validity of our predictions, we further


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tested our solvation model and its effect on HT barriers by adding more H2O molecules hydrogenbonded to adsorbed CO2 reduction intermediates (HCOOH*, CH2(OH)2*, CH2O* and CH3OH*). (Up to this point, these intermediates were only able to hydrogen-bond to coadsorbed water or hydroxide.) We chose HCOOH* + 2-PyH–* as the first test case, with three H2O molecules added in the solvent region, hydrogen-bonded to the two O atoms and one acidic H atom in HCOOH*, just as done for HCOOH in solution. The new HT activation energy is 0.36 eV (only increasing by 0.13 eV compared to the case of HCOOH* + 2-PyH–* without explicit H2O molecules in the solvent region). Section 3 of the SI provides the free energy diagram for this test case. This small change of ~0.1 eV does not impact our conclusions about the reaction pathway (Figure 5). For the other three HT reactions occurring on the surface (CH2(OH)2* + 2-PyH–*, CH2O* + 2-PyH–*, CH3OH* + 2-PyH–*), we argue there is no need to carry out analogous further tests of this kind. The HT barriers indeed may increase slightly upon adding more explicit H2O molecules to those adsorbed species. However, as the activation energies for CH2(OH)2* + 2-PyH–* and CH3OH* + 2-PyH–* are already higher than 1.1 eV, adding more explicit H2O molecules simply will make the barriers even higher and therefore will not impact our conclusions. For CH2O* + 2-PyH–*, the upper limit for the HT barrier is already set by CH2Osol + 2-PyH–* (with one H2O molecule hydrogen-bonded to the carbonyl oxygen in CH2O), which is 0.21 eV. Therefore, even if the barrier for CH2O* + 2-PyH–* increases by adding more H2O molecules in the solvent region, the qualitative conclusion of fast HT to CH2O* from 2-PyH–* would remain.


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-

H

N

H

= 0.33-0.59 eV

N

HT

H T, PT

H T, PT

CO 2

H H

surface

surface

G ╪

-

HCOO H

G ╪

= 0.23-0.36 eV

CH 2(OH) 2 dehydr ation G ╪ = 0.58 eV

Final Pr oduct

H T, PT

G ╪ < 0.21 eV

CH 4

HT

CH 3OH

or

CH 2O

-

H

N

H

surface

HT

Figure 5. Complete reaction pathway for CO2 reduction to CH3OH on functionalized GaP photoelectrodes catalyzed by 2-PyH–* serving as the hydride donor. Green arrows represent kinetically accessible steps, while grey arrows represent kinetically forbidden steps. The barriers for producing formic acid are from Ref. 27 (0.33 eV on GaP(110)) and Ref. 25 (0.59 eV on reconstructed GaP(111)).

Discussion This work has revealed the most probable mechanism for 2-PyH–*-catalyzed CO2 reduction to CH3OH on GaP and CdTe photoelectrodes. We now discuss rate-limiting factors for CH3OH production. A significant body of work now points to 2-PyH–* as the most likely catalyst for HT to CO2 reduction intermediates at semiconductor electrodes.25,27,30 However, a major issue is the relative stability of 2-PyH–* vs. its relatively inactive, protonated form DHP*. We predict the protonation reaction 2-PyH–* + H+sol  DHP* to be thermodynamically favorable (G = -0.44 eV


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at pH = 5.2) within our solvated cluster model, consistent with both DHP*’s rather large predicted pKa (13.4) on GaP(110)39 and the experimentally observed drop in CO2 reduction activity at pH < 5.2.16 The analogous 2-PyH-* protonation free energy on GaP(111) was predicted to be -0.6 eV (pH = 5.2),25 quite close to our prediction for GaP(110). Therefore, the equilibrium between DHP* and its deprotonated form 2-PyH–* greatly favors DHP*. The limited amount of the transient species 2-PyH–* at the GaP surface is probably the origin of the low yield of CO2 reduction product.13 Aside from the thermodynamic stability of 2-PyH–*, the predicted barrier to its formation (0.59 eV on GaP(110))27 is higher than the barriers for HT to the CO2 reduction intermediates. Although this moderate barrier is surmountable, it is still 0.2-0.3 eV higher than the barriers for HT to CO2, HCOOH*, and CH2O/CH2O*. Formation of 2-PyH–* therefore might be rate-limiting in the overall CO2 reduction mechanism (Figure 5). Our results further suggest that dehydration of CH2(OH)2 to CH2O in aqueous solution must precede subsequent HT to produce CH3OH. However, the equilibrium between CH2(OH)2 and CH2O favors CH2(OH)2 and the measured barrier for CH2(OH)2 dehydration is 0.58 eV,41 indicating that conversion of CH2(OH)2 to CH2O might also be rate-limiting. Furthermore, as the adsorption free energy for CH2(OH)2 is Gads = -0.4 eV, the likelihood of desorption of CH2(OH)2 or CH2(OH)O- from the Ga site needs to be considered as well. HT to HCOOH* produces CH2(OH)O-*. Our predicted adsorption energy for CH2(OH)O- is Gads = -1.48 eV, indicating that direct desorption of CH2(OH)O- would be difficult. However, the free energy cost to protonate CH2(OH)O-* to CH2(OH)2* at pH = 5.2 is predicted to be only +0.18 eV. A brief derivation is shown here: G = G(CH2(OH)2*) – G(CH2(OH)O-*) – G(H+, pH=5.2) = G(CH2(OH)2*) – G(CH2(OH)O-*) – G(H+, pH=0) + 5.2  0.059 eV = - pKa  0.059 eV + 5.2  0.059 eV = +0.18


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eV, using the pKa of 2.1 calculated for CH2(OH)2* from our mixed implicit-explicit solvation model. Thus it seems plausible that CH2(OH)O-* could be protonated to CH2(OH)2* first and then desorb from the surface, given that the adsorption free energy for CH2(OH)2* is moderate, Gads = -0.4 eV. Because protonation and desorption steps are unlikely to have additional kinetic barriers, we estimate an effective activation energy of 0.58 eV to produce dissolved CH2(OH)2. This barrier is essentially identical to that for 2-PyH-* formation (0.59 eV) and for conversion of CH2(OH)2 to CH2O (0.58 eV). We therefore propose that CH2(OH)O-* protonation followed by desorption might be limiting as well. We next propose some ideas to overcome these rate-limiting factors identified above. First, noting that 2-PyH–* and DHP* formation from Py* can be written as Py* + H+sol + 2e–  2-PyH–* and Py* + 2H+sol + 2e–  DHP*, then 2-PyH–* may be considered an intermediate along the reaction path to DHP* formation. If we can devise a strategy to stabilize 2-PyH–* and destabilize DHP*, then more 2-PyH–* would be available to catalyze HT to CO2 reduction intermediates and product yield might increase. Moreover, stabilization of 2-PyH–*, i.e., decreasing the formation free energy for 2-PyH–*, might reduce the 2-PyH–* formation barrier, based on the Bell-EvansPolanyi (BEP) principle.44,45 Surface doping of the cation adsorption site (i.e., substitutional doping to replace a fraction of Ga atoms on the GaP surface) and functionalization of the Py molecule can be used to tune the relative stability of 2-PyH–* vs. DHP*, by perturbing the electron distribution on the Py-derived species. More electrophilic substitutions should stabilize the anion. However, too much stabilization might increase the HT barriers, making 2-PyH–* less active in terms of reducing CO2. An optimal design strategy will balance 2-PyH–* stability (vs. DHP*) and HT kinetics. With respect to CH2(OH)2 dehydration in aqueous solution, a nonreactive chemical


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additive that could shift the equilibrium toward CH2O must be found that also does not poison the catalytic intermediate 2-PyH–* and the surface Ga site.

Conclusions The complete reaction pathway for Py-co-catalyzed CO2 reduction to CH3OH on GaP photoelectrodes was elucidated from vetted quantum mechanical models. CO2 reduction to CH3OH consists of a series of steps involving HT from the catalytic intermediate 2-PyH–* to CO2, then to HCOOH, and then to CH2O. Calculated HT barriers indicate that HCOOH must adsorb on a surface cation site after its formation (produced via HT from 2-PyH–* to CO2 to form formate, followed by protonation) in order to allow further reduction to CH2(OH)2. A subsequent HT to CH2(OH)2 to form CH3OH is kinetically hindered, whether or not the former is adsorbed or in solution. However, adsorbed or dissolved CH2O, the dehydrated form of CH2(OH)2, easily undergoes HT from 2-PyH–* to form methoxide, which then is protonated to form CH3OH. The barriers to reduce CH3OH to CH4 (via either CH3OH* or CH3OHsol) are so high that CH4 production is kinetically forbidden, consistent with the experimental observation that CH3OH is the final product of CO2 reduction on GaP photoelectrodes. The indispensable role of the cation adsorption site for HT to HCOOH explains the experimental observation of HCOOH as the final product of CO2 reduction on CdTe photoelectrodes: we find that HCOOH does not adsorb on CdTe and, because the barrier to HT from 2-PyH–* to formic acid in solution is prohibitively large, the reaction stops at formic acid. Finally, we conclude that the lower stability of 2-PyH–* (vs. its protonated form DHP*, a relatively inactive species), the relative dominance of CH2(OH)2 over CH2O, and the required desorption of CH2(OH)2* are the main reasons for the low observed yield of CO2 reduction


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products. Future work should focus on optimizing surface composition and Py functionalization to tune the relative stability of these species. By doing so, photoelectrocatalytic recycling of carbon dioxide may become a reality.

Methods All of the density functional theory (DFT) calculations were performed within the ORCA software package (version 3.0.3).46 The B3LYP47–49 hybrid exchange-correlation (XC) functional was employed, as it reasonably balances computational cost and accuracy. As all species we evaluated are closed shell, we always restricted the overall spin to zero. Tests had been conducted earlier,27 comparing the relative energies of singlet vs. triplet states in the modeling of 2-PyH–* hydricity, adsorption energy, and HT barrier to CO2. The singlet states were always more stable than the triplets, validating our choice of spin-multiplicity here. We used all-electron Pople 631G** basis sets50,51 for all atoms except Ga in geometry optimizations. Ga was described with the Stuttgart effective core potential (ECP) and its corresponding valence double-zeta basis for the three valence electrons.52,53 The larger, all-electron aug-cc-pVDZ basis set54,55 was employed subsequently for all atoms in single-point calculations (to refine the energetics), except for Ga atoms that were described with the same ECP and valence basis used in the geometry optimization. The geometry was optimized with Grimme’s a posteriori, semi-empirical D2 dispersion correction,56 which was applied in all of the calculations to capture the lateral interactions between adsorbates more accurately. We used the same mixed implicit-explicit solvation approach as justified in previous work,26,39 which consists of the SMD implicit solvation model36 coupled with a full monolayer of half-dissociated water molecules adsorbed on the cluster surface. This adsorbed water monolayer configuration was demonstrated to be the most thermodynamically


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stable, based on previous computational and experimental characterization of the GaP (110)/water interface.28,29 We refer the reader to our group’s previous studies26,39 for further details and validation of this mixed implicit-explicit solvation approach. We conducted solvated cluster model calculations to obtain the free energy diagrams of selected reactions occurring on the GaP(110) surface. The solvated cluster contains 24 Ga atoms, 24 P atoms, and 40 H atoms. The H atoms in the solvated cluster were used to passivate the dangling bonds at the edges of the cluster. The same model had been validated in our group’s earlier work.57 There are two possible isomers (ortho/para) of 2-PyH–*; here, we only considered the ortho position for the hydride addition to Py* for simplicity, based on previous discussions regarding o-DHP* vs. p-DHP*.20,30,57,58 Frequency calculations were performed to verify that both the optimized initial-state (IS) and final-state (FS) structures are local minima and that the transition-state (TS) structures exhibit a single imaginary mode. TS structures were found by using the eigenvector-following method implemented in ORCA. Because the eigenvector-following method is only appropriate for a local optimization of a TS, the drag method was employed first to scan the energy surface along a postulated reaction pathway, which is typically some simultaneous chemical bond-breaking and bond-forming, starting from a fully relaxed IS. We chose the atomic structure with the maximum energy along the scanned energy surface as the initial guess for the TS optimization executed by the eigenvector-following method. Once the TS optimization was converged, we conducted a frequency calculation to verify that the converged TS structure exhibits only a single, non-trivial imaginary mode. Lastly, the TS structure was relaxed along the TS imaginary mode to obtain the IS and FS structures to which it is connected. This new IS structure then was compared to the original IS structure used for the energy surface scan to further verify our TS optimization. We did


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not do the comparison for the FS structure because the energy surface scan starts from a postulated fully relaxed IS instead of a FS. The FS geometry was produced by relaxing along the TS imaginary mode (toward the FS). Multiple imaginary frequencies below 100i cm-1 were present in some of the IS, TS, or FS structures. These modes, with such small imaginary frequencies, corresponded to rotations of the H2O molecules in the explicit solvation adlayer or in the solvent region (hydrogen-bonded to the CO2 reduction intermediates) and were not involved in any of the reaction processes. We could not remove these imaginary modes despite numerous attempts to refine the IS/TS/FS structure. The corresponding influence on the system’s energy is smaller than 10 meV. Therefore, all imaginary modes with frequencies below 100i cm-1 were ignored and considered to be numerical noise in our kinetics calculations. For the FS of HCOOHsol + 2-PyH–* / CH2(OH)2 sol + 2-PyH–*, there was an imaginary mode with a frequency of 258.6i cm-1/244.0i cm-1 that could not be removed despite numerous attempts to refine the FS structure. These two imaginary modes were not associated with HT to HCOOH or CH2(OH)2. The corresponding influence on the system’s energy is smaller than 20 meV, based on our attempted calculations for removal of these two imaginary modes, so this minor issue of the FS structural relaxation does not affect any of our results or conclusions drawn. We calculated zero-point energy (ZPE), thermal corrections, and entropy terms (at T = 298.15 K) based on the harmonic oscillator approximation, i.e., only including vibrational contributions to the free energy. Because the cluster is meant to represent a semi-infinite crystal surface, the atoms of the adsorbed species and surface cluster do not have rotational and translational degrees of freedom. We also ignored the rotational and translational contributions of the non-adsorbed species to the free energy. This “super-molecule (SM) approach” has the advantage that the TS is well connected to the IS and FS, thereby offering a more rigorous way of


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computing the activation energetics. In contrast, a “separate-reactants (SR) approach,” where the energy of each individual reactant species is calculated separately, considers the rotational and translational free energies of the non-adsorbed species. However, we lose the reasonable connection of the IS to the TS in this SR approach. Our group’s previous study26 tested the SR approach and compared the free energies and activation energies to those of the SM approach. The energies given by these two approaches differ by only 0.17 eV (at most), thus suggesting that neglecting rotational and translational contributions from non-adsorbed species does not significantly change the results. The SM approach therefore was employed to compute free energy changes (Gs) and activation barriers (G╪s) in this study to maintain the feature that the IS and FS are well connected to the TS. Our recent work30 demonstrated that only the configuration of 2-PyH–* situated next to an OH–* has a sufficient lifetime to transfer hydride to CO2. Thus, the 2-PyH–*-OH–* configuration was employed in this work to conduct the kinetic barrier calculations.

Data availability Cartesian coordinates of all the IS/TS/FS structures in the simulated HT reactions are available at XXXX.ZIP. Supporting information is available at DOI: XXXX including: free energy diagrams of the kinetically hindered HT steps, atomic structures of HCOOH adsorption on the reconstructed CdTe(111) surface, calculation setup information, and the free energy diagram for HCOOH* + 2PyH–*  CH2(OH)O–* + Py* with three explicit H2O molecules hydrogen-bonded with HCOOH*.

Acknowledgements


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The authors acknowledge financial support from the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0254. We thank Dr. John Mark P. Martirez and Ms. Nari Baughman for helping to edit this manuscript.


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Table of Contents -

H

N

H

surface

G ╪ = 0.33-0.59 eV

H H

HT

H T, PT

H T, PT

CO 2

N

surface

HCOO H

G ╪ = 0.23-0.36 eV

CH 2(OH) 2 dehydr ation G ╪ = 0.58 eV

Final Pr oduct

H T, PT

G ╪ < 0.21 eV

CH 4

HT

CH 3OH

or

CH 2O

-

H

N

H

surface

HT


27

Why and How Carbon Dioxide Conversion to Methanol Happens on

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