Ab initio molecular dynamics of solvation effects on …2016/08/05  · Ab initio molecular dynamics...

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Ab initio molecular dynamics of solvation effects on reactivity at electrified interfaces Jeffrey A. Herron a , Yoshitada Morikawa b,1 , and Manos Mavrikakis a,1 a Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, WI 53706; and b Division of Precision Science & Technology and Applied Physics, Osaka University, Osaka 565-0871, Japan Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved June 29, 2016 (received for review March 20, 2016) Using ab initio molecular dynamics as implemented in periodic, self-consistent (generalized gradient approximation PerdewBurkeErnzerhof) density functional theory, we investigated the mechanism of methanol electrooxidation on Pt(111). We investi- gated the role of water solvation and electrode potential on the energetics of the first proton transfer step, methanol electrooxi- dation to methoxy (CH 3 O) or hydroxymethyl (CH 2 OH). The results show that solvation weakens the adsorption of methoxy to un- charged Pt(111), whereas the binding energies of methanol and hydroxymethyl are not significantly affected. The free energies of activation for breaking the C-H and O-H bonds in methanol were calculated through a Blue Moon Ensemble using constrained ab initio molecular dynamics. Calculated barriers for these elementary steps on unsolvated, uncharged Pt(111) are similar to results for climbing-image nudged elastic band calculations from the litera- ture. Water solvation reduces the barriers for both C-H and O-H bond activation steps with respect to their vapor-phase values, although the effect is more pronounced for C-H bond activation, due to less disruption of the hydrogen bond network. The calcu- lated activation energy barriers show that breaking the C-H bond of methanol is more facile than the O-H bond on solvated nega- tively biased or uncharged Pt(111). However, with positive bias, O-H bond activation is enhanced, becoming slightly more facile than C-H bond activation. AIMD | DFT | methanol | Pt(111) | electrocatalysis M ethanol presents a promising fuel alternative to hydrogen for low-temperature polymer electrolyte membrane fuel cells (1, 2). However, the slower reaction kinetics, compared with hydrogen, requires high overpotentials, which reduces the overall energy efficiency of these fuel cells. Additionally, CO molecules that are formed during the reaction poison Pt catalysts, limiting their activity (2). To address this problem, improved electro- catalysts must be developed (3, 4). Toward this goal, it is essential to understand the sequence of elementary steps that comprise the reaction mechanism. Through knowledge of the detailed mech- anism on a particular catalyst, one can determine the rate- determining step(s). In turn, this information can be used to de- sign improved catalysts that are targeted toward facilitating the rate-determining step. First-principles density functional theory (DFT) calculations have become invaluable for working toward understanding these detailed reaction mechanisms at the atomic scale. In particular, these methods have successfully been used to calculate the thermochemistry (reaction energies) and kinetics (activa- tion energy barriers) of a variety of different catalytic reactions in the vapor phase. However, modeling electrocatalytic reactions poses a greater challenge due to the complexity of the reaction environment at the active site. That is, the reaction occurs on a charged substrate that is surrounded by solvent (and elec- trolyte). A variety of methods have been developed to model the effects of solvent and charged surfaces on the thermo- chemistry and kinetics of electrocatalytic reactions (513), and the strengths of these approaches were summarized in a recent publication (13). Although these methods have made advances toward model- ing this complicated active site, one of the many deficiencies is the inability to capture the effect of dynamic solvent reorganization (14). Here, we use ab initio (constrained) molecular dynamics (AIMD) to study the effect of water solvent reorganization and substrate charge toward methanol electrooxidation on a model Pt surface, Pt(111). In particular, we use a Blue Moon Ensemble (1520) to evaluate the free energy of activation for breaking the CH and OH bonds in methanol. This systematic study applies AIMD to a heterogeneously catalyzed reaction accounting for both sol- vent and charged surfaces. Results and Discussion In this section, we report the results of our AIMD simulation. The results are organized into three sections. First, we discuss calculated reaction thermochemistry and binding energies of ad- sorbates on uncharged, solvated Pt(111). Then, we present cal- culated activation energy barriers using the Blue Moon Ensemble for uncharged, solvated Pt(111). Finally, we present activation energy barriers for the same elementary steps on a charged, sol- vated Pt(111) surface using the Blue Moon Ensemble. Reaction Thermochemistry on Uncharged, Solvated Pt(111). First, we calculated the minimum energy binding geometries and the corresponding binding energies of methanol, methoxy (OCH 3 ), and hydroxymethyl (CH 2 OH) on Pt(111) in the absence of ad- ditional charge or solvation. The equation for the binding energy (BE) was BE = E adsorbate E clean E isolated , where E adsorbate is the total energy of the adsorbate and surface when the adsorbate is adsorbed, E clean is the total energy of the clean slab, and E isolated is the total energy of the isolated gas-phase adsorbate. We found Significance Low-temperature fuel cells are efficient energy conversion devices that face a number of hurdles toward commercializa- tion, including difficulties in storing hydrogen. Methanol rep- resents a liquid-phase fuel alternative to hydrogen, yet the high cost of Pt-based catalysts limits fuel cellseconomic viability. Toward improved, lower-cost catalyst design, a fundamental understanding of the methanol electrooxidation reaction mech- anism is necessary. Density functional theory calculations have become invaluable in elucidating these reaction mechanisms, although the complex reaction environment including solvation of a charged electrode has been a challenge to model. Using ab initio molecular dynamics, via the Blue Moon Ensemble, we have investigated methanol electrooxidation on a solvated and charged Pt(111) surface to understand the effect of solvation and charge on the reaction energetics. Author contributions: Y.M. and M.M. designed research; J.A.H. performed research; J.A.H., Y.M., and M.M. analyzed data; and J.A.H., Y.M., and M.M. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. 1 To whom correspondence may be addressed. Email: [email protected] or morikawa@ prec.eng.osaka-u.ac.jp. www.pnas.org/cgi/doi/10.1073/pnas.1604590113 PNAS Early Edition | 1 of 9 ENGINEERING PNAS PLUS Downloaded by guest on December 19, 2020

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Page 1: Ab initio molecular dynamics of solvation effects on …2016/08/05  · Ab initio molecular dynamics of solvation effects on reactivity at electrified interfaces Jeffrey A. Herrona,

Ab initio molecular dynamics of solvation effects onreactivity at electrified interfacesJeffrey A. Herrona, Yoshitada Morikawab,1, and Manos Mavrikakisa,1

aDepartment of Chemical and Biological Engineering, University of Wisconsin–Madison, Madison, WI 53706; and bDivision of Precision Science & Technologyand Applied Physics, Osaka University, Osaka 565-0871, Japan

Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved June 29, 2016 (received for review March 20, 2016)

Using ab initio molecular dynamics as implemented in periodic,self-consistent (generalized gradient approximation Perdew–

Burke–Ernzerhof) density functional theory, we investigated themechanism of methanol electrooxidation on Pt(111). We investi-gated the role of water solvation and electrode potential on theenergetics of the first proton transfer step, methanol electrooxi-dation to methoxy (CH3O) or hydroxymethyl (CH2OH). The resultsshow that solvation weakens the adsorption of methoxy to un-charged Pt(111), whereas the binding energies of methanol andhydroxymethyl are not significantly affected. The free energies ofactivation for breaking the C−H and O−H bonds in methanol werecalculated through a Blue Moon Ensemble using constrained abinitio molecular dynamics. Calculated barriers for these elementarysteps on unsolvated, uncharged Pt(111) are similar to results forclimbing-image nudged elastic band calculations from the litera-ture. Water solvation reduces the barriers for both C−H and O−Hbond activation steps with respect to their vapor-phase values,although the effect is more pronounced for C−H bond activation,due to less disruption of the hydrogen bond network. The calcu-lated activation energy barriers show that breaking the C−H bondof methanol is more facile than the O−H bond on solvated nega-tively biased or uncharged Pt(111). However, with positive bias,O−H bond activation is enhanced, becoming slightly more facilethan C−H bond activation.

AIMD | DFT | methanol | Pt(111) | electrocatalysis

Methanol presents a promising fuel alternative to hydrogenfor low-temperature polymer electrolyte membrane fuel

cells (1, 2). However, the slower reaction kinetics, compared withhydrogen, requires high overpotentials, which reduces the overallenergy efficiency of these fuel cells. Additionally, CO moleculesthat are formed during the reaction poison Pt catalysts, limitingtheir activity (2). To address this problem, improved electro-catalysts must be developed (3, 4). Toward this goal, it is essentialto understand the sequence of elementary steps that comprise thereaction mechanism. Through knowledge of the detailed mech-anism on a particular catalyst, one can determine the rate-determining step(s). In turn, this information can be used to de-sign improved catalysts that are targeted toward facilitating therate-determining step.First-principles density functional theory (DFT) calculations

have become invaluable for working toward understandingthese detailed reaction mechanisms at the atomic scale. Inparticular, these methods have successfully been used to calculatethe thermochemistry (reaction energies) and kinetics (activa-tion energy barriers) of a variety of different catalytic reactionsin the vapor phase. However, modeling electrocatalytic reactionsposes a greater challenge due to the complexity of the reactionenvironment at the active site. That is, the reaction occurs ona charged substrate that is surrounded by solvent (and elec-trolyte). A variety of methods have been developed to modelthe effects of solvent and charged surfaces on the thermo-chemistry and kinetics of electrocatalytic reactions (5–13), andthe strengths of these approaches were summarized in a recentpublication (13).

Although these methods have made advances toward model-ing this complicated active site, one of the many deficiencies isthe inability to capture the effect of dynamic solvent reorganization(14). Here, we use ab initio (constrained) molecular dynamics(AIMD) to study the effect of water solvent reorganization andsubstrate charge toward methanol electrooxidation on a model Ptsurface, Pt(111). In particular, we use a Blue Moon Ensemble (15–20) to evaluate the free energy of activation for breaking the C−Hand O−H bonds in methanol. This systematic study applies AIMDto a heterogeneously catalyzed reaction accounting for both sol-vent and charged surfaces.

Results and DiscussionIn this section, we report the results of our AIMD simulation.The results are organized into three sections. First, we discusscalculated reaction thermochemistry and binding energies of ad-sorbates on uncharged, solvated Pt(111). Then, we present cal-culated activation energy barriers using the Blue Moon Ensemblefor uncharged, solvated Pt(111). Finally, we present activationenergy barriers for the same elementary steps on a charged, sol-vated Pt(111) surface using the Blue Moon Ensemble.

Reaction Thermochemistry on Uncharged, Solvated Pt(111). First, wecalculated the minimum energy binding geometries and thecorresponding binding energies of methanol, methoxy (OCH3),and hydroxymethyl (CH2OH) on Pt(111) in the absence of ad-ditional charge or solvation. The equation for the binding energy(BE) was BE=Eadsorbate −Eclean −Eisolated, where Eadsorbate is thetotal energy of the adsorbate and surface when the adsorbate isadsorbed, Eclean is the total energy of the clean slab, and Eisolated isthe total energy of the isolated gas-phase adsorbate. We found

Significance

Low-temperature fuel cells are efficient energy conversiondevices that face a number of hurdles toward commercializa-tion, including difficulties in storing hydrogen. Methanol rep-resents a liquid-phase fuel alternative to hydrogen, yet the highcost of Pt-based catalysts limits fuel cells’ economic viability.Toward improved, lower-cost catalyst design, a fundamentalunderstanding of the methanol electrooxidation reaction mech-anism is necessary. Density functional theory calculations havebecome invaluable in elucidating these reaction mechanisms,although the complex reaction environment including solvationof a charged electrode has been a challenge to model. Using abinitio molecular dynamics, via the Blue Moon Ensemble, we haveinvestigated methanol electrooxidation on a solvated andcharged Pt(111) surface to understand the effect of solvationand charge on the reaction energetics.

Author contributions: Y.M. and M.M. designed research; J.A.H. performed research; J.A.H.,Y.M., and M.M. analyzed data; and J.A.H., Y.M., and M.M. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence may be addressed. Email: [email protected] or [email protected].

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generalized gradient approximation Perdew–Burke–Ernzerhof(21) (GGA-PBE) binding energies of −0.09 eV (methanol, phys-isorbed), −1.58 eV (OCH3, top site), and −2.10 eV (CH2OH, topsite), for the minimum energy structure of each species. Notethat we found that CH2OH is more stable than OCH3 in the gasphase by 0.27 eV, and it is more stable by 0.80 eV on Pt(111), inthe absence of solvation. These values are compared with ex-perimental (22) and theoretical (23, 24) results from the liter-ature in Table 1.We then explored how solvation in water affects these binding

energies. To do this, we performed unconstrained AIMD simu-lations of the adsorbate over the Pt(111) surface that is surroundedby four layers of water. To maintain the density in the aqueousphase close to 1 g/cm3, we removed a single water molecule (cor-responding to 15 water molecules) when adding an adsorbate. Wecalculated the average energy of the slab with an adsorbate and 15water molecules (hEadsorbatei), and the slab with 16 water molecules(hEcleani). The average binding energy (hB.E. i) was calculated

from the following equation: hB.E. i= hEadsorbatei− hEcleani−Eisolated + μH2O, where μH2O is the chemical potential of water, andEisolated is the total energy of the isolated gas-phase adsorbate. Thechemical potential of water was calculated from bulk water viaAIMD (2-ps trajectory) using a 9.85 Å × 9.85 Å × 9.85 Å cubicsupercell with 32 water molecules (1.00 g/cm3 density). For ref-erence, from our simulation we derived a chemical potential forbulk water of −0.67 eV.The ultimate goal of our study is to understand how solvation

and surface charge affect adsorption and reactions on the sur-face. Therefore, we tested the convergence of the average en-ergy, with respect to averaging time length, using CH2OH* with15 water molecules on Pt(111) with 0.15 e− in excess charge inthe surface (Fig. 1). Previous studies have found that, by addingor subtracting, this electron density corresponds to a potentialrange of ±0.4 VSHE (25). This calculation was chosen, ratherthan a clean surface with 16 H2O molecules, to ensure that boththe adsorbate reorganization on the surface and water dynamicswere converged. From the results, we see that, as expected, theoscillations in the average energy are dampened as the averagingtime length is increased. Averaging the total energy over a 2-pssimulation (after allowing the simulation to equilibrate, i.e., lo-cate the local minima) was deemed sufficient because the cal-culated average energy fluctuated by plus or minus ∼0.1 eVaround the more stable average obtained with 3-ps averagingtime. For other calculations, we used the equilibrated watergeometry obtained here as an initial guess to reduce theequilibration time.Following this procedure, we calculated the average bind-

ing energies of CH3OH, CH2OH, and OCH3 as shown in Ta-ble 1. The corresponding AIMD trajectories are shown in Fig. 2.From these results, we see that the binding energies of CH3OHand CH2OH on uncharged Pt(111), in the aqueous phase, aresimilar to the respective ones in the vapor-phase case. On theother hand, OCH3 is significantly destabilized (by 0.58 eV)in the aqueous phase. These differences are mainly due tothe binding geometries and chemical functionalities of theseadsorbates. In Fig. 3A, we show the optimized geometries ofCH3OH, OCH3, and CH2OH on uncharged Pt(111), and in

Fig. 1. Convergence of average energy of CH2OH* (hECH2OHi) with 15 H2O molecules on Pt(111) with 0.15 e− excess charge (per six surface Pt atoms) included.Running average of total energy (hECH2OHi) over a varying length of averaging time, from 1 ps to 3.5 ps (see figure key) is shown. The time coordinate for theaverage energy corresponds to the point in simulation time from which the averaging window begins (e.g., for an averaging length of 1 ps, the average thatis shown at 500 fs corresponds to the average from 500 fs to 1,500 fs). The total energy from AIMD simulation is shown in the dotted line.

Table 1. Average (GGA-PBE) binding energies (<B.E.>) ofCH3OH, CH2OH, and OCH3 on solvated, uncharged Pt(111)derived from AIMD simulation

Parameter

Vapor phase

This work LiteratureAqueous phase,

uncharged

<B.E.> CH3OH, eV −0.09 −0.33*, −0.61† −0.14<B.E.> CH2OH, eV −2.10 −1.98*, −1.93† −2.09<B.E.> OCH3, eV −1.58 −1.54* −1.00<ΔE> CH3OH → CH2OH + H −0.04 −0.16* 0.02<ΔE> CH3OH → OCH3 + H 0.76 0.62*, 0.59† 1.53

Included is thermochemistry (<ΔE>) for CH3OH dehydrogenation toCH2OH and OCH3 on uncharged Pt(111) with hydrogen balance from isolatedH2(g). Vapor-phase binding energies on uncharged Pt(111) are presentedalong with previous theoretical and experimental results for comparison.*Perdew–Wang 91 (PW91) GGA DFT calculations from refs. 23 and 24.†Microcalorimetry experiments from ref. 22.

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Fig. 3B, we show representative snapshots from the AIMD tra-jectories of the respective species. In the vapor phase, CH3OHbinds with the CH3 group oriented toward the Pt(111) surfacewith the C–O bond axis 56° from the surface normal. The O–Hbond is pointed toward the surface. Interestingly, in the aqueousphase, the C–O bond axis is nearly parallel with the surface, andthe O–H bond is oriented toward the surface, rather than toward

the aqueous phase. A neighboring water molecule has its O–Hbond pointing toward the O atom of CH3OH, forming a hy-drogen bond. In the vapor phase, CH2OH is bound to a top siteof Pt(111) through its C atom, the C–O bond axis is 68° awayfrom the surface normal, and the O–H bond is oriented parallelto the surface. In the aqueous phase, the geometry is not sig-nificantly changed. The O–H bond is oriented slightly away fromthe surface, toward a neighboring water molecule, and the O–Hbond of a neighboring water molecule is pointed toward the Oatom, like with CH3OH. In the vapor phase, OCH3 is bound tothe top site on Pt(111) through its O atom and the C–O bondoriented 57° with respect to the surface normal, exposing theCH3 group to the environment. In the aqueous phase, theequilibrated binding geometry is on the top site, where the C–Obond is oriented nearly parallel to the surface, like in the vaporphase. However, unlike CH2OH, there are no hydrogen-bondedwater species. Therefore, the differences in binding energytrends due to water solvation are because of stabilizing hy-drogen bonding in CH3OH and CH2OH, whereas OCH3 isnot stabilized.From the calculated binding energies, we evaluated the ther-

mochemistry,hΔEi, for dehydrogenation of CH3OH to CH2OHand to OCH3. The hydrogen balance was taken from vapor-phase H2, i.e., CH3OH → CH2OH + 1/2H2 and CH3OH →OCH3 + 1/2H2. Therefore, the thermochemistry includes onlysolvation for methanol, methoxy, and hydroxymethyl species. Forexample, the hΔEi for methanol dehydrogenation to hydoxymethylis given by hΔEi= hB.E. CH2OHi− hB.E. CH3OHi+Eisolated CH2OH +ð1=2ÞEisolated H2 −Eisolated  CH3OH, where Eisolated CH2OH, Eisolated CH3OH,and Eisolated H2 are the energies of isolated vapor-phaseCH2OH, CH3OH, and H2, respectively. Substituting our defi-nition of binding energy into this equation, the overall result ishΔEi= hECH2OHi− hECH3OHi+ ð1=2ÞEisolated H2. As a result, the

Fig. 3. (A) Vapor phase—the optimized binding geometries of CH3OH, CH2OH, and OCH3 on uncharged Pt(111). (B) Aqueous phase—representativesnapshots from AIMD trajectory of equilibrated CH3OH, CH2OH, and OCH3 on uncharged Pt(111). The snapshots were constructed by repeating the unit celltwice each in the x and y directions (2 × 2) to enhance clarity of the water network. The adsorbates are highlighted in red ellipses in the aqueous phase.

Fig. 2. Dynamic change of binding energies for CH3OH (black line), CH2OH(red line), and OCH3 (blue line) on uncharged solvated Pt(111). Averagebinding energies (hB. E. i) (averaged over 2 ps) of these adsorbates are shownwith dotted lines.

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only energies included are the average total energies takenfrom the AIMD simulations and the energy of isolated H2. Theresults are shown in Table 1.The reaction energies for CH3OH dehydrogenation to CH2OH

on solvated, uncharged Pt(111) are similar to the vapor-phaseenergetics because the binding energies of CH3OH and CH2OHare similar in the two phases. However, dehydrogenation to OCH3is significantly more endothermic in the aqueous phase, by 0.77 eV,due to the lack of stabilization of surface-adsorbed OCH3 by theaqueous phase.

Free Energies of Activation for Vapor-Phase Chemistry on CleanPt(111). First, we used constrained molecular dynamics toevaluate the free energies of activation for CH3OH de-hydrogenation on uncharged, Pt(111) without solvation usinga Blue Moon Ensemble (15–20) as described in detail inMethods. This is analogous to vapor-phase methanol de-hydrogenation. The chosen reaction coordinate involvesstretching of the C–H (to form CH2OH) or O–H (to formOCH3) bonds. The purpose of this calculation is to compare the

results versus the traditional nudged elastic band (NEB)method (26) and to isolate the effects of solvation, charge, andcalculation method.

Fig. 4. Constrained AIMD trajectories for CH3OH dehydrogenation to CH2OH on Pt(111) at (A) −0.4 VSHE with solvation, (B) 0.0 VSHE with solvation, (C) 0.4VSHE with solvation, and (D) uncharged and without solvation. (E) The mean constraint force derived from the trajectories is plotted versus the reactioncoordinate, C–H bond stretching.

Table 2. Free energies of activation (<EA>) for CH3OHdehydrogenation to CH2OH and OCH3 on uncharged, solvatedPt(111) derived from Blue Moon Ensemble via constraining C–Hand O–H bond lengths, respectively

Parameter

Vapor phase

This work LiteratureAqueous phase,

uncharged

<EA> CH3OH → CH2OH + H 0.60 0.67* 0.52<EA> CH3OH → OCH3 + H 0.71 0.81* 0.67

Vapor-phase results on unsolvated Pt(111) are calculated using the BlueMoon Ensemble for comparison purposes.*PW91 GGA DFT calculations from refs. 23 and 24. Note that product isadsorbed H rather than gas-phase H2.

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The constraint forces obtained from these AIMD simulationsas a function of bond length are shown in Figs. 4 and 5 for C–Hbond and O–H bond activation, respectively [we note that thesefigures also show the corresponding trajectories for the simulationsusing solvated and charged Pt(111), as will be described in FreeEnergies of Activation on Uncharged, Solvated Pt(111) and FreeEnergies of Activation on Charged, Solvated Pt(111). The corre-sponding activation energies are included in Table 2.CH3OH → CH2OH + H: Uncharged Pt(111) without solvation. The re-action coordinate resembles that of previous vapor-phase resultsusing more traditional NEB methods (26), although the transi-tion state occurs at a C–H bond length of 1.48 Å, whereas a tran-sition state bond length of 1.56 Å has been calculated in thevapor phase using the NEB method (23, 24). We find a freeenergy of activation of 0.60 eV (at 300 K), whereas the NEBmethod predicts an activation energy barrier of 0.67 eV (at 0 K)(23, 24). This shows that one is able to accurately calculateactivation energy barriers for surface phenomena using theBlue Moon Ensemble.CH3OH → OCH3 + H: Uncharged Pt(111) without solvation.As with C–Hbond activation, the O–H bond activation reaction coordinate re-

sembles that of previous vapor-phase results using NEB method(23, 24). However, the transition state O–H bond length is 0.18 Åshorter than what has been reported using the NEB method (23,24). The vapor-phase barrier from the NEB method has beencalculated as 0.81 eV (23, 24), 0.10 eV more than our calculatedvapor-phase barrier using the AIMD approach.

Free Energies of Activation on Uncharged, Solvated Pt(111).We usedconstrained molecular dynamics to evaluate the free energies ofactivation for CH3OH dehydrogenation on charged, solvatedPt(111) using a Blue Moon Ensemble (15–20), as describedin detail in Methods. The chosen reaction coordinate involvesstretching of the C–H (to form CH2OH) or O–H (to form OCH3)bonds; characteristic snapshots of the molecular geometry fromeach Blue Moon simulation are shown in Figs. 6 and 7, respectively.CH3OH → CH2OH + H: Solvated, uncharged Pt(111). When the C–Hbond has been stretched from its equilibrium bond length of1.06 Å to 1.22 Å, the carbon atom interacts weakly with thePt(111) surface. (Characteristic snapshots from the constrainedAIMD trajectories are shown in Fig. 6.) The CH3OH moleculeremains close to the surface, while the orientation of the C–H

Fig. 5. Constrained AIMD trajectories for CH3OH dehydrogenation to OCH3 on Pt(111) at (A) −0.4 VSHE with solvation, (B) 0.0 VSHE with solvation, (C) 0.4 VSHE

with solvation, and (D) uncharged and without solvation. (E) The mean constraint force derived from the trajectories is plotted versus the reaction coordinate,O–H bond stretching.

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bond fluctuates considerably, that is, there is near-free rotationof the C–H bond axis. As the bond elongates to 1.37 Å, the Catom is positioned above a top site of Pt(111) with the elongatedC–H bond oriented toward the top site. The C–O bond axisfluctuates from nearly parallel to the surface to almost perpen-dicular with the O–H bond hydrogen-bonded with the neigh-boring water molecules. When the C–H bond has been stretchedto 1.53 Å, we are beyond the transition state. The C atom ofCH2OH is bound to the top site of Pt(111) with the elongatedC–H bond pointed toward a neighboring bridge site, where theH atom is bound. Based on linear interpolation of the calcu-lated mean constraint force, the transition state (where themean constraint force crosses from negative to positive; Fig. 4E)corresponds to a C–H bond length of 1.46 Å and an activationenergy barrier of 0.52 eV.The reaction coordinate resembles that of previous vapor-

phase results using more traditional NEB methods (26), althoughthe transition state in the aqueous phase occurs at a C–H bondlength that is ∼0.10 Å shorter (at 1.56 Å in the vapor phase usingthe NEB method) and with a barrier that is 0.15 eV less activated(24). Our Blue Moon Ensemble calculations on a Pt(111) surfacewithout any solvation identified a transition state bond length of1.48 Å. Therefore, the differences in bond lengths between theaqueous phase Blue Moon results and the vapor-phase NEB

results are likely due to differences in the specific computationalmethodologies rather than being an effect of solvation. Com-paring the Blue Moon Ensemble vapor-phase result with thesolvated result, we conclude that the activation energy barrierfor H–CH2OH activation is reduced by 0.08 eV as a result ofaqueous solvation.CH3OH → OCH3 + H: Solvated, uncharged Pt(111). The equilibriumO–H bond length in CH3OH is 0.95 Å. As the bond is stretchedslightly, to 1.11 Å, the molecule remains close to the Pt(111)surface, the O–H bond is oriented away from the surface, andhydrogen is bonded with water. (Characteristic snapshots fromthe constrained AIMD trajectories are shown in Fig. 7.) Thiscontinues up until the O–H bond is stretched to 1.43 Å, wherethe O atom becomes bound to the top site of Pt(111). Here, theO–H bond is oriented toward a neighboring top site. As the bondelongates further, the O atom binds to the top site with the O–Hbond oriented toward a neighboring top site, although the Hatom becomes bound to that neighboring top site rather than tothe bridge site between the two top sites. Through linear in-terpolation of the mean constraint force, we estimate that thetransition state corresponds to an O–H bond length of 1.58 Åwith an activation energy barrier of 0.67 eV.Again, the reaction coordinate resembles that of previous vapor-

phase results using NEB methods (26), although the transition

Fig. 6. Characteristic snapshots from constrained AIMD simulations of H–CH2OH bond breaking reaction on uncharged, solvated Pt(111) at fixed C–H bondlengths of 1.22 Å, 1.37 Å, and 1.53 Å. The snapshots were constructed by repeating the unit cell twice each in the x and y directions (2 × 2) to enhance clarity ofthe water network. Each snapshot shows approximately one unit cell length in the x direction (three Pt atoms wide). A second methanol molecule in eachimage was removed to improve clarity. The orientation of the camera with respect to the surface is approximately the same for each of these images. Thestretched H–CH2OH molecule is highlighted in a red circle in each image, with the constrained H highlighted in yellow.

Fig. 7. Characteristic snapshots from constrained AIMD simulations of CH3O–H bond-breaking reaction on uncharged, solvated Pt(111) at fixed O–Hbond lengths of 1.11 Å, 1.27 Å, 1.43 Å, and 1.59 Å. The snapshots were constructed by repeating the unit cell twice each in the x and y directions (2 × 2) toenhance clarity of the water network. Each snapshot shows approximately one unit cell length in the x direction (3 Pt atoms wide). The second methanolmolecule in each image was removed to improve clarity. The angle of orientation of the camera with respect to the surface normal in the latter threeimages is slightly increased with respect to the first image. The stretched CH3O–H molecule is highlighted in a red circle in each image, with the con-strained H highlighted in yellow.

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state occurs at a bond length that is ∼0.2 Å shorter (1.80 Å inthe vapor phase from the NEB method). Our vapor-phase re-sult using the Blue Moon Ensemble predicts a transition-statebond length that is only 0.02 Å longer than the aqueous phaseresult. Therefore, comparing the vapor-phase result calculatedusing the Blue Moon Ensemble versus the solvated result, wefind that solvation has little effect on the transition state bondlength. Additionally, we find that solvation lowers the free en-ergy of activation by only 0.04 eV (while the barrier for C–Hbond activation was lowered by 0.08 eV). The preferential low-ering of the C–H bond activation step suggests that water sol-vation tends to enhance selectivity toward the CH2OHdeprotonation product (as opposed to the CH3O deprotonationproduct). As the O–H bond is elongated, it becomes oriented to-ward the surface, rather than the solvent, breaking the hydrogen-bond interactions.

Solvation Center of Mass. To provide additional quantification tothis solvation effect, we calculated the time-averaged, z-coordinatecenter of mass of oxygen atoms from the surrounding watermolecules from the constrained AIMD simulations. The resultsare referenced relative to the z coordinate of the top layer of Ptatoms, as shown in Fig. 8. Additionally, we calculated the resultfrom the surface without a methanol molecule in the unit cell andfound a value of 8.40 Å. We note that, to maintain near-constantdensity in our simulations, when adding the methanol molecule,we removed one water molecule from our unit cell (leaving 15water molecules, corresponding to approximately four layers).Therefore, the exact numerical values in this analysis should beread with caution. The z coordinate of the oxygen atoms’ centerof mass provides an indication of how far the water moleculesare from the surface or surface-bound adsorbate. For adsorbatesthat present a hydrophobic group to the surrounding water, wewould expect the z coordinate to increase (i.e., the water mol-ecules move away from the surface). For adsorbates that presenta hydrophilic group to the surrounding water, we would expect adecrease in the z coordinate as the water molecules solvatethe adsorbate.For O–H bond activation, as the O–H bond is stretched from

1.11 Å to 1.27 Å, the z coordinate of the oxygen center of massincreases from 8.34 Å to 9.17 Å as the water molecules moveaway from the surface-bound, hydrophobic methyl group. Thez coordinate of the center of mass increases slightly more, by0.12 Å, as the O–H bond is stretched past the transition state.For comparison, we calculated an equilibrium value of 9.89 Åfrom the unconstrained AIMD simulations of methoxy adsorbedon solvated uncharged Pt(111) (note that there is one fewerhydrogen atom in the binding energy simulations).For C–H bond activation, as the C–H bond is stretched to 1.37 Å,

the z coordinate increases to 9.00 Å. Then, as the bond isstretched beyond the transition state (1.46 Å), the CH2 group isbound closer to the surface and the center of mass of the oxygenatoms moves closer to the surface, at 8.62 Å. From our uncon-strained AIMD simulations of CH2OH adsorbed on solvateduncharged Pt(111), an equilibrium center of mass coordinateof 8.65 Å is found. We note that this value is much closer to ourconstrained AIMD result than the CH3O–H activation result(although we have not extended our constrained AIMD simu-lations past the transition state all of the way to the local minimafinal states, as they are in the unconstrained simulations).Importantly, these results show that the center of mass of

oxygen is closer to the surface-bound CH2OH species than to theOCH3 species, except at very small stretching of the constrainedbond. Based on linear interpolation of the results presented inFig. 8, at the transition states (1.46 Å for H–CH2OH bond ac-tivation and 1.58 Å for CH3O–H bond activation), the center ofmass of oxygen is much closer to the surface for C–H bond ac-tivation than O–H bond activation.

Overall, we find that breaking the C–H bond is more facilethan breaking the O–H bond in methanol on solvated unchargedPt(111). The transition state for breaking the C–H bond occursafter the C–H bond is stretched by 0.39 Å, whereas the O–Hbond must be stretched by 0.50 Å. The surrounding aqueousenvironment stabilizes the reaction coordinates for both of theseelementary steps. The barrier for C–H bond activation is 0.08 eVless with solvation, whereas O–H bond activation is only 0.04 eVless with solvation; the reason for this is that the OH group ofCH2OH remains hydrogen-bonded throughout the elementarystep. In contrast, the water stabilization of OCH3 diminishesonce the O–H bond is stretched and O binds to the Pt surface.Our calculated oxygen center of mass for the surrounding watermolecules relative to the Pt demonstrates that the water mole-cules move farther away from the surface as the O–H bond isincreasingly stretched, whereas the water molecules do moveaway during C–H stretching, but only at intermediate stretchinglevels, coming back closer to the surface after the C–H bond inCH3OH is broken.

Free Energies of Activation on Charged, Solvated Pt(111). Next,we used constrained molecular dynamics to evaluate thefree energies of activation for CH3OH dehydrogenation oncharged, solvated Pt(111) using a Blue Moon Ensemble. ThePt(111) surface was charged by adding or subtracting 0.15 e−

to the surface. Previous studies have found that, by adding(or subtracting), this electron density corresponds to a potentialrange of −0.4 VSHE (+0.4 VSHE when removing electron density)(25). The chosen reaction coordinate involves stretching ofthe C–H (to form CH2OH) or O–H (to form OCH3) bonds.The reaction coordinates for the two elementary steps stud-ied, with a charged surface, did not change significantly fromthe uncharged, solvated results (Figs. 6 and 7). The constraintforces obtained from these constrained AIMD simulations

Fig. 8. Time-averaged z-coordinate center of mass of oxygen atoms fromwater molecules (15 water molecules per unit cell) with respect to the Ptsurface as a function of constrained bond length, calculated from con-strained AIMD simulations on uncharged Pt(111). Black (squares) shows theresults from stretching the O–H bond, and red (circles) shows the results fromstretching the C–H bond. As the z coordinate of the center of mass increases,water molecules move away from the surface and the surface-bound ad-sorbate, corresponding to a weakening of hydrogen bond stabilization of theadsorbate. For reference, the transition state bond length is 1.46 Å forH–CH2OH and 1.58 Å for CH3O–H. The green dashed line shows the oxygencenter of mass without methanol in the simulation (corresponding to 16 watermolecules in the unit cell).

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as a function of bond length (reaction coordinate) are shownin Figs. 4 and 5, respectively, and the corresponding activa-tion energies are provided in Table 3. We note that the valueat 0.0 VSHE corresponds to the uncharged results that werepresented in Free Energies of Activation on Uncharged, Sol-vated Pt(111).CH3OH → CH2OH + H: Charged, solvated Pt(111).We find that the freeenergy of activating the C–H bond in CH3OH to form CH2OH isnot significantly affected by the electrode potential. The barrieris ∼0.50 eV for all potentials probed, and the reaction coordinateis similar for the entire potential range we studied. Based on linearinterpolation of the calculated mean constraint force, the transi-tion state (where the mean constraint force crosses from negativeto positive; Fig. 4E) corresponds to a C–H bond length of 1.49 Åat −0.4 VSHE, 1.46 Å at 0.0 VSHE, and 1.46 Å at +0.4 VSHE.We note that these differences are likely within the error of ourcalculation.CH3OH→OCH3 + H: Charged, solvated Pt(111). Unlike the H–CH2OHbond activation, here we find that the activation energy forCH3O–H bond activation is lowered at positive potentials. Thecalculated activation energy barrier is 0.62 eV at −0.4 VSHE and0.67 eV at 0.0 VSHE, and decreases to 0.48 eV at +0.4 VSHE.Through linear interpolation of the mean constraint force, weestimate that the transition state corresponds to an O–H bondlength of 1.58 Å at −0.4 VSHE, 1.58 Å at 0.0 VSHE, and 1.59 Å at+0.4 VSHE. We note that these bond length differences arelikely within the error of our calculation. Therefore, althoughthe reaction coordinate does not change noticeably (northe transition state bond length), there is a ∼0.2-eV reductionin the activation energy barrier at positive potentials. In-terestingly, we find that on uncharged and negatively biasedPt(111), C–H bond activation is more facile than the O–Hbond activation. However, at +0.4 V, the barrier for O–H bondactivation becomes slightly lower than that for C–H bondactivation.We note that when either the C–H or O–H bonds are oriented

away from the surface and are constrained at a significant stretchfrom their equilibrium, often a neighboring water molecule willspontaneously dissociate during the AIMD simulation. The Hatom of the water binds to the strained methanol species, regen-erating the C–H or O–H bond. At the same time, the constrainedH atom binds to the dissociated water, regenerating its O–H bond.The end result is the formation of an unphysical, constrainedmethanol–water couple. This phenomena was found for bothuncharged and charged Pt(111). As a result, we were unable toobtain an activation energy barrier for dissociation of these C–Hor O–H bonds into the liquid phase directly (forming an H3Ospecies). Rather, we were only able to obtain results where theC–H or O–H bonds break to form a surface-bound H; thismay be the reason why we did not find a strong potential de-pendence for these barriers.

ConclusionsUsing AIMD simulations, we investigated the first elementarysteps for methanol electrooxidation in an aqueous environmenton Pt(111). In particular, we compared the effect of electrodepotential and solvation on the first proton/electron transferevent, electrooxidation of methanol to methoxy or hydrox-ymethyl. We found that for an uncharged, solvated Pt(111)surface, the binding energies of methanol and hydroxymethyl aresimilar to their vapor-phase values, but the binding of methoxy isdestabilized by ∼0.6 eV. As a result, the reaction thermochem-istry for methanol electrooxidation to methoxy is more endo-thermic in the aqueous phase, whereas electrooxidation tohydroxylmethyl is approximately isoenergetic when comparingthe aqueous phase and vapor-phase results. Further, we calcu-lated the activation energy barriers for breaking the C–H andO–H bond of methanol using the constrained molecular dy-namics approach through a Blue Moon Ensemble in the vaporphase and in the aqueous phase [on both charged and unchargedPt(111)]. The vapor phase results calculated using the BlueMoon Ensemble provide activation energy barriers that aresimilar to traditional static NEB methods. Furthermore, we findthat solvation on uncharged Pt(111) lowers the barrier for C–Hbond activation by 0.08 eV, while only lowering the barrier forO–H bond activation by 0.04 eV. For both the vapor-phase andaqueous phase results, C–H bond activation is more facile. Whencharging the surface, we find that C–H bond activation remainsmore facile than O–H bond activation with negative bias, but,with positive bias, O–H bond activation becomes slightlymore facile.

MethodsAll calculations in this work were performed using planewave DFT asimplemented in the Simulation Tool for Atom Technology (STATE) (27, 28).The Pt(111) surface is represented using a periodic 3 × 2 unit cell with threeatomic layers. The optimized Pt lattice constant of 3.949 Å was used, which isin close agreement with the experimental value (29), 3.92 Å. The ionic coreswere described using ultrasoft Vanderbilt pseudopotentials (30, 31). Thewave functions and the augmentation charge were expanded by a planewave basis set with the cutoff energies of 25 Ry and 225 Ry, respectively.GGA-PBE was used for the exchange–correlation energy functional. Thesurface Brillouin zone was sampled with a 2 × 4 × 1 Monkhorst–Pack k-pointmesh (32). The first-order Methfessel–Paxton scheme was used to deal withthe Fermi level (33). The smearing width was set to 0.054 eV.

To simulate the charged surface with solvation, the “effective screeningmedium” method was applied (14). In the surface-normal direction (in the zaxis), a semiinfinite continuum with an infinite dielectric constant, i.e., aclassical conductor, was located at z1 = ∼18.4 Å away from the center of thePt atoms in the top layer of the surface (z = 0 Å), while another region (z <z1) was characterized by the dielectric constant of unity, i.e., the vacuummedium. In this region, we introduced 16 water molecules. To apply a po-tential bias, excess electrons per supercell were introduced, whereby thesame amount of charge with the opposite sign was induced at the surface ofthe classical conductor.

AIMD simulations were performed using STATE software (27, 28). The toptwo layers of surface atoms, along with all water molecules and adsorbates,were freely allowed to move, whereas the bottom layer of the slab wasfixed. An artificial boundary was placed above the surface and water mol-ecules, restricting their movement and maintaining the density at ∼1 g/cm3.In the molecular dynamics simulations, we have adopted the deuteriummass for H atoms to implement a longer time step (1.2 fs). Velocities werescaled to maintain the temperature around 300 K.

Constrained molecular dynamics were performed while constraining theC–H or O–H bond in methanol using the SHAKE (34) and RATTLE (35) al-gorithms. These bonds were stretched from their equilibrium bond lengthsin increments of 0.30 Bohr Radii (0.158 Å) and constrained to those bondlengths while performing AIMD: 1.2 fs time step, 300 K temperature. Themean constraint force (<Fc>) as a function of the constrained bond length(reaction coordinate) was evaluated from a 2-ps AIMD trajectory after equili-bration was established (at least a 300-fs period). To determine the free energyof activation, the <Fc> was integrated over the reaction coordinate using thetrapezoidal rule.

Table 3. Free energies of activation (<EA>) for CH3OHdehydrogenation to CH2OH and OCH3 on charged, solvatedPt(111) derived from Blue Moon Ensemble via constraining C–Hand O–H bond lengths, respectively

Parameter

Aqueous phase

−0.4 V 0.0 V 0.4 V

<EA> CH3OH → CH2OH + H 0.48 0.52 0.51<EA> CH3OH → OCH3 + H 0.62 0.67 0.48

Pt(111) surface was charged by adding or subtracting 0.15 e− to the sur-face. Previous studies have found that, by adding (or subtracting), this elec-tron density corresponds to a potential range of −0.4 VSHE (+0.4 VSHE whenremoving electron density) (34).

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ACKNOWLEDGMENTS. Work at University of Wisconson–Madison has beensupported by the US Department of Energy - Basic Energy Sciences (DOE-BES),Division of Chemical Sciences, Grant DE-FG02-05ER15731, and was initiatedby National Science Foundation East Asia and Pacific Summer Institutes TravelGrant 1014611. The computational work was performed, in part, using super-computing resources at the following institutions: the Environmental Molec-ular Sciences Laboratory (EMSL), a national scientific user facility at Pacific

Northwest National Laboratory (PNNL); the Center for Nanoscale Materials(CNM) at Argonne National Laboratory; and the National Energy ResearchScientific Computing Center (NERSC). EMSL is sponsored by the Departmentof Energy’s Office of Biological and Environmental Research located atPNNL. CNM and NERSC are supported by the US Department of Energy, Officeof Science, under Contracts DE-AC02-06CH11357 and DE-AC02-05CH11231,respectively.

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