用VASP计算的方法

This paper is published as part of Faraday Discussions volume 140: Electrocatalysis - Theory and Experiment at the Interface

Preface

Preface

Andrea E. Russell, Faraday Discuss., 2009 DOI:10.1039/b814058h

Introductory Lecture

Electrocatalysis: theory and experiment at the interface

Marc T. M. Koper, Faraday Discuss., 2009 DOI:10.1039/b812859f

Papers

The role of anions in surface electrochemistry

D. V. Tripkovic, D. Strmcnik, D. van der Vliet, V. Stamenkovic and N. M. Markovic, Faraday Discuss., 2009

DOI:10.1039/b803714k

From ultra-high vacuum to the electrochemical interface: X-ray scattering studies of model electrocatalysts Christopher A. Lucas, Michael Cormack, Mark E. Gallagher, Alexander Brownrigg, Paul Thompson, Ben Fowler, Yvonne Gründer, Jerome Roy, Vojislav Stamenkovi? and Nenad M. Markovi?, Faraday Discuss., 2009

DOI:10.1039/b803523g

Surface dynamics at well-defined single crystal microfacetted Pt(111) electrodes: in situ optical studies

Iosif Fromondi and Daniel Scherson, Faraday Discuss., 2009

DOI:10.1039/b805040f

Bridging the gap between nanoparticles and single crystal surfaces

Payam Kaghazchi, Felice C. Simeone, Khaled A. Soliman, Ludwig A. Kibler and Timo Jacob, Faraday Discuss., 2009

DOI:10.1039/b802919a

Nanoparticle catalysts with high energy surfaces and enhanced activity synthesized by electrochemical method

Zhi-You Zhou, Na Tian, Zhi-Zhong Huang, De-Jun Chen and Shi-Gang Sun, Faraday Discussion

General discussion

Faraday Discuss., 2009,

DOI:10.1039/b814699n

Papers

Differential reactivity of Cu(111) and

Cu(100) during nitrate reduction in acid electrolyte

Sang-Eun Bae and Andrew A. Gewirth,

Faraday Discuss., 2009

DOI:10.1039/b803088j

Molecular structure at electrode/electrolyte solution interfaces related to

electrocatalysis

Hidenori Noguchi, Tsubasa Okada and Kohei Uosaki, Faraday Discuss., 2009

DOI:10.1039/b803640c

A comparative in situ195Pt electrochemical-

NMR investigation of PtRu nanoparticles supported on diverse carbon nanomaterials Fatang Tan, Bingchen Du, Aaron L. Danberry,

In-Su Park, Yung-Eun Sung and YuYe Tong, Faraday Discuss., 2009

DOI:10.1039/b803073a

Spectroelectrochemical flow cell with temperature control for investigation of electrocatalytic systems with surface-

enhanced Raman spectroscopy

Bin Ren, Xiao-Bing Lian, Jian-Feng Li, Ping-

Ping Fang, Qun-Ping Lai and Zhong-Qun

Tian, Faraday Discuss., 2009

DOI:10.1039/b803366h

Mesoscopic mass transport effects in electrocatalytic processes

Y. E. Seidel, A. Schneider, Z. Jusys, B. Wickman, B. Kasemo and R. J. Behm,

Faraday Discuss., 2009

DOI:10.1039/b806437g

Discussion

General discussion

Faraday Discuss., 2009,

DOI:10.1039/b814700k

View Article Online / Journal Homepage / Table of Contents f

Papers

On the catalysis of the hydrogen oxidation E. Santos, Kay P?tting and W. Schmickler, Faraday Discuss., 2009

DOI:10.1039/b802253d

Hydrogen evolution on nano-particulate transition metal sulfides

Jacob Bonde, Poul G. Moses, Thomas F. Jaramillo, Jens K. N?rskov and Ib Chorkendorff, Faraday Discuss., 2009

DOI:10.1039/b803857k

Influence of water on elementary reaction steps in electrocatalysis

Yoshihiro Gohda, Sebastian Schnur and Axel Gro?, Faraday Discuss., 2009

DOI:10.1039/b802270d

Co-adsorbtion of Cu and Keggin type polytungstates on polycrystalline Pt: interplay of atomic and molecular UPD Galina Tsirlina, Elena Mishina, Elena Timofeeva, Nobuko Tanimura, Nataliya Sherstyuk, Marina Borzenko, Seiichiro Nakabayashi and Oleg Petrii, Faraday Discuss., 2009

DOI:10.1039/b802556h

Aqueous-based synthesis of ruthenium–selenium catalyst for oxygen reduction reaction

Cyril Delac?te, Arman Bonakdarpour, Christina M. Johnston, Piotr Zelenay and Andrzej Wieckowski, Faraday Discuss., 2009

DOI:10.1039/b806377j

Size and composition distribution dynamics of alloy nanoparticle electrocatalysts probed by anomalous small angle X-ray scattering (ASAXS)

Chengfei Yu, Shirlaine Koh, Jennifer E. Leisch, Michael F. Toney and Peter Strasser, Faraday Discuss., 2009

DOI:10.1039/b801586d

Discussion

General discussion

Faraday Discuss., 2009,

DOI:10.1039/b814701a

Papers

Efficient electrocatalytic oxygen reduction by the blue copper oxidase, laccase, directly attached to chemically modified carbons

Christopher F. Blanford, Carina E. Foster, Steady state oxygen reduction and cyclic voltammetry

Jan Rossmeisl, Gustav S. Karlberg, Thomas Jaramillo and Jens K. N?rskov, Faraday Discuss., 2009

DOI:10.1039/b802129e

Intrinsic kinetic equation for oxygen

reduction reaction in acidic media: the

double Tafel slope and fuel cell applications

Jia X. Wang, Francisco A. Uribe, Thomas E. Springer, Junliang Zhang and Radoslav R. Adzic, Faraday Discuss., 2009

DOI:10.1039/b802218f

A first principles comparison of the mechanism and site requirements for the electrocatalytic oxidation of methanol and formic acid over Pt

Matthew Neurock, Michael Janik and Andrzej Wieckowski, Faraday Discuss., 2009

DOI:10.1039/b804591g

Surface structure effects on the electrochemical oxidation of ethanol on platinum single crystal electrodes

Flavio Colmati, Germano Tremiliosi-Filho,

Ernesto R. Gonzalez, Antonio Berná, Enrique Herrero and Juan M. Feliu, Faraday Discuss., 2009

DOI:10.1039/b802160k

Electro-oxidation of ethanol and

acetaldehyde on platinum single-crystal electrodes

Stanley C. S. Lai and Marc T. M. Koper,

Faraday Discuss., 2009

DOI:10.1039/b803711f

Discussion

General discussion

Faraday Discuss., 2009,

DOI:10.1039/b814702g

Concluding remarks

All dressed up, but where to go?

Concluding remarks for FD 140

David J. Schiffrin, Faraday Discuss., 2009

DOI:10.1039/b816481a

View Art

A ?rst principles comparison of the mechanism and site requirements for the electrocatalytic oxidation of methanol and formic acid over Pt

Matthew Neurock,*a Michael Janik b and Andrzej Wieckowski c

Received 17th March 2008,Accepted 13th May 2008

First published as an Advance Article on the web 22nd August 2008DOI:10.1039/b804591g

First principles density functional theoretical calculations were carried out to examine and compare the reaction paths and ensembles for the electrocatalytic oxidation of methanol and formic acid in the presence of solution and applied electrochemical potential.Methanol proceeds via both direct and indirect pathways which are governed by the initial C–H and O–H bond activation,respectively.The primary path requires an ensemble size of between 3–4Pt atoms,whereas the secondary path is much less structure sensitive,requiring only 1–2metal atoms.The CO that forms inhibits the surface at potentials below 0.66V NHE.The addition of Ru results in bifunctional as well as electronic effects that lower the onset potential for CO oxidation.In comparison,formic acid proceeds via direct,indirect and formate pathways.The direct path,which involves the activation of the C–H bond followed by the rapid activation of the O–H bond,was calculated to be the predominant path especially at potentials greater than 0.6V.The activation of the O–H bond of formic acid has a very low barrier and readily proceeds to form surface formate intermediates as the ?rst step of the indirect formate path.Adsorbed formate,however,was calculated to be very stable,and thus acts as a spectator species.At potentials below 0.6V NHE,CO,which forms via the non-Faradaic hydrolytic splitting of the C–O bond over stepped or defect sites in the indirect path,can build up and poison the surface.The results indicate that the direct path only requires a single Pt atom whereas the indirect path requires a larger surface ensemble and stepped sites.This suggests that alloys will not have the same in?uence on formic acid oxidation as they do for methanol oxidation.

1.Introduction

The electrocatalytic oxidation of methanol and formic acid has been analyzed in great detail over the past decade as the result of their importance in the development of both methanol and formic acid fuel cells.1–4There are a number of elegant studies for methanol as well as formic acid oxidation over well-de?ned Pt and Pt alloy surfaces that have helped to establish the overall pathways and provide insights into the mechanisms that control these reactions.4–34Although much is known about the paths involved,a detailed understanding of the elementary steps in the mecha-nism as well as the explicit surface ensembles needed to carry out these steps is still

a

Departments of Chemical Engineering and Chemistry,University of Virginia,Charlottesville,VA,22904-4741,USA b

Department of Chemical Engineering,Pennsylvania State University,University Park,PA,16802,USA PAPER https://www.360docs.net/doc/9f5392889.html,/faraday_d |Faraday Discussions

P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

somewhat speculative and often debated.While theory and simulation have begun to provide mechanistic insights into some of the elementary steps for methanol decomposition in vapor phase over different metals,35–37there have been very few studies that have examined the in?uence of the solution phase or more realistic electrochemical interfaces.8,38–42

Methanol oxidation is thought to follow a dual path mechanism over Pt at suf?-ciently high potentials 4,6–8,18,20,43–45involving both ‘‘indirect’’and ‘‘direct’’pathways.The indirect path proceeds via the formation of CO followed by its subsequent oxidation to CO 2.The direct path proceeds instead through the formation of soluble intermediates,such as formaldehyde and formic acid,which can subsequently oxidize to form CO 2.In the indirect path,methanol weakly adsorbs to platinum and dehydrogenates via a sequence of steps to form adsorbed CO.At lower poten-tials,CO builds up on the surface and blocks the sites necessary for the activation of water to form surface hydroxyl intermediates that could oxidize it from the surface.At higher potentials,CO can be oxidized and both the indirect and the direct pathways can proceed.The direct path involves the formation of soluble secondary intermediates such as formaldehyde and formic acid 5–7,18,31,46–48Experiments by Korzeniewski et al.31,46–48carried out in different electrolytes suggested that formal-dehyde is the predominant secondary product and the path to formaldehyde requires signi?cantly smaller ensemble sizes than that for the primary path.Osawa et al.,10,21,26on the other hand,used surface enhanced infrared spectroscopy to suggest that the dual path proceeds through the formation of surface formate inter-mediates which form on polycrystalline Pt electrodes at potentials greater than 0.7V RHE and subsequently oxidize to form formic acid rather than formaldehyde.Koper et al.15,20showed that formaldehyde and formic acid could react to one another.Formaldehyde,for example,can be hydrolyzed to form methylene glycol which can oxidize over Pt to form formic acid.

The oxidation of formic acid is thought to be signi?cantly easier than methanol since CO 2is already present intact in the molecule.This would eliminate the need for an external oxidant.This is consistent with the fact that formic acid can oxidize at potentials signi?cantly less than those required to activate water.Three different paths for the oxidation of formic acid to CO 2have been suggested in the litera-ture.9,10,24,26,43,44,49,68The ?rst,which is known as the ‘‘direct path’’,involves the adsorption of formic acid onto the metal followed by the direct removal (dehydro-genation)of both protons to form CO 2.The direct path was clearly identi?ed by carrying out labeling studies in which 13CO was preadsorbed onto Pt before the oxidation of 12C-formic acid.The resulting product was found to be unlabeled,thus demonstrating that CO 2was formed via a ‘‘direct’’path from formic acid rather than from the adsorbed 13CO.25The second path,which is termed the ‘‘indirect path’’,involves the non-Faradaic dehydration of formic acid to CO and the subse-quent oxidation of CO to CO 2.In the ?nal ‘‘formate path’’,the O–H bond of formic acid is cleaved to form a surface formate intermediate which can subsequently react to form CO 2.9,49

Osawa et al.carried out the ?rst in-situ internal re?ection infrared studies using surface enhanced infrared adsorption spectroscopy (SEIRS)combined with cyclic voltammetry and chronoamperometry to determine the nature of the surface inter-mediates that form during formic acid oxidation under reaction conditions.They suggest that the reaction proceeds at potentials from 0.35V–0.9V NHE through the formation of an adsorbed formate intermediate.21,25,26More recent studies by Behm et al.,9,11,49,24–26however,indicate that formate decomposition is not involved in the dominant reaction path and plays a very minor role,if any,in the formation of CO 2.They found that the changes in the formate adlayer were similar to those found by acetate and (bi)sulfate and suggest that the adlayer is controlled by a fast adsorption/desorption equilibrium with the electrolyte.The changes in the formate coverage were considered to be the result of this exchange rather than the P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

predominantly through the direct activation of the formic acid.The formate that forms was thought to be a spectator species that inhibits the direct decomposition of formic acid to CO 2.They showed,via isotopic labeling,that C–H bond activation was involved in the rate determining step for the oxidation of formic acid.

Many of the different rate controlling processes outlined here for both methanol and formic acid oxidation can proceed over different surface ensembles.As such,the development of alloys which lead to enhanced electrocatalytic activity for methanol oxidation may have little in?uence on formic acid oxidation and vice-versa .In order to better understand the mechanisms that control both methanol and formic acid oxidation and the nature of the active sites necessary to carry out the controlling reaction steps,we have carried out ?rst-principle density functional theoretical calculations over the well-de?ned Pt(111)surface in the presence of solution and over a range of applied potentials.We compare the results for methanol oxidation on different alloys to the results on formic acid in an effort to understand the simi-larities and differences between the two and how alloying might change the results.

https://www.360docs.net/doc/9f5392889.html,putational methods

Plane wave density functional theoretical calculations were carried out herein using the Vienna Ab Initio Software Program (VASP)by Kresse and Hafner 50–52within the Generalized Gradient Approximation to determine the potential-dependent reac-tion energies and activation barriers for the oxidation of formic acid over Pt(111)and methanol over Pt(111)and PtRu(111)alloys at the electri?ed aqueous/metal inter-face.The metal surface was modeled using a 3?3super cell comprised of 3metal

layers with 9metal atoms per layer together with a 10A

?vacuum inserted between the top and bottom surfaces.The bottom layer of the surface was held ?xed at the experimental bulk distances.All of the calculations were carried out using the RPBE functional form of the exchange–correlation function,53–55ultrasoft pseudopo-tentials 56to model the electron ion interactions and a plane-wave basis cutoff energy of 396.0eV.The ?rst Brillioun zone was sampled using a 3?3?1Monkhorst–Pack 57k-point mesh,which was tested to converge a subset of the relative energies reported herein.Previous tests comparing 3and 5layer metal slabs found little change on the structural or potential dependent behavior of water over an Ni(111)surface.58

The aqueous solution phase was modeled using 24explicit solvent molecules chosen to ?ll up the vacuum region.The height of the background region was chosen so as to match the density of water near a metal surface of 0.86g cm à3at 0K.8,39,40,59,60The water molecules were initially oriented in a hexagonal bilayer in registry with the (111)surface and subsequently optimized.In order to study the adsorption of the reactant methanol and formic acid molecules and their subsequent reactions,model systems were created containing speci?c intermediates.One water molecule on the surface was therefore replaced with reactant,intermediate or product species.The surface metal layers,the adsorbates,and the aqueous interface were optimized for all the structures explored.

The potential dependence on the reaction energies and activation barriers were determined using the double-reference method of Filhol and Neurock.39,59,61The double-reference method involves calculating two different reference potentials in order to relate the electri?ed aqueous metal interface to a neutral aqueous system and a vacuum reference potential.The potential is controlled by systematically adding or subtracting fractions of an electron from the unit cell and applying an equal but opposite compensating,homogeneous background charge.The details of the approach and the approximations are described in detail elsewhere.59

3.Results

We report below on the results for the oxidation of methanol and formic acid over P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

compared and used to provide an understanding of how alloying may in?uence each of these systems.3.1Methanol oxidation

In a previous communication,we presented DFT-calculated overall reaction ener-gies for methanol decomposition over Pt(111).8Herein,we highlight just the salient features of the overall pathways and focus predominantly on the active surface ensembles necessary to carry out methanol decomposition and its subsequent oxida-tion,as this was not discussed previously.The general pathways involved in the oxidation of methanol are shown in Fig.1.The decomposition of methanol over Pt proceeds by the adsorption of methanol to the surface.The energies for both the C–H and O–H bond activation paths of methanol were calculated over a range of different potentials.Herein,we summarize the results for an arbitrary potential of 0.5V NHE.The reaction energies in Fig.2indicate that the elementary steps that make up the primary involve:

CH 3O*H /*CH 2O*H +H ++e à/*CHOH +H ++e à/

*COH +H ++e à/*CO +H ++e à

The elementary steps that make up the secondary reaction path proceeds via the initial activation of the O–H bond to form methoxy which then reacts to formalde-hyde:

CH 3O*H /CH 3O*+H++e à/*CH 2]O +H ++e à

The structure and speci?c surface site requirements to carry out both the primary C–H bond activation and the secondary O–H bond activation steps are illustrated in Fig.3.The primary reaction path requires an ensemble site comprised of at least 4Pt metal atoms to accommodate the formation of the formyl intermediate that forms.This is consistent with experimental results from Cuesta,62who used cyanide to modify the ensembles in the Pt(111)surface and showed that the indirect path for methanol to CO required a surface ensemble with three contiguous Pt atoms arranged

triangularly.

Fig.1The initial steps in dual path mechanism for the electrocatalytic oxidation of methanol over Pt to CO 2.The indirect path which proceeds through the formation of CO is shown in the center (in black).The direct paths which lead to the formation of formaldehyde and formic acid are shown at the top and bottom,respectively (in blue).The formaldehyde that forms can also react in solution to form formic acid or adsorbed formate.The interconversion of these steps is sketched in red.A more detailed set of possible elementary steps and how they may interconvert P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

The O–H activation of methanol,as well as the subsequent C–H activation of me-thoxy to formyl,take place over the top of a single metal atom,as is shown in Fig.3B and thus require signi?cantly smaller ensemble sizes which consist of at most 1to 2metal atoms.This secondary path is much less structure sensitive.This is consistent with the experimental results from Iwasita et al.6,7and Koper et al.15,20who demon-

Fig.2DFT-calculated reaction energies over Pt(111)for the initial methanol decomposition paths to CO and formaldehyde at a constant potential of 0.5V.The indirect path through CO shown in purple is favored.The path to formaldehyde is a secondary route that becomes more favorable at potentials slighter greater than 0.5V.

8

Fig.3DFT optimized structures on Pt(111)in solution at 0.5V for the intermediates in the electrocatalytic oxidation of methanol to:(A)CO via the primary path and (B)formaldehyde via the secondary path.The primary path requires 3–4metal atoms whereas the secondary route requires 1–2metal atoms.

P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s

other soluble intermediates was greatest on the Pt(111)surface in sulfuric acid and lowest in perchloric acid.They demonstrated that the (bi)sulfate strongly adsorbs and inhibits the sites that lead to the indirect path to form CO.Perchlorate,on the other hand,is more weakly held to the surface and requires fewer sites,and there-fore does not strongly inhibit the CO formation path for the Pt(111)surface.

In addition to the dehydrogenation pathways depicted in Fig.2,each of the inter-mediates that form can subsequently undergo oxygen addition to form CO 2contain-ing intermediates.We have carried out subsequent DFT calculations to examine most of these paths in the vapor phase.In addition,we have explored,in much greater detail,the potential dependent reaction energies and activation barriers for CO oxidation over Pt(111)in solution,as this is the most dif?cult and important oxidation step.The overall reaction energies for the oxidation of CO via adsorbed atomic oxygen,hydroxyl intermediates,or water to CO 2and COOH are shown as a function of potential in the phase diagram in Fig.4.These results indicate that on the Pt(111)surface,hydroxyls are the active oxidants and appear at above 0.6V NHE as the result of the oxidation of water.CO is oxidized at a just slightly higher potential of 0.66V NHE.Theory was subsequently used to calculate potential dependent activation barriers for CO oxidation.The calculated activation barriers for CO +OH coupling,suggested to be the rate-limiting step,decreases from about 0.52eV to 0.38eV in moving from 0.0V to 1.0V NHE,which is consistent with experimental results.633.2In?uence of ruthenium

The oxidation of methanol is typically carried out by the introduction of Ru which aids in the oxidation of CO to CO 2via both bifunctional and electronic or ligand effects.29,63,64In the bifunctional mechanism,water is activated over Ru atoms in the surface to form active hydroxyl intermediates that can then oxidize CO bound to neighboring Pt sites.We used theory to analyze various arrangements of Pt

Fig.4The DFT-calculated electrochemical surface phase diagrams for the reaction energy vs.potential for the reaction of CO and water to CO 2and COOH over Pt(111).39

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potential dependent energies for both water activation and CO oxidation steps over Pt,Ru,a well dispersed Pt 66.7%Ru 33.3%surface alloy on Pt,Ru surface ensembles with the Pt(111)surface,and a pseudomorphic Pt overlayer on Ru in order to eluci-date the effects on the CO oxidation kinetics.A summary of the DFT calculated results are presented in Table 1which shows the onset potentials for the activation of water and the subsequent oxidation of adsorbed CO by surface OH groups over each of the alloy structures.The results indicate that the addition of Ru into the surface helps to lower the onset potential for the activation of water,and in addition,subsurface Ru atoms weaken the adsorption of CO.Both effects help promote CO oxidation.There appears to be an optimal balance of the amount of Ru needed in the surface which will enhance the formation of OH without poisoning the surface.This is consistent with other experimental studies.65–67While the addition of Ru to the surface decreases the Pt surface ensemble size which is important for methanol decomposition,it also results in a greater number of nearest neighbor Pt-CO and Ru-OH pairs that can enhance the reactivity.Subsurface Ru lowers the barriers for the CO +OH reaction via electronic interactions 3.3Oxidation of formic acid

As was discussed earlier in the introduction,formic acid can be oxidized via three different paths.These three paths are illustrated in Fig.5(a schematic adopted from Behm 9).The ?rst path,depicted in the middle of Fig.5,involves the direct decomposition of formic acid into CO 2+2H +and 2e à.The second,depicted at the bottom of Fig.5,involves an indirect route in which formic acid is ?rst dehydrated to CO.CO is then re-oxidized in a second step to form CO 2.The third path,depicted at the top of Fig.5,involves the conversion of formic acid into

Table 1The DFT calculated onset potentials for the activation of water to OH +H ++e àand CO oxidation to CO 2over model Pt(111)and Ru(0001)alloy surfaces.The higher than expected potentials for the activation of water are due to the fact that activation is carried out directly next to CO.The activation of water on Pt(111)in the absence of CO was calculated to be 0.66V SURFACE H 2O OH*+H ++e à

CO +H 2O CO 2+2H ++2e àPt ML

/Ru(0001) 1.22V 0.01

V Pt(111)

1.29V 0.55V Ru dispersed 0.74V 0.66V Ru islands

0.55

V

0.78

V

Fig.5The reaction paths involved in the electrocatalytic oxidation of formic acid to CO 2.(A)Direct path (center)to CO 2,(B)indirect path through CO (bottom),and (C)formate path though the formation of the formate intermediate (top).Schematic follows that proposed by Behm et al.9,11,49Experimental evidence also suggests the possible exchange of formate with P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

a surface formate intermediate that can subsequently react to form CO 2.All three of these paths were examined herein using density functional theory together with the double-reference method in order to determine their potential dependence reaction energies and activation barriers.Formic acid can adsor

b on Pt(111)in the presence of solution in an upright fashion through both the OH group and the oxygen of the C ]O,or lying parallel to the surface via a di-s interaction with C ]O.While the adsorption in the upright mode is more favorable in the gas phase,the two modes of adsorption in solution are nearly equivalent.Formi

c aci

d readily reacts from th

e upright mode to form a surface formate intermediate and proton.The formate intermediate is bound to a bridge site in a di-s mode through its two oxygen atoms.The molecule is quite rigid in this con?guration with the C–H bond oriented along the surface normal,thus making it very dif?cult to activate the C–H.The dissocia-tion o

f the C–H bond from this state requires bendin

g the CH group out of the CO 2plane,whic

h costs over 1eV in energy.Alternatively,the molecule can reorient itself to be bound through one of the oxygen atoms and the hydrogen.This also costs over 1eV in energy at positive potentials as it requires breaking the very highly polarized Pt–O bond.

If the C–H bond of formic acid is activated initially instead,the hydroxy carbonyl (COOH)intermediate is formed.At positive potentials,it readily deprotonates to form CO 2and H +with a very low activation barrier.While we did not observe the simultaneous activation of both C–H and O–H bonds,we did ?nd that the O–H bond was very easy to activate subsequent to C–H bond activation.We can not,however,rule out the possible simultaneous activation of both bonds.In either case,the COOH intermediate is unstable and unlikely to be observed,therefore sug-gesting this path is the direct path of Fig.5.

Formic acid,formate as well as hydroxy carbonyl intermediates can all potentially react via the activation of the C–O bond,which would result in the formation of adsorbed CO.While we have not calculated this path in solution,we would expect,based on vapor phase calculations for the activation of the RC–OH bond over Pt(111),the barrier to be greater than 1eV.The C–O bond is quite strong and there is little reason to expect the rate of this non-Faradiac process to be substantially altered by variations in potential.Experimentally,it has been reported that formic acid dehydration can not occur over Pt(111).69The reaction instead requires defect sites on the surface.Based on the theoretical estimates and the experimental results,we conclude that CO activation requires step edges or defect sites.Despite the higher activation barrier the calculated overall reaction energy for this step is highly exothermic.The CO intermediates that form are very strongly bound to Pt,and therefore would be expected to be observed at low potentials at which the CO oxida-tion rate is slow.

The ensemble requirements for both the direct as well as the formate pathways shown along the middle and the top branches in Fig.6,respectively,are 1to 2metal atoms.The indirect path,which is shown along the bottom branch in Fig.6,on the other hand,requires a signi?cantly larger ensemble size in order to activate the C–O bond.As such,the direct and formate pathways are characteristically different than that for methanol activation,which requires at least 4metal atoms along its primary path.The indirect path would be similar to methanol since both require the oxida-tion of CO.

3.4The effect of potential

We carried out calculations over a range of potentials in order to establish the potential dependence of all three paths,compare with experiment and establish which path ultimately controls the chemistry at different conditions.For the sake of comparison,we report here on the energies at three very different potentials:0.0,0.5and 1.0V NHE in order to see the changes that can occur as a function P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

The resulting elementary reaction energies calculated at 0V are shown in Fig.7.Formic acid can react to form adsorbed CO and water,which is thermodynamically the most favored path,with an overall reaction energy of à2.06eV.Based on previous gas phase C–OH activation calculations over Pt(111),the activation barrier is speculated to be over 1eV and will thus require step or defect sites.Any CO that forms on the surface at lower potentials will be strongly bound,build up over time and inhibit the surface.The activation of the O–H bond of formic acid,on the other hand,occurs quite easily in solution with a barrier of less then 0.05eV and is exothermic at the surface by à0.78eV.The resulting formate group can either continue on and break its C–H bond to form CO 2and a proton or undergo the reverse recombination reaction and form formic acid.As was discussed above,the barrier to break the C–H bond of the bound formate intermediate is very

dif?cult

Fig.7DFT-calculated potential energy surface for the direct,indirect,and formate paths for

Fig.6DFT calculated surface ensembles required for the direct oxidation path (center),the indirect path through CO (bottom)and the formate path (top)for the oxidation of formic acid over Pt(111)to CO 2at a constant potential of 0.5V.

P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

as it requires overcoming a barrier of 1.2eV.At lower potentials,the adsorbed formate would preferentially react with a proton and come off as formic acid.The barrier for this reverse reaction is 0.78eV.The adsorption and dissociation of formic acid to form formate may be expected to reach equilibrium.The equilibrium to form surface formate at this low of a potential is signi?cantly smaller than that found at higher potentials,and both surface formate or formic acid species will compete with strongly-bound CO.We would therefore expect a very low surface concentration of formate intermediates and instead a much larger concentration of CO at steady state conditions.The initial activation of the C–H bond of formic acid has a barrier of 0.5eV,which is greater than that required to break the O–H bond of the acid.The hydroxyl carbonyl surface intermediate that forms,however,is quite reactive and can readily cleave the O–H bond to form CO 2directly with an activation barrier of 0.52eV.At lower potentials,this path should dominate any CO 2production but its activity will be quite low due to inhibition by CO,which covers the surface.The formate path at lower potentials is simply inactive.These results are consistent with the experimental results of Osawa et al.10,24–26and Behm et al.,9–11,49which show that the surface is highly covered in CO at potentials up to 0.9V but has a decreasing intensity from 0.3V to 0.7V RHE.Formate,which slowly begins to appear at 0.3V,dominates the surface at 0.9V RHE.

As the potential is increased to 0.5V NHE,both the direct path and the formate path become thermodynamically more favorable as both involve electro-oxidation steps (Fig.8).The formation of surface formate becomes 0.22eV more exothermic in moving from 0V to 0.5V NHE.The subsequent activation barrier for breaking the C–H bond of formate to form CO 2is reduced by 0.1eV (from 1.2to 1.1eV).The direct activation of formic acid to the hydroxyl carbonyl becomes more exothermic by 0.26eV and its barrier is lowered by 0.03eV (from 0.50to 0.47eV).There is no change in the thermodynamics or the kinetics for the dehydration of formic acid to CO and water as it is a non-Faradaic reaction.The barrier for the subsequent oxidation of CO to CO 2is reduced from 1.42eV at 0V to 0.99eV at 0.5V.While this is a signi?cant drop,there is likely little change in the macroscopic behavior of the system,as CO will still build up and act to poison the surface.The direct path is still the most favorable and remains strongly inhibited by chemisorbed CO on the

surface.

Fig.8DFT-calculated potential energy surface for the direct,indirect,and formate paths for P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

As the potential is increased to 1V NHE more signi?cant changes occur in the importance of the various paths,as seen in Fig.9.The greatest change is that the indirect route,which produces CO,no longer acts to poison the surface.The activa-tion barrier to oxidize CO is reduced down to 0.5eV.Earlier,we showed that CO is readily oxidized at potentials greater then 0.66V.CO therefore no longer acts to inhibit the surface.This is well established experimentally as well,since CO is oxidized from Pt(111)at potentials above 0.6V RHE.29,63Other oxidation reactions also become more favorable as the potential is increased.The reaction energies for the initial steps of both the formate (O–H activation)and direct (C–H activation)paths become more exothermic by 0.21eV.The C–H activation barrier for the conversion of formic acid to the hydroxyl carbonyl intermediate (*COOH)is lowered down to 0.05eV.

The barrier for the activation of the C–H bond of adsorbed formate to CO 2is decreased by 0.1eV.The change in this barrier is rather small in comparison with the change in the overall thermodynamics for creating the formate intermediate.The activation barrier for the reverse reaction,i.e.the reduction of formate back to formic acid is energetically very unfavorable at 1.0V NHE as the barrier is increased to over 1.2eV.The result is that formate has a low barrier to form but is energetically a very stable surface intermediate.As such,it remains bound very strongly to the surface and acts as a spectator.The formation of CO 2is therefore dominated by the direct path.The higher coverage of formate may lead to minor contributions from the indirect path.The results from the calculations suggest that the surface should become covered with formate intermediates at higher poten-tials as they form faster than they can be removed.They are thermodynamically stable and no longer inhibited from forming by adsorbed CO.The barriers for the C–H as well as the subsequent O–H activation of formic acid via the direct path are both lower than 0.5eV and can readily occur.Formate appears to predomi-

Fig.9DFT-calculated potential energy surface for the direct,indirect,and formate paths for the oxidation of formic acid over Pt(111)to CO 2held at a constant potential of 1.0V.

P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

The results reported here are consistent with the results for formic acid oxidation over Pt by Behm and colleagues 9that suggest that formate is a spectator species which does not undergo further oxidation.The predominant path to CO 2over a wide potential range is through the direct path which involves the activation of the C–H bond followed by the facile activation of the O–H bond.While the results reported herein indicate that these are two distinct steps,they do not rule out a path involving the simultaneous C–H and O–H activation.In either case,we refer to this path as the direct path as our results are consistent with the fact that this path proceeds without the formation of a stable,observable,intermediate at any potential of interest.The barriers for both steps are less than 0.5eV and become more favor-able at higher potentials.At potentials lower than 0.66V,CO forms as the result of dehydration and builds up on the surface,thus inhibiting the oxidation of formic acid to CO 2.This is consistent with the presence of CO seen from the SEIRS data from Osawa and Behm at potentials between 0.1V RHE up to 0.9V RHE.At higher potentials,the CO can be oxidized from the surface.It is removed more rapidly than it is formed and as such does not build up on the surface at potentials greater then 0.66V.

3.5.Ensemble requirements

While the oxidation of formic acid to CO 2can proceed through any one of the three paths presented,the direct route appears to be the dominant path.For completeness,however,we discuss the site requirements for all three pathways.The direct activa-tion of formic acid to CO 2,which involves the initial activation of the C–H bond,was found to require just one metal atom.The hydroxyl carbonyl intermediate that forms,binds to the surface through the carbon atom in an atop con?guration.The subsequent O–H activation of the bound *COOH intermediate occurs quite easily with a low activation barrier and also involves only 1or 2metal atoms.The site requirement for the activation of the O–H bond of formic acid to form formate is quite similar,requiring only a single Pt atom.As such,both the direct and the formate paths have much lower ensemble requirements than that for the primary (indirect)path for the activation of methanol over Pt.The ?nal path,which involves the indirect activation of formic acid to CO,requires defect sites as well as a much larger surface ensemble in order to activate the C–O bond.The oxidation of CO that forms is very dif?cult at lower potentials and CO acts to poison the surface.The difference in ensemble requirements can lead to signi?cant differences upon alloying.We found that the primary oxidation path for methanol to CO 2was considerably enhanced by the presence of Ru in the surface.Ru aids in the activation of water to form hydroxyl intermediates which enhances the oxidation of CO to CO 2,i.e.the bifunctional mechanism.The most active surfaces would therefore be ones in which Pt and Ru were intimately mixed in the surface in order to enhance their activity.This is the result of both the large surface ensemble requirements as well as the fact that all of the intermediates that might form require an oxidant to ultimately be converted to CO 2.Subsurface Ru signi?cantly enhances the reaction by weakening the adsorption of CO,which is known as the ligand or electronic effect.While the ligand effect is very important,it is not a very strong function of ensemble size and tends to be localized to nearest neighbor surface and subsurface sites.

In the oxidation of formic acid,CO 2is already explicitly present in the fuel.The site requirements for simple C–H and O–H scission are only 1–2metal atoms.As such,one might expect small changes in activity as the result of alloying.While the addition of Ru to the surface will help to enhance the oxidation of CO formed via the indirect pathway,the results reported here indicate that there is a very small effect,if any,on the overall rates of oxidation as the presence of Ru would not be expected to enhance the activation of the C–H bond of formic acid to initiate the P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

do not require the activation of water to produce a sacri?cial oxidant as the CO 2in the reactant remains intact.Both,however,do require the activation of C–H and O–H bonds of the acid.The C–H bond activation in general is preferentially carried out over Pt and should therefore not be explicitly in?uenced by Ru with exception of weaker electronic in?uences.The activation of the O–H bond of an acid occurs readily over Pt and therefore does not require Ru either.

4.Summary and conclusions

First principles density functional theoretical calculations help to con?rm that meth-anol oxidation proceeds through both the direct and indirect pathways that are governed by C–H and O–H bond activation,respectively.The indirect path,which proceeds via the formation of CO,is the dominant path.The rate,however,is signif-icantly lower at potentials less than 0.6V NHE over Pt(111)where CO is strongly chemisorbed and inhibits oxidation.The ensemble size required for the indirect and direct paths are characteristically different.The indirect path requires 3–4Pt atoms in order to accommodate the carbon bound CH x O fragments that form,which is consistent with experimental ?ndings by Cuesta.62The direct path,which leads to the production of formaldehyde and/or formic acid,on the other hand,requires only 1–2Pt atoms,and as such,is much less in?uenced by anion adsorption.This is consistent with the experimental results presented by Iwasita et al.6,7and Koper et al.,15,20which indicate that bi(sulfate)blocks Pt sites and inhibits the indirect path but has much less or an in?uence on the direct path.

Formic acid oxidation can also proceed to CO 2through both direct and indirect pathways.In addition,there is a third path which involves the formation of formate intermediates on the surface.Similar to that for methanol,the direct path proceeds via the initial activation of the C–H bond.The barrier for this step is 0.5eV at 0V NHE and drops down to 0.4eV at higher potentials.The barrier to activate the O–H bond of the hydroxyl carbonyl that forms is very low over a range of potentials,and is thus readily activated to form CO 2directly.Based on the calculated activation barriers,this path appears to be the dominant route to form CO 2and requires the presence of step or defect sites.At lower potentials,however,the direct path is blocked by CO which covers the surface.The indirect path,which is responsible for CO formation,is controlled by the non-Faradaic hydrolytic activation of the C–O bond of formic acid or formate.At potentials lower than 0.67V NHE,the CO that forms is thermodynamically quite stable and can not be oxidized from the surface.At potentials greater than 0.6V,CO is oxidized and the direct path is much less inhibited.The formate path,which involves the activation of the O–H bond of formic acid,initiates quite readily over Pt,thus resulting in the formation of formate intermediates.At lower potentials,this path is blocked due to the pres-ence of CO on the surface.As the potential is increased between 0.5–1.0V NHE,formate formation becomes preferable.The activation barrier for the subsequent activation of the formate C–H bond,however,is prohibitive.The computational results indicate that formate forms on the surface at these higher potentials but acts as a spectator,which is consistent with experimental results from Behm.9

The site speci?city for the direct and indirect paths involved in the oxidation of formic acid were found to be quite similar to those found for methanol.The direct path appears to occur over just 1or 2Pt atoms.The indirect path,however,requires splitting the C–O bond which requires a signi?cantly greater ensemble size and defect sites.There is one signi?cant difference,however,between the ensemble size effects for methanol and formic acid.In methanol,the indirect path through CO is the major path to CO 2over a wide range of potentials.In formic acid,however,the direct path is actually the dominant path to CO 2over nearly all poten-tials.This helps to explain the signi?cant differences concerning the in?uence of Ru on methanol and formic acid oxidation.Ru signi?cantly enhances the methanol P u b l i s h e d o n 22 A u g u s t 2008. D o w n l o a d e d b y S u n Y a t -S e n (Z h o n g s h a n ) U n i v e r s i t y o n 20/10/2015 14

Acknowledgements

We gratefully acknowledge the Army Research Of?ce for funding through a MURI grant (DAAD19-03-1-0169)for fuel cell electrocatalysis research as well as the computational time from the U.S.Army Research Laboratory Major Shared High Performance Computing Resource Center,the Molecular Sciences Computing Facility in the William R.Wiley Environmental Molecular Sciences Laboratory at Paci?c Northwest Laboratory,and National Center for Computational Sciences at Oak Ridge National Laboratory.AW acknowledges support from the National Science Foundation under grant NSF CHE06-51083.We also kindly acknowledge the very helpful discussions with Sally Wasileski,Christopher Taylor,Jean Sebastian Filhol,and Tom Zawodzinski.

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