Theroleofatomicorbitalsofdopedearth-abundantmetalsondesignedcoppercatalyticsurfacesDequanXiaoa,*andTrevorCallahana

a. HIGAPolymerMaterialsLaboratory,CenterforIntegrativeMaterialsDiscovery,DepartmentofChemistryandChemicalEngineering,UniversityofNewHaven,WestHaven,CT06516

*correspondingauthor:dxiao@newhaven.edu

Abstract It isageneralchallengetodesignhighlyactiveorselectiveearth-abundantmetals forcatalytichydrogenation.Here, we demonstrated an effective computational approach based on inverse molecular design theory todeterministicallysearchforoptimalbindingsitesonCu(100)surfacethroughthedopingofFeand/orZn,andastableZn-dopedCu(100)surfacewasfoundwithminimalbindingenergytoH-atoms.Weanalyzetheelectronicstructurecauseoftheoptimalbindingsitesusinganewquantumchemistrymethodcalledorbital-specificbindingenergyanalysis.Comparedto the 3d-orbitals of surface Cu atoms, the 3d-orbitals of surface Zn-atoms show less binding energy contribution andparticipation,andaremuchlessinfluencedbytheelectroniccouplingsofthemediaCuatoms.Ourstudyprovidesvaluablegreenchemistryinsightsondesigningcatalystsusingearth-abundantmetals,andmayleadtothedevelopmentofnovelCu-based earth-abundant alloys for important catalytic hydrogenation applications such as lignin degradation or CO2transformation.

Introduction

Designing highly active or selective catalysts using earth-abundant metals is consistent with one of the principles ofgreenchemistrythatistodesigngreenerandsaferchemicals.1Afewexperimental2-7andcomputational8,9designeffortshavebeen made to develop novel heterogeneous Cu-basedcatalysts by dopingwith othermetals, for various interestingapplications such as the hydrogenation of lignin2,3, syngas4,5,CO2

10,andbiomassderivatives6,7.

However, it remains a general challenge to design highlyactive or selective earth-abundant alloy catalysts throughdeterministic search. In this work, we will demonstrate aneffective way to computationally search for optimal catalyticactive sitesonCu (100) surface through thedopingof earth-abundantmetalsusingtheinversemoleculardesignmethod11,and then reveal the role of atomic orbitals of the effectivedopedmetalsusinganewquantumchemistrymethodcalledorbital-specificbindingenergyanalysis.Ourworkwillprovidevaluablegreenchemistry insightsondesigningcatalystsusingearth-abundantmetals,whichcanleadtothedevelopmentofnovelearth-abundantalloycatalystsforimportantapplicationssuch as lignin degradation and CO2 transformation. Forexample,thedesignedoptimalsurfacebindingsitesheremaybecreatedbythefabricationtechniquesusedforsingle-atomcatalysis12-14 or by the search of potential bulk alloys inpractice.

Doping Cu with other metals (either noble or earth-abundantmetals)hasshowntobeaneffectivewayinpractice

togeneratepromisinghydrogenationcatalysts.Forexamples,Cu/ZnO/Al2O3 is an industrial catalyst formethanol synthesisthrough the hydrogenation of CO4, and was also used forwater-gas shift reactionat relatively low temperature5.Cu-Znalloy was used to synthesize ethylene glycol from thehydrogenationofdimethyloxalatewith remarkableactivity.15AbimetallicCu-Pdalloywasplacedonthesupportofreducedgrapheneoxideshowingimprovedcatalyticactivity.7Recently,a Cu-doped hydrotalcite show high yields of liquid fuels andaromatics from the hydrogenation of lignin under relativelymild condition,2,3 was later found to have high selectivity oncarbonyl groups or double-bonds at relatively lowtemperature.16

WeaimtominimizethebindingaffinitytoH-atomsontheCu-basedcatalyticsurfaces,whichcanleadtotheloweringofactivation energy barrier for catalytic hydrogenation. H-adsorption energy is correlated to catalytic activities as“volcano” plots17. Strong hydrogen bindingmay increase theconcentration of H-atoms adsorbed on catalytic surfaces;however, itmay also increaseH-atom release barrier for thenexthydrogenationstep.Weakhydrogenbindingonsurfacesfacilitates the release of H-atoms; however, it may alsodecrease the concentrationof adsorbedH-atomson catalyticsurfaces. This is known as Sabatier principle.18 Here, we willminimize the H-release barrier on catalytic surfaces whilemaintains the favorable binding of H-atoms on the surfaces,which can lead to highly active hydrogenation catalysts thatworkunderlowtemperature.

Wewillusetheinversemoleculardesignmethodsintight-binding framework to optimize the Cu (100) surface. Themotivationofinversemoleculardesignistosearchforoptimal

molecular or chemical structures guided by efficientoptimization algorithms.19 Early studies of inverse designmethodologies focused on finding optimum bandgap atomicconfigurationsusingtheeffective inverseband-structure(IBS)approach, based on the simulated annealing algorithm20 orgeneticalgorithm.21 In2006,anewapproachbasedon linearcombinationofatomicpotential (LCAP)wasdevelopedat thelevel of density functional theory (DFT),22 for optimizingpolarizabilities and hyperpolarizabilities. In 2008, Xiao et al.implemented the LCAP approach into the tight-binding (TB)framework using aHückel-typeHamiltonian that is shown tobe effective at locating optimal structures within a largenumber (102–1016) of possible chemical structures.23 Later,Xiaoetal.developedtheextended-Hückel(EH)generalizationoftheHückelTB-LCAPapproachforthepurposeofoptimizingmolecularsensitizersfordye-sensitizersolarcellapplications.24Herein, we will adapt the TB-LCAP inverse molecular designapproach to search for optimal binding sites to H-atomsthroughthedopingofearth-abundantmetals(i.e.,Fe,andZn)onCu(100)surfaces.

Wewillanalyzetheroleofatomicorbitalsofdopedearth-abundantmetals on Cu (100) surfaces using a new quantumchemistry analysis method called orbital-specific bindingenergy analysis. Revealing the role of specific atomic orbitalsof surface atoms at active sites upon chemisorption of H-atomswillhelpustounderstandtheelectronicstructurecauseof the optimal active site, leading to new insights (thusinnovativeapproaches)todesignhighlyactive/selectiveearth-abundant catalysts. The influence of atomic orbitals to thebinding of H-atoms has been studied through the analysis ofdensityofstates(DOS)ofbulkalloysbeforeandafterH-atomsbinding.25 However, this analysis can only use the change inDOS or projected DOS (e.g., DOS of d-bands) to imply theinteraction of surface metallic orbitals with the adsorbed H-atoms, and there is no quantitative analysis of the bindingenergy contributionper atomicorbital or electronicband. Tosolve this problem, a d-band model was successfullydeveloped by Hammer and Nørskov 26 by decomposing theoverall adsorbate-surface binding energy into thecontributions from sp bands and d-band based using a fewfittingparameters,where thed-band is treatedasa singled-electronic level with energy εd and one coupling matrixelement V between d-band and bonding or anti-bondingorbital of the adsorbate. The d-bandmodel was successfullyused to design novel heterogeneous catalysts. Here, wedeveloped a computational analysis method based on theanalytic derivation of the electronic structure theorywithoutadditionalfittingparameters.Wewillprovidenewinsightsforthebindingenergycontributionsfortheoverallbindingenergyfromtheviewpointofindividualatomicorbitals,insteadoftheviewpointofelectronicbands.

In the orbital-specific binding energy analysis, the overallH-atombindingenergyisdecomposedintothebindingenergycontributions of all (e.g., hundreds of) the atomic orbitals ofthemetallicatoms,withoutadditionalapproximationorfittingparameters.Inaddition,wewilluseGreen’sfunctionmethodstoanalyzetheinfluenceofmediaatomstothesurfaceatomicorbitals that are in direct contactwith H-atoms. TheGreen’s

function methods have been previously used to study theatomic orbital couplings between donor-and-acceptor forelectrontransferstudy,27andelectrontransportformoleculesor solid-statematerials28.Using theGreen’s functionmethodintheorbital-specificbindingenergyanalysishere,wecantellthe role of earth-abundant metal atomic orbitals upon thechemisorptionofH-atoms.

In the followings, we first describe the theory of inversemolecular design in tight-binding framework for H-atombinding energyminimization, and also the theory for orbital-specific binding-energy analysis. We will then show the TB-LCAP search for optimal surface binding sites through thedopingofZnand/orFeatomsontheCu(100)surfaces.Afterthat, we will verify the accessibility of such surface and itsoptimal H-binding property using geometry relaxation at thedensity functional theory level in the framework of infinitelyrepeatedlatticeunits.WewillanalyzethechangeofdensityofstatesuponthebindingofH-atomsforpureCu(100)surfacesand the designed Zn-doped Cu (100) surfaces, respectively.Lastly, we will analyze the role of surface atomic orbitals byperforming the orbital-specific binding energy analysis for H-atomadsorptiononpureCu (100) surface,and theZn-dopedCu(100)surface,respectively.

Theories

Minimization of the H-atom binding energy by the Inversemoleculardesignapproach

The followingdescribes theextended-Hückel tight-bindinglinear combination of atomic potential (TB-LCAP) method asimplementedforoptimizingH-atombindingenergyontheCu(100)surfaces.

The extended Hückel approach29,30 has been widely usedforelectronicstructureandmaterialspropertycalculationsformolecules and solid-state structures.31 The time-independentSchrödinger equation in the extendedHückel framework canberepresentedinmatrixform,

𝑯𝑪= 𝑬𝑺𝑪(1)

where𝑯 is the extended Hückel Hamiltonian in the basis ofSlater-type atomic orbitals (AO’s), 𝑪 is the matrix ofeigenvectors,𝑬 is thediagonalmatrixof eigenstateenergies,and𝑺istheoverlapmatrixoftheSlater-typeatomicorbitals.

In the extended Hückel TB-LCAP formula,24 participationcoefficients 𝒃𝒊

(𝑨) are introduced to represent the occurrenceprobability of𝑵𝒕𝒚𝒑𝒆𝒊 optional atom-type A at site i, with the

conditionsof 𝒃𝒊(𝑨)

𝑨 = 𝟏and𝟎≤ 𝒃𝒊𝑨 ≤1.

The diagonal matrix elements of the EH/TB-LCAPHamiltonian are related to the participation coefficients𝒃𝒊

(𝑨)by

𝑯𝒊𝜶,𝒊𝜶 = 𝒃𝒊(𝑨)𝑵𝒕𝒚𝒑𝒆𝒊

𝑨!𝟏 𝒉𝒊𝜶,𝒊𝜶(𝑨) (2)

where α is the index for atomic orbitals. Note that for thespecialcaseofbi

(A)=1,𝑯𝒊𝜶,𝒊𝜶 = 𝒉𝒊𝜶,𝒊𝜶(𝑨) ,thesiteenergyofAOα

ofatomtypeA. Theoff-diagonalmatrixelementsare relatedtotheparticipationcoefficients𝒃𝒊

(𝑨)by

𝑯𝒊𝜶,𝒊𝜷 = 𝒃𝒊(𝑨)𝒃𝒋

(𝑨!)𝑵𝒕𝒚𝒑𝒆𝒋

𝑨!!𝟏 𝒉𝒊𝜶,𝒋𝜷(𝑨,𝑨!)𝑵𝒕𝒚𝒑𝒆𝒊

𝑨!𝟏 (3)

Here, β is the index of atomic orbitals, 𝒉𝒊𝜶,𝒋𝜷(𝑨,𝑨!) is the original

extended Hückel AO interaction energy between atomicorbitalsα (ofatomtypeAatsite i)andβ(ofatomtypeA’atsitej).24

Theoverlapmatrix(S)elementiscomputedby

𝑺𝒊𝜶,𝒊𝜷 = 𝒃𝒊(𝑨)𝒃𝒋

(𝑨!)𝑵𝒕𝒚𝒑𝒆𝒋

𝑨!!𝟏 𝑺𝒊𝜶,𝒋𝜷(𝑨,𝑨!)𝑵𝒕𝒚𝒑𝒆𝒊

𝑨!𝟏 (4)

where𝑺𝒊𝜶,𝒋𝜷(𝑨,𝑨!)istheoverlapmatrixelementbetweenα(ofAat

sitei)andβ(ofA’atsitej).

Theparticipationcoefficients𝒃𝒊(𝑨)are initializedrandomly,

and subsequently optimized for the target property, i.e., H-atom binding energy here. The chosen Slater-type atomicorbitals include 3d, 4s and 4p atomic orbitals for Cu, Fe, Zn,and1sorbitalforH.

The goal of TB-LCAP search here to minimize is themagnitude of binding energy of H-atoms on the catalyticsurface.

𝑬𝒃𝒊𝒏𝒅𝒊𝒏𝒈 = 𝑬𝑪𝒖!𝑯− 𝑬𝑪𝒖+𝑬𝑯 (5)

Here, 𝑬𝑪𝒖!𝑯 denotes the electronic energy of the Cu (100)surface bound by H-atoms, 𝑬𝑪𝒖 is the energy of Cu (100)surfacewithoutthebindingofH-atoms,and𝑬𝑯 istheenergyoffreeH-atoms.

𝑬𝒃𝒊𝒏𝒅𝒊𝒏𝒈isminimized(intermsofmagnitude)withrespect

to the participation coefficients 𝒃𝒊(𝑨) using standard

optimization techniques (e.g., the quasi-Newton BFGSalgorithm), computing the gradients of 𝑬𝒃𝒊𝒏𝒅𝒊𝒏𝒈 in terms of𝒃𝒊(𝑨)usingstandardfinitedifferenceexpressions.

Computationalmethods The accessibility and optimal property of the designedsurfaceswereverifiedbyadvancedDFTcalculations.Geometryrelaxation of the pure Cu or Zn-doped Cu lattice wasperformed by the DFT (PW91/GGA) method with periodicboundary condition using a plane-wave basis set and theultrasoft Vanderbilt pseudopotential approximation, asimplemented in the Vienna ab initio Simulation Package(VASP/VAMP).32Alatticeunitwith32metallicatomswasusedto simulate the pure Cu or Zn-doped Cu lattice. The wavefunction cutoff was 400 eV, and single γ-point sampling wasused when surface geometry was performed. For theprojected density of states analysis using VASP, we usedMonkhorstPack9x9x1forthenumberofk-points.

ThefollowingistheprocedureforcomputingtheH-bindingenergyonZn-doped(orpure)Cu(100)surfacesusingtheDFTcalculationswithperiodicboundarycondition.Wefirstrelaxedthe lattice parameters of the Zn-doped Cu lattice unit. Afterthat,wecomputedtheenergyoftheZn-dopedCusurface(𝑬𝑪𝒖in equation 5) without the adsorbed H-atoms. Then, weattached the H-atom and relaxed only the surface atoms byfixing the atomic configuration of the bottom two layers. Inthisway,weobtainedthetotalenergyofthesurface(𝑬𝑪𝒖!𝑯inequation5)afterthebindingofH-atoms.

Resultsanddiscussion1. TB-LCAP search for optimal binding sites through atomicdopingonCu(100)surfaces

As shown in Figure 1,we first define a search frameworkbasedonthecopper(100)surface.Onthissurface,ahydrogenatomisplacedinthefour-foldhollowbindingsite,asit isthemostlikelybindingsitesontheCu(100)surface.Weallowfouratom sites to vary with the choice of Cu, Fe, and Zn atomtypes, where the four atoms sites are in direct-contact withtheH-atomashighlightedinblueinFigure1a.WethenruntheTB-LCAP inverse molecular design program with 10 randominitiations. A typical search path that leads to the lowestbinding energy is shown Figure 1c. The search frameworkstartsatarandomstructurewithabindingenergyof-3.2eV,and then vary deterministically to a structurewith a bindingenergy of -2.5 eV. The optimized active site has three Znatoms, in anatomic configuration shown in Figure1d. Here,wefoundthatgradientofH-atombindingenergyhypersurfaceversus the alchemical structure variation is well-defined,providinggradientstothefinalstructure.

2.AnalyzingaccessibilityandvalidityofthesearchresultsonthesurfacesofperiodicunitsusingDFTcalculations

Based on the searched optimal binding site, weconstructedaminimalrepeatlatticeunitcontainingthewholeoptimalbindingsite(seeFigureS1).Weperformedthesurfacegeometry relaxation of the lattice repeat unit, using the DFT

calculations with periodic boundary condition. The atomconfigurationoftherelaxedsurface-bindingsiteremainstobestable, suggesting the accessibility of such binding site inexperiments. Based on the relaxed geometry, we computedthebindingenergyofHatomsonthefourfoldhollow-site(site2 in Figure 2) under periodic boundary condition, which is -0.427 eV. Similarly, we calculated the binding energy of H-atomsonthefour-foldhollow-site(site8inFigure2)onpureCu (100) surface using the same-size lattice under periodicboundary condition, and the calculated binding energy is -0.814 eV. Therefore,we verify that the Zn-doped Cu bindingsiteindeedhaslessbindingenergythantheCufourfoldhollowsites on periodic solid-state surfaces, indicating the effectivesearchbytheTB-LCAPinversemoleculardesignmethod.

The doping of Zn atoms to the Cu (100) surfaces alsocreatesothernewpotentialH-atombindingsites(seesites1-7inFigure2).So,wecomputedtheH-atombindingenergiesforall these sites, and compared them to the possible bindingsites (seesites8-10,Figure2)onpureCu (100) surfaces.ThecalculatedbindingenergiesarelistedinTable1.Wefoundthatthe highest possible binding energy (-0.702 eV) on the Zn-doped Cu (100) surface is actually lower than the highestpossible binding (-0.814 eV) on pure Cu (100) surface. Thus,theentireZn-dopedCu(100)surfaceisexpectedtohaveloweroverallbindingaffinitytoH-atomsthanpureCu(100)surface,indicatingameaningfuldesignofoptimalcatalyticsurfacesbytheTB-LCAPsearch.ThefindingoflowH-atombindingaffinityof Zn-dopedCu surfacemaybe related to the reportedhigh-activityofZn-dopedCucatalystsintheliterature.4,5,15

Forbetterunderstandingtheelectronicstructurecauseoflowering H-atom binding energy through the doping of Zn-atoms, we will perform the following quantum chemistryanalyses based on the found fourfold hollow site (site 2 inFigure2)onZn-dopedCu(100)surfaceandcompareittothefourfoldhollowsiteonpureCu(100)surface.

Table1.Bindingenergy(eV)ofH-atoms indifferentpositionsonthepureandZn-dopedCu(100)surfacesinFigure2.

SiteNo. Sitetype Binding energy(eV)

1 fourfoldhollow -0.655

2 fourfoldhollow -0.427

3 fourfoldhollow -0.702

4 top -0.175

5 top -0.328

6 bridge -0.490

7 bridge -0.328

8 fourfoldhollow -0.814

9 top -0.140

10 bridge -0.664

3.DensityofstatesanalysisforthebindingofH-atoms

Weperformedthedensityofstate(DOS)analysisforpureandZn-dopedCu(100)surfaces,attheDFTlevelwithperiodicboundary condition.As shown in Figure 3, there are changesonprojectedDOS forall thevalenceorbitals (i.e.,3d,4s,and4p) of Cu and Znupon thebindingofH-atoms. For example,theCu4s-orbitalshowsclearchange (seeFigures3aand3d).InFigure3f,theprojectDOSofCu3d-orbitalsontheZn-dopedCu (100) surface shows a spread-out upon the binding of H-atoms.InFigure3i,thereisanapparentenergyshiftoftheZn3d-orbitals upon the binding ofH-atoms. From theprojectedDOS analysis, we can observe changes of projected orbitaldensityorenergyshiftsofatomicorbitals.However,wedonotknowqualitativelywhat are thebinding energy contributionsby specific atomic orbitals. In the following, we will performtheorbitalspecificbindingenergyanalysisforthepureCuandZn-doped(100)surfaces,respectively.

4. Orbital-specific binding energy analysis for H-atomadsorption

Weperformed theorbital-specific binding energy analysisat the level of the extendedHückel tight-binding theory.We

usedthelatticerepeatunitsoptimizedattheDFTlevelasthecrystal slab models to perform the orbital-specific bindingenergy analysis. By the extendedHückel calculations, the Zn-doped Cu slab (28 Cu atoms and 3 Zn atoms) shows lowerbinding energy (-2.64 eV) than the pure Cu slab (-3.29 eV),which is consistent with the DFT calculation with periodicboundary condition. Thus, the binding energy analysis basedon extendedHückel tight-binding calculations and the crystalslab models is appropriate for us to understand the role ofatomicorbitalsuponthebindingofH-atoms.

As shown in Figure 4, upon the binding of H-atoms onpure-Cu (100) surface, it is interesting to find that electronpopulation change on the AO is closely correlated to thenatureofbindingenergycontribution:anincreaseofelectronpopulation(positiveΔρ)onanatomicorbitalcausesanegative(favored)contributiontothebindingenergy;whileadecreaseof electron population (negative Δρ) on an atomic orbitalcauses a position (disfavored) contribution to the bindingenergy.OntheadsorbedH-atom,thereisnetgainofelectronpopulationafterbindingtothefour-foldhollowsite,indicatingelectrontransferfromtheCu(100)surfacetotheadsorbedH-atom. The increase of electron population on H-atom 1s-orbital causes a negative contribution to the binding energy.Overall, hydrogen 1s-orbital contributes a negative bindingenergy (-4.0 eV). We find that some atomic orbitals(particularly those on Cu-atoms in direct-contactwith the H-atom) have apparent positive binding energy contributions,e.g.,Cu64s-orbitals(+1.52eV),andCu74s-orbital(+1.52eV).Some atom orbitals contribute negatively to the bindingenergy, e.g., Cu104s-orbital (-1.04eV) andCu194s-orbital (-1.06eV).

Overall,theatomicorbitalsonpureCu(100)surfacehaveanet positive binding energy contribution. The Cu atomicorbitals in direct-contact with the H 1s-orbital show largepositive contributions. The atomic orbitals distant to the H-atomtendtoshowminorcontributionstothebindingenergy.But,somedistantCu-atomorbitalscanstillshowrelativelargebindingenergycontributionssuchasCu14s-orbital.

ThebindingenergycontributionforeachatominthepureCu slab can be found in Figure S2. The five direct contactatoms Cu6, Cu7, Cu8, Cu15, and Cu22 make positivecontributions to thebindingenergy. The rest indirect-contactCu atoms contribute positively or negatively to the overallbinding energy. The H-atom shows the largest bindingparticipation percentage (60%), followed by direct-contactatomsCu6(10%)andCu7(10%).

Figure5showstheorbitalspecificbindinganalysis fortheZn-doped Cu slab. Again, we found that positive Δρ iscorrelated to a negative contribution to the binding energy;negative Δρ is correlated to a position contribution to thebindingenergy.UponthebindingoftheH-atom,theZn-doped

Figure3.Projecteddensityof states (DOS)of 4s, 4p, and3dorbitalsforCu-atomsonpureCu(100)surface(a-c),Cu-atomson Zn-doped Cu (100) surface (d-f), and Zn-atoms on Zn-doped Cu (100) surface (g-i). The blue curves are the DOSwithout adsorbed H-atoms, and the red curves are the DOSwithadsorbedH-atomsonthesurfaces.

Cu binding site shows slightly higher gain of electronpopulationontheH-atom1s-oribtal thanthatof thepureCu(100) surface. Meanwhile, a much higher loss of electronpopulation is found on Cu27 atomic orbitals than any theatomicorbitalsofanyotherCuatomonthepureCuslab.ForZn atoms, Zn30 4s and Zn31 4s orbitals show strong positivecontributiontotheoverallbindingenergy.

Overall, as shown in Figure S3, the H-atom contributes anegative binding energy (-5.0 eV). For the direct-contact Cu-atoms,Cu28showsapositivebindingenergycontribution,andCu27 showsanegativebindingenergy contribution.All threeZn-atoms show positive binding energy contributions. Theadsorbed H-atom shows the highest binding participation(60%), followed by Cu28 (18%) and Zn30 (5%). The indirect-contact atom Cu1 shows a moderately high bindingparticipation(4%).

5.Theinfluenceofmediaatomsorbitals

As shown in Figures 4 and 5, the media atoms activelyparticipate inthebindingofH-atom. IfweassumethattheH1s-orbital is localized(avalidapproximation),themediaatomorbitals can influence the H-binding through the atomicorbitals of Cu atoms that are in direct-contact with theadsorbedH-atom.

Figure 6a (top panel) shows the orbital-specific bindingenergy contribution of the core atoms on Zn-doped copper

(100) surface,without thepresenceof anymedia atoms.Wefound that H 1s orbital has the largest binding energycontribution, followed by Cu2 4s-orbital. Figure 6a (bottompanel) shows the orbital-specific binding energy contributionofthecoreatomsonZn-dopedcopper(100)surface,withthecouplingofallthemediaatoms.Wefoundverystrongbindingenergycontribution fromCu24s-orbital (+50.08eV),𝟑𝒅𝒙𝟐!𝒚𝟐-orbital (-56.5eV), and 3dyz-orbital (-67.2eV). Zn5 4s-ortialshowslargebindingenergycontribution(+20.0eV).Indeed,inFigure S4a (bottom panel), atom Cu2 shows the highestbinding participation percentage (61%), with a large positivecontribution to the overall binding energy. Other direct-contact atoms (e.g., Zn) show moderate binding energycontributions,andweaklowbindingparticipationpercentage.Afterthecouplingofmediaatoms,thebindingparticipationofthe H-atom decreases to 10%. The overall binding energycalculatedby theGreen’s functionmethod is -2.64eV for theZn-dopedCuslab, reproducing thebindingenergycalculationresultbasedonallatoms.

Incontrast,forthepureCu(100)labwithoutthepresenceof media atoms (see Figure 6b, top panel), Cu1 4s-orbitalshows the strongest binding energy contribution, while theother core Cu atoms show similar strong AO binding energycontributions. Atoms Cu2-5 show strong binding energycontributions from the4s-orbitals.Meanwhile, Cu 3d-orbitalsalso show strong binding energy contributions, e.g., 3dxzorbitalsofCu2andCu4,and3dyz-orbitalsofCu3andCu5.

After the coupling of the media Cu atoms (Figure 6b,bottom panel), all of the core Cu 4s-orbitals show strongbindingenergycontributions,withstrongercontributionsfromCu2andCu5thanthosefromtheotherthreecoreCu-atoms.Overall,theHatomshowsa35%ofbindingparticipation,witha negative contribution (-35.0 eV) to the overall bindingenergy. The five core Cu atoms show similar bindingparticipationpercentages:Cu1:12%,Cu2:18%,Cu3:8%,Cu4:7%,andCu5:20%.TheoverallbindingenergybytheGreen’sfunctionmethod is -3.29eV for thepureCuslab, reproducingthebindingenergycalculationresultbasedonalloftheatoms.

By the analysis in Figure 6, we notice the difference inbindingenergy contributionsof the3d-orbitals between coreCu and Zn atoms.Without themedia atom coupling, Cu 3d-oribtalsshowstrongbindingenergycontributionsonthepureCu binding site. However, Zn 3d-orbitals do not show strongbindingenergycontributionsontheZn-dopedCubindingsite.After the coupling by the media atoms, at the Zn-doped Cubinding site,onlyCu23d-oribtals showstrongbindingenergycontribution. Zn 3d-orbitals do not shows strong bindingenergycontribution, indicatingthemediaCu-atomshaveveryweekcouplingtotheZn3d-orbitals.However,atthepureCubinding sites, all the core Cu 3d-orbitals show strong bindingenergy contribution and binding participation, indicating themediateCuatomshasstrongcouplingtothecoreCu-atoms.Inaddition,weclearlyobserveinFigure6thatthe4sorbitalsofCu and Zn show strong participation in the binding energy,which isconsistentwiththeresultsofprojectDOSanalysisattheDFTlevelwithperiodicboundarycondition.

Conclusions

We demonstrated an effective design of optimal H-atombinding sites on Cu (100) surface by doping Zn or Fe atomsusingTB-LCAPinversemoleculardesignmethods,andfoundafour-fold hollow site with 2 Cu atoms and 3 Zn atoms withlowerbindingenergy toH-atoms thanpureCu (100) surface.WeverifiedtheaccessibilityandoptimalpropertiesoftheZn-doped Cu (100) surface using density functional theory withperiodic boundary condition. Project DOS analysis indicatesthe3d,4s,and4porbitalsofCuandZnactivelyparticipateinthebindingofH-atoms.

We provided new insights on understanding the role ofatomicorbitalsuponthebindingofH-atomsonthepureorZn-doped Cu (100) surfaces through a new quantum chemistrymethod called the orbital-specific binding energy analysis.First,we found that the electron population change (Δρ) peratomicorbital iscloselycorrelatedtotheAO’sbindingenergycontribution: positive Δρ usually causes negative-value(favored) contribution; and negative Δρ usually causespositive-value (disfavored) contribution. Second, the netbindingenergy contributionofH-atoms isnegative (favored),and positive (disfavored) by the pure or Zn-doped Cu (100)

surface.Thecoreatomicorbitalsusuallyshowstrongerbindingenergycontributionsand thushighbindingparticipation thanthe media atomic orbitals. Even though the mediate atomsshowminorbindingcontributionindividually,theycollectivelyshowimportantinfluencetotheoverbindingenergy.

The effectiveness of lowing of H-atom affinity by thedoping of Zn on Cu (100) surface can be understood by thedistinguished atomic orbitals binding energy contributionsbetweenZnandCuatoms.OntheZn-dopedCu(100)surface,thebindingparticipation isdominatedbyoneofthecoreCu-atoms (Cu28, see Figure S2), and the three Zn-atoms showweak binding participations. The Zn atomic orbitals have lessbinding participation than Cu atomic orbitals, without thepresenceofmediaatoms.Afterthecouplingofmediaatoms,the binding energy contributions by the Zn atomic orbitals(particularly 3d orbitals) are less influenced compared to thecore Cu atomic orbitals. In general,we found that Zn atomicorbitalsshowlessparticipationinthebindingofH-atomsthanCuatomicorbitals.

Insummary,wedesignedanoptimalcatalyticbindingsurfacesthrough the doping of Zn atoms on Cu (100) surfacess, and weexplored the effectiveness of the doping of earth-abundantmetalZn through a thourough analysis of the role of metallic atomicorbitalsuponthebindingofH-atoms.Ourstudyprovidesvaluablegreen chemistry insights on designing catalysts using earth-abundantmetals, andmay lead to the development of novel Cu-basedearth-abundantalloys for importantcatalytichydrogenationapplicationssuchaslignindegradationorCO2transformation.

Acknowledgements

Thisworkwas supportedby thenew faculty startup fundandUniversityResearchScholarawardtoDXprovidedbytheUniversity of New Haven, and the research fund supportprovidedbyHigasketPlasticsGroupCo.Ltd.DXacknowledgessupercomputertimefromOpenScienceGrid.

Notesandreferences

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