Does a Higher Density of Active Sites Indicate a Higher Reaction Rate?

Abstract

A consensus view in catalysis is that a higher density of catalytically active sites indicates a higher reaction rate. Using molecular dynamics simulations capable of mimicking the electrochemical formation of gas molecules, we herein demonstrate that this view is problematic for electrocatalytic gas production. Our simulation results show that a higher density of catalytic active sites does not necessarily indicate a higher reaction rate─a high density of active sites could lead to a reduction in the rate of reaction. Further analysis reveals that this abnormal phenomenon is ascribed to aggregation of the produced gas molecules near catalytic sites. This work challenges the consensus view and lays the groundwork for better developing gas-producing reaction electrocatalysts.

Doesa HigherDensityof ActiveSitesIndicatea HigherReactionRate?TengLiu, YingheZhao,*and TianyouZhai*CiteThis:J. Am. Chem.Soc.2024,146,6461−6465ReadOnlineACCESSMetrics& MoreArticleRecommendations*sıSupportingInformationABSTRACT:A consensusviewin catalysisis that a higherdensityof catalyticallyactivesites indicatesa higherreactionrate.Usingmoleculardynamicssimulationscapableof mimickingthe electrochemicalformationof gas molecules,we hereindemonstratethatthis viewis problematicfor electrocatalyticgas production.Our simulationresultsshowthat a higherdensityof catalyticactivesitesdoesnot necessarilyindicatea higherreactionrate�ahighdensityof activesitescouldlead to a reductionin the rate of reaction.Furtheranalysisrevealsthat this abnormalphenomenonis ascribedto aggregationof the producedgas moleculesnear catalyticsites.Thisworkchallengesthe consensusviewand lays the groundworkfor betterdevelopinggas-producingreactionelectrocatalysts.Electrocatalyticproductionof gas playsa keyroleinprocessesthat are crucialfor energyand synthesis,suchashydrogenevolution,1−12oxygenevolution,13−25and CO2-to-CH4conversion.26−31Elucidatingthe relationshipsbetweenthe reactionrate and the propertiesof the catalystsis vitaltothe developmentof high-efficiencycatalysts.A consensusviewin catalysisis that a higherdensityof activesitesindicatesahigherreactionrate.32−40In this work,however,we demonstratethatthis viewisproblematicfor the electrocatalyticproductionof gas by usingmoleculardynamics(MD)simulationscapableof mimickingtheelectrochemicalformationof gasmolecules.Morespecifically,dueto the aggregationof the producedgasmoleculesnearcatalyticsites,a highdensityof catalyticactivesitescouldresultin a reductionin the rateof reaction.Therefore,a higherdensityof activesitesdoesnot necessarilyindicatea higherreactionrate.The simulationdetails,togetherwitha benchmarkwiththe experimentalresult,41can be foundin the SupportingInformation.The simulationsystemwas constructedby referenceto thepreviousstudyon the electrocatalyticproductionof gas(Figure1).42,43The systemcan be initiallydividedinto threeregionsalongthezdirection:(i) a solidslab,(ii) an aqueoussolution,and(iii)an emptyreservoir.Thesolidslabiscomposedof a catalystandan inertsupport,whicharearrangedin a Pt-likeface-centeredcubic(fcc)crystallinestructureexposedto the aqueoussolution.Thesupportconsistsof inertatoms.Thecatalystis locatedat the centerof the solidslaband has a radiusof 2.5 nm.Thecatalystconsistsof activesites and inertatoms.The coverageis definedas the ratioof the numberof activesites to the totalnumberofatomsin the catalyst.We consideredtwo representativecasesfor the distributionof activesites:regulardistributionandrandomdistribution.We generateda sequenceof randomnumbersusinga Pythonscript.Thesequencewas usedtodeterminewhichatomsin the catalystwouldbe chosenasactivesites.Duringthe gas-producingreactionprocess,gasmoleculesare producedcontinuouslyand graduallydiffuseintothe emptyreservoir.A gas deletionregion(locatedat the topof the emptyreservoir)servesto avoida high-pressureenvironment.Thatis, gas moleculesthatdiffuseintothisregionwill be deleted.We first calculatedthe relationshipbetweenthe coverageofthe catalyticsitesand the gas-producingrate at the initialstage(i.e.,the initial100 ps). As expected,the rate increaseswithincreasingcoverage(Figure2a). Unexpectedly,at steadystate,a volcano-shapedrelationshipis observedbetweenthecoverageof catalyticsitesand the gas-producingrate (Figure2b).Thecoveragecorrespondingto the turningpointof thevolcanocurveis 20/32,afterwhichthe rate decreaseswithincreasingcoverage.In otherwords,for electrocatalyticgasproduction,a higherdensityof catalyticactivesitesdoesnotnecessarilyindicatea higherreactionrate.Similarresultsarealso foundfor the case wherethe distributionof catalyticsitesis in a regularpattern.Thegas-producingrate increaseswiththe coverageof catalyticsitesin the initialstage(Figure2c).Also,a volcano-shapedrelationshipis observedbetweenthecoverageand the rate at steadystate(Figure2d).Notably,coverages(18/32,22/32,and 23/32)are not accountedfor inFigure2c,d,sincethe correspondingdistributionscannotbeconstructedfor the regularpattern.Next,we focuson analyzingthe reasonfor the abnormalphenomenon.The producedgas moleculesdo not immediatelymovefar fromthe catalyticsites.In otherwords,a certainamountof timeis requiredfor stayingfar awayfromcatalyticsites.Wespeculatethatthe abnormalrelationbetweenthecoverageof catalyticsitesand the gas-producingrate couldReceived:December23, 2023Revised:February23, 2024Accepted:February26, 2024Published:February28,2024Communicationpubs.acs.org/JACS© 2024AmericanChemicalSociety6461https://doi.org/10.1021/jacs.3c14625J. Am. Chem.Soc.2024,146,6461−6465

be relatedto the producedgas molecules.To testthisspeculation,we performeda seriesof MD simulationsin whichthe producedgas moleculeswereremovedfromthe simulationsystemin an extremelyshorttime(10 ps). As shownin Figure2e, the gas-producingrateincreaseswithincreasingthecoverageof catalyticsitesat the initialstage,similarto theresultin Figure2a. However,in contrastto the resultin Figure2b, Figure2f still showsa trendof increasingthe rate withincreasingcoveragein the steadystate.Thisindicatesthat theabnormalphenomenonpresentedin Figure2b is indeedcloselyassociatedwiththe producedgas molecules.A gradualincreasein the densityof gas moleculesnear the catalystcan beobserved,as shownin the statisticalanalysispresentedinFigureS3. Fromthe resultsin FigureS3, it can be concludedthat a highercoverageof catalyticsitesrendersgas moleculesmoreproneto aggregationnearthe catalyst.Freshlyproducedgas moleculesshouldbe distributednearcatalyticsites.Ahighercoveragemeansa smallerdistancebetweentwo catalyticsites.Accordingly,it also meansa smallerdistancebetweentwofreshlyproducedgas molecules.Consequently,thiswellFigure1.Schematicillustrationof the initialmultiphasemodelusedin this work.(a) Sideviewof the schematicof the usedmodel.The catalyst,inertsupport,and aqueoussolutionare coloredin orange,gray,and azure,respectively.The gas deletionregionis markedwitha red dashedbox.The catalystis a circulardisk and it is surroundedby the inertsupport.(b) Top viewof the top layerof the solidslab consistingof the catalystandinertsupport.Notably,only the top layeris presentedin (b). The catalystconsistsof activesites and inertatoms(denotedby orangeand gray balls,respectively).Tworepresentativecasesare consideredfor the distributionof catalyticsites:a randomdistributionand a regulardistribution.Therandomdistributionis constructedby generatinga sequenceof randomnumbersvia a pythonscript.Theregularand randomdistributionsofcatalyticsites at coverage= 8/32and 16/32are presentedin (b), and the distributionscorrespondingto the othercoveragescan be foundin FigureS1 and S2.Figure2.Relationshipsbetweenthe coverageand the gas-producingrate at−0.25V vs the standardhydrogenelectrode(SHE).(a−d)CoveragedependencyofRgas(calculatedbasedon normalkineticMonteCarlo(kMC)simulations)at the initialstage(a,c)and at the steadystate(b,d).(e−h) CoveragedependencyofRgas(calculatedbasedon kMCsimulationswherethe producedgas moleculesare removedfromthe simulationsystemin an extremelyshorttime)at the initialstage(e,g)and at the steadystate(f,h).Rgasrepresentsthe gas-producingrate relativeto that at coverage=1/32.(a,b,e,f)Resultsfor the casewherethe distributionof catalyticsitesis in a randompattern.(c,d,g,h)Resultsfor the casewherethedistributionof catalyticsitesis in a regularpattern.The specificdistributionsare presentedin Figures1, S1, and S2.Journalof theAmericanChemicalSocietypubs.acs.org/JACSCommunicationhttps://doi.org/10.1021/jacs.3c14625J. Am. Chem.Soc.2024,146,6461−64656462

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