Applicationsof SurfaceScience1 (1978) 471—478© North-HollandPublishingCompany

HEATS OF ADSORPTION OF HYDROGEN ON POLYCRYSTALLINEIRON FILMS *

G. WEDLER, K.-P. GEUSS,K.G. COLB andG. McELHINEYInstitut für Physikalischeund TheoretischeChe,nie,LehrstuhlPhysikalischeChemieII,UniversitatErlangen-Nurnberg, 8520Erlangen, WestGermany

Received11 January1978Revisedmanuscriptreceived13 March 1978

A calorimetric method has beenused to measurethe heat of adsorptionof hydrogenat273 K and 77 K on iron films preparedunderUHV conditions.The initial heat of adsorptionwas98 ~- 3 kJ moI~at 273 K and 88 + 3 kJ mor

1 at 77 K. Film resistancechangesweresimul-taneously measuredand a comparisonof changesin the two parametersallowed the coverageregion to be divided into threedistinct parts.A tentativeexplanationis madeof the causesofthesechangesin termsof the stateof the adsorbedspecies.Data for deuteriumshoweda zero-point energyeffect on theadsorptionheat, this beinginitially 104 3 kJ moF1 at 273 K.

I. Introduction

Iron surfacesareknown to be catalytically activein manyprocessesof industrialvalue involving hydrogen [1,21. Despite this fact and theconsequentinterestthesystem Fe/H

2 remainspoorly understood.Onereasonfor this is that manyof theinvestigationshave beencarried out using technicalcatalyst samplesin which the

stateof theiron surfaceis uncertain.Anotherimportant factoris thehigh reactivityof iron itself, complicatingthe preparationof a clean surfaceon which to performcontrolledadsorptionor catalyticexperiments.

An exampleof theresultingconfusedsituation is providedby publisheddataforthe heat of adsorptionof hydrogenon iron, a parameterof generalinterest.Theliterature containsinformation obtainedon complicatedcatalytic iron samples[3]and films producedunder inadequatevacuum conditions for surfacecleanliness[4—7]. Only recentlyhavea few valuesbeenpublishedwhere thesurfacewasrea-sonablyfree from contamination[8,9].

In this papera direct, calorimetric,measurementof theheatof adsorptionofhydrogenon iron films asa function of coverageis reported,the iron surfacesbeing

* Presentedin part at theCentennialMeetingof theAmericanChemicalSociety, San Francisco,

CA, 1976.

472 G. Wedlereta!. / Heatsof adsorption of hydrogenon iron surfaces

producedunderUHV conditionswith precautionsto ensureminimal contamination.

This information is correlatedwith resistancechangesandthermaldesorptiondataalso measuredfor films, allowing an interpretationin termsof differing statesforthe adsorbedhydrogen.

2. Experimental

The measurementswereperformedusing a bakeableall glassvacuumsystemca-pable of a backgroundpressureof 4 X 1 0--8 Pa.The techniqueandcell usedto mea-sure the heats of adsorptionhave already beendescribed [10,11]. With this cell

changesin film resistancedue to adsorptioncould be concurrentlymeasured,aswell as thecoverage.A topatronmassspectrometerallowed partial pressureanalysisof thegasphase.

The iron films weredepositedunderUHV at a rate of I nm min~to a thicknessof approximately10 nm on Duran glass surfacescooled to 77 K. After depositionthe films wereannealedat 343 K for onehour.Hydrogenanddeuteriumwerepuri-

fied immediately prior to useby diffusion througha palladium thimble.

3. Results

3.1. Measurementsat 273K

The heatsof adsorptionobtainedon four typical films of thickness7—12nm arepresentedin fig. 1. Here theabscissais the coveragen, calculatedusingthegeomet-rical areaof the film, arbitrarily assuminga roughnessfactor of unity for all films.

In each casetheheat is seento be initially independentof coverage.Then it fallssharply by a small amount (~10kJ mol~1),after which the value falls slowly withincreasingcoverage.A further break in this fall occurs at highercoveragesin theform of a short plateau,and thereafterthe adsorptionheat falls further. At cover-ages greaterthan 3/4 maximumsignificant amountsof hydrogenremained in thegasphaseafter eachdosedueto theonsetof reversibleadsorption(fig. 2). This pre-ventedan accuratedeterminationof the amountof hydrogentaken up and hence

of the heatof adsorption.The heatdatashownthereforeonly extendto a coverageof roughly 3/4 maximum.Resistancechangesfor the samefour films are given infig. 3. With theadsorptionof hydrogenthe film resistanceincreasesto a maxiniuniafter which further adsorptioncausesa slight fall. The coveragecorrespondingtothe maximumis seento be largerfor thicker films. The dataof fig. 1 showeda sim-ilar effect of film thicknesson the coverageat which thenotedfeaturesappeared.the correspondingcoveragebeinghigher for thicker films. This is probablyduetothe higherroughnessfactorof the thicker filnis. In fig. 4 theheatsand relativeresis-tancechanges(~RI~Rmax)areplotted againstthequantity~1/FI~ fl~j\ ,where0R~

G. Wedlereta!. / Heatsofadsorption ofhydrogenon iron surfaces 473

104 • 7nmI ~ . Onm

o 10r~m

75[ ~— ~‘.__ o l2nm

~I EI ~ 5O~ _•• ~100I.~

.-.--—-~-‘U’.- •

•—.-‘. I\ I

U F 50D.___

75~

50[ ______ 100

I — ~

~ 7550

0 5 10 15 20

~ lQ14molecules cm2

Fig. 1. The dependenceof the heat of adsorptionq on the coveragen for four films at 273 K.

L-2 12

‘~‘ ~ ~

I, I‘ -3 I 10 I~

191-‘ ~./J ~

I,’ / ~6/5

3

/ 1-8 -~

0 025 05 0.75 10

—0

Fig. 2. Isothermsfor hydrogenand deuteriumat 273 K. The openpointsrepresenttotal pres-sure.

474 G. Wedlereta!. /Heatsof adsorptionof hydrogenon iron surfaces

I0 I2~

C __________________________

U 5 10 15 20

—1014 mnecu!esc.’rr

2

Fig. 3. Resistancechangesfollowing hydrogenadsorptionat 273 K for thesamefour films asinfig. 1.

I ii IIIto ‘~

1•

/

: ~ 05 o~. ~

Fig. 4. The coveragedependenceof the heat of adsorptionq and relative resistancechange~R/~5.Rmaxat273 K.

is thecoveragecorrespondingto themaximumin the resistancechangecurves.Thisfigure showsthat data from the different films can be representedby a single line,eliminating theinfluence of film thickness.Similarly thequantity 0 = ~1~~maxcould

be used,whereumax is the maximum coverageof hydrogenobserved.However,~/~Rmax was chosenbecausethe value of ~Rmax is more accuratelymeasurablethan that of ~1maxdue to the reversibleadsorptionmentionedabove.The heatofadsorptionandrelative resistancechangeresultsfor hydrogenareplotted alongsidedatapointsobtainedfor theadsorptionof deuteriumin fig. 5.

G. WedleretaL /Heatsofadsorption ofhydrogenon iron surfaces 475

100 ~•~•

50

// — H

2datO

0 ~0U 05 1.0 1.5 20

A max

Fig. 5. The coveragedependenceof the heat of adsorptionq and relative resistancechange~R/~Rmax at 273 K for deuteriumcomparedwith hydrogen.

3.2. Measurementsat 77K

Data obtainedat liquid nitrogen temperaturefor two films are shown in fig. 6

where thehydrogenheatsof adsorptionand relative resistancechangeare plottedagainstnInRmax~Again this removestheinfluenceof film thicknessandallowsthetwo setsof points to be representedby a single line. Theheatof adsorptionline isof the sameoverall shapeas at 273 K, althoughthe first stagesoccurat a differentvalue of ~/~Rmax~As at 273 K thehighercoveragevaluesof theadsorptionheatwere not sufficiently accurateand arethereforeomitted. Again the relativeresis-tancechangecurve passesthrougha maximum,althoughat 77 K thereis a markedkneein thepre-maximumregionwhich is muchlessclearly definedat 273K.

100 a-o~--- 1~0

—a- aV° ~ ~a .,,/~ O5~,,/

50 //

/00 10

__________ n0RmOx

Fig. 6. The coveragedependenceof the heat of adsorptionq and relative resistancechange~R/~XRmaxat 77 K.

476 G. Wedlereta!. /Heatsofadsorptionofhydrogenon iron surfaces

4. Discussion

The dataof fig. 4 showthat it is possibleto divide thecoveragerangeinto sepa-

rate regionshaving boundariesat which someparticularchangein thebehaviourofthe adsorptionheator therelativeresistancechangeoccurs.The clearestboundaries

are at ~/~Rmax = 0.45 in theadsorptionheatcurveandatn/nRmax= I in therela-tive resistancechangecurve. On closerinspectionit can be seenthat thesebounda-ries are more general.A kneein the relativeresistancechangecurve alsooccursinthe region of ~/~Rmax 0.45 and the adsorptionheatcurveshows an increaseingradientat ~/~Rmax = 1. Evidently the threeregionsI, II and III of fig. 4, createdby the two boundariesmentioned,denotethe occurenceof somebasic changesinthe adsorbedphase.As the iron films were polycrystallineit is possible that these

regions denotesequentialadsorptionon different crystallographicplanes, i.e. ana priori heterogeneity.Alternatively the changesoccurring in theadsorbedphasecould be due to lateral interactionswithin that phasecausinga restructuringatcer-

tain coverages,i.e. an inducedheterogeneity.As an aid to theinterpretationof thedatapresentedheresomeresultsfrom thermaldesorptionspectroscopywill beused,bearingin mind that suchdata is obtainedby depopulatingtheadsorbedphase,incontrastto the adsorptionheat and relative resistancechangedata which derivefrom a gradualpopulationprocess.

Thermaldesorptiondatahasbeenobtainedfor iron films preparedunderanalo-gous conditionsto thoseof the presentpaper [12]. The spectrashow that, as the

coverageof hydrogen increases,a state which desorbsat ~430 K is first formed,designatedthe02 state.The height of the l~2desorptionpeakgrowsuntil a maxi-mum value is reachedat thecoverage~/~Rmax = 1 andthereafterit remainscon-stant.At highercoveragesa seconddesorptionpeakbeginsto appearat 350 K dueto a state termedf~~•A physisorbedy stateof hydrogenwasfound to desorbat90 K and wasonly formedat coveragesgreaterthanmonolayer.Consequently,inregions I and It of fig. 4 only 02 hydrogenis present,whilst in regions 111 the ~phaseexists on the surface.

The presenceof a stepbetweenregionsI and 11 doesnot, therefore,seemto bedue to a new adsorptionstatebut is more likely dueto the formation of a slightlydifferent02 stateat ~/°Rmax = 0.45. Either of the abovementionedcauses,a priorior inducedheterogeneity,couldaccountfor such a difference.It is interestingtonote that the calorimetric method is able to detect this slight difference in theadsurbate-adsorbentinteraction whilst thermal desorptionspectroscopy.with itsneedto sweepthe temperatureof the adsorbedphase,shows only onedesorptionmaximumin this coverageregion.

Comparison with thermal desorptionspectrashowsregion III can be associatedwith the growth of the 13i state. This, in contrast to the 02 state,producesa de-creasein film resistance.The existenceof two such adsorptionstates,~ and02.could again be due to either of the heterogeneities,a priori or induced.ErtI et al.

[9] have found analogousadsorptionstateson singlecrystal iron samples.It seems

G. Wedlereta!. /Heatsofadsorptionof hydrogenon iron surfaces 477

likely that the 02 -÷ 01 transition reflects the onsetof lateral interactionsashasbeenproved for Fe(110) planes.The useof single crystal planesdid not allow an

unambiguousassignmentof thenatureof thevariousstateswhen no LEED struc-turewasobserved.

The shapesof the curvesof fig. 6, measuredat 77 K, agreewell with thoseof

fig. 4. However, theheat curve showsthe stepat a coverageof ~/~Rmax 0.6,cf.0.45and 273 K andthe reasonfor this particulardiscrepancybetweenthe two setsof data remainsunclear.It can be clearly seenin fig. 6 that the kneeatfl/t~Rmax0.45 in the relativeresistancechangecurveis muchmore markedthan at the highertemperature,probablydueto the decreasedadsorbatemobility.

Dataobtainedfor deuteriumadsorption,fig. 5, showanisotopeeffect on thead-

sorption heatbut not on the resistancechange.Thevalue of theheat for deuterium

was reproducibly ~a~6kJ mol1 higher than for hydrogenover theentire coveragerangemeasuredand it shows the samethreeregions discussedfor hydrogen.The

differencelies outsidetheexperimentalerror andis dueto thedifferent zero-pointenergiesof the H

2 andD2 moleculesandthe Fe—H andFe—Dsurfacecomplexes.A similar behaviourwasfound in the Ni/H2 andNi/D2 system [131, althoughwithPd(l 10)an oppositeeffect wasfound, i.e.a higherheatfor hydrogen 1141.

As mentionedin theintroduction,publisheddatafor the iron—hydrogensystem

is scarce.The value of the initial heatof adsorptionfound here,98 ±3 kJ mol~at273 K (86 ±3 kJ mol~at 77 K) can be comparedwith avalue of 88 ±4 kJ mol

1found for a polycrystallinefilament cleanedin UHV conditions [8]. Theheatsforthe iron filament were derived from measuredadsorptionisobarsand showedasteadydecline with increasingcoverage,no step-like fine structurebeingobserved.These authorsfound only one peak in the thermal desorptionspectrumfrom ahydrogenadlayeradsorbedat 298 K. Resultsobtainedin our laboratoryhaveshownthat with adsorptionat 273 K the ~ desorptionpeakis concealedunderthe 02peakproducinga largeand broaddesorptionmaximumin the spectrum[15]. Theonly published data for adsorptionheatson iron single crystals [9] showedinitialvaluesof 109 kJ mol~, 100 kJ rnol~and 88 kJ mo11 for the (110), (100)and(111) planesrespectively,and thesevalues comparewell with the data reportedhere.Thermal desorptionspectrawere also recordedfrom the single crystalsamplesafter variousdegreesof coverageat ~t140 K. Onthe (110)and(100)planestwo de-sorption stateswerefound correspondingwith the~ and137 statesdiscussedabove.At saturation the relativepopulationof the two stateswasquite different for thetwo planes.The (Ill) planewasfound to exhibit a third desorptionmaximum,due

is a state only populatedat highercoveragesanddesorbingat a temperaturebelowthe normal ~l value. From a comparisonwith their thermaldesorptiondataErtI etal. [91concludedthat the (110) facegavespectraclosestto thosereportedfor ironfilms [12]. However the results for work function change were found to be even

more face specificthan the desorptionspectra(saturationvaluesof 0.310V, 0.075V and —0.095 V for (111), (100) and (110) respectively)and here the (ill) faceapproximatesbetterthebehaviourobservedon our films [15]. In theolderpublica-

478 G. Wedlereta!. /Heatsof adsorptionof hydrogenon iron surfaces

tions valuesof ~l20 kJ mol~areto be foundfor the adsorptionheatof hydrogen

[4—71.It is likely that theseiron surfaceswerecontaminatedwith carbonmonox-ide, or its residues,becauseof the poorervacuumconditions.Thepreadsorptionofcarbonmonoxidehasbeenfoundto increasetheheatof adsorptionof hydrogenoniron [161.Resistancechangesoccurringfollowing adsorptionare in good agreementwith publishedresults[12,17,181.

Acknowledgements

The authors wish to thank the Deutsche Forschungsgemeinschaftand theVerband der ChemischenIndustrie for their financial assistance,and the RoyalSociety of London for a Fellowship (G.M.) underthe EuropeanScienceExchangeProgramme.

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