J.C.S. Faraday I, 1980, 76, 49-59Influence of Dipole Inter actions on Surface ReactionsBY JAY B. BENZIGER~Department of Chemical Engineering, Stanford University,Stanford, California 94305, U.S.A.Received 19th December, 1978Electrostatic interactions between dipoles of adsorbed molecules can influence reaction mechanismand kinetics. Repulsive interactions can increase the rate of surface reactions, whereas attractiveinteractions can decrease the rate of reaction because of an increased stability of the adsorbed species.With formic acid decomposition on Ni surfaces attractive dipole interactions resulted in the formationof a condensed surface phase, which decomposed with autocatalytic kinetics. The formation of thecondensed phase was affected by both crystallographic structure as well as adsorption temperatureby affecting the approach to an equilibrium configuration.Attractive dipole interactions also affect the orientation of molecules on a surface, thus facilitatingreactions that might not otherwise occur.The formation of methyl formate from formaldehyde on aW(100) - (5 x l)C surface has been attributed to the favourable alignment of formaldehyde moleculesstemming from attractive dipole interactions.Adsorbate-adsorbate (A-A) interactions affect the adsorption/desorptionbehaviour of gases on metal surfaces. LEED studies have shown that orderedphases are formed due to these interacti0ns.l Large changes in the isosteric heatsof adsorption result from repulsive interactions between adsorbates.2 In tem-perature programmed desorption experiments A-A interactions result in multipledesorption peaks.The effects of these interactions can be treated by statisticalmechanics and models have been developed to explain temperature programmeddesorption results in terms of adsorbate-adsorbate interaction^.^'^Most models have assumed the adsorbate interactions to be the result of indirectinteractions through the metal surface. Theoretical evidence for oscillatory indirectA-A interactions uia the substrate was first presented by Grimley.’ More recentlyEinstein and Schrieffer * have shown that the periodicity of this oscillatory behaviouris related to the periodicity of the substrate. These models have been useful inproviding a basis for the existence of A-A interactions, correlating well with theobservations for many ordered LEED patterns.Over forty years ago both Langmuir and Roberts examined some implications ofA-A interactions which they attributed to dipole-dipole interactions. 9-1 Mostmodern investigators have dismissed the importance of these dipole interactions ;however, recent studies of adsorption and reactions of organic molecules on singlecrystal metal surfaces indicate that dipole interactions are important in determiningreaction kinetics and in affecting reaction mechanisms. This paper discusses thenature of dipole-dipole interactions and how these interactions affected adsorption,surface reactions and reaction kinetics.DIP 0 LE-D I P 0 LE INTER A CTI ON SIn order to have dipole-dipole interactions an adsorbed species must possesseither a permanent dipole (e.g., the dipole of formaldehyde) or a dipole due tot Present address : Department of Chemical Engineering, Princeton University, Princeton, NewJersey 08540, U.S.A.450 DIPOLE INTERACTIONS I N SURFACE REACTIONSadsorption (e.g., oxygen adsorption on a metal will result in a polar M-0 bond).The adsorption of two such molecules can be represented as in fig.1. The totaladsorption energy for these two molecules can be written asEtotal = Eo + Econfigwhere Etotal is the total adsorption energy, Eo is the adsorption energy for isolatedmolecules and Econfig is the configurational energy due to adsorbate-adsorbateinteractions. The configurational term will include contributions both from theindirect interactions and the direct dipole-dipole interactions ; only the latter will beconsidered in the following discussion.FIG.1 .-Dipole orientation for adsorbed species.The interaction energy between two dipoles with dipole moments pl and p2 atlarge separation is given by classical electrostatics asPlP2 U = - -$2 cos O1 cos O2 - sin el sin O2 cos (41 - &)Iwhere Y is the distance between dipoles and the angles are as indicated in fig. 1. Ifit is assumed that the point dipole approximation is valid and that both adsorbedmolecules are in the same configuration then the above expression reduces toPLr3 u = --(3 cos 2e- 1). (3)The significance of eqn (3) is that it shows that the molecular orientation affects theconfiguration energy.If the dipoles were aligned normal to the surface the con-figuration would be repulsive ; whereas, if the dipoles lay in the plane of the surfacethe configuration would be attractive. The important feature to note is that if thedipole has a significant component in the plane of the surface it can lead to attractiveinteractions between adsorbates.The net effect of dipole-dipole interactions can be either attractive or repulsive,depending on the orientation with the surface. Both attractive or repulsive inter-actions will result in the binding energy being dependent on the adsorbate coverage ;however, the sign of the interaction will produce profoundly different effects in theadsorption process. At low coverage, repulsive interactions will result in adsorptionbeing approximately spatially random as that will minimize the effect of the repulsiveforces.Attractive interactions, on the other hand, will result in the adsorbedmolecules condensing into islands where the binding energy is enhanced comparedwith random adsorption. For a given attractive interaction potential (a) there willbe a critical temperature (T,) defined byu) Tc = -2R (4J . B. BENZIGER 51below which a condensed phase will exist in equilibrium with a diffuse phase on thesurface.12 Above the critical temperature a single diffuse phase will exist, withapproximately random distribution.The effects of A-A interactions can best be seen in temperature programmeddesorption experiments. Several previous investigators have treated the effects ofrepulsive A-A interactions on temperature programmed desorption using a quasi-chemical approximation.Adams has derived the following expression for therate of desorption from a square lattice when only nearest neighbour interactions areconsideredd8 l+& - - dt = r$)e[ 1 -1 +ys exp (-E,/itT)where 8 is the fractional coverage, Eo is the adsorption energy of an isolated moleculeand OJ is the interaction energy, This expression is also applicable for attractiveinteractions when the temperature is above the critical temperature, where randomdistribution is a reasonable approximation. Below the critical temperature, desorp-tion can be described as occurring from two phases, a condensed phase where the rateof desorption should be pseudo first-order and a diffuse phase where the rate ofdesorption is described as above.Desorption from the condensed phase can alsobe described by the above expression, but using a local fractional coverage which isunity in the condensed phase.Fig. 2 shows the desorption spectra as a function of adsorbate coverage for thethree cases of (a) no interactions, (b) attractive interactions and (c) repulsive inter-actions. For all three cases E,, was 102 kJ mol-l, the pre-exponential factor was1 x 1 O l 3 s-l and the heating rate (p) was 10 K s-l. An attractive potential of 2.6 kJmol-1 and a repulsive potential of 4.2 kJ mo1-1 were chosen as typical values ofdipole interactions in next nearest neighbour positions to generate the spectra shownin fig.2(b) and (c). (These values were obtained from assuming a dipole moment of1.7 D separated by 5 A. The orientations were 8 = 90" for the repulsive configura-tion and 0 = 30" for the attractive configuration.) The three cases can be seen togive quite distinctive desorption behaviour as a function of adsorbate coverage. Inthe absence of any interactions the dcsorption peak temperature shows no variationwith coverage. This was first pointed out by Redhead l3 and is the distinguishingfeature of a simple first-order reaction process. Atttractive interactions result inthe adsorbate being more stable on the surface than if no interactions were present.The enhanced stability causes the desorption peak to shift to higher temperaturewith increasing coverage, as shown in fig.2(b). This distinguishing feature ofattractive interactions has only been reported for polar molecules like formaldehyde,or formic acid,14* which has suggested the importance of dipole-dipole interactions.Repulsive interactions have been discussed previously with respect to through surfaceinteraction^.^'^ The desorption spectra in fig. 2(c) show that the binding energydecreased with increased coverage, due to repulsive interactions causing desorptionat lower temperatures than if no interactions occurred. In particular one observesa low temperature desorption peak above a fractional coverage of 8 = 3, as increasingadsorption above this point results in adsorbates being in nearest neighbour positionswhere the repulsive interactions are most significant.As will be discussed belowboth attractive and repulsive interactions have been observed experimentally andcan be explained as the result of dipole-dipole interactions52 DIPOLE INTERACTIONS I N SURFACE REACTIONStemperature/Kl.." t I I t 1 I I300 350 400temperature/J . B . BENZIGER 53I I I 1 I 1 I300 350 400temperature/KFIG. 2.-Temperature programmed desorption as a function of coverage for (a) 8 = 1 .O, (6) 8 = 0.75,(c) 0 = 0.50, (d) 8 = 0.25. (A) First-order desorption. (B) First-order desorption with attractiveinteractions w = 2.6 kJ mol-I. (C) First-order desorption with repulsive interactions w = 4.2 kJmol- l.EXPERIMENTAL EVIDENCE FOR DIPOLE-DIPOLE INTERACTIONSFORMIC ACID ON Ni(ll0) AND Ni(100)Attractive dipole-dipole interactions for adsorbed species were shown by Benzigerand Madix to have affected the kinetics of the decomposition of formic acid on Nisurfaces.Formic acid decomposed on Ni(ll0) and Ni( 100) via the dehydrationof two formic acid molecules to give a formic anhydride intermediate which sub-sequently decomposed by an autocatalytic process to yield H2, C02 and C0.15* l6Formic anhydride is a highly polar molecule and should display effects of dipole-dipole interactions. Benziger and Madix proposed that the dipoles are tilted at anangle of 37” to the surface such that there is an attractive interaction between theadsorbed intermediates, which explained the unusual kinetics observed. Thecalculated attractive interaction potential was 11 kJ mol-1 when all pairwise inter-actions were accounted for.The temperature for decomposition of the anhydridewas well below the critical temperature (T, = 660 K) so that one would predict thata condensed phase would form as the coverage increased. Condensation was clearlydemonstrated by the work of Ying l7 shown in fig. 3. At low coverages COz wasformed by the decomposition of the formic anhydride in the diffuse phase at 75OC.As the coverage was increased a condensed phase was formed, resulting in greaterstability of the formic anhydride intermediate, which thus decomposed at a highertemperature. The emergence of the high temperature peak was the indication ofcondensation54 DIPOLE INTERACTIONS IN SURFACE REACTIONSThe formation of a condensed phase on the surface was further exemplified bythe work of Falconer and Madix.18 Deuterated formic anhydride (derived fromDCOOH) decomposed at a higher temperature than normal formic anhydride.When DCOOH was first adsorbed on Ni(ll0) followed by adsorption of HCQOHtwo desorption peaks were observed for the CQ2 product, one coincident with D2desorption where the deuterated intermediate normally decomposed and the othercoincident with H2 desorption where normal formic anhydride decomposed.In25 50 75 100 125 150temperature/oCFIG. 3.-COZ desorption from formic acid decomposition as a function of c0verage.l'contrast to this behaviour was that for coadsorption of DCQOH and HCOOH,where a single C 0 2 desorption peak was found coincident with H2, D2 and HD at atemperature intermediate to those normally observed for HCOOH and DCOOHadsorption.These results clearly showed the formation of a condensed phase;consistent with the model for attractive dipole interactions proposed by Benzigerand Madix.One unique feature of these attractive interactions was the autocatalytic reactionkinetics. As desorption occurred the binding energy of the remaining adsorbatesdecreased as the configurational interaction decreased. This led to an accelerationof the rate with decreasing surface concentration, or in other words an autocatalyticprocess. Such autocatalytic behaviour was observed by Falconer et a P 9 where thJ . B. BENZIGER 55rate of reaction was observed to increase with decreasing coverage at constant tern-perature.To complete the discussion of formic acid decomposition on Ni the effect ofcrystallographic structure on the dipole alignment should be mentioned.TheNi(l10) and Ni(100) surfaces are shown schematically in fig. 4. Ying and Madix 2ohave shown that a four Ni atom cluster stabilized the formic anhydride on Ni(ll0)and proposed that the molecule was oriented such that the dipoles were alignedalong the (110) direction. As the (110) surface is misotropic there was a preferredOrientation for adsorption of the formic anhydride so that island condensation wasreadily effected. On the other hand, the (100) surf= is isotropic so that no preferredorientation for adsorption was indicated.The saturation coverage of the anhydrideon Ni(100) suggested that a six Ni atom cluster was required for adsorption of theanhydride. Initial adsorption in the diffuse phase on the Ni(100) surface wouldresult in the orientation of the molecules being random, inhibiting the subsequentalignment of the dipoles at higher coverages as shown in fig. 4 and hence reducing theoverall attractive interaction in the condensed phase. This was observed by Benzigerand Madix where the attractive interactions for Ni(100) were reduced by 50 % fromthose observed on Ni(110).15 . . . . .e l . .I. . @ t @ .(A) (B)atom; 'l , formic anhydride.FIG. 4.-Formic anhydride island structure on Ni surfaces. (A) Ni(l10), (B) Ni(100). e, NickelMETHANOL ON Ag(110)The oxidation of methanol to formaldehyde on an Ag(ll0) surface with adsorbedoxygen was studied by Wachs and Madix.21 They observed that the adsorption ofmethanol was induced by the presence of preadsorbed oxygen.Employing iso-topically labelled 1 8 0 2 and CH30D they were able to show that each adsorbedoxygen atom induced the adsorption of two methanol molecules which subsequentlyformed methoxy intermediates as shown in fig. 5. This figure shows that the methoxyintermediates were formed in pairs so that a pairwise dipole interaction was expected.The tetrahedral coordination of the carbon in the methoxy group forces the dipoleto be nearly normal to the surface so as to avoid repulsive interactions between themethyl group and the surface resulting in a repulsive force between the methoxyintermediates. The repulsive dipole interaction should then result in the decomposi-tion of the methoxy pairs by two sequential processes.In the first step the rate i56 DIPOLE INTERACTIONS I N SURFACE REACTIONSenhanced by the repulsive forces which reduce the stability of the methoxy inter-mediate. After the first methoxy has decomposed the second gains added stabilitybecause the repulsive dipole force has been eliminated and so the rate of decompositionis slower. The rate expression would bedo - = v(o, + 62) exp (- E,/RT) + V O ~ exp [ -(Ea- a)/RT]dt (6)where o1 is the coverage of the isolated methoxy, o2 is the coverage of the methoxypairs and u) is the repulsive interaction energy. In a temperature programmeddesorption experiment the dipole forces would result in two desorption peaks.D D CH,+ 2 H Z C O + H 2FIG.5.-Mechanism for methanol oxidation on Ag(l10).Furthermore, if repulsive dipole forces were important then one would expect theadsorbed oxygen sequentially to induce adsorption of one molecule, then another,resulting in the high temperature desorption peak growing to saturation followed bythe emergence of the low temperature peak. This behaviour, which is shownschematically in fig. 6 for v = 1013 s-l, E, = 105 kJ mol-1 and o = 10 kJ mol-l,is identical to the observations of Wachs and Madix for formaldehyde formationfrom methanol on Ag(l10). The repulsive interaction of 10 kJ mol-1 was deriveJ . B . BENZIGER 57by assuming a dipole moment of 1.7 D for the methoxy supposing it to be orientednormal to the surface and a separation of 4 A, which are typical values for methanoladsorbed at next nearest neighbour sties on the Ag(ll0) surface.22I I l a350 400 45 0temperature/KFIG.6.-Effect of pairwise repulsive dipoles on temperature programmed desorption.The repulsive interactions between methoxys were observed for low surfacecoverages (< 20 % of saturation coverage), indicating that the adsorbed intermediatesremained adsorbed in adjacent positions and did not establish an equilibrium con-figuration during the time period of the experiment. Establishment of equilibriumis dependent on the rate of diffusion across the surface. Diffusion is temperaturedependent so that the adsorption temperature will affect the approach to equilibriumand hence the interactions between adsorbates.This effect was also observed forformic acid decomposition on Ni( 110) where decreasing the adsorption temperatureof formic acid from 310 to 210 K resulted in an increase in the rate of decompositionof the formic anhydride and suppression of the autocatalytic kinetics.16 The loweradsorption temperature inhibited the diffusion of the formic anhydride to form acondensed phase, resulting in the increased reaction rate which occurred from thediffuse phase.FORMALDEHYDE O N w(100) -(5 X 1)cThe two previous examples have shown the effects of dipole-dipole interactionson the reaction kinetics. As a last example we consider a case where attractive dipoleinteractions influence the reaction mechanism.The adsorption of formaldehydeon a W(100) -(5 x l)C surface resulted in a complex reaction scheme in whichvarious hydrocarbons as well as CO and H2 were formed.14 The initial reactionstep was the decomposition of formaldehyde to CO and hydrogen, with much of thehydrogen reacting with adsorbed formaldehyde to form intermediates which led tohydrocarbon formation. The product desorption spectra were studied as a functionof formaldehyde exposure and showed the CO and H2 peaks, corresponding to theinitial decomposition step, shifted to higher temperature with increasing formaldehyd58 DIPOLE INTERACTIONS I N SURFACE REACTIONSevosure, similar to the effect seen in fig. 2 for attractive interactions. Furthermore,as the coverage was increased above some critical coverage methyl formate wasobserved to desorb at low temperature by a desorption limited process (i.e., methylformate desorbed at the same temperature as when methyl formate was adsorbed),below the temperature at which formaldehyde decomposed.H HHIRo.7.--Mechanism of methyl formate formation from formaldehyde.The coverage dependence of formaldehyde decomposition indicated that attractiveinteractions were important, so the alignment of the dipoles of two formaldehydemolecules was considered. Fig. 7 shows the alignment of formaldehyde moleculeson a surface due to attractive dipole-dipole interactions. As shown in fig. 7 thisorientation is favourable for a hydrogen transfer leading to the formation of methylformate.Attractive dipole interactions can thus influence the configuration ofadsorbed species facilitating reactions which might otherwise not occur.CONCLUSIONSA simple approach to the interactions between the dipoles of adsorbed moleculeshas been presented. In reviewing the experimental results for reactions of simpleorganic molecules on metal surfaces the importance of these dipole-dipole interactionshas been clearly demonstrated. The dipole interactions affect the orientation ofadsorbed molecules as well as the adsorption energy. The distances over which thesedipole interactions are of importance are much greater than the indirect interactionsdiscussed by Einstein and Schrieffer, so that for polar molecules the through spacedipole interactions would be expected to dominate.Furthermore, the dipole inter-actions can be strongly attractive, resulting in the formation of condensed surfacephases, as seen in the case of formic acid decomposition of Ni(l10).The author thanks Prof. M. Boudart and R. J. Madix for their encouragement inpreparing this manuscript. He also thanks the surface reactivity group of Prof. R. J.Madix for providing the fine experimental results supporting the ideas proposed here.This work has been done under the financial support of the Natonal Science Founda-tionJ . B . BENZIGER 59l G. A. Somorjai and H. H. Farrell, Ado. Chem. Phys., 1971,20, 215.J. C. Tracy and P. W. Palmberg, J. Chem. Phys., 1969,51,4852.J. C. Tracy, J. Chem. Phys., 1972,56,2736.D. L. Adams, Surface Sci., 1974, 42, 12.C. G. 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Weast (Chemical Rubber Co., Cleveland, 1968).Cambridge, 1956).P. A. Redhead, Vacuum, 1962,12,203.J. L. Falconer and R. J. Madix, Surfme Sci., 1974,46,473.(PAPER 8/2182