www.elsevier.com/locate/susc

Surface Science 601 (2007) 341–345

The dissociation of molecularly adsorbed CO and CN overthe 4d transition metals: A universal relationship between

the reaction barriers and the reaction enthalpies

Paul Crawford, P. Hu *

School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast BT9 5AG, UK

Received 11 July 2006; accepted for publication 1 October 2006Available online 27 October 2006

Abstract

Density functional theory calculations are used to study the stability of molecularly adsorbed CO and CN over transition metal sur-faces. The minimum energy reaction pathways, corresponding reaction barriers (Ea), and reaction enthalpies (DH) for the dissociation ofCO and CN to atomic products over the 4d transition metals from Zr to Pd have been determined. CO is found to be the more stableadsorbate on the right hand side of the period (from Tc onwards), whereas CN is the more stable surface species on the early metals (Zr,Nb and Mo). A single linear relationship is found to exist that correlates the barriers of both reactions with their respective reactionenthalpies.� 2006 Elsevier B.V. All rights reserved.

Keywords: Density functional theory; Transition metal surfaces; Dissociation reactions; CO; CN

1. Introduction

The propensity of an adsorbed molecule to dissociate ona metal surface is a fundamental issue in heterogeneouscatalysis [1,2]. It is also of general interest throughoutchemistry as it highlights the catalytic effect of the metalon particular chemical bonding. To date, most researchaddressing molecular dissociation on transition metalshas focused on the direct dissociation of the molecule fromthe gas phase (dissociative adsorption) [3–5]. Thus, ques-tions still remain as to how the stability of the molecularlyadsorbed state affects the energy barrier to dissociation. Inthis paper, we address molecular dissociation from a pre-adsorbed state by studying CO(ad) and CN(ad) dissociationover the flat surfaces of the 4d transition metals.

The chemistry of CN on transition metals is of interestfor a number of reasons. HCN is an undesirable productin the reduction of NO by hydrocarbons on Rh based cat-

0039-6028/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2006.10.006

* Corresponding author.E-mail address: p.hu@qub.ac.uk (P. Hu).

alysts in the exhausts of cars [6,7]. HCN is however impor-tant industrially, and is produced by the reaction of CH4

and NH3 over Pt based catalysts [8]. Molecularly adsorbedCO is also a topical system. The stability of CO on transi-tion metals is significant in hydrocarbon combustion. Visteet al. have recently studied the kinetics of benzene combus-tion over Rh(1 11) under UHV conditions [9]. They foundthe CO product to dominate over CO2 by a factor of 15.Indeed, CO2 formation was limited by the amount of COadsorbed on the surface. Molecularly adsorbed CO is alsoknown to have a poisoning effect in some systems. The pro-duction of H2 by steam reforming of methanol in fuel celltechnology is one example. CO formation in this systemhas been considered to occur by two different schemes: (i)CO is produced directly by the decomposition of methanol[10] and (ii) CO is produced as a secondary product [11].However, recent kinetic studies of CO formation duringmethanol reforming over Cu based catalysts by Purnamaet al. have shown that CO formation is indeed a secondaryreaction; they suggest that CO is produced by a reversewater gas shift reaction [12].

342 P. Crawford, P. Hu / Surface Science 601 (2007) 341–345

In this work, the stability of molecularly adsorbed COand CN on transition metal surfaces is approached by deter-mining the energetics of dissociation from density func-tional theory calculations. We have calculated the energybarriers to dissociation (Ea) and the enthalpies of reaction(DH) for both CN(ad)! C(ad) + N(ad) and CO(ad)! C(ad) +O(ad) over Zr(001), Nb(11 0), Mo(11 0), Tc(001), Ru(0 01),Rh(1 11) and Pd(111). By examining the respective trendsacross the 4d transition series we aim to address the follow-ing questions: (i) Do the dissociation barriers increase or de-crease on moving from left to right across the period? (ii)How does the stability of CO(ad) compare with that ofCN(ad) on each surface? (iii) Is there a relationship betweenthe reaction enthalpy and the energy barriers analogous tothat observed in dissociative adsorption?

2. Calculation details

The density functional theory calculations were per-formed using the CASTEP code [13]. The effects of electronexchange and correlation were approximated using theGGA-PBE functional [14]. The valence states were ex-panded in a plane wave basis set with a cut off energy of360 eV, and the ionic cores were described by ultra-softpseudo-potentials. The metal surfaces were modelled bythree layers of metal atoms repeated periodically withinp(2 · 2) unit cells. 3 · 3 · 1 k-point sampling was foundto provide sufficient accuracy. The metal atoms were fixedat their bulk truncated positions. Previous work on thechemistry of CO and CN over close packed transition me-tal surfaces addressed the effect of layer relaxation andlayer number; it was found that reaction barriers wereaffected by less than 0.2 eV [15]. A vacuum spacing of

Fig. 1. The calculated minimum energy structures of CO and CN adsorbed onon Zr(001) and Nb(110), (b) that of CO on Mo(110) and (c) shows the most sminimum energy structure of CN adsorbed on Zr(001), Nb(110), Mo(110) anRh(111), and (f) the minimum energy structure on Pd(111). The red spheres rlarger green spheres represent the metal atoms. (For interpretation of the refereof this article.)

approximately 10 A was left between the metal slabs. Thetransition states were identified using the constrained-minimisation technique [16], which constrains the distancebetween the reacting atoms, whilst optimising the remain-ing degrees of freedom. The transition states were identifiedwhen: (i) the forces on the atoms reached a convergencelimit of 0.05 eV/A, and (ii) the energy was a maximumalong the reaction coordinate, but a minimum with respectto all the remaining degrees of freedom. Previous work hasshown that the above settings provide sufficient accuracyfor the type of surface chemical reactions studied in thiswork [17,18].

3. Results and discussion

Calculations were firstly carried out in order to deter-mine the lowest energy reaction pathways, namely, the ener-getically most stable reactants, products and transitionstates. In the interest of conciseness, only the minimumenergy reaction pathways are reported in this paper. If wefirstly consider the initial reactant states, namely the CX(X = O, N) molecule adsorbed on the metal surfaces(Fig. 1), an interesting trend is observed. The calculatedminimum energy structures for CO(ad) are shown inFig. 1a–c. Fig. 1a shows the equilibrium geometry of COon Zr(001) and Nb(1 10), as can be seen, on these surfacesthe CO molecule lies flat, and the O atom interacts with twosurface metal atoms. Moving to the right across the periodwe see that the CO begins to sit upright on the surface.Fig. 1b shows the adsorption geometry of CO onMo(110), here the O atom is interacting with only 1 surfacemetal atom. From Tc onwards the CO molecule is observedto bond perpendicular to the surface via C. A similar trend

the flat surfaces of the 4d metals. (a) Shows the most stable geometry of COtable geometry on Tc(001), Ru(001), Rh(111) and Pd(111). (d) Shows thed Tc(001), (e) the most stable adsorption geometry of CN on Ru(001) andepresent oxygen, the blue spheres represent N and the grey spheres C; thences in colour in this figure legend, the reader is referred to the web version

P. Crawford, P. Hu / Surface Science 601 (2007) 341–345 343

is observed for the equilibrium geometry of CN on thesesurfaces. Fig. 1d shows the calculated minimum energystructure of CN on Zr(00 1), Nb(1 10), Mo(110) andTc(001). Here we can see that the CN molecule lies flaton the surface, and the N atom interacts with two surfacemetal atoms. Again, on moving right across the transitionseries the molecule begins to sit upright on the surface.Fig. 1e shows the minimum energy geometry of CN onRu(0 01) and Rh(111), here the molecule is observed tosit at about 45� to the surface, and interacts with the surfacemetal atoms mainly through C atom. On reaching Pd(1 11)the CN molecule bonds perpendicular to the metal surface

Table 1Calculated geometrical parameters of the minimum energy adsorptionmodes of CO on the close packed surfaces of the 4d metals from Zr to Pd

Surface Adsorption mode ofCO

d(C–M)(A)

d(C–O)(A)

d(O–M)(A)

Zr(001) Flat 2.179 1.369 2.218Nb(110) Flat 2.187 1.294 2.306Mo(110) Flat 2.173 1.278 2.349Tc(001) Upright 2.131 1.202 3.064Ru(001) Upright 2.141 1.194 3.078Rh(111) Upright 2.067 1.191 2.982Pd(111) Upright 2.052 1.187 2.950

d(C–M) is the C–metal distance in angstroms, d(C–O) is the C–O bonddistance in angstroms and d(O–M) is the O–metal distance in angstroms.

Table 2Calculated geometrical parameters of the minimum energy adsorptionmodes of CN on the close packed surfaces of the 4d metals from Zr to Pd

Surface Adsorption mode ofCN

d(C–M)(A)

d(C–N)(A)

d(N–M)(A)

Zr(001) Flat 2.208 1.360 2.125Nb(110) Flat 2.234 1.277 2.236Mo(110) Flat 2.182 1.284 2.240Tc(001) Flat 2.091 1.291 2.190Ru(001) Flat 2.108 1.259 2.300Rh(111) Flat 2.123 1.236 2.397Pd(111) Upright 2.092 1.207 3.009

d(C–M) is the C–metal distance in angstroms, d(C–N) is the C–N bonddistance in angstroms and d(N–M) is the N–metal distance in angstroms.

Fig. 2. Calculated minimum energy transition state geometries for the dissociametals from Zr to Pd. (a) Shows the lowest energy transition state geometryRu(001), (b) Shows the geometry of both CO and CN on Nb(110) and MoRu(001), Rh(111) and Pd(111) and of CN on Rh(111) and Pd(111). The bluspheres represent the metal atoms. (For interpretation of the references in coarticle.)

through C (Fig. 1f). As these initial states are the most sta-ble adsorption modes of CO and CN on the surfaces consid-ered in this work, we have provided the calculatedgeometrical parameters of each structure in Tables 1 and2 respectively.

The transition states to dissociation from the above ini-tial reactant states were located using the aforementionedtechniques. The minimum energy structures on each sur-face are shown in Fig. 2. As can be seen from Fig. 2 thereare basically three different types of transition state config-urations. Fig. 2a shows a geometry where both C and X(X = O, N) are adsorbed at adjacent 3-fold hollow sites.In the case of CO dissociation this is the lowest energystructure on Zr(001) and Tc(0 01); in the case CN dissoci-ation it is the lowest energy structure on Zr(001), Tc(001)and Ru(0 01). Fig. 2b shows the transition state geometryof both CO and CN on the 110 surfaces of the BCC metalsNb and Mo. Here it can be seen that both C and X (X = O,N) sit at bridge sites and do not share the bonding of anysurface metal atoms. Fig. 2c shows a transition state geom-etry where C sits at a 3-fold hollow site and X (X = O, N)sits at a bridge site and shares the bonding of one surfacemetal atom with C. In the case of CO dissociation, thisstructure is the most stable on Ru(00 1), Rh(1 11) andPd(111); whereas in the case of CN dissociation it is theminimum energy structure on Rh(1 11) and Pd(111).

Our calculated energy barriers to dissociation and reac-tion energies for both reactions over the 4d metals are givenin Table 3, and the reaction barriers are plotted in Fig. 3.We can see that this plot has a number of noteworthy fea-tures. The first thing to notice is that, on the perfect sur-faces considered in this work, the energy barriers forboth CO(ad)! C(ad) + O(ad) and CN(ad)! C(ad) + N(ad)

are much greater on the right-hand side of the periodictable (from Ru onwards), thus the kinetic stability ofmolecularly adsorbed CO and CN will be greater on metalsfrom this side of the transition series. It is also clear fromexamination of Fig. 3 that the energy barriers for CN dis-sociation are greater than those for CO dissociation on theearly transition metals: by �0.7 eV on Zr(001) andNb(1 10), and by �0.3 eV on Mo(110). The turning point

tion of CX (X = N, O) over the close packed surfaces of the 4d transitionof CO on Zr(001) and Tc(001), and of CN on Zr(001), Tc(001) and

(110), (c) shows the minimum energy transition state structure of CO one spheres represent X (X = N, O) and the grey spheres C; the larger green

lour in this figure legend, the reader is referred to the web version of this

Table 3Calculated reaction enthalpies (DH), and calculated energy barriers (Ea)for the dissociation of molecularly adsorbed CN and CO over the 4dtransition metals from Zr to Pd

Surface CN(ad)! C(ad) + N(ad) CO(ad)! C(ad) + O(ad)

DH (eV) Ea (eV) DH (eV) Ea (eV)

Zr(001) �0.986 1.265 �2.249 0.540Nb(110) �0.725 1.394 �1.575 0.675Mo(110) �1.167 1.314 �1.572 1.020Tc(001) �1.249 0.921 �1.332 1.348Ru(001) �0.332 1.645 0.348 2.383Rh(111) 0.287 2.368 1.783 3.444Pd(111) 1.027 3.227 2.732 4.328

00.5

11.5

22.5

33.5

44.5

5

Zr Nb Mo Tc Ru Rh Pd

E a (e

V)

COCN

Fig. 3. Variation in the dissociation energy barriers (Ea) for CO(ad) andCN(ad) on the surfaces of the 4d transition metals from Zr to Pd.

R2 = 0.98

00.5

11.5

22.5

33.5

44.5

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3ΔH (eV)

E a (e

V)

Fig. 4. A universal correlation between the energy barriers to dissociation(Ea) and the reaction enthalpies (DH).

344 P. Crawford, P. Hu / Surface Science 601 (2007) 341–345

is Tc, as on Tc(00 1) the energy barrier for CO dissociationis �0.4 eV greater than that for CN dissociation. The en-ergy barriers for CO dissociation are subsequently greaterthan those for CN dissociation: by �0.7 eV on Ru(00 1)and by �1 eV on Rh(111) and Pd(111).

The above results suggest that CN is a more kineticallystable adsorbate than CO on the early 4d transition metals(Zr, Nb and Mo). Our calculations therefore imply that ata finite temperature the rate of CO dissociation will conse-quently be much greater than CN dissociation over theseparticular metals. Furthermore, its is noticed that the en-ergy barriers for CO dissociation are particularly low onZr(001) and Nb(1 10) (�0.5 eV and �0.7 eV respectively);it is therefore possible that CO may spontaneously dissoci-ate to atomic products over these particular surfaces at fi-nite temperatures due to the thermodynamic and kineticinstability. On the far right of the transition series (fromRu onwards) the dissociation barriers for both reactionsare particularly high, ranging from �1.6 eV (CN dissocia-tion on Ru(001)) to �4.3 eV (CO dissociation onPd(111)). Thus, the calculations suggest that higher tem-peratures may be required to produce a significant dissoci-ation rate on these surfaces. We appreciate that the abovediscussion does not take account of the fact that on realsurfaces defects such as steps and kinks may affect the over-all stability of surface species; nonetheless the above resultsprovide insight into the relative stability of molecularly ad-sorbed CO and CN at terrace regions on real surfaces.

It remains to address the trends in the energy barriers,namely, how the reactivity of both processes varies acrossthe periodic table. To this end, we will consider the data gi-ven in Table 3. The first observation is that the reactionenthalpies for both processes are more endothermic onthe right side of the transition series (from Ru onward).More interestingly, however, is the fact that the energy bar-riers are seen to follow closely the reaction enthalpies. Spe-cifically, in the case of both reactions, the energy barriers todissociation are seen to diminish as the reactions becomemore exothermic. Furthermore, when the energy barriersto both reactions are plotted together as a function of reac-tion enthalpy, all the points are observed to fall along thesame line (Fig. 4). That is to say, there exists a single linearrelationship between the reaction enthalpies and the energybarriers to dissociation. This type of correlation is knownas a Bronsted–Evans–Polanyi (BEP) relation [19], andhas been observed to occur in the direct dissociation of cer-tain molecules form the gas phase (dissociative adsorption)[20]. To understand this relationship we must consider thegeometry of the transition state. When the transition stategeometrically resembles the product, the transition state issaid to occur ‘‘late’’ along the reaction coordinate. In thecase of CN(ad) and CO(ad) dissociation, the transition statesare clearly product-like (Fig. 2) in the respect that at thetransition state X (X = N, O) and C are interactingstrongly with the surface atoms and quite weakly with eachother. Indeed, the barriers of reactions with ‘late’ transitionstates are know to be influenced strongly by the reactionenthalpy, i.e. are known to obey the Hammond postulate[21]. It is particularly interesting, however, that in thecase of CO(ad)! C(ad) + O(ad) and CN(ad)! C(ad) + N(ad)

a single linear relationship describes both reactions.

4. Conclusion

First principles DFT calculations have been used tostudy the dissociation of molecularly adsorbed CO andCN over the 4d transition metals. The minimum energyreaction pathways, corresponding reaction barriers andreaction enthalpies have been determined for both reactions

P. Crawford, P. Hu / Surface Science 601 (2007) 341–345 345

over flat surfaces from Zr to Pd. From our analysis thefollowing conclusions can be made:

(i) The dissociation barriers for both CO and CN aregreater on the right hand side of the period (fromRu onwards), thus the kinetic stability of CO andCN is greatest on this side of the transition series.

(ii) On the early 4d transition metals (Zr, Nb and Mo)CN is the more kinetically stable molecule; whereasfrom Tc onwards, CO is the more kinetically stablesurface species.

(iii) The reaction enthalpies are found to be more endo-thermic on the right of the period (from Ruonwards); and, interestingly, the dissociation barriersare seen to diminish with increasing exothermicity ofthe respective reaction enthalpy.

(iv) A universal linear relationship is observed betweenthe reaction barriers and the reaction enthalpies. Itshows that the dissociation barriers for both reactionsare related to their respective reaction enthalpy in thesame way.

References

[1] R.I. Masel, Principles of Adsorption and Reaction on Solid Surfaces,Wiley, New York, 1996.

[2] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis,Wiley, New York, 1994.

[3] V. Logadottir, T.H. Rod, J.K. Norskov, B. Hammer, S. Dahl, C.J.H.Jacobsen, J. Catal. 197 (2001) 229.

[4] C.J. Jacobsen, S. Dahl, B.S. Clausen, S. Bahn, A. Logadottir, J.K.Norskov, J. Am. Chem. Soc. 123 (2001) 8404.

[5] J.K. Norskov, T. Bligaard, A. Logadottir, S. Bahn, L.B. Hansen, M.Bollinger, H. Bengaard, B. Hammer, Z. Sljivancanin, M. Mavrikakis,Y. Xu, S. Dahl, C.J. Jacobsen, J. Catal. 209 (2002) 275.

[6] R. Burch, P.J. Millington, Catal. Today 26 (1995) 185.[7] R. Burch, T.C. Watling, Catal. Lett. 37 (1996) 51.[8] J.J. McKetta, Encyclopedia of Chemical Processing and Design, vol.

27, M. Dekker, New York, 1988, p. 7.[9] M.E. Viste, K.D. Gibson, S.J. Sibener, J. Catal. 191 (2000) 237.

[10] R. Peters, H.G. Dusterwald, B. Hohlein, J. Power Sources 86 (2000)507.

[11] J.P. Breen, J.R.H. Ross, Catal. Today 51 (1999) 521.[12] H. Purnama, T. Ressler, R.E. Jentoft, H. Soerijanto, R. Schlogl, R.

Schomacker, Appl. Catal. A 259 (2004) 83.[13] M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Joannopoulos,

Rev. Mod. Phys. 64 (1992) 1045.[14] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.[15] A. Michaelides, P. Hu, J. Chem. Phys. 114 (2001) 5719.[16] P. Crawford, P. Hu, J. Chem. Phys. 124 (2006) 044705.[17] P. Crawford, P. Hu, J. Phys. Chem. B 110 (2006) 4157.[18] A. Michaelides, P. Hu, J. Am. Chem. Soc. 122 (2000) 9866.[19] M.G. Evans, N.P. Polanyi, Trans. Faraday Soc. 32 (1936) 1333.[20] A. Michaelides, Z-P. Liu, C.J. Zhang, A. Alavi, D.A. King, P. Hu,

J. Am. Chem. Soc. 125 (2003) 3705.[21] G.S. Hammond, J. Am. Chem. Soc. 77 (1955) 334.