IndIan Journal of ChemIstryVol. 16A, Aul&ust 1978, pp. 641-644

Orbital Symmetry & Its Role in Catalysis: Decomposition ofNitrous Oxide & Oxidation of Carbon Monoxide

G. NAGASUBRAMANIAN, B. VISWANATHAN· & M. v. c. SASTRIMaterials Science Research Centre, Indian Institute of Technology, Madras 600036

Received 24 October 1977; accepted 20 March 1978

The importance of localized d-orbitals occupancy of proper symmetry, depending uponwhether desorption of O. (for the decomposition of NzO at pressures >200 torr) or the adsorp-tion of N.O (at pressure <50 torr) is the rate-determining step, has been discussed in depth.These studies were made on the perovskite type system (ATiO.; where A = Ba, Sr or Ca), Forthe decomposition of N.O the activity (as measured by the Ea) has been predicted at both pres-sure ranges and is found to be in concurrence with experimental observations. The results areanalysed for the oxidation of CO in the same way on the above catalysts.

WOODWARD and Hoffmann- have shownthat for a reaction to occur, in both direc-tions in the ground state, orbital symmetry

should be conserved along the reaction coordinate.Later Morin and Wolfram- showed that where areaction cannot proceed because of lack of symmetrybetween the reactants and products as well as alongthe reaction coordinate, then the catalyst can playa significant role in converting this symmetry-for-bidden into a symmetry-allowed class by providingsuitable orbitals or surface states of such symmetrythat an activated complex is formed which allowsthe reaction to proceed with symmetry conservation.For reactants where simple molecular species cannotbe considered, there should be suitable localizedsurface orbitals available for partaking in the cata-lysed reactions. The catalyst can therefore beconsidered as an electron relay switch and shouldnot suffer significant electron loss, charge generationor in this sense even electronic excitation. In whatfollows, a qualitative account is presented of theconcepts of orbital symmetry and its role in pro-moting certain reactions in the presence of a catalyst.For the purposes of the present investigation we havechosen to elucidate the mechanism of the decom-position of nitrous oxide and oxidation of carbonmonoxide on titanates with perovskite type oxides.

Considerable attention has been focused recentlyon the transition metal perovskite type oxides andtheir electronic structures, The structural featuresof perovskite type oxides have been studied indetail by the authors (unpublished data). In fact,Wolfram et al», have even suggested that the d-bandperovskite insulators such as SrTi03 would be simplemodel materials for studies of catalytic properties ofd-band perovskites since the d-band surface states inthese materials are normally empty but may bepopulated in a controlled manner by suitable solidstate or photochemical excitation methods. Morinand Wolfram- using a simple LCAO energy band

·Department of Chemistry, Indian Institute of Technology,Madras 600036.

calculation have demonstrated the presence of alarge density of localized electronic states of d-symmetry in [001] face which may be responsiblefor the remarkable catalytic properties of d-bandperovskites.

In AB03 type perovskites the anion orbitals thatare p7t relative to B cations are Po relative to Acations such that the A-O bonding decreases themagnitude of the covalent mixing of the cationic d-orbitals with anionic p-orbitals. Stronger the A-Obonding the weaker will be the covalent mixingbetween d-orbitals of the B cation and p-orbitals ofanion. This essentially means that both A and Bcations compete for the P electron of the anions inpa and P7t modes respectively. The competition isstronger the more acidic the A cation and the relativeacidity increase as one goes up in any column of theperiodic table. Therefore in the iso-structuralseries of Ca, Sr and Ba titanates the mixing of cationicd-orbitals with anion p7t-orbitals will be expected toincrease in the same order.

The ground state of N20 can be considered to belinear", The 16 valence electrons 2S2, 2p3 of nitrogenatom, 2S2, 2p4 of oxygen atom are distributed invarious molecular orbitals and the highest occupiedand lowest unoccupied orbitals are anti-bonding7t*-orbitals only.

For the CO molecule the highest occupied orbitalis localized on carbon atomic nucleus while thelowest unoccupied level is anti-binding 7t*-orbitals.

Results and DiscussionDecomposition of nitrous oxide - For elucidating

the reaction mechanism" N20 decomposition onternary oxides of titanates with perovskite typestructure has been studied in two pressure ranges.At lower pressure range, <50 torr, the rate has afirst order dependence on the pressure of N20 whilethe rate at higher pressure range (>200 torr) can heexpressed as

PRate =k ~(PoY

NAGASUBRAMANIAN et al.: ORBITAL SYMMETRY & CATALYSTS

At pressures > 200 torr the desorption of oxygen isfound to be the rate-determining step while atpressures <50 torr the rate is determined by theadsorption of N20.

Desorption of oxygen as rate determining - In thiscase the typical surface reactions can be expressedby Eqs. (1-3):

O-(ads) + O'{ads) -.. O.+2e- (to the catalyst)N20-(ads) + O'{ads) -->- N2+O.+2e- (to the catalyst)N.O(gas) + O'{ads) -->- N.+O.+e- (to the catalyst)

The first reaction involves two adsorbed oxygenspecies (an incipient O~' species) at the neighbouringsites as a molecule leaving two electrons to thecatalyst, while in the second and third reactionsthe adsorbed oxygen species interacts with theadsorbed or gas phase N20 molecule to give rise toN2 and O2, For reaction (1) one has to consider themolecular orbital scheme for O~-which has 14 valenceelectrons. The presence of empty localized dx.- or dyz-orbitals in a catalyst can function as a switch deviceaccepting the two electrons from the anti-bondingn*-orbitals of Oi-. The presence of such localizedempty surface d-orbitals in insulating perovskitesensures the possibility of this reaction. The pictorialrepresentation and alignment of orbitals, given inFig. 1 is deliberate. It has been shown earlier thatin the series Ba, Sr and Ca titanates the localizationand density of such localized surface states increasesin the order Ba-c Sr-c Ca,

Therefore, one can expect that CaTi03 can providesuch localized surface orbitals with appropriateorientation so that the reaction will experience aminimum barrier for desorption of oxygen and theenergy barrier will increase in the order Ca-c Sr-c Ba.The activation energy for the decomposition of N20at initial pressures >200 torr, wherein the reactionis controlled by the desorption of oxygen is in thesame order (vide Table 1).

If reaction (2) or (3) is considered as rate-determining step then also the energy barrier will bein the same order since (i) the pn-orbitals of 0- caneasily match with one of the d-orbitals of the catalystto strip away its excess electron and (ii)N20- anionas shown in Fig. 2, with its anti-bonding orbitalsoccupancy should be capable of overlapping withtwo cations at next neighbour position with localizedd-orbitals and as discussed above the localized emptyd-orbitals availability decreases along the series ofCa, Sr, Ba titanates.

Fig. 1 - Exchange of electron .pairs .betwee':l the me;al dy•orbital and one of the anti-bonding orbitals of 0.-

TABLE 1 - VALUES OF ACTIVATION ENERGY FOR THEDECOMPOSITION OF N20 AND OXIDATION OF CO

System Ea (kcal/rnole) forN.O decomposition at

s; (kcal/mole) for

oxidation ofCO>200 torr <50 torr

... (1)

... (2)

... (3)

57·50

39·90

30·70

10·00

19'20

24·30

32·24

26·3219·19

BaTiO s

SrTi03

csrto,

This accounts for the Ea for the decomposition ofN20 at pressures >200 torr.

N20 adsorption as the rate-determining step - Ifreaction (4) is assumed, as the rate-determining step,then the anti-bonding (n*) orbitals of N20 should beaccepting the electron from either localized occupiedd-orbitals of the cations (occupation in insulatingtitanates being achieved by thermal or other methodsof excitation). In the case of the series Ba, Sr, Catitanates the occupancy of localized d:"orbitals willinvolve increasing energy barrier along the seriesbecause of the fact that the covalent mixing para-meter An for cationic (n) d-orbitals with anionic P1t-orbitals decreases thus narrowing the 1t bands or inother words increasing the energy gap between thehighest occupied 1t-band and the lowest unoccupiedlocalized d-Ievels. Therefore, with reaction (4) asN.O(gas) +e- (from catalyst) -->- N.O- ... (4)

the rate-determining step one would expect theactivation energy (Ea) to increase in the order BaTiOa<SrTi03<CaTi03. This has been realized as seenfrom the values in Table 1.

Oxidation of co - The highest occupied orbital islone pair on the carbon atom and the lowest un-occupied levels are a pair of 1t*-orbitals. These twostates of appropriate symmetry for bond formationwith surface states such that CO can be chemisorbedas an ionic (probably cationic) species which is anessential step in the mechanism of oxidation of COby the redox process. The order of activation

Fig. 2 - Exchange of electron from It*-orbital of N.O- tometal d-orbitals one above and one below the plane of paperorient toward the anti-bonding orbital of N.O for the exchange

of electron

643

INDIAN J. CHEM., VOL. 16A, AUGUST 1978

energy, if this Werethe rate-determining step, wouldbe CaTiOa<SrTiOa < BaTiOa· If the desorption ofCO2 is the rate-determining step the same order(as in the previous case) in activation energy will berealized (vide Table 1).

The arguments presented in this section may notbe suitable for explaining the behaviour of materialslike ZnO and other oxides because of the followingreasons:

(i) The surface states of these materials basicallyof p-character rather than of d-character may beinefficient for interactions with the "It-orbitalsof thetypical reactants. (ii) The density of surface statesmay be smaller than that expected for perovskitetitanates. (iii) The presence of surface irregularitieslike point defects may be responsible for enhancedsurface activity in these cases.

Acknowledgement

The award of research fellowship by lIT, Madras,to one of the authors (G.N.S.) is gratefully acknow-ledged.

References1. WOODWARD,R. B. & HOFFMANN,R., The conservation of

orbital symmetry ('Veriag-Chemie, Weinheim), 1970.2. MORIN,F. J. & WOLFRAM,T.,Phys. Rev. u«..30(1970), 1214.3. GOODENOUG~, J. B., Progress in solid state chemistry,

Vol. 5, edited by H. Reiss (Pergamon Press, Oxford),1971, 230, 320.

4. WOLFRAM,T., KRANT, E. A. & MORIN, F. J., Phys. Rev.,B7 (1973), 1677.

5. CARTMELL,E. & FOWLES,G. W. A., Valency and molecularstructure (Butterworths, London), 1956.

6. NAGASUBRAMANIAN,G., Ph.D. thesis, Indian Institute ofTechnology, Madras, 1977.