Gopalakrishnan, R.

Vetrivel, R.

Viswanathan, B.

English

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NotesChemisorption of Nitrous Oxide on Transition MetalSurfaces-Bond Energy Bond Order Model CalculationR. VETRIVEL, R. GOPALAKRISHNAN & B. VISWANATHAN*Department of Chemistry, Indian Institute of Technology,Madras 600 036Received 20 March 1981; revised 11 JUlie 1981; accepted4 July 1981Using the Bond Energy Bond Order (BEBO) model ealcela-tion for the chemisorption of nitrous oxide on various transitionmetals, it has been shown that there is no stable molecularly ad·sorbed state on the surface and the molecules undergo rapid dls-sociation on most of these metal surfaces. The predicted activitypattern in terms of the heat of adsorption for various transitionmetals is in good agreement with the pattern observed expert-mentally.AMONG the theoretical methods available forthe study of adsorption of gaseous moleculeson solid surfaces, the Bond Energy Bond Order(BEBO) methodv ' has been increasingly used dueto its simplicity and its ability to give results com-parable to those obtained by semi- empirical quantummechanical calculations. In this note the chemi-sorption of nitrous oxide on various transition metalshas been treated within the framework of the BEBOmodel, with a view to predicting the activity orderof the transition metals towards adsorption anddecomposition of nitrous oxide. The adsorption ofnitrous oxide on platinum metal alone has beenstudied by Weinberg'.In BEBO model calculation of weinbergs, nitrousoxide molecule is considered to approach the metalsurface through nitrogen and the results predict thereaction to be endothermic and its heat of chemi-sorption is unrealistic. In the present calculations,it has been assumed that nitrous oxide approachesand gets adsorbed on the metal through the oxygenatom, which seems more probable.The molecular orbital diagram of nitrous oxideshows that 2 ai and doubly degenerate hTa mole-cular orbitals are used to form O=NeN bonds.Further there are three lone pairs of electrons namely,2a~ on oxygen and doubly degenerate hTI7 on ter-minal nitrogen atom. It is presumed that the 2a~electrons on oxygen are utili sed for bonding with themetal surfaces.In gaseous nitrous oxide molecule the bond order'rjO-N of O~N bond is 1.5 (ref. 5) and that of N-Nbond 'rjN-Nis considered as 2.35 in order that theinteraction energy VNso calculated by step (A) andstep (B) are the same at the bond order 'rjM-O= 1.0.As the nitrous oxide approaches the metal throughoxygen atom, the 'l0-Na decreases and simultaneouslythe 'rjM-O of forming ~M-O increases. The wholeprocess is assumed to Ctake place in the followingtwo steps :M + 0 = NiSN -*M-O-NeN (A)M-O - N= N -* M = 0 + N=N (B)The interaction energy between the gaseous moleculeand the metal for the various bond order values arecalculated in terms of the Born-Haber cycle.Bond order variations considered in step (A) are :O:;:;;;'rjM-O<;1.0; and 1.5 > 'rjO-Na"> 1.0. The ener-getics involved in step (A) may be written asVNaO = DNaO - EM_ON a - EO_Na, surf ... (1)where VNao is the interaction energy, DNSO is thedissociation energy of the nitrous oxide", EO_Na, surfis the energy of O-N2 bond in the adsorbed state;EM_oNa is obtained from the relationEM•ONI= 'rjM_ONa'EM_oNa,Burf ... (2)where '1M.ONa is calculated in terms of O-N2 bondorder through the relation,'rjM_ONa= A('rj'oO.N2 - YjO.N2) ••• (3)where A is the ratio between forming bond order toloosening bond order (in this case ~ = 1/0.5 = 2)and llo0_N2is the bond order of O·N2 bond in freegaseous molecule. EM.ONa, surf is the single bondenergy of M-O bond and it has been calculated andtabulated by Miyazaki! for various transition metalsusing parameterised Pauling-Eley equation.Eo•Ni, Burf has been calculated- as a function ofthe bond orders of O·N2 bond using the relation(4).EO-N.,surf = -6'rjtNa + 43'rjtNa - 3'lO_Na ... (4)Thus the interaction energies have been calculatedusing Eq. (1) for the bond order variation of 1.5to 1.0 for the bond O·N2• The bond order variationsconsidered in step (B) are: 1.0 <;;YjM.O<;; 2.0;1.0 ">'lO_N">0.0; and 2.35 <;;YjN.N<;;3.0. The ener-getics involved in step (B) may be written as :VNaO= DNaO --,-EM•ONa - EO-N2,llurf - EN-N ... (5)EN-N corresponds to the increase in the bond energyof N-N bond, due to increase in its bond order from2.35 to 3.0 in the step (B) and is calculated fromthe relation (6)EN_N= [0.65 - ('rjO-Nx 6.65)]. EN_N(g) ... (6)where EN_N(g) is the single bond energy of N-N bondin gaseous state (=38 kcal/mol).The interaction energy between NaO and metalsurface is calculated from Eq, (5) for the bond ordervariation from 1.0 to 0.0 for the bond O-Na.157INDIAN J. CHEM., VOL. 2lA, FEBRUARY 1982The interaction energies calculated at various bondorders are plotted as a function of bond order of theloosening bond (O-Nz) and the pattern is shown inFig. I.The absence of any shallow in the patterns at theearly stages (near '7jO-Na = 1) reveals that there isno stable molecularly adsorbed species. The breaksin the curves indicate that there may be a metastablemolecular state, which immediately undergoes furtherdissociation. Thus the BEBO calculation predictsthat nitrous oxide molecule undergoes mostly anexothermic dissociative chemisorption on transitionmetals.It has been experimentally shown7-l1 that therate of decomposition of nitrous oxide on transitionmetal surfaces is directly proportional to the pressureof nitrous oxide. Further, it was also observed'that nitrous oxide undergoes rapid dissociativechemisorption with nitrogen evolution, leaving anoxide surface layer. These experimental observa-tions justify the assumptions made for the modelcalculations.The activity of various metals to decompose nitrousoxide as predicted by the magnitudes of heats ofadsorption calculated by BEBO model is W> FeesNi>Pd:::::Pt:::::Ir. The activity predicted experi-mentally=w is in the order Fe:::::Ni> W> Pd:::::Pt> Ir.Thus, the activity pattern agrees with the observedOr-r---,---,---,---,----r---r---,-15-30! -459..•>.eo•. -60c•.c:~:s -75-0,..>z-90-105-120Lt.L.,--,!t2:--1:i=.0---::lo.':""s --:().-f-;6:--.l..0;;<.4~~O';':;2:->'-~'lO-N2(Bond <>nHt 01 O-N2 band)Fig. 1 - Variation of interaction energy as a function of theloosening bond order of N.O as a result of adsorption on transi-tion metal surfaces.158trend with a few exceptions like Wand Ir. The excep-tions can be due to additional complications like :(i) simultaneous formation of nitric oxide, duringadsorption of nitrous oxide on these surfaces; (ii)inhibition due to strongly adsorbed oxygen on somemetals; and (iii) condensation of nitrous oxide overthe surface.These calculations suggest a trend for makingfurther rigorous quantum mechanical calculations.However, the model requires major modifications soas to be applicable to ionic solids.The authors are thankful to the CSIR, New Delhifor the grant of fellowships.Referencesl.(a) WEINBERG, W. H. & MERRILL, R. P., Surface Sci.,33 (1972), 493.(b) WEINBERG, W. H., J. Vac. Sci. Technol.• 10 (1973), 89.2. MIYAZAKI, E. & YASUMORI, I., Surface Sci .. S5 (1976),747.3. MIYAZAKI, E. & YASUMORI, I., Surface Sci., 57 (1976),755.4. MIYAZAKI, E., J. Catalysis, 6S (1980), 84.5. WEINBERG, W. H., J. Catalysis, 28 (1973), 49 .6. CRC handbook of chemistry and physics, (CRC Press,Florida), 1979, F-219.7. REDMOND, J. P., J. Catalysis, 7 (1967), 297.8. IsA, S. A. & SALEH, J. M., J. phys. Chem .. 76 (1972),2530.9. HAYES, K. E., Can. J. Chem., 37 (1959), 583.10. REDMOND, J. P., J. phys, Chem., 67 (1963), 788.11. GASSER, R. P. H. & LAWERANCE, C. P., Surface Sci., 10(1968), 91.Electrolytically Generated 4-(2' -Thienyl)quinazolineRadical : Decay in Dipolar Aprotic MediaT. PRIYAMVADA Devr, C. KALIDAS & C. S. VENKATACHALAM·Department of Chemistry, Indian Institute of Technology,Madras 600 036Received 12 JUlie 1981; accepted 28 August 1981The electrolytically generated radical from 4-(2' -thienyl)-quinazoline in dimethylformamide and acetonitrile solutionscontaining O.IM tetrabutylammonium iodide as supportingelectrolyte has been Investigated by EPR spectroscopy. Thevarious possibilities for the observed decay pattern of theradical are discussed.THE key intermediate, 4-(2'-thienyl)quinazoline(4-TQ)t, in the preparation of 4-(2'-thienyl)-quinazolinone is reduced in two one-electron stepsat d.m.e. to give 3,4-dihydro-4-TQ as the finalproduct. in amphiprotic" (methanol-water mixtures)and dipolar aprotic [N,N-dimethylformamlde1 (D MF)and acetonitrile (AN)] media. The first step of reduc-tion in DMF and AN solutions gives 4-TQ neutralradical, the presence of which has been confirmed byEPR experiments", In this note, we present theEPR results for the 4-TQ radical decay in dipolaraprotic media.The EPR spectrum of 4-TQ radical obtained at thelimiting region of the first polarographic wave (at-1.300 V versus silver wire electrode) in DMF at25°C in the presence of 0.1M tetrabutylammonium

Chemisorption of Nitrous Oxide on Transition Metal Surfaces-Bond Energy Bond Order Model Calculation

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Chemisorption of Nitrous Oxide on Transition Metal Surfaces-Bond Energy Bond Order Model Calculation

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