Indian Journal of ChemistryVol. 22A, August 1983, pp. 693-694

Molecular Orbital Study of FormateChemisorption on a Pt(11l) Surface

H C TANDON & N K RAY·

Department of Chemistry, University of Delhi, Delhi 110007

Received 15 February 1983; accepted 23 March 1983

A molecular orbital study, using atom superposition electrondelocalization (ASED) technique, has been made of the structuresand energy levels of formate species on a clean Ptl l l l l surface.Formate species is predicted to be preferentially adsorbed at a bridgeposition in a highly symmetric C2. configuration with its two-foldaxis perpendicular to the image plane of the Pt(lll)surface. This is ingood agreement with the recent high resolution electron.energy lossspectroscopic studies of Avery [ArP! SIII:lSci, 11/12 (1982) 774]. Thetwo oxygen atoms are predicted to be symmetrically placed above abridge position at a height of 1.81 A.

The decomposition of formic acid over many catalyticsurfaces has drawn considerable attention in recentyears. Many workers have studied thechemisorptionand decomposition of HCOOH on Ni(lIO)!.2,W(OOl)3, CU(OOl)4, Ag(lIO)5, Pt(110)6 and Pt(1II)7surfaces. Recently Avery" has shown that thechemisorbed species on Ptf l l l ) surface is the formate.In our laboratory, we have started a theoretical studyof the electronic structure of chemisorbed formatespecies on various metal surfaces and in this note wereport the results of our study for HCOO/Pt(\ II)system.

We have used atom superposuion electrondelocalisation technique (ASED), which combines one-electron orbital energies with atom-atom repulsionenergies". Here the total energy consists of two parts.The binding part due to electron delocalization is givenby the extended Hiickel one-electron energy, and theatom superposition part is a sum of repulsiveinteratomic interactions. Although this method is notsophisticated as compared to other quantummechanical methods (i.e. X, and ab-initio), it is useful inpredicting trends and general conclusions: preciseenergy values are not obtainable by this method. Themethod has been found useful for determiningstructures, binding energies and electronic energylevels for numerous transition metal complexes andsurface systems, including studies of structures andreaction of water!", carbon monoxide I I - 13 andacetylene I

4 on Pt(1ll) surface.A Pt4 model (see Fig.!) of Pt(lll) surface is used in

the present study. This cluster is bulk superimposableand has four unpaired electrons II -13 at the top of thed-band. Theory parameters are given in Table I.

(

Notes

Geometry optimization have been carried out bykeeping the cluster distance (Pt- Pt) and geometry ofHCOO fixed. at experimental distances.

Calculations are made for both horizontal andvertical orientations (Fig. 1) of HCOO over Pt(lll)

H H HI I I

/c" C /c~

~

o 0 0/:"---0 o : 0

0 0.

COII ill

Pt.VERTICAL ARRANGEMENT

!Y

HORIZONTAL ARRANGEMENT

Table I--Principal Quantum Number, Ionization Potential,Orbital Exponents and Respective Coefficients (£I only) Used

in Present Study

[For all adsorption studies Pt ionization potentials are increased 1.5eV. H.C and 0 ionization potentials are decreased by 1.5 eV and 0and H exponents arc decreased by 0.1: all to mimic sell-consistency

(see ref. II)] .

Atom p d--'-------

Pt 6 9.0 2.55 6 4.96 2.25 5 9.6, 2.39, 6.013,0.5715,0.6567

H I 13.6 1.30

C 2 20.00 1.658 2 11.26 .J.6180 2 28.48 2.246 2 13.62 2.227

Table 2~Formate Chemisorption on Pt(lll) Surfaces:Optimized Distances and Corresponding Energies of the

System

Structure(Fig. I)

(Vertical approach)(J)

(II)(III)

Distance (.1\) Energy (eV)

Pt -0 = 1.83Pt-C=2.41Pt(lll) plane toC distance = 2.38

-738.33- 737.38-738.95

(Horizontal approach)(IV) PtOI!) plane to

C distance = 2.00Pt-C=3.10

-736.73

(V) -736.93

693

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INDIAN J. CHEM., VOL. 22A, AUGUST 1983

surface. The vertical (I, II, III) arrangements are all thetime predicted to be more stable than the horizontalones (IV, V). In coordination chemistry, three types ofcarboxylate ligands ha ve been recognised 15, i.e.monodentate (I), bidentate (II) and bridged (III) (Fig.1). Energy minimizations for all three verticalconfigurations have been made and results of thesecalculations are summarised in Table 2. It can be seenthat HCOO is predicted to be preferentially adsorbedat a bridge position (configuration III). This predictionis in good agreement with the recent high resolutionelectron energy loss spectroscopic studies of Avery".

One of the authors (HCT) is grateful to the CSIR.New Delhi for a fellowship. Thanks are also due to theDST, New Delhi for a computational grant.

694

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