React. Kinet. Ca'}al. Lett., Vo.L 27, No.. 2, 321--324 (1985) OXIDATION OF CARBON MONOXIDE ON RARE EARTH COBALTITES -- ROLE OF SPIN STATE EQUILIBRIUM B. Viswanathan and Susan George Department of Chemistry, Indian Institute of Technology; Madras 600 036, INDIA l~eeeived April 13, ] 984 Accepted June 20, 1984 The catalytic oxidation of CO on a series of LnCoO 3 compounds (where Ln =La, Yr, Nd, Sin, Eu, Gd, Dy or Ho) has been studied. The Arrhenius plots show gradient changes around 180--200 ~ which is the temperature range wherein ordering of low spin and high spin states starts. The activation energy for CO oxidation in the low temperature region varies linearly with oxygen deficiency. IR-Spectra of the adsorbed species in both the high an(] low temperature regions show the presence of carbonate species, l~aTa~HT[~qecKoe OKHcJleHHe CO 6bU]O 14CCJle~IOBaHOB cepm~ LnCoO~(rae Ln = =La, Pr, Nd, Sin, Eu, Gd, Dy ~nu Ho). Appeur~ycoBc~<He 3aB!4CI4MOCTH CBHg, e- TeYIt, CTBylOT 0 nocTeneHHoM uaMeHenm~ npn 180--200 ~ qTo npeAcTaBa~eT co6o~ TeMnepaTypHbI.~ HHTepBaY!, rg, e nacTynaeT ynop~aoqe~He HH3KocrIHHO- BOFO 14 BblCOKOCHI4HOBBIX C0r 3HeprH~ a~<THBat~gH OKr~C:~eHgg CO B l~H3KOTeMnepaTypH0.~ 06:]acTr~ H3MeH~eTC~ ~HHe.~HO C ~e~I4ttHTOM ~<HcJ~0p0zta. IdI~-CneKTpbl a/Icop6HpoBauHb~XUaCTHt[ B 06eHx BblCOKO- H HH3F<OTeMnepaTyp- HbIX 063aCTflX yr<aabiBarOTna rlpHcyTCTBHeKap6OHaTHb~X qacTr~l. The catalytic activity of mixed oxide perovskites for both oxidation and reduction [1, 2] reactions has been examined extensively recently especially because the valence band structures of these compounds are similar to those of transition metals. Rare earth orthocobaltites (LnCoOa) are interesting perovskite compounds with varying magnetic and electrical properties [3] thus providing a scope for establishing interrelations useful for catalyst selection. It has been postulated [4] that the catalytic properties of these systems are related to the spin state equilibrium existing in these systems especially to the relative concentration of high spin Co 3+ states. The experimental evidecne (5) for this postulate is insufficient at the moment. The data obtained on the oxidation of CO on a series of LnCoO 3 compounds (where Ln = La, Pr, Nd, Sm, Eu, Gd, Dy and Ho) are presented in this communication from this point of view. The samples were prepared by calcining the mixture of oxalates at 900 ~ for 24 h. The products were analysed by X-ray diffraction. The oxygen defi- ciency in these samples was calculated from the analysis of the total and the cobMt(III) contents. Catalytic oxidation of CO was carried out in a reeircula- tory static reactor [6]. After activation, stoichiometric mixtures of CO and 321