III International Conference

PP-I-18where brackets mean the integer part of the inner expression, and n ads is the fractional part ofthe same expression. In our simulation advance N ads - n ads , N des - n des , N dif - n dif etc. quantitieswere calculated before each successive Monte Carlo step. Then all type of Monte Carlo trialswere realized randomly and numbers n ads, n des etc. were added to corresponding quantitiesN ads , N des etc. for the next Monte Carlo step.The good agreement between the experimental and Monte Carlo generated time variationof fractions of differently labeled oxygen molecules (in the outlet flow)f34( t)16 18= O O16 16 18 18O + O O+Oand22αg( t)=2(1616 18 18O O + 2 O216 18 18O + O O+Owas obtained with following values of the rate constants of isotope exchange elementary steps22)TABLE 1. Rate constants of elementary steps of isotopic exchange.Stages Expression of the reaction rates Rate constants (s -1 )Adsorption: x O 2 + 2Z -> 2[ x OZ] orW ads = k ads C O2 Θ 2 Z or k ads = 7.80016 O 18 O + 2Z -> Z 16 O + Z 18 O2W ads = k ads C OO Θ ZDesorption: 2[ x OZ] -> x O 2 + 2Z orW des = k des Θ 2 xO k des = 0.03216 OZ + 18 OZ -> 16 O 18 O + 2ZSurface diffusion: Z x 1 O + Z 2 -> Z 1 + Z x 2 O W sdif = k sdif Θ xO Θ Z k sdif = 0.030Lattice diffusion: Z x i O + Z j -> Z i + Z x j O W vdif = k vdif Θ iO Θ jV k vdif = 0.034Here x is 16 or 18; Z - oxygen vacancy; i,j - numers of layers; Θ xO - fraction of surface sitesoccupied by x O atoms; Θ Z - fraction of surface oxygen vacancies; Θ iO - fraction of oxygenatoms in i-th layer; Θ jV - fraction of oxygen vacancies in neighbouring j-th layer.Support by RFBR-CNRS 05-03-34761 Project is gratefully acknowledged.Figure 1. Closed squares, experimental f 34 (t) andα g (t) for 18 O 2 and 18 O 16 O over CeO 2 -ZrO 2 -La 2 O 3 ;open squares are the same over CeO 2 -ZrO 2 .Ascending curve, calculated α g (t) for18 O 2 ;descending curve, the same for 16 O 2 ; hump-backedcurve, calculated f 34 (t) for 18 O 16 O.41