The oxidation of CO on silica-supported hematite (Fe 2 O 3 ) was studied by the step-response method in a tubular fixed-bed reactor, at temperatures ranging between 270 and 350°C. The oxidation process appeared to proceed through two stages. Firstly, oxygen atoms adsorbed on the surface of hematite react with gas phase CO according to an Eley-Rideal mechanism. Once that adsorbed oxygen has been consumed to some extent, surface oxygen from the lattice of iron oxide is removed in a second stage involving CO adsorption and CO reactive desorption steps, thus generating surface oxygen vacancies. Further reduction of hematite proceeds through diffusion of subsurface oxygen into surface oxygen vacancies. On this basis, a kinetic model was developed, which quantitatively describes the transient behavior of the oxidation process. The activation energies and pre-exponential factors of the rate constants and characteristic subsurface oxygen diffusion time could be determined.

The steady-state catalytic reduction of NO with CO on iron oxide was described with three different models, which all fit the experimental data equally well. By applying the transient method to this reaction, it has been shown that for reduced catalysts without adsorbed CO, the N 2 0 reduction step is well described qualitatively by two of the models, whereas only one of them can describe the NO reduction step. In the presence of adsorbed CO, however, none of these models can explain the observed transient behaviour. Therefore, a modification of the best model was proposed and tested via computer simulation, which showed good agreement with transient experiments.On a decrit la reduction catalytique en regime permanent du NO avec du CO sur de l'oxyde de fer au moyen de trois modeles qui montrent tous un bon accord avec les donnees experimentales. En appliquant la methode transitoire a cette reaction, il a ete montre que pour des catalyseurs reduits sans CO adsorbe, I'etape de reduction du N 2 0 est bien decrite qualitativement par deux de ces modeles, tandis qu'un seul d'entre eux est capable de decrire I'etape de reduction du NO. Cependant, en presence de CO adsorbe, aucun des modeles ne peut expliquer le comportement transitoire observe. On propose donc une modification du meilleur modele, qui a ete testee par simulation sur ordinateur et qui concorde bien avec les experiences transitoires.

No abstract