Boreskov Institute of Catalysis of the Siberian Branch of Russian ...

PP-I-21THEORETICAL RESEARCH OF SURFACE REACTIONS KINETICSVitalina KukuevaFire Safety Academy, Onoprienko str., 8, Cherkassy, Ukraine,fax: (80472)55-09-71, e-mail: kukueva@yahoo.comOne of the aspects about solid surfaces that have the largest fundamental and technicalimportance is the way in which chemical reactions are affected by the presence of a surface.At the most general level the role of the surface can often be regarded as a mean of stabilizingintermediates in the reaction. The synthesis of NH 3 directly from N 2 and H 2 , for instance, isextremely slow in the gas phase because it requires an extremely large energy to break theN-N bond. At a metal surface the N atoms are stabilized by the chemisorptive bond to thesurface and this means that reaction can proceed with reasonable activation energy [1].A kinetic description of chemical reactions is much simpler than a full dynamicaldescription. Because of the thermal averaging, only certain features of the adiabatic potentialenergy surfaces, namely the minimum energy barriers separating the minima, enter into therate [2]. We also get a chance to use our understanding of the adiabatic potential energysurface with the ability of relating the properties of chemisorbed molecules and atoms directlyto the kinetics of high pressure catalytic reactions for chemisorptions to explain some of thegeneral questions in the field of catalysis. This is what we shall attempt to illustrate in the thispaper. The kinetics developed for the ammonia synthesis will be used to explain why thetransition metals of iron type are the most active catalysts for this reaction and in which formoxygen (atomic or molecular) presents in such reactions.The collision complexes: Fe…NH 3 , Fe…O 2 , Fe…O have been calculated by quantumchemicalMNDO methods for evidence of the first stage of catalytic transformation, namely –particles adsorption on the metal surface (table 1). The Fe … NH 3 collision complex has beencalculated at the distance between particles from 1 till 3.2Å. The calculation of ammoniummolecule on the clear Fe shows the insignificant interaction. The comparison of activationenergy shows that the less barrier is for ammonia and molecular oxygen adsorption(Е аct = 4,03 eV and Е аct = 4,45 eV, accordingly). For atomic oxygen adsorption the activationenergy has slightly bigger value Е аct = 6,74 eV. It is necessary to note, that there is attractionbetween atomic oxygen and iron. The activation energy analysis of investigated adsorptioncomplexes (table 1) shows that almost at the same distance between particles (R = 1,9Å) theactivation barrier of Fe…O complex is a little bit bigger than analogical values for otherinteracting components. Therefore at the adsorption stage in the competition between particles250