III International Conference

PP-I-10CatalystЕа 1 ,kJ/mol(77÷140 К)Т ofmechanismchange,КProperties of the Studied CatalystsЕа 2 ,kJ/mol(140÷690 К)К s·10 -13 ,molecule/(cm 2·s)77 К 300 КRu/Al 2 O 3 0,14 273 3,1 53,7 107,0Ru/sibunitPd/Al 2 O 3Pd/sibunit0,5 343 4,4 32,2 50,3Т, КPressureordern77 0,9273 0,6403 0,7577 0,87343 0,21693 0,95Metalparticlesize,nm4,6 - - 0,21 25,11 - - 244,6 - - 0,15 26,3 - - 3,5Ni/Al 2 O 3 2,5 143 8,8 0,39 94,6Ni/SiO 277 0,9143 0,35293 0,96,16,934005,5 - - 0,033 - - - 1500The necessary condition of the isotope exchange is the formation of hydrogen atoms onmetal surfaces. They can be either the products of dihydrogen dissociation in a precursor stateor fragments formed immediately when a gas-phase molecule falls down to the surface.Predomination of the first or the second mechanism depends on temperature, pressure andconcentration of reaction sites since the active centers on a surface responsible for the twomechanisms may be not the same. However, in both cases the hydrogen atoms appeared afterdissociation are “hot” and possess kinetic energy enough for their motion along the surfacebefore strong adsorption. Quite a long motion of atoms and their collisions to each other givethem an opportunity to recombine and form different isotopomers (for example HD if the gasmixture contained initially the Н 2 and D 2 molecules only).The equations for calculating the catalytic activities of metal particles were obtainedwithin the transition state theory framework invoking the two mechanisms of dissociationdescribed above. Capability of the model is proven by a good correlation between calculatedand experimental values of activities as well as by realistic values of such fitting parameterslike hydrogen-metal bond energies, mean vibrational temperature of adsorbed species,activation energies of elementary steps etc.26