Interpretation of the Multiplicity of Kinetics in Heterogeneous Catalysis*

The multiplicity of reaction kinetics in heterogeneous catalysis has been studied by using the oxidation of propene over Pt/SiO_2 of widely varying dispersion, relating to the adsorption behavior of reaction components at temperature a range of 290〜350℃. The reaction consists of a partial oxidation to propanal and complete oxidation to CO_2 and H_20, each of which progresses independently in parallel on own active sites. The rate of propanal formation indicates a characteristic oscillation. The oscillation seems to be a multiplicity caused by the switching between two different surface structures on each of which the partial oxidation progresses at different rate, high rate (γCHO^H) and low rate (γCHO^L). For γCHO^H andγCHO^L, the L-H and E-R models consistently explained all experimental data. Three rate equations for the two rates to form propanal and the complete oxidation are expressed γCHO^H =κCHO^HPc_3H_6^<0.25>Pn_2O^<0.42> γ^<CHOL>=κCHO^LPc_3H_6^<0.29>Pn_2O^<0.31> γCO_2=κCO_2Pc_3H_6^<0.36>Pn_2O^<0.81> The activation energies for propanal formation at high and low rates and for CO_2 formation were evaluated to be 9(±4)kJ/mol 92〜218kJ/mol and 85(±5)kJ/mol, respectively. All the surface coverages of N_20, C_3H_6 and propanal during the reaction were evaluated to be θN_2O=0.13, θC_3H_6=0.69−1.17 and θCHO=0.15, respectively. θC_3H_6 is presumed to be a sort of trigger for the oscillating rate