THE MECHANISM OF THE EXPLOSIVE NO+CO REACTION ON PT(100) - EXPERIMENTS AND MATHEMATICAL-MODELING

Coadsorbed NO and CO on a Pt(100) surface react upon heating to form extremely narrow TPR product peaks of CO2 and N2 at approximately 400 K whose position and half-width are almost independent of the initial coverages. Video-LEED, TDS and work-function measurements were used to study the adsorption behavior of NO and the surface reaction between coadsorbed NO and CO. The LEED experiments demonstrated that at higher coverages the "explosive" formation of CO2 and N2 is not accompained by the 1 x 1 --> hex phase transition which is shifted up by approximately 50 K to higher temperature. This result rules out the phase transition as a possible explanation for the narrowness of the product peaks in TPR. A new model is proposed based on the vacant site requirement for NO dissociation in which the autocatalytic increase in the number of vacant sites during the surface reaction is responsible for the "surface explosion". This simple vacancy model is modified by taking island formation into account in order to reproduce the coverage independent position and shape of the product peaks. Island formation occurs in the NO/CO-induced lifting of the hex reconstruction as a consequence of the higher adsorption energy of NO/CO on the 1 x 1 phase. Based on these mechanistic steps a mathematical model was developed consisting of five coupled differential equations whose solutions reproduce the experimental results quite accurately.