DIFFERENTIAL-CONVERSION TEMPERATURE-PROGRAMMED DESORPTION - A NEW METHOD FOR OBTAINING BIMOLECULAR SURFACE RATE CONSTANTS

For bimolecular surface reactions, temperature programmed desorption ( TPD) cannot be used efficiently to detect the product if either of the reactants desorbs before the product. A modification of TPD (differen tial-conversion TPD, or DCTPD) that circumvents this problem is descri bed in this paper. The reactant desorbing at high temperature is first adsorbed as in normal TPD. The surface is then exposed to a continuou s flux of the other reactant, and the rate of product desorption is mo nitored at the same time. The flux is kept so low that the coverage of the reactant first adsorbed scarcely changes. The rate constant is th en determined using this measured coverage and that calculated for the impinging species from its previously-determined adsorption/desorptio n kinetics. A formalism is also developed for cases in which a continu um of related pathways with a distribution of activation energies is p resent. Experimental application of DCTPD is demonstrated in the parti cular case of HCl production from SiH4 and TiCl4 on TiSi2.