Radiotracer mechanistic studies of heterogeneous adsorption and catalytic reactions

Abstract

Both the hydrogen adsorption and the catalytic reactions over Palladium have been studied with tritium as a radiotracer. The work is divided into two major portions: (i) The interactions of hydrogen with Pd black as well as Pd supported on activated carbon (Pd/C); (ii) The mechanistic studies of hydrogen, hydrogenolysis, and exchange reactions over Pd/C in a flow system. In the first portion, the types of hydrogen adsorption on Pd black and Pd/C were studied by using the Temperature Programmed Desorption Technique (TPD) with a H(,2)/HT/T(,2) mixture. For the Pd black system, five energetically different states of adsorbed hydrogen, arbitrarily defined as B, C, D, E, and F sites, were observed due to the appearance of five peaks on the TPD spectrum, where peak D is the major peak in size. For the Pd/C system, six TPD peaks, A', B', C', D', E', and F' were observed. Peak A', which appeared at -120(DEGREES)C, was shown to be in a molecular form adsorbed on the supporting material, activated carbon. The latter information was obtained from a series of isotopic separation experiments which also indicated that the hydrogens adsorbed on the other active sites on Pd were all in the atomic form. Judging from the desorption temperatures, peaks B, C, and D from Pd black showed a correspondence to peaks B', C', and D' from Pd/C. However, in the Pd/C system, the major peak was shifted from peak D to peak C. On the other hand, Peak E and F from Pd black were considerably higher in their desorption temperatures than those of peaks E' and F' from Pd/C. This indicated that supporting Pd on activated carbon may change the properties of the adsorption sites. Other experimental results indicated that hydrogen adsorbed at the higher temperature sites could not shift to the lower temperature sites. In addition, kinetic parameters such as the activation energies of desorption for the various sites were also evaluated. In the second portion, hydrocarbons such as C(,2)H(,4) and fluorocarbons such as C(,2)H(,3)F and C(,2)H(,5)F, were used to investigate the mechanisms of hydrogenation, exchange, and hydrogenolysis reactions in a flow type system. From the information of the tritium content in the products, some insights about these catalytic processes could be obtained.