Mixed adsorption of D2 and C2H4 on polycrystalline tungsten

The adsorption and reaction of hydrogen and ethylene on polycrystalline tungsten were studied by means of flash filament spectroscopy. For an ethylene exposure just sufficient to form a monolayer on a bare surface, a D2 covered surface adsorbs 12 monolayer of ethylene without loss of D2; for the inverse process, preadsorption of C2H4, no D2 is adsorbed. In flash filament spectra taken subsequent to a dose of hydrogen followed by ethylene, the major gas phase products are hydrogen and small quantities of ethane. The hydrogen spectra contain peaks at 300°K (ethylene decomposition), 450°K (both ethylene decomposition and hydrogen desorption), and 650°K (hydrogen desorption) with H2 predominant in the 300° peak and D2 predominant in the high temperature peaks. Since coverage studies show that ethylene decomposition occurs via transfer of hydrogen to the surface where H2 and D2 are known to equilibrate, this concentration of isotopes in specific peaks indicates that C2H4 adsorption does not occur uniformly over the D2-covered surface. These results are interpreted in terms of a surface composed of two regions: region A where D2 and C2H4 coadsorb on independent sites, and region B where D2 and C2H4 occupy the same sites. Arguments are presented for associating W(100) with region A and W(110) with region B. The production of ethane during the flash is consistent with a Langmuir-Hinshelwood mechanism with migration of hydrogen as the rate limiting step.