AN INFRARED STUDY OF THE INTERACTION OF DINITROGEN WITH DIHYDROGEN OVER SUPPORTED RHODIUM FILMS

The adsorption of dinitrogen in the presence of dihydrogen over suppor ted rhodium catalyst films has been studied at pressures up to 8000 To rr using infrared spectroscopy as the analytical probe. The integrated area of the Rh-N-2 band at 2248-2258 cm(-1) has been employed to esti mate coverages and adsorption equilibrium constants as a function of t emperature. These data were used to determine enthalpies and entropies of adsorption for the Rh-N-2 species. It was found that the presence of Rh-H species coadsorbed with the Rh-N-2 species caused intensity en hancement of the Rh-N-2 infrared band, a blue shift in its frequency, increased coverage of N-2 relative to a Rh-H-free surface, and a stron ger interaction of N-2 With Rh. Two forms of hydrogen interacting with Rh appear to be important in causing these observations-a weakly-boun d reversible form with a Rh-H infrared band at 2013 cm(-1) and a stron gly-bound form postulated by others which has no detectable infrared b and. It was also observed that the support TiO2 induced stronger N-2 i nteraction with Rh than did Al2O3; the trend referred to above concern ing coadsorbed Rh-H was observed for both supports.