On the H-exchange of ammonia and silica hydroxyls in the presence of Rh nanoparticles

Attenuated total reflection infrared spectroscopy (ATR-FT-IR) is employed to study the H/D-exchange of planar hydroxylated silica during ND₃ and D₂ dosing, and catalyzed by Rh nanoparticles. For ND₃ dosing, it is observed that the H/D-exchange is about 10 times more efficient in the presence of Rh nanoparticles on the hydroxylated silica than for bare hydroxylated silica. When the Rh adsorption sites are blocked by CO, the H/D-exchange is similar to the case without Rh nanoparticles. No H/D-exchange is observed for exposure to D₂ regardless of the presence of Rh nanoparticles. Hydrogen spillover, involving the decomposition of D₂ on the Rh and subsequent transfer of atomic D to the oxide support, therefore does not explain the observed effects. Alternatively, we conjecture that for ND₃, the exchange is through a mechanism in which ND₃ adsorption on the edge of the Rh particles takes place, followed by direct H/D exchange with the hydroxyls of the support. This exchange is possibly aided by the formation of ammonium complexes with the help of hydrogen from the hydroxyls.