Unique adsorption and catalytic behavior of H2 and CO over hollow Silica-Rh, -Ir, and -Ru nanocomposites prepared by Reversed Micelle Technique

Hollow silica nano spheres containing Rh, Ir or Ru metal particles were synthesized by Rh(NH₃)₆Cl₃ aq, Ir(NH₃)₃Cl₃ aq or Ru(NH₃)₆Cl₃ aq/NP-6/cyclohexane reversed micelle system. Hydrolysis of TEOS surrounding metal ammine complex crystals inside the micelle caused the formation of the hollow, which contained small metal particles inside and tiny metal clusters in the silica network. The amounts of H₂ adsorption over Rh and Ir nanocomposites were two to three times more in the cases of hollow-SiO₂ catalysts compared with those of non-hollow ones, suggesting the occlusion of hydrogen inside the hollows of Rh–SiO₂ or Ir–SiO₂. CO molecules could also permeate into the silica wall and be adsorbed on the metal clusters in the silica wall after 573 K pretreatment. Especially in the case of Ru nanocomposite the amount of adsorbed CO was much more than that of H₂, suggesting some unique character of Ru metal nanoparticles. After 773 K pretreatment, however, the amount of CO(a) decreased drastically to less than 1/10 of H(a), indicating the densification of Si–O–Si bonds and the formation of ultra-micropores in the silica wall where only H₂ can selectively permeate. Selective formation of methane was observed in the CO–H₂ reaction over these nanocomposite catalysts, provably because of the higher concentration of hydrogen inside the hollow and silica network.