Photoinduced Radical Processes on the Spinel (MgAl2O4) Surface Involving Methane, Ammonia, and Methane/Ammonia

The present study explored photoinduced radical processes caused by interaction of CH4 and NH3 with a photoexcited surface of a complex metal oxide: magnesium–aluminum spinel (MgAl2O4; MAS). UV irradiation of MAS in vacuo yielded V -type color centers as evidenced by the 360 nm band in difference diffuse reflectance spectra. Interaction of these H-bearing molecules with photogenerated surface-active hole states (OS –•) yielded radical species which on recombination produced more complex molecules (including heteroatomic species) relative to the initial molecules. For the MAS/CH4 system, photoinduced dissociative adsorption of CH4 on surface-active hole centers produced •CH3 radicals that recombined to yield CH3CH3. For MAS/NH3, a similar dissociative adsorption process led to formation of •NH2 radicals with formation of NH2NH2 as an intermediate product; continued UV irradiation ultimately yielded N2. For the mixed MAS/CH4/NH3 system, however, interaction of adsorbed NH3 and CH4 on the UV-activated surface of MAS yielded •NH2 and •CH3 radicals, respectively, which produced CH3–NH2 followed by loss of the remaining hydrogens to form a surface-adsorbed cyanide, CNS, species. Recombination of photochemically produced radicals released sufficient energy to re-excite the solid spinel, generating new surface-active sites and a flash luminescence (emission decay time at 520 nm, τ ∼ 6 s for the MAS/NH3 case) referred to as the PhICL effect.