Role of O- Ions in Charge-Transfer Reactions at the Surface of Silica-Supported Molybdenum Catalysts Prepared by the Grafting Method

Article abstract of DOI:10.1021/j100267a023

In order to elucidate the possible order of O^- ions in charge-transfer processes, the reaction of adsorbed O^- ions with electron acceptor molecules has been investigated on grafted Mo/SiO2 catalysts.It is shown that their high reactivity might explain the fact that O^- cannot be detected in charge-transfer reactions on oxide surfaces between acceptor molecules (A) and surface oxygen in low coordination (O_LC^2-) A + O_LC^2- -> O^- + A^-.Tetracyanoethylene (TCNE) readily reacts with adsorbed O^- ions to form a new intermediate species (g = 2.003, doublet with a^H = 50 +/- 1 G).The hyperfine splitting and experiments performed with various molecules suggest that TCNE undergoes successive reactions with adsorbed O^- species and OH^- surface ions to lead to the formation of an intermediate species assumed to be (CN)-CH=C^.(CN) (a^H is the coupling constant between the unpaired electron and the proton located trans to it on the β carbon).This species is stable at room temperature, but can react in the presence of excess TCNE and lead to the formation of TCNE^- radical.The reaction between fumaronitrile H(CN)=(CN)H and adsorbed O^- ions induces the same doublet with the same coupling constant and confirms the identification of the intermediate species observed with TCNE.It further indicates that the proton is more easily abstracted by O^- than the CN group.This work also shows that on reduced Mo/SiO2 catalysts, the Mo⁵⁺ ions act as electron donor centers while on the oxidized catalysts, the surface O^2- ions do not lead to charge-transfer reactions with TCNE.