Intermolecular Electron Transfer on the Surface of MgO Nanoparticles

A systematic study of electron-transfer reactions on the surface of MgO nanoparticles, exposed to H 2 /O 2 atmosphere in the dark, was performed. The evolving products are two different types of surface complexes consisting each of a paramagnetic superoxide anion (O 2 - ) on a Mg 2+ cation and an adjacent infrared active hydroxyl group (OH). Via the electronic (H-bond-like) influence of O 2 - on the OH group (red shift of the respective IR signal) the process of O 2 - evolution can be monitored IR spectroscopically on a time scale of minutes. Additional structural information on the complexes is obtained via the magnetic influence of the OH proton on O 2 - (superhyperfine splitting of the respective EPR signals). On the bases of these data as well as on a kinetic evaluation of the time-dependent IR and EPR signals, a description of two local surface structures involved in a complicated reaction scheme was obtained: two hydride groups as intermediate products are constituents of a low coordinated anion vacancy or of a (110) microplane, respectively. A fraction of the products (O 2 - ···HO) is located on ion pairs adjacent to or in these structure elements. Because of an intermediately produced mobile H radical, further product evolves on similar ion pairs (same coordination), but remote from the local surface structures where H 2 may be split.