A method for surface characterization using solid-state nuclear magnetic resonance spectroscopy demonstrated on nano-crystalline ZnO:Al

Nano-scale zinc-oxide doped with aluminium ZnO:Al is studied by different techniques targeting surface changes induced by the conditions at which ZnO:Al is used as support material in the catalysis of methanol. While it is well established, that a variety of 1H and 27Al resonances can be found by solid-state NMR for this material, it was not clear yet which signals are related to species located close to the surface of the material and which to species located in the bulk. To this end, a method is suggested which makes use of a paramagnetically impregnated material to suppress NMR signals close to particle surface in the blind-sphere around the paramagnetic metal atoms. It is shown that it is important to use conditions which guarantee a stable reference system relative to which it can be established whether the coating procedure is conserving the original structure or not. This method, called Paramagnetically Assisted Surface Peak Assignment (PASPA), helped to assign the 1H and 27Al NMR peaks to the bulk and the surface layer defined by the blind- sphere of the paramagnetic atoms. The assignment results are further corroborated by the results from heteronuclear 27Al{1H} dipolar dephasing experiments, which indicate the hydrogen atoms are preferentially located in the surface layer and not in the particle core.