Surface Characterisation from Adsorbate Knight Shifts

Abstract

Recent works on catalyst surface characterisation from adsorbate Knight shifts are reviewed. It has been shown how for supported catalysts the proton NMR gives a better estimate of the metal dispersion than the classical volumetric technique where the amount of hydrogen adsorbed is measured to determine the dispersion. It has also been shown that adsorption on multiple sites can be probed by the adsorbate Knight shifts. Again, for bimetallic systems the Knight shifts has been shown to indicate the composition of the surfaces. This has been corroborated by a microscopic theory using Friedel sum rule. The role of promoters on active site blocking, the evaluation of kinetic parameters and the effect of segregation on the adsorption energy may also be found from NMR Knight shifts. The works reviewed here concern generally hydrogen as an adsorbate. However, some work using 2 H , 13 C and 129 Xe NMR are also discussed. Deuterium NMR helps to find the kinetic parameters in exchange reaction and is complementary to the proton NMR in surface studies. Similarly, Carbon-13 NMR helps to understand the organic reactions on metal surfaces as well as the surface structure of catalysts. 129 Xe NMR helps to understand the internal structure of zeolites.