DIRECT DIFFERENTIATION OF SURFACE AND BULK COMPOSITIONS OF POWDER CATALYSTS - APPLICATION OF ELECTRON-YIELD AND FLUORESCENCE-YIELD NEXAFS TO LIXNI1-XO

We have applied near-edge X-ray absorption fine structure (NEXAFS) to characterize the surface and bulk properties of LixNi1-xO catalysts. I n our experimental set-up, NEXAFS spectra of powder materials could be obtained by measuring the intensity of either electron-yield or fluor escence-yield. While the electron-yield method is sensitive only to th e top few atomic layers, the fluorescence-yield method can detect spec ies up to a few thousands angstroms deep into the bulk structure. The ability to distinguish surface and bulk compositions is demonstrated i n studies of a number of Li0.5Ni0.5O samples, of which the surface com positions vary as a function of preparation procedures. In addition, N EXAFS investigations following the reaction of LixNi1-xO with CH4 have also been carried out and the results indicate that the initial surfa ce reaction intermediates are Li2CO3.