Surface structure and composition of high-surface-area molybdenum nitrides

Abstract

The determination of surface structure is critical in elucidating structure-function relationships for catalytic materials. However, such materials often consist of active regions of very limited spatial extent, rendering conventional bulk structural characterization techniques of limited utility. In this work, we have employed high-resolution transmission electron microscopy coupled with Fourier analysis, and X-ray photoelectron spectroscopy to determine the near-surface structures and compositions of a series of molybdenum nitride catalysts. The results show the near-surface to differ both in crystal structure and composition from the bulk. The bulk structure was [gamma]-Mo2N (fcc) while the lattice structure near the surface was body-centered. Many of the materials contained nitrogen in excess of that expected for phases in the Mo-N phase diagram. As the amount of nitrogen decreased, the oxygen content increased, and the calculated lattice parameter increased. Taken together, the results suggested the presence of Mo2N3-xOx, a hypothetical primitive cubic structure, near the surface. This structure, which would produce a diffraction pattern that approximates that of a body-centered lattice, accounts for the near-surface structural and compositional properties of the high-surface-area molybdenum nitrides.