X-ray diffraction study of Kr, N2, and H2O adsorbed on pyrogenic silica

To examine the structures and growth modes of adsorbed layers on heterogeneous surfaces, we performed simultaneous measurements of adsorption isotherms and X-ray diffraction patterns of Kr, N-2, and H2O on the poorly hydroxylated surface of pyrogenic silica at 80, 50, and 270 K, respectively. The adsorption of N-2 resulted in a type II isotherm, whereas that of H2O led to a type III isotherm. The adsorption isotherm of Kr was intermediate between them. The analysis of the diffraction patterns showed that the molecules of Kr and N-2 adsorbed on pyrogenic silica formed simple triangular lattices of medium-range order, and thus, they were nearly close packed on the heterogeneous surface. On the other hand, the H2O monolayer did not possess even short-range order. At a monolayer capacity of the Brunauer-Emmett-Teller (BET) equation, the nitrogen cross-sectional area was 15.5 Angstrom (2); the difference with the frequently used value of 16.2 Angstrom (2) was only 4%. At the BET monolayer capacity, neither Kr nor H2O molecules covered completely the surface of the hydroxylated silica.