Characterizing Hf _X Zr_1−X O₂ by EXAFS: Relationship Between Bulk and Surface Composition, and Impact on Catalytic Selectivity for Alcohol Conversion

Abstract

A series of mixed Hf _X Zr_1−X O₂ oxide catalysts was prepared according to a recipe that yields the monoclinic structure. The samples were examined by EXAFS spectroscopy at the Zr K and Hf L_III edges. A fitting model was used that simultaneously fits data from both edges, and makes use of an interdependent mixing parameter X _mix to take into account substitution of the complementary atom in the nearest metal-metal shell. For XPS analysis, Scofield factors were applied to estimate the relative atomic surface concentrations of Zr and Hf. EXAFS results suggested that a solid bulk solution was formed over a wide range of X for Hf _X Zr_1−X O₂ binary oxides, and that the relative ratio was retained in the surface shell (i.e., including some subsurface layers by XPS) and the surface (e.g., by ISS). The increase in selectivity for the 1-alkene from dehydration of alcohols at high Zr content does not correlate smoothly with the tuned relative atomic concentration of Hf to Zr. The step change at high Zr content appears to be due to other indirect factors (e.g., surface defects, oxygen vacancies).