On the nature of the rate limiting step in the reaction of cracking water molecules on a metallic surface

F. Ghezzi

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It is well known that the chemical interaction between gas and solid occurs principally through two steps: surface associated processes (viz. adsorption, dissociation) and bulk dependent processes (viz. diffusion, bulk sorption). Earlier works on metallic alloy-hydrogen systems have shown that the hydrogen sorption rate by the getter alloy is decided by the surface related processes at low pressures and that it is diffusion dependent at higher pressures. In the series of batch conversion experiments conducted on cracking isotopic waters using a metallic (Zr-V-Fe) getter alloy, it has been observed that, over a wide range of pressure (1-500 Pa) and temperature (100-400°C), the conversion reaction rate remains governed by surface processes (characterized by P1dependence of the absorption rate) at the beginning of the process, i.e., when the water vapor pressure is high, and that it shifts itself to become diffusion dominated (characterized by the P1/2dependence) at later times, i.e., when the water vapor pressure becomes low. This observed behavior is attributed to the amount and nature of change in the hydrogen and oxygen concentrations in the alloy occurring during the conversion process. This work presents the experimental observations and discusses the occurrence and relevance of shift in the rate-limiting step during the conversion process. © 1998 American Vacuum Society.
Original languageEnglish
Pages (from-to)3096 - 3103
Number of pages8
JournalJournal of Vacuum Science and Technology A: Vacuum, Surfaces and Films
Issue number5
Publication statusPublished - 1998
Externally publishedYes


All Science Journal Classification (ASJC) codes

  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Surfaces, Coatings and Films

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