| Research Abstract |
It is well known that under low oxygen pressure and at high temperature, dioxygen is evolved from some special mixed oxide, such as perovskite type and spinel type, and from oxide of Co, Mn, Pr, Tb. On the other hand some oxide at low oxidation number (metal was excluded in this study) can reduce water to produce dihydrogen. From thermodynamical consideration, oxide of Ce, Nb, V,Ti, Fe and Sn can have such special low oxidation number. If mixed metal consisted from metals of these two groups are synthesized, it is possible that dioxygen is evolved at high temperature and water is reduced at low temperature to produce hydrogen with regenerating the original oxide. Totally water should be decomposed with thermal cycle.
In this study variety of oxides were tested by periodic reaction where inert gas (Ar) and water/Ar were alternately supplied for oxygen evolution and re-oxidation, respectively. From Co_3, O_4, PrO_<22>, MnO_2 and mixed oxide containing Fe, Co, Mn, Pr or Ag, dioxygen was ev
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olved in Ar at high temperature. From oxide of Sn or Ce and mixed oxide containing Sn, Ce or Nb, hydrogen was generated in water/Ar atmosphere. The behavior of simple oxide could be predicted from thermodynamical calculation. It is revealed that such calculation is a useful guideline for a search of oxides. Because cerium oxide was assumed to be most suitable site for water reduction among the oxide mentioned above, mixed cerium oxide was mainly surveyed. When it is mixed with Ag or Rh, oxygen was evolved from cerium oxide. However, hydrogen was not produced by the successive water feed.
The purpose of this study is to develop a process to produce hydrogen without waste product. If lattice oxygen of oxide is not used for combustion but is used to produce useful chemicals, usage of a reducing agent doesn't degrade the merit of this process. In this study methane was used for the reducing agent to produce ethylene by partial oxidation. From the thermodynamical calculation only oxide of Sn or Fe can produce ethylene from methane and can reduce water in normal reaction temperature range. The latter showed high activity for methane oxidation but selectivity was quite low. Tin oxide showed high selectivity when it was mixed with alkaline earth metal oxide. Especially K_2NiF_4 type oxide showed high activity for C2 hydrocarbon production. Preparation procedure and additives were investigated and the activity was successfully increased without deterioration of selectivity. Less
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