Use of Oxides in Thermochemical Water-Splitting Cycles for Solar Heat Sources. Cobalt Oxides

摘要

The concept of utilizing oxide decompositions in advanced thermochemical hydrogen cycles for solar heat sources is introduced. It has particular interest in allowing direct transmission of energy to the process through an air window. A cycle for the Co sub 3 O sub 4 -CoO pair would be, schematically: (1) Co sub 3 O sub 4 = 3CoO + 1/2 O sub 2 ; (2) I sub 2 (s,1) + Mg(OH) sub 2 + 3CoO = MgI sub 2 (aq) + Co sub 3 O sub 4 + H sub 2 O(1); (3) H sub 2 O + MgI sub 2 (aq) = MgO + 2HI; (4) 2 HI = H sub 2 + I sub 2 ; and (5) MgO + H sub 2 O = Mg(OH) sub 2 . Reaction (2) should give a high concentration of MgI sub 2 that would be favorable for (3). The solutions would also contain iodine dissolved as polyiodide, partly offsetting this advantage. Preliminary results indicate that reaction (2) is slow at 150 exp 0 I sub 2 (4); and Mg. It is surmised that the mechanism of (2) consists of the iodine disproportional reaction (6), followed by reaction (7); (6) I sub 2 (s,1) + Mg(OH) sub 2 = 5/6 MgI sub 2 (aq) + 1/6 Mg(IO sub 3 ) sub 2 (aq) + H sub 2 O(1); and (7) 1/6 Mg(IO sub 3 0 sub 2 (aq) + 3 CoO = 1/6 MgI sub 2 (aq) + Co sub 3 O sub 4 . Other workers have found (6) to be relatively fast and with a good yield at 150 exp 0 C. We have found reaction (7) to be sufficiently slow at 150 exp 0 C to account for the slowness of (2). The yield of (7) was found to be proportional to the square root of the time, which suggests that iodate must diffuse through an adherent, accumulating Co sub 3 O sub 4 layer. Since (7) is much faster when Mg(IO sub 3 ) sub 2 is replaced by KIO sub 3 , the magnesium ion may catalyze formation of an adherent Co sub 3 O sub 4 spinel layer or enter into the structure of the layer. The reactivity of CoO in the KIO sub 3 analog of (7) is greatly decreased by exposure to high temperature. (ERA citation 07:046084)