Kinetics of the Catalytic Decomposition of Hydrogen Iodide in the Magnesium–Iodine Thermochemical Cycle

Abstract

The decomposition of hydrogen iodide serves as the hydrogen-evolution step in several thermochemical water-splitting cycles, including the Magnesium–Iodine cycle. A kinetic analysis of the catalytic decomposition of hydrogen iodide has been carried out by the use of a flow method at 500–700 K. The platinum-supported active carbon catalyst (1 wt%) and the active carbon catalyst which have been found effective in the research reported previously, are used as the catalysts. The contact time-conversion relationships for both the catalysts are simulated successfully on the basis of an assumed reaction scheme. The influence of water vapor on the rate and the equilibrium of the decomposition of hydrogen iodide is negligibly small. The inhibition effect of iodine on the rate of the decomposition of hydrogen iodide over the platinum-supported active carbon catalyst is remarkable below 550 K.

Kinetics of hydrogen iodide decomposition over activated carbon catalysts in pellets

“…We consider previously reported equations for HI decomposition kinetics, both from Osawa over carbon and Pt/C catalyst [13] and from Shindo over Pt/␥-alumina catalyst [14], to be unsuitable for the description of our system, in that they cannot predict an increase in conversion with HI pressure, on the contrary they predict a decrease due to the raise of I 2 relative adsorption enhancing the backward reaction in respect to the forward one. In their models, each HI molecule followed an individual path, decomposing over the surface of the catalyst irrespective of the other molecules; in our model, the positive effect of the pressure means that a kind of cooperation among HI molecules shall be present, in that each HI molecule can be helped in decomposition by the already adsorbed ones.…”

“…In the previous kinetic studies [13,14], only the iodine term has been considered, since it has been clearly demonstrated to be the most significant of all. We neglect, for the moment, only hydrogen adsorption term, surely the smallest one.…”

“…(13) and (14) have not been solved by mathematical integration to find t as a function of x; they were instead solved numerically, fixing a thick range of x values, calculating for each x the value (dt/dx), and then obtaining each corresponding t adding this value to the previous t, with the starting condition that when x is 0, t equals 0 too.…”

“…Once A and D have been fixed, it becomes possible to calculate the exact values of C to substitute in Eq. (13) to match the experimental values of x.…”

“…In order to get a considerable reaction rate, several catalytic systems have been investigated [11] and shown to be active, in particular Pt supported over different substrates, like carbon, alumina and ceria [12][13][14][15][16][17][18][19][20]. To reduce the cost of the metal, also Nickelsupported catalysts have been studied, both over alumina [21,22] and ceria [23,24]: our group of work showed however that Nickel catalysts suffer from deactivation problem at temperature below 700 • C due to the formation of the stable and rather volatile NiI 2 [22].…”