Reactivity of Doped Ceria-Based Mixed Oxides for Solar Thermochemical Hydrogen Generation via Two-Step Water-Splitting Cycles

Ceria-type materials were investigated as reactive chemical intermediates, in view of solar thermochemical hydrogen production via two-step water-splitting. Ceria/zirconia mixed oxides and ceria doped with yttrium, lanthanum, praseodymium, or gadolinium were studied using a thermobalance to evaluate their thermal reduction capacity in inert atmosphere and their subsequent reactivity with water steam to generate hydrogen. Ceria/zirconia materials present the highest reduction yields with a noticeable linear increase as a function of the zirconium content (in the range 0-54% Zr), while the gravimetric amount of 02 released during reduction tends to level off for Zr atomic contents above 25%. Temperature-programmed reduction experiments demonstrate that the zirconium insertion favors the bulk reduction. The addition of different dopants (among Y, La, Pr, and Gd) did not affect the global materials reducibility, although it should favor the material thermal stability during repeated cycles. The marked effect of the synthesis method of the material and of the temperature of the reduction reaction on the reactivity of ceria/zirconia was highlighted. In addition, the beneficial influence of decreasing the system total pressure for improving the thermal reduction of ceria/zirconia was experimentally evidenced, offering new prospects for operating a solar thermochemical reactor.