Solid oxygen-carrier materials for chemical-looping applications have been examined by reduction with CH 4 and oxidation with air in a fixed-bed quartz reactor at 900ºC. Four perovskite materials, three metal-oxide materials and four metal-oxide mixtures have been studied. It was found that La x Sr 1─x FeO 3─δ perovskites provided very high selectivity towards CO/H 2 and should be well suited for chemical-looping reforming. Substituting La for Sr was found to increase the oxygen capacity of these materials, but reduced the selectivity towards CO/H 2 and the reactivity with CH 4 . La 0.5 Sr 0.5 Fe 0.5 Co 0.5 O 3─δ was found to be feasible for chemical-looping combustion applications. NiO/MgAl 2 O 4 propagated formation of solid carbon, likely due to the catalytic properties of metallic Ni. Fe 2 O 3 /MgAl 2 O 4 had properties that made it interesting both for chemical-looping combustion and chemical-looping reforming. Adding 1% NiO particles to a bed of Fe 2 O 3 -particles increased both reactivity with CH 4 and selectivity towards CO/H 2 for reforming applications. Mn 3 O 4 /Mg-ZrO 2 was found to be suitable for chemical-looping combustion applications, but it could not be verified that adding NiO produced any positive effects.
The mechanism and structure requirements of selective and total oxidation of methane in a chemical looping process are both experimentally and theoretically examined on La 1−x Sr x FeO 3−δ (x = 0, 0.2, and 0.5) and La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0.5 and 1) perovskites. The oxygen mobility in the perovskites described by the formation energy of oxygen vacancy is found to have a pronounced effect on the catalytic activity and selectivity. In particular, the selectivity is controlled largely by the surface oxygen concentration or the oxygen vacancy concentration on perovskites, which depends strongly on the bulk oxygen concentration and the relative rate of the lattice oxygen diffusion with respect to the surface reaction. The substitution of Sr for La at the A site and the substitution of Co for Fe at the B site of the ABO 3 perovskites dramatically increase the oxygen mobility. A higher oxygen diffusion rate, and hence enrichment of oxygen on the surface, would improve the catalyst selectivity toward total oxidation.
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