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Iron oxide has been widely used as an oxygen carrier material (OCM) for hydrogen production by chemical looping due to its favourable thermodynamic properties. In spite of this, iron oxide loses much of its activity after redox cycling mainly due to sintering and agglomeration. Perovskites, such as La 0.7 Sr 0.3 FeO 3-δ (LSF731), have been suggested as potential candidate OCMs for hydrogen production due to their excellent oxygen transport properties and stability under cycling. However, hydrogen production per cycle for a similar carrier weight is lower than with iron oxide. This work proposes the use of composite OCMs made of iron oxide clusters embedded in an LSF731 matrix. The perovskite matrix facilitates oxygen transport to the iron oxide clusters while preventing agglomeration. Two preparation methods, mechanical mixing and a modified Pechini method, were used to obtain composite materials with different iron oxide weight fractions, 11 and 30 wt.%. The reactivity of these OCMs was studied in a thermogravimetric analyser. Hydrogen production and carrier stability were investigated in a microreactor over 25 redox cycles while periodically feeding carbon monoxide and water in order to produce carbon dioxide and hydrogen in separate streams. Hydrogen production was stable over 25 cycles for LSF731 and the composite OCM with 30 wt.% iron oxide produced by the modified Pechini method but iron oxide particles alone underwent a decrease in the hydrogen production with cycling. The hydrogen production during the 25th cycle was eight times higher for the composite material than for iron oxide alone and four times higher than for LSF731. The hydrogen production was therefore also higher than that expected from a simple combination of the iron oxide and LSF731 alone, indicating a synergetic effect whereby the LSF731 may have a higher effective oxygen capacity when in the form of the composite material. [ABSTRACT FROM AUTHOR]

Graphical abstract Ni x Cu 1- x Al 2 O 4 spinel solution suppresses carbon deposition during the chemical looping oxidation of CH 4 by Ni-Cu-Al 2 O 3 oxygen carriers. The Ni-Cu-Al 2 O 3 oxygen carriers also contain highly active redox-active lattice oxygen compared to a single metal oxide (CuO or NiO) supported on Al 2 O 3. Highlights • Ni-Cu-Al 2 O 3 oxygen carriers with stable performance over 100 redox cycles. • Fully characterized phase interactions in Ni-Cu-Al 2 O 3 oxygen carriers. • CuO-NiO interaction promotes lattice oxygen activity. • Ni 1- x Cu x Al 2 O 4 spinel solid solution is resistant against carbon deposition. Abstract Chemical looping processes present great potentials to achieve carbon capture and fuel conversion with high thermodynamic efficiencies. Well-known applications of chemical looping include combustion and methane reforming, where phase interactions in oxygen carriers play important roles in determining the process performance. In this study, we systematically investigate the interactions between various phases in Ni-Cu-Al 2 O 3 mixed oxides oxygen carriers, which were prepared from layered double hydroxides precursors, synthesized hydrothermally using urea and metal nitrates. It appears that the addition of 32–45 wt% Al 2 O 3 was sufficient to prevent sintering effects over 100 redox cycles at 800 °C, 1 atm, using methane as the fuel. The oxide phases and their compositions were determined using a set of complementary analytical techniques, allowing us to establish relationships between (i) the compositions of the mixed oxides, (ii) the chemical activity of the various types of lattice oxygen present and (iii) the distributions of gaseous products of chemical looping methane oxidation. We found that the mutual doping between NiO and CuO leads to enhanced lattice oxygen activities, whilst the solid solution of NiAl 2 O 4 and CuAl 2 O 4 leads to reduced lattice oxygen activity in the spinel phase, which also turns out to be particularly resistant to carbon deposition. The generality of the composition – activity - performance relationship is demonstrated by the successful prediction of the product distributions of methane oxidation based on solely the elemental compositions of the oxygen carriers. These findings enable the rational formulation of Ni-Cu-Al 2 O 3 oxygen carriers for methane conversion with precise control of product selectivity. [ABSTRACT FROM AUTHOR]