ZrO₂-Supported Fe₂O₃ for Chemical-Looping-Based Hydrogen Production: Effect of pH on Its Structure and Performance As Probed by X-ray Absorption Spectroscopy and Electrical Conductivity Measurements

Abstract

Chemical looping is a promising process to produce high purity H 2 while simultaneously capturing CO 2 . The key requirement for this process is the availability of oxygen carriers that possess a high cyclic redox stability, resistance to carbon deposition, and thermal sintering. In this study, ZrO 2 -supported Fe 2 O 3based oxygen carriers were developed using a coprecipitation technique. We assess in detail the influence of the key synthesis parameter, i.e., the pH value at which the precipitation was performed, on the morphological properties, chemical composition, local structure, and cyclic redox stability. The performance of the new oxygen carriers was compared to unsupported Fe 2 O 3 and Al 2 O 3 -supported Fe 2 O 3 . A higher degree of disorder in the local structure of oxygen carriers precipitated at low pH values was confirmed by X-ray absorption spectroscopy (XAS) measurements. Electrical conductivity measurements showed that supporting Fe 2 O 3 on ZrO 2 lowered significantly the activation energy for charge transport when compared to pure Fe 2 O 3 . In line with this observation, ZrO 2supported oxygen carriers displayed a very high and stable H 2 yield over 15 redox cycles when precipitation was performed at pH > 5.

Chemical looping beyond combustion – a perspective

“…144 The utilization of modern catalysis and surface science techniques in a few recent studies have already started to reveal fundamental insights on the mechanisms of the redox reactions. 78,128,[145][146][147][148][149][150][151] However, significant progress still needs to be made to gain additional fundamental insights and this should be a topic of focus for redox catalyst studies within the foreseeable future.…”

Na-β-Al2O3 stabilized Fe2O3 oxygen carriers for chemical looping water splitting: correlating structure with redox stability

“…15 Sintering leads to an appreciable decrease in the reduction rate of iron oxide, which results in an incomplete reduction and in turn a low H2 yield in the subsequent reoxidation step. 7,25,26 To mitigate deactivation by sintering, strategies based on the addition of a high Taman temperature stabilizing oxide, e.g. Al2O3, 7,10,20,24,27 SiO2, 7,9 TiO2, 28 CeO2, 14 MgAl2O4 19,29 and ZrO2.…”

“…Al2O3, 7,10,20,24,27 SiO2, 7,9 TiO2, 28 CeO2, 14 MgAl2O4 19,29 and ZrO2. 13,21,25 have been proposed. Indeed, these oxides have shown to increase the stability of iron based oxygen carriers over multiple cycles by reducing FeOx/Fe particle sintering.…”

Inhibition of carbon deposition using iron ore modified by K and Cu in chemical looping hydrogen generation

“…Carbon formation possibly happens when the conversion of fuel and the reactivity of oxygen carrier are low . Much work has focused on developing the methods to enhance the redox rate of iron oxides . The common attempt to increase the reactivity of iron oxides is to introduce foreign additives.…”

“…15 Much work has focused on developing the methods to enhance the redox rate of iron oxides. 12,[16][17][18][19][20][21] The common attempt to increase the reactivity of iron oxides is to introduce foreign additives. The addition of copper based on the iron oxygen carriers had been proven to be capable to accelerate the rate and reactivity of reduction in the CLC process.…”