Na⁺ doping induced changes in the reduction and charge transport characteristics of Al₂O₃-stabilized, CuO-based materials for CO₂ capture

Abstract

Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are emerging CO2 capture technologies that could reduce appreciably the costs associated with the capture of CO2. In CLC and CLOU, the oxygen required to combust a hydrocarbon is provided by a solid oxygen carrier. Among the transition metal oxides typically considered for CLC and CLOU, copper oxide (CuO) stands out owing to its high oxygen carrying capacity, exothermic reduction reactions and fast reduction kinetics. However, the low Tammann (sintering) temperature of CuO is a serious drawback. In this context, it has been proposed to support CuO on high Tammann temperature and low cost alumina (Al2O3), thus, reducing the morphological changes occurring over multiple CLC or CLOU redox cycles and stabilizing, in turn, the high activity of CuO. However, in CuO-Al2O3 systems, phase stabilization and avoiding the formation of the CuAl2O4 spinel is key to obtaining a material with a high redox stability and activity. Here, we report a Na(+) doping strategy to phase stabilize Al2O3-supported CuO, yielding in turn an inexpensive material with a high redox stability and CO2 capture efficiency. We also demonstrate that doping CuO-Al2O3 with Na(+) improves the oxygen uncoupling characteristics and coke resistance of the oxygen carriers. Utilizing in situ and ex situ X-ray absorption spectroscopy (XAS), the local structure of Cu and the reduction pathways of CuO were determined as a function of the Na(+) content and cycle number. Finally, using 4-point conductivity measurements, we confirm that doping of Al2O3-supported CuO with Na(+) lowers the activation energy for charge transport explaining conclusively the improved redox characteristics of the new oxygen carriers developed.

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.…”

A‐ and B‐site Codoped SrFeO3 Oxygen Sorbents for Enhanced Chemical Looping Air Separation

“…Among various factors affecting CLAS performance, the metal oxide‐based oxygen carrier, also known as oxygen sorbent, plays the most critical role . For example, CuO and Cu 2 O have been identified as promising oxygen sorbents, owing to their reversible reaction with gaseous oxygen at a suitable oxygen partial pressure range (i.e., 0.01–1 atm) and at below 1000 °C, which has also been demonstrated for chemical‐looping oxygen uncoupling for solid fuel combustion and CO 2 capture . Although these monometallic oxides have very high theoretical oxygen capacity, the slow redox kinetics limit their actual oxygen capacity and require high operating temperatures.…”

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

“…21 Among the investigated oxides, Al2O3 is particularly interesting owing to its relatively low cost, high Tammann temperature of ∼885 °C and high mechanical resistance. [30][31][32] However, Al2O3 can react with iron and iron oxide under CL conditions forming the spinel phase FeAl2O4 (hercynite) which leads to a decay in the H2 yield of the material as it cannot be re-oxidized with steam. 10,20,24 Approaches to prevent the formation of FeAl2O4 during redox operation have included the addition of alkali or alkali earth metal oxides to the oxygen carrier, yet the mechanisms behind the mitigation of FeAl2O4 formation remains elusive.…”

“…10,20,24 Approaches to prevent the formation of FeAl2O4 during redox operation have included the addition of alkali or alkali earth metal oxides to the oxygen carrier, yet the mechanisms behind the mitigation of FeAl2O4 formation remains elusive. 21,32,33 In particular, the effect of the addition of sodium on the redox a. Laboratory of Energy Science and Engineering, ETH Zurich, Leonhardstrasse 27,8092, Zurich, Switzerland.…”