Luongo Giancarlo, Bork Alexander H, Abdala Paula M, Wu Yi-Hsuan, Kountoupi Evgenia, Donat Felix, Müller Christoph R
Laboratory of Energy Science and Engineering, Department of Mechanical and Process Engineering, ETH Zurich Leonhardstrasse 21 8092 Zürich Switzerland
J Mater Chem A Mater. 2023 Feb 27;11(12):6530-6542. doi: 10.1039/d2ta09102j. eCollection 2023 Mar 21.
We elucidate the underlying cause of a commonly observed increase in the rate of oxygen release of an oxygen carrier with redox cycling (here specifically for the perovskite SrCaFeO ) in chemical looping applications. This phenomenon is often referred to as activation. To this end we probe the evolution of the structure and surface elemental composition of the oxygen carrier with redox cycling by both textural and morphological characterization techniques (N physisorption, microscopy, X-ray powder diffraction and X-ray absorption spectroscopy). We observe no appreciable changes in the surface area, pore volume and morphology of the sample during the activation period. X-ray powder diffraction and X-ray absorption spectroscopy analysis (at the Fe and Sr -edges) of the material before and after redox cycles do not show significant differences, implying that the bulk (average and local) structure of the perovskite is largely unaltered upon cycling. The analysis of the surface of the perovskite X-ray photoelectron and Raman spectroscopy indicates the presence of surface carbonate species in the as-synthesized sample (due to its exposure to air). Yet, such surface carbonates are absent in the activated material, pointing to the removal of carbonates during cycling (in a CO-free atmosphere) as the underlying cause behind activation. Importantly, after activation and a re-exposure to CO, surface carbonates re-form and yield a deactivation of the perovskite oxygen carrier, which is often overlooked when using such materials at relatively low temperature (≤500 °C) in chemical looping.
我们阐明了在化学链应用中，氧化还原循环（此处具体针对钙钛矿SrCaFeO ）导致氧载体氧气释放速率普遍增加的潜在原因。这种现象通常被称为活化。为此，我们通过结构和形态表征技术（N物理吸附、显微镜、X射线粉末衍射和X射线吸收光谱）来探究氧化还原循环过程中氧载体的结构和表面元素组成的演变。我们观察到在活化期间样品的表面积、孔体积和形态没有明显变化。氧化还原循环前后材料的X射线粉末衍射和X射线吸收光谱分析（在Fe和Sr边）没有显示出显著差异，这意味着钙钛矿的体相（平均和局部）结构在循环后基本未改变。对钙钛矿表面的X射线光电子能谱和拉曼光谱分析表明，在合成的样品中存在表面碳酸盐物种（由于其暴露于空气中）。然而，在活化材料中不存在这种表面碳酸盐，这表明在循环过程中（在无CO气氛中）碳酸盐的去除是活化背后的潜在原因。重要的是，活化后再暴露于CO中，表面碳酸盐会重新形成并导致钙钛矿氧载体失活，在化学链中相对低温（≤500°C）使用此类材料时，这一点常常被忽视。
