Through continuous flow reactor experiments, materials characterization, and theoretical calculations, we provide new insights into the reduction of hematite (α-Fe₂O₃) nanoparticles by methane (CH₄) during chemical looping combustion (CLC). Across CLC-relevant temperatures (500–800 °C) and gas flow rates (2.5–250 h^–1), decreasing α-Fe₂O₃ particle size (from 350 to 3 nm) increased the duration over which CH₄ was completely converted to CO₂ (i.e., 100% yield). We attribute this size-dependent performance trend to the greater availability of lattice oxygen atoms in the near-surface region of smaller particles with higher surface area-to-volume ratios. All particle sizes then exhibited a relatively rapid rate of reactivity loss that was size- and temperature-independent, reflecting a greater role for magnetite (Fe₃O₄), the primary α-Fe₂O₃ reduction product, in CH₄ oxidation. Bulk (X-ray diffraction, XRD) and surface (X-ray photoelectron spectroscopy, XPS) analysis revealed that oxygen carrier reduction proceeds via a two-stage solid-state mechanism; α-Fe₂O₃ reduction to Fe₃O₄ followed the unreacted shrinking core model (USCM) while subsequent reduction of Fe₃O₄ to wüstite (FeO) and FeO to iron metal (Fe) followed the nucleation and nuclei growth model (NNGM). Atomistic thermodynamics modeling based on density functional theory supports that reduction initiates via the USCM, as partially reduced α-Fe₂O₃ surfaces exhibited a wide range of stability relative to bulk Fe₃O₄. Reduction and reoxidation cycling experiments were also performed to explore more practical aspects related to the long-term performance of unsupported α-Fe₂O₃ nanoparticles as oxygen carriers for CLC.

中文翻译：

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的α-Fe ₂ ö ₃纳米微粒作为氧载体的化学环流式燃烧：一个综合材料表征方法了解载氧体的性能，还原机理，及粒径的影响

通过连续流动反应器的实验中，材料特性，和理论计算，我们提供了新的见解的赤铁矿的还原（的α-Fe ₂ ö ₃）由甲烷的纳米颗粒（CH ₄化学循环燃烧（CLC）期间）。跨CLC-相关温度（500-800℃）和气体流量（2.5-250ħ ^-1），降低的α-Fe ₂ ö ₃粒径（从350至3nm）增加的持续时间在其CH ₄完全转化为CO ₂（即100％的收率）。我们将这种与尺寸有关的性能趋势归因于具有较高的表面积体积比的较小颗粒的近表面区域中晶格氧原子的可用性更高。然后，将所有的颗粒尺寸显示反应性损失的相对快的速率，这是尺寸-和温度无关的，这反映了磁铁矿更大的作用（FE ₃ Ò ₄），初级的α-Fe ₂ ö ₃还原产物，在CH ₄氧化。本体（X射线衍射，XRD）和表面（X射线光电子能谱，XPS）分析表明，氧载流子的还原是通过两阶段的固态机理进行的。的α-Fe ₂ ö ₃还原成Fe ₃O ₄遵循未反应的收缩核模型（USCM），随后遵循成核和核生长模型（NNGM）将Fe ₃ O ₄还原为白铁矿（FeO）和将FeO还原为铁金属（Fe）。原子论热力学建模设计基于密度泛函理论的支持经由USCM还原发起，如部分还原的α-Fe _2个ö ₃表面表现出宽范围的相对于本体的Fe稳定性的₃ ö ₄。还原和再氧化循环实验也在进行探讨有关不受支持的α-Fe的长期性能更实用方面₂ ö ₃纳米颗粒作为氧载体为CLC。