High entropy oxides (HEOs) are an emerging class of materials constituted by multicomponent systems that are receiving special interest as candidates for obtaining novel and desirable properties. In this study we present a detailed investigation of the relevant intermediates arising at the surface of the prototypical HEO Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O during low-temperature CO oxidation. By combining Cu L_2,3-edge operando soft X-ray absorption spectroscopy (soft-XAS) with density functional theory simulations and in situ FT-IR spectroscopy, we propose that upon HEO exposure to CO at 235 °C reduced Cu(I) sites arise mostly coordinated to activated CO molecules and partly to bidentate carbonate species. When the HEO surface is then exposed to a stoichiometric mixture of CO + 1/2O₂ at 250 °C, CO₂ is produced while bidentate carbonate moieties remain interacting with the Cu(I) sites. We structurally characterize the carbonate and CO preferential adsorption geometries on the Cu(I) surface metal centers, and find that CO adopts a bent conformation that may energetically favor its subsequent oxidation. The unique surface, structural and electronic sensitivity of soft-XAS coupled with the developed data analysis work-flow and supported by FT-IR spectroscopy may be beneficial to characterize often elusive surface properties of systems of catalytic interest.

中文翻译：

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通过 Cu L-edge 操作软 X 射线吸收光谱对原型高熵氧化物上的低温 CO 氧化的结构和机械见解

高熵氧化物 (HEO) 是由多组分系统构成的一类新兴材料，作为获得新奇和理想特性的候选材料而受到特别关注。在这项研究中，我们详细研究了在低温 CO 氧化过程中在原型 HEO Mg _0.2 Co _0.2 Ni _0.2 Cu _0.2 Zn _0.2 O表面产生的相关中间体。通过将 Cu L _2,3 -edge 操作软 X 射线吸收光谱 (soft-XAS) 与密度泛函理论模拟和原位FT-IR 光谱相结合，我们提出在 HEO 暴露于 235 °C 的 CO 时，Cu( I) 位点的出现主要与活化的 CO 分子协调，部分与双齿碳酸盐物种协调。然后，当 HEO 表面在 250 °C 下暴露于化学计量的 CO + 1/2O ₂混合物中时，会产生CO ₂，而双齿碳酸盐部分仍然与 Cu( I ) 位点相互作用。我们在结构上表征了 Cu( I) 表面金属中心，并发现 CO 采用弯曲构象，可能在能量上有利于其随后的氧化。soft-XAS 独特的表面、结构和电子灵敏度加上开发的数据分析工作流程并得到 FT-IR 光谱的支持，可能有利于表征通常难以捉摸的催化系统的表面特性。