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Surface Conditions That Constrain Alkane Oxidation on Perovskites
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-05-29 , DOI: 10.1021/acscatal.0c01289 Gregor Koch 1 , Michael Hävecker 1, 2 , Detre Teschner 1, 2 , Spencer J. Carey 1 , Yuanqing Wang 1, 3 , Pierre Kube 1 , Walid Hetaba 1, 2 , Thomas Lunkenbein 1 , Gudrun Auffermann 4 , Olaf Timpe 1 , Frank Rosowski 5 , Robert Schlögl 1, 2 , Annette Trunschke 1
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-05-29 , DOI: 10.1021/acscatal.0c01289 Gregor Koch 1 , Michael Hävecker 1, 2 , Detre Teschner 1, 2 , Spencer J. Carey 1 , Yuanqing Wang 1, 3 , Pierre Kube 1 , Walid Hetaba 1, 2 , Thomas Lunkenbein 1 , Gudrun Auffermann 4 , Olaf Timpe 1 , Frank Rosowski 5 , Robert Schlögl 1, 2 , Annette Trunschke 1
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The crystal structure of perovskites can incorporate a wide variety of cations, which makes this class of materials so interesting for studies of links between solid-state chemistry and catalysis. Perovskites are known as typical total combustion catalysts in hydrocarbon oxidation reactions. The fundamental question that we investigate here is whether surface modifications of perovskites can lead to the formation of valuable reaction products in alkane oxidation. We studied the effect of segregated two-dimensional surface nanostructures on selectivity to propene in the oxidative dehydrogenation of propane. Manganese-based perovskites AMnO3 (A = La, Sm) were prepared by combustion and hydrothermal synthesis. Bulk and surface structures were investigated by X-ray diffraction, temperature-programmed reduction, aberration-corrected scanning transmission electron microscopy (STEM), multiwavelength Raman, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) in combination with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Surface oxygen species responsible for C–H activation were distinguished by AP-XPS on the basis of a rigorous in situ analysis of the O 1s spectra recorded under a broad range of reaction conditions. Signals at 529.2, 530.1, 530.9, 531.2, and 531.8 eV were attributed to lattice O, defect-affected O, surface O, oxygen in carbonates, and hydroxyl groups, respectively. Operando AP-XPS revealed critical surface features, which occur under catalyst operation. The catalyst performance depends on the synthesis technique and the reaction conditions. In presence of a two-dimensional MnOx surface phase, addition of steam to the feed resulted in an increase in selectivity to the partial oxidation product propene to practically relevant values. The selectivity increase is related to the presence of Mn in a low oxidation state (2+/3+), an increased concentration of hydroxyl groups, and a higher abundance of adsorbed activated oxygen species on the catalyst surface. The surface analysis of a working catalyst highlights the importance of the termination layer of polycrystalline perovskites as a genuine property implemented by catalyst preparation. Such a termination layer controls the chemical properties and reactivity of perovskites. The information provides input for the development of realistic models that can be used by theory to predict functional properties.
中文翻译：
限制钙钛矿上烷烃氧化的表面条件
钙钛矿的晶体结构可以结合多种阳离子，这使得这类材料对于固态化学与催化之间联系的研究非常有趣。钙钛矿被公认为是烃氧化反应中典型的全燃烧催化剂。我们在此研究的基本问题是钙钛矿的表面改性是否会导致烷烃氧化过程中形成有价值的反应产物。我们研究了分离的二维表面纳米结构对丙烷氧化脱氢中丙烯选择性的影响。锰基钙钛矿AMnO 3（A ＝ La，Sm）通过燃烧和水热合成来制备。通过X射线衍射，程序升温还原，像差校正扫描透射电子显微镜（STEM），多波长拉曼光谱和环境压力X射线光电子能谱（AP-XPS）结合近红外光谱研究了大块和表面结构边缘X射线吸收精细结构（NEXAFS）光谱。AP-XPS在严格的原位基础上区分了负责C–H活化的表面氧种类在宽范围的反应条件下记录的O 1s光谱分析。529.2、530.1、530.9、531.2和531.8 eV处的信号分别归因于晶格O，缺陷影响的O，表面O，碳酸盐中的氧和羟基。Operando AP-XPS显示出关键的表面特征，这些特征是在催化剂操作下发生的。催化剂的性能取决于合成技术和反应条件。存在二维MnO x在表面相中，向进料中添加蒸汽导致对部分氧化产物丙烯的选择性增加到实际相关的值。选择性的增加与低氧化态（2 + / 3 +）中Mn的存在，羟基浓度的增加以及催化剂表面上吸附的活化氧种类的较高丰度有关。工作催化剂的表面分析突出了多晶钙钛矿终止层作为通过催化剂制备实现的真实性能的重要性。这种终止层控制钙钛矿的化学性质和反应性。该信息为实际模型的开发提供了输入，理论可将其用于预测功能特性。
更新日期：2020-07-02
中文翻译：
限制钙钛矿上烷烃氧化的表面条件
钙钛矿的晶体结构可以结合多种阳离子，这使得这类材料对于固态化学与催化之间联系的研究非常有趣。钙钛矿被公认为是烃氧化反应中典型的全燃烧催化剂。我们在此研究的基本问题是钙钛矿的表面改性是否会导致烷烃氧化过程中形成有价值的反应产物。我们研究了分离的二维表面纳米结构对丙烷氧化脱氢中丙烯选择性的影响。锰基钙钛矿AMnO 3（A ＝ La，Sm）通过燃烧和水热合成来制备。通过X射线衍射，程序升温还原，像差校正扫描透射电子显微镜（STEM），多波长拉曼光谱和环境压力X射线光电子能谱（AP-XPS）结合近红外光谱研究了大块和表面结构边缘X射线吸收精细结构（NEXAFS）光谱。AP-XPS在严格的原位基础上区分了负责C–H活化的表面氧种类在宽范围的反应条件下记录的O 1s光谱分析。529.2、530.1、530.9、531.2和531.8 eV处的信号分别归因于晶格O，缺陷影响的O，表面O，碳酸盐中的氧和羟基。Operando AP-XPS显示出关键的表面特征，这些特征是在催化剂操作下发生的。催化剂的性能取决于合成技术和反应条件。存在二维MnO x在表面相中，向进料中添加蒸汽导致对部分氧化产物丙烯的选择性增加到实际相关的值。选择性的增加与低氧化态（2 + / 3 +）中Mn的存在，羟基浓度的增加以及催化剂表面上吸附的活化氧种类的较高丰度有关。工作催化剂的表面分析突出了多晶钙钛矿终止层作为通过催化剂制备实现的真实性能的重要性。这种终止层控制钙钛矿的化学性质和反应性。该信息为实际模型的开发提供了输入，理论可将其用于预测功能特性。
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