Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K<sub>2</sub>WO<sub>4</sub>/SiO<sub>2</sub> Catalyst Revealed by In Situ Characterization
Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K<sub>2</sub>WO<sub>4</sub>/SiO<sub>2</sub> Catalyst Revealed by In Situ Characterization
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原位表征揭示未熔融 K<sub>2</sub>WO<sub>4</sub>/SiO<sub>2</sub> 催化剂上甲烷高选择性氧化偶联中的瞬态过氧化钾物质
DOI：
10.1021/acscatal.1c04206
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发表时间：
2021
期刊：
影响因子：
12.9
通讯作者：
Takanabe Kazuhiro
中科院分区：
文献类型：
--
作者：
Li Duanxing;Yoshida Shintaro;Siritanaratkul Bhavin;Garcia-Esparza Angel T.;Sokaras Dimosthenis;Ogasawara Hirohito;Takanabe Kazuhiro
使用 CH4-O2 流系统研究了 K2WO4/SiO2 催化剂上甲烷的氧化偶联 (OCM)。向反应物流中添加 H2O 导致 CH4 转化率和 C2 选择性增加，这与平衡 OH 自由基生成相关的动力学一致，然后用 OH 自由基从 CH4 中夺取 H。当 CH4 转化率为 47.1% 时，C2+（C2 和高级烃的总和）收率达到最大值 24.7%（C2H4 收率 17.0%），与之前报道的 Na2WO4/SiO2 催化剂相当。在接近环境压力的原位 X 射线光电子能谱在 560 °C 及以上、气相 O2 存在的情况下发现了 K2O2 和 KO2 物质。这些 K2O2/KO2 物质被认为可以催化 H2O 活化以产生 OH 自由基。通过原位高温 X 射线衍射，K2WO4 在 850 °C 的 OCM 反应过程中保持了块状结晶相，这与之前报道的 Na2WO4 的熔融状态相反。这表明只要对OCM使用惰性载体材料，负载组分的熔化和载体的覆盖对于在负载碱金属钨酸盐催化剂中实现高C2产率并不是必需的。
The oxidative coupling of methane (OCM) over a K2WO4/SiO2catalyst was investigated using a CH4–O2flow system. The addition of H2O to the reactant stream caused an increase in the CH4conversion rate and C2selectivity, consistent with the kinetics associated with the equilibrated OH radical generation, followed by H-abstraction from CH4with OH radicals. The C2+(a sum of C2and higher hydrocarbons) yield reaches a maximum of 24.7% (C2H4yield 17.0%) at a CH4conversion of 47.1%, comparable to those over the previously reported Na2WO4/SiO2catalyst. In situ near ambient pressure X-ray photoelectron spectroscopy uncovered K2O2and KO2species at and above 560 °C in the presence of gas-phase O2. These K2O2/KO2species are considered to catalyze H2O activation for OH radical generation. By in situ high-temperature X-ray diffraction, K2WO4maintained a crystalline phase in the bulk during the OCM reaction at 850 °C in contrast to the molten state of Na2WO4previously reported. This indicates that the melting of the supported component and the covering of support are not essential to achieve a high C2yield in supported alkali metal tungstate catalysts as long as inert support materials are used toward OCM.