Molecular Structure and Catalytic Promotional Effect of Mn on Supported Na2WO4/SiO2 Catalysts for Oxidative Coupling of Methane (OCM) Reaction
Molecular Structure and Catalytic Promotional Effect of Mn on Supported Na2WO4/SiO2 Catalysts for Oxidative Coupling of Methane (OCM) Reaction
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DOI：
10.1016/j.cattod.2022.07.005
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发表时间：
2022-07
期刊：
影响因子：
5.3
通讯作者：
S. Sourav;Daniyal Kiani;Yixiao Wang;J. Baltrusaitis;R. Fushimi;I. Wachs
中科院分区：
文献类型：
--
作者：
S. Sourav;Daniyal Kiani;Yixiao Wang;J. Baltrusaitis;R. Fushimi;I. Wachs
负载型Mn-Na 2 WO 4/SiO2催化剂中Mn的结构及其对甲烷氧化偶联（OCM）反应的促进作用长期以来一直存在争议。本文通过多位原位表征研究发现，在低温和氧化环境下，新焙烧的负载型1.2Mn-5 Na 2 WO 4/SiO2催化剂具有结晶的Na 2 WO 4、Mn 2 O3和SiO2（方石英相）沿着出现，同时还存在表面MnOx和Na-WOx位。在OCM反应环境下（T > 800 °C），结晶Na 2 WO 4相熔化，Mn 2 O3相还原。相比之下，表面的MnOx和Na-WOx网站表现出优异的热稳定性和化学稳定性。将1.2Mn-5 Na 2 WO 4/SiO2催化剂暴露于OCM反应环境使熔融的Na 2 WO 4相重新分散在SiO2载体上以形成新的表面WOx位点。有趣的是，在OCM反应过程中，稳定的MnOx物种与熔融Na 2 WO 4相和表面Na-WOx位点相互作用。在TAP和详细的稳态OCM固定床反应研究中的受控瞬态动力学实验揭示了Mn在1.2Mn-5 Na 2 WO 4/SiO2催化剂中的作用和促进作用。W氧化物（熔融的Na_2WO_4和表面的Na-WOx位）是催化OCM反应的活性位，而MnOx物种仅起促进作用。MnOx的促进作用强烈依赖于气相O2分压，MnOx物种充当气相分子O2与催化剂晶格氧之间氧交换的介体。温度依赖的MnOx促进表明，MnOx物种选择性地促进熔融Na 2 WO 4相在较低的反应温度和表面Na-WOx位在较高的温度。
The structure and promotional effect of Mn in supported Mn-Na2WO4/SiO2catalysts for the oxidative coupling of methane (OCM) reaction has been debated for a longtime in the literature. In the current investigation, with the aid of multiplein-situcharacterization studies, we show that the freshly calcined supported 1.2Mn-5Na2WO4/SiO2catalyst possesses crystalline Na2WO4, Mn2O3and SiO2(cristobalite phase) along with surface MnOxand Na-WOxsites at low temperature and oxidizing environments. Under the OCM reaction environment (T > 800 °C), the crystalline Na2WO4phase melts and Mn2O3phase reduces. In contrast, the surface MnOxand Na-WOxsites exhibit excellent thermal and chemical stability. Exposure of the 1.2Mn-5Na2WO4/SiO2catalyst to the OCM reaction environment redisperses the molten Na2WO4phase on the SiO2support to form new surface WOxsites. Interestingly, the stable MnOxspecies interacts with both molten Na2WO4phase and surface Na-WOxsites during OCM reaction. Controlled transient kinetic experiments in TAP and detailed steady state OCM fixed-bed reaction studies reveal the role and promotional effect of Mn in the 1.2Mn-5Na2WO4/SiO2catalyst. The W-oxides (both molten Na2WO4and surface Na-WOxsites) are the active sites for the catalytic OCM reaction and the MnOxspecies only function as promoters. The promotion of MnOxstrongly depends on the gas phase O2partial pressure and the MnOxspecies act as mediators for oxygen exchange between the gas phase molecular O2and catalyst lattice oxygen. The temperature dependent MnOxpromotion reveals that the MnOxspecies selectively promote the molten Na2WO4phase at lower reaction temperature and the surface Na-WOxsites at higher temperature.