Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)
Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)
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DOI：
10.1039/d0cy00289e
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发表时间：
2020-05-21
影响因子：
5
通讯作者：
Baltrusaitis, Jonas
中科院分区：
文献类型：
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作者：
Kiani, Daniyal;Sourav, Sagar;Baltrusaitis, Jonas
甲烷氧化偶联反应（OCM）用负载型Na_2WO_4/SiO_2催化剂表面活性中心的分子结构、电子结构和化学性质尚不清楚。采用不同的Na/W摩尔比，以SiO_2为载体，系统地制备了Na促进的氧化钨催化剂（Na-WOx/SiO_2），并用原位拉曼、UV-vis DR、CO_2-TPD-DRIFT和NH_3-TPD-DRIFT光谱进行了表征。以Na 2 WO 4中心点2 H（2）O水溶液为前驱体，制备了相应于5%Na2WO4标称负载量、Na/W摩尔比为2的传统制备的催化剂。在800 ℃煅烧后，初始无定形SiO2载体结晶成方石英相，并且负载的钨酸钠相由结晶Na 2 WO 4纳米颗粒（Na/W = 2）和分散相Na-WO 4表面位点（Na/W < 2）组成。另一方面，通过使用NaOH + AMT的单独前体的改进的浸渍方法制备的催化剂，使得Na/W摩尔比保持远低于2，导致：（i）SiO2保持无定形（ii）仅分散相Na-WO 4表面位点。分散的Na-WO 4表面位点是孤立的，更多的几何扭曲，更少的基本性质，和更可还原的结晶Na 2 WO 4纳米粒子。CH 4 + O-2-TPSR结果表明，分离的分散相Na-WO（4）表面位点具有明显更高的C（2）选择性，但活性略低于传统制备的含有结晶Na 2 WO 4纳米颗粒（Na/W = 2）的催化剂。这些研究结果表明，孤立的，分散相Na-WO 4网站的SiO2载体表面上的OCM反应的选择性活性位点。
The molecular and electronic structures and chemical properties of the active sites on the surface of supported Na2WO4/SiO2 catalysts used for oxidative coupling of methane (OCM) are poorly understood. Model SiO2-supported, Na-promoted tungsten oxide catalysts (Na-WOx/SiO2) were systematically prepared using various Na- and W-precursors using carefully controlled Na/W molar ratios and examined with in situ Raman, UV-vis DR, CO2-TPD-DRIFT and NH3-TPD-DRIFT spectroscopies. The traditionally-prepared catalysts corresponding to 5% Na2WO4 nominal loading, with Na/W molar ratio of 2, were synthesized from the aqueous Na2WO4 center dot 2H(2)O precursor. After calcination at 800 degrees C, the initially amorphous SiO2 support crystallized to the cristobalite phase and the supported sodium tungstate phase consisted of both crystalline Na2WO4 nanoparticles (Na/W = 2) and dispersed phase Na-WO4 surface sites (Na/W < 2). On the other hand, the catalysts prepared via a modified impregnation method using individual precursors of NaOH + AMT, such that the Na/W molar ratio remained well below 2, resulted in: (i) SiO2 remaining amorphous (ii) only dispersed phase Na-WO4 surface sites. The dispersed Na-WO4 surface sites were isolated, more geometrically distorted, less basic in nature, and more reducible than the crystalline Na2WO4 nanoparticles. The CH4 + O-2-TPSR results reveal that the isolated, dispersed phase Na-WO(4)surface sites were significantly more C(2)selective, but slightly less active than the traditionally-prepared catalysts that contain crystalline Na2WO4 nanoparticles (Na/W = 2). These findings demonstrate that the isolated, dispersed phase Na-WO4 sites on the SiO2 support surface are the selective-active sites for the OCM reaction.