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Existence and Properties of Isolated Catalytic Sites on the Surface of β-Cristobalite-Supported, Doped Tungsten Oxide Catalysts (WOx/β-SiO2, Na-WOx/β-SiO2, Mn-WOx/β-SiO2) for Oxidative Coupling of Methane (OCM): A Combined Periodic DFT and Experimental Study
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-03-16 , DOI: 10.1021/acscatal.9b05591
Daniyal Kiani 1 , Sagar Sourav 1 , William Taifan 1 , Monica Calatayud 2 , Frederik Tielens 3 , Israel E. Wachs 1 , Jonas Baltrusaitis 1
Affiliation  

The nature of isolated tungsten oxide (WOx) sites in the dispersed phase on the surface of a β-cristobalite (β-SiO2) support in undoped and Na- or Mn-doped WOx/SiO2 model catalysts used for the oxidative coupling of methane (OCM) has not been explored previously. This work provides a computational model for isolated surface WOx sites (doped and undoped) supported on β-cristobalite (β-SiO2) and computationally explores their molecular structure, degree of hydration, and energetics over a wide range of OCM-relevant temperatures from 500 to 1300 K. Ab initio thermodynamic analysis showed that the most stable molecular configuration of the surface sites in all cases (WOx, Na-WOx, Mn-WOx) was the digrafted, dioxo pseudotetrahedral WO4. The thermal stability of the surface WO4 sites was in the order Na-WO4 ≫ WO4 > Mn-WO4 in the OCM-relevant temperature range of 850–1300 K. A spin analysis of an Mn-WO4 isolated surface site indicates a paramagnetic high-spin Mn2+–O–W6+ electronic state, in line with literature reports. The computed frequencies for isolated surface WO4 sites agree well with the experimental in situ Raman spectra of the corresponding model catalysts, proving their existence. Finally, steady-state OCM studies showed that the C2 selectivity was highest for Na-WO4 sites, followed by Mn-WO4 and WO4, a trend mimicking the degree of distortion in the molecular geometry of each site. Na-WO4 exhibited the lowest reactivity toward CH4 and the highest degree of W═O bond elongation.

中文翻译:

存在,而位于该面隔离催化位点的性质β-方石英支持的,掺杂的氧化钨的催化剂(WO X /β-的SiO 2,钠-WO X /β-的SiO 2,锰-WO X /β-的SiO 2)甲烷氧化偶联(OCM)的合成:周期性DFT和实验研究的结合

在未掺杂和钠或锰掺杂的WO x / SiO 2模型催化剂中,β-方石英(β - SiO 2)载体表面分散相中孤立的氧化钨(WO x)位点的性质甲烷(OCM)的偶联以前未曾探索过。这项工作提供WO为分离面的计算模型X位点(掺杂和未掺杂)支撑在β方石英(β-的SiO 2)和计算探索了它们的分子结构,水化程度,和能量学在宽范围OCM-相关温度的从500到1300K。从头算热力学分析表明,在所有情况下(WO x,Na-WO x,Mn-WO x),表面位点最稳定的分子构型是二嫁接的二氧代伪四面体WO 4。表面WO的热稳定性4位点是在顺序的Na-WO 4 »WO 4 >的Mn-WO 4在OCM-相关温度范围内的850-1300的Mn-WO的K.自旋分析4分离表面位点表示与文献报道一致的顺磁性高自旋Mn 2+ -O-W 6+电子态。隔离表面的计算频率WO 4这些位点与相应模型催化剂的实验原位拉曼光谱非常吻合,证明了它们的存在。最后,稳态OCM研究表明,Na-WO 4位点的C 2选择性最高,其次是Mn-WO 4和WO 4,这种趋势模仿了每个位点的分子几何结构的畸变程度。Na-WO 4对CH 4的反应性最低,W═O键的延伸度最高。
更新日期:2020-04-23
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