Kiani Daniyal, Sourav Sagar, Wachs Israel E, Baltrusaitis Jonas
Department of Chemical and Biomolecular Engineering, Lehigh University B336 Iacocca Hall, 111 Research Drive Bethlehem PA 18015 USA
Chem Sci. 2021 Oct 5;12(42):14143-14158. doi: 10.1039/d1sc02174e. eCollection 2021 Nov 3.
The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface NaWO catalytic active sites for CH heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO catalytic active sites resulting in surface WO sites with retarded kinetics for C-H scission. On the other hand, dimeric MnO surface sites, identified and studied molecular dynamics and thermodynamics, were found to be more efficient in activating CH than the poisoned surface WO sites or the original WO sites. However, the surface reaction intermediates formed from CH activation over the MnO surface sites are more stable than those formed over the NaWO surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the NaWO surface sites towards CH activation but can yield MnO surface sites that activate CH faster than NaWO surface sites, but unselectively.
本文首次通过实验验证了所开发的计算模型，结果表明，在氧化偶联甲烷（OCM）过程中，锰促进作用不会增强表面NaWO催化活性位点对CH异裂解离的活性。与之前的认识相反，研究表明锰促进作用会使表面WO催化活性位点中毒，导致表面WO位点的C-H断裂动力学迟缓。另一方面，通过分子动力学和热力学方法识别并研究的二聚体MnO表面位点，被发现比中毒的表面WO位点或原始WO位点在活化CH方面更有效。然而，在MnO表面位点上由CH活化形成的表面反应中间体比在NaWO表面位点上形成的更稳定。表面中间体的更高稳定性使得它们的脱附变得不利，增加了过度氧化为CO 的可能性，这与文献中关于锰促进催化剂的实验结果一致。因此，锰促进剂在协同调节NaWO表面位点结构以实现CH活化方面似乎没有本质上的积极作用，但可以产生比NaWO表面位点更快活化CH的MnO表面位点，但这种活化是非选择性的。
