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  1. Abstract

    The oxidative coupling of methane to higher hydrocarbons offers a promising autothermal approach for direct methane conversion, but its progress has been hindered by yield limitations, high temperature requirements, and performance penalties at practical methane partial pressures (~1 atm). In this study, we report a class of Li2CO3-coated mixed rare earth oxides as highly effective redox catalysts for oxidative coupling of methane under a chemical looping scheme. This catalyst achieves a single-pass C2+yield up to 30.6%, demonstrating stable performance at 700 °C and methane partial pressures up to 1.4 atm. In-situ characterizations and quantum chemistry calculations provide insights into the distinct roles of the mixed oxide core and Li2CO3shell, as well as the interplay between the Pr oxidation state and active peroxide formation upon Li2CO3coating. Furthermore, we establish a generalized correlation between Pr4+content in the mixed lanthanide oxide and hydrocarbons yield, offering a valuable optimization strategy for this class of oxidative coupling of methane redox catalysts.

     
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  2. The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na 2 WO 4 catalytic active sites for CH 4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO 4 catalytic active sites resulting in surface WO 5 sites with retarded kinetics for C–H scission. On the other hand, dimeric Mn 2 O 5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH 4 than the poisoned surface WO 5 sites or the original WO 4 sites. However, the surface reaction intermediates formed from CH 4 activation over the Mn 2 O 5 surface sites are more stable than those formed over the Na 2 WO 4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , 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 Na 2 WO 4 surface sites towards CH 4 activation but can yield MnO x surface sites that activate CH 4 faster than Na 2 WO 4 surface sites, but unselectively. 
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