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Catalytic conversion of CH4 to transportable liquid hydrocarbons via partial oxidation is a promising avenue towards efficient utilization of natural gas. Single Fe atoms on N-functionalized graphene (FeN4/GN) have recently been shown to be active for partial CH4 oxidation with H2O2 at room temperature. Here, density functional theory (DFT) calculations combined with mean-field microkinetic modeling (MKM) have been applied to obtain kinetic understanding of partial CH4 oxidation with H2O2 to CH3OH and CH3OOH over FeN4/GN. CH3OH and CH3OOH are found to be minor and major reaction products, respectively, with a selectivity in agreement with reported experimental data. The kinetic modeling reveals two pathways for CH3OH production together with a dominant catalytic cycle for CH3OOH formation. The selectivity is found to be sensitive to the temperature and H2O2 concentration, with the CH3OH selectivity increasing with increasing temperature and decreasing H2O2 concentration. Turnover frequencies of both CH3OH and CH3OOH are found to decrease over time, due to a change in the Fe formal oxidation state from +6 to +4; Fe(+6) is more active, but less stable than Fe(+4). The present work unravels the detailed reaction mechanism for partial oxidation of methane by FeN4/GN, rationalizes experimental observations and provides guidance for efficient room-temperature methane conversion by single-atom Fe-catalysts. 

Modifiers are commonly used in natural, biological, and synthetic crystallization to tailor the growth of diverse materials. Here, we identify tautomers as a new class of modifiers where the dynamic interconversion between solute and its corresponding tautomer(s) produces native crystal growth inhibitors. The macroscopic and microscopic effects imposed by inhibitor-crystal interactions reveal dual mechanisms of inhibition where tautomer occlusion within crystals that leads to natural bending, tunes elastic modulus, and selectively alters the rate of crystal dissolution. Our study focuses on ammonium urate crystallization and shows that the keto-enol form of urate, which exists as a minor tautomer, is a potent inhibitor that nearly suppresses crystal growth at select solution alkalinity and supersaturation. The generalizability of this phenomenon is demonstrated for two additional tautomers with relevance to biological systems and pharmaceuticals. These findings offer potential routes in crystal engineering to strategically control the mechanical or physicochemical properties of tautomeric materials.

 

The functionalization of methane to value-added liquid chemicals remains as one of the “grand challenges” in chemistry. In this work, we provide insights into the direct methane-to-methanol conversion mechanisms with H 2 O 2 as an oxidant on single Fe-atom centers stabilized on N-functionalized graphene, using first principles calculations. By investigating a series of different reaction paths on various active centers and calculating their turnover frequencies, we reveal that a H 2 O 2 -mediated radical mechanism and a Fenton-type mechanism are energetically the most plausible pathways taking place on di- and mono-oxo centers, respectively. Due to the thermodynamic preference of the mono-oxo center formation over the di-oxo under reaction conditions, the Fenton-type mechanism appears to determine the overall catalytic activity. On the other hand, the hydroxy(oxo) center, which is thermodynamically the most favorable center, is found to be catalytically inactive. Hence, the high activity is attributed to a fine balance of keeping the active centers as oxo-species during the reaction. Moreover, we reveal that the presence of solvent (water) can accelerate or slow down different pathways with the overall turnover of the dominant Fenton-type reaction being decreased. Importantly, this work reveals the nature of active sites and a gamut of reaction mechanisms for the direct conversion of methane to methanol rationalizing experimental observations and aiding the search for room temperature catalysts for methane conversion to liquid products.