Theoretical Insights into Dual-Metal-Site Catalysts for the Nonoxidative Coupling of Methane

Abstract

Direct conversion of methane to C2 hydrocarbons under nonoxidative conditions is an attractive technology but is challenging due to high reaction temperature, severe coke deposition, and low selectivity. Here, we report three dual-metal-site catalysts (DMSCs) based on nitrogen-doped graphene (FeCo–N–C, Fe2–N–C, and Co2–N–C) for nonoxidative coupling of methane to C2 hydrocarbons from a theoretical perspective. Our calculated results reveal that DMSCs present universally better performance in methane activation than single-metal-site catalysts (Fe–N–C and Co–N–C). Among the three DMSCs, Co2–N–C exhibits the best catalytic activity and superior selectivity to ethane in the whole reaction pathway. Our microkinetic modeling reveals that the Co2–N–C catalyst can convert methane to CH3, C2H6, and H2 at 1200 K. The electronic structure analysis and ab initio molecular dynamics simulations demonstrate that Co2–N–C possesses both intrinsic stability and high-temperature stability. Moreover, Co2–N–C also manifests better coke resistance compared with larger Co clusters, indicated by the difficult kinetics of methane deep dehydrogenation to naked carbon. This work provides a potential catalyst prototype for the selective conversion of methane to C2 hydrocarbons under nonoxidative conditions.

First-Principles Microkinetic Modeling Unravelling the Performance of Edge-Decorated Nanocarbons for Hydrogen Production from Methane

“…It is possible to observe from Figure 2a that the adsorption of methane is an exothermic process for all the decorated EDNCs considered in this study, leading to more thermodynamically stable products when CH 3 and H are chemisorbed on different edge sites. It is noteworthy that the CH 4 adsorption was reported to be endothermic on doped metal sites on graphene 11,56 and bimetallic catalysts 55,57 in previous works. A large energetic difference was found between the two adsorption configurations of CH 3 and H on N-EDNC and B-EDNC.…”

“…4,10 This process generally requires high temperatures to overcome the kinetic barrier for methane cracking and hydrogen desorption, which can result in the severe deactivation of the catalyst due to coking, thus being a major drawback of metal-based catalysts. 1,9,11,12 Therefore, significant effort has been made toward the design of efficient catalysts that are expected to improve the selectivity activation of C−H bonds, while being resistant to carbon poisoning. 13−16 Much attention has been directed to the improvement of the process of catalytic methane decomposition (CMD) into hydrogen H 2 production in the past decade.…”

“…The direct conversion of methane, CH 4 , to hydrogen (so-called catalytic methane decomposition, or CMD) is an attractive process for converting natural gas into high-value chemicals, mainly due to the cleanliness of the process , and the large availability of natural and shale gas reserves. , The main challenge for thermally cracking methane is the large energy required for the C–H bond cleavage; therefore, the usage of a catalyst is crucial for achieving lower temperatures for efficient conversion . The direct catalytic conversion of methane can proceed through oxidative routes, such as oxidative coupling of methane (OCM) and partial oxidation of methane (POM), and nonoxidative routes. ,, For the former, undesirable C 2 and C 3 byproducts as well as CO 2 can be formed via reactions between methyl radicals and oxygen, making the process less sustainable. , On the other hand, the nonoxidative conversion of methane can yield high selectivity toward target products. , This process generally requires high temperatures to overcome the kinetic barrier for methane cracking and hydrogen desorption, which can result in the severe deactivation of the catalyst due to coking, thus being a major drawback of metal-based catalysts. ,,, Therefore, significant effort has been made toward the design of efficient catalysts that are expected to improve the selectivity activation of C–H bonds, while being resistant to carbon poisoning. − …”

Toward Rational Design of Dual-Metal-Site Catalysts: Catalytic Descriptor Exploration

“…In addition, using microkinetic analysis, Huang et al have revealed that the NGr-supported Fe−Co, Fe−Fe, and Co−Co DMSCs are all good catalysts for the nonoxidative coupling of methane, and the homonuclear Co−Co DMSC exhibits better stability and superior activity than the others. 24 Mechanistic exploration using density functional theory (DFT) simulations by Wang et al determined that the Fe−Co dual-site structure can result in an energetically preferential side-on adsorption reactant configuration. 25 This special structure preactivates the bond on the bridge site, reducing the oxygen reduction barrier.…”

“…In this regard, a variety of metal combinations including Fe–Co, Fe–Cu, Ni–Cu, Co–Pt, and Co–Zn on NGr have been investigated theoretically and been identified as good candidates for heterogeneous catalysts. In addition, using microkinetic analysis, Huang et al have revealed that the NGr-supported Fe–Co, Fe–Fe, and Co–Co DMSCs are all good catalysts for the nonoxidative coupling of methane, and the homonuclear Co–Co DMSC exhibits better stability and superior activity than the others …”