A Theoretical Study Of Single And Double Atom Catalysts For Methane Conversion

Methane（CH₄）is the major component of natural gas.It is abundant in the earth and has been widely used as a feedstock and fuel in the chemical industry.CH₄ has played an important role in the global energy system.Currently,the widespread use of methane still faces difficulties,such as storage,transportation,low combustion efficiency and high pollutant emissions.Thus it is of great significance to convert methane into high-value transportable chemicals.It is well known that the activation of the first C-H bond in CH₄ requires high energy（4.51 e V）,which will need to choose a suitable catalyst.As an important development direction in the field of heterogeneous catalysis,single and diatomic catalysts have shown excellent performance in regulating reaction activity and selectivity.Although some single atom catalysts have been reported for methane conversion,researches about their reaction processes and activation mechanisms are still in the early stages.In order to obtain highly active and stable catalysts for methane conversion,we performed a systematic investigation about the activation pathways based on the high-precision electronic structure calculations and molecular dynamics simulations.During the studies,the single and double metal atoms were introduced to the common two-dimensional materials,such as black phosphorus and graphdiyne.Our studies not only helps to understand the reaction mechanism of methane conversion to methanol and multi-carbon products（ethane）but also provides technical support for the design of efficient catalysts.The main content of this article is the following:（1）The single transition metal atoms（TM=Fe,Co,Ni,Cu）anchored on black phosphorus（TM@BP）were studied as catalysts to assist the methane oxidation to methanol by means of density functional theory calculations and molecular dynamics simulations.The results indicate that the 3p orbitals of phosphorus can hybridize with the 3d orbitals of metals,leading to the structural stability of TM@BP.Using N₂O as the oxidant,the single atom catalysts will generate the metal-oxygen（TM-O）active sites.The results indicate that Cu@BP exhibits an outstanding catalytic activity through the radical reaction pathways and the formation of the Cu-O active site is the rate-determining with an energy barrier of 0.48 e V.In addition,the calculations show that Cu@BP can effectively suppress the occurrence of side reactions during the conversion of methane to methanol.Thus the selectivity is improved.（2）The single,double and triple metal atoms（Cu_x@GDY,x=1,2,3）anchored on graphdiyne were studied as catalysts to assist the methane conversion by means of density functional theory calculations.The calculations show that the diatomic catalyst Cu₂@GDY has the best catalytic performance for C-H bond activation.Meanwhile,Cu₂@GDY can further facilitate the methyl coupling to generate C2product（ethane）.The energy barrier for the rate-determining step is 1.66 e V.The calculations show that the steric hindrance effect during the methane activation coupling process can be effectively reduced by the two catalytic active centers in Cu₂@GDY,which will promote the carbon-carbon coupling to produce the multi-carbon products.