Small Molecule Activation On 2D Modified Mxenes:Multi-scale Simulation Study

The continuous increasing of global energy demand is attributed to the fast rising of world population and the rapid development of high technology.Building a renewable green energy system is an important way to get rid of traditional energy（e.g.,coal and fossils）and achieve global energy transition.One promising method is to convert small molecules（e.g.,oxygen,methane and water in the nature）into higher value products（e.g.,methanol and hydrogen）by catalysts.The greatest challenge is how to quickly screen and design the high efficiency catalysts.The modified MXenes show good catalytic performance in various small molecule activation reactions due to high conductivity,adjustable surface structure,adjustable electric potential,easily doped and supported properties.Combined density functional theory（DFT）calculations with machine learning（ML）algorithms,the catalysts are quickly and efficiently screened from the 2D modified MXenes system for the direct conversion of methane to methanol and hydrogen evolution reaction,respectively.And the reaction mechanism on the surface have been studied in depth.The main findings are as follows:（1）The activity of hydrogen evolution reaction（HER）was able to regulate via the functional groups on MXenes,and 4 potential HER catalysts were screened from 299MXenes by DFT calculations and ML algorithms.Bader charge analysis indicted that charge transfer between the surface and adsorbed H atom resulted in the difference of HER activity with and without S functional groups.The ML models were built to predict accurateΔG_H*by ML algorithms and a database containing 132 MXenes.TheΔG_H*of 167 additional MXenes were accurately predicted by random forest model using elemental information.Finnally,the HER activity of 8 potential catalysts were considered over different hydrogen coverages,including Os₂C,Os₂B,Sc₂N₁S₂,Sc₃N₂S₂,were potentially an excellent catalyst for the HER over high hydrogen coverages.This study not only provided 4 potential HER catalyst for the experimental researchers,but also profoundly explained that S functional groups affected the HER performance of MXenes materials by molecular orbital theory.（2）Previous work showed that the HER activity was promoted by the interaction between active center and support,and 2 ideal HER catalysts were screened from 205MXenes by DFT calculations and ML algorithms.TheΔG_H*of 66 MXenes and 46MBenes were calculated by DFT.Based on a statistical approach using combined descriptors,an equation was established to explain the difference of HER activity between bare MBenes and MXenes.The equation also indicated that Bader charge transfer and d band center were the effective predictors for this system.The HER performance of Bare MBenes was further improved by doping of single metal atom.The ML algorithms showed the predicted accuracy with a database containing 180MBenes,especially support vector machine.Finnally,2 ideal HER catalysts were found from 28 potential ones after two rounds of screening.This study provided a theoretical guidance of single metal doped catalysts for the experiments,and offered an advanced idea for explaining the differences of catalytic performance for different materials.（3）As for the impact on the catalytic activity due to different catalytic active centers,Pd₁/Mo₂CO₂ and Pd₂/Mo₂CO₂ were designed as the catalysts of methane partial oxidation to methanol（POM）.The stability of Pd at different adsorption sites on Mo₂CO₂ surfaces were firstly evaluted.The calculation results indicated that Pd dimer provided multiple adsorption sites for co-adsorption and activation of methane and oxygen.Based on transition state theory,the reaction mechanism of POM on Pd₂/Mo₂CO₂was systematically studied,including oxygen dissociation,proton transfer and hydrogen spillover reaction pathways,respectively.The optimal reaction path showed a low energy barrier of C-H activation and avoided the extra energy for oxygen dissociation before methane activation.In addition,methane on Pd₂/Mo₂CO₂was difficult to be deeply oxidized according to the comparison of activation energy barriers.This study provided a new design strategy with the atomical clusters supported-catalyst for POM.