A DFT study of methane activation on graphite surfaces with vacancy defects

The activation of methane on graphite surfaces with monovacancies and 5-8-5 vacancies have been investigated using density functional theory. Sixteen different initial adsorption configurations were investigated to identify the most favorable activation site. It is found that methane tends to be activated on the defective graphite surfaces, and the most stable configuration is that methane activation happened in the center hole of the monovacancy site, with a reaction energy of 1.13 eV. Electron transfer and weaker electrostatic potential of the vacancy region indicate that carbon atom of methane tends to fill the vacancy and makes the system more stable.

Raman probing carbon&aqueous electrolytes interfaces and molecular dynamics simulations towards understanding electrochemical properties under polarization conditions in supercapacitors

Raman probing of carbon electrode and electrolyte under dynamic conditions is performed here using different aqueous electrolytes to elucidate the fundamental events occurring in electrochemical supercapacitor during charge–discharge processes.The areal capacitance ranges from 1.54 to 2.31μF cm^(-2)μm and it is determined using different techniques.These findings indicate that the Helmholtz capacitance governs the overall charge-storage process instead of the space charge(quantum)capacitance commonly verified for HOPG electrodes in the range of~3 to 7μF cm^(-2).Molecular dynamics simulations are employed to elucidate the origin of the reversible Raman spectral changes during the charge–discharge processes.A correlation is verified between the reversible Raman shift and the surface excesses of the different ionic species.A theoretical framework is presented to relate the effect of the applied potential on the Raman shift and its correlation with the surface ionic charge.It is proposed that the Raman shift is governed by the interaction of solvated cations with graphite promoted by polarization conditions.It is the first time that a comparative study on different aqueous electrolyte p H and cation ion size has been performed tracking the Raman spectra change under dynamic polarization conditions and contrasting with comprehensive electrochemistry and dynamic molecular simulations studies.This study shines lights onto the charge-storage mechanism with evidence of Kohn anomaly reduction in the carbon electrode during the reversible adsorption/desorption and insertion/extraction of ionic species.

Sorption kinetics of 1,3,5-trinitrobenzene to biochars produced at various temperatures

Sorption kinetics of organic compounds on biochars is important for understanding the retardation of mobility and bioavailability of organic compounds.Herein,sorption kinetics of 1,3,5-trinitrobenzene on biochars prepared from 200 to 700℃was investigated to explore the sorption process.Loose partition matrix and condensed partition matrix were formed at relatively low and moderate temperatures,respectively.However,biochars produced at relatively high temperatures formed rich pore structures.Therefore,sorption equilibrium time of 1,3,5-trinitrobenzene increased with increasing preparation temperature from 200 to 350℃due to the slower diffusion rate in the more condensed matrix,and then decreased when preparation temperature was higher than 400℃because of the faster adsorption rate in the greater number of pores.Linear positive relationship between matrix diffusion rates of 1,3,5-trinitrobenzene on biochars prepared at 200,250,300,350℃and H/C ratios of biochars was observed,suggesting that the inhibition of partition process was caused by the condensed matrix in biochars.Linear positive relationships between adsorption rates(i.e.,fast outer diffusion rate and slow pore diffusion rate)of 1,3,5-trinitrobenzene on biochars prepared at 400,450,550,700℃and graphite defects of biochars were observed,because the increase of graphite defects of biochars could promote the adsorption by increasing the quantity of fast diffusion channels and sorption sites.This study reveals the underlying mechanisms of sorption kinetics for organic compounds with relatively large size on biochars,which has potential guidance for the application of biochars and prediction of the environmental risks of organic compounds.