Gas Molecules Adsorption Mechanism At Solid Materials: A First Principle Study

Small gas molecules can be adsorbed both on the surface and into the inner pores of solidmaterials. The former case can be applied for surface functionalization, and the latter case can beused for gas capture, separation and catalyse. Accordingly, we explored dechlorniation mechanismof perchloroethylene （C2Cl4） on the surface of Si（100） and CO₂gas adsorptive capacity of metal-salts chelated MOF-253by first principle calculation. Finally, activating CO₂molecule by singleNi atom chelated MOF-253was discussed.Silicon surface can adsorb some organic gas molecules, which is important for siliconesurface chemistry research. In our previous studies, adjacent Si dimer didechlorination wasproposed as the major dissociation mechanism for the adsorption of dichloethylene （C₂H₂Cl₂） onSi（100）-2×1. To further understand the multi-chlorine effect on the mechanism, dechlorniationprocess of perchloroethylene on Si （100）-2×1was studied in this work based on first principlescalculation. By analyzing the adsorption energies, dechlorniation barriers and C-1s ionizationenergies of the potential adsorption states, the adjacent Si dimer didechlorination process ofproducing intra or inter-dimer tetra-σ adspecies were found to be the major reaction process atroom temperature, while the tetradechlorination would take place at higher temperature due to itshigher Cl dissociation barriers. Finally, in order to further validate the proposed adsorptionstructures, we compared the calculated vibrational frequencies with EELS （electron energyloss spectroscopy） experimental results.MOFs （Metal Organic Frameworks） with porous structure and high surface area havebeen explored for gas storage and separation. MOF-253contains open2,2’-bipyridine sites. Twobipyridine rings could be rotated180ofrom each other. Using cNEB （climbing nudged elasticband） method, the rotating energy barrier was calculated to be0.42eV. By dispersion forcecorrected DFT-D2method, CO₂molecule adsorption energy of MOF-253was calculated to rangefrom-0.15eV to-0.50eV, and double-N of2,2’-bpy ligand had an enhanced CO₂moleculeadsorptive capacity compared to single-N site. Cu（BF₄）₂chelated MOF-253had an enhancedadsorption energy of0.49eV over CO₂, which indicated that Cu（BF₄）₂chelating over MOF-253could improve CO₂adsorptive capacity. NiCl₂@MOF-253had a limited CO₂adsorption energy（-0.29^-0.35eV）. That would be because that Cu（BF₄）₂is more ionic than NiCl₂. After chelations,F took more electrons than Cl and Cu was more positive than Ni. That made Coulomb force ofCO₂/Cu（BF₄）₂@MOF-253stronger than that of CO₂/NiCl₂@MOF-253. Finally, single Ni atom was chelated onto open2,2’-bpy ligand, and found that Ni@MOF-253could distorted linear CO₂molecule to142.7o. Therefore, It is promising that doping MOF-253with some activation groupscan be applied to activate or catalyze CO₂molecule.