Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO(2) and CH(4) to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH(2), H(2)PO(3), NO(2), and CH(3). Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO(2) to exceed that of CH(4) by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH(2), H(2)PO(3), NO(2), and COOH functional groups can significantly enhance gas binding, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO(2)/CH(4) selectivity are discussed.     

Atomic carbon adsorption on Ni nanoclusters: a DFT study

Atomic carbon is a key intermediate interacting with transition metal clusters during the growth of carbon nanotube (CNT). Present density functional calculations studied the initial carbon adsorption on four Ni nanoclusters (N₁₃, N₁₅, N₃₈, and N₅₅). Our results show that carbon atoms preferentially adsorb on high-coordination sites, and carbon adsorption energies are larger on smaller Ni clusters. Ni cluster reconstruction plays an important role in creating more stable subsurface adsorption sites. The migration of adsorbed carbon atom on the surface threefold hollow site into the underlying interstitial subsurface positions is thermodynamically and kinetically feasible. The results indicate that the investigation of CNT growth mechanism should include both surface and subsurface carbon atoms, coupled with surface reconstruction of Ni nanoclusters.     

Regioselective control of the nickel-mediated coupling of acetylene and carbon dioxide - A DFT study

The nickel-mediated coupling of asymmetric alkynes with carbon dioxide is known to be highly regioselective with respect to the formation of nickelacycle intermediates and α,β-unsaturated carboxylic acid products. Using density functional theory (DFT), we have investigated the effect that parameters such as acetylene-substituent, ancillary ligand and solvent have on the potential energy surface of the nickelacycle coupling reaction. 3-R-substituted nickelacycles are the thermodynamically preferred product in all cases surveyed, however, the transition structure characterised by the attack of CO₂ on the alkyne carbon distal from the R-group is generally lower in energy, making the 2-R-substituted nickelacycle the kinetically favoured product. Ligating the zerovalent nickel species with the diazabicyclo[5.4.0]undec-7-ene (DBU) ancillary ligand in preference to 2,2′-bipyridine (BIPY) leads to lower activation energies for the coupling reaction and products that are less susceptible to steric bulk in the 2-position of the nickelacycle. Solvation with dimethylformamide (DMF) has the advantage of lowering the activation barrier for the coupling reaction when compared to tetrahydrofuran (THF).     

Carbon dioxide adsorption on lanthanum zirconate nanostructured coating surface: a DFT study

Pyrochlore lanthanum zirconate (La₂Zr₂O₇) is a very promising candidate material for thermal barrier coating applications. However it may deteriorate by oxidizing gas such as CO₂ during operating conditions. This paper investigates CO₂ gas adsorption on La₂Zr₂O₇ nanostructured coating surfaces using the density functional theory calculations. CO₂ adsorption energies on (001), (011) and (111) planes in the La–Zr bridge positions have been calculated. The most favorable CO₂ adsorption occurs on the (111) plane, which is confirmed by electron charge transfer and charge density difference analyses. La₂Zr₂O₇ surface energies on (001), (011) and (111) planes have been calculated. Results show that (011) plane is the most thermodynamically stable plane due to its lowest surface energy.     

Capture and separation of CO2 on BC3 nanosheets: A DFT study

In order to reduce the greenhouse effect caused by the rapid increase of CO₂ concentration in the atmosphere, it is necessary to develop more efficient, controllable, and highly sensitive adsorbing materials. In this study, the adsorption behavior of CO₂ on BC₃ nanosheets under an external electric field was explored based on density functional theory (DFT). It was found that CO₂ experienced a transition from physisorption to chemisorption in the electric field range of 0.0060-0.0065 a.u.. In addition, the adsorption/desorption of CO₂ is reversible and can be precisely controlled by switching on/off at the electric field of 0.0065 a.u.. The selective adsorption of CO₂/H₂/CH₄ by BC₃ can also be used to realize gas separation and purification under different electric fields. This study highlighted the potential application of BC₃ nanosheets as a high-performance, controllable material for CO₂ capture, regeneration, and separation in an electric field.     

纳米二氧化钛光催化降解喹啉动力学

利用溶胶-凝胶法制备纳米二氧化钛光催化剂,探讨了其光催化降解废水中典型含氮杂环化合物喹啉的动力学行为。实验条件下,TiO₂光催化降解喹啉的反应为准一级反应,可用Langmuir-Hinshelwood动力学模型描述,其表达式为r=0.296kc_t/(1+0.296c_t),其中,lnk =-0.411 1lnc₀+2.278。     

Carbon dioxide hydrogenation catalyzed by a ruthenium dihydride: a DFT and high-pressure spectroscopic investigation

Reaction pathways during CO(2) hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)(2)H(2)] (dmpe=Me(2)PCH(2)CH(2)PMe(2)) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120 bar in toluene at 300 K. CO(2) and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, cis-[Ru(dmpe)(2)(OCHO)(2)] and trans-[Ru(dmpe)(2)H(OCHO)], were formed at low CO(2) pressure (<5 bar). The latter occurred exclusively when formic acid reacted with the complex. A RuHHOCHO dihydrogen-bonded complex of the trans form was identified at H(2) partial pressure higher than about 50 bar. The trans form of the complex is suggested to play a pivotal role in the reaction pathway. Potential-energy profiles along possible reaction paths have been investigated by static DFT calculations, and lower activation-energy profiles via the trans route were confirmed. The H(2) insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H(2) insertion is attributed to the elongated Ru--O bond. The H(2) insertion and the subsequent formation of formic acid proceed via Ru(eta(2)-H(2))-like complexes, in which apparently formate ion and Ru(+) or Ru(eta(2)-H(2))(+) interact. The bond properties of involved Ru complexes were characterized by natural bond orbital analysis, and the highly ionic characters of various complexes and transition states are shown. The stability of the formate ion near the Ru center likely plays a decisive role for catalytic activity. Removal of formic acid from the dihydrogen-bonded complex (RuHHOCHO) seems to be crucial for catalytic efficiency, since formic acid can easily react with the complex to regenerate the original formate complex. Important aspects for the design of highly active catalytic systems are discussed.     

Synthesis of nanosized titanium dioxide from tetrabutoxytitanium

Mesoporous TiO₂ has been obtained by template synthesis. The introduction of a surfactant in the hydrolysis of tetrabutoxytitanium in aqueous ethanol allows the structure of the resulting material to be controlled. The amorphous TiO₂ resulting from hydrolysis turns into anatase on being calcined at 300°C. As determined by X-ray diffraction, the anatase crystallite size is 70 nm in the presence of the highest surfactant concentration examined and 210 nm in the absence of a surfactant. Amorphous materials have been characterized by electron microscopy and differential thermal analysis. The ultimate benzene adsorption value has been determined for calcined mesoporous TiO₂.     

Density functional study on the sensing properties of nano-sized BeO tube toward H2S

Using density functional calculations, we have investigated the adsorption of a H₂S molecule on the pristine and Si-doped BeO nanotubes (BeONT). It was found that the H₂S molecule is physically adsorbed on the pristine BeONT with adsorption energies ranging from 3.0 to 4.2 kcal/mol. Substituting a Be or O atom of the tube by Si increases the adsorption energy to 6.9–17.2 kcal/mol. We found that substituting an O atom by Si makes the electronic properties of the BeONT strongly sensitive to the H₂S molecule. Therefore, the process of Si doping provides a good strategy for improving the sensitivity of BeONT to toxic H₂S, which cannot be trapped and detected by the pristine BeONT. Also, the emitted electron current density from the Si_O–BeONT will be significantly increased after the H₂S adsorption.     

IR spectroscopy study of the radiation-thermal decomposition of water on nanosized zirconium dioxide

Radiation-thermal decomposition of water on nanosized ZrO₂ in the temperature range of 300–673 K has been studied by FTIR spectroscopy. It has been revealed by the X-ray diffraction method that the sample used has the centrsymmetric monoclinic crystal structure. It has been shown that nanosized zirconium dioxide adsorbs water on via the molecular and dissociative mechanisms. Intermediate products of the radiation-induced heterogeneous decomposition of water, namely, the molecular oxygen and hydrogen peroxide radical ions, zirconium hydride, and hydroxyl radicals have been detected. A comparative analysis of changes in the absorption bands of molecular water and surface hydroxyl groups with temperature has been conducted, and the stimulating role of radiation in the radiation-thermal process of water decomposition has been revealed.     

Microsolvation of glycine: a DFT study

PBE1PBE/6-311+G(d,p) computations exploring the microsolvation of neutral and zwitterionic glycine are reported. A broad configuration search was performed to identify the lowest energy clusters of glycine with one to seven water molecules. The structures of the clusters are analyzed on the basis of the hydrogen-bonding network established between the water molecules and between water and glycine. Neutral glycine is favored when associated with zero to six water molecules, but with seven water molecules the two structures are isoenergetic.     

Mechanism of formation of organic carbonates from aliphatic alcohols and carbon dioxide under mild conditions promoted by carbodiimides. DFT calculation and experimental study

Dicyclohexylcarbodiimide (CyN=C=NCy, DCC) promotes the facile formation of organic carbonates from aliphatic alcohols and carbon dioxide at temperatures as low as 310 K and moderate pressure of CO2 (from 0.1 MPa) with an acceptable rate. The conversion yield of DCC is quantitative, and the reaction has a very high selectivity toward carbonates at 330 K; increasing the temperature increases the conversion rate, but lowers the selectivity. A detailed study has allowed us to isolate or identify the intermediates formed in the reaction of an alcohol with DCC in the presence or absence of carbon dioxide. The first step is the addition of alcohol to the cumulene (a known reaction) with formation of an O-alkyl isourea [RHNC(OR')=NR] that may interact with a second alcohol molecule via H-bond (a reaction never described thus far). Such an adduct can be detected by NMR. In alcohol, in absence of CO2, it converts into a carbamate and a secondary amine, while in the presence of CO2, the dialkyl carbonate, (RO)2CO, is formed together with urea [CyHN-CO-NHCy]. The reaction has been tested with various aliphatic alcohols such as methanol, ethanol, and allyl alcohol. It results in being a convenient route to the synthesis of diallyl carbonate, in particular. O-Methyl-N,N'-dicyclohexyl isourea also reacts with phenol in the presence of CO2 to directly afford for the very first time a mixed aliphatic-aromatic carbonate, (MeO)(PhO)CO. A DFT study has allowed us to estimate the energy of each intermediate and the relevant kinetic barriers in the described reactions, providing reasonable mechanistic details. Calculated data match very well the experimental results. The driving force of the reaction is the conversion of carbodiimide into the relevant urea, which is some 35 kcal/mol downhill with respect to the parent compound. The best operative conditions have been defined for achieving a quantitative yield of carbonate from carbodiimide. The role of temperature, pressure, and catalysts (Lewis acids and bases) has been established. As the urea can be reconverted into DCC, the reaction described in this article may further be developed for application to the synthesis of organic carbonates under selective and mild conditions.     

From carbon dioxide to methane: homogeneous reduction of carbon dioxide with hydrosilanes catalyzed by zirconium-borane complexes

A mixture of a zirconium benzyl phenoxide complex and tris(pentafluorophenyl)borane is reported that catalyzes the hydrosilation reaction of carbon dioxide to generate methane via a bis(silyl)acetal intermediate.     

Dilation of polycarbonate by carbon dioxide

A new technique is described for dilatometry under high pressure. The technique is based on optical interferometry and is analogous to measuring the thickness of thin, nonabsorbing films and coatings. The procedure is demonstrated for the well-characterized system of n-pentane sorption by polyisobutylene, and then results for the dilation of polycarbonate by the sorption of carbon dioxide are presented. The dilation of polycarbonate by CO₂ is nearly linear with concentration; the partial molar volume of CO₂ decreases slightly with increasing pressure. This result indicates that all sorbed CO₂ molecules contribute equally to the dilation of the polymer matrix and that none reside in microvoids or in preexisting free-volume elements which do not contribute to volume expansion of the polymer.     

机械力化学反应法制备纳米金红石型二氧化钛的研究

以Ti (C4H9O)4 和 C2H2O4·2H2O 为原料在室温下利用机械力化学反应法制备前驱体,利用热重-微商热重法(TG-DTG)决定前驱体的分解温度,通过在特定温度下对前驱体进行热分解得到TiO2.利用XRD,SEM,ZETA 电位粒度分析仪以及紫外光谱仪对所得样品物相,形貌,粒度和光学性能进行分析和表征.结果显示所得粉体微观形貌为球形,其直径(＞ 95%)小于60nm,粒径较小的粉体对紫外线UVA波段的吸收效果较好.     

Polar bromination of cyclopropane: a DFT study

Hitherto the polar addition of bromine to cyclopropane has been considered as a two-step process. Current calculations have established the energetics for the proposed cation-anion pairs required by these mechanisms. An energetically lower pathway is proposed here in the form of a syn-cycloaddition process. Compared to the two-step process, a significantly lower activation enthalpy for this process has been found. The stereochemical consequences of the cyclic mechanism are retention-retention for the two adding moieties. This result is consistent with published experimental data on the bromination of a deuterated cyclpropane.