表面改性对Cu/活性炭催化剂表面Cu物种和催化活性的影响

通过对活性炭表面改性,研究了醋酸铜溶液浸渍后热分解制备的Cu/AC无氯催化剂对甲醇氧化羰基化合成碳酸二甲酯催化性能的影响。通过低温氮吸附－脱附、FTIR等表征对改性前后活性炭的比表面积及表面官能团变化进行了分析,发现氨水改性的活性炭比表面积略有增大,表面不饱和还原性官能团(C≡N、C≡C)增加,而KMnO₄改性的活性炭比表面积减小7%,表面C=O、-COOH显著增加。进一步通过XRD、H₂-TPR、XPS等表征分析了催化剂表面铜物种(Cu⁰/Cu^I/Cu^Ⅱ)的形态,发现表面铜物种的价态和配比与活性炭表面性质密切相关,影响催化剂活性。活性炭改性后,表面微观结构和表面化学性质的变化对浸渍醋酸铜的表面分布和热分解过程产生明显影响,导致催化剂表面活性Cu物种发生变化。和其它表面改性方法相比,氨水改性的活性炭更有利于催化剂活性的提高,碳酸二甲酯的时空收率及甲醇转化率分别达到了152.8 mg·g^-1·h^-1和7.4%。     

β-环糊精修饰的壳聚糖对氯酚吸附的动力学和热力学

制备了β-环糊精(β-CD)修饰的壳聚糖(CS)—β-环糊精-6-壳聚糖(CS-CD), 并用傅里叶变换红外(FT-IR)光谱仪, 扫描电镜(SEM), X 射线衍射(XRD)仪和比表面分析仪(BET)进行了表征. 详细研究了其对2-氯酚(2-CP)、2,4-二氯酚(DCP)和2,4,6-三氯酚(TCP)的吸附行为和机理. 研究表明其吸附较好地满足Langmuir 和Freundlich 吸附模型, β-环糊精的引入能够较大地提高吸附效率, 2-CP、DCP和TCP在CS-CD上吸附的最大吸附量分别为14.51、50.68和74.29 mg·g^-1. 动力学研究表明其吸附速率快, 在1 h内能达到吸附平衡, 并符合假二级动力学模型. 计算出了热力学参数ΔG⁰、ΔH⁰和ΔS⁰的值, ΔG⁰为负值表明吸附剂对氯酚的吸附是一个自发的过程. 电解质和溶液pH值对吸附的影响说明在吸附过程中主要是氯酚与吸附剂之间形成了氢键, 并进一步讨论了可能的吸附机理. 改性吸附剂易重复利用, 重复使用六次后的质量和吸附效率与初次相比分别保持在90%和82%以上, 然而CS的质量有较大的损失, 吸附效率也明显降低.     

活性炭的孔径分布对CH4和CO2的吸附性能的影响

采用不同炭化温度和活化温度,以椰壳作为前驱体制备了系列结构性能相同,表面吸附基团相似,不同孔结构的活性炭。分别采用密度函数理论(DFT)吸附法和BJH估算了系列活性炭的孔径分布。结果表明,随炭化温度和活化温度的升高系列活性炭中微孔量先增加后减少。当炭化温度为700 ℃,活化温度为800 ℃时,制备的活性炭微孔量达到最大。随炭化温度和活化温度的升高,系列活性炭的中孔依次增加。考查了CH₄,CO₂在系列活性炭上的吸附性能。结果表明该系列活性炭对CO₂有很强的吸附能力,在常温常压下对CO₂的吸附量均高于1.0 mmol·g^-1;系列活性炭对CH₄的吸附能力有较大的差异,在对CO₂具有最大吸附量的活性炭上对CH₄具有最小的吸附量。采用变压吸附法测试了该系列活性炭在25 ℃时对n_CH4∶n_CO2=9∶1的混合气体的分离性能。结果表明炭化温度为700 ℃,活化温度为800 ℃时制备的活性炭对CH₄-CO₂混合气具有最好的分离效果,是变压吸附分离CH₄,CO₂混合气的优异吸附剂。     

用扩展的Istomin-Palm模型估算双向延伸化合物R1-Y-R2生成焓

Istomin和Palm曾提出用模型Δ_fH⁰(RX)=h[R]+h[X]+φ[R]φ[X](式中h[R]和h[X]分别为烷基R 和取代基X对单取代烷烃生成焓Δ_fH⁰(RX)的贡献, φ[R]φ[X]则表示R与X之间的相互作用对Δ_fH⁰(RX)的贡献)来表示单取代烷烃生成焓Δ_fH⁰(RX). 对于双向延伸化合物R1-Y-R2, 其取代基Y位于分子链的中间, 与两个烷基(R1和R2)相连. 此类化合物分子内取代基与烷基之间的相互作用, 较单取代烷烃的相比更为复杂. 因此, Istomin-Palm模型在R1-Y-R2体系中应用必须进行修正. 本文把取代基Y、烷基R1和R2三者之间的相互作用对R1-Y-R2类化合物生成焓Δ_fH⁰(R1-Y-R2)的贡献分为三部分: R1Y与R2之间的相互作用(φ[R2]φ[R1Y]), YR2与R1之间的相互作用(φ[R1]φ[YR2]), 以及两烷基R1与R2之间的相互作用(ψ[R1]ψ[R2]). 用以上三项替换φ[R]φ[X], 扩展Istomin-Palm模型, 建立一个新的经验模型Δ_fH⁰(R1-Y-R2)=h[R1]+h[R2]+h[Y]+φ[R1]φ[YR2]+φ[R2]φ[R1Y]+ψ[R1]ψ[R2], 来表示Δ_fH⁰(R1-Y-R2)(式中h[R1]、h[R2]和h[Y]分别为烷基R1、R2和取代基Y对Δ_fH⁰(R1-Y-R2)的贡献, 后三项则表示烷基R1、R2和取代基Y两两之间相互作用对Δ_fH⁰(R1-Y-R2)的贡献). 进而, 采用本研究组最近报道的相互作用势指数IPI(X)(Wu, Y. X.; Cao, C. Z.; Yuan, H. Chin. J. Chem. Phys. 2012,25 (2), 153.)表示取代基Y对烷基的固有作用(φ[Y]), 从而建立两个定量估算生成焓的通用模型. 其中, 一个用于估算硫醚、仲胺、醚和酮类化合物生成焓, 另一个用于估算酯类化合物生成焓. 这两个模型均得到良好的结果, 与采用G3和G3MP2方法相比具有同样的精度, 还可以避免大量繁琐的计算.     

二维TiC 层表面H2的化学吸附与物理吸附

第一性原理计算研究发现由于二维TiC 单原子层具有高的比表面积与大量的暴露在表面的Ti 原子,其是一种非常有潜力的储氢材料. 计算结果显示H₂可以在二维TiC 单原子层表面进行物理吸附与化学吸附. 其中化学吸附能为每个氢分子0.36 eV,物理吸附能是每个氢分子0.09 eV. 覆盖度为1和1/4层（ML）时,H₂分子在二维TiC 单原子层表面的离解势垒分别为1.12 和0.33 eV. 因此,除了物理吸附与化学吸附,TiC 表面还存在H单原子吸附. 最大的H₂储存率可以达到7.69%（质量分数）. 其中,离解的H原子、化学吸附的H₂、物理吸附的H₂的储存率分别为1.54%、3.07%、3.07%. 符合Kubas吸附特征的储存率为3.07%. 化学吸附能随覆盖度的变化非常小,这有利于H₂分子的吸附与释放.     

密度泛函理论研究NO在CuCr2O4(100)表面的吸附

运用密度泛函理论研究了NO在CuCr₂O₄(100)表面4个可能吸附位的顶位吸附。结果表明:表面铜(Cu)和铬(Cr)为活性吸附位，吸附能分别为98.1 kJ·mol^-1和92.9 kJ·mol^-1。对活性吸附位Cu位和Cr位考虑了NO以N端和O端2种吸附取向的吸附，发现N端吸附比O端吸附更为有利，O端吸附为简单的物理吸附。在2种吸附取向情况下，N-O键的伸缩振动频率都发生了红移。Mulliken布居分析表明，N端吸附时NO分子失去电子;对NO吸附前后的态密度分析可知，对Cu位和Cr位N端吸附NO的2π轨道得到电子。本文并进一步讨论了NO分子在CuCr₂O₄ (100)表面Cu位和Cr位的成键机理。     

螺吡喃功能化UiO-66光响应吸附剂的制备及其性能

将光响应分子甲基螺吡喃SP-CH₃引入UiO-66的非极性孔笼中,构筑吸附活性位可光控调节的光响应吸附剂。SP-CH₃功能化的吸附剂完好保留了载体UiO-66的骨架和孔道结构。以阴离子染料甲基橙为探针,研究了吸附剂在不同光照条件下的吸附和解吸性能。结果表明,经紫外光照后,吸附剂对甲基橙的吸附量为41.99 mg·g^-1,相较于可见光照后样品的吸附量提升57.56%,吸附作用增强;经可见光照后,甲基橙的脱附量为81.6%。本策略通过光照刺激改变UiO-66孔笼中SP-CH₃的构型及表面电荷性质,即对吸附活性位进行光控调节,在不同光照条件下实现对吸附质的高效吸附和有效脱附。     

统计方法校正O3LYP方法计算的生成热

由于引入各种内在近似, 密度泛函理论存在固有误差. 本文采用O3LYP/6-311+G(3df, 2p)//O3LYP/6-31G(d)计算了220个中小型有机分子的生成热(Δ_fH_calc^Θ), 随后应用神经网络(ANN)和多元线性回归(MLR)方法对Δ_fH_calc^Θ进行校正. 采用计算得到的生成热、零点能、分子中原子总数、氢原子个数、双中心成键电子数、双中心反键电子数、单中心价层孤对电子数、单中心内层电子数作为ANN和MLR的描述符. 以180个分子作为训练集构造ANN或MLR模型, 并对40 个独立测试集分子的Δ_fH_calc^Θ进行了预测. 结果表明: 经过ANN和MLR校正后,训练集分子生成热的理论计算值和实验值间的均方根偏差(RMSD)从24.7 kJ·mol^-1分别降低到11.8、13.0 kJ·mol^-1; 独立测试集分子的RMSD从21.3 kJ·mol^-1分别降低到10.4、12.1 kJ·mol^-1. 因此ANN模型的拟合和预测能力要明显优于MLR模型.     

螺吡喃功能化UiO-66光响应吸附剂的制备及其性能

将光响应分子甲基螺吡喃SP-CH₃引入UiO-66的非极性孔笼中,构筑吸附活性位可光控调节的光响应吸附剂。SP-CH₃功能化的吸附剂完好保留了载体UiO-66的骨架和孔道结构。以阴离子染料甲基橙为探针,研究了吸附剂在不同光照条件下的吸附和解吸性能。结果表明,经紫外光照后,吸附剂对甲基橙的吸附量为41.99 mg·g^-1,相较于可见光照后样品的吸附量提升57.56%,吸附作用增强;经可见光照后,甲基橙的脱附量为81.6%。本策略通过光照刺激改变UiO-66孔笼中SP-CH₃的构型及表面电荷性质,即对吸附活性位进行光控调节,在不同光照条件下实现对吸附质的高效吸附和有效脱附。     

糠醛在Pt(111)表面的吸附和脱碳反应

采用广义梯度近似的密度泛函理论并结合平板模型的方法, 优化了糠醛分子在Pt(111)面的吸附模型,并探究了糠醛脱碳反应形成呋喃的机理. 结果表明: 吸附后糠醛分子环上的C―H(O)键及支链―CHO相对于金属表面倾斜上翘, 分子平面被扭曲, 易于呋喃的形成; 同时, 糠醛分子向Pt表面转移电子0.765e, 环中的大π键与Pt(111)表面的d轨道发生较强的相互作用, 使得糠醛的芳香性被破坏, 环上的碳原子呈现准sp³杂化. 此外, 对糠醛脱碳反应中的各反应步骤进行过渡态搜索, 通过比较各步骤的活化能, 得出糠醛更易先失去支链上的H形成酰基中间体(C₄H₃O)CO, 中间体继续脱碳加氢形成产物呋喃. 该过程的控速步骤为(C₄H₃O)CO^*+^*→C₄H₃O^*+CO^* (^*为吸附位),活化能为127.65 kJ·mol^-1.     

Cluster-Mediated Water Adsorption on Carbon Nanopores

The adsorption isotherms of water at 303 K and N2 at 77 K on various kinds of porous carbons were compared with each other. The saturated amounts of water adsorbed on carbons almost coincided with amounts of N2 adsorption in micropores. Although carbon aerogel samples have mesopores of the great pore volume, the saturated amount of adsorbed water was close to the micropore volume which is much small than the mesopore volume. These adsorption data on carbon aerogels indicated that the water molecules are not adsorbed in mesopores, but in micropores only. The adsorption isotherms of water on activated carbon having micropores of smaller than 0.7 nm in width had no clear adsorption hysteresis, while the water adsorption isotherms on micropores of greater than 0.7 nm had a remarkable adsorption hysteresis above P/P0 = 0.5. The disappearance of the clear hysteresis for smaller micropores suggested that the cluster of water molecules of about 0.7 nm in size gives rise to the water adsorption on the hydrophobic micropores; the formation and the structure of clusters of water molecules were associated with the adsorption mechanism. The cluster-mediated pore filling mechanism was proposed with a special relevance to the evidence on the formation of the ordered water molecular assembly in the carbon micropores by in situ X-ray diffraction.     

Adsorption of valeric acid from aqueous solution onto activated carbons: role of surface basic sites

Adsorption of valeric acid was studied on two activated carbons of different origins, wood and bituminous coal. The effect of oxidation on the adsorption uptake was investigated. Both initial samples were oxidized with nitric acid, which caused the introduction of a significant number of oxygen-containing groups onto the carbon surface. Boehm titration, potentiometric titration, thermal analysis, and sorption of nitrogen were used for detailed surface characterization. Valeric acid adsorption from aqueous solution was measured at 333 K. The calculated isotherms showed a good fitting to the Freundlich equation. The capacity coefficients revealed a direct correlation with the number of basic groups present on the surface. The amount of valeric acid adsorbed at its low concentration is dependent on the number of basic groups, whereas at high concentration the amount adsorbed depends on the volume of micropores smaller than 10 A, where the adsorption potential is the strongest.     

Adsorption of NH3 onto activated carbon prepared from palm shells impregnated with H2SO4

Adsorption of ammonia (NH3) onto activated carbons prepared from palm shells impregnated with sulfuric acid (H2SO4) was investigated. The effects of activation temperature and acid concentration on pore surface area development were studied. The relatively large micropore surface areas of the palm-shell activated carbons prepared by H2SO4 activation suggest their potential applications in gas adsorption. Adsorption experiments at a fixed temperature showed that the amounts of NH3 adsorbed onto the chemically activated carbons, unlike those prepared by CO2 thermal activation, were not solely dependent on the specific pore surface areas of the adsorbents. Further adsorption tests for a wide range of temperatures suggested combined physisorption and chemisorption of NH3. Desorption tests at the same temperature as adsorption and at an elevated temperature were carried out to confirm the occurrence of chemisorption due to the interaction between NH3 and some oxygen functional groups via hydrogen bonding. The surface functional groups on the adsorbent surface were detected by Fourier transform infrared spectroscopy. The amounts of NH3 adsorbed by chemisorption were correlated with the contents of elemental oxygen present in the adsorbents. Mechanisms for chemical activation and adsorption processes are proposed based on the observed phenomena.     

Density of He adsorbed in micropores at 4.2K

The density of He adsorbed in the cylindrical micropores of zeolites NaY and KL has been determined by He adsorption at 4.2K. He adsorption isotherms were then compared with N₂ adsorption isotherms at 77K. Crystallographic considerations of the micropore volumes gave the density of the He adsorbed layer, which is necessary for assessment of ultramicroporosity of less-crystalline microporous solids, such as activated carbons. The determined density of He adsorbed in the cylindrical micropores of the zeolite was in the range 0.22 to 0.26 gml^–1, greater than that of He adsorbed on a flat surface (0.202 gml^–1). A value for the density of He between 0.20 to 0.22 gml^–1 is recommended for evaluation of ultramicroporosity of a slit-shaped microporous system such as activated carbon.     

Voltammetric and FT-IR studies of modified activated carbon systems with phenol, 4-chlorophenol or 1,4-benzoquinone adsorbed from aqueous electrolyte solutions

Cyclic voltammetric and spectral FT-IR studies of the influence of surface chemistry on the adsorption and electrochemical behaviour of powdered activated carbon electrodes (PACE) in the presence of phenol, 4-chlorophenol and 1,4-benzoquinone were carried out. The variety of surface chemical properties was achieved through modification of the carbon samples by heat treatment under vacuum and in an oxygen–ammonia atmosphere as well as by oxidation with conc. nitric acid. Adsorption processes were carried out from aqueous solutions (in air and an oxygen-free atmosphere) in the presence acidic, neutral or basic electrolytes. Some electrochemical parameters of electrode systems with adsorbed organics were estimated and correlated with the adsorption ability of carbon materials. The changes in FT-IR spectra of the carbons caused by adsorption and/or coupling organic molecules are presented and discussed.     

Hydrogen adsorption in transition metal carbon nano-structures

Templated microporous carbons were synthesized from metal impregnated zeolite Y templates. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to characterize morphology and structure of the generated carbon materials. The surface area, micro- and meso-pore volumes, as well as the pore size distribution of all the carbon materials were determined by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. All the hydrogen adsorption isotherms were Type 1 and reversible, indicating physisorption at 77 K. Most templated carbons show good hydrogen storage with the best sample Rh-C having surface area 1817 m²/g and micropore volume 1.04 cm³/g, achieving the highest as 8.8 mmol/g hydrogen storage capacity at 77 K, 1 bar. Comparison between activated carbons and synthesized templated carbons revealed that the hydrogen adsorption in the latter carbon samples occurs mainly by pore filling and smaller pores of sizes around 6 Å to 8 Å are filled initially, followed by larger micropores. Overall, hydrogen adsorption was found to be dependent on the micropore volume as well as the pore-size, larger micropore volumes showing higher hydrogen adsorption capacity.     

Importance of structural and chemical heterogeneity of activated carbon surfaces for adsorption of dibenzothiophene

The performance of various activated carbons obtained from different carbon precursors (i.e., plastic waste, coal, and wood) as adsorbents for the desulfurization of liquid hydrocarbon fuels was evaluated. To increase surface heterogeneity, the carbon surface was modified by oxidation with ammonium persulfate. The results showed the importance of activated carbon pore sizes and surface chemistry for the adsorption of dibenzothiophene (DBT) from liquid phase. Adsorption of DBT on activated carbons is governed by two types of contributions: physical and chemical interactions. The former include dispersive interactions in the microporous network of the carbons. While the volume of micropores governs the amount physisorbed, mesopores control the kinetics of the process. On the other hand, introduction of surface functional groups enhances the performance of the activated carbons as a result of specific interactions between the acidic centers of the carbon and the basic structure of DBT molecule as well as sulfur-sulfur interactions.     

Cadmium ion adsorption on different carbon adsorbents from aqueous solutions. Effect of surface chemistry, pore texture, ionic strength, and dissolved natural organic matter

Adsorption of Cd(II) species at pH = 5 was studied on three carbon adsorbents: granular activated carbon, activated carbon fiber, and activated carbon cloth. As-received and oxidized adsorbents were used. Cd(II) adsorption greatly increased after oxidation due to the introduction of carboxyl groups. The use of a buffer solution to control the pH introduced some changes in the surface chemistry of carbons through the adsorption of one of the compounds used, biphthalate anions. The increase in ionic strength reduced Cd(II) uptake on both as-received and oxidized carbons due to a screening of the electrostatic attractions between the Cd(II) positive species and the negative surface charge, which in the case of as-received carbons derived from the biphthalate anions adsorbed and in the oxidized ones from the carboxyl groups. Tannic acid was used as a model compound for natural organic matter. Its adsorption was greatly reduced after oxidation, and most of the carbon adsorbents preadsorbed with tannic acid showed an increase in Cd(II) uptake. In the case of competitive adsorption between Cd(II) species and tannic acid molecules, there was a decrease in Cd(II) uptake on the as-received carbon whereas the contrary occurred with the oxidized carbons. These results illustrate the great importance of carbon surface chemistry in this competitive adsorption process. Finally, under all experimental conditions used, when the adsorption capacity of carbons was compared under the same conditions it increased in the following order: granular activated carbon < activated carbon fiber < activated carbon cloth.     

Adsorption of tetraalkylammonium ions on microporous and mesoporous activated carbons prepared from vinylidene chloride copolymer

The adsorption of tetraalkylammonium ions on microporous (AC-micro) and mesoporous (AC-meso) activated carbons prepared from vinylidene chloride copolymer was investigated. The adsorbed amounts on AC-micro decreased in the order of tetraethyl-, tetrapropyl-, hexadecyltrimetyl-, and tetrahexylammonium bromide. Consequently it is suggested that the pore size of the activated carbon plays an important role in the adsorption. The adsorbed amounts on AC-meso increased with increasing alkyl chain length. In the case of mesoporous activated carbon, hydrophobic interaction between tetraalkylammonium ions and the surface of activated carbons contributes to in the adsorption.     

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of the Water Molecules in Active Carbons: 2. The State of Water in Active Carbon with Relatively Large Pores According to the NMR Relaxation Data

The state of water upon adsorption on FAS-3 active carbon with relatively large micropores is studied by the NMR relaxation method. The dependences of the times of spin–lattice (T ₁) and spin–spin (T ₂) NMR relaxation of adsorbed water molecules on the adsorption value are established. The character of the dependences of T ₁ and T ₂ on the number of adsorbed water molecules per primary adsorption site reflects the specific features of the volume filling of micropores and the formation of a continuous adsorption layer on the mesopore surface due to cluster coalescence on the one wall of a pore. The results obtained are compared with the data for typical microporous active carbons, as well as with the data obtained by the adsorption method.