热重分析技术测定二苯并呋喃在活性炭上的吸附相平衡

以吸附动力学为基础,建立了热重分析技术测定难挥发性有机物吸附等温线的方法.通过热重分析实验测定了不同升温速率下二苯并呋喃从Norit RB1和Chemviton BPL活性炭上脱附速率曲线,及对应的脱附峰温度.运用所提出的方法,对二苯并呋喃在Norit RB1和ChemvironBPL活性炭上的Langmuir吸附等温线方程进行了估算.估算结果与静态吸附实验测定结果相比,误差不超过10%.     

用热脱附方法测定氧气、乙醇在Porapak样品上的脱附级数和脱附活化能

本文采用热脱附(TDS的实验方法,以多孔性Porapak微球为样品,测定了氧气、乙醇分子在PorapakQ、N两种样品上的脱附级数n和脱附活化能E_d,并与气相色谱法测定的吸附热作了比较。得出E_d≥ΔH令人满意的结果。     

最小二乘法计算苯、噻吩和正辛烷在NaY上程序升温脱附活化能

采用程序升温脱附(TPD)技术测定了苯、噻吩和正辛烷在NaY上以不同升温速率升温时的TPD谱图. 利用TPD谱图的峰形和其微分曲线判断了程序升温脱附过程中的脱附级数. 提出了一种利用最小二乘法计算吸附剂/催化剂的脱附活化能及其动力学参数的方法. 以这些TPD谱图为基础, 分别采用传统TPD计算模型、最小二乘法以及一阶微分曲线法计算了苯、噻吩和正辛烷在NaY上的脱附活化能和动力学参数. 结果表明, 最小二乘法对在不同线性升温速率时的程序升温脱附活化能的计算结果是一致的.     

红外光谱法测定CO在Ni/AL2O3催化剂上的吸附热和脱附活化能

本文介绍了用红外光谱(IR)和程序升温脱附原位红外(TPD-IR)技术测定吸附热(Q_a)和脱附活化能(E_d)的实验方法,并测定了CO在Ni/Al_2O_3催化剂上弱吸附线式态的吸附热和强吸附线式态的脱附活化能。结果表明弱吸附线式态的吸附热约为21kJ/mol,强吸附线式态的脱附活化能在10%Ni/Al_2O_3上约为84kJ/mol,在20%Ni/Al_2O_3上约为100kJ/mol。对本方法及实验结果进行了讨论。     

程序升温热脱附法研究HZSM-5和HM分子筛的酸性质

程序升温热脱附法测定HZSM-5和HM分子筛吸附氨的程脱谱均有两个峰。HZSM-5的峰Ⅰ和峰Ⅱ的T_M分别为199和418℃,HM的为207和526℃,它们代表了两种强弱不同的吸附中心。通过分析脱附的氨量,计算了强弱两种吸附中心数,并与两种分子筛中所含的理论酸中心数作了比较,得到了与两个峰相应的酸中心性质的信息。 另外用电子计算机模拟热脱附曲线的方法,测定了HZSM-5和HM分子筛上氨的脱附活化能E_d与覆盖度θ的函数关系,提出用E_d和E_d～θ表征酸强度和酸强度分布。结果表明:HM的强酸中心的强度较HZSM-5强,而且酸强度分布更不均匀。但两种分子筛的弱酸中心的性质从能量上看很相似,也较强酸中心均匀。     

热脱附-气相色谱-质谱法同时测定工作场所空气中10种醇类和醚类化合物的含量北大核心CSCD

通过对比填充不同吸附剂的不锈钢吸附管对10种醇类和醚类化合物的吸附能力,优化热脱附条件,考察不同色谱柱和样品保存时间对测定结果的影响,提出了热脱附-气相色谱-质谱法同时测定工作场所空气中10种醇类和醚类化合物含量的方法。用填装Tenax GR的吸附管采集气体样品,采集完成后将吸附管两端封好,设置一次热脱附温度为220℃,二次热脱附温度为240℃,吹扫时间为4 min,进样时间为50 s,在上述条件下进行热脱附,脱附的气体进入气相色谱仪,采用DB-624毛细管色谱柱分离目标物,并用配电子轰击离子源的质谱仪在选择离子监测模式下检测。结果显示:10种醇类和醚类化合物的质量在10~300 ng内与其对应的峰面积呈线性关系,检出限(3.143s)为0.5~1.9 ng;对空白样品进行3个浓度水平的加标回收试验,回收率为70.5%~125%,测定值的相对标准偏差(n=6)为1.0%~20%;方法用于实际样品分析,正丁醇、异丁醇和仲丁醇被检出,检出量依次为0.013,0.007,0.008 mg·m^(-3)。     

用IR/TPD法测定氢型丝光沸石上氨的脱附活化能

通过IR/TPD的改进方法, 测定了氨在氢型丝光沸石(HM)上的Bronsted酸位上的脱附活化能和相应的集居数。结果表明, 在HM上有三种不同强度的Bronsted酸位, 分别对应脱附活化能为63.6、49.8和35.4kJ/mol。     

烷基化催化剂表面酸性及催化性能的动力学研究

在确定关联升温速率、脱附峰温和脱附峰覆盖率的程序升温脱附动力学模型的基础上，通过TPD实验和模型参数估值，建立了表征催化剂酸密度、酸强度及强度分布情况的方法。研究表明，随着活化温度的提高，固体酸催化剂表面酸中心强度分布先变宽后趋于均匀，350?℃活化催化剂的强度分布最宽；催化剂表面酸强度和酸密度随活化温度提高均呈先增大后降低、分别在350 ℃和250 ℃活化温度达到极大值的变化规律。催化剂酸性与催化性能关联的结果表明，随着活化温度的提高，烷基化反应速率常数与总脱附量的变化趋势相同，而催化剂失活速率常数与脱附活化能变化趋势相同；催化剂活性稳定性随其酸强度的增大而变差，催化剂活性与催化剂酸量和酸强度有关。     

β-环糊精修饰SiO_2复合材料固相萃取-高效液相色谱法测定环境水样中芴、菲和荧蒽

采用β-环糊精修饰SiO_2复合材料(β-CD@SiO_2)固相萃取,高效液相色谱法测定,建立了环境水样中3种多环芳烃芴、菲和荧蒽的分析方法。考察了影响该复合材料固相萃取芴、菲、荧蒽的各种因素,在选定实验条件下,方法对目标物的富集倍数为30倍,芴、菲和荧蒽线性范围分别为17.0~3 300ng/mL、7.0~3 300ng/mL和3.0~3 300ng/mL,检出限分别为2.10、0.50和0.47ng/mL,回收率范围为96.3%~106.0%。所建立的方法成功用于湖水和下水道污水中芴、菲和荧蒽的检测。     

吸汞载银活性炭纤维和吸汞活性炭纤维的热脱附特性研究

70℃下分别对载银活性炭纤维(载银量14.07%)和活性炭纤维的片状吸附体进行气态汞吸附实验,测定出载银活性炭纤维汞饱和吸附量为192.3 mg/g,活性炭纤维汞饱和吸附量为29.4 mg/g,分别为普通活性炭的48倍～192倍和7倍～29倍.采用热重分析法(TGA)研究了两种吸附剂汞饱和后的热脱附再生特性.结果表明,汞饱和载银活性炭纤维的汞脱附发生在100℃～650℃,在70 min内从50℃升温至650℃,才乏脱附率为94.73%;汞饱和活性炭纤维的汞脱附发生在100℃～230℃,在40 min内从50℃升温至350℃ ,汞脱附率为69.93%.扫描电镜分析发现,载银活性炭纤维因吸附汞而富集的银,经热脱附后变成均匀弥散于纤维表面的亚微米级和纳米级球状银颗粒;吸汞活性炭纤维经热脱附后物理吸附汞基本消失,而氧化汞颗粒反而变多,说明物理吸附的汞易于脱附,氧化汞难以脱附,同时在热脱附过程中存在金属汞向氧化汞的转化.     

Interactions and binding energies of dimethyl methylphosphonate and dimethyl chlorophosphate with amorphous silica

The fundamental interactions of dimethyl methylphosphonate (DMMP) and dimethyl chlorophosphate (DMCP) on amorphous silica nanoparticles have been investigated with transmission infrared spectroscopy and temperature-programmed desorption (TPD). DMMP and DMCP both adsorb molecularly to silica through the formation of hydrogen bonds between isolated silanols and the phosphoryl oxygen of the adsorbate. The magnitude of the shift of the ν(OH) mode upon simulant adsorption is correlated to the adsorption strength. The activation energies for desorption for a single DMMP or DMCP molecule from amorphous silica varied with coverage. In the limit of zero coverage, after the effects of defects were excluded, the activation energies were 54.5 ± 0.3 and 48.4 ± 1.0 kJ/mol for DMMP and DMCP, respectively.     

Fundamental study on the thermal regeneration stages of exhausted activated carbons: kinetics

In this work the thermal regeneration of activated carbons saturated with p-nitrophenol has been analysed. By thermogravimetry, it was possible to elucidate the different events taking place during the thermal treatment, and relate them to the type of adsorption in the interfacial system. It was found that the mass loss during thermal treatment comprises a complex process in which different stages are involved, such as drying, desorption of physisorbed adsorbate, breaking up of surface functional groups, cracking of products from adsorbate–surface-specific interactions, etc. The analysis of the textural and surface chemistry characteristics of the pristine and regenerated adsorbent confirmed the thermal desorption mechanisms. Moreover, a kinetic study based on temperature-programmed desorption and Suzuki models was performed, using thermogravimetry data at different heating rates (5–20 K min^?1). From this analysis, the values of activation energy involved in each degradation step were estimated.     

Potential energy surface for an electron transfer reaction mediated by a metal adlayer

A model Hamiltonian for electron transfer from a metal electrode to a solvated reactant via a metallic adsorbate is proposed. The adsorbates are distributed randomly over the electrode surface, and a coherent-potential approximation has been employed to treat this randomness. Both the adsorbates and the reactant are assumed to interact with the solvent, which is modeled as a bath of phonons with frequencies in the classical regime. Both the adiabatic and the nonadiabatic potential energy surfaces are calculated, and their dependence on the adsorbate coverage is highlighted. In the low coverage regime the potential-energy surfaces exhibit features similar to a bridge-assisted electron transfer reaction, whereas for higher coverages the surfaces resemble those for direct heterogeneous transfer. This change of shape is caused by the metallization of the adsorbate layer at higher coverages.     

Implementation of new TPD analysis techniques in the evaluation of second order desorption kinetics of cyanogen from Cu(001)

The interactions of cyanide species with a copper (001) surface were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Adsorbed cyanide species (CN(a)) undergo recombinative desorption evolving molecular cyanogen (C(2)N(2)). As the adsorbed CN species charge upon adsorption, mutually repulsive dipolar interactions lead to a marked desorption energy reduction with increasing CN(a) coverages. Two new TPD analysis approaches were developed, which used only accurately discernible observables and which do not assume constant desorption energies, E(d), and pre-exponential values, ν. These two approaches demonstrated a linear variation of E(d) with instantaneous coverage. The first approach involved an analysis of the variations of desorption peak asymmetry with initial CN coverages. The second quantitative approach utilized only temperatures and intensities of TPD peaks, together with deduced surface coverages at the peak maxima, also as a function of initial surface coverages. Parameters derived from the latter approach were utilized as initial inputs for a comprehensive curve fit analysis technique. Excellent fits for all experimental desorption curves were produced in simulations. The curve fit analysis confirms that the activation energy of desorption of 170-180 kJ/mol at low coverage decreases by up to 14-15 kJ/mol at CN saturation.     

The kinetic study of specific adsorption of phosphate species on Pt(111) in acidic solutions

The study of the adsorption/desorption mechanism of phosphate anions at Pt(111) in acidic solution of pH 4.3 and 0.8 was performed by the potential step method in order to reveal the kinetics of anion adsorption. The current-time curve due to phosphate adsorption/desorption showed various decay features, being dependent on the potential region. The rate of current decay depended on pH, being faster in a lower pH solution. Specific adsorption processes were analyzed by the Langmuir and Elovich adsorption equations and also in terms of a two-dimensional nucleation-growth mechanism in different adsorption/desorption regions. In the case of adsorption in 0.3M phosphate buffer solution of pH 4.3, random adsorption without interaction following the Langmuir adsorption, takes place at low coverage, while random adsorption with repulsive force was observed at high coverage. In the desorption process, random desorption with repulsive force takes place at high coverage, and the repulsive force disappears where random adsorption without interaction takes place at medium coverage. When the surface coverage becomes further lower, the desorption mechanism changes dramatically into a two-dimensional nucleation-growth type, suggesting that an ordered adsorbate structure is formed after a rapid discharge process of anion adsorption.     

Spontaneous desorption of vibrationally excited molecules physically adsorbed on surfaces

The energy within a vibrationally excited physisorbed molecule often exceeds that needed to break its bond to the surface. Energy transfer from the vibrating chemical bond to the surface bond causes the surface bond to rupture and the vibrationally relaxed adsorbate is released from the surface. We present a theoretical model which allows an estimation of the residence time of a vibrationally excited adsorbate on a surface. Because of uncertainties in the nature of the surface bond, the lifetimes obtained from the analytical expressions presented have only qualitative significance. The results are interpreted in terms of Franck-Condon overlaps between the wavefunctions which describe the adsorbate-substrate complex and the released adsorbate. Lifetimes are calculated for hydrogen isotopes adsorbed on sapphire surfaces. Guide-lines are given for estimating lifetimes of other systems in terms of a few easily calculated parameters.Let us summarize this guide to spontaneous desorption of physically adsorbed vibrationally excited molecules. The most efficient desorption processes will occur for adsorbates with a small number of bound states (d₀ small) and when released the adsorbate has small translational momentum (small q_m). This momentum gap correlation is most succinctly revealed by fig. 3. Smaller translational momentum will be achieved if the adsorbate can take up energy into its internal motions. Absorption of energy into lattice modes of the substrate will also serve to reduce the translational momentum and provide for more efficient desorption. However, if the vibrational frequency of the adsorbate is in near resonance with surface polarons or plasmons of the substrate, energy transfer to the solid will be so efficient that desorption will be quenched.A test of these possible relaxation channels awaits the first experimental measurements of desorption of vibrationally excited molecules.     

Interaction between water molecules and zinc sulfide nanoparticles studied by temperature-programmed desorption and molecular dynamics simulations

We have investigated the bonding of water molecules to the surfaces of ZnS nanoparticles (approximately 2-3 nm sphalerite) using temperature-programmed desorption (TPD). The activation energy for water desorption was derived as a function of the surface coverage through kinetic modeling of the experimental TPD curves. The binding energy of water equals the activation energy of desorption if it is assumed that the activation energy for adsorption is nearly zero. Molecular dynamics (MD) simulations of water adsorption on 3 and 5 nm sphalerite nanoparticles provided insights into the adsorption process and water binding at the atomic level. Water binds with the ZnS nanoparticle surface mainly via formation of Zn-O bonds. As compared with bulk ZnS crystals, ZnS nanoparticles can adsorb more water molecules per unit surface area due to the greatly increased curvature, which increases the distance between adjacent adsorbed molecules. Results from both TPD and MD show that the water binding energy increases with decreasing the water surface coverage. We attribute the increase in binding energy with decreasing surface water coverage to the increasing degree of surface under-coordination as removal of water molecules proceeds. MD also suggests that the water binding energy increases with decreasing particle size due to the further distance and hence lower interaction between adsorbed water molecules on highly curved smaller particle surfaces. Results also show that the binding energy, and thus the strength of interaction of water, is highest in isolated nanoparticles, lower in nanoparticle aggregates, and lowest in bulk crystals. Given that water binding is driven by surface energy reduction, we attribute the decreased binding energy for aggregated as compared to isolated particles to the decrease in surface energy that occurs as the result of inter-particle interactions.     

The effect of adsorbate-induced surface reconstruction on the apparent arrhenius parameters for desorption

The effect of adsorbate-induced surface reconstruction on the apparent preexponential factor and activation energy for desorption is discussed. The reconstruction is assumed to be a continuous phase transition. The adsorbate-induced changes in the surface are shown to result in a rather strong (of the order of 10²) coverage dependence of the preexponential factor for desorption. The direction of the variation is in accordance with the compensation effect (the preexponential factor and the activation energy decrease with increasing coverage).     

Water desorption from an oxygen covered Pt(111) surface: multichannel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample.