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Spectroscopic Measurement of Diffusion Kinetics through Subnanometer and Larger Al 2 O 3 Particles by a New Method: The Interaction of 2-Chloroethylethyl Sulfide with γ-Al 2 O 3
Abstract
A new method to study the diffusion properties of molecules into porous materials using transmission IR spectroscopy is employed. A measurement of the diffusion of the 2-chloroethylethyl sulfide (2-CEES) molecule into two types of gamma-Al2O3 powder is performed, showing that the diffusion rate into subnanometer crystallite particle size gamma-Al2O3 powders (subnano-Al2O3) is higher than that into the larger crystallite particle size powder. It is shown that a surface diffusion mechanism can be used to model the diffusion process giving good agreement with the experimental results, where Dsubnano-Al2O3 is approximately 5 times larger than Dmultinano-Al2O3 at 170 K for the 2-CEES molecule.
... A diffusion analysis using this experimental method has been previously established for similar porous materials. [206][207][208][209][210] Briefly, the diffusion of propene through ...
Developing a fundamental understanding of the interactions between catalytic surfaces and adsorbed molecules is imperative to the rational design of new materials for catalytic, sorption and gas separation applications. Experiments that probed the chemistry at the gas-surface interface were employed through the utilization of in situ infrared spectroscopic measurements in high vacuum conditions to allow for detailed and systematic investigations into adsorption and reactive processes. Specifically, the mechanistic details of propene epoxidation on the surface of nanoparticulate Au supported on TiO2 and dimethyl chlorophosphate (DMCP) decomposition on the surface of TiO2 aerogel-supported Cu nanoparticles were investigated. In situ infrared spectroscopy illustrates that TiO2-supported Au nanoparticles exhibit the unprecedented ability to produce the industrially relevant commodity chemical, propene oxide, through the unique adsorption configuration of propene on the surface of Au and a hydroperoxide intermediate (-OOH) in the presence of gaseous hydrogen and oxygen. Whereas, TiO2-supported Cu aerogels oxidize the organophosphate-based simulant, DMCP, into adsorbed CO at ambient environments. Through a variety of spectroscopic methods, each step in these oxidative pathways was investigated, including: adsorption, oxidation and reactivation of the supported-nanoparticle systems to develop full mechanistic pictures. Additionally, the perturbation of vibrational character of the probe molecule, CO, was employed to characterize the intrinsic µ3-hydroxyls and molecular-level defects associated with the metal-organic framework (MOF), UiO-66. The adsorption of CO onto heterogeneous surfaces effectively characterizes surfaces because the C-O bond vibrates differently depending on the nature of the surface site. Therefore, CO adsorption was used within the high vacuum environment to identify atomic-level characteristics that traditional methods of analysis cannot distinguish.
... With the goal of evaluating the transport dynamics of CWAs in Zr-based MOFs, our research efforts began by developing a method for monitoring molecular transport under pristine conditions. Our approach, adapted from Kim et al., 46 utilized in situ infrared (IR) spectroscopy operating under ultrahigh vacuum (UHV) conditions to monitor changes that occur during exposure, diffusion, and desorption of the analytes of interest within porous materials, specifically Zr-based MOFs. 30,46−48 Initial diffusion studies were performed on UiO-66 with a series of linear and aromatic hydrocarbons to validate the method. ...
The threat of chemical warfare agents (CWAs), assured by their ease of synthesis and effectiveness as a terrorizing weapon, will persist long after the once-tremendous stockpiles in the U.S. and elsewhere are finally destroyed. As such, soldier and civilian protection, battlefield decontamination, and environmental remediation from CWAs remain top national security priorities. New chemical approaches for the fast and complete destruction of CWAs have been an active field of research for many decades, and new technologies have generated immense interest. In particular, our research team and others have shown metal-organic frameworks (MOFs) and polyoxometalates (POMs) to be active for sequestering CWAs and even catalyzing the rapid hydrolysis of agents. In this Forum Article, we highlight recent advancements made in the understanding and evaluation of POMs and Zr-based MOFs as CWA decontamination materials. Specifically, our aim is to bridge the gap between controlled, solution-phase laboratory studies and real-world or battlefield-like conditions by examining agent-material interactions at the gas-solid interface utilizing a multimodal experimental and computational approach. Herein, we report our progress in addressing the following research goals: (1) elucidating molecular-level mechanisms of the adsorption, diffusion, and reaction of CWA and CWA simulants within a series of Zr-based MOFs, such as UiO-66, MOF-808, and NU-1000, and POMs, including Cs8Nb6O19 and (Et2NH2)8[(α-PW11O39Zr(μ-OH)(H2O))2]·7H2O, (2) probing the effects that common ambient gases, such as CO2, SO2, and NO2, have on the efficacy of the MOF and POM materials for CWA destruction, and (3) using CWA simulant results to develop hypotheses for live agent chemistry. Key hypotheses are then tested with targeted live agent studies. Overall, our collaborative effort has provided insight into the fundamental aspects of agent-material interactions and revealed strategies for new catalyst development.
The high-energy demand of benzene, toluene, and xylene (BTX) separation highlights the need for improved non-thermal separation techniques and materials. Due to their high surface areas, tunable structures, and chemical stabilities, metal-organic frameworks (MOFs) are a promising class of materials for use in more energy efficient, adsorption-based separations. In this work, BTX compounds in the pore environment of UiO-66 were systematically examined using in situ infrared (IR) spectroscopy to understand the fundamental interactions that influence molecular transport through the MOF. Isothermal diffusion experiments revealed BTX diffusivities between 10⁻⁸ - 10⁻¹² cm² s⁻¹, where the rate follows the trend: o-xylene < m-xylene < p-xylene. Corresponding activation energies of diffusion (Ediff) were determined to be 44 kJ mol⁻¹ for the xylene isomers and 34 kJ mol⁻¹ for both benzene and toluene with the diffusion-limiting barrier identified to be molecular passage through the small triangular pore apertures of UiO-66. Furthermore, IR spectroscopy and computational methods showed the formation of two types of hydrogen bonds between BTX molecules and the μ3-OH groups located in the tetrahedral cavities of UiO-66, which indicates BTX molecules are capable of fully accessing the inner pore environment of the MOF. The molecular-level insight into the diffusion mechanism and energetics of BTX transport through UiO-66 presented in this work provide rich insight for the design of next-generation MOFs for cost-effective separation processes.
Hydrocarbon diffusion and binding within porous molecular networks are critical to catalysis, separations, and purification technologies. Fundamental insight into n-butane uptake and mobility within a new class of materials for separations, metal-organic frameworks (MOFs), has been gained through in situ infrared spectroscopy. These UHV-based measurements revealed that adsorption of n-butane within UiO-66 proceeds through the formation of hydrogen bonds between the alkane molecules and hydroxyl groups located at the inorganic node of UiO-66. Modeling the gas transport of n-butane with Fick’s 2nd law yielded diffusion coefficients at several temperatures. The Arrhenius parameter for the activation energy for diffusion was found to be 21.0 ± 1.2 kJ/mol. These studies have further shown that the rate-determining step for diffusion is the dissociation of n-butane from a binding site located within the tetrahedral pores of UiO-66.
The study of the surface behavior of high area solids can be carried out using extensions of the fundamental measurement methods developed for studies of low area single crystal surfaces. Among the most widely used methods in single crystal studies is temperature-programmed desorption (TPD), often called thermal desorption spectroscopy (TDS).
The study of supported catalysts using transmission IR spectroscopy is a well-established technique employed widely throughout the field. Over 50 cell designs have been devised and reported in the literature (Basu et al. Rev Sci Instrum 59:1321, 1988 [1]). The cell described in Fig. 47.1 has special features of importance for some types of applications, especially where a simple design for use
at cryogenic temperatures is desired.
Nanoscience and nanotechnology are transforming materials science in a broad way, in a manner similar to polymer chemistry’s transformation of materials science over the preceding century. The continuous development of novel nanostructured materials and the extensive study of physicochemical phenomena at the nanoscale are creating new approaches to innovative technologies that are constantly resulting in products with a wide range of applications [1–5].
CO2 diffusion limitations and readsorption of desorbed CO2 during removal from immobilized amine sorbents could significantly reduce the effectiveness of CO2 capture processes. To decouple CO2 diffusion from desorption/readsorption on silica and tetraethylenepentamine (TEPA)/silica sorbents, a new transient diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method was carried out by using benzene as a surrogate probe molecule. Comparison of the infrared intensity profiles of adsorbed CO2 and Si–OH (which adsorbs benzene) revealed that slow rates of CO2 uptake and desorption are a result of (i) CO2 diffusion through an interconnected network produced from CO2 adsorbed inside of the amine/silica sorbent pores and (ii) readsorption of CO2 on the amine sites inside of the pores and at the external surface of the sorbents. High rates of CO2 adsorption/desorption onto/from the immobilized amine sorbents could be achieved by sorbents with low amine density at the external surfaces and pore mouths.
The influence of water vapor on the surface diffusion behavior of pyridine adsorbed on powdered MgO surfaces has been studied by Fourier transform IR (FTIR) absorption spectroscopy. It has been found that the introduction of water vapor significantly increases the pyridine surface diffusion coefficient. FTIR spectra showed that water vapor converted Lewis acid Mg2+ sites to Brønsted Mg−OH sites. These measurements also detected the change in surface bonding of pyridine to the two types of sites. The activation energy for escape of chemisorbed pyridine into a mobile precursor state is lower for pyridine bound to Mg−OH sites than for binding to Mg2+ sites, causing the hydroxylated MgO to exhibit a higher diffusivity than that found for dry MgO containing no surface hydroxyl groups. This effect was confirmed by DFT calculations of the binding energy of pyridine to MgO(100) sites and to defect sites on MgO(100), where hydroxylation decreases the binding energy by 30 kJ mol−1 on each type of site.
The molecular transport of pyridine through nanosized MgO particles has been investigated by Fourier transform IR spectroscopy. Two regimes of chemisorbed pyridine diffusion are observed. A fast diffusion process is associated with pyridine bound to nondefective smooth MgO(100) facets which exhibit an activation energy barrier of 35.7 kJ mol−1 for escape into a mobile precursor. The slow pyridine diffusion process occurs from defect sites, where the activation energy of escape to the mobile precursor is measured to be 64.6 kJ mol−1. The escape barriers controlling pyridine diffusion through MgO powders are similar to the calculated adsorption energy on MgO(100) and to the calculated adsorption energies on various defect sites on MgO(100) having lower Mg2+ coordination numbers than those on MgO(100) facets.
FTIR spectroscopy has been used to monitor the transport of CO to the Pt cores of Pt@CoO nanoparticles forming CO/Pt species. It was found that external Pt sites are not present on the outer surfaces of the approximately 10 nm diameter nanostructures and that CO transports to Pt adsorption sites by an activated surface diffusion process through the CoO shells surrounding approximately 2 nm diameter Pt cores. The CO transport process is not due to gas-phase transport below 300 K. The weakly bound adsorbed CO/CoO species responsible for transport was directly observed at approximately 2147 cm(-1) during transport through the CoO shells.
2-Chloroethyl ethyl sulfide (Cl–CH2–CH2–S–CH2–CH3 or C4H9ClS) is used as an analogue for mustard gas in biomedical studies. The Cl—C—C—S and the two C—C—S—C torsion angles are −177.38 (9), 83.17 (14) and 73.81 (16)°, respectively, and are similar to values that have been predicted, by ab initio quantum calculations, for the corresponding parameters in mustard gas. There are no strong intermolecular interactions, and the packing in the crystal structure bears some resemblance to hexagonal close packing.
Nuclear magnetic resonance has been applied to study the details of molecular motion of low-molecular-weight polar and nonpolar organic liquids in nanoporous silicon crystals of straight cylindrical pore morphology at different pore loadings. Effective self-diffusion coefficients as obtained using the pulsed field gradient nuclear magnetic resonance method were found to pass through a maximum with increasing concentration for all liquids under study. Taking account of a concentration-dependent coexistence of capillary condensed, adsorbed and gaseous phases a generalized model for the effective self-diffusion coefficient was developed and shown to satisfactorily explain the experimental results. An explicit use of the adsorption isotherm properties within the model extends its applicability to the mesoporous range and highlights the role of surface interaction for the transport of molecules in small pores. The problem of surface diffusion and diffusion of multilayered molecules is also addressed.
The pulsed field gradient nuclear magnetic resonance method has been employed to probe self-diffusion of organic guest molecules adsorbed in porous silicon with a 3.6 nm pore size. The molecular self-diffusion coefficient and intrapore adsorption were simultaneously measured as a function of the external vapor pressure. The latter was varied in a broad range to provide pore loading from less than monolayer surface coverage to full pore saturation. The measured diffusivities are found to be well-correlated with the adsorption isotherms. At low molecular concentrations in the pores, corresponding to surface coverages of less than one monolayer, the self-diffusion coefficient strongly increases with increasing concentration. This observation is attributed to the occurrence of activated diffusion on a heterogeneous surface. Additional experiments in a broad temperature range and using binary mixtures confirm this hypothesis.
This chapter describes that the use of thermal desorption methods for the study of species adsorbed on surfaces is widespread. The study of thermal desorption supplies useful information to basic science as well as information of importance to technology. The study of thermally activated desorption phenomena represents a natural combination of chemical kinetics, statistical thermodynamics, and molecular dynamics. Knowledge of the nature of the desorption process has fundamental implications to understanding the nature of elementary chemical processes in the layer, to the energetics of bonding and the specification of the chemical nature of the bound species, and to the nature and magnitude of interactional effects between adsorbate species. The chapter discusses that the extension of thermal desorption methods to the study of the interaction of more complex molecules, with surfaces, has now occurred. Studies of desorption kinetics from these systems can suggest the nature of surface-stabilized molecular fragments of importance to a fundamental understanding of catalytic chemistry at surfaces. It concludes that in the future, further refinement in experimental methods for studying desorption processes will be devised, so that thermal desorption may be understood on a more fundamental molecular basis. In addition, continued widespread application to the technological problems is anticipated, particularly to high-area powdered substrates.
Adsorption experiments for benzotriazole, (BTAH) on the γ-Al2O3 surface were carried out at 150 and 293 K. The thermal stability of the adsorbed layer was investigated. At low temperatures, adsorption results in the production of a thick condensed layer of BTAH. Strong intermolecular hydrogen bonds are observed. With increasing sample temperature, the BTAH molecules diffuse into the Al2O3 pore structure. Hydrogen bonding of the BTAH molecule specifically to isolated Al−OH groups is observed. Deprotonation of the BTAH molecule begins to occur through interaction, with the surface oxide anions near 300 K. As a result of such deprotonation-type interaction the production of associated surface OH groups and of the BTA- anion are observed. The BTA- anion is bound to the surface Lewis acid site via a nitrogen atom lone pair. Extensive thermal treatment (T = 573−623 K) causes decomposition of the adsorbed BTA- species, and modes related to the nitrogen ring disappear. After thermal treatment up to 873 K, IR spectroscopy detects weak and broadened features of aromatic molecules on the surface. Auger spectroscopy of the sample after this thermal treatment shows that nitrogen containing species have disappeared whereas carbon remains. Infrared features due to aromatic residues persist to higher temperatures (T = 873−1023 K).
The chemisorption of trimethylamine and pyridine on a commercial silica-alumina catalyst was studied by infrared techniques. Trimethylamine was found to be unsuitable as an agent for differentiating between Lewis and Brønsted sites because it is dissociatively adsorbed with concomitant generation of protons which form protonated chemisorbed species. Pyridine, on the other hand, is not dissociatively adsorbed and is, therefore, useful for determining the relative numbers of Lewis and Brønsted acid sites by making use of the differences in the spectra of the pyridinium ion and coordinately bonded pyridine. Approximately equal numbers of Lewis and Brønsted sites were observed on the highly dehydrated silica-alumina. The perturbing effects of potassium poisoning and irreversible H2O adsorption on the distribution of acid sites were studied. It was found that potassium poisoning weakens the majority of acid sites and eliminates apparent Brønsted acidity. Irreversibly adsorbed H2O converts Lewis sites to Brønsted sites; however, the interaction is relatively weak since the H2O can be removed by evacuation. These observations provide considerable insight into the nature of the acid sites and have led to the formulation of the following model. It is suggested that all of the primary acid sites on a silica-alumina are of the Lewis type centered on active surface aluminum atoms and that apparent Brønsted sites are produced by a second-order interaction between the molecule chemisorbed on a Lewis site and a nearby surface hydroxyl group. This model qualitatively fits the experimental observations to date.
Even after drying at 1000° γ-alumina continues to evolve traces of water on further heating. This water appears to bound to the surface of alumina and is known to affect its catalytic properties. To investigate the nature of bound water present in less than monolayer amounts, infrared study was made of transparent sheets of alumina aerogel. Hydroxyl groups, as well as water molecules, are initially held on the alumina surface. Heating at 400° removes all water molecules, but leaves many of the hydroxyl groups. Alumina dried above 650° gives absorption bands at 3698, 3737 and 3795 cm.-1, assignable to "isolated" hydroxyl groups. These three bands can be eliminated from the spectrum either by further heating or by deuterium exchange, but they are not all removed at the same rate. Spectral changes ranging from perturbations to removal or creation of hydroxyl bands are observed as a result of interactions of the hydroxyl groups with adsorbed molecules other than water. Independent variations observed in the intensities of the three hydroxyl bands indicate that these bands represent chemically distinct groups on at least three different types of surface sites. Such groups may play different roles in catalytic reactions on alumina.
A discussion covers the crystallographic structure of bulk ..gamma..- and g-aluminas and the configurations of alumina crystal faces; IR spectroscopic studies of surface hydroxyl groups; surface densities of hydroxyl groups, oxygen atoms, and anion vacancies as a function of dehydroxylation temperature; identifcation of active sites as special configurations of multiple vacancies or clusters of oxygen atoms; mechanisms of defect site generation during dehydroxylation; studies of carbon monoxide chemisorption as a surface probe for the nature of defect sites; and surface mechanisms of chemisorption of hydrogen, deuterium, olefins, and aromatics, and deuterium exchange with surface hydroxyls and adsorbed olefins or aromatics. Diagrams, graphs, spectra, tables, and 96 references.
A method for the preparation of high surface area solids for transmission infrared spectroscopic surface chemistry studies is described. This method consists of mechanically pressing the solid into tiny holes of a support grid, resulting in many tiny self-supported pellets surrounded by the metal grid framework. This allows for maximum efficiency heat transport in the temperature range of 100-1500 K in the sample. Studies of adsorption and desorption phenomena from the pressed powder indicate that diffusion of gases into and out of the pressed sample is rapid. There are no apparent differences in the adsorption and desorption behavior of samples produced by this method and by a spray deposition method. Furthermore, contaminant-free samples can be prepared by this method since no solvents are used.
The room-temperature reactions of the chemical warfare agents VX (O-ethyl S-2-(diisopropylamino)ethyl methylphosphonothioate), GD (3,3-dimethyl-2-butyl methylphosphonofluoridate, or Soman), and HD (2,2‘-dichloroethyl sulfide, or mustard) with nanosize MgO have been studied using solid-state MAS NMR. All three agents hydrolyze on the surface of the very reactive MgO nanoparticles. VX yields ethyl methylphosphonic acid (EMPA) and methylphosphonic acid (MPA), but no toxic S-(2-diisopropylamino)ethyl methylphosphonothioate (EA-2192). GD forms both GD-acid and MPA. For HD, in addition to hydrolysis to thiodiglycol, about 50% elimination to divinyl sulfide occurs. The reaction kinetics for all three agents are characterized by a fast initial reaction followed by gradual slowing to a steady-state reaction with first-order behavior. The fast reaction is consistent with liquid spreading through the porous nanoparticle aggregates. The steady-state reaction is identified as a gas-phase reaction, mediated by evaporation, once the liquid achieves its volume in the smallest available pores.
The reaction of 2-chloroethylethyl sulfide (CEES) with a high-area Al2O3 surface was investigated. Two different reactive sites were created by thermally pretreating the Al2O3 powder:? isolated hydroxyl sites and Lewis acid?base pair sites. The reaction of CEES with the isolated hydroxyl groups at 303 and 473 K produced a surface-bound species, which is characterized by a carbon?oxygen stretching mode near 1100 cm-1. This assignment was confirmed by isotopic substitution of 18O into the isolated hydroxyl groups prior to reaction with CEES. The other reaction product detected at 473 K was HCl. For a more highly dehydroxylated Al2O3 surface, the reaction of CEES with the Lewis acid?base pairs produced a surface?bound species also exhibiting a carbon?oxygen stretching mode near 1100 cm-1. This species is postulated to form at O2- Lewis base sites as Al3+?Cl- bonds form on adjacent Lewis acid sites. The adsorption of pyridine onto the Lewis acid sites prior to CEES exposure effectively closes this reaction
The interaction of the various hydroxyl groups on the Al2O3 surface with a bifunctional adsorbate, 2-chloroethylethyl sulfide, was studied by transmission infrared spectroscopy. Al2O3 has five unique Al?OH groups, differentiated by their coordination to the surface, which give independent AlO?H stretching modes. Monitoring intensity changes in these modes as 2-chloroethylethyl sulfide (CEES) molecules diffuse into the porous structure of alumina shows distinctly different stages of surface diffusion and reaction at the Al?OH groups. Three sequential steps have been separated by studies at low temperature, where the rate of the processes was kinetically retarded to allow observation. The final stage of the CEES interaction with the surface occur when the CEES molecules adsorbed on the surface via the sulfide moiety at Al3+ Lewis acid sites undergo a hydrolysis reaction with the neighboring basic Al?OH groups.
A new design for an infrared cell useful for studies of the spectrum of surface species on high area solids is presented. The cell is well suited over a wide temperature range (100–1000 K). Other demonstrated features of the cell include ultrahigh‐vacuum operation, temperature control to ±1 K, linear and rapid temperature programmability and low‐temperature gradients across the powdered sample. The method of sample preparation and support minimizes both heat and mass transport effects. A detailed literature search of previous infrared cell designs is included. Results of the application of the new cell design to the high‐temperature dehydroxylation of Al 2 O 3 are given as an example of the performance.
In a recent priority communication [M. Digne et al., J. Catal. 211 (2002) 1], we proposed the first ab initio constructed models of γ-alumina surfaces. Using the same density-functional approach, we investigate in further detail the acid–basic properties of the three relevant γ-alumina (100), (110), and (111) surfaces, taking into account the temperature-dependent hydroxyl surface coverages. The simulations, compared fruitfully with many available experimental data, enable us to solve the challenging assignment of the OH-stretching frequencies, as obtained from infrared (IR) spectroscopy. The precise nature of the acid surface sites (concentrations and strengths) is also determined. The acid strengths are quantified by simulating the adsorption of relevant probe molecules such as CO and pyridine in correlation with surface electronic properties. These results seriously challenge the historical model of a defective spinel for γ-alumina and establish the basis for a more rigorous description of the acid–basic properties of γ-alumina.
Methods for determining the activation energy, rate constant and order of reaction from “flash-filament” desorption experiments are examined. Two heating schedules are considered: (a) a linear variation of sample temperature with time (T = T0+ßt), and (b) a reciprocal temperature variation . The activation energy of desorption is estimated from the temperature (Tp) at which the desorption rate is a maximum (for first-order reactions) and from the change of Tp with surface coverage (for second-order reactions). The order of the reaction is determined from the shape of the desorption rate versus time curve, or the variation of Tp with coverage. Applications of these methods to experiments on chemisorption and the desorption of ionically trapped gases are discussed. The effect of finite pumping speed in the system is analysed and the necessary corrections derived.
The adsorption of triethylenediamine (TEDA) at 300 K is observed to occur via hydrogen bonding to isolated Al-OH groups on the surface of partially dehydroxylated high area gamma-Al(2)O(3) powder. This form of bonding results in +0.3 to +0.4% blue shifts in the CH(2) scissor modes at 1455 cm(-1) and a -0.4% red shift in the CN skeletal mode at 1060 cm(-1), compared to the gas-phase frequencies. Other modes are red shifted less than 0.1%. The isolated OH modes are red shifted by -200 to -1000 cm(-1) due to the strong hydrogen bonding association of Al-OH groups with an N atom in TEDA. Thermal desorption of adsorbed TEDA from the surface occurs in the range 300-700 K. Mass spectral and infrared studies indicate that the decomposition of TEDA occurs on Al(2)O(3) above 725 K, and that C-H bonds are broken, forming adsorbed species with N-H bonds which are stable to 1000 K or above. In contrast to adsorption at 300 K, adsorption of TEDA at 85 K results in the formation of a condensed ice of TEDA, which covers the outer surface of the porous Al(2)O(3) and which does not interact with Al-OH groups inside the porous powder due to immobility.