Distribution Of Active Centers Research Articles

Transition-metal‐doped ceria carried on two-dimensional vermiculite for selective catalytic reduction of NO with CO: Experiments and density functional theory

Selective catalytic reduction (SCR) reduces oxynitrides from power plant and vehicle emissions, and enhancements in efficiency would lessen pollution further. We prepared a series of transition metal (TM) doped CeO2 catalysts by impregnating them with vermiculite (VMT) as a carrier for reducing NOx by SCR with carbon monoxide (CO–SCR). The catalyst performance was in the following order: Zn < Cr < Fe < no dopant < Mn < Ni < Co < Cu. In other words, Mn, Ni, Co, and Cu greatly promoted NO conversion compared with Ce/VMT alone, whereas Zn, Cr, and Fe dopants hindered NO conversion. We applied density functional theory (DFT) by structural optimization and potential configuration analyses of CeO2 (111) and TM–CeO2 (111), which enabled us to propose the reaction pathway and the potential energy distribution of the transition state. Our DFT analyses are in accordance with the sequence of the NO + CO reaction. The performance of the Cu catalyst was superior to the others. The CeO2 (111) lattice plane is primarily in the cerium species, whereas the Cu–O–Ce interface forms in two phases, which indicates a complex interplay between the copper and cerium. Furthermore, the catalyst has numerous surface oxygen vacancies (Ovs) and active *O species, and exhibits an impressive reduction capacity: the NO conversion reaches 100% with a gas hourly space velocity of 102,000 h−1 at 300 °C. The CO–SCR reaction pathway on the Cu–CeO2 (111) surface is as follows: R1: CO + Olattice → Ov; R2: 2NO → *ONNO → N2O + *O; R3: N2O → N2 + *O; R4: *O + CO → CO2; R5: *O + Ov → Olattice. The synergy of the dopants on the CeO2 (111) surface modulated the distribution of active centers in the catalyst, which in turn modulated the catalyst performance. Our research will be useful for flue gas remediation.

Poly(ionic liquid)s based nano core-shell catalyst SiO2@V-PIL for efficient oxidative desulfurization of diesel

For efficient oxidative desulfurization (ODS) of fuel oils with cumene hydroperoxide (CHP), a poly(ionic liquid)s based nano core-shell catalyst SiO2@V-PIL was prepared by reacting poly vinylimidazole (PVIM) with peroxovanadic acid on nano-particles of SiO2. The structure and composition of SiO2@V-PIL, V-PIL/SiO2 and V-PIL were studied through TEM, SEM, XRD and other methods. Compared to micron-sized V-PIL and V-PIL/SiO2, the core-shell Nano catalyst can significantly enhance the oxidation rate of the sulfides. The catalytic difference may arise from their specific surface exposure and active center distribution. Dibenzothiophene (DBT) can be oxidized completely within 60 min under appropriate conditions, and 99 % of catalytic performance can be retained after 5 recycled uses. The Nano core-shell structuring might be used as a viable and universal way to significantly upgrade the catalytic activity of the composite catalysts.

ВЛИЯНИЕ СОСТАВА КРИСТАЛЛОХИМИЧЕСКОЙ СТРУКТУРЫ КАЛЬЦИЙ-ФОСФАТНОГО НАНОКОМПОЗИТА НА ФОРМИРОВАНИЕ ЭЛЕКТРОПОВЕРХНОСТНЫХ И КИСЛОТНО-ОСНОВНЫХ СВОЙСТВ

The single-phase modified nanostructured hydroxyapatites and biomimetic calcium-phosphate nanocomposite (BMHAP) doped by silicate and carbonate anions are synthesized by chemical precipitation in aqueous solutions. The chemical composition of the obtained samples is studied by energy dispersive X-ray spectroscopy using a scanning electron microscope. It is shown that the obtained molar ratios are close to the design values in all samples. The influence of different modifying ions in the crystal-chemical structure of hydroxyapatite (HAp) on the electrosurface properties of the synthesized products has been studied. Unmodified hydroxyapatite, silicate-substituted hydroxyapatite, and BMHAP particles are found to carry a negative charge in an aqueous suspension at pH = 7, while carbonate-substituted HAp particles had a positive charge. It is revealed that the introduction of silicate and carbonate anions into the HAp structure is accompanied by an increase in negative values of the zeta-potential from -1.05 to -4.29 mV. The change in the distribution of active centers on the surface of modified nanostructured hydroxyapatites is studied by the adsorption of acid-base indicators method with pKax in the range from -0.3 to +12.8. In contrast to stoichiometric hydroxyapatite, in BMHAP the neutral Bronsted centers prevails, and their concentration is 48.38 mmol-eq/g

Green synthesis of zinc ferrite

Zinc ferrite (ZF) was obtained by a green synthesis using the Ginkgo Biloba extract as reductant and fuel. ZF was characterized by XRD, SEM, EDS, and IR-spectroscopy. The XRD analysis indicates that single spinel structure is formed. The crystallite size of ZF is calculated by Scherer formula and Williamson-Hall method. SEM showed agglomerated morphology of ZF particles. Adsorption measurements were carried out using model solutions of Congo red dye with different concentrations. Adsorption process is described by Langmuir and Freundlich models. It was shown that Langmuir model fitted well the adsorption isotherm. This indicates the homogeneous distribution of surface active centers.

Development of an Integrated Framework for Multiscale, Multiphase Modeling of Industrial Slurry‐Phase Reactors for Polyethylene Production

AbstractA framework based on the Monte Carlo/random‐pore polymeric flow model is proposed to simulate both single‐particle and continuous slurry reactor industrial polymerizations. The Sanchez–Lacombe equation of state describes the distributions of components in the different phases of these systems. The developed process model is applied to describe heterogeneously catalyzed polymerizations of ethylene in n‐hexane diluent with or without 1‐hexene as a comonomer, but the proposed methodology is applicable to any ethylene/1‐olefin copolymerization in slurry reactors. In addition to the effects of catalyst particle size and reactor residence time distributions, the proposed hybrid model is used to investigate the impact of several catalyst characteristics under different process conditions on polymer yield and microstructure. Particular attention is paid to the catalyst fragmentation process and active center distribution through the particle. These simulations demonstrate the versatility and thoroughness of combining Monte Carlo simulation with single‐particle models to analyze and predict the behavior of commercial polyolefin reactors.

Radial distribution and absorption cross section of active centers in bismuth-doped phosphosilicate fibers.

Determination of the active centers distribution across the fiber core as well as calculation of absorption cross sections is a challenging task for all types of bismuth-doped fibers. This is due to the low concentration of active centers and the ability of the bismuth ions to form various centers in silica-based glasses. In this work, we demonstrate the results of experimental measurement of radial distribution of bismuth active centers associated with phosphorus in fiber core using the luminescence spectroscopy. The shape of the distribution turned out to have prominent reduction of the active centers in the middle of the core. With these data, absorption cross section spectra were calculated by two methods. Both approaches demonstrated close values of absorption cross sections regardless the bismuth concentration and fiber geometry. The maximum of the absorption cross section was found to be 2.1 ± 0.3 pm2.

Effects of titanium dispersion state on distribution and reactivity of active centers in propylene polymerization with MgCl2‐supported Ziegler‐Natta catalysts: A kinetic study based on active center counting

AbstractKinetics of propylene polymerization with two TiCl4/MgCl2 model catalysts of different Ti load (Cat‐1: Ti 0.1 wt%; Cat‐2: Ti 1.0 wt%) was investigated in this work. Time‐dependent variations of active centers fraction ([C*]/[Ti]) and active center distribution of the two catalysts were compared. In propylene polymerization with Cat‐1, [C*]/[Ti] was only 7.8 % in the initial stage, but increased for more than three times in reaction period of 600 s. Propylene polymerization with Cat‐2 had much lower [C*]/[Ti]. Active center distributions of the two catalysts were markedly different. Cat‐1 has evidently higher proportion of aspecific active centers and less isospecific ones than that of Cat‐2. These catalysts also showed different time‐dependent variations of active center distribution. The chain propagation rate constant of isospecific centers in Cat‐2 was evidently higher than that in Cat‐1, but propagation rate constants of either aspecific and medium‐isospecific centers were only slightly changed by enhancing Ti content of the catalyst. The results suggest that isospecific centers are mainly originated from clustered Ti species, but non‐clustered Ti species are the main source of active centers with lower stereoselectivity. By considering factors like dispersion states of Ti species and release of shielded active species through disintegration of catalyst particles, active center models have been proposed to explain the different kinetic behaviors of the two model catalysts.

Claisen‐Schmidt Condensation using Green Catalytic Processes: A Critical Review

AbstractClaisen‐Schmidt condensation have historical significance for over140 years and the scope was expanded due to the realization that chalcones so synthesized possess biological activity. The principles of green chemistry with emphasis on use of different solid catalysts with proper distribution of active centers such as acid, base, acid‐base, organo‐ were used along with solvent‐less systems. Among solid bases layered double hydroxides (LDH)/hydrotalcites are the most common. The limitations of solid bases are overcome by solid acids and superacids. A number of metal oxide supports are used. MOFs are increasingly being used. Complex organo‐catalysts are employed for getting excellent regio‐selectivity. In certain metal oxides, both acidic and basic sites co‐exist on the pore surface of catalyst which offer superior results. Synergism of solid catalysis using microwaves and ultrasound to intensify rates and selectivity is in vogue.Controlled pore structure and distribution of active catalytic centers, whether mono‐, bi‐ or tri‐functional catalysts, will give better green and smart processes. The review gives a critical analysis of the area and presents future directions for research.

Enhancement of the Direct Synthesis of Dimethyl Ether (DME) from Synthesis Gas by Macro- and Microstructuring of the Catalytic Bed

This work reports on a modelling study of the influence of the distribution of metallic and acidic active centers within a catalytic fixed-bed reactor for the direct synthesis of dimethyl ether (DME), conducted to demonstrate the potential of reactor-level and pellet-level structuring of catalytic active centers in process integration and intensification. To account for the pellet structure, the analysis was performed with the aid of a heterogeneous model considering both interphase and intrapellet mass transport resistances. The study evaluated, in terms of DME and methanol yield and selectivity, the performance of a tubular reactor loaded with a physical mixture of monofunctional catalyst pellets or structured bifunctional catalyst pellets with different arrangements of the catalytic centers. It was confirmed that bifunctional catalysts overperform significantly a physical mixture of monofunctional particles. Moreover, it was shown that the internal structure of a bifunctional catalyst pellet is an important feature that deserves to be exploited deeper, in view of further intensification of the DME synthesis process to be achieved with a better reactor design.

Моделирование и численное исследование физико-химических закономерностей 1,4-цис-полиизопрена, полученного в присутствии модифицированных каталитических систем

Построена математическая модель, описывающая процесс получения полиизопрена в присутствии микрогетерогенных титансодержащих каталитических систем с учетом динамики активных центров. Построение модели носило поэтапный характер, при котором модель, описывающая периодический процесс, была дополнена рекуррентными соотношениями с целью описания процесса в каскаде реакторов идеального перемешивания. Численное решение прямой задачи позволило провести анализ молекулярно-массового распределения получаемого продукта в зависимости от исходной загрузки. The aim of this work is building a mathematical model that describes the process of producing polyisoprene in the presence of microheterogeneous titanium-containing catalytic systems, taking into account the dynamics of active centers and numerical calculation of the main physicochemical properties of the resulting product. The main methodology for compiling the mathematical model is the application of the kinetic approach, which consists of compiling and numerically solving the kinetic equations for all types of particles involved in the process. Previously, the distribution of active centers of the applied catalytic system was studied. The confirmed polycentricity of the used catalyst, represented by two types of active centers, is reflected in the nature of the description of the mathematical model. To reduce the system of differential equations to the final form, the method of moments was applied, which allows evaluating the complex properties of the obtained product by its averaged molecular characteristics. A model that permits describing the scale of batch reactors was modified by a macrokinetic module that takes into account possible energy and hydrodynamic laws of the process. For this, recurrence relations were introduced that are valid for the description of continuous mixing reactors. The software package developed by the authors of the work allows carring out a computational experiment for various technological conditions for conducting a continuous process. Based on the results of its launch, the dependences of conversion and averaged molecular characteristics on time in the context of each polymerizer were obtained. Thus, the numerical solution of the direct problem is capable analyzing the molecular weight distribution of the obtained product depending on various parameters of the initial load, for both the batch and continuous modes of the process. The dependences obtained by calculation showed satisfactory agreement with experimental data

Interfacial Engineering for Gradient Optical Fiber

AbstractGradient functionalization is a widely applied process in materials science that involves generating gradient distribution of composition or structure to yield materials with new functions or improved properties. Here an interfacial engineering is described for construction of gradient fiber, which exhibits gradient distribution of active centers, bonding configuration, and the resultant optical response. It is shown that the interfacial diffusion of protype niobium (Nb) in the core and cladding region of the fiber can be tuned via rational control of the viscosity of melt and the subsequent thermal activation. This leads to gradient distribution of Nb species and active [NbO6]7− centers within 10–20 µm region. As a result, unique optical properties including the tunable energy transfer between active centers and special annular emission beam can be obtained. These results highlight a fundamental principle to develop novel photonic fibers and provide a great step in expanding the scope of gradient materials.

Influence of Structure Sensitivity on Apparent Activation Energy of Parallel Heterogeneous Catalytic Reactions

Analysis of apparent activation energy is presented for different heterogeneous catalytic reactions with parallel reaction routes. In the case of kinetic coupling between catalytic cycles the activation energy in a particular route depends not only on the activation energies of the elementary steps comprising this route, but also on the frequency of the steps in a parallel route. Expressions were derived for coupling between routes through irreversible adsorption of the substrate, quasi-equilibrated binding as well as different substrate adsorption modes. Theoretical analysis of the apparent activation energy was extended for the reaction network with two routes possessing mechanistically different rate determining steps (i.e. monomolecular vs bimolecular). For structure sensitive reactions an expression for the apparent activation energy for parallel reactions was developed for cases with a continuous distribution of active centers and a cubo-octahedral representation of the metal clusters. A comparison between the theoretical analysis and experimental data on transformations of furfural to furfuryl alcohol and furan on ruthenium clusters shows applicability of the developed theoretical framework.Graphic

ALUMINOSILICATES FROM RICE STRAW: OBTAINING AND SORPTION PROPERTIES

In order to solve the problem of complex utilization of rice straw – waste of rice production – the task was to obtain from vegetable silicon-containing raw materials samples of aluminosilicates that are promising for use as highly efficient sorbents. A series of sodium aluminosilicate samples were synthesized from alkaline hydrolysates of rice straw of various strains of Far Eastern breeding, the effect of synthesis conditions on the properties of the final product was investigated. The chemical and phase composition of the samples were determined, the particle morphology was observed by scanning electron microscopy, the specific surface area was measured, the IR spectra were recorded, the thermal properties were studied, the limits of thermal stability were determined, and the products of thermal transformations were identified. The acid-base properties of the surface were investigated by the pH-metry method and the Hammett indicator method, and the distribution of active centers was determined. The dependence of the sorption capacity of the obtained aluminosilicates on rice varieties and the conditions of the extraction of silicon from the straw (the duration of alkaline hydrolysis) was established on model aqueous solutions of methylene blue. The results expand the possibilities of comprehensive utilization of agricultural wastes with obtaining valuable products and deepen knowledge about the processing biogenic silicon-containing raw materials and the sorption mechanisms of organic pollutants on aluminosilicate materials.

Impact of AlCl3 and FeCl2 Addition on Catalytic Behaviors of TiCl4/MgCl2/THF Catalysts for Ethylene Polymerization and Ethylene/1-Hexene Copolymerization

The present research focuses on elucidating of the impact of Lewis acids including AlCl3 and FeCl2 addition on catalytic behaviors during ethylene polymerization and ethylene/1-hexene copolymerization over the TiCl4/MgCl2/THF catalyst (Cat. A). In this study, the Cat. A with the absence and presence of Lewis acids was synthesized via the chemical route. Then, all catalyst samples were characterized and tested in the slurry polymerization. For ethylene polymerization, using the Cat. A with the presence of AlCl3 apparently gave the highest activity among other catalysts. In addition, the activity of catalysts tended to increase with the presence of the Lewis acids. This can be attributed to an enhancement of active center distribution by the addition of Lewis acids leading to larger amounts of the isolated Ti species. Moreover, with the presence of Lewis acids, the effect of hydrogen on the decreased activity was also less pronounced. Considering ethylene/1-hexene copolymerization, it revealed that the catalyst with the presence of mixed Lewis acids (AlCl3 + FeCl2) exhibited the highest activity. It is suggested that the presence of mixed Lewis acids possibly caused a change in acidity of active sites, which is suitable for copolymerization. However, activities of all catalysts in ethylene/1-hexene copolymerization were lower than those in ethylene polymerization. The effect of hydrogen on the decreased activity for both polymerization and copolymerization system was found to be similar with the presence of Lewis acids. Based on this study, it is quite promising to enhance the catalytic activity by addition of proper Lewis acids, especially when the pressure of hydrogen increases. The characteristics of polymers obtained upon the presence of Lewis acids will be discussed further in more detail.

Quantum-Chemical Analysis of the Mechanism of Degradation of Binary Platinum Nanoclusters with Sulfur-Containing Compounds

We propose a quantum-chemical model of poisoning of binary Pt42Me13 platinum nanoclusters (where Me is a transition metal, namely, Fe, Co, or Ni) with core-shell structure by hydrogen sulfide and sulfur dioxide in media of low-temperature fuel cells. This model is based on the evaluation of the adsorption characteristics of interaction of H2S and SO2 molecules with the surfaces of nanoclusters. It is shown that their susceptibility to the formation of strong chemisorption bonds with H2S and SO2 molecules depends on the type of nanocluster core and the sites of adsorption on their surfaces. By using the quantum-chemical method of density functional, we establish the regularities of influence of the cores of binary nanoclusters of transition metals Fe, Co, or Ni on the geometric and energy characteristics of the adsorption of molecules. We also propose the mechanism of interaction of H2S and SO2 molecules with binary platinum nanoclusters based on the changes in the electronic properties of the surface platinum atoms or in the distribution of active adsorption centers on the surface of nanoclusters depending on the type of the core. The accumulated results confirm the prospects of the density functional method in the theoretical evaluation of the influence of chemical composition and the structure of binary nanoparticles on their properties in the reactions of low-temperature fuel cells.

Mechanism of internal and external electron donor effects on propylene polymerization with MgCl2‐supported Ziegler–Natta catalyst: New evidences based on active center counting

ABSTRACTTwo TiCl4/Di/MgCl2 type supported Ziegler–Natta catalysts were prepared by loading dibutylphthalate or dicyclopentyldimethoxysilane (DCPDMS) (internal donor, Di) and TiCl4 on activated δ‐MgCl2 in sequence, and a blank catalyst was prepared by loading TiCl4 on the same δ‐MgCl2 without adding Di. These catalysts have similar specific surface area and pore size distribution, thus form a suitable base for comparative studies. Propylene polymerization with the catalysts was conducted in n‐heptane slurry using triethylaluminum (TEA) as cocatalyst, and the effects of Di as well as De (external donor, in this work it was DCPDMS) on the number of active centers, the distribution of active centers among three polypropylene (PP) fractions (isotactic, medium isotactic, and atactic PP chains), and chain propagation rate constants of the PP fractions were studied by counting the number of active centers in the PP fractions using a method based on selective quench‐labeling of the propagation chains by 2‐thiophenecarbonyl chloride. When De was not added in the polymerization, introducing a phthalate type Di in the catalyst evidently changed the active center distribution by enhancing the proportion of active centers producing isotactic PP (iPP) ( ), but scarcely changed reactivities of the three groups of active centers forming the three fractions. When the De was added in the polymerization system with TiCl4/phthalate/MgCl2 catalyst, further shifting of active center distribution in favor of took place, meanwhile reactivities of the three groups of active centers also remarkably changed in favor of . Mutual effects of these changes led to overwhelming dominance of iPP production in the TiCl4/Di/MgCl2–TEA/De system (Di = phthalate, De = alkoxysilane). In contrast, though using alkoxysilane as Di also caused shifting of active center distribution in favor of when De was not added, addition of alkoxysilane De caused reverse shifting of active center distribution in favor of those producing PP of lower stereoregularity. This unfavorable change largely counteracted the reactivity changes in favor of caused by the De, rendering the catalytic system rather poor isospecificity. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46605.

Kinetics and mechanism of ethylene polymerization with TiCl4/MgCl2 model catalysts: Effects of titanium content

Two MgCl2-supported Ziegler–Natta catalysts were prepared by loading TiCl4 onto activated δ-MgCl2 supports, which contained 0.1 wt% (Cat-1) and 1.0 wt% (Cat-2) of titanium (Ti), respectively. Cat-1 has a slightly larger specific surface area (88.1 m2/g) and nearly the same pore volume (0.45 cm3/g) as the δ-MgCl2 support, but the catalyst with 1.0% Ti (Cat-2) has smaller specific surface area (66.3 m2/g) and pore volume (0.40 cm3/g). The kinetics and active center distribution of ethylene polymerization with the catalysts were compared. Cat-1 showed much higher activity per gram of titanium than Cat-2, which can be attributed mainly to the much higher [C∗]/[Ti] value of Cat-1 (60%) than of Cat-2 (12%). In both polymerization systems, the [C∗]/[Ti] value increased with time, but the Cat-2 system showed a much larger [C∗]/[Ti] increase than the Cat-1 system. Active center distribution of the catalysts disclosed by deconvolution of the molecular weight distribution into multiple Flory components was evidently different in the initial stage (0–200 s). The Cat-1 system produced more polymer with low molecular weight than the Cat-2 system, and the Flory components of low molecular weight decayed faster than those with high molecular weight. Influence of Ti content on number and structure of active centers formed on clustered Ti species is discussed with reconsideration of the current models of catalyst structure.

1-Hexene polymerization with supported Ziegler-Natta catalyst: Correlation between catalyst particle fragmentation and active center distribution

1-Hexene was polymerized with a commercial MgCl2-supported Ziegler-Natta catalyst using triethylaluminum as cocatalyst at different monomer/catalyst mass ratio (mhexene/mcat = 5, 15, 30), and the number of active centers ([C*]/[Ti]) was determined by quench-labeling the catalyst with 2-thiophenecarbonyl chloride and monitoring sulfur content of the polymer. Changes of polymerization rate (Rp) and propagation rate constant (kp) with time were also determined. [C*]/[Ti] was found to increase with time for 3 − 5 folds in the first 10 min of polymerization, but the maximum [C*]/[Ti] value reached after the induction period markedly enhanced with increase of monomer/catalyst mass ratio. Gradual fragmentation of the catalyst particles in parallel with the [C*]/[Ti] increase was observed by SEM analysis. It means that a large proportion of active center precursors are inaccessible to cocatalyst and monomer at the beginning of reaction, and fragmentation of the particles by hydraulic force of the growing polymer chains leads to exposure and activation of these precursors. Raising the monomer concentration can provide larger hydraulic force to expose those active site precursors that are more tightly buried inside the catalyst particle. The polymer molecular weight distribution and related active center distribution was found to shift with increase of the monomer/catalyst ratio, meaning that there are different types of buried active center precursors, and their exposure requires different extent of particle fragmentation.

Acid and Base Resistant Zirconium Polyphenolate-Metalloporphyrin Scaffolds for Efficient CO2 Photoreduction.

A series of zirconium polyphenolate-decorated-(metallo)porphyrin metal-organic frameworks (MOFs), ZrPP-n (n = 1, 2), featuring infinite ZrIV -oxo chains linked via polyphenolate groups on four peripheries of eclipse-arranged porphyrin macrocycles, are successfully constructed through a top-down process from simulation to synthesis. These are the unusual examples of Zr-MOFs (or MOFs in general) based on phenolic porphyrins, instead of commonly known carboxylate-based types. Representative ZrPP-1 not only exhibits strong acid resistance (pH = 1, HCl) but also remains intact even when immersed in saturated NaOH solution (≈20 m), an exceptionally large range of pH resistance among MOFs. The metallation at the porphyrin core gives rise to materials with enhanced sorption and catalytic properties. In particular, ZrPP-1-Co, with precise and uniform distribution of active centers, exhibits not only high CO2 trapping capability (≈90 cm3 g-1 at 1 atm, 273 K, among the highest in Zr-MOFs) but also high photocatalytic activity for reduction of CO2 into CO (≈14 mmol g-1 h-1 ) and high selectivity over CH4 (>96.4%) without any cocatalyst under visible-light irradiation (λ > 420 nm). Given the strong chemical resistance under extreme alkali conditions, these catalysts can be recycled without appreciable loss of activity. The possible mechanism for photocatalytic reduction of CO2 -to-CO over ZrPP-1-Co is also proposed.

Modeling the dose dependence of the vis-absorption spectrum of EBT3 GafChromic™ films.

The aim of this work was to model the dose dependence of the darkening of GafChromic™ EBT3 films by combining the optical properties of the polydiacetylene polymer phases, and a modified version of the single-hit model, which will take the stick-like shape of the monomer microcrystals into account. Second, a comparison is made between the quantification of the film darkening by flatbed scanning and by UV-vis absorption spectroscopy. GafChromicTM EBT3 films were irradiated with a 6 MV photon beam at dose levels between 0 and 50 Gy. The radiation-induced darkening of the films is quantified by a flatbed scanner, and by UV-vis absorption spectroscopy in the wavelength range of 220-750 nm. From the UV-vis absorption spectra, the contribution of each polymer phase to the absorbance was deduced. Next, the dose dependence of the polymer content is described by a modified single-hit model where the size distribution of polymerizable centers is approximated by way of the size distribution of the monomer microcrystals in the film. The absorption properties of the film can be accurately quantified by UV-vis spectroscopy for dose levels between 0 and 10 Gy. Over 10 Gy, the absorption spectrum saturates due to the limited sensitivity of the spectrometer. The modified single-hit model was successful in describing the increasing polymer concentration with radiation dose, using a log-normal distribution for the length of the stick-like monomer microcrystals. The dose dependence of the polymer content, deduced from the UV-vis absorption spectrum, differs from that of the flatbed scanning method and is more sensitive to changes in dose. The dose dependence of the polymer concentration can be modeled by taking into account the distribution of active centers using the microstructure of the active layer for dose levels between 0 and 10 Gy. The dissimilar dose dependence of the polymer concentration and the absorbance must be accounted for when modeling darkening from the kinetics of the photopolymerization reaction.