Search

Polygranular nuclear graphite, manufactured at 2800–3000 °C from a carbonized filler and binder mix, has high graphitization degree, crystalline structure close to perfect graphite, and about 20% porosity. The pore surfaces expose large regions of rough, defective prismatic edges of graphite crystallites which are the locus of graphite materials surface sites active for oxidation, chemisorption, and electron transfer. However, we show that high-resolution N2 and Kr first monolayer adsorption on polygranular graphite (P/P0

Silicon carbide is widely appreciated for its high temperature strength, radiation tolerance and neutronic transparency in applications for fuel particles and core internals of nuclear reactors. In the Fluoride Salt-Cooled High Temperature Reactor, silicon carbide ceramic matrix composites are candidate construction material for regions of higher neutron fluxes. Silicon carbide is wettable and reacts electrochemically with dissolved metals. Metallic impurities, tritium, moisture-based impurities and fission products, as well as thermal gradients can accelerate hot corrosion of silicon carbide in molten fluoride salt. Tritium can become trapped in radiation defects of silicon carbide. Thus, an understanding of the potential for tritium absorption, impurities reactions and thermal gradient-assisted corrosion mechanisms along with tritium recovery and redox control systems are necessary to mitigate silicon carbide corrosion in molten fluoride salt systems. Here, we survey current research on silicon carbide corrosion in molten fluoride salts and critically evaluate the research and development gaps.

Molten salt reactors (MSRs) are being considered as one of the potential nuclear options to meet future energy demands. While the MSR designs are drastically different from the more traditional light water reactor, many of the waste streams are similar between the concepts. The purpose of this paper is to outline strategies for the treatment and processing of MSR-type wastes from concepts of reconditioning and recycle of certain components to partitioning and direct immobilization of other waste components. To help bridge science and technology gaps, knowledge gained from similar efforts such as pyroprocessing of Experimental Breeder Reactor II salt wastes can be leveraged to develop concept-to-disposition pathways.

Oxidation of nuclear graphites produces microstructural changes that affect the elastic moduli of these materials. It is widely accepted that the primary effect of oxidation is to increase porosity, but the related effect on the moduli cannot be explained satisfactorily by simply noting changes to porosity. In this work, models describing the elastic moduli of porous, polycrystalline graphite materials are developed to interpret experimental determinations of Young's modulus and shear modulus in two grades of nuclear graphite – IG-110 and NBG-18 – that were oxidized to produce varying levels of porosity. Experimental measurements were carried out using laser-based ultrasonic methods and were interpreted successfully using models that take into account the effects of preferential oxidation of different elements of the graphite microstructure. The results indicate the importance of the processes that lead to increased porosity since these can heavily influence the nature of the resulting structure-property relationships.

Microstructural changes induced by neutron irradiation of superfine grain graphite G347A (Tokai Carbon, Japan) were examined by nitrogen adsorption at 77 K and by three microscopy techniques (SEM, TEM and FIB-SEM tomography). The specimens were irradiated at doses of up to 30 dpa, covering stages before and after the turnaround fluence at three temperatures (300, 450, 750 °C) of their irradiation envelope. The initial graphite densification at low fluences did not produce any detectable effect in the pore size range ( 50 nm) at high irradiation fluences and more structural changes on multiple scales, from nanometers to microns. This work demonstrates the unique ability of gas adsorption techniques to analyze open pores with sizes between sub-nanometer and sub-micron in bulk nuclear graphite, with supporting microscopy results.

Understanding “Where?” and “How much?” oxidation has occurred in a nuclear graphite component is critical to predicting any deleterious effects to physical, mechanical, and thermal properties. A key factor in answering these questions is characterizing the effective mass transport rates of gas species in nuclear graphites. Effective gas diffusion coefficients were determined for twenty-six graphite specimens spanning six modern grades of nuclear graphite. A correlation was established for the majority of grades examined allowing a reasonable estimate of the effective diffusion coefficient to be determined purely from an estimate of total porosity. The importance of Knudsen diffusion to the measured diffusion coefficients is also shown for modern grades. Knudsen diffusion has not historically been considered to contribute to measured diffusion coefficients of nuclear graphite.

Four grades of nuclear graphite were oxidized in helium with traces of moisture and hydrogen in order to evaluate the effects of slow oxidation by moisture on graphite components in high temperature gas cooled reactors. Kinetic analysis showed that the Langmuir-Hinshelwood (LH) model cannot consistently reproduce all results. In particular, at high temperatures and water partial pressures, oxidation was always faster than the LH model predicts. It was also found empirically that the apparent reaction order for water has a sigmoid-type variation with temperature which follows the integral Boltzmann distribution function. This suggests deviations from the LH model are apparently caused by activation with temperature of graphite reactive sites, which is probably rooted in specific structural and electronic properties of graphite. A semi-global kinetic model was proposed, whereby the classical LH model was modified with a temperature-dependent reaction order for water. This new Boltzmann-enhanced Langmuir-Hinshelwood (BLH) model consistently predicts oxidation rates over large ranges of temperature (800–1100 °C) and partial pressures of water (3–1200 Pa) and hydrogen (0–300 Pa). The BLH model can be used for modeling chronic oxidation of graphite components during life-time operation in high- and very high temperature advanced nuclear reactors.

For the next generation of nuclear reactors, HTGRs specifically, an unlikely air ingress warrants inclusion in the license applications of many international regulators. Much research on oxidation rates of various graphite grades under a number of conditions has been undertaken to address such an event. However, consequences to the reactor result from the microstructural changes to the graphite rather than directly from oxidation. The microstructure is inherent to a graphite's properties and ultimately degradation to the graphite's performance must be determined to establish the safety of reactor design. To understand the oxidation induced microstructural change and its corresponding impact on performance, a thorough understanding of the reaction system is needed. This article provides a thorough review of the graphite-molecular oxygen reaction in terms of kinetics, mass and energy transport, and structural evolution: all three play a significant role in the observed rate of graphite oxidation. These provide the foundations of a microstructurally informed model for the graphite-molecular oxygen reaction system, a model kinetically independent of graphite grade, and capable of describing both the observed and local oxidation rates under a wide range of conditions applicable to air-ingress.

We report results of vibrational neutron spectroscopy investigation aimed to identify the state of hydrogen adsorbed in ultramicroporous carbon. The mobility of hydrogen confined in carbon pores was probed as a function of temperature and pressure using inelastic neutron scattering, and the molecular translational and rotational motions were studied. At low loading rotation of H 2 molecules adsorbed in the smallest carbon pores (∼4–5 A) is severely hindered, suggesting that the interaction between H 2 and the host matrix is anisotropic. At higher loading, H 2 molecules behave as a nearly free rotor, implying lower anisotropic interactions with adsorption sites. At 77 K where bulk H 2 is a gas, deconvolution of elastic/quasielastic signal provide evidence of pressure-dependent fractions of immobile (solid-like) and partially mobile (liquid-like) hydrogen, which correlate with the excess adsorption isotherm at 77 K. Effective H 2 density in pores changes from solid-like to liquid-like with increasing pressure at 77 K. Surprisingly, immobile and partially mobile H 2 is present even at temperatures as high as ∼110 K where bulk hydrogen exists only in gas form.

Laser ultrasonic line source methods have been used to study elastic anisotropy in nuclear graphites by measuring shear wave birefringence. Depending on the manufacturing processes used during production, nuclear graphites can exhibit various degrees of material anisotropy related to preferred crystallite orientation and to microcracking. In this study, laser ultrasonic line source measurements of shear wave birefringence on NBG-25 have been performed to assess elastic anisotropy. Laser line sources allow specific polarizations for shear waves to be transmitted – the corresponding wavespeeds can be used to compute bulk, elastic moduli that serve to quantify anisotropy. These modulus values can be interpreted using physical property models based on orientation distribution coefficients and microcrack-modified, single crystal moduli to represent the combined effects of crystallite orientation and microcracking on material anisotropy. Ultrasonic results are compared to and contrasted with measurements of anisotropy based on the coefficient of thermal expansion to show the relationship of results from these techniques.

Even when not functionalized intentionally, most carbon materials are not hydrophobic and readily adsorb water molecules from atmospheric water vapor. We have equilibrated an ultramicroporous carbon at several levels of relative humidity, thereby attaining various hydration levels. The water molecules were adsorbed on the pore walls (but did not fill completely the pore volume) and thus could be better described as hydration, or surface, rather than confined, water. We used quasielastic neutron scattering to perform a detailed investigation of the dependence of microscopic dynamics of these adsorbed water species on the hydration level and temperature. The behavior of hydration water in ultramicroporous carbon clearly demonstrates the same universal traits that characterize surface (hydration) water in other materials that are surface-hydrated. Thus, unless special treatment is intentionally applied to ultramicroporous carbon, the species filling its pores in various applications, ranging from hydrogen molecules to electrolytes, likely find themselves in contact with non-freezing water molecules characterized by rich microscopic dynamics.

Laser ultrasonic methods have been used to measure the elastic moduli of various nuclear graphites. Measurements were made to assess wavespeeds for longitudinal and shear waves in different propagation directions and these were used along with density measurements to compute the longitudinal and shear moduli as well as Young's modulus. All moduli decreased with increasing graphite porosity and these variations could be interpreted using models describing the effect of porosity on material stiffness. Extrapolations for these models to zero porosity were used to infer the moduli for theoretically dense graphite; the results were far below predicted values reported in the literature for fully dense, polycrystalline, isotropic graphite. Differences can be attributed to microcracking in the graphite microstructure. Using models for the effects of microcracking on modulus, estimates for microcrack populations indicate that the number of cracks per unit volume must be much greater than the number of pores per unit volume. Experimental results reported in the literature for irradiated graphites as well as for the stress dependence of graphite modulus are consistent with the influence of microcracking on elastic behavior and could be interpreted using concepts developed here. Results in this work for graphite structure-property relationships should allow for more sophisticated characterization of nuclear graphites using ultrasonic methods.

The reactor core, moderator and reflectors of a thermal spectrum advanced molten salt reactor will constitute multi-tons of graphite. Porous reactor-grade graphite, if unprotected, can be permeated by molten salt depending on the infiltrating pressure differential and entrance diameters of accessible graphite pores. Salt and gas permeation of graphite can affect microstructural properties and radiation behavior but also facilitate diffusion, deposition and retention of fission products and tritium. Because of the significant void volume of nuclear graphite, fission products and tritium retention due to salt permeation necessitates seal coatings or pore impregnation to reduce open porosity. Alternatively, very fine-grained graphite grades with low Xe permeability are being developed. Here, we survey the current technologies for mitigating salt and gas transport into graphite.

The high pressure adsorption behavior of CO2 at T = 296 K in microporous carbon was investigated by small-angle neutron scattering (SANS) technique. A strong densification of CO2 in micropores accompanied by non-monotonic adsorption-induced pore deformation was observed. The density of confined CO2 increases rapidly with pressure and reaches the liquid –like density at 20 bar, which corresponds to the relative pressure of P/Psat∼0.3. At P > 20 bar density of confined CO2 increases slowly approaching a plateau at higher pressure. The size of micropores first increases with pressure, reaches a maximum at 20 bar, and then decreases with pressure. A complementary SANS experiment conducted on the same microporous carbon saturated with neutron-transparent and non-adsorbing inert gas argon shows no deformation of micropores at pressures up to ∼200 bars. This result demonstrates that the observed deformation of micropores in CO2 is an adsorption-induced phenomenon, caused by the solvation pressure - induced strain and strong densification of confined CO2.

Small angle neutron scattering (SANS), scanning transmission electron microscopy (STEM) and gas adsorption, were applied to study the morphology and hydrogen adsorption properties of a wood-based ultramicroporous carbon (UMC) and a poly(furfuryl alcohol) derived carbon (PFAC). The polydispersed spherical model and the Guinier analysis of the scattering profiles were applied to obtain morphological parameters such as average pore size and pore size distribution of the two carbons; the results agreed reasonably well with independent gas sorption measurements and structural analysis by electron microscopy. The density of hydrogen physisorbed in these two carbons at room temperature and at moderate pressures was investigated by in situ SANS measurements. The experimental data, analyzed using a modified Kalliat model for decoupling scattering contributions from pores of different sizes, indicate that the molecular hydrogen acquires high densities preferentially in pores of subnanometer size at all measured pressures. These results support existing quantum mechanical and thermodynamical models that have predicted that the narrowest pores enhance the adsorption due to the overlapping of the potential fields from both wall sides in the slit-like pores. The structural information at a nanometer level gained via this work could guide the new development of porous-carbon based materials for hydrogen storage.

Accelerated oxidation tests were performed to determine kinetic parameters of the chronic oxidation reaction (i.e. slow, continuous, and persistent) of PCEA graphite in contact with helium coolant containing low moisture concentrations in high temperature gas-cooled reactors. To the authors’ knowledge such a study has not been done since the detailed analysis of reaction of H-451 graphite with steam (Velasquez, Hightower, Burnette, 1978). Since that H-451 graphite is now unavailable, it is urgently needed to characterize chronic oxidation behavior of new graphite grades that are being considered for use in gas-cooled reactors. The Langmuir–Hinshelwood mechanism of carbon oxidation by water results in a non-linear reaction rate expression, with at least six different parameters. They were determined in accelerated oxidation experiments that covered a large range of temperatures (800–1100 °C), and partial pressures of water (15–850 Pa) and hydrogen (30–150 Pa) and used graphite specimens thin enough (4 mm) in order to avoid diffusion effects. Data analysis employed a statistical method based on multiple likelihood estimation of parameters and simultaneous fitting of non-linear equations. The results show significant material-specific differences between graphite grades PCEA and H-451 which were attributed to microstructural dissimilarity between the two materials. It is concluded that kinetic data cannot be transferred from one graphite grade to another.

Natural graphite powders were subjected to a series of thermal treatments to improve the anode irreversible capacity loss and capacity retention during long-term cycling of lithium-ion batteries. A baseline thermal treatment in inert Ar or N2 atmosphere was compared to cases with a proprietary additive to the furnace gas. This additive substantially altered the surface chemistry of the uncoated natural graphite powders and resulted in significantly improved long-term cycling performance of the lithium ion batteries over the commercial, carbon-coated natural graphite baseline. Different heat-treatment temperatures were investigated ranging from 950 to 2900 °C to achieve the desired long-term cycling performance with a significantly reduced thermal budget. A detailed summary of the characterization data is also presented, which includes X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and temperature-programmed desorption-mass spectroscopy. Characterization data was correlated to the observed capacity fade improvements over the course of long-term cycling at high charge–discharge rates in full lithium-ion cells. It is believed that the long-term performance improvements are a result of forming a more stable solid electrolyte interface (SEI) layer on the anode graphite surfaces, which is directly related to the surface chemistry modifications imparted by the proprietary gas environment during thermal treatment.

A series of gasification experiments, using two right cylinder specimens (� 12.7 � 25.4 mm and 25.4 � 25.4 mm) of PCEA nuclear graphite in ambient airflow, measured the total gasification flux at weight losses up to 41.5% and temperatures (893–1015 K) characteristics of those for in-pores gasification Mode (a) and in-pores diffusion-limited Mode (b). The chemical kinetics parameters for the gasification of PCEA graphite are determined using a multi-parameters optimization algorithm from the measurements of the total gasification rate and transient weight loss in experiments. These parameters are: (i) the pre-exponential rate coefficients and the Gaussian distributions and values of specific activation energies for adsorption of oxygen and desorption of CO gas; (ii) the specific activation energy and pre-exponential rate coefficient for the breakup of stable un-dissociated C(O2) oxygen radicals to form stable (CO) complexes; (iii) the specific activation energy and pre-exponential coefficient for desorption of CO2 gas and; (iv) the initial surface area of reactive free sites per unit mass. This area is consistently 13.5% higher than that for nuclear graphite grades of NBG-25 and IG-110 and decreases inversely proportional with the square root of the initial mass of the graphite specimens in the experiments. Experimental measurements successfully validate the chemical-reactions kinetics model that calculates continuous Arrhenius curves of the total gasification flux and the production rates of CO and CO2 gases. The model results at different total weight losses agree well with measurements and expand beyond the temperatures in the experiments to the diffusion-limited mode of gasification. Also calculated are the production rates of CO and CO2 gases and their relative contributions to the total gasification rate in the experiments as functions of temperature, for total weight losses of 5% and 10%. Published by Elsevier B.V.

The oxidation-induced porosity development in nuclear graphite has great effect on its mechanical behavior, thus it is very important to understand the pore structure development of nuclear graphite during oxidation. This paper reports on the oxidation kinetics of grade IG-110 nuclear graphite and the porosity development in oxidized samples. The distribution of the oxidized layer in IG-110 specimens oxidized at 600–750 °C was studied using optical microscopy coupled with automated image analysis technique, and the mechanism of porosity development was determined. The thickness of oxidized layer decreased with the oxidation temperature but was independent of the weight loss level. Oxidation caused consumption of graphite structure and development of porosity, which was initiated from the binder phase. Statistical analysis indicated that generation and growth of pores was dominant at low temperatures, while merging and collapse of pores was the main effect at high temperatures. Compared with medium-grained PCEA graphite, the fine-grained IG-110 graphite demonstrates deeper penetration of the oxidant because of its higher pore density and greater porosity.

Potassium-doped activated carbon was prepared by mixing potassium salt with polyfurfuryl alcohol precursor followed by carbonization and activation. Several experimental techniques, such as thermogravimetric analysis, gas adsorption, X-ray diffraction and Raman scattering spectroscopy were employed to understand the effects of potassium on the texture and pore structures of the activated carbon, which ultimately affects the hydrogen adsorption properties. After doping with potassium, the activated carbon exhibits higher surface area, higher micropore volume, and enhanced hydrogen adsorption capacity. Understanding of how alkali metals affect surface area and micropore development in activated carbon may help to clarify the hydrogen adsorption mechanism and improve the design of suitable carbon-based hydrogen storage materials.

Oxidation resistance in air of three grades of nuclear graphite with different structures was compared using a standard thermogravimetric method. Differences in the oxidation behavior have been identified with respect to both (i) the rate of oxidation in identical conditions and the derived apparent activation energy and pre-exponential factor and (ii) the penetration depth of the oxidant and the development of the oxidized layer. These differences were ascribed to structural differences between the three graphite grades, in particular the grain size and shape of the graphite filler, and the associated textural properties, such as total BET surface area and porosity distribution in the un-oxidized material. It was also found that the amount of strongly bonded surface oxygen complexes measured by thermodesorption significantly exceeds the amount afforded by the low BET surface area, and therefore low temperature oxygen chemisorption is not a reliable method for determining the amount of surface sites (re)active during air oxidation. The relationship between nuclear graphite microstructure and its oxidation resistance demonstrated in this work underlines the importance of performing comprehensive oxidation characterization studies of the new grades of nuclear graphite considered as candidates for very high temperature gas-cooled reactors.

Owing to several disadvantages of traditional hard template based synthesis, soft-template or self-assembly was adopted to synthesize mesoporous carbon. In this work, we have introduced hexaphenol as a new and large organic precursor for the synthesis of mesoporous carbon by self-assembly with Pluronic P123 as structure dictating agent. The resultant mesoporous carbon is bimodal in nature with median pore widths of 29 and 45 A and BET surface area of 312 m2/g. Unlike previously synthesized mesoporous carbon, this carbon possesses negligible micropore volume. This mesoporous carbon is very suitable candidate for several applications including membrane separation, chemical sensor or selective sorption of larger molecules.

Quasi-elastic neutron scattering was used for characterization of dynamics of molecular hydrogen confined in narrow nanopores of two activated carbon materials: a carbon derived from polyfurfuryl alcohol and an ultramicroporous carbon. Fast, but incomplete ortho–para conversion was observed at 10 K, suggesting that scattering originates from the fraction of unconverted ortho isomer which is rotation-hindered because of confinement in nanopores. Hydrogen molecules entrapped in narrow nanopores (

Aberration corrected scanning transmission electron microscopy was used to demonstrate the feasibility of imaging individual Pd atoms that are highly dispersed throughout the volume of activated carbon fibers. Simultaneous acquisition of high-angle annular dark-field and bright-field images allows correlation of the location of single Pd atoms with microstructural features of the carbon host material. Sub-Angstrom imaging conditions revealed that 18 wt% of the total Pd content is dispersed as single Pd atoms in three re-occurring local structural arrangements. The identified structural configurations may represent effective storage sites for molecular hydrogen through Kubas complex formation as discussed in detail in the preceding article.

Palladium-modified activated carbon fibers (Pd-ACF) were synthesized by melt-spinning, carbonization and activation of an isotropic pitch carbon precursor premixed with an organometallic Pd compound. The hydrogen uptake at 25 °C and 20 bar on Pd-ACF exceeded the expected capacity based solely on Pd hydride formation and hydrogen physisorption on the microporous carbon support. Aberration-corrected scanning transmission electron microscopy (STEM) with sub-Angstrom spatial resolution provided unambiguous identification of isolated Pd atoms occurring in the carbon matrix that coexist with larger Pd particles. First principles calculations revealed that each single Pd atom can form Kubas-type complexes by binding up to three H 2 molecules in the pressure range of adsorption measurements. Based on Pd atom concentration determined from STEM images, the contribution of various mechanisms to the excess hydrogen uptake measured experimentally was evaluated. With consideration of Kubas binding as a viable mechanism (along with hydride formation and physisorption to carbon support) the role of hydrogen spillover in this system may be smaller than previously thought.

The effect of mixing chemically-activated ultramicroporous carbon (UMC) with Pd nanopowder is investigated. Results show that Pd addition doubles the rate of hydrogen uptake, but does not enhance the hydrogen capacity or improve desorption kinetics. The effect of Pd on the rate of hydrogen adsorption supports the existence of the hydrogen spillover mechanism in the Pd – UMC system.

Hydrogen adsorption on ultramicroporous carbon was investigated at near-ambient temperatures using volumetric and gravimetric methods. The results showed that the main process, physisorption, is accompanied by a slow process of different nature, that causes slow uptake at high pressures and hysteresis on desorption. The combined result is unusually high levels of hydrogen uptake at near-ambient temperatures and pressures (e.g. up to 0.8 wt.% at 25 °C and 2 MPa). The heat of adsorption corresponding to the slow process leading to high uptake (17–20 kJ/mol) is higher than usually reported for carbon materials; the adsorption kinetics is slow, and the isotherms exhibit pronounced hysteresis. These unusual properties were attributed to contributions from polarization-enhanced physisorption induced by traces of alkali metals residual from chemical activation. The results support the hypothesis that polarization-induced physisorption in high surface area carbons modified with traces of alkali metal ions is an alternate route for increasing the hydrogen storage capacity of carbon adsorbents.

Equations are derived for the approach to equilibrium in the oxidation of graphite under assumptions of constant graphite density and linearized oxidation kinetics. A two-factor expression is assumed for the effective diffusivity. Equilibration may be estimated by observing the convergence of profiles with time or by means of an algebraic approximation. At large times, the profiles converge to the steady state. Oxidation depths show fair agreement with published measurements and follow closely the observed temperature trend.

The efforts for designing a meaningful and acceptable standard test method for characterization of kinetic parameters of air oxidation of graphite helped identify several practical issues that must be considered for the development of such a test. Using standard size (and shape) specimens, large enough in size to accommodate the inherent local microstructure differences between graphite samples, resulted in non-uniform oxidation profiles and preferential binder oxidation; this was not expected based on the linearity of Arrhenius plots and the (large) values of activation energy. It was found that the transition between the regimes 1 and 2 of graphite oxidation occurs gradually, depending both on the oxidation temperature and rate of oxygen supply. Nevertheless, measuring oxidation rates obtained on standard size samples provides a basis for a meaningful comparison among materials, which may serve as much needed information for predictive models.

Fabrication of uranium oxicarbide microspheres, a component of TRISO fuel particles for high temperature nuclear power systems, is based on the internal gelation of uranium salts in the presence of carbon black. In order to obtain a high quality product, carbon black should remain dispersed during all phases of the gelation process. In this study, the surface and structural properties of several commercial carbon black materials, and the use of dispersing agents was examined with the goal of finding optimal conditions for stabilizing submicron-sized carbon black dispersions. Traditional methods for stabilizing dispersions, based on the use of dispersing agents, failed to stabilize carbon dispersions against large pH variations, typical for the internal gelation process. An alternate dispersing method was proposed, based on using surface-modified carbons functionalized with strongly ionized surface groups (sodium sulfonate). With a proper choice of surface modifiers, these advanced carbons disperse easily to particles in the range of 0.15–0.20 μm and the dispersions remain stable during the conditions of internal gelation.

Palladium-doped activated carbon fibers are being evaluated as candidate materials for enhanced hydrogen storage at near ambient conditions. Pd-doped fibers were spun using a Pd salt mixed with an isotropic pitch precursor. Experimental techniques such as in situ X-ray analysis, thermogravimetric studies, scanning transmission electron microscopy and gas adsorption were employed to understand how processing conditions for the production of Pd-doped activated carbon fibers affect the microstructure, pore development, and dispersion of metal particles throughout the fibers. The results showed that PdO phase is present in the stabilized fibers and that this oxide phase is stable up to about 250 °C. The oxide phase transforms into Pd metal with increasing heat treatment temperature, going through the formation of an intermediate carbide phase. Sintering of Pd particles was observed with heat treatment at temperatures over 750 °C. It was also found that pore development during physical activation with CO2 was not significantly affected by the presence of Pd particles within the fibers.

A conducting polymer based liquid membrane electrochemical cell is presented in which reversible water transport in propylene carbonate (PC) is achieved by applying a dc voltage across the cell. In a cell two electrochemically deposited polypyrrole (PPy) films are utilized together with a porous separator filled with an electrolyte solution to make a sandwich structure, and the water vapor pressure adjacent to each electrode is measured. Under a cycling electrode potential the movement of solvated ions across the cell reversibly transports water. PPy films with different dopant anions (BF4−, PF6−, DBS−) are tested in PC solutions containing corresponding anions and different cations (Li+, Na+, TBA+). Water activity increases at the cathode and decreases at the anode. Water transport numbers for different liquid membrane systems are estimated and discussed.

In a previous paper [Carbon, 1997, 35, 83]it was shown for a series of activated carbons with acidic behavior that surface characterization with a continuous pK distribution was possible based on potentiometric titration provided that several experimental precautions were taken during data collection. The number and strength of acid functionalities calculated from the proton affinity distribution (PAD) were in quantitative agreement with those measured by Boehm titration of the same samples. In this work the surface basicity of an activated carbon was investigated using the same analytical procedures. The factors that may denature the results of potentiometric titration have been analyzed; it is found that incremental addition controlled by a pH stability criterion (the “fully automatic” titration) provides reproducible results even for large particle granulation. From a practical point of view this shows that the results of this optimized analytical method are reliable. The acid buffering capacity of basic carbon, with the quality and quantity of proton binding groups, reveals that the most important proton binding processes occur in the ranges of pH 4–7, 8.4–8.6 and >9.5; the second process accounts for most of the acid buffering capacity of this carbon. The basic properties of the carbon surface are proposed to be due to the combination of redox reactions and proton transfer to/from the surface and the supporting electrolyte.

Acidic properties (acid sites' strength and concentration) of three series of mixed oxide catalysts (titania-silica, zirconia-silica and alumina-silica) were analyzed by proton affinity distributions. Catalytic properties under mild dehydration conditions (1-butene isomerization at 423 K) of these materials were also evaluated. A Bronsted-type correlation was found between first-order apparent isomerization rate constants and acid strengths of certain components of the pK spectra of the catalysts. This was explained in terms of proton transfer processes that take place at both wet and dry interfaces, i.e. at the surface hydroxyl layer. The results were discussed in terms of Fourier transform infrared data which indicate the presence of large surface hydroxyl populations on all catalysts under the reaction conditions.

In a recent paper, the authors demonstrated that the intensity of a specific peak (pK {approximately} 6) in proton affinity distributions (PADs) measured for a series of Co-Mo/Al{sub 2}O{sub 3} catalysts could be correlated to the variation in HDS activity within the series. PADs access information about species on the catalyst`s surface from analyses of proton binding data collected using potentiometric titration. If the species determined at the solid/aqueous interface are present after catalyst activation and are active during catalytic testing, then this methodology can provide a simple, surface sensitive procedure for catalysts characterization which should correlate with other characterization techniques. To test this hypothesis the authors provide herein results from more conventional catalyst characterization techniques in order to corroborate the PAD results. The techniques of temperature-programmed reduction, X-ray photoelectron spectroscopy, and laser Raman spectroscopy were used to provide additional characterization of Co-Mo/Al{sub 2}O{sub 3} catalysts in their oxide state. PADs were obtained for this catalyst system in their oxide and sulfided states. The HDS activity for each member of this series was measured using hydrosulfurization of thiophene as a test reaction. The effect of cobalt loading, molybdenum content, presulfidation conditions, and pH during catalyst preparation were the variables used.more » Based on the experimental data the author`s propose that the active site for HDS activity is a Co-Mo interaction species consisting of molybdate octahedra and a dispersed Co which could also be octahedrally coordinated. This species can be transformed reversibly during the process of sulfidation and reoxidation and its formation is controlled by pH. 48 refs., 9 figs., 1 tab.« less

Potentiometric titration data from pure rutile, anatase, and a commercial fumed titania (Degussa P25) were analyzed in terms of proton binding isotherms from which proton affinity distributions (PADs) of surface sites were obtained. As-received samples, whose thermal and storage history were not systematically controlled, as well as samples subjected to controlled calcination–rehydration–drying treatments were studied. The results indicated the occurrence of a limited number of surface groups on the two polymorphs. The behavior of pure rutile and anatase could be admixed to simulate the acid–base behavior of the commercial sample; on this basis the surface of fumed titania consists largely of anatase-like structures with a small contribution (7%) of rutile-like groups. The region of νODstretching vibrations of isolated -OD groups on extensively dehydroxylated samples was found to correlate with the pK’s determined from PADs. A qualitative assignment of measured pKvalues based on either the original MUSIC model (Hiemstra, T., de Wit, J. C. M., and Van Riemsdijk, W. H.,J. Colloid Interface Sci.133,105 (1989)) or a refined version of it is presented.

A series of MoO3/Al2O3catalysts of varying weight loading were characterized after calcination by their buffering capacity during potentiometric titration. Strong buffering of the aqueous electrolyte occurred in distinct pH ranges, which indicated the formation of hydrolysis products of the surface compounds formed in these pH windows. Addition of cobalt, followed by calcination, revealed a new feature which signaled the formation of a possible surface heteropolymolybdate compound. If cobalt was added first this compound was not formed, however, if cobalt and molybdenum were coimpregnated it was detectable. As a promoter, Ni(II) had an effect similar to cobalt, but Fe(III) did not. The series of catalysts were further tested for their hydrodesulfurization activity using thiophene as a reactant. The thiophene conversion, as a function of the amount of surface compound formed by addition of the cobalt promoter, resulted in a linear relation, indicating that the Co/Mo compound detected in the oxidic state could be correlated with the HDS activity of these catalysts.

Proton-releasing properties at the oxide/aqueous solution interface were examined as a function of pH for two series of mixed oxide aerogels and their pure oxide components using potentiometric titrimetry. Reports of synthesis, surface characterization, and catalytic activity (1-butene isomerization) have appeared recently for similar preparation batches of titania-silica and zirconia-silica. After deconvolution of potentiometric titration data, the pK spectra of the oxide surfaces were obtained; they reveal the number and strength of various proton donor sites present under "wet" conditions. The activity in 1-butene isomerization measures weak Bronsted sites of the "pseudo-dry" catalyst (reaction at 423 K after drying at 473 K). We find that the activity for 1-butene isomerization is linearly related to the density of a particular type of proton donor site determined from pK spectra. This shows that only a limited number of the total Bronsted sites accessed by potentiometric titrimetry is specifically active for 1-butene isomerization under the conditions used here. The pK range of active sites and the corresponding TOF numbers are 6.3 < pK < 8.7, (8.2 ± 2.1) × 10(-3) s(-1) for titania-silica and 6.4 < pK < 6.5, (93 ± 24) x 10−3 s−1 for zirconia-silica. In the two catalysts series the catalytic performance varied with either composition (titania-silica) or preparation parameters (zirconia-silica) at constant composition.

The hydrolysis of trimethylorthobenzoate was studied over the pH range 4–7. In homogeneous experiments, overall first-order kinetics was found for both constant and variable proton concentrations, with an average intrinsic rate constant of 6· 10 3 1/mol·min. Within the above pH range, it was also found that a series of alumina/silica oxides could act as general catalysts for the same reaction. Proton binding on the oxides determined from potentiometric titration data revealed two types of potentially active, proton-donor, sites with acid strengths p K mI a ≈ 4 and p K II a ≈ 5.5. A detailed kinetic analysis is presented which accounts for the time dependent mass, charge and site balance in the system. Kinetic scanning, which involves a pH perturbation and subsequent analysis of the heterogeneous system relaxation, is proposed as a general method for evaluation of the concentration of surface sites active for general acid catalysis. Active site densities, determined by means of this approach, increased from 0.085 to 0.13 mmol/g as the aluminum content in the mixed oxides increased from 3 to 10% by weight. The proton transfer rate constant's value, essentially independent of oxide composition, was estimated at ≈ 1500 g/mol·min.

The effect of electronegative (F) and electropositive (Na) dopants on the proton affinity distribution (PAD) of surface hydroxyls at the alumina/water interface was determined and compared with the changes in the hydroxyl region of infrared spectra collected after dehydration of the same samples. The comparison between the two is based on the assumption that the local configuration of OH groups at the oxide/solution interface is similar to that used in the assignment of IR bands for hydroxyls on partially dehydrated surfaces, i.e., assignments from models assuming perfect crystal planes. A common structural model is used to correlate the changes observed in proton affinity distributions with those recorded in IR spectra after doping alumina with either F or Na.

A method for characterization of carbon surfaces' acidity using a continuous distribution of acidity constants is proposed. The method is based on potentiometric titration measurements. Titration curves are transformed into proton adsorption isotherms, and are analyzed to yield the distribution of acidity constants. Calculation of the distribution function is sensitive to experimental errors, and therefore a careful smoothing treatment must be applied to the experimental data. The method is tested by application to organic compounds and the calculated pK values are in excellent agreement with literature data. The method is then applied to study the evolution of acidic groups, in terms of their pK values, subsequent to modification processes such as oxidation and reduction of activated carbons. It is demonstrated that the method is sensitive to the changes in the number and character of surface acidic groups.

The objective of this study is to compare the properties and performance of a series of alumina supported nickel catalysts where the only variable is the origin of the alumina. Three commercial aluminas were subjected to a battery of characterization techniques including dissolution studies, X-ray diffraction, nickel adsorption, and measurements of the pH of their point of zero charge. Nickel catalysts formed on these supports were characterized by temperature-programmed reduction, temperature-programmed reaction of carbon monoxide, and temperature-programmed surface reaction. The performance of the catalysts was assessed by CO/H2 methanation studies. The results from the characterization and performance studies demonstrated that two types of nickel can exist on these alumina supports: a surface nickel and an incorporated nickel. The extent of partitioning between these two states is controlled by the dissolution properties of each alumina when in contact with nickel electrolytes at low pH.