Rate Law Parameters Research Articles

Kinetic modeling of dual-stage oxidative extractive denitrogenation of model fuel

The petroleum fraction of crude oil is laden with notorious nitrogen compounds, whose removal facilitates fuel oil transport and enhances the longevity of hydrodesulfurization catalysts. Petroleum refining operation thus demands the adoption of unconventional techniques, such as coupled oxidative–extractive denitrogenation. Kinetic models can provide insights into the reaction mechanism and the design and performance of reactors. In this work, nine different kinetic models were developed by applying the Langmuir–Hinshelwood and Eley–Rideal approaches to various reaction pathways. The screening of the kinetic models was done based on the values of the rate law parameters and corresponding R 2 and adjusted R 2 values calculated through a multi-parametric, non-linear regression based on the Levenberg–Marquardt algorithm. Out of all the models, the Eley–Rideal model corresponding to the rate expression rs9 was found to be the best fit, reflecting that hydrogen peroxide remained in bulk while pyridine alone was adsorbed on the catalyst. The selection of suitable extractants for the extractive removal of oxidation products was made based on their extraction efficiency and partition coefficients. The ternary liquid–liquid equilibrium (LLE) data for the (acidulated water-pyridine-isooctane) and (acidulated water-pyridine N-oxide-isooctane) systems were plotted, and Othmer–Tobias, Hand, and Bachman correlations explained the consistency of the experimental LLE data.

Measurement of the interfacial energy release rate (ERR) and cohesive law parameters of polyvinyl butyral laminated glass using a novel beam-type test specimen

The polyvinyl butyral (PVB) laminated glass (LG) structures are widely used in architectural and automotive sectors. Determination of the interfacial fracture energy (i.e. G) between the PVB layer and glass is one of the most vital procedures for ensuring the structural safety and fracture resistance of these kinds of components. The present study investigates the adhesion characteristics between glass and PVB interface in a LG composite structure. A new simple test fixture is introduced to measure the mixed-mode fracture energy from pure mode I (i.e. GI) to pure mode II (i.e. GII). By making use of the concept of Equivalent material Concept (EMC), linear elastic fracture mechanics (LEFM) formulations together with finite element (FE) analysis can be used to compute the compliance parameter of the system and pure modes I/II fracture energies. Besides, the interaction of cohesive forces between the PVB layer and glass are simulated by using a FE method based on a bi-linear cohesive zone model (CZM) technique. In this regard, the cohesive law parameters are computed for mode I and mode II in an iterative process. Then, the obtained pure mode cohesive law parameters are utilized to predict the mixed-mode load–displacement diagrams. The obtained results demonstrate the satisfactory accuracy of the cohesive law parameters and fracture energies in predicting the mixed-mode loading results across four different loading rates (i.e. LR = 2, 5, 10, and 15 mm/min).

Novel semi-analytical model for solving the C*-integral of specimens with mode-I crack under various constraints

The energy rate line integral C* is an essential fracture parameter that characterizes the stress and strain rate field intensity at a crack tip for materials undergoing creep deformation, and is also a crucial parameter describing the creep crack growth rate. This study proposes a method for determining the characteristic parameters of the creep displacement rate model for specimens with mode-I crack under various constraints. Based on the displacement rate model, a semi-analytical model of the C*-integral related to the displacement rate, load, specimen sizes, and Norton's law parameters of materials is proposed. Furthermore, finite element analysis (FEA) models were established for compact tension (CT), single-edged notched bending (SEB), C-shaped tension (CST), and single-edge notched tension (SET) specimens, and the proposed models were verified using FEA results and those predicted by the ASTM method. The explicit correlation between the creep displacement rate and crack length a in the model can contribute to developing novel testing methods for real-time crack length and creep crack growth rate.

On-Demand Transformation of Carbon Dioxide into Polymers Enabled by a Comb-Shaped Metallic Oligomer Catalyst

In addition to harsh conditions, CO2 is an excess in most chemical transformations due to its sluggish reactivity. Herein, we realize the on-demand transformation of CO2 in the synthesis of poly(ether carbonate) polyols through a comb-shaped metallic oligomer catalyst, displaying quantitative reaction and product specificity. In contrast to conventional single-site catalysts that suffer from direct depolymerization long before quantitative CO2 conversion, the metallic oligomer catalyst anchoring multiple aluminum porphyrin complexes at the side chain maintains excellent polyol selectivity (>99%), exhibits high polymerization rate (turnover number of 50,000), and exquisitely places all CO2 monomers in the middle of the polyol backbone with oligoetherol end groups. The composition of polyol exactly corresponds to initial CO2 feed ratios of 0.07–0.39. Further studies on the kinetic rate law and activation parameters of key intermediates decipher the multimetallic synergistic catalysis. Hence, this work offers a fresh view of CO2 as an indeed countable monomer rather than ambiguous pressure as normally considered.

Oxidation Kinetics of Magnesium‐Manganese Oxides for High‐Temperature Thermochemical Energy Storage

In this article, the high‐temperature (≥1000 °C) oxidation kinetics of porous magnesium‐manganese oxide structures considered for large‐scale thermochemical energy storage are determined. For this analysis, oxides with Mn/Mg molar ratios of 2/3, 1/1, and 2/1 are synthesized via solid‐state reaction and crushed to a powder with particle sizes ranging from 125 to 180 μm. The powder is thermally reduced at 1500 °C inside a heated alumina tube under argon flow. Subsequently, the resulting porous bed is oxidized at temperatures between 1000 and 1500 °C with an oxygen to argon molar ratio of 1:4 (leading to an oxygen partial pressure of 0.2 atm). An Arrhenius‐type kinetic rate law is derived using the theory of internal oxidation assuming spherical particles. Subsequently, the kinetic rate law parameters are identified by comparing measured oxygen flow at the reactor outlet to the oxygen output computed using a 1D plug flow reactor model incorporating the postulated kinetic model. Results indicate that the derived bulk kinetic rate law describes the measured oxidation kinetics well and is suited for designing thermochemical energy storage modules based on magnesium‐manganese oxides.

Synthesis of multilayer polymer-immobilised nanosilver for catalytic study in condensation reaction of aniline and acetylacetone

Silver nanoparticles were synthesized in green route with non-hazardous polyvinyl alcohol and polyvinylpyrrolidone as stabilizing as well as reducing agents and water as green solvent. This silver nanoparticle-embedded polymer composite film, Ag/PVA–PVP was characterized by UV–Vis spectroscopy, SEM and TEM. The catalytic activity of this film was evaluated in reduction of p-nitrophenol, acylation of aniline and synthesis of a β-enaminone, 4-phenylamino-pent-3-en-one with appreciably good results. β-enaminone synthesis reaction was chosen to study the effects of kinetic parameters such as reactant quantity, catalyst loading, solvents and temperatures. In a typical reaction, 89% conversion was achieved. A probable chemical reaction mechanism is suggested. Kinetic model fitting for the synthesis of β-enaminone reaction was done for the first time here. Heterogeneous kinetic model following Eley-Rideal pathway showed an excellent data fitting for the reaction. The rate law parameters were estimated. Scheme. Synthesis and catalytic application of multilayer polymer-immobilised nanosilver in condensation reaction of aniline and acetylacetone.

Dioxygen Reduction to Hydrogen Peroxide by a Molecular Mn Complex: Mechanistic Divergence between Homogeneous and Heterogeneous Reductants.

The selective electrocatalytic reduction of dioxygen (O2) to hydrogen peroxide (H2O2) could be an alternative to the anthraquinone process used industrially, as well as enable the on-demand production of a useful chemical oxidant, obviating the need for long-term storage. There are challenges associated with this, since the two-proton/two-electron reduction of H2O2 to two equivalents of water (H2O) or disproportionation to O2 and H2O can be competing reactions. Recently, we reported a Mn(III) Schiff base-type complex, Mn(tbudhbpy)Cl, where 6,6'-di(3,5-di- tert-butyl-2-phenolate)-2,2'-bipyridine = [tbudhbpy]2-, which is active for the electrocatalytic reduction of O2 to H2O2 (ca. 80% selectivity). The less-than-quantitative selectivity could be attributed in part to a thermal disproportionation reaction of H2O2 to O2 and H2O. To understand the mechanism in greater detail, spectrochemical stopped-flow and electrochemical techniques were employed to examine the catalytic rate law and kinetic reaction parameters. Under electrochemical conditions, the catalyst produces H2O2 by an ECCEC mechanism with appreciable rates down to overpotentials of 20 mV and exhibits a catalytic response with a strong dependence on proton donor p Ka. Mechanistic studies suggest that under spectrochemical conditions, where the homogeneous reductant decamethylferrocene (Cp*2Fe) is used, H2O2 is instead produced via a disproportionation pathway, which does not show a strong acid dependence. These results demonstrate that differences in mechanistic pathways can occur for homogeneous catalysts in redox processes, dependent on whether an electrode or homogeneous reductant is used.

A heterogeneous chemical reactor analysis and design laboratory: The kinetics of ammonia decomposition

A laboratory module for senior-level reaction engineering/reactor design students is described. Students use low-conversion experimental data to explore and characterize the kinetics of ammonia decomposition over various supported catalysts at atmospheric pressure in a packed-bed reactor. Each student team is assigned one of four catalyst types, a reactor temperature, and a series of feed flow rates and compositions. Aggregate data from all student groups is then summarily analyzed per catalyst type. In each experimental trial, the reactor conversion is determined by a thermal conductivity measurement applied to the feed (reactor bypass) and reactor effluent gases. An analysis of the reaction rate across a range of temperatures and varying feed gas partial pressures allows students to test various reaction mechanisms, to suggest rate-determining steps, and to statistically determine rate law parameters. Students typically use the Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach to derive rate law expressions, and determine rate constants through application of the Arrhenius equation. High student numbers (ca. 140) are accommodated through the availability of four experimental stations — each sharing a common source of feed gas and equipped with independent flow controllers and gas analyzers.

Cast composite solid propellants with different combustion stabilizers

The effect of four different compounds as the combustion stabilizers of composite solid propellants based on the AP/HTPB/IPDI has been presented in this paper. Except that, various total solid content, ammonium-perchlorate bimodal and trimodal combination of particle sizes ratio have been made by preparing the eleven formulations. For all of them, the burning rate law's parameters and temperature sensitivity, uniaxial mechanical characteristics, apparent viscosity of uncured propellants and physic-chemical and energetic values have been determined and compared.

Will Coral Islands maintain their growth over the next century? A deterministic model of sediment availability at Lady Elliot Island, Great Barrier Reef.

A geomorphic assessment of reef system calcification is conducted for past (3200 Ka to present), present and future (2010–2100) time periods. Reef platform sediment production is estimated at 569 m3 yr−1 using rate laws that express gross community carbonate production as a function of seawater aragonite saturation, community composition and rugosity and incorporating estimates of carbonate removal from the reef system. Key carbonate producers including hard coral, crustose coralline algae and Halimeda are mapped accurately (mean R2 = 0.81). Community net production estimates correspond closely to independent census-based estimates made in-situ (R2 = 0.86). Reef-scale outputs are compared with historic rates of production generated from (i) radiocarbon evidence of island deposition initiation around 3200 years ago, and (ii) island volume calculated from a high resolution island digital elevation model. Contemporary carbonate production rates appear to be remarkably similar to historical values of 573 m3 yr−1. Anticipated future seawater chemistry parameters associated with an RCP8.5 emissions scenario are employed to model rates of net community calcification for the period 2000–2100 on the basis of an inorganic aragonite precipitation law, under the assumption of constant benthic community character. Simulations indicate that carbonate production will decrease linearly to a level of 118 m3 yr−1 by 2100 and that by 2150 aragonite saturation levels may no longer support the positive budgetary status necessary to sustain island accretion. Novel aspects of this assessment include the development of rate law parameters to realistically represent the variable composition of coral reef benthic carbonate producers, incorporation of three dimensional rugosity of the entire reef platform and the coupling of model outputs with both historical radiocarbon dating evidence and forward hydrochemical projections to conduct an assessment of island evolution through time. By combining several lines of evidence in a deterministic manner, an assessment of changes in carbonate production is carried out that has tangible geomorphic implications for sediment availability and associated island evolution.

Kinetic modeling of laboratory CO2‐exposure experiments performed on whole rock reservoir samples

AbstractMineralogical and geochemical observations from laboratory CO2‐exposure experiments on reservoir rocks are compared with predictions from geochemical modeling that was performed using PHREEQC software. The Pitzer‐based Eq 3/6 thermodynamic database, provided by Quintessa Ltd., was applied. For kinetic modeling, a Lasaga‐type rate equation was implemented and different models were parameterized taking kinetic rate law parameters from literature. Based on previous modeling studies a modified inverse modeling approach is presented here. This comprises several different Fe‐proxies and improved statistical ranking preferences that were implemented in particular to better match modeled and measured concentrations of dissolved K+, Fe2+ and Al3+. Compared to the previous approach, the presented modeling results are in good (better) agreement with experimental data. Systematic discrepancies between modeling and observation still occur regarding K‐bearing mineral phases and corresponding K+ brine concentrations. Despite missing correlation between K+ and Cl− concentrations, potential reasons for these discrepancies may be increased K+ brine concentrations during the experiments due to dissolution of K‐rich salt(s), such as sylvite. Much better matches were generated for dissolved Fe2+ concentrations. Goethite mainly controls the chemical behavior of dissolved Fe2+ in kinetic simulations. Based on both the available equilibrium and kinetic modeling results, the ultimate fate of dissolved Al3+ and the analysis of Al‐bearing mineral phases potentially controlling dissolved Al3+ brine concentrations cannot be conclusively determined. The overall best ranked kinetic model comprises anhydrite, dolomite, goethite, K‐feldspar and kaolinite. Despite minor inconsistencies dissolved Fe2+, Al3+ and Si4+ were in particular much better reproduced by the best ranked kinetic models.

Comparison of the kinetic rate law parameters for the dissolution of natural and synthetic autunite in the presence of aqueous bicarbonate ions

This research evaluated the effect of aqueous hydrogen carbonate solutions on the uranium rate of release from natural Ca-autunite and quantified the process kinetic rate law for a better prediction of the stability of autunite-group minerals. Testing was accomplished via a single-pass flow-through (SPFT) apparatus using buffered aqueous bicarbonate solutions (0.0005 to 0.003M) at temperatures of 23–90°C and pH values of 7–11. The release rate of uranium from Ca-autunite was directly correlated to increasing concentrations of hydrogen carbonate solutions and showed strong pH dependency. Ca-autunite kinetic rate law parameters were compared to the values obtained for synthetic Na-autunite. The power law coefficient and intrinsic rate constant were higher at pH9–11 for Ca-autunite than for Na-autunite. The lower stability of Ca-autunite was attributed to the high Ca-autunite surface cracking, fractures and basal plane cleavages as compared to Na-autunite and the combined effect of the formation of aqueous uranyl–carbonate and calcium uranyl carbonate species as a driving force for uranium(VI) detachment and the formation of secondary Ca–P hydroxyapatite and uranyl phosphate mineral phases as a driving force for phosphate and calcium detachment controlling the net release of elements.

Quantification of kinetic rate law parameters of uranium release from sodium autunite as a function of aqueous bicarbonate concentrations

Environmental context Uranium is a key contaminant of concern because of its high persistence in the environment and toxicity to organisms. The bicarbonate ion is an important complexing agent for uranyl ions and one of the main variables affecting its dissolution. Results from this investigation provide rate law parameters for the dissolution kinetics of synthetic sodium autunite that can influence uranium mobility in the subsurface. Abstract Hydrogen carbonate (also known as bicarbonate) is one of the most significant components within the uranium geochemical cycle. In aqueous solutions, bicarbonate forms strong complexes with uranium. As such, aqueous bicarbonate may significantly increase the rate of uranium release from uranium minerals. Quantifying the relationship of aqueous bicarbonate solutions to the rate of uranium release during dissolution is critical to understanding the long-term fate of uranium within the environment. Single-pass flow-through experiments were conducted to estimate the rate of uranium release from Na meta-autunite as a function of bicarbonate solutions (0.0005–0.003M) over the pH range of 6–11 and temperatures of 5–60°C. Consistent with the results of previous investigations, the rate of uranium release from sodium autunite exhibited minimal dependency on temperature, but was strongly dependent on pH and increasing concentrations of bicarbonate solutions. Most notably at pH 7, the rate of uranium release exhibited a 370-fold increase relative to the rate of uranium release in the absence of bicarbonate. However, the effect of increasing concentrations of bicarbonate solutions on the release of uranium was significantly less under higher pH conditions. It is postulated that at high pH values, surface sites are saturated with carbonate, thus the addition of more bicarbonate would have less effect on uranium release. Results indicate that the activation energies were unaffected by temperature and bicarbonate concentration variations, but were strongly dependent on pH conditions. As the pH increased from 6 to 11, the activation energy values were observed to decrease from 29.94 to 13.07kJmol–1. The calculated activation energies suggest a surface controlled dissolution mechanism.

A facile synthesis of nearly monodisperse ruthenium nanoparticles and their catalysis in the hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage

A facile method for the preparation of nearly monodisperse Ru nanoparticles (NPs) and their record catalytic activity in the hydrolytic dehydrogenation of ammonia borane (AB) for chemical hydrogen storage are reported herein. Ru NPs were prepared by a novel protocol comprising the thermal decomposition and concomitant reduction of ruthenium(III) acetylacetonate (Ru(acac)3) in the presence of oleylamine (OAm) and benzylether (BE). In the protocol, OAm served as both a stabilizer and reducing agent and BE used as a solvent. Ru NPs were supported on aluminum oxide nanopowder (Ru@Al2O3) via a simple liquid impregnation method before their use as catalyst for the dehydrogenation of AB in water. Without any special treatment to remove the surfactants, Ru@Al2O3 showed high catalytic activity in the hydrolytic dehydrogenation of AB providing an initial turnover frequency (TOF) of 39.6molH2(molRumin)−1. Their catalytic performance was further enhanced dramatically by the acetic acid treatment and the initial TOF value is increased to 83.3molH2(molRumin)−1 that is the highest among the reusable Ru catalysts and even higher than Pt-based catalysts tested in the hydrolysis of AB. Additionally, the work reported here includes a wealth of kinetic data to determine the rate law and apparent activation parameters for the catalytic dehydrogenation of AB in water.

A streamline approach to two-dimensional steady non-ideal detonation: the straight streamline approximation

Detonations in non-ideal explosives tend to propagate significantly below the nominal one-dimensional detonation speed. In these cases, multi-dimensional effects within the reaction zone are important. A streamline-based approach to steady-state non-ideal detonation theory is developed. It is shown in this study that, given the streamline shapes, the two-dimensional problem reduces to an ordinary differential equation eigenvalue problem along each streamline, the solution of which determines the local shock shape that, in turn, leads to the solution of the detonation speed as a function of charge diameter. A simple approximation of straight but diverging streamlines is considered. The results of the approximate theory are compared with those of high-resolution direct numerical simulations of the problem. It is shown that the straight streamline approximation is remarkably predictive of highly non-ideal explosive diameter effects. It is even predictive of failure diameters. Given this predictive capability, one potential use of the method is in the determination of rate law parameters by fitting to data from unconfined rate stick experiments. This is illustrated by using data for ammonium nitrate fuel oil explosives.

High-Temperature Photocatalytic Ethylene Oxidation over TiO2

Photocatalytic oxidation of ethylene was conducted in a novel two-dimensional packed bed reactor at higher temperatures (60 °C < T < 520 °C) than previously reported. Degussa P25 TiO2 was used as a photocatalyst; samples of this TiO2 were heat treated to different temperatures to obtain different phase compositions, surface areas, and surface hydroxyl concentrations. Maximum photocatalytic ethylene oxidation rates were observed between 100 and 200 °C. Further, oxidation rates greater than 75% of the maximum oxidation rate were observed over a large temperature range: 60 °C < T < 300 °C. At higher temperatures (T > 300 °C), decreasing oxidation rates were attributed to the nonradiative, multiphonon recombination of photogenerated charge carriers. Regression analysis of a nonlinear, multiphonon recombination model and a simplified chemical rate law were used to determine apparent reaction rate law parameters for the photocatalytic oxidation of ethylene and recombination parameters for the TiO2 photocatalyst. Insights for the development and operation of novel high-temperature photocatalysts are also discussed.

An enantioselective Claisen rearrangement catalyzed by N-heterocyclic carbenes.

In the presence of a chiral azolium salt (10 mol %), enols and ynals undergo a highly enantioselective annulation reaction to form enantiomerically enriched dihydropyranones via an N-heterocyclic carbene catalyzed variant of the Claisen rearrangement. Unlike other azolium-catalyzed reactions, this process requires no added base to generate the putative NHC-catalyst, and our investigations demonstrate that the counterion of the azolium salt plays a key role in the formation of the catalytically active species. Detailed kinetic studies eliminate a potential 1,4-addition as the mechanistic pathway; the observed rate law and activation parameters are consistent with a Claisen rearrangement as the rate-limiting step. This catalytic system was applied to the synthesis of enantioenriched kojic acid derivatives, a reaction of demonstrated synthetic utility for which other methods for catalytic enantioselective Claisen rearrangements have not provided a satisfactory solution.

Electrochemical Oxidation of Saccharose on Copper (Hydr)oxide-Modified Electrode in Alkaline Media

A stable copper (hydr)oxide-modified electrode was prepared in 0.5 mol/L NaOH solution by cyclic voltammetry in the range of −250 to 1000 mV. It can be used for electrochemical studies in the range of −250 to 1 000 mV without interfering peaks because there is no oxidation of copper. During an anodic potential sweep, the electro-oxidation of saccharose on Cu occurred by the formation of Cu III and this reaction also occurred in the early stages of the reversed cycle until it is stopped by the negative potentials. A mechanism based on the electro-chemical generation of Cu III active sites and their subsequent consumption by saccharose was proposed, and the rate law and kinetic parameters were obtained. The charge transfer resistance from theoretical and impedance studies was used to verify the mechanism. Under chronoamperometry regimes, the reaction followed Cottrellian behavior. The transfer of up to 21 electrons was observed in further investigations of the electro-oxidation of saccharose on a (hydr)oxide Cu rotating disk electrode. The electrochemical oxidation of saccharose on a copper (hydr)oxide-modified electrode by the electrochemical generation of Cu III active sites and their consumption by saccharose is discussed.

Dimethylammonium Hexanoate Stabilized Rhodium(0) Nanoclusters Identified as True Heterogeneous Catalysts with the Highest Observed Activity in the Dehydrogenation of Dimethylamine−Borane

Herein we report the discovery of a superior dimethylamine-borane dehydrogenation catalyst, more active than the prior best heterogeneous catalyst (Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 9776) reported to date for the dehydrogenation of dimethylamine-borane. The new catalyst system consists of rhodium(0) nanoclusters stabilized by C(5)H(11)COO(-) anions and Me(2)H(2)N(+) cations and can reproducibly be formed from the reduction of rhodium(II) hexanoate during dehydrogenation of dimethylamine-borane at room temperature. Rhodium(0) nanoclusters in an average particle size of 1.9 +/- 0.6 nm Rh(0)(approximately 190) nanoclusters) provide 1040 turnovers over 26 h with a record initial turnover frequency (TOF) of 60 h(-1) (the average TOF value is 40 h(-1)) in the dehydrogenation of dimethylamine-borane, yielding 100% of the cyclic product (Me(2)NBH(2))(2) at room temperature. The work reported here also includes the full experimental details of the following major components: (i) Characterization of dimethylammonium hexanoate stabilized rhodium(0) nanoclusters by using TEM, STEM, EDX, XRD, UV-vis, XPS, FTIR, (1)H, (13)C, and (11)B NMR spectroscopy, and elemental analysis. (ii) Collection of a wealth of previously unavailable kinetic data to determine the rate law and activation parameters for catalytic dehydrogenation of dimethylamine-borane. (iii) Monitoring of the formation kinetics of the rhodium(0) nanoclusters by a fast dimethylamine-borane dehydrogenation catalytic reporter reaction (Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382) at various [Me(2)NH.BH(3)]/[Rh] ratios and temperatures. Significantly, sigmoidal kinetics of catalyst formation was found to be well fit to the two-step, slow nucleation and then autocatalytic surface growth mechanism, A --> B (rate constant k(1)) and A + B --> 2B (rate constant k(2)), in which A is [Rh(C(5)H(11)CO(2))(2)](2) and B is the growing, catalytically active rhodium(0) nanoclusters. (iv) Mercury(0) and CS(2) poisoning and nanofiltration experiments to determine whether the dehydrogenation of dimethylamine-borane catalyzed by the dimethylammonium hexanoate stabilized rhodium(0) nanoclusters is homogeneous or heterogeneous catalysis.

An experimental study of the dissolution rates of simulated aluminoborosilicate waste glasses as a function of pH and temperature under dilute conditions

Abstract Single-pass flow-through tests were conducted to determine the pH (7–12) and temperature (23–90 °C) dependence of kinetic rate law parameters; ko, η, and Ea, for the dissolution of glass in aqueous solution. Experiments were performed with three prototypic nuclear waste glasses that span a wide compositional range, which covers, with high probability, the expected processing composition range for candidate immobilized low-activity waste (ILAW) glasses. Comparison of the B to Na release rates for one glass was incongruent at 23 and 40 °C, and pH (23 °C) = 7.0 and 8.0, suggesting two distinct mechanisms are responsible for the Na+ release, namely Na+–H+ ion-exchange and matrix dissolution. Matrix dissolution became the dominant dissolution mechanism for all glasses at the forward rate and pH values greater than 9.0 as evident by the congruent release of Al, B, Na and Si to solution. By combining the results collected for each ILAW glass at pH values greater than 9.0, pH and temperature-dependent rate law parameters were determined for Al, B, Na and Si release. A comparison of the pH power-law coefficient for Al, B, Na and Si at each temperature suggest that η does not depend on temperature within experimental error and suggests the release of these elements into solution is controlled by the same dissolution mechanism at the forward rate of reaction. The activation energies (Ea), based on B release, range from 52 ± 4 to 56 ± 6 kJ/mol which suggest that dissolution is a surface-controlled reaction mechanism. The data presented in this manuscript suggest that for these three ILAW glasses the chemical durability for each glass is similar under these test conditions. A lack of compositional dependence on the forward dissolution rate is observed even though there is as much as a 39 kJ/mol difference in the free energy of hydration (ΔGhyd) among the borosilicate waste glasses tested. This similarity in the forward dissolution rate despite the large ΔGhyd difference is almost certainly because these glasses have similar, if not identical, polymerization states. This is evident from the almost identical 29Si chemical shifts for each of these glasses. The polymerization state is an indication of the number of framework SiO4 linkages contained in the glass network. In general, the greater the number of framework SiO4 linkages the more durable the glass. Finally, in agreement with previous work, these results suggest breakage of the Si–O bond is the rate-determining dissolution mechanism under alkaline conditions [pH (23 °C) > 9.0] far from saturation with respect to an alteration phase or phases.