First-order Reaction Rate Constants Research Articles

Characterization and hydrodeoxygenation properties of Co promoted Ni–Mo–B amorphous catalysts: influence of Co content

Different contents of promoter (Co) in Co–Ni–Mo–B amorphous catalysts were prepared by chemical reduction of the precursors of metal salts with a sodium borohydride aqueous solution. The catalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Adding a proper content of the promoter Co into Ni–Mo–B amorphous catalyst could increase the MoO2 content and decrease the particle size, but doping excess Co would cover some of the active sites and increase the particle size of the catalysts. The effect of Co content on the catalytic hydrodeoxygenation activity of the amorphous catalysts was studied using phenol as a model compound. The main route for the HDO of phenol on these amorphous catalysts was hydrogenation–dehydration and the content of aromatic compounds in HDO products was decreased obviously. The pseudo first-order reaction rate constant of the phenol HDO on Co–Ni–Mo–B amorphous catalyst was much greater than that of MoS2 when adding proper promoter Co.

Catalytic activity of rare earth oxides in flameless methane combustion

The following rare earth oxides, which were prepared by the precipitation of hydroxides, were studied as catalysts for the reaction of complete methane oxidation: CeO2, a hydrated phase of La2O3, Pr6O11, Tb4O7, and Gd2O3. The catalytic activities of the oxides were compared in terms of first-order reaction rate constants per unit surface area of the catalyst. With consideration for data on the reduction of CeO2, Pr6O11, and Tb4O7, the previously proposed redox reaction mechanism on transition metal oxides was supported. The high activity of hydrated La2O3 was found, and it was hypothesized that, in this case, the process occurred by the mechanism of oxidative methane condensation followed by the rapid oxidation of the resulting intermediate products.

Rheological behavior and color stability of anthocyanins from Merlot (Vitis vinifera L.) and Bordô (Vitis labrusca L.) grapes in a jam model system

Anthocyanins are the pigments responsible for the color of most red grapes and are easily degraded following various reaction mechanisms affected by oxygen, enzymes, pH, and temperature among other variables. In this study, a jam model system was developed using Merlot and Bordô grape extracts and polysaccharides (xanthan and locust bean gums) and different temperatures (45, 55 and 65 °C). The stability of the anthocyanin pigments and the rheological behavior of the jam model system were studied. For the determination of the stability, the half-life time and first-order reaction rate constants for the anthocyanin pigments were calculated. The rheological behavior was determined through the Power law model. The jam model system produced using a temperature of 45 °C showed the best results for the anthocyanin half-life time. The first-order reaction rate constants for the 45, 55, and 65 °C treatments were not significantly different among each other (p > 0.05). It was observed that with an increase in the jam model system temperature there was an increase in the index of consistency.

Development of HPLC and UV spectrophotometric methods for the determination of ascorbic acid using hydroxypropyl-β-cyclodextrin and triethanolamine as photostabilizing agents

In this study, the effect of complex formation with triethanolamine (TEA) alone and in combination with hydroxypropyl-β-cyclodextrin (HP-β-CD) on the photostability of ascorbic acid was evaluated for exposure to artificial and diffuse daylight. The first-order rate constants for the photodegradation reactions were determined. The data obtained showed that these complexes strongly reduced the photodegradation process with an 11- and 35-fold increase in the photostability of ascorbic acid, depending of the ligand concentration and the irradiation source. The multicomponent complex gave a significantly better stabilization for exposure to light than TEA alone. Due to the fact that the complexation extended the exposure of ascorbic acid to light (without molecular changes), UV spectrophotometric and reversed phase high performance liquid chromatographic (HPLC) methods were developed for the quantitative determination of the vitamin in pure form and in pharmaceutical preparations. These methods were statistically validated, all the validation parameters were found to be within the acceptance range. These results demonstrate that the proposed methods are suitable for the quality control of ascorbic acid, providing simple, rapid, precise, accurate and convenient approaches for routine analysis of bulk drug and pharmaceutical formulations.

Quantitative Production of Compound I from a Cytochrome P450 Enzyme at Low Temperatures. Kinetics, Activation Parameters, and Kinetic Isotope Effects for Oxidation of Benzyl Alcohol

Cytochrome P450 enzymes are commonly thought to oxidize substrates via an iron(IV)-oxo porphyrin radical cation transient termed Compound I, but kinetic studies of P450 Compounds I are essentially nonexistent. We report production of Compound I from cytochrome P450 119 (CYP119) in high conversion from the corresponding Compound II species at low temperatures in buffer mixtures containing 50% glycerol by photolysis with 365 nm light from a pulsed lamp. Compound I was studied as a reagent in oxidations of benzyl alcohol and its benzylic mono- and dideuterio isotopomers. Pseudo-first-order rate constants obtained at -50 degrees C with concentrations of substrates between 1.0 and 6.0 mM displayed saturation kinetics that gave binding constants for the substrate in the Compound I species (K(bind)) and first-order rate constants for the oxidation reactions (k(ox)). Representative results are K(bind) = 214 M(-1) and k(ox) = 0.48 s(-1) for oxidation of benzyl alcohol. For the dideuterated substrate C(6)H(5)CD(2)OH, kinetics were studied between -50 and -25 degrees C, and a van't Hoff plot for complexation and an Arrhenius plot for the oxidation reaction were constructed. The H/D kinetic isotope effects (KIEs) at -50 degrees C were resolved into a large primary KIE (P = 11.9) and a small, inverse secondary KIE (S = 0.96). Comparison of values extrapolated to 22 degrees C of both the rate constant for oxidation of C(6)H(5)CD(2)OH and the KIE for the nondeuterated and dideuterated substrates to values obtained previously in laser flash photolysis experiments suggested that tunneling could be a significant component of the total rate constant at -50 degrees C.

Roof-harvested rainwater for potable purposes: application of solar disinfection (SODIS) and limitations

Efficiency of solar disinfection (SODIS) was evaluated for the potability of rainwater in view of the increasing water and energy crises especially in developing countries. Rainwater samples were collected from an underground storage tank in 2 L polyethylene terephthalate (PET) bottles and SODIS efficiency was evaluated at different weather conditions. For optimizing SODIS, PET bottles with different backing surfaces to enhance the optical and thermal effects of SODIS were used and different physicochemical parameters were selected and evaluated along with microbial re-growth observations and calculating microbial decay constants. Total and fecal coliforms were used along with Escherichia Coli and Heterotrophic Plate Counts (HPC) as basic microbial and indicator organisms of water quality. For irradiance less than 600 W/m(2), reflective type PET bottles were best types while for radiations greater than 700 W/m(2), absorptive type PET bottles offered best solution due to the synergistic effects of both thermal and UV radiations. Microbial inactivation did not improve significantly by changing the initial pH and turbidity values but optimum SODIS efficiency is achieved for rainwater with acidic pH and low initial turbidity values by keeping air-spaced PET bottles in undisturbed conditions. Microbial re-growth occurred after one day only at higher turbidity values and with basic pH values. First-order reaction rate constant was in accordance with recent findings for TC but contradicted with previous researches for E. coli. No microbial parameter met drinking water guidelines even under strong experimental weather conditions rendering SODIS ineffective for complete disinfection and hence needed more exposure time or stronger sunlight radiations. With maximum possible storage of rainwater, however, and by using some means for accelerating SODIS process, rainwater can be disinfected and used for potable purposes.

Sorption and Biodegradation of 17β-Estradiol by Acclimated Aerobic Activated Sludge and Isolation of the Bacterial Strain

Abstract Batch experiments were conducted to investigate the sorption and biodegradation behavior of 17β-estradiol (E2) at low μg·L−1 levels by acclimated aerobic activated sludge as well as the effect of temperature on the sorption and biodegradation. Results showed that E2 was completely transformed after 2 h under aerobic conditions. E2 attenuation was achieved by sorption onto the sludge and the subsequent biodegradation by micro-organisms in the activated sludge. Sorption of E2 onto both the inactivated sludge and the activated sludge was well described by both the Freundlich and linear isotherms. The biodegradation of E2 by the acclimated activated sludge can be described by first-order reaction kinetics with the first-order reaction rate constant of 3.49 h−1 at 20°C. The sorption isotherms and biodegradation kinetics indicated that partitioning played a dominant role in the sorption of E2 on the activated sludge. Temperature exerted significant effects on both the sorption and biodegradation of E2....

Indirect electrochemical reduction of perchlorate and nitrate in dilute aqueous solutions at the Ti–water interface

Perchlorate and nitrate are contemporary contaminants in drinking water. In most perchlorate containing surface or ground waters, nitrate was also present at significant concentrations. The present study investigated the removal of perchlorate and nitrate at the Ti–water interface by indirect electrochemical reduction process. Results indicated that perchlorate and nitrate could be reduced readily at the surface of a Ti anode simultaneously. Upon the application of an anodic current at the Ti electrode, Ti species such as Ti(III) or Ti(II) were generated. These multivalent Ti species are strong reducing agent that can reduce perchlorate and nitrate ion. The dominant end product of the indirect electrochemical reduction was chloride or nitrite. However, when these two anions were co-existing, the nitrite concentration was negligible. The reduction for both anions followed typical Langmuir–Hinshelwood kinetics; zero-order and first-order reaction at high and low concentrations, respectively. The first-order reaction rate constant was in the range of 10 −5 s −1. Perchlorate and nitrate at initial concentration of 200 and 1000 ppm individually were reduced to final concentration of <20 and <200 ppb, respectively, over a short reaction time of 6–8 h. The reaction mechanism was studied with the aid of surface analysis of the solid end products (e.g., TiO 2 particles) remaining on the Ti electrode using XPS and SEM/EDX. It was observed that doping of chlorine and nitrogen atoms onto the solid TiO 2 particles contributed partially to imbalance of the total Cl and N mass in the system. Additionally, formation of volatile chlorine and hypochlorite species brought mass imbalance to total chlorine.

Kinetics of the reactions of soot surface-bound polycyclic aromatic hydrocarbons with NO2

The kinetics of heterogeneous reactions of NO2 with 17 polycyclic aromatic hydrocarbons (PAHs) adsorbed on laboratory generated kerosene soot surface was studied over the temperature range (255–330) K in a low pressure flow reactor combined with an electron-impact mass spectrometer. The kinetics of soot-bound PAH consumption due to their desorption and reaction with NO2 were monitored using off-line HPLC measurements of their concentrations in soot samples as a function of reaction time, NO2 concentrations in the gas phase being analyzed by mass spectrometer. No measurable decay of PAHs due to the reaction with NO2 was observed under experimental conditions of the study (maximum NO2 concentration of 5.5 × 1014 molecule cm−3 and reaction time of 45 min), which allowed to determine the upper limits of the first-order rate constants for the heterogeneous reactions of 17 soot-bound PAHs with NO2: k < 5.0 × 10−5 s−1 (for most PAHs studied). Comparison of these results to previous studies carried on different carbonaceous substrates, showed that heterogeneous reactivity of PAHs towards NO2 is, probably, dependent on the substrate nature even for resembling, although different carbonaceous materials. Results show that particulate PAHs degradation by NO2 alone is of minor importance in the atmosphere

Regularities in the stabilization by hemoglobin of binuclear iron complexes [Fe2(μ-N—C—SR)2(NO)4] containing benzimidazolylthiol and benzothiazolylthiol ligands

The NO-donor ability of new binuclear tetranitrosyl complexes of the μ-N—C—S type, namely, bis(5-methylbenzimidazol-2-ylthio)- (1), bis(benzimidazol-2-ylthio)- (2), and bis(benzothiazol-2-ylthio)(tetranitrosyl)diiron (3), was studied in aqueous solutions by spectrophotometry. All kinetic regularities obtained for complexes 1–3 are well described in terms of formalism of pseudo-first-order reactions. The apparent first-order reaction rate constants for NO evolution by the complexes to solution were determined. Complexes 1–3 are good donors of NO. The structures of the complexes and the effect of their stabilization by hemoglobin were compared. The stabilization effect is explained by different basicities of the sulfur-containing ligands in the complexes studied.

Absorption of carbon dioxide into aqueous potassium carbonate promoted by boric acid

Abstract The absorption of carbon dioxide with alkanolamine or potassium carbonate solvents has gained widespre ad acceptance for the removal of CO2 from natural gas and H 2. However, alkanolamine solutions are prone to oxidative degradation at high temperature. The main advantages of potassium carbonate solution for CO2 removal are the high chemical solubility of CO2 in the carbonate/ bicarbonate system, low solvent costs and the low energy requirement for solvent regeneration. The major challenge concerning absorption of CO2 into aqueous solutions of potassium carbonate is a relatively slow rate of reaction in the liquid phase causing low mass transfer rates. It is often advantageous to add a promoter to increase the absorption rate. While piperazine is often used for this purpose, there are some environmental health concerns with this approach. In this work we consider boric acid as an alternative promoter. Absorption of CO2 in a wetted -wall column by aqueous potassium carbonate solution with and without various concentrations of boric acid as promoter was measured under conditions in which the reaction of CO2 was of pseudo-first order. The equilibrium partial pressure and the rate of absorption of CO2 were measured in 30.0 wt% potassium carbonate and 1.0 to 5.0 wt% of boric acid at 50 to 80 ∘C. The overall pseudo -first-order reaction rate constants were determined fro m the kinetic measurements. The addition of small amounts of boric acid to potassium carbonate resulted in a significant enhancement of CO2 absorption rates.

Kinetics study of carbon dioxide absorption into aqueous solutions containing N-methyldiethanolamine + diethanolamine

Kinetics of the absorption of CO 2 into N-methyldiethanolamine (MDEA) + diethanolamine (DEA) + water were investigated at 30, 35, and 40 °C using a wetted wall column apparatus. Eight systems of which 1.0 and 1.5 kmol/m 3 MDEA mixed with various DEA concentrations (0.1, 0.2, 0.3, and 0.4) kmol/m 3 were studied. Densities and viscosities of eight blended amine systems were measured. The solubilities and diffusivities of N 2O in the studied amine systems were also measured. The N 2O analogy was applied to estimate the solubilities and diffusivities of CO 2 in the investigated amine systems. Based on the pseudo first-order for the CO 2 absorption, the overall pseudo first-order reaction rate constants were determined from the measured kinetic data. The addition of small amounts of DEA to MDEA results in a significant enhancement of CO 2 absorption rates. A hybrid reaction rate model, a zwitterion mechanism for DEA and a first-order reaction for MDEA was used to model the kinetic data. The model satisfactory represented the CO 2 absorption rates in MDEA + DEA aqueous systems.

Kinetics of the Absorption of Carbon Dioxide into Mixed Aqueous Solutions of Piperazine and Triethanolamine

The reaction kinetics of the absorption of CO2 into mixed aqueous solutions of piperazine (PZ) and triethanolamine (TEA) are investigated using the wetted wall column at temperatures of 303.2 to 313.2 K. The physical properties such as density and viscosity of the aqueous alkanolamine solutions, and the solubility and diffusivity of the nitrous oxide in aqueous alkanolamine solutions are also measured. The N2O analogy is applied to estimate the solubilities and diffusivities of CO2 in aqueous amine systems. Based on the pseudo-first-order for CO2 absorption, the overall pseudo first-order reaction rate constants were determined from the kinetics measurements. For CO2 absorption into mixed aqueous solutions of PZ and TEA, it is found that the addition of small amounts of PZ to aqueous TEA solutions has a significant effect on the enhancement of the CO2 absorption rate. For the CO2 absorption rate model, a second-order reaction rate model is applied for both reactions of CO2 with PZ and TEA. The overall absolute percentage deviation for the calculation of the apparent rate constant kapp was 4.6% for the kinetics data measured. The model is satisfactory in representing the CO2 absorption into mixed aqueous solutions of PZ and TEA.

A Real-Time Bioluminescent HTS Method for Measuring Protein Kinase Activity Influenced Neither by ATP Concentration Nor by Luciferase Inhibition

The firefly luciferin-luciferase reaction has been used to set up an assay for protein kinase based on measuring ATP consumption rate as the first-order rate constant for the kinase reaction. The assay obviates the problems encountered with previous bioluminescent protein kinase assays such as interference with the luciferase reaction from library compounds, nonlinear standard curves, and limited dynamic ranges. In the assay described in the present paper luciferase and luciferin are present during the entire kinase reaction, and the light emission can be measured continuously. In an HTS situation the light emission is measured only twice, i.e., initially and after a predetermined time. After a fivefold reduction of the ATP concentration a Z' value of 0.96 was obtained. Light emission data from samples with kinase are normalized with light emission data from blanks without kinase. First-order rate constants for the kinase reaction calculated from normalized light emission are not affected by a moderate degree of inactivation of luciferase and luciferin during the measuring time. The constants have the same value at all ATP concentrations much lower than the K(m) of the luciferase and the kinase. These factors make the assay very robust and influenced neither by ATP concentration nor by luciferase inhibition. The measuring time depends on the kinase activity and can be varied from minutes to more than 8 h provided the kinase is stable and the evaporation of water from the wells is acceptable. The assay is linear with respect to kinase activity over three orders of magnitude. The new reagents also allowed us to determine K(m) values for ATP and for Kemptide.

Oscillatory and nonoscillatory behavior of a simple model for cool flames, Sal'nikov's reaction, P → A → B, occurring in a spherical batch reactor with varying intensities of natural convection

When cool flames, or indeed any exothermic chemical reaction, occur in a fluid inside an unstirred vessel, the heat from the reaction often induces temperature gradients and consequently motion, i.e., natural convection. The intensity of the resulting flow is governed by the Rayleigh number (Ra). This work simulates numerically the behavior of Sal'nikov's reaction, P → A → B, under the influence of natural convection in an unstirred spherical reactor. This reaction is the simplest to exhibit the thermokinetic oscillations characterizing cool flames. The behavior of this system can be represented on a three-dimensional regime diagram, whose axes are ratios of the characteristic timescales ( τ) for chemical reaction, diffusion (of both heat and mass), and natural convection. Previous work has identified a region of oscillations on this diagram in the purely diffusive limit, when Ra = 0. This work extends this analysis to the general 3D space, where diffusion and natural convection are both important. A region in which oscillations are observed has been found for fixed values of the first-order rate constants for Sal'nikov's reaction. There is a distinct change in the shape of the region of oscillations around the critical value of Ra ∼ 500, when natural convection becomes important. When diffusion dominates transport (Ra < 500), the boundaries between oscillatory and nonoscillatory solutions are largely independent of the ratio of timescales τ step 2 / τ convection and agree well with the values found previously in the purely diffusive limit. When natural convection is important (Ra > 500), the oscillations occur over a wider range of parameters than is the case for a diffusive system. The presence of natural convection also leads to various, more complex behaviors than are seen in the diffusive or well-mixed limits. A region in the regime diagram was found where the oscillations in temperature and the concentration of A have small amplitudes and a frequency that is quite different from those generated in a well-mixed system. It is possible that these oscillations are caused by natural convection, i.e., are not thermokinetic oscillations produced by the chemical reaction. It was also found that sometimes the oscillations in the temperature and the concentration of A are in phase; more generally they are in anti-phase. The evolution of nonoscillatory behavior with relatively small increases in temperature was found to be always fairly similar, regardless of the intensity of natural convection. The shape of the temperature profile along the vertical axis of the reactor did, however, change with the intensity of natural convection. Finally, the nonoscillatory solutions with a large rise in temperature in the presence of natural convection were found to be very much like those seen in the purely diffusive limit for small times, due to the relatively long induction time (∼3.5 s in a vessel with diameter 0.1 m) for the onset of natural convection.

Controlling the extent of π-backbonding in platinum(ii) terpyridyl systems: a detailed kinetic, mechanistic and computational approach

The rate of substitution of the chloride ligand from [Pt(terpy)Cl](+) () (where terpy = 2,2':6',2''-terpyridine) and its corresponding analogue [Pt((t)Bu(3)terpy)Cl](+) () (where (t)Bu(3)terpy = 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine) by a series of neutral and anionic nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I(-)) and thiocyanate (SCN(-)), was determined under pseudo first-order conditions as a function of concentration and temperature using standard stopped-flow spectrophotometric techniques. The observed pseudo first-order rate constants for the substitution reactions obeyed the simple rate law k(obs) = k(2)[nucleophile]. Second-order kinetics and negative activation entropies support an associative mode of activation. The rate of substitution of chloride is observed to decrease with an increase in the steric bulk of the neutral nucleophiles, whilst rate of substitution by I(-) was observed to be faster than that by SCN(-), in correlation with their polarizability and the softness of the metal centre. A comparison of the second-order rate constants, k(2), at 298 K, obtained for the substitution reactions of and shows that the introduction of strong sigma-donating groups on the periphery of the terpyridyl backbone in results in a corresponding decrease in the reactivity. DFT calculations at the B3LYP/LACVP** level of theory for the two complexes, and , and a series of similar analogues containing either electron-donating or electron-withdrawing groups in the periphery positions demonstrate that the introduction of electron-donating groups decreases the positive charge on the metal centre and increases energy separation of the frontier molecular orbitals (E(HOMO)-E(LUMO)) of the ground state platinum(ii) complexes leading to a less reactive metal centre whilst the introduction of electron-withdrawing groups has an opposite effect leading to increased reactivity of the metal centre.

Limitations of the Transition State Variation Model. Part 8. Dual Reaction Channels for Solvolyses of 3,4-Dimethoxybenzenesulfonyl Chloride

Solvolyses of 3,4-dimethoxybenzenesulfonyl chloride (DSC) in water, D2O, CH3OD, and in aqueous binary mixtures of acetone, acetonitrile, 1,4-dioxane, ethanol, methanol, and 2,2,2-trifluoroethanol (TFE) have been investigated at 25.0 oC. Kinetic solvent isotope effects (KSIE) in water and in methanol and product selectivities in alcohol-water mixtures are also reported. The Grunwald-Winstein plot of first-order rate constants for the solvolyic reaction of DSC with YCl shows marked dispersions into separated lines for various aqueous mixtures. With use of the extended Grunwald-Winstein equation, the l and m values obtained are 1.12 and 0.58 respectively for the solvolyses of DSC. The relatively large magnitude of l is consistent with substantial nucleophilic solvent assistance. From Grunwald-Winstein plots the rate data are dissected approximately into contributions from two competing reaction channels. This interpretation is supported for alcohol-water mixtures by the trends of product selectivities, which show a maximum for ethanol-water mixtures. From the KSIE of 1.45 in methanol, it is proposed that the reaction channel favored in methanolwater mixtures and in all less polar media is general-base catalysed and/or is possibly (but less likely) an addition-elimination pathway. Also, the KISE value of 1.35 for DSC in water is expected for SN2-SN1 processes, with minimal general base catalysis, and this mechanism is proposed for solvolyses in the most polar media.

Evaluation of the Kinetics of Xylose Formation from Dilute Sulfuric Acid Hydrolysis of Forest Residues of Eucalyptus grandis

Dilute acid hydrolysis studies were performed on forest residues of Eucalyptus grandis, in a cylindrical reactor of stainless steel. The kinetics of this hydrolysis reaction was investigated employing 0.65% sulfuric acid, a residue/acid solution ratio of 1/9 (w/w), temperatures of 130, 140, 150, and 160 °C, and reaction times in the range 20−100 min. The results showed that, under the optimized conditions of acid hydrolysis employed in this study, the variables temperature and reaction time had a strong influence on hemicellulose removal and a small influence on the degree of lignin and cellulose removal. The highest xylose extraction yield was 87.6% attained at 160 °C, after 70 min reaction time, simultaneously with the formation of decomposition products, namely 2.8% acetic acid, 0.6% furfural, and 0.06% 5-hydroxymethylfurfural. A similar xylose extraction yield (82.8%) was observed at 150 °C after 100 min, with the formation of 3.2% acetic acid, 1.0% furfural, and 0.07% 5-hydroxymethylfurfural. The kinetic parameters determined at 130, 140, 150, and 160 °C for degradation of xylan present in the hemicellulose of the eucalyptus forest residue during the formation of xylose were the first-order reaction rate constants (k) for each temperature, 1.22 × 10-4, 2.12 × 10-4, 5.43 × 10-4, and 9.05 × 10-4 s-1, respectively, and an activation energy (Ea) of 101.3 kJ mol-1.

Does increased chelation enhance the rate of ligand substitution at PtII (N,N,N) centres? A detailed kinetic &amp; mechanistic study

The rate of substitution of the chloride and aqua moieties from the platinum(II)-amine complexes, viz. [Pt(dien)Cl]Cl(Pt1-Chloro) and [Pt(en)(NH3)Cl]Cl (Pt2-Chloro) and their corresponding aqua analogues, viz. [Pt(dien)(OH2)] (ClO4)2 (Pt1-Aqua) and [Pt(en)(NH3)(OH2)](ClO4)2 (Pt2-Aqua), by a series of neutral and anionic nucleophiles,viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I−) and thiocyanate (SCN−), was determined under pseudo first-order conditions as a function of concentration and temperature using UV/Visible spectrophotometry and standard stopped-flow techniques. The observed pseudo first-order rate constants for the substitution reactions obeyed the simple rate law k obs = k 2[Nucleophile]. Second-order kinetics and negative activation entropies, ca. −93 J K−1 mol−1 and −71 J K−1 mol−1, for the chloro and aqua complexes respectively, support an associative mode of activation. The rate of substitution of both the chloro and aqua moieties are observed to decrease with an increase in the steric bulk of the neutral nucleophiles, whilst rate of substitution by SCN− was observed to be faster than that of I−, in correlation with the observed nucleophilicities of the two nucleophiles. A comparison of the second-order rate constants, k 2, at 298 K, obtained for the substitution reactions of Pt1and Pt2 shows that an increase in chelation in moving from Pt2 to Pt1 results in a corresponding increase in the reactivity, by a factor of ca. 3, (28.31 ± 0.15 and 8.02 ± 0.13 m −1 s−1 for Pt1 and Pt2 respectively, in the case of substitution of the aqua species by TU). Computational analysis of the chloro complexes, viz. Pt1-Chloro, Pt2-Chloro and [Pt(NH3)3Cl]Cl (Pt3) support this conclusion by demonstrating that the Pt–N bond trans to the leaving group is shortened and that the Pt–Cl bond is lengthened when chelation is increased from Pt3 to Pt1. Consequently, these results suggest that the increase in reactivity of Pt1 over Pt2, promoted by increased chelation, is as a result of ground state destabilization.

First-order kinetic gas generation model parameters for wet landfills

Landfill gas collection data from wet landfill cells were analyzed and first-order gas generation model parameters were estimated for the US EPA landfill gas emissions model (LandGEM). Parameters were determined through statistical comparison of predicted and actual gas collection. The US EPA LandGEM model appeared to fit the data well, provided it is preceded by a lag phase, which on average was 1.5 years. The first-order reaction rate constant, k, and the methane generation potential, L o, were estimated for a set of landfills with short-term waste placement and long-term gas collection data. Mean and 95% confidence parameter estimates for these data sets were found using mixed-effects model regression followed by bootstrap analysis. The mean values for the specific methane volume produced during the lag phase ( V sto), L o, and k were 33 m 3/Megagrams (Mg), 76 m 3/Mg, and 0.28 year −1, respectively. Parameters were also estimated for three full scale wet landfills where waste was placed over many years. The k and L o estimated for these landfills were 0.21 year −1, 115 m 3/Mg, 0.11 year −1, 95 m 3/Mg, and 0.12 year −1 and 87 m 3/Mg, respectively. A group of data points from wet landfills cells with short-term data were also analyzed. A conservative set of parameter estimates was suggested based on the upper 95% confidence interval parameters as a k of 0.3 year −1 and a L o of 100 m 3/Mg if design is optimized and the lag is minimized.