Slow Step Of Reaction Research Articles

A Robust Method for the Estimation of Kinetic Parameters for Systems Including Slow and Rapid Reactions—From Differential-Algebraic Model to Differential Model

Reliable estimation of kinetic parameters in chemical systems comprising both slow and rapid reaction steps and rapidly reacting intermediate species is a difficult differential-algebraic problem. Consequently, any conventional approach easily leads to serious convergence and stability problems during the parameter estimation. A robust method is proposed to surmount this dilemma: the system of ordinary differential equations and nonlinear algebraic equations is converted to ordinary differential equations, which are solved in-situ during the parameter estimation. The approach was illustrated with two generic examples and an example from green chemistry: synthesis of dimethyl carbonate from carbon dioxide and methanol.

Monitoring a Mechanochemical Reaction Reveals the Formation of a New ACC Defect Variant Containing the HCO3– Anion Encapsulated by an Amorphous Matrix

Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO3. In nature, it serves as a storage material or as a permanent structural element, whose lifetime is regulated by an organic matrix. The relevance of ACC in materials science is primarily related to our understanding of CaCO3 crystallization pathways and CaCO3/(bio)polymer nanocomposites. ACC can be synthesized by liquid–liquid phase separation, and it is typically stabilized with macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. Phosphate “impurities” were added in the form of monetite (CaHPO4) to substitute the carbonate anions, thereby stabilizing ACC by substitutional disorder. The phosphate anions do not simply replace the carbonate anions. They undergo shear-driven acid/base and condensation reactions, where stoichiometric (10%) phosphate contents are required for the amorphization to be complete. The phosphate anions generate a strained network that hinders ACC recrystallization kinetically. The amorphization reaction and the structure of BM-ACC were studied by quantitative Fourier transform infrared spectroscopy and solid state 31P, 13C, and 1H magic angle spinning nuclear magnetic resonance spectroscopy, which are highly sensitive to symmetry changes of the local environment. In the first—and fast—reaction step, the CO32– anions are protonated by the HPO42– groups. The formation of unprecedented hydrogen carbonate (HCO3–) and orthophosphate anions appears to be the driving force of the reaction, because the phosphate group has a higher Coulomb energy and the tetrahedral PO43– unit can fill space more efficiently. In a competing second—and slow—reaction step, pyrophosphate anions are formed in a condensation reaction. No pyrophosphates are formed at higher carbonate contents. High strain leads to such a large energy barrier that any reaction is suppressed. Our findings aid in the understanding of the mechanochemical amorphization of calcium carbonate and emphasize the effect of impurities for the stabilization of the amorphous phases in general. Our approach allowed the synthesis of new amorphous alkaline earth defect variants containing the unique HCO3– anion. Our approach outlines a general strategy to obtain new amorphous solids for a variety of carbonate/phosphate systems that offer promise as biomaterials for bone regeneration.

Transient Kinetic Experiments within the High Conversion Domain: The Case of Ammonia Decomposition

In the development of catalytic materials, a set of standard conditions is needed where the kinetic performance of many samples can be compared. This can be challenging when a sample set covers a broad range of activity. Precise kinetic characterization requires uniformity in the gas and catalyst bed composition. This limits the range of convecting devices to low conversion (generally <20%). While steady-state kinetics offer a snapshot of conversion, yield and apparent rates of the slow reaction steps, transient techniques offer much greater detail of rate processes and hence more information as to why certain catalyst compositions offer better performance. In this work, transient experiments in two transport regimes are compared: an advecting differential plug flow reactor (PFR) and a pure-diffusion temporal analysis of products (TAP) reactor. The decomposition of ammonia was used as a model reaction to test three simple materials: polycrystalline iron, cobalt and a bimetallic preparation of the two. These materials presented a wide range of activity and it was not possible to capture transient information in the advecting device for all samples at the same conditions while ensuring uniformity. We push the boundary for the theoretical estimates of uniformity in the TAP device and find reliable kinetic measurement up to 90% conversion. However, what is more advantageous from this technique is the ability to observe the time-dependence of the reaction rate rather than just singular points of conversion and yield. For example, on the iron sample we observed reversible adsorption of ammonia and on cobalt materials we identify two routes for hydrogen production. From the time-dependence of reactants and product, the dynamic accumulation was calculated. This was used to understand the atomic distribution of H and N species regulated by the surface of different materials. When ammonia was pulsed at 550 °C, the surface hydrogen/nitrogen, (H/N), ratios that evolved for Fe, CoFe and Co were 2.4, 0.25 and 0.3 respectively. This indicates that iron will store a mixture of hydrogenated species while materials with cobalt will predominantly store NH and N. While much is already known about iron, cobalt and ammonia decomposition, the goal of this work was to demonstrate new tools for comparing materials over a wider window of conversion and with much greater kinetic detail. As such, this provides an approach for detailed kinetic discrimination of more complex industrial samples beyond conversion and yield.

Electrochemical Nanoreactors of Mixed Metal-Oxide-Supported Noble- Metal-Nanoparticles for Efficient Oxidation of Dimethyl Ether and Ethanol

Noble metal-based (e.g. Pt, Pd, PtRu or PtSn) nanoparticles have been recognized as the most active catalytic systems towards oxidation of small organic molecules (ethanol, dimethyl ether) at low and moderate temperatures. Although all of them can be ideally oxidized to carbon dioxide, their actual electrooxidation mechanisms can significantly differ and may result in distinct electrocatalytic efficiencies and final reaction products. There is a need to develop functionalized electrocatalytic systems exhibiting specific reactivity and capability to induce the slow reaction steps at ambient conditions. Platinum based nanoparticles are readily poisoned by the strongly adsorbed CO-type intermediate species requiring fairly high overpotentials for their removal. For example, to enhance activity of Pt-based catalysts toward the ethanol oxidation, additional metal or metal oxide nanostructures (rhodium, iridium, tin or molybdenum and tungsten oxides) will be intentionally introduced to the electrocatalytic interface. We are also going to demonstrate that catalytic activity of platinum-based nanoparticles can be significantly enhanced through their interfacial modification with ultra-thin monolayer-type films of mixed-metal-oxo-species of titanium, cerium, zirconium or molybdenum and tungsten in their simple and polyoxometallate forms Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account possibility of specific interactions of noble metals with transition metal oxide species as well as existence of active hydroxyl groups in the vicinity of catalytic noble metal sites. It is noteworthy that certain metal and mixed-metal oxides can generate –OH groups at low potentials: they induce oxidation of passivating CO adsorbates (e.g. on Pt) or breaking C-H bonds (e.g. during oxidation of methanol or dimethyl ether). When combined with dispersed Rh or Ir, immobilized in the organized manner in the vicinity Pt sites, they tend to weaken C-C bonds during ethanol oxidation. We will also demonstrate that deposition of Pt and bimetallic PtRu nanoparticles onto nanostructured zirconia support results in the enhancement of their catalytic activities toward electrooxidation of dimethyl ether, another possible alternate fuel. Once more, it is reasonable to expect that the abilities of zirconia to facilitate proton mobility and to donate active hydroxyl groups at the electrocatalytic interface in acid media are responsible for the observed enhanced catalytic effects. Specific interactions with platinum and ruthenium components are also postulated.

Drying activity of oxovanadium(IV) 2-ethylhexanoate in solvent-borne alkyd paints

Oxovanadium(IV) 2-ethylhexanoate was established as a new primary drier for alkyd-based paints. A series of experiments has demonstrated an excellent drying activity toward solvent-borne alkyd resins of different oil length at considerably lower metal concentration than optimal for commercial cobalt(II) 2-ethylhexanoate. The behavior in real paint was simulated on formulations bearing inhibitor of autooxidation. The detailed studies of the drying process have shown that the relatively slow initial step of autooxidation reaction, when compared with cobalt compounds, is compensated for by the absence of the induction times. Furthermore, the active oxovanadium(IV) species are not consumed during the drying processes as evidenced by EPR spectroscopic measurements.

Kinetics and Mechanism of Oxidation of l‐Histidine by Permanganate Ions in Sulfuric Acid Medium

ABSTRACTThe reaction kinetics for the oxidation of l‐histidine by permanganate ions have been investigated spectrophotometrically in sulfuric acid medium at constant ionic strength and temperature. The order with respect to permanganate ions was found to be unity and second in acid concentration, whereas a fractional order is observed with respect to histidine. The reaction was observed to proceed through formation of a 1:1 intermediate complex between oxidant and substrate. The effect of the acid concentration suggests that the reaction is acid catalyzed. Increasing the ionic strength has no significant effect on the rate. The influence of temperature on the rate of reaction was studied. The presence of metal ion catalysts was found to accelerate the oxidation rate, and the order of effectiveness of the ions was Cu2+ > Ni2+ > Zn2+. The final oxidation products were identified as aldehyde (2‐imidazole acetaldehyde), ammonium ion, manganese(II), and carbon dioxide. Based on the kinetic results, a plausible reaction mechanism is proposed. The activation parameters were determined and discussed with respect to a slow reaction step.

KINETICS AND MECHANISM OF FORWARD EXTRACTION OF MN(II) FROM SULFATE-ACETATO MEDIUM BY CYANEX 272 DISSOLVED IN KEROSENE USING THE SINGLE-DROP TECHNIQUE

The flux of Mn(II) transfer in -Cyanex 272-kerosene system was investigated using the single-drop technique and the flux method of data treatment. The empirical flux equation at 303 K is: F f = 10−3.6 [Mn(II)] (1 + 104.5 [H+])−1 (1 + 1.58 [])−1. The activation energy (E a ) varies within 15–59 kJ/mol depending on experimental conditions. On analyzing the flux equation and with the help of E a values, it is concluded that either the extraction reaction step Mn2+ + A− → MnA+ or the diffusion of reactants (or products) to (or from) the reaction site is rate controlling depending on the reaction parameters. At high pH and at intermediate pH-higher temperature regions, low E a (≤20 kJ/mol) suggests that the process is under diffusion control. But at low pH and at intermediate pH-lower temperature regions, high E a (≥50 kJ/mol) suggests that the process is under chemical control. High negative ΔS ± value indicates that the slow extraction reaction step occurs by means of an SN2 mechanism.

Kinetic, Thermodynamic, and Structural Characterizations of the Association between Nrf2-DLGex Degron and Keap1

Transcription factor Nrf2 (NF-E2-related factor 2) coordinately regulates cytoprotective gene expression, but under unstressed conditions, Nrf2 is degraded rapidly through Keap1 (Kelch-like ECH-associated protein 1)-mediated ubiquitination. Nrf2 harbors two Keap1-binding motifs, DLG and ETGE. Interactions between these two motifs and Keap1 constitute a key regulatory nexus for cellular Nrf2 activity through the formation of a two-site binding hinge-and-latch mechanism. In this study, we determined the minimum Keap1-binding sequence of the DLG motif, the low-affinity latch site, and defined a new DLGex motif that covers a sequence much longer than that previously defined. We have successfully clarified the crystal structure of the Keap1-DC-DLGex complex at 1.6 Å. DLGex possesses a complicated helix structure, which interprets well the human-cancer-derived loss-of-function mutations in DLGex. In thermodynamic analyses, Keap1-DLGex binding is characterized as enthalpy and entropy driven, while Keap1-ETGE binding is characterized as purely enthalpy driven. In kinetic analyses, Keap1-DLGex binding follows a fast-association and fast-dissociation model, while Keap1-ETGE binding contains a slow-reaction step that leads to a stable conformation. These results demonstrate that the mode of DLGex binding to Keap1 is distinct from that of ETGE structurally, thermodynamically, and kinetically and support our contention that the DLGex motif serves as a converter transmitting environmental stress to Nrf2 induction as the latch site.

Investigating the Residues that are Responsible for PH Dependent Activity of the ATPase Domain of DnaK

Hsp70 proteins have essential roles in cells such as de novo folding of newly synthesized proteins, refolding or ubiquitination of denatured proteins, protein tarfficking and translocation through membranes. DnaK, Escherichia coli homolog of Hsp70 molecular chaperone, is comprised an N-terminal ATPase domain (NBD), a C-terminal substrate-binding domain (SBD) and a partially conserved hydrophobic linker that connects the domains. Substrate-binding affinities on SBD are driven by ATP-ADP conversion cycles in NBD. Allosteric communication between the two domains is provided by the conserved 389VLLL392 sequence on the linker region. Previous studies done using truncated DnaK(1-392) construct, containing the 389VLLL392 sequence, showed a pH-dependent enhanced ATPase activity, similar to the substrate-stimulated activity of the full-length protein; whereas construct lacking this sequence, DnaK(1-388) showed an activity resembling to the unstimulated-form of full-length. In the same study, it was proposed that linker binding to the ATPase domain causes a change in the slow step of the ATP hydrolysis reaction by rearranging the ATPase domain to a conformation where ADP release becomes the rate-limiting step. Here, we are investigating the molecular details of the reason of pH dependence and enhanced ATPase activity upon linker interactions with the domain using ATPase domain constructs both in in vitro and in silico. We observed with molecular dynamic simulations significant upshift in the pKa values of Asp194 and Asp201 compared to their expected pKa values as negatively charged residues, and it seems like that protonation states of these residues at different nucleotide-bound forms are important in the linker derived conformational changes leading, speculatively, variations in the Pi and ADP affinities. Our results will be discussed with overall ATPase rate as well as ADP off-rate measurements on DnaK(1-388) and DnaK(1-392) constructs.

Kinetics of Partitioning of Zn(II) in Zn(II)/SO4 2− - BTMPPA/Kerosene and H2SO4 - Zn-BTMPPA Complex/BTMPPA/Kerosene Systems

Kinetics of Zn(II)-extraction from SO4 2− medium by purified Cyanex 272 (BTMPPA, H2A2) and of Zn(II)-stripping from Zn(II)-BTMPPA complex by H2SO4 solution have been investigated using the single falling drop technique and F-method of data treatment. At 303 K, the empirical extraction and stripping flux equations are: Ff (kmol/m2s) = 10−8.4 [Zn(II)] [H+ ] (int) −1 ([H2A2 ] (o) 0.5 + 1.07[H2A2 ] (o)) (1+0.708 [SO4 2− ])−1 and Fb (kmol/m2s) = 10−5.25 [Zn-BTMPPA complex] (o) [ H+ ] [H2A2 ] (o) −0.5 (1 + 1.5 [HSO4 −]), respectively. In both cases, values of Ea, ΔH±, and ΔS± have been estimated. With the help of flux equations and Ea-values, the mechanisms of extraction and stripping are proposed. The extraction is diffusion controlled in all parametric conditions at high temperature region (htr). At lower temperature region (ltr), it may be under diffusion, intermediate, or chemical control depending on the experimental parameters. The chemical control step is: . High negative ΔS± values suggest that this step occurs via an SN2 mechanism. On the other hand, the stripping is intermediate control at htr and chemical control at ltr. The chemical controlling step is either: ZnA+ → Zn2+ + A−; or, ZnA+ + HSO4 −→ ZnHSO4 + + A−; depending on [HSO4 −] in the medium. The slow reaction step in stripping occurs via an SN2 mechanism. The equilibrium constant derived from the rate study matches well with that obtained from the equilibrium study.

Allosteric Control of cAMP Receptor Binding Dynamics

The intrinsic fluorescence of the cyclic AMP receptor is a sensitive indicator of the reaction with DNA, but signals are perturbed by a photoreaction. A ratio procedure is shown to be useful for correction. The reaction of the protein with DNA indicated by corrected transients extends over a broad time range not only at low salt concentrations but also at physiological salt concentrations. The initial binding step can be recorded preferentially at low salt pH 7 and is shown to be very similar for specific and nonspecific DNA. The rate constant for initial binding at 13.5 mM salt pH 7 is 2 × 10(8) M(-1) s(-1). Slow reaction steps up to times of several hundred seconds are observed both at low and high salt; the magnitude and sign of fluorescence amplitudes are strongly dependent on salt and pH. At 100 mM salt pH 8, the slow reaction step observed for the binding of the cyclic AMP receptor protein to promoter DNA is strongly shifted to longer times upon reduction of the cAMP concentration. The observed cAMP dependence is described quantitatively by a model implying that binding of the receptor to promoter DNA requires two cAMP molecules per protein dimer and is not consistent with a model assuming that a single cAMP is sufficient for activation. The rate constant for binding of the protein·dimer·(cAMP)(2) complex to the promoter is 1.3 × 10(8) M(-1) s(-1), close to the limit of diffusion control. Equilibration of specific complexes takes ~100 s at physiological concentrations of the reaction components.

Kinetics and Mechanism of the Autocatalyzed Oxidation of Theophylline by Permanganate in Aqueous Perchloric Acid Medium

The reaction kinetics for the oxidation of theophylline by permanganate ions have been investigated in perchloric acid medium using spectrophotometric techniques at 25 ∘C, and at constant ionic strength 1.60 mol⋅dm−3, under pseudo first order conditions. An autocatalyzed reaction is observed due to one of the products formed is Mn(II). The orders with respect to theophylline and Mn(VII) were both found to be unity, whereas fractional order is observed with respect to the autocatalyst, Mn(II). The rate of the reaction increases as the concentration of acid increases, but the order with respect to acid concentration is less than unity. The influence of temperature on the rate of reaction was studied. Based on the experimental results a suitable mechanism is proposed. The activation and thermodynamic parameters were determined with respect to slow reaction step.

Spectroscopic and catalytic characterization of extra large pore zeotype H-ITQ-33

The ITQ-33 structure comprises 18-ring channels, and this material constitutes one of very few zeolites/zeotypes with extra large pores. Such materials are of interest due to their ability to catalyze reactions involving bulky species and the prospect of novel shape selective properties. In this work, the recently discovered and little studied H-ITQ-33 catalyst has been synthesized and characterized. Emphasis has been placed on characterization of the fundamental properties of this material and its catalytic properties in the industrially relevant conversion of methanol to hydrocarbons. An unusually harsh calcination procedure (5 h at 650 °C) is required to fully remove the structure directing agent. Using FTIR, it is shown that the Brønsted acid sites become available once persistent NH 3 / NH 4 + formed during the decomposition is removed. It is demonstrated that H-ITQ-33 is an active catalyst in the conversion of methanol to hydrocarbons. The catalyst is fully regenerable during three deactivation/regeneration cycles. The presence of extra large 18-ring channels is not reflected in the observed product shape selectivity, which is similar to that commonly observed for 12-ring zeolites. This indicates that the formation of bicyclic species as undesired byproducts (coke) is a slow reaction step relative to the reaction leading to arenes. Remarkably, the H-ITQ-33 catalyst contains unusually little carbonaceous deposits (coke) when fully deactivated and the mode of catalyst deactivation has been studied in detail using in situ FTIR, UV–Vis, thermogravimetry and a catalyst dissolution/coke extraction protocol.

Structure sensitivity of the Fischer–Tropsch reaction; molecular kinetics simulations

Strong interest in the Fischer–Tropsch reaction that converts synthesis gas into hydrocarbons is reappearing because it is basic to one of the major routes that convert natural gas into liquid energy carriers. For catalytic science it provides amongst others an opportunity to revisit still open mechanistic issues of the Fischer–Tropsch conversion reaction. New approaches as computational advances and development of model systems are tools that may provide new insights. In this paper we will review our current understanding of the kinetics and its relation to catalyst structural parameters that determine the selectivity of the reaction. In the introductory section we formulate the key questions that we will address. Especially we will discuss the reason for particle size dependence of metallic catalysts as Co and Ru when particles are in the nanosize range and also the apparent paradox that step–edge sites are necessary for the chain growth reaction, where the CO molecule has to dissociate but that such sites should not be poisoned by the presence of the growing hydrocarbon chains or deactivating carbonaceous residue. One of the main selectivity issues of this reaction is the desire to produce long chain hydrocarbon molecules, without co-production of light gas molecules as methane. We will begin the presentation with the elementary kinetic expressions that enable calculation of the selectivity from a microkinetics reaction scheme. This will highlight that the rate of chain growth termination has to be one of the slow reaction steps and also that CO dissociation has to be fast. Since the past decade has seen major advances in the understanding of the structure sensitivity of transition metal catalysed surface reactions, the kinetic analysis helps to understand how structure sensitivity affects Fischer–Tropsch selectivity. Quantum chemical computational studies now can be used to analyse reaction paths and estimate reaction intermediate adsorption energies. Also activation free energies can be deduced for elementary surface reactions. We will illustrate this by discussing a so-called dual site model of the reactive catalyst center. On this reaction center we will discuss in detail CO dissociation and initiation of the chain growth reaction. It appears that synchronized subsequent reaction events involving reaction intermediate diffusion to different positions at the reaction center leads to accommodation of hydrocarbon chain growth while CO dissociation is not suppressed. Ultimately the mechanistic model deduced from the quantum-chemical studies will have to be used in kinetic equations to predict overall catalytic conversion rates. We will demonstrate how this can be done in a Kinetic Monte Carlo scheme, in which no assumption on the rate limiting step has to be made. We will present the results of some initial simulations where we allow for growth of short hydrocarbon chains. These results can be used in an insightful way using the kinetic model equations presented earlier in the paper. The paper is concluded by discussing the implications of these model results for our general mechanistic understanding of the Fischer–Tropsch reaction.

The water-gas shift reaction on Pt/γ-Al 2O 3 catalyst: Operando SSITKA-DRIFTS-mass spectroscopy studies

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry ( operando) were performed for the first time to study essential mechanistic aspects of the water-gas shift reaction (WGSR) over a 0.5 wt.% Pt/γ-Al 2O 3 catalyst. The chemical structure of active and inactive reaction intermediate species present in the carbon-path (“C-path”) from CO to the CO 2 product gas (use of 13CO) and in the hydrogen-path (“H-path”) from H 2O to the H 2 product gas (use of D 2O) of the reaction mechanism were determined. In addition, the concentrations (μmol/g) of active species in both the “C-path” and “H-path” of the WGSR at 350 °C were measured. Based on the large concentration of active species present in the “H-path” (OH/H located on the alumina support), the latter being larger than 28 equivalent monolayers of the exposed Pt metal surface, the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol/g) of active species present in the “C-path” (adsorbed CO on Pt and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support surface towards Pt catalytic sites present in the “H-path” of reaction (back-spillover process) might be considered as a slow reaction step. The latter process was evidenced after conducting the WGS reaction (CO/H 2O) in a partially deuterated alumina surface (Pt/γ-Al 2O 3). At least two kinds of formate (–COOH) species residing on the alumina surface have been identified, one of which was active and leads to the formation of CO 2(g) and H 2(g), whereas the other kind(s) is/are considered as inactive (spectator) adsorbed reaction intermediate species.

A study of the kinetics and mechanism of oxidation of L-tryptophan by diperiodatonickelate(IV) in aqueous alkaline medium

The kinetics of oxidation of L-tryptophan by diperiodatonickelate(IV) (DPN) in an aqueous alkaline medium at a constant ionic strength of 0.30 mol dm−3 was studied spectrophotometrically. The reaction was first order in diperiodatonickelate(IV) and less than first order in tryptophan and the OH− ion. The addition of periodate had no effect on the reaction, and nickel(II) produced did not influence the reaction rate significantly. An increase in ionic strength and decrease in medium permittivity did not affect the reaction rate. A mechanism involving the formation of a complex between L-tryptophan and reactive DPN species was proposed. The constants characterizing the mechanism were evaluated. The activation parameters for the slow reaction step were computed and discussed.

Mechanistic aspects of the water–gas shift reaction on alumina-supported noble metal catalysts: In situ DRIFTS and SSITKA-mass spectrometry studies

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with mass spectrometry were performed for the first time to study essential mechanistic aspects of the water–gas shift (WGS) reaction over alumina-supported Pt, Pd, and Rh catalysts. In particular, the concentrations (μmol g −1) of active intermediate species found in the carbon-path from CO to the CO 2 product gas (use of 13CO), and in the hydrogen-path from H 2O to the H 2 product gas (use of D 2O) of the reaction mechanism were determined. It was found that by increasing the reaction temperature from 350 to 500 °C the concentration of active species in both the carbon-path and hydrogen-path increased significantly. Based on the large concentration of active species present in the hydrogen-path (OH/H located on the alumina support), the latter being larger than six equivalent monolayers based on the exposed noble metal surface area ( θ > 6.0), the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol g −1) of active species present in the carbon-path (adsorbed CO on the noble metal and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support towards catalytic sites present in the hydrogen-path of reaction mechanism might be considered as a slow reaction step. The formation of labile OH/H species is the result of dissociative chemisorption of water on the alumina support, where the role of noble metal is to activate the CO chemisorption and likely to promote formate decomposition into CO 2 and H 2 products. It was found that there is a good correlation between the surface concentration and binding energy of CO on the noble metal (Pt, Pd or Rh) with the activity of alumina-supported noble metal towards the WGS reaction.

Kinetics of Solvent Extraction of Iron(III) from Sulfate Medium by Purified Cyanex 272 using a Lewis Cell

The kinetics of the forward and backward extraction of the title process have been investigated using a Lewis cell operated at 3 Hz and flux or (F) – method of data treatment. The dependences of (F) in the forward extraction on [Fe3+], [H2A2](o), pH, and [HSO4 −] are 1, 0.5, 1, and −1, respectively. The value of the forward extraction rate constant (k f ) has been estimated to be 10−7.37 kmol3/2 m−7/2 s−1. The analysis of the experimentally found flux equation gives the following simple equation: F f =100.13 [FeHSO4 2+] [A−], on considering the monomeric model of BTMPPA and the stability constants of Fe(III)‐HSO4 − complexes. This indicates the following elementary reaction occurring in the aqueous film of the interface as rate determining: [FeHSO4]2++A−→[FeHSO4.A]+. The very high activation energy of 91 kJ mol−1 supports this chemical reaction step as rate-determining. The negative value of the entropy change of activation (−94 J mol−1 K−1) indicates that the slow chemical reaction step occurs via the SN2 mechanism. The backward extraction rate can be expressed by the equation: F b =10−5.13 [[FeHSO4A2]](o) [H+] [H2A2](o) −0.5. An analysis of this equation leads to the following chemical reaction step as rate-determining: [FeHSO4A2](int)→[FeHSO4A]+A(i) −. However, the activation energy of 24 kJ mol−1 suggests that the backward extraction process is intermediate controlled with greater contribution of the diffusion of one or the other species as a slow process. The equilibrium constant obtained from the rate study matches well with that obtained from the equilibrium study.

Redox and complexation chemistry of the Cr(VI)/Cr(V)-D-galacturonic acid system.

The oxidation of d-galacturonic acid by Cr(VI) yields the aldaric acid and Cr(III) as final products when a 30-times or higher excess of the uronic acid over Cr(VI) is used. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and the two intermediate species reacting with galacturonic acid at comparable rates. The rate of disappearance of Cr(VI), Cr(IV) and Cr(V) depends on pH and [substrate], and the slow reaction step of the Cr(VI) to Cr(III) conversion depends on the reaction conditions. The EPR spectra show that five-coordinate oxo-Cr(V) bischelates are formed at pH < or = 5 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form or the ring oxygen of the pyranose form. Six-coordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-coordinated oxo-Cr(V) bischelates only for galacturonic acid : Cr(VI) < or =10 : 1, in 0.25-0.50 M HClO(4). At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the vic-diol groups of Galur participate in the bonding to Cr(V). At pH 3-5 the Galur-Cr(V) species grow and decay over short periods in a similar way to that observed for [Cr(O)(alpha-hydroxy acid)(2)](-). The lack of chelation at any vic-diolate group of Galur when pH < or = 5 differentiates its ability to stabilise Cr(V) from that of neutral saccharides that form very stable oxo-Cr(V)(diolato)(2) species at pH > 1.

Kinetics and Mechanism of Oxidative Decarboxylation of Benzilic Acid by Alkaline Permanganate Using the Stopped-Flow Technique

A mechanism involving the formation of complex between benzilic acid and permanganate has been proposed for the title reaction and activation parameters for the slow step of reaction and reaction constants involved in the mechanism were calculated and discussed.