Catalytic Acceleration Research Articles

Role of adsorption complexes in catalytic acceleration of heterogeneous reactions

The universal equation that imposes restrictions on the kinetics of heterogeneous chemical reactions has been derived. A method of studying the participation of chemisorbed or physisorbed gas molecules in heterogeneous chemical reactions has been proposed. It has been shown that the heterogeneous reaction rate is independent of the degree of occupation of the catalyst surface by the reactants as long as the product is formed by chemisorbed complexes and physisorbed molecules. It has been found that the formation of CO2 in the heterogeneous chemical reactions CO+ O → CO2 and 2CO + O2 → 2CO2 is participated by physisorbed particles and chemisorbed complexes of oxygen atoms and CO molecules.

Catalytic Effect of PbO Glass Frit on the Degradation of the Carbon Nanotubes in a Field Emitter Paste

This study investigated an oxidative reaction of carbon nanotubes (CNTs) with other ingredients in a CNT paste, which was developed for eld-emission applications, during ring in air at 420 C. In the thermogravimetric analysis of the mixtures of CNTs and other ingredients of the paste, PbO brought the oxidation temperature of CNTs down by as much as 79 C from that of pristine CNTs, probably due to catalytic acceleration of their oxidation. Ethyl cellulose (EC), which was used as an organic binder, also lowered the burning temperature of the CNTs by 62 C. When both PbO and EC were mixed together with CNTs at the same time, they had a synergetic e ect on the decomposition of the CNTs. This problem was overcome by replacing the PbO frit with a Pb-free, SnO frit in such a way that most of CNTs could survive during the ring of the paste. Consequently, the eld-emission current of the CNT paste prepared from the Pb-free, SnO frit was improved by as much as 250 %, compared to that of the PbO-containing paste. This study seems to be of great signi cance not only for improving the eld-emission characteristics but also for eliminating Pb for environmental protection.

Hydrogen generation: catalytic acceleration and control by light

The ruthenium-catalyzed generation of hydrogen from formic acid is significantly accelerated by light.

Kinetic Spectrophotometric Determination of Biotin in Pharmaceutical Preparations

A simple accurate kinetic spectrophotometric method was developed for the determination of biotin in pure form and pharmaceutical preparations. The proposed method is based on a catalytic acceleration of biotin on the reaction between sodium azide and tri-iodide in an aqueous solution. Concentration range of 4-16 µg/mL for biotin was determined by measuring the decrease in the absorbance of tri-iodide at 348 nm by a fixed time method. The decrease in absorbance after 14 min from the initiation of the reaction was markedly correlated to the concentration with correlation coefficient of 0.9999. The detection limit (LOD) of biotin was 0.18 µg/mL while quantitation limit (LOQ) was 0.54 µg/mL. The percentage recovery of the spiked samples was 100.08 ± 0.761. The proposed procedure was successfully applied for the determination of biotin in its pharmaceutical preparations with mean recoveries of 101.17 ± 2.05 and 97.87 ± 1.50 for biotin ampoules and capsules, respectively. The results obtained were in good agreement with those obtained using official method.

Evaporation and carbonic anhydrase activity recorded in oxygen isotope signatures of net CO2 fluxes from a Mediterranean soil

AbstractThe oxygen stable isotope composition (δ18O) of CO2 is a valuable tool for studying the gas exchange between terrestrial ecosystems and the atmosphere. In the soil, it records the isotopic signal of water pools subjected to precipitation and evaporation events. The δ18O of the surface soil net CO2 flux is dominated by the physical processes of diffusion of CO2 into and out of the soil and the chemical reactions during CO2–H2O equilibration. Catalytic reactions by the enzyme carbonic anhydrase, reducing CO2 hydration times, have been proposed recently to explain field observations of the δ18O signatures of net soil CO2 fluxes. How important these catalytic reactions are for accurately predicting large‐scale biosphere fluxes and partitioning net ecosystem fluxes is currently uncertain because of the lack of field data. In this study, we determined the δ18O signatures of net soil CO2 fluxes from soil chamber measurements in a Mediterranean forest. Over the 3 days of measurements, the observed δ18O signatures of net soil CO2 fluxes became progressively enriched with a well‐characterized diurnal cycle. Model simulations indicated that the δ18O signatures recorded the interplay of two effects: (1) progressive enrichment of water in the upper soil by evaporation, and (2) catalytic acceleration of the isotopic exchange between CO2 and soil water, amplifying the contributions of ‘atmospheric invasion’ to net signatures. We conclude that there is a need for better understanding of the role of enzymatic reactions, and hence soil biology, in determining the contributions of soil fluxes to oxygen isotope signals in atmospheric CO2.

A Billion-fold Range in Acidity for the Solvent-Exposed Amides of Pyrococcus furiosus Rubredoxin

The exchange rates of the static solvent-accessible amide hydrogens of Pyrococcus furiosus rubredoxin range from near the diffusion-limited rate to a billion-fold slower for the non-hydrogen-bonded Val 38 (eubacterial numbering). Hydrogen exchange directly monitors the kinetic acidity of the peptide nitrogen. Electrostatic solvation free energies were calculated by Poisson-Boltzmann methods for the individual peptide anions that form during the hydroxide-catalyzed exchange reaction to examine how well the predicted thermodynamic acidities match the experimentally determined kinetic acidities. With the exception of the Ile 12 amide, the differential exchange rate constant for each solvent-exposed amide proton that is not hydrogen bonded to a backbone carbonyl can be predicted within a factor of 6 (10 (0.78)) root-mean-square deviation (rmsd) using the CHARMM22 electrostatic parameter set and an internal dielectric value of 3. Under equivalent conditions, the PARSE parameter set yields a larger rmsd value of 1.28 pH units, while the AMBER parm99 parameter set resulted in a considerably poorer correlation. Either increasing the internal dielectric value to 4 or reducing it to a value of 2 significantly degrades the quality of the prediction. Assigning the excess charge of the peptide anion equally between the peptide nitrogen and the carbonyl oxygen also reduces the correlation to the experimental data. These continuum electrostatic calculations were further analyzed to characterize the specific structural elements that appear to be responsible for the wide range of peptide acidities observed for these solvent-exposed amides. The striking heterogeneity in the potential at sites along the protein-solvent interface should prove germane to the ongoing challenge of quantifying the contribution that electrostatic interactions make to the catalytic acceleration achieved by enzymes.

Sensitive kinetic spectrophotometric determination of captopril and ethamsylate in pharmaceutical preparations and biological fluids

A highly sensitive kinetic spectrophotometric method was developed for the determination of captopril (CPL) and ethamsylate (ESL) in pharmaceutical preparations and biological fluids. The method is based on a catalytic acceleration of the reaction between sodium azide and iodine in an aqueous solution. Concentration range of 0.1–1.5 μgml –1 for CPL and 0.3–3 μgml –1 for ESL was determined by measuring the decrease in the absorbance of iodine at 348 nm by a fixed time method. The decrease in absorbance after 5 min was markedly correlated to the concentration. The relative standard deviations obtained were 1.30 and 1.87 for CPL and ESL, respectively, in pure forms. Correlation coefficients were 0.9997 and 0.9999 for CPL and ESL, respectively. The detection limits were determined as ( S/N = 3) were 20 ngml –1 for CPL and 50 ngml –1 for ESL. The proposed procedure was successively applied for the determination of both drugs in pharmaceutical preparations and in biological fluids.

The Molecular Roots of Compositional Inheritance

Non-covalent compositional assemblies, made of monomeric mutually catalytic molecules, constitute an alternative to alphabet-based informational biopolymers as a mechanism of primordial inheritance. Such assemblies appear implicitly in many “Metabolism First” origin of life scenarios, and more explicitly in the Graded Autocatalysis Replication Domain (GARD) model [Segréet al. (2000). Proc. Natl Acad. Sci. U.S.A.97, 4112–4117]. In the present work, we provide a detailed analysis of the quantitative molecular roots of such behavior. It is demonstrated that the fidelity of reproduction provided by a newly defined heritability measure η*s, strongly depends on the values of molecular recognition parameters and on assembly size. We find that if the catalytic rate acceleration coefficients are distributed normally, transfer of compositional information becomes impossible, due to frequent “compositional error catastrophes”. In contrast, if the catalytic acceleration rates obey a lognormal distribution, as actually predicted by a statistical formalism for molecular repertoires, high reproduction fidelity is obtained. There is also a clear dependence on assembly size N, whereby maximal η is seen in a narrow range around N∼3.5NG /λ, where NGis the size of the primordial molecular repertoire and λ is a molecular interaction statistical parameter. Such relationships help define the physicochemical conditions that could underlie the early steps in pre-biotic evolution.

Determination of the surface area of silver sols based on the rate of the electrocatalytic oxidation of N, N-dimethyl- p-phenylenediamine with chloropentaamine-cobalt(III) chloride

The rate of the electrocatalytic reaction between N, N-dimethyl- p-phenylenediamine and the cobalt(III) complex [Co III(NH 3) 5Cl] 2+ is proportional to the surface area of silver which acts as catalyst. For a monodisperse colloid the surface area can be calculated from the size of the particles. Thus, a calibration line can be obtained by carrying out several measurements with the same silver sol but in different concentration. Silver sols of both large particles (4.6 μm diameter) and small particles (22.6 nm) were used. These sols were prepared by reduction of silver nitrate with ascorbic acid and borohydride respectively. The calibration line allows the determination of the blank surface area of silver sols from the catalytic acceleration of the chosen standard redox reaction.

Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials

Nanocrystalline MgH 2/Me x O y - and Mg 2NiH 4/Me x O y -powders were produced by high energy ball milling (Me x O y =Sc 2O 3, TiO 2, V 2O 5, Cr 2O 3, Mn 2O 3, Fe 3O 4, CuO, Al 2O 3, SiO 2). The hydrogen absorption and desorption kinetics of the nanocomposite materials were determined with respect to a technical application. Some of the selected oxides lead to an enormous catalytic acceleration of hydrogen sorption compared to pure nanocrystalline materials. In absorption, the catalytic effect of TiO 2, V 2O 5, Cr 2O 3, Mn 2O 3, Fe 3O 4, and CuO is comparable. Concerning desorption, the composite material containing Fe 3O 4 shows the fastest kinetics followed by V 2O 5, Mn 2O 3, Cr 2O 3 and TiO 2. Only 0.2 mole% of the catalysts is sufficient to provide a fast sorption kinetics.

Mg-Based Hydrogen Storage Materials with Improved Hydrogen Sorption

Nanocrystalline MgH 2 /Me x O y composite powders were produced by high energy ball milling (Me x O y = Sc 2 O 3 , TiO 2 , V 2 O 5 , Cr 2 O 3 , Mn 2 O 3 , Fe 3 O 4 , CuO, Al 2 O 3 , SiO 2 ). The hydrogen absorption and desorption kinetics were determined in view of a technical application. The addition of selected oxides lead to an enormous catalytic acceleration of hydrogen sorption compared to pure nanocrystalline hydrides. In absorption, the catalytic effect of TiO 2 , V 2 O 5 , Cr 2 O 3 , Mn 2 O 3 , Fe 3 O 4 , and CuO is comparable. Conceming desorption, the composite material containing Fe 3 O 4 shows the fastest kinetics followed by V 2 O 5 , Mn 2 O 3 , Cr 2 O 3 and TiO 2 . Only 0.2 mol% of the catalysts is sufficient to provide a fast sorption kinetics. Additionally, Mg absorbs hydrogen already at room temperature by the use of metal oxides as catalysts. Furthermore, it is demonstrated for the first time that MgH 2 /Me x O y -powders release hydrogen at 200°C.

Kinetic analysis of systems containing aromatic amine, phenol, and hydroperoxide

Processes and phenomena in three-component systems containing a secondary aromatic amine, a sterically hindered phenol, and a hydroperoxide are analyzed from the standpoint of kinetics. The following three phenomena, which are well known, are related to each other: (1) simultaneous phenol and hydroperoxide consumption during the generation of amyl radicals; (2) consecutive consumption of phenol and amine as antioxidants in the initiated oxidation of hydrocarbons; and (3) catalytic acceleration of phenoxyl radical reaction with hydroperoxide by aromatic amines. These three phenomena reflect the process behavior in the title systems in the generation or addition of the corresponding free radicals. It was shown that the main catalytic feature of the reactions in three-component systems is a trend toward their coupled nature. The requirements to the experimental conditions are formulated under which coupling either takes place in full measure or does not play any role at all.

Recombination of preadsorbed oxygen atoms at the surface of solids

Relaxation measurements are used to establish that for a concentration of gas-phase oxygen atoms n=1014 cm−3 heterogeneous recombination of oxygen atoms at the surface of copper and zinc sulfide takes place by a mechanism involving recombination of preadsorbed (precursor state) atoms. A hypothesis is put forward for the existence of a universal mechanism for the catalytic acceleration of heterogeneous chemical reactions.

Catalytic acceleration of graphitisation of amorphous carbon during synthesis of tungsten carbide from tungsten and excess amorphous carbon in a solar furnace

Amorphous carbon is one of the allotropes of carbon possessing carbon activity a(C) higher than that of graphite (standard state with a(C) = 1). Amorphous carbon is in a metastable state but, under normal circumstances, it takes several hours to be graphitised to an extent detectable by X-ray diffraction even at temperature higher than 1500°C. In the present work, we report the accelerated graphitisation of amorphous carbon induced apparently by the catalytic action of tungsten (W) or tungsten carbide (WC) during synthesis of WC started from W and active carbon in solar furnace under controlled atmosphere (Ar or N 2). This degree of graphitisation of amorphous carbon did not proceed by the similar reaction undertaken in the traditional laboratory furnace under the comparable conditions.

Unique push-pull mechanism of dealkylation in soman-inhibited cholinesterases.

The pH-dependence and solvent isotope effects of dealkylation in diastereomeric adducts of Electric eel (Ee) and fetal bovine serum (FBS) acetylcholinesterase (AChE) inactivated with P(-)C(+) and P(-)C(-) 2-(3,3-dimethylbutyl) methylphosphonofluoridate (soman) were studied at 4.0 +/- 0.2 degrees C. The rate constant versus pH profiles were fit to a bell-shaped curve for all adducts. Best fit parameters are pK1 4.4-4.6 and pK2 6.3-6.5 for Ee AChE and pK1 4.8-5. 0 and pK2 5.8 for FBS AChE. The pKs are consistent with catalytic participation of the Glu199 anion and HisH+440. Maximal rate constants (kmax) are 13-16 x 10(-3) s-1 for Ee AChE and 8 x 10(-3) s-1 for FBS AChE. The solvent isotope effects at the pH maxima are 1.1-1.3, indicating unlikely proton transfer at the enzymic transition states for the dealkylation reaction. Slopes of log rate constant versus pH plots are near 1 at 25.0 +/- 0.2 degrees C between pH 7.0 and 10.0. In stark contrast, the corresponding adducts of trypsin are very stable even at 37.0 +/- 0.2 degrees C. The rate constants for diastereomers of soman-inhibited trypsin at 37.0 +/- 0.2 degrees C are pH independent and approximately 10(4) times smaller than kmax for analogous adducts with AChE. Dealkylation in soman-inhibited AChEs is estimated to occur at >10(10) times faster than a plausible nonenzymic reaction. Up to 40% of the catalytic acceleration can be attributed to an electrostatic push, and an electrostatic pull provides much of the balance. The results of this work together with results of a product analysis by Michel et al. (1969) can be explained by an initial and rate-determining methyl migration from Cbeta to Calpha. This is driven by the high electron density of residues (Glu199 and Trp84) at a crowded active site and may be concerted with C-O bond breaking. The positive charge at the rate-determining transition state is distributed between Cbeta and His440. A tertiary carbocation may have a fleeting existence before it is trapped by water or neighboring electrons which is likely to be promoted by Glu199 as the proton acceptor.

Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials

New reversible hydrogen storage systems are proposed, based on catalyzed reactions (Eqs. 4–6). The catalytic acceleration of the reactions in both directions is achieved by doping alkali metal aluminium hydrides with a few mol% of selected Ti compounds. The PCI diagrams of the Ti catalyzed systems show an absence of hysteresis and nearly horizontal pressure plateaus. The PCI of the NaAlH 4 system reveals two temperature-dependent pressure plateaus, corresponding to the two-step reversible dissociation of NaAlH 4. The PCI of the Na 3AlH 6 system shows only one pressure plateau; the latter can be lowered by partial substitution of Na by Li. In cyclic tests, reversible H 2 capacities of 4.2–3.1 and 2.7–2.1 wt% H have been achieved.

Catalytic acceleration for CO 2 decomposition into carbon by Rh, Pt or Ce impregnation onto Ni(II)-bearing ferrite

Catalyzing effects of impregnated Rh, Pt and Ce on Ni(II)-bearing ferrite (NF) activated with H 2 have been studied for CO 2 decomposition to carbon at 300°C. The activation rate of the Rh- or Pt-impregnated NF with H 2 gas was about 1.6 times as large as that of the NF alone at 300°C. At 250°C, the Rh- and Pt-NFs were also activated at 1.6 and 1.3 times higher rates than that of NF control, respectively. The activation of NF was, however, not enhanced by the impregnation of Ce. The rates of CO 2 decomposition using these activated materials were 1.2–3.0 times as large as that of the activated NF. The effect of metal impregnation on the activation and CO 2 decomposition increased in the order of Ce < Pt < Rh. This catalyzing effect is attributed to hydrogen and oxygen spillovers through accelerated dissociations of H 2 and CO 2 molecules, respectively, on the surface of Rh, Pt or Ce deposited on the NF.

EPR imaging study of paramagnetic centre distribution in thiokol-epoxy hermetics

The distribution of paramagnetic centres in carbon black filler in the interphase layer of the thiokol-epoxy hermetics on the border of brass or glass substrate was studied using EPR-imaging method. It was shown that the relative content of radicals decreases near the hermetic-“rigid” surface contact border. The thickness of the layer with a low concentration of radicals is estimated as 0.5±0.3 mm. The inhomogeneous distribution of radicals is more obvious in the case of hermetic hardening on a brass surface. These results are explained by a catalytic acceleration of the thiokol-epoxy polymerization reaction in the region of hermetic-metal surface contact.

Modeling of Ignition and CO Oxidation in the Boundary Layer of a Single Char Particle

A model is developed for CO oxidation in the boundary layer of a single char particle. The model includes char oxidation and a 56 reaction gas phase kinetic scheme which is coupled with the diffusive properties of the 12 species involved in the CO oxidation. The model is compared with the experimental data of Tognotti and co-workers. The temperature reached on ignition, the resulting CO 2 /CO ratio, and the effect of changing water vapor concentrations are well described. Studies on the effect of water concentration show that significant CO oxidation at low temperatures requires a high surrounding water concentration. The presence of water vapor or hydrogen is found to be necessary to obtain a high degree of CO oxidation at low temperatures; however, there is a minimum temperature below which significant CO oxidation in the boundary layer does not occur irrespective of how high the water concentration is. In addition, CO oxidation is negligible even at a surface temperature as high as 2500 K when water and hydrogen are absent. The model has also been applied to predict the effects of changing parameters. Catalytic acceleration of the rate of carbon oxidation, for example by the addition of calcium, leads to both a high particle temperature overshoot and a significant increase in CO oxidation over the particle surface.

Isocyanate/Phenolics Wood Adhesives by Catalytic Acceleration of Copolymerization

The reaction of MDI with the hydroxybenzyl alcohol group of phenol-formaldehyde resol (PF) resins can be accelerated by the use of catalysts of urethane formation such as triethylamine and dibutyltin dilaureate (DBTDL). A kinetic study for MDI (4,4'-diphenylmethane diisocyanate) and p-hydroxybenzyl alcohol as a model compound for phenol-formaldehyde resins confirmed this reaction. These catalysts have been used to upprade the performance of MDI/phenolic thermosetting wood adhesives for particleboard, in amounts varying between 0.5% and 2% of PF resin solids content. The use of these catalysts allows the use of simpler, usually slow-reacting, PF resins at pressing times as fast as 5 seconds per mm of board thickness, hence faster than urea-formaldehyde resins. These pressing times are achieved while still mantaining the exterior grade properties of the board