Second Stage Of Reaction Research Articles

Highly Selective Reduction of Nitrate by Zero-Valent Aluminum (ZVAI) Ball-Milled Materials at Circumneutral pH: Important Role of Microgalvanic Cells for Depassivation of ZVAl and N2-Selectivity.

The passivation of zero-valent aluminum (ZVAl) limits its application in environmental remediation. Herein, a ternary composite material Al-Fe-AC is synthesized via a ball-milling treatment on a mixture of Al0, Fe0, and activated carbon (AC) powders. The results show that the as-prepared micronsized Al-Fe-AC powder could achieve highly efficient nitrate removal and a nitrogen (N2)-selectivity of >75%. The mechanism study reveals that, in the initial stage, numerous Al//AC and Fe//AC microgalvanic cells in the Al-Fe-AC material could lead to a local alkaline environment in the vicinity of the AC cathodes. The local alkalinity depassivated the Al0 component and enabled its continuous dissolution in the subsequent second stage of reaction. The functioning of the AC cathode of the Al//AC microgalvanic cell is revealed as the primary reason accounting for the highly selective reduction of nitrate. The investigation on the mass ratio of raw materials manifested that an Al/Fe/AC mass ratio of 1:1:5 or 1:3:5 was preferable. The test in simulated groundwater suggested that the as-prepared Al-Fe-AC powder could be injected into aquifers to achieve a highly selective reduction of nitrate to nitrogen. This study provides a feasible method to develop high-performance ZVAl-based remedial materials that could work in a wider pH range.

Calorimetric study on electrolytic decomposition of hydroxylammonium nitrate (HAN) ternary mixtures

Electrolytic decomposition of hydroxylammonium nitrate (HAN) is appealing for development of chemical micropropulsion system due to its effectiveness in thermal management. In this paper we present the decomposition characteristics and behaviour of various HAN ternary mixtures prepared according to 0 Oxygen Balance (0 OB). There are multiple stages of decomposition depending on the type of fuels added into the HAN solution. While concentrated HAN solution (73 wt%) has only single stage of decomposition, the saccharides-based HAN ternary mixtures has three stages reaction with increased energy release. The addition of nitrogen-rich compounds has sustained the electrolytic decomposition process into the second stage of reaction, which produced the highest decomposition temperature. The study also reveals a linear relationship between the electrical resistivity of HAN ternary mixture and reaction rate in the first stage of reaction, indicating the presence of Joule heating in the process. The influence of electrical resistivity of the ternary mixture became negligible in the second stage of reaction. This work concludes the importance of combined electrical and thermal energy in the first stage decomposition of HAN ternary mixtures.

Gasification characteristics of petcoke and coal blended petcoke using thermogravimetry and mass spectrometry analysis

The particle size effect of petcoke and blended samples of coal and petcoke with similar sizes were investigated during steam gasification. A simultaneous thermal analyzer and mass spectrometry was used to characterize the tested samples and the identification of the volatiles evolved during the heating up to 1150 °C under combustion cum gasification related conditions. The TG and DTA results are discussed for the investigations in steam gasification process. The effect of blending petcoke with coal was studied in order to determine the possible presence of synergetic phenomena. The TG-MS profile of the petcoke provides information on gasification performance on different particle size, reaction temperature and time. It gives the chemical changes of petcoke during gasification and gas evolution profiles. The size effects on temperature of petcoke and coal blended petcoke during gasification are discussed. It was found that temperature and particle size had the greatest influence on the gasification rate and final products. With the smaller sized particles, initial higher reaction stage is continued up to larger conversion level, whereas it is shifted to second stage of reaction for larger particles. The appropriate temperature range to the petcoke and blended petcoke gasification was evaluated for different particle sizes. A synergistic effect was ascertained during the gasification of coal and petcoke mixture in steam ambience.

Six-coordinate nitrito and nitrato complexes of manganese porphyrin.

Reaction of small increments of NO2 gas with sublimed amorphous layers of Mn(II)(TPP) (TPP = meso-tetra-phenylporphyrinato dianion) in a vacuum cryostat leads to formation of the 5-coordinate monodentate nitrato complex Mn(III)(TPP)(η(1)-ONO2) (II). This transformation proceeds through the two distinct steps with initial formation of the five coordinate O-nitrito complex Mn(III)(TPP)(η(1)-ONO) (I) as demonstrated by the electronic absorption spectra and by FTIR spectra using differently labeled nitrogen dioxide. A plausible mechanism for the second stage of reaction is offered based on the spectral changes observed upon subsequent interaction of (15)NO2 and NO2 with the layered Mn(TPP). Low-temperature interaction of I and II with the vapors of various ligands L (L = O-, S-, and N-donors) leads to formation of the 6-coordinate O-nitrito Mn(III)(TPP)(L)(η(1)-ONO) and monodentate nitrato Mn(III)(TPP)(L)(η(1)-ONO2) complexes, respectively. Formation of the 6-coordinate O-nitrito complex is accompanied by the shifts of the ν(N═O) band to lower frequency and of the ν(N-O) band to higher frequency. The frequency difference between these bands Δν = ν(N═O) - ν(N-O) is a function of L and is smaller for the stronger bases. Reaction of excess NH3 with I leads to formation of Mn(TPP)(NH3)(η(1)-ONO) and of the cation [Mn(TPP)(NH3)2](+) plus ionic nitrite. The nitrito complexes are relatively unstable, but several of the nitrato species can be observed in the solid state at room temperature. For example, the tetrahydrofuran complex Mn(TPP)(THF)(η(1)-ONO2) is stable in the presence of THF vapors (∼5 mm), but it loses this ligand upon high vacuum pumping at RT. When L = dimethylsulfide (DMS), the nitrato complex is stable only to ∼-30 °C. Reactions of II with the N-donor ligands NH3, pyridine, or 1-methylimidazole are more complex. With these ligands, the nitrato complexes Mn(III)(TPP)(L)(η(1)-ONO2) and the cationic complexes [Mn(TPP)(L)2](+) coexist in the layer at room temperature, the latter formed as a result of NO3(-) displacement when L is in excess.

Removal of Vanadium from Molten Aluminum—Part II. Kinetic Analysis and Mechanism of VB2 Formation

Transition metal impurities such as vanadium (V), titanium (Ti), zirconium (Zr), and chromium (Cr) reduce the electrical conductivity of smelter grade aluminum. These impurities are removed by the addition of boron-bearing substances (AlB12/AlB2) and the process is called boron treatment. The quality of the alloy and the economy of boron treatment can be improved by understanding the kinetics and mechanism of VB2 formation in molten aluminum. Currently, there is limited published information on the heterogeneous kinetics of the boron treatment. In this study, an investigation of the kinetics of V removal from molten Al-1 wt pct V alloys during boron treatment at 1023 K (750 °C) was carried out. The effects of stirring (natural, mechanical, and gas), reaction temperature, and the levels of boron addition were analyzed. The kinetics of the process could be described in two stages. The rate was faster in the early stage of reaction and was halted after 10 minutes. It was concluded that the early stage of reaction was controlled by the mass transfer of V in molten aluminum. In the second stage of reaction, it was concluded that the kinetics was controlled by the diffusion of B through a VB2 layer formed in the early stage of reaction. It was observed from microscopy analysis that the ring of VB2 disintegrated during gas stirring of bulk melt and consequently the kinetics of reaction was enhanced.

Hydrogen Generation from Aluminum Corrosion in Aqueous Solutions

The H2 can be generated from the aluminum water reaction. Hydrogen generation rate by using aluminum from soft drink was measured by immersing samples in H2SO4, Na2SO4, KOH, and NaOH solutions. No observable amount of hydrogen is generated in acidic or in neutral solutions. At a given concentration, the generation rate of H2 is higher in NaOH solution than in KOH solution. High generation rate of H2 is obtained in concentrated NaOH as well as in concentrated KOH solutions. The gas volume of H2 generation can be fitted into a semi-empirical equation as function of reaction time. The H2 generation rate can be divided into two stages. During the first stage, hydrogen generation rate increases as the reaction continued. It reaches a maximum then gradually reduced in the second stage of reaction.

CaO/NaA combined with enzymatic catalyst for biodiesel transesterification

Abstract We combined CaO/NaA with enzymatic catalyst to produce biodiesel at moderate temperature and studied the Arrhenius model of the combined reaction. This transesterification reaction with molar ratio of methanol to olive oil 3:1, was speculated to be the fourth order reaction by formula calculation. The Ea of the first stage catalyzed by CaO/NaA was 37.92 kJ/mol, and that of the second stage catalyzed by lipase was 73.98 kJ/mol. The lipase activity was repressed in the second stage. The combined transesterification reaction did not completely inactivate the enzymatic catalyst, which was still active in the second stage of reaction and further improved the conversion efficiency. The maximum FAME product conversion yield was optimal 94.5% after reaction catalyzed by CaO/NaA at 40 °C for the first 3 h and catalyzed by lipase for the next 3 h.

Synthesis and Characterization of New Bolaform Surfactant from Palm Oil Derivative

In this research, new chemistry is explored through a simple, efficient and cheap method to synthesize new surfactants from palm oil fatty acids. Oleic acid, which is one of the major fatty acids in palm oil, has been used as a starting material for the synthesis of surfactants. This study focuses on the synthesis and characterization of bolaform surfactants from oleic acid. For this purpose, two steps reaction will be carried out: Firstly, consisted of hydrobromination of oleic acid followed by reaction of 10-bromo-octadecanoic acid with ethane-1,2-dithiol and butane-1,2-dithiol. During the first stage of the reaction, hydrobromination of oleic acid was carried out by adding hydrobromic acid into the oleic acid. The reaction was carried out in benzene as solvent at 30°C and atmospheric pressure. This reaction gives 73% yield of 10-bromo-octadecanoic acid and it yield a black brownish liquid which have the m/z 363.2056 by ESI-MS(ToF) and –CH2-CHBr-CH2-shift (4.02 ppm) was well defined in1H-NMR. The separation of 10-bromo-octadecanoic acid was done through thin layer chromatography (TLC) with chloroform and methanol as the eluent and silica plate grade 60 F254as the stationary phase. The second stage of reaction was reacting the 10-bromo-octadecanoic acid with ethane-1,2-dithiol and butane-1,2-dithiol in THF as the solvent and 1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU) as the catalyst respectively. These reactions were carried out at 30°C and atmospheric pressure. Product obtained was a two layers black brownish liquid with white colour precipitate gives m/z: 659.4813 for ethane-1,2-dithiol and m/z:686.5278 for butane-1,4-dithiol respectively using ESI-MS (ToF) mass spectroscopy. Separation was done on both bolaform surfactants through thin layer chromatography (TLC) with hexane and ethyl acetate as eluent and silica plate grade 60 F254as stationary phase, and finally followed by column chromatography. These two new surfactants will be tested for their physical-chemical properties as well as apply on nanomaterial stabilizer.

Unexpected reactivity of Au25(SCH2CH2Ph)18 nanoclusters with salts

We report some interesting results of the chemical reactivity of thiolate-protected [Au(25)(SCH(2)CH(2)Ph)(18)](0) nanoclusters with two types of salts, including tetraoctylammonium halide (TOAX) and NaX. At the early stage of the reaction, [Au(25)(SCH(2)CH(2)Ph)(18)](0) was found to spontaneously convert to its anionic form ([Au(25)(SCH(2)CH(2)Ph)(18)](-)) in the presence of either type of salt. However, a large difference was observed in the second stage of the reaction. With NaX, we observed decomposition of anionic clusters, while with TOAX, the clusters show excellent stability. We have gained some insight into the reaction mechanism. The X(-) ions seem to attack [Au(25)(SCH(2)CH(2)Ph)(18)](q) surface and displace some thiolates, evidenced by the observation of halide-bound clusters such as Au(25)(SCH(2)CH(2)Ph)(18-x)Br(x) in mass spectrometry analysis. These halide-bound clusters show a reduced stability, and their decomposition into Au(I) complexes leads to the release of gold valence electrons of the clusters; concurrently, the non-halide-bound [Au(25)(SCH(2)CH(2)Ph)(18)](0) clusters are reduced into [Au(25)(SCH(2)CH(2)Ph)(18)](-). For the second stage of reaction with organic salts such as TOA-Br, after [Au(25)(SCH(2)CH(2)Ph)(18)](0) clusters are converted to [Au(25)(SCH(2)CH(2)Ph)(18)](-)) the TOA(+) counterions surprisingly protect the anionic clusters from further attack by halide ions, hence, TOA(+) cations can stabilize [Au(25)(SCH(2)CH(2)Ph)(18)](-) clusters. In contrast, with NaX salts the Na(+) ions do not provide any steric stabilization of the [Au(25)(SCH(2)CH(2)Ph)(18)](-) clusters, hence, degradation occurs when being further attacked by halide ions, especially Br(-) and I(-).

The chemical oxidative polymerization of aniline in water: Raman spectroscopy

AbstractThe structural changes of aniline oligomers and polyaniline, associated with oxidation of aniline with ammonium peroxydisulfate in aqueous solutions without any added acid, were studied. The reaction was quenched at various times and the films deposited on silicon windows have been studied in detail by Raman spectroscopy, at excitation wavelengths 633 and 514 nm. The presence of substituted phenazine structural units, in addition to ordinary benzenoid, quinonoid, and semiquinonoid structures, has been proved by the appearance of characteristic Raman bands at 1645–1630, 1420–1400, 1380–1365, ∼575, and ∼415 cm−1. The remarkable differences of the Raman spectra of oligoanilines, precipitated in the first phase of oxidative polymerization, and polyaniline nanotubes and nanorods, formed in the second stage of reaction, are discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Photocatalytic sterilization of TiO 2 films coated on Al fiber

Photocatalytic TiO 2 films were coated on Al fiber by sol–gel dip-coating method, and then annealed. The crystal structure and morphology of the films were performed by XRD, TEM and SEM. Photocatalytic sterilization of the films was investigated in O 2 atmosphere through purifying the aqueous solution with facultative aerobe ( Bacillus cereus), aerobe ( Pseudomonas aeruginosa) and anaerobe ( Staphylococcus aureus, Enterococcus faecalis and Escherichia coli). In the presence of O 2, it benefits to generate O 2 − and OH at the first stage of the photocatalytic reaction, while the excess O 2 restrains the anaerobe from reproducing and accelerates the reproducing for the aerobe at the second stage of reaction. As a result, it was found that the crystal of TiO 2 films is anatase phase and the films have excellent sterilization effect against facultative aerobe and anaerobe. Nevertheless, it only decreased the bioactivity against aerobe in a short time.

Experimental investigation of silane combustion and particle nucleation using a rapid-compression facility

Abstract Ignition and reaction kinetics of silane/oxygen/diluent (SiH 4 /O 2 /Ar/N 2 ) mixtures have been studied using a rapid compression facility (RCF). Time-resolved pressure profiles and absolute, quantitative hydroxyl radical mole fraction ( χ OH ) time histories were obtained for a range of conditions representing different heating rates. At the moderate temperatures studied (∼630 K), ignition was observed to occur in two stages with no OH detected during the first stage. The separation of the two stages was typically >5 ms, thus the long test times provided by the RCF were critical to the studies. The second stage of reaction is identified by significant production of OH ( χ OH = 45–82 ppm). Experimental uncertainty in χ OH was 32/−29% with a 15 ppm detectivity limit. The results were examined in the context of proposed silane combustion and particle nucleation reaction mechanisms. Existing silane combustion kinetics mechanisms capture many of the experimentally observed features of the system, yet fail to quantitatively reproduce the OH time histories in their entirety.

Nucleation of product phase in reactive diffusion of Al/Co

The reactive interdiffusion of Co/Al bilayers is studied by atom probe tomography. For that, metallic thin films were deposited on tungsten substrates preshaped by field evaporation. Owing to the outstanding resolution of the method and its real three-dimensional analysis, the nucleation of the first product could be characterized in detail. It is clearly seen that interdiffusion of the initial Al and Co layers precedes the formation of the first nuclei. A critical gradient for the onset of nucleation is determined to be $0.3{\mathrm{nm}}^{\ensuremath{-}1}.$ The ${\mathrm{Al}}_{9}{\mathrm{Co}}_{2}$ phase is identified as the first reaction product by direct chemical analysis. Instead of a layer, this phase forms as globular nuclei, which quickly grow to a thickness of about 10 nm before a dense layer has developed. The observed growth is understood by fast transport along incoherent interphase boundaries. For this condition, a kinetic model is discussed that describes the growth of the particles. The thickness at impingement is controlled by the ratio of the transport coefficients along and across the boundary. Choosing realistic values for the latter parameters, noninteger growth exponents ranging from 2 to 3 are predicted. Thus, the reaction proceeds much faster than in the case of ordinary parabolic or linear growth. After consumption of pure Al, the formation of B2-ordered AlCo is observed as the second stage of reaction.

Glycolytic aminolysis of poly(ethylene terephthalate) waste for recovery of value‐added comonomer at atmospheric pressure

AbstractReaction of poly(ethylene terephthalate) waste (PETW) powder with ethylene glycol (EG) using 0.003 mol lead acetate as a catalyst was carried out in a batch reactor at 470 K under atmospheric pressure. Reactions were undertaken with various particle sizes ranging from 50 to 512.5 μm and reaction times ranging from 10 to 60 min at 10‐min intervals. A low molecular weight product of PETW was obtained using this reaction. Then hydrazine monohydrate, chlorobenzene, and cyclohexylamine (CHA) were introduced to convert the low molecular weight product of PETW into terephthalohydrazide (TPHD). To increase the PETW conversion rate, an external catalyst (lead acetate) was introduced during the reaction. The reaction product was deposited onto the surface of unreacted PETW that was removed from the surface by introducing dimethyl sulfoxide. To accelerate the reaction rate CHA was introduced during the second stage of reaction, which has industrial significance. Depolymerization of PETW was proportional to the reaction time and inversely proportional to the particle size of PETW. Analyses of value‐added products (TPHD and EG) as well as PETW were undertaken. A kinetic model was developed and experimental data were simulated consistent with the model. A thermodynamic study was undertaken because this is required during the transfer of laboratory data through the pilot plant for commercialization. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3437–3444, 2003

Kinetic and thermodynamic studies of the nonisothermal decomposition of anhydrous copper(II) formate in different gas atmospheres

The thermal decomposition of anhydrous (orthorhombic) copper(II) formate was studied by programmed rising-temperature methods (TG, DTG, DTA and DSC) to about 250 °C in flowing gas atmospheres of nitrogen (inert), hydrogen (reducing) and air (oxidizing). The degradation reaction, anion breakdown, proceeded to completion in two distinct, but partially overlapping, rate processes and apparent Arrhenius parameters, calculated by the Ozawa nonisothermal kinetic method, agreed satisfactorily with the literature results. It was concluded that the two consecutive processes, contributing to the overall reaction, involved stepwise cation reduction: Cu 2+→Cu +→Cu 0, with copper(I) formate as intermediate. This mechanism is similar to that proposed in previous studies of the decompositions of copper(II) oxalate, malonate, maleate, fumarate, mellitate and squarate. For all of these reactants, the Cu + salt has been identified as an intermediate, exhibiting a (slightly) lower relative reactivity than the corresponding Cu 2+ salt. For copper(II) formate the response curves in the three different gaseous atmospheres were generally similar, showing that neither oxidizing nor reducing conditions caused a marked change in reactivity. The temperature of reaction initiation in H 2 was slightly diminished and the temperature of the second stage of reaction in O 2 was raised appreciably. It is believed that electron transfer contributed to the control of reactivity and that the gases present appreciably influence the rates of the contributory reactions occurring.

Kinetic and thermodynamic studies of the nonisothermal decomposition of anhydrous copper(II) formate in different gas atmospheres

The thermal decomposition of anhydrous (orthorhombic) copper(II) formate was studied by programmed rising-temperature methods (TG, DTG, DTA and DSC) to about 250 °C in flowing gas atmospheres of nitrogen (inert), hydrogen (reducing) and air (oxidizing). The degradation reaction, anion breakdown, proceeded to completion in two distinct, but partially overlapping, rate processes and apparent Arrhenius parameters, calculated by the Ozawa nonisothermal kinetic method, agreed satisfactorily with the literature results. It was concluded that the two consecutive processes, contributing to the overall reaction, involved stepwise cation reduction: Cu 2+ →Cu + →Cu 0 , with copper(I) formate as intermediate. This mechanism is similar to that proposed in previous studies of the decompositions of copper(II) oxalate, malonate, maleate, fumarate, mellitate and squarate. For all of these reactants, the Cu + salt has been identified as an intermediate, exhibiting a (slightly) lower relative reactivity than the corresponding Cu 2+ salt. For copper(II) formate the response curves in the three different gaseous atmospheres were generally similar, showing that neither oxidizing nor reducing conditions caused a marked change in reactivity. The temperature of reaction initiation in H 2 was slightly diminished and the temperature of the second stage of reaction in O 2 was raised appreciably. It is believed that electron transfer contributed to the control of reactivity and that the gases present appreciably influence the rates of the contributory reactions occurring.

Kinetics of Hydrocracking and Hydrotreating of Coker and Oilsands Gas Oils

Two-stage hydrocracking of gas oils involves a complex set of hydrogenation, heteroatom removal, and cracking reactions. Feeds from coking processes and oilsands bitumen are richer in aromatics and heteroatoms, which increases the importance of the reactions of these components. The hydrocracking and hydrotreating kinetics of a series of conventional, coker and oilsands gas oils were measured using a laboratory microreactor. The kinetics were then correlated with the composition of the feed oils. The conversion of gas oil to middle distillate during hydrotreating was correlated with sulfur content and polyaromatic hydrocarbon content. Hydrocracking of the gas oil in the second stage of reaction followed a Langmuir-Hinshelwood rate expression based on the inlet concentration of total nitrogen. The resulting composite rate expression gave good prediction of gas oil conversion kinetics for the full range of gas oils. The inhibition of hydrocracking by nitrogen was more important for cracking than the differences in the homologous series in the gas oils of different origins.

High-molecular-weight copolyesters of dihydroxybenzophenones by ?induced? copolyesterification using TsCl/DMF/Py as a condensing agent

Dihydroxybenzophenones and 4,4'-dihydroxydiphenylsulfone, which yield high-molecular-weight polyesters only difficultly by conventional methods, gave high-molecular-weight copolyesters through a two-stage copolycondensation involving initial induction of oligomerization with selected monomers followed by copolycondensation. The copolycondensation was studied by examining such factors as the kinds of initial monomers, their amounts, initial reaction times, and the order of reaction, and the results are explained in terms of oligomer distribution in the initial oligomerization as determined by GPC. Copolycondensation was also facilitated by lowering the temperature at the second stage of reaction.

Enantioselective Hydrogenation: V. Hydrogenation of Butane-2,3-dione and of 3-Hydroxybutan-2-one Catalysed by Cinchona-Modified Platinum

Pt/silica modified by cinchonidine and cinchonine is active for the enantioselective hydrogenation of butane-2,3-dione to butane-2,3-diol in dichloromethane at 268–298 K and 10 bar pressure. Reaction proceeds in three stages. In the first, about 85% of the butane-2,3-dione is converted to 3-hydroxybutan-2-one and 15% to three higher molecular weight products by hydrodimerisation. The initial enantiomeric excess in the hydroxybutanone is modest (20 to 40%(R) with cinchonidine as modifier, 10%(S) with cinchonine as modifier) and dependent on the amount of alkaloid used in catalyst preparation. In the second stage, 3-hydroxybutan-2-one is converted to butane-2,3-diol; a marked kinetic effect is observed whereby the minority enantiomer is converted preferentially to butanediol and the enantiomeric excess in the remaining hydroxybutanone increases dramatically to values in the range 62 to 89%(R) and to 30%(S). Under all conditions, the most abundant stereochemical form of the final product is meso-butane-2,3-dione. In the third stage the three dimers are slowly converted by hydrogenation, dissociation, and further hydrogenation to butane-2,3-diol. In the absence of alkaloid, butane-2,3-dione hydrogenation to racemic products in dichloromethane solution proceeds in two distinct stages with no dimer formation. Butane-2,3-dione hydrogenation has also been studied over Pt/silica modified anaerobically by exposure to cinchonidine in ethanol under propyne at 2 bar. This catalyst is remarkably active for the conversion of diketone to diol in ethanol at 293 K and 10 bar and kinetic selection in the second stage of reaction is again observed. The hydrogenation of racemic 3-hydroxybutan-2-one in dichloromethane over cinchonine-modified Pt/silica at 273 K and 10 to 40 bar pressure also showed kinetic selection, an enantiomeric excess of up to 70%(S) appearing in the reactant as it was consumed. Mechanisms which account for these hydrogenations and dimerisations and for the enantioselectivities observed and their variation are presented. This diketone hydrogenation provides an example of consecutive thermodynamic and kinetic control of enantioselectivity in a multistage catalytic reaction.

Kinetics of the conversion of NO to N 2 during the oxidation of iron particles by NO in a hot fluidised bed

The kinetics of the conversion of poisonous NO gas to N 2 in Fe+q NO→ FeO q+ q 2 N 2 have been measured in hot fluidised beds of sand from 973 to 1173 K at 1 bar. The reaction involves NO (in N 2) reacting with tiny (diam. 139–400 μm) iron particles and, because NO is thereby converted to harmless N 2, the reaction is of interest for removing NO from, e.g. fluidised bed or other combustors. Whether the oxide produced is FeO, Fe 3O 4 or Fe 2O 3 is one subject of discussion and it is concluded that the product is mainly FeO. Two stages were found for the reaction. Initially, the rate of oxidation of iron is first order in NO and, in fact, is kinetically controlled. The outer layer of oxide produced on each iron particle is found to be very porous at the highest temperature (1173 K), but much less porous at the lowest temperature (973 K). The associated rate constant has a low activation energy (41±4 kJ/mol) and a pre-exponential factor close to the theoretical upper limit of the collision frequency. During the second stage of reaction, the observed rate is independent of the concentration of NO. This is because the rate is controlled by diffusion of Fe 2; ions and electrons from a particle’s metallic core, through vacancies in the layer of FeO (covering the unreacted Fe), to the outer surface of the particle. Interestingly, the second stage has a larger activation energy (155±20 kJ/mol) than the first stage. At the highest temperature (900°C) these iron particles can be oxidised completely, but at 700°C the reaction stops after only a conversion of ∼20% of Fe to FeO.