Change In Reaction Order Research Articles

Kinetics Study on the Hydrogen Reduction of Bauxite Residue-Calcite Sintered Pellets at Elevated Temperature

In this study, the isothermal reduction of bauxite residue-calcite sintered pellets by hydrogen at elevated temperatures and different gas flow rates was investigated. A thermogravimetric technique was applied to study the kinetics of the direct reduction by H2 at 500–1000 °C. It was observed that iron in sintered oxide pellets mainly exists in the form of brownmillerite, srebrodolskite and fayalite. The reduction of brownmillerite, the dominant Fe-containing phase, with hydrogen produces mayenite, calcite and metallic iron. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), BET surface area, pycnometer and mercury intrusion porosimeter analyses were adopted on reduced pellets to interpret the experimental results. The order of the reduction process changes from first-order reaction kinetics to second-order with an increasing reduction temperature. The change in reaction order may be due to sintering at higher reduction temperatures and corresponding physical and microstructural changes in pellets. The activation energy of reduction was calculated as 55.1–96.6 kJ/mol based on the experimental conditions and using different kinetic model equations. From the experimental observations, it was found that 1000 °C with 60 min is the most suitable condition for bauxite residue-CaO sintered pellets’ reduction with hydrogen.

Curing Kinetics of Methylene Diphenyl Diisocyanate-Based Polyurethane Elastomers.

The curing kinetics of MDI-based polyurethane elastomers were studied by non-isothermal differential scanning calorimetry (DSC). The kinetic parameters of the reaction system were calculated by the Kissinger method. The changing activation energy was observed by the Flynn–Wall–Ozawa method and the Friedman method. The results of model free fitting showed that the curing reaction could be divided into two stages, showing a change in reaction order when α > 0.45 and a piecewise curing mechanism function of the MDI-based polyurethane elastomers reaction system was deduced by autocatalytic model. The extrapolation method was used to determine the optimum curing conditions for the system, which can accurately describe the curing process. In addition, the optimal curing conditions are when: the constant temperature curing temperature of the system is 81 °C, the curing time is 29 min, and the post-curing temperature is 203 °C.

First-Order or Second-Order? Disproportionation of Lithium Superoxide in Li-O2 Batteries.

The disproportionation of LiO2 to Li2O2 is a key step in Li-O2 batteries, and it is regarded as a second-order reaction. However, its mechanism is not well addressed, and its kinetics is rarely studied due to the difficulties of quantifying the rate constants, particularly for high concentrations of superoxide (>10 mM). Here, we quantified the kinetic rate constant by a microkinetic model using a microelectrode tip with a thin diffusion layer and fast response. We report that the reaction order of LiO2 transitions from 1 at high concentrations of superoxide (∼20 mM) to 2 at low concentrations of superoxide (∼1 mM). LiO2 is chemically reduced by free superoxides to form Li2O2 and O2, instead of reacting with another LiO2 via a disproportionation step. This chemical-reduction mechanism explained the change of reaction order and the kinetics profile. As a rate-determining step, this step restricts the overall kinetics of the discharging process and should be the focus of future catalyst design.

Catalytic and kinetic study of the liquid-phase oxidation of lactose over Au/Al2O3 nanostructured catalysts in a monolithic stirrer reactor

The liquid-phase oxidation of lactose (LA) to lactobionic acid (LB) was studied over Au(2 %)/Al2O3 nanostructured catalyst. Catalytic tests were carried out at 65 °C, varying O2 partial pressures in the range of 0.21−1 bar and LA initial concentrations between 0.007 and 0.089 M. In all of the cases, the selectivity to LB was always 100 %. It was observed that the trends for the LA conversion (XLA) with time depends on the ratio of catalyst weight to initial LA moles W/nLA0 and the initial LA concentration CLA0. Experimental data were interpreted by kinetic modelling applying pseudo-homogeneous and Langmuir-Hinshelwood-Hougen-Watson (LHHW) models. It was found that irreversible surface oxidation of LA rules the reaction rate when W/nLA0= 30 g mol−1. At the early stages, the reaction rates did not depend on the LA concentration, and thus the reaction order with respect to LA was zero. In this case, an LHHW model considering total coverage of the active sites gives a very good fitting of the experimental data. As LA was consumed, the progress of conversion with time is different for each case and the reaction order changes to positive. Now, a better fitting is obtained with an LHHW model in which partial coverage of the active sites is considered. When liquid phase was diluted in LA and W/nLA0= 95 g mol−1, the LA chemisorption becomes the controlling step. Therefore, the results obtained by kinetic modelling helped to explain the dependence of LA oxidation rate with CLA0 and W/nLA0.

Trapping AsPh3 via reaction with NiS/γ-Al2O3 in the presence of H2: Reaction mechanism and kinetics

Removal of As from petroleum feedstocks is an important process which can be realized using As trapping mass containing supported nickel sulfide. In order to understand the mechanism of the trapping we studied the reaction of AsPh3 with NiS/γ-Al2O3 in the presence of H2 in a batch reactor in toluene solution at 230 °C. This reaction results in formation of NiAs, benzene and H2S. Also, the intermediate species, thiophenol and diphenylsulfide, were observed. Despite formation of NiAs layer in the course of reaction, the rate of AsPh3 decomposition is not affected by the solid state diffusion up to ∼ 50 % of nickel conversion. The rate determining step in these conditions appears to be the hydrogenolysis of As-C bonds. It was found that the reaction order changes in the course of reaction from zero to one. This trend is described by a kinetic model assuming adsorption of AsPh3 molecules followed by decomposition.

Catalytic Oxidation of Methylene Blue by Use of Natural Zeolite-Based Silver and Magnetite Nanocomposites

This work reports the synthesis of natural zeolite-based silver and magnetite nanocomposites and their application for the catalytic oxidation of methylene blue in water. The zeolite was impregnated with 5.5 wt.% Fe in the form of magnetite nanoparticles with size of 32 nm, and with 6.4 wt.% Ag in the form of silver oxide and metallic silver nanoparticles with sizes of 42 and 20 nm, respectively. The results showed that physical adsorption contributed to the removal of methylene blue by 25–36% and that Fe3O4@NZU is superior to Ag2O@NZU and Ag0@NZU, leading to 55% removal without oxidant and 97% in the presence of H2O2. However, there is no evidence of significant mineralization of methylene blue. The application of reaction rate models showed that the reaction order changes from zero to first and second order depending on the H2O2 concentration.

Hydrogen dependence of the reaction mechanism and kinetics of water gas shift reaction on Ni catalyst: Experimental and DFT study

This paper presents a combined experimental and density functional theory study of the relative reactivity of surface species O*, OH*, and H* with CO* on nickel catalysts and their catalytic consequence in reaction mechanism and kinetics of water gas shift reaction. The kinetic study illustrates the hydrogen reaction order changes from 0.5 at relatively low hydrogen pressures to -1 at high hydrogen pressures. Detailed kinetic analysis indicated a hydrogen-induced change of the corresponding reaction pathway from hydrogen assisted CO activation to the redox mechanism with CO*+O* as a rate-determining step. The DFT investigation revealed that the surrounding surface H* atoms destabilize more significantly O* adsorption than H* adsorption, thus enhance more the reactivity of O* than H* towards reaction with CO* at high H* coverage. This kinetic study provides an insightful depiction for the future study of CO activation on other transition metals and the catalyst development for WGS reaction.

Emulsion copolymerization of vinyl monomers from soybean and olive oil: Effect of counterpart aqueous solubility

Highly hydrophobic vinyl monomers from soybean (SBM) and olive (OVM) oil with different amounts of unsaturation were copolymerized in emulsion with significantly more soluble in water comonomers, methyl methacrylate (MMA), vinyl acetate (VA), and methyl acrylate (MA). Our intention was to demonstrate that higher aqueous solubility of the comonomer impacts the mechanism of the emulsion process by balancing latex particle homogeneous and heterogeneous (micellar) nucleation.Using an experimental method based on water insoluble dye Blue 70, it is shown that particle nucleation indeed depends to a great extent on aqueous solubility of the comonomer, along with the plant oil-based monomer (POBM) content in the feed. Upon addition of POBM into the monomer mixture with MMA (aq.solubility 1.5 wt%), the latex polymer particles originating from micellar nucleation essentially increased. In copolymerization of POBM with MA (aqueous solubility 4.9 wt%), this effect is even more pronounced. Those findings clearly indicate that copolymerization loci change when monomer feed hydrophobicity increases, and the magnitude of this effect is dependent on the comonomer aqueous solubility.Experimentally determined changes in reaction order with respect to emulsifier and initiator upon addition of POBM confirm that kinetics of emulsion copolymerization is impacted by comonomer aqueous solubility. The latter finding can be indicative of changing latex particle nucleation mode when monomer feed hydrophobicity changes.Overall, the portion of latex particles originating through micellar nucleation is higher in the copolymerization of more unsaturated SBM (with both MMA and MA) in comparison to OVM. No impact of POBM content on the reaction kinetics was observed in copolymerization with VA (aq. solubility 2.5 wt%). Here, high reactivity of growing PVA radicals in chain transfer reactions makes the presence of POBM in monomer feed kinetically irrelevant.

Kinetic Investigation of the Sustainable Electrocatalytic Hydrogenation of Benzaldehyde on Pd/C: Effect of Electrolyte Composition and Half-Cell Potentials

Electrocatalytic reduction of benzaldehyde to benzyl alcohol on Pd supported on carbon felt was conducted in the aqueous phase using a continuous flow fixed-bed reactor at room temperature and atmospheric pressure. Methanol, ethanol, or isopropanol was added to the electrolyte to study the impact of alcohol type and concentration on the rates of benzaldehyde electrocatalytic hydrogenation (ECH) and H2 evolution, which is the prevalent side reaction. Whereas the ECH rates and Faradaic efficiency decreased with increasing alcohol concentrations, H2 evolution rates remained constant. The impact of the alcohol on hydrogenation was greater as the length of the alcohol’s hydrocarbon chain increased. Increasing the benzaldehyde concentration allows for high ECH rates and high Faradaic efficiency. The reaction order increased from ∼0.13 to ∼0.66 with half-cell potential increasing from −650 to −1150 mV (vs Ag/AgCl). Kinetic analysis reveals that the changes in reaction order are due to changes in benzaldehyde (an...

Catalytic carbon gasification: Understanding catalyst-carbon contact and rate jump behavior with air

It is easier to optimize reactions when the mechanism is well understood. Nowadays, catalytic carbon gasification is an area of industrial importance. The mechanism based on C-bulk diffusion has been recently updated. The relevance of the Tammann temperature to get efficient carbon/catalyst nanoparticle contact is now better understood. However, the interaction between kinetics and thermodynamics still needs some clarification. Rate jump is a kinetic phenomenon observed in some cases in catalytic carbon gasification by air or oxygen following a minor increase in temperature (v.g. ΔT = 5 °C). This occurrence has been reported, but the phenomenon is not well understood. In this short review, we show that the rate jumps can be consistently explained by the “carbon-worm” mechanism due to a jump in the temperature of the moving nanocatalyst particles. The carbon bulk diffusion step is then much faster and the external film mass transfer becomes the rate-limiting step. The reaction order changes from zero to one. The nature and role of catalyst‑carbon contact in catalytic carbon gasification is discussed.

The catalyzing effect of chromate in the chlorate formation reaction

Electrolyte used in the manufacturing process of sodium chlorate contains chromate primarily to inhibit cathodic loss reactions. Chromate also accelerates the chlorate formation reaction which reduces the concentration of hypochlorite species in the chlorate cell leading to enhanced anodic current yield, reduced risk of oxygen explosions and reduced cost for the purification of the cell gas.Laboratory scale trials show that the rate of chlorate formation can be described as an ‘uncatalyzed’ third order reaction with respect to hypochlorite species and a parallel chromate catalyzed reaction of apparent reaction order 2.2. Validation is presented based on data from electrolysis trials in pilot plant and bench scale at technically relevant conditions. If the dichromate concentration is increased from 0 to 5g/L, more than 50% of the chlorate may be formed via the chromate catalyzed path and the apparent reaction order changes from 3 to about 2.4. The kinetic effect of the chromate species also lowers the optimum reaction pH below that of the uncatalyzed reaction. Furthermore, anionic hypochlorite does not seem to be necessary to generate chlorate via the chromate catalyzed path.Other hypochlorite consuming reactions such as anodic and homogeneous oxygen formation as well as cathodic reduction and desorption of chlorine species in the cell gas only have a marginal effect on the hypochlorite concentration in the chlorate cell.

Deactivation of Pt/Al2O3 during propane oxidation at low temperatures: Kinetic regimes and platinum oxide formation

The kinetics of propane oxidation over Pt/Al2O3 are investigated in this work as a function of O2/C3H8 ratio in the 150–300°C temperature range. At high O2/C3H8 ratios, the platinum nanoparticles are saturated with oxygen and the reaction rate is zero-order with respect to the oxygen partial pressures in this regime. As the oxygen coverage decreases with decreasing O2/C3H8 ratio, the reaction rate increases and the reaction order changes from zero-order to negative-order in the oxygen partial pressure. The reaction rate is controlled to a large extent by the oxygen coverage on the platinum nanoparticles. However, at lower temperatures and higher oxygen pressures there is a slow deactivation of the catalyst that cannot be explained by a slow change in the oxygen coverage. Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) of adsorbed CO was performed to track the evolution of the nanoparticle structure over the course of the propane oxidation reaction and to determine whether the slow deactivation was caused by reconstruction of the platinum nanoparticles. We found that the platinum nanoparticles are significantly reconstructed during the course of the reaction, including the formation of a platinum oxide (PtO) which has a characteristic CO-DRIFTS band at 2123cm−1. The extent of PtO formation decreases with increasing temperature and, as a result, deactivation of the catalyst is less severe at higher temperatures. Unexpectedly, increasing the oxygen partial pressure resulted in less PtO formation. We believe that a different platinum oxide phase (e.g. PtO2 or Pt3O4) is formed at higher oxygen pressures, which is reduced to metallic platinum during CO exposure at 25°C, and therefore is not detectable by CO-DRIFTS. These results are unique because they show how the nanoparticle structure evolves over many hours of propane oxidation, and how the temperature and oxygen pressure influence the reconstruction of the nanoparticles, which has implications for a wide range of reactions not limited to propane oxidation.

On-line pyrolysis kinetics of swine manure solid samples collected from rearing farm

The on-line dynamic pyrolysis kinetics of swine manure solid samples was investigated in this paper. It has been found that the model which includes combined Friedman’s isoconversional and multi-distributed reactivity approaches can best describe the pyrolysis process. Pyrolysis kinetics of swine manure samples proceeds through two major pyrolysis zones, where in first zone exists the kinetically complex reactions related to decompositions of hemicelluloses and fat/proteins, as well as decomposition reactions related to degradation of cellulose and lignin structures. Second zone includes the reactions attached to follow-up devolatilization and carbonization processes. It was found that identified high reaction orders represent the measure of complexity and multiplicity of the mechanism. It has been found that the change in reaction order (n) values is the result of reactions that are taking place due to the instability of products, or the secondary reactions, change in apparent activation energy, and effect of varying of the pre-exponential factor values.

The mechanism of dissolution of forsterite, olivine and minerals of the orthosilicate group

The dissolution of forsterite and other minerals of the olivine and phenakite silicate groups are described by a novel mechanism of dissolution. For many of these minerals, the order of reaction with respect to H+ is close to 0.5 in the acidic region. The mechanism of dissolution proposed here correctly predicts these orders of reaction without any adjustable parameters. Recent work has shown that the order of reaction of forsterite with respect to H+ changes from 0.5 in the acidic region to 0.25 in the region above a value of pH of approximately 6. Previously proposed models of dissolution cannot predict this change in order of reaction, whereas the mechanism proposed here does predict this change in reaction order, again without any adjustable parameters. The mechanism proposes that the reason for the change in order of reaction is that the H+ needs to be positioned at the inner Helmholtz plane to be effective at higher values of pH. The acceleration of the rate of dissolution by organic acids and the retardation of the rate by dissolved silica and carbon dioxide are also correctly predicted. The mechanism predicts a change in the interfacial potential difference at the same value of pH that the order of reaction changes. This prediction is verified by measurements of the zeta potential, which reflects the predicted change in surface potential at a pH of approximately 6. The proposed mechanism provides a framework for the interpretation of the correlation between the rate of dissolution of the orthosilicates and the exchanges rates of water in the inner sphere of the corresponding metal aqua-ion.

The mechanism of dissolution of minerals in acidic and alkaline solutions: Part II Application of a new theory to silicates, aluminosilicates and quartz

The kinetics of the dissolution of silicates, aluminosilicates and quartz is described by a novel theory of dissolution. The experimental data for the rate of dissolution of these minerals shows a remarkable pattern: for many of these minerals, the order of reaction with respect to H+ is close to 0.5 in the acidic region, and the order of reaction with respect to OH− is close to 0.5 in the alkaline region. It is proposed that the site of bond breaking in the rate-determining step of dissolution is the weakest bond, and this is frequently the metal–oxygen bond because of the higher bond energy of silicon–oxygen bonds. Alternatively, the least number of silicon–oxygen bonds is broken. This means that silicate groups react intact as a unit. Both metal atoms and silicate groups react and are removed independently. The rates of these independent processes are coupled by the potential at the surface. In the acid region, H+ reacts with silicate groups at the surface. In the alkaline region, OH− ions react with the metal atom at the surface. The proposed theory of dissolution correctly predicts the observed orders of reaction with respect to H+ ions and OH− ions in solution. The order of reaction of forsterite with respect to H+ changes from 0.5 in the acidic region to 0.25 in the region above a value of pH of approximately 6. The proposed mechanism suggests that the reason for the change in order of reaction is that the H+ needs to be positioned at the inner Helmholtz plane to be effective in alkaline solutions.

On the variation of dissolution rates at the orthoclase (0 0 1) surface with pH and temperature

The pH- and temperature-dependent variations of the dissolution rates of orthoclase (001) surfaces in acidic aqueous solutions were examined in situ by using X-ray reflectivity and compared to density functional theory results. A phenomenological analysis of these data using the conventional relation Rd(pH,T)=pH−α exp(−ΔEapp/kT), with an apparent activation energy of ΔEapp∼65kJ/mol, requires a variable pH-dependent order of dissolution having a value of α∼1 for pH⩾2.5, and α∼0.37 for pH⩽2.5. These data also can be described by a two-term Arrhenius model characterized by activation energies, ΔEA and ΔEB, with prefactors in the form of Langmuir adsorption isotherms controlled by the proton adsorption enthalpies, δE. This analysis reveals that dissolution at the primary reactive site (site A) for pH>0.5 has an apparent activation energy of ΔEA=67±3kJ/mol and a proton adsorption enthalpy of δEA=−13.0±0.5kJ/mol. A secondary reactive site (site B), which dominates dissolution at pH<0.5, is described by the sum of these two energies, ΔEB+δEB=66±3kJ/mol. Density functional theory calculations provide insight into activation energies of water reactions with the protonated sites as a function of site coordination. These results suggest that the dominant reactive site for pH>0.5 is the bridging oxygen between Al and Si sites, while the dominant reactive for pH<0.5 is identified as the bridging oxygen between Si surface sites. The superposition of these two reactions leads to the change in the apparent order of reaction near pH 2. Comparison to kinetic theory reveals that the pre-exponential factor for dissolution reaction at site A is well-reproduced by the proton impingement rate, suggesting that the sticking coefficient is effectively unity.

Study of thermal behavior of phytic acid

Phytic acid is a natural compound widely used as depigmenting agent in galenic cosmetic emulsions. However, we have observed experimentally that phytic acid, when heated to 150 ºC for around one hour, shows evidence of thermal decomposition. Few studies investigating this substance alone with regard to its stability are available in the literature. This fact prompted the present study to characterize this species and its thermal behavior using thermal analysis (TG/DTG and DSC) and to associate the results of these techniques with those obtained by elemental analysis (EA) and absorption spectroscopy in the infrared region. The TG/DTG and DSC curves allowed evaluation of the thermal behavior of the sample of phytic acid and enabled use of the non-isothermal thermogravimetric method to study the kinetics of the three main mass-loss events: dehydration I, dehydration II and thermal decomposition. The combination of infrared absorption spectroscopy and elemental analysis techniques allowed evaluation of the intermediate products of the thermal decomposition of phytic acid. The infrared spectra of samples taken during the heating process revealed a reduction in the intensity of the absorption band related to O-H stretching as a result of the dehydration process. Furthermore, elemental analysis results showed an increase in the carbon content and a decrease in the hydrogen content at temperatures of 95, 150, 263 and 380 °C. Visually, darkening of the material was observed at 150 °C, indicating that the thermal decomposition of the material started at this temperature. At a temperature of 380 °C, thermal decomposition progressed, leading to a decrease in carbon and hydrogen. The results of thermogravimetry coupled with those of elemental analysis allow us to conclude that there was agreement between the percentages of phytic acid found in aqueous solution. The kinetic study by the non-isothermal thermogravimetric method showed that the dehydration process occurred in two stages. Dehydration step I promoted a process of vaporization of water (reaction order of zero), whereas dehydration step II showed an order of reaction equal to five. This change in reaction order was attributed to loss of chemically bonded water molecules of phytic acid or to the presence of volatile substances. Finally, the thermal decomposition step revealed an order of reaction equal to one. It was not possible to perform the kinetic study for other stages of mass loss.

Variation of Avrami parameter during non-isothermal surface crystallization of glass powders with different sizes

The influence of grain size on the surface crystallization kinetics is investigated by non-isothermal Differential Thermal Analysis and Scanning Electron Microscopy. For this purpose, different powder fractions of a model glass forming diopside via surface crystallization and crystallization induced porosity, are used. The activation energy of crystallization, EC, is determined from Kissinger plot, and the Avrami parameter, m, is estimated by both Ozawa and of Augis–Bennett methods. The values of EC decrease from 530 to 300kJ/mol when grain size increases. Simultaneously, the m value drops from 2.5 to 1.3. This result contradicts to the widespread fallacy that the reaction order should be equal to one in the case of surface crystallization. During the initial stage of surface crystallization three dimensional growth takes place, a fact confirmed by the SEM investigations. Later on, one dimensional growth inwards the grain is observed, leading to the reaction order changes from 3 to 1.

Characterization and kinetics study of the photochlorination of polyethylene

The radical chlorination of polyethylene was carried out in perchloroethylene solvent using chlorine gas and ultraviolet photoradiation to investigate a reaction kinetics model. The recurrent structure of chloromethylene (–CHCl–) and methylene (–CH2–) groups and the percentage of substituted chlorine were determined using hydrogen-nuclear magnetic resonance (H-NMR) (500 MHz) spectra. A kinetics model has been proposed for this reaction in which the conversion of methylene (–CH2–) units to chloromethylene (–CHCl–) units was considered to be the main reaction step. The simplified version of the obtained kinetics model indicates that the reaction is first order with respect to the chlorine gas concentration and zero order with respect to the methylene concentration up to 40% chlorination. At the first stage of the reaction, the concentration of γ-methylene sequences is high. Once the degree of chlorination exceeds 40%, the reaction order changes and becomes zero order with respect to the chlorine gas concentration and first order with respect to the methylene group concentration. At this stage of the reaction, the concentration of γ-methylene groups has decreased considerably.

Coke analysis by temperature-programmed oxidation: Morphology characterization

Temperature-programmed oxidation (TPO) profiles obtained using high final temperature are usually very similar in shape and it is not possible to easily distinguish among different kinetic models. Typically, statistical criteria are used to select the one that best fits the experimental TPO profile. In this work, we show that using final temperatures selected in such a way that the coke is still reacting with oxygen, relevant information can be obtained from the experimental profile. Coked naphtha reforming catalysts, obtained from an industrial reactor, are characterized by running TPO analyses using intermediate temperatures. Results clearly show that the coke reaction order changes as the coke conversion increases. When the initial coke content is approximately 5 wt% or higher, coke has a tridimensional structure that leads to a low reaction order, close to 0.2. At high conversion levels, approximately 80%, the coke reaction order starts increasing until reaching a value of 1 at conversion close to 1. This information is easily obtained by plotting the experimental coke reaction order using the data at constant temperature. In this way, it is possible to easily distinguish among different models, and to replicate the experimental results.