Effective Reaction Order Research Articles

Kinetics of dehydrogenation of n-heptane over GaPt supported catalytically active liquid metal solutions (SCALMS).

The concept of Supported Catalytically Active Liquid Metal Solutions (SCALMS) was explored for the catalytic dehydrogenation of n-heptane. For this purpose, a GaPt on alumina (Ga84Pt/Al2O3) was compared with a Pt on alumina catalyst at different reaction temperatures and feed compositions. While the observed activation energies with both catalysts for the overall n-heptane depletion rate were similar with both catalysts, the SCALMS systems provides a lower activation energy for the desired dehydrogenation path and significantly higher activation energies for the undesired aromatization and cracking reaction. Thus, the SCALMS catalyst under investigation shows technically interesting features, in particular at high temperature operation. The partial pressure variation revealed an effective reaction order of around 0.7 for n-heptane for both catalysts, while the effective order for hydrogen was 0.35 for Pt/Al2O3 and almost zero for SCALMS.

Silicon Carbide Supported Liquid Phase (SLP) Hydroformylation Catalysis – Effective Reaction Kinetics from Continuous Gas‐phase Operation

AbstractThe hydroformylation reaction is one of the most important applications of homogeneous catalysis on an industrial scale with worldwide production capacities exceeding 15 Mio tons per year. Efficient catalyst immobilization becomes mandatory for such large‐scale processes. In this work, a supported liquid phase (SLP)‐type catalyst on a silicon carbide support material was applied in the continuous Rh‐bpp (bpp=biphephos) catalyzed gas‐phase hydroformylation of but‐1‐ene. A parameter variation was carried out in the pressure range from 0.5 to 3.0 bara for but‐1‐ene, 0.5 to 5 bara for H2, 0.5 to 5 bara for CO, and in the temperature range from 90 to 125 °C. Activation energies were determined for the individual reactions showing a shift in activation energy most significantly for the hydroformylation to n‐pentanal for temperatures of 110 °C and above. For all substrates the effective reaction orders were calculated and employed in a power‐law rate model giving good agreement of measured and modeled data within a ±20 % error margin. Similar results were obtained using a microkinetic model.

PEM Single Cells under Differential Conditions: Full Factorial Parameterization of the ORR and HOR Kinetics and Loss Analysis

The anode and cathode kinetics are parameterized based on differential cell measurements. Systematic parameter variations are evaluated to disentangle the dependencies of the electrochemical impedance spectroscopy (EIS) signatures in H2/H2 mode. We introduce a new CO recovery protocol for both electrodes that enables to accurately characterize the hydrogen oxidation reaction (HOR) kinetics. Then, we demonstrate that a compact Tafel kinetics law captures the oxygen reduction reaction (ORR) kinetics for a full factorial grid of conditions, covering a wide range of relative humidities (rH), temperatures, oxygen partial pressures and current densities. This yields the characteristic activation energy and effective reaction order, and we reconcile models that make different assumptions regarding the rH dependency. Moreover, we analyze O2 transport contributions by steady-state and transient limiting current techniques and heliox measurements. Although the rising uncertainty of loss corrections at high current densities makes it impossible to unambiguously identify an intrinsic potential-dependent change of the Tafel slope, our data support that such effect needs not be considered for steady-state cathodic half-cell potentials above 0.8 V.

Modeling investigation of Sm0.5Sr0.5CoO3−δ–Ce0.8Sm0.2O2−δ cathode for proton-conducting ceramic fuel cell

Simulation of proton ceramic fuel cell is of great importance in understanding their working mechanisms. Cathode activation polarization is a critical concern in the simulation. In this paper, Sm0.5Sr0.5CoO3−δ–Ce0.8Sm0.2O2−δ cathode is selected as studied case. The effective reaction order, which is the exponential of the oxygen partial pressure in cathode Butler-Volmer equation, is determined via our model. The dominant process in cathode reaction is investigated and identified. Among the involved reaction steps, model result points out that the surface diffusion of absorbed oxygen ion is dominant, which is in accord with the report. Moreover, single button cells with BaCe0.7Zr0.1Y0.2O3−δ electrolyte and Sm0.5Sr0.5CoO3−δ–Ce0.8Sm0.2O2−δ cathode are prepared, and the measured cell output voltages are compared with the simulated ones. The predicted cell outputs via our full cell simulation match the measurements under diverse experimental settings, suggesting that the method and parameters in our simulation are applicable. Our models facilitate both the analysis of experimental results and the development of proton-conducting solid oxide fuel cell model.

Effect of oxygen concentration on combustion behavior of single coal pellets from three different ranks in a concentrating photothermal reactor

Comparative combustion studies were performed in a concentrating photothermal platform (room temperature chamber type experimental bench with less volumetric reactions) on coal pellets (>6mm) from three different ranks: bituminous coal, sub-bituminous coal, and lignite. The combustion of single coal pellet in 21 ~ 50% oxygen concentration was observed with IR pyrometry and high-speed cinematography to obtain temperature–time–size histories. Based on the combined mass measurement and gaseous product analysis, a comprehensive understanding of the effects of oxygen concentration on single coal pellet combustion behaviours was developed. Distinctive three-stage processes were proposed with characteristical temperature and cinematography variations. No obvious flame duration was detected in 21% or 30% O2 while transient but luminous flames were clearly observed in 40% and 50% O2 and formed notable temperature peaks in the second stage during combustion of all the three ranks coal pellets. Volume and density both changed non-linearly during combustion and such processes were accelerated with the increase of oxygen concentration. High concentrations of CO and CH4 were detected due to the suppression of volumetric reactions. CO accounted for about 9.3% of the gaseous products and consistently exhibited two peaks with the first peak independent from oxygen concentration. Three-stage combustion process was further validated by CO2/CO ratio and the effective reaction orders of oxygen concentration were 1.91 (bituminous coal), 1.81 (sub-bituminous coal) and 1.26 (lignite).

Detailed Kinetic Investigations on the Selective Oxidation of Biomass to Formic Acid (OxFA Process) Using Model Substrates and Real Biomass

A detailed kinetic analysis of the selective catalytic oxidation of different biogenic model substrates to formic acid (OxFA process) is presented using two different polyoxometalate catalysts, HPA-2 (H5PV2Mo10O40) and HPA-5 (H8PV5Mo7O40). By using high oxygen pressures (30–60 bar) and temperatures of 80–90 °C in aqueous solution, we were able to investigate the rate-determining substrate oxidation step catalyzed by the oxidized form of the polyoxometalates (POMs) by keeping the concentration of the catalytic active species constant. Under these conditions, kinetic parameters like effective reaction order and reaction rate constants could be determined by the differential method for model substrates like glucose, fructose, sorbitol, and gluconic acid. Furthermore, kinetic experiments using the two different catalysts showed a different behavior for various functionalities within the carbon framework of the substrate with the tendency of higher reaction rates with the HPA-5 catalyst. This effect was more p...

Importance of metal-oxide interfaces in heterogeneous catalysis: A combined DFT, microkinetic, and experimental study of water-gas shift on Au/MgO

The water-gas shift (WGS) reaction is central to a spectrum of industrially important catalytic processes, ranging from the manufacture of hydrogen to the processing of biomass-derived feedstocks. Recently, oxide-supported Au catalysts have attracted attention for low-temperature WGS reactions, but the role of the Au/oxide interface in promoting this chemistry remains under debate. In this contribution, we combine periodic Density Functional Theory (DFT) calculations, detailed microkinetic modeling, and rigorous kinetic measurements to elucidate the impact of this interface on the molecular-level features of WGS chemistry of a gold nanowire supported on a MgO(100) substrate. The results demonstrate that the barrier to activate water, which is prohibitively high (∼2eV) on a clean Au(111) surface, is decreased to essentially zero at the Au/MgO interface. From the DFT-calculated energetics, a dual-site microkinetic model of the Au/MgO interface is constructed. Rate and thermodynamic control analysis demonstrate both the high degree of kinetic control of COOH formation at the interface and a strong influence of competitive adsorption between CO and H. A procedure to refine the microkinetic predictions by iterative replacement of the energies of kinetically sensitive steps with a higher accuracy hybrid HSE06 prediction is introduced, and the determined effective activation barriers and reaction orders agree well with the results of detailed kinetic measurements on Au nanoparticles on MgO substrates. The results clearly show the critical role that the metal/oxide interface plays in WGS catalysis, and the approach introduced to predict kinetics at the metal/oxide interface should be applicable to a variety of catalytic processes on oxide-supported metal nanoparticles.

Characterization of the dominating bulk recombination in bulk-heterojunction blends using photoinduced absorption

In this work we show how to clarify the dominating bulk recombination in organic solar cells by using photoinduced absorption. We show how to use the intensity and frequency dependence of the in-phase and quadrature signals to obtain the effective reaction order. For trap-assisted recombination, we can show using a multiple trapping and retrapping model with an exponential tail-state distribution that a temperature dependent reaction order is obtained which allows for determination of the characteristic energy of the exponential distribution of trap-states. In the model system pBTTT:PC60BM, we show that trap-assisted recombination is the dominating bulk recombination in 1:1 blends with a characteristic energy of the exponential trap distribution Ech=44±5 meV. The 1:4 blend, on the other hand, shows temperature independent behavior in good agreement with a dominating 2D Langevin bulk recombination.

Online Monitoring of Vinyl Chloride Polymerization in a Microreactor Using Raman Spectroscopy

AbstractA novel capillary‐based microfluidic device has been designed to follow the vinyl chloride polymerization reaction. Monodisperse droplets of 200 μm diameter could be obtained by means of a co‐flow generation system, each one being considered as a polymerization reactor. Monomer droplets were visualized in a microchannel with a high‐speed camera. At the end of the reaction, PVC grains were observed with a scanning electron microscopy technique. Real‐time non‐invasive Raman measurement was performed on stationary vinyl chloride monomer droplets and provided values of effective reaction orders and rate constants. This microdevice allowed reaction investigation under difficult conditions of pressure and temperature with a minimal amount of reagents.

Improved Method for Kinetic Studies in Microreactors Using Flow Manipulation and Noninvasive Raman Spectrometry

A novel method has been devised to derive kinetic information about reactions in microfluidic systems. Advantages have been demonstrated over conventional procedures for a Knoevenagel condensation reaction in terms of the time required to obtain the data (fivefold reduction) and the efficient use of reagents (tenfold reduction). The procedure is based on a step change from a low (e.g., 0.6 μL min(-1)) to a high (e.g., 14 μL min(-1)) flow rate and real-time noninvasive Raman measurements at the end of the flow line, which allows location-specific information to be obtained without the need to move the measurement probe along the microreactor channel. To validate the method, values of the effective reaction order n were obtained employing two different experimental methodologies. Using these values of n, rate constants k were calculated and compared. The values of k derived from the proposed method at 10 and 40 °C were 0.0356 ± 0.0008 mol(-0.3) dm(0.9) s(-1) (n = 1.3) and 0.24 ± 0.018 mol(-0.1) dm(0.3) s(-1) (n = 1.1), respectively, whereas the values obtained using a more laborious conventional methodology were 0.0335 ± 0.0032 mol(-0.4) dm(1.2) s(-1) (n = 1.4) at 10 °C and 0.244 ± 0.032 mol(-0.3) dm(0.9) s(-1) (n = 1.3) at 40 °C. The new approach is not limited to analysis by Raman spectrometry and can be used with different techniques that can be incorporated into the end of the flow path to provide rapid measurements.

Reaction Mechanism of Monoethanolamine with CO2 in Aqueous Solution from Molecular Modeling

We present a theoretical study of the reaction mechanism of monoethanolamine (MEA) with CO₂ in an aqueous solution. We have used molecular orbital reaction pathway calculations to compute reaction free energy landscapes for the reaction steps involved in the formation of carbamic acids and carbamates. We have used the conductor-like polarizable continuum model to calculate reactant, product, and transition state geometries and vibrational frequencies within density functional theory (DFT). We have also computed single point energies for all stationary structures using a coupled cluster approach with singles, doubles, and perturbational triple excitations using the DFT geometries. Our calculations indicate that a two-step reaction mechanism that proceeds via a zwitterion intermediate to form carbamate is the most favorable reaction channel. The first step, leading to formation of the zwitterion, is found to be rate-determining, and the activation free energies are 12.0 (10.2) and 11.3 (9.6) kcal/mol using Pauling (Bondi) radii within the CPCM model at the CCSD(T)/6-311++G(d,p) and CCSD(T)/6-311++G(2df,2p) levels of theory, respectively, using geometries and vibrational frequencies obtained at the B3LYP/6-311++G(d,p) level of theory. These results are in reasonable agreement with the experimental value of about 12 kcal/mol. The second step is an acid-base reaction between a zwitterion and MEA. We have developed a microkinetic model to estimate the effective reaction order at intermediate concentrations. Our model predicts an equilibrium concentration for the zwitterion on the order of 10⁻¹¹ mol/L, which explains why the existence of the zwitterion intermediate has never been detected experimentally. The effective reaction order from our model is close to unity, also in agreement with experiments. Complementary ab initio QM/MM molecular dynamics simulations with umbrella sampling have been carried out to determine the free energy profiles of zwitterion formation and proton transfer in solution; the results confirm that the formation of the zwitterion is rate-determining.

Effect of reactivity loss on apparent reaction order of burning char particles

Considerable debate still exists in the char combustion community over the expected and observed reaction orders of carbon reacting with oxygen. In particular, very low values of the reaction order (approaching zero) are commonly observed in char combustion experiments. These observations appear to conflict with porous catalyst theory as first expressed by Thiele, which suggests that the apparent reaction order must be greater than 0.5. In this work, we propose that this conflict may be resolved by considering the decrease in char reactivity with burnout due to ash effects, thermal annealing, or other phenomena. Specifically, the influence of ash dilution of the available surface area on the apparent reaction order is explored. Equations describing the ash dilution effect are combined with a model for particle burnout based on single-film nth-order Arrhenius char combustion and yield an analytical expression for the effective reaction order. When this expression is applied for experimental conditions reflecting combustion of individual pulverized coal particles in an entrained flow reactor, the apparent reaction order is shown to be lower than the inherent char matrix reaction order, even for negligible extents of char conversion. As char conversion proceeds and approaches completion, the apparent reaction order drops precipitously past zero to negative values. Conversely, the inclusion of the ash dilution model has little effect on the char conversion profile or char particle temperature until significant burnout has occurred. Taken together, these results suggest that the common experimental observation of low apparent reaction orders during char combustion is a consequence of the lack of explicit modeling of the decrease in char reactivity with burnout.

UC oxidation with HNO2 in aqueous solutions of HNO3 and HClO4

The oxidation of UC powder was studied in 0.5–6.0 M HNO 3 and HClO 4 , containing 0.01–0.1 M NaNO 2 , at ambient temperature. The addition of nitrous acid caused the oxidation potential to exceed 400 mV/SCE and thus accelerated the dissolution and increased the dissolution yield to exceed 95%. One hundred percent dissolution at room temperature was achieved within 30 to 40 min when 3.0–6.0 M HNO 3 was used that contained more than 0.005 M HNO 2 .The dissolution rate increased with increasing HNO 3 concentration and initial solid-to-liquid ratio. The effective dissolution rate constants and reaction orders were estimated for the UC oxidation with H + , NO 3 - and HNO 2 in 0.5–6.0 M HClO 4 . The rate of UC oxidation with HNO 2 exceeded 50–80 times its oxidation rates with NO 3 - ions. During UC oxidation with HNO 2 in 0.5–6.0 M HClO 4 , 60–85% carbon was oxidized to CO 2 . The C/U(VI) ratio in the dissolver solution decreased from 0.5 to 0.1 mg eq/mol with increasing of HClO 4 and HNO 2 solution concentration. Two to five percent of the carbon remained in the insoluble residue of the HNO 3 dissolver solutions, even when a 100% U(VI) yield was observed. Scanning electron microscopy of the insoluble residue showed the carbon particle size between 1.0 and 5.0 μm.

Catalytic wet oxidation of phenol over lanthanum strontium manganite

The catalytic performances of lanthanum strontium manganite (La0.8Sr0.2)Mn0.98O3, in catalytic wet oxidation (CWO) of a phenol solution under milder conditions of temperature (398–498K) and pressure (Po2=4 bar), in a batch reactor, have been investigated. Aim of this study is the evaluation of the effect of temperature, catalyst loading, phenol concentration and stirrer speed on phenol conversion. Experimental data obtained from the different test conditions are best-fitted to evaluate the effective reaction order and apparent activation energy.

Quartz crystal microbalance measurements of kinetic parameters of anodic dissolution of gold in thiocarbamide electrolytes in the presence of sulfide ions

As shown by quartz-crystal microbalance measurements, in the potential range from 0.0 to 0.55 V (NHE), sulfide ions adsorbed on the gold electrode surface accelerate the electrode reaction of anodic dissolution of gold in acidic thiocarbamide solutions. The microbalance determination of kinetic parameters at a constant electrode surface coverage with sulfide ions includes a special procedure developed for the determination of the gold dissolution rate. The conditions (the potential range and the potential scan rate) of independence of the dissolution rate from the diffusion limitations associated with the ligand delivery is determined. Under these conditions, the polarization curve is shown to be linear on semilogarithmic coordinates and correspond to the Tafel equation. In this potential range, the transfer coefficient α and the reaction order with respect to the ligand p are determined at a constant electrode surface coverage θ with adsorbed sulfide ions. It is shown that with the transition from the surface coverage with sulfide ions θ = 0.1 to θ = 0.8, the transfer coefficient α changes from 0.25 to 0.55, the exchange current (i 0) changes from 10−5 to 5 × 10−5 A/cm2, and the effective reaction order p with respect to the ligand changes from 0.2 to 1.3. The mentioned changes are associated not only with the acceleration of gold dissolution in the presence of chemisorbed sulfide ions but also with the changeover in the mechanism of this process. Quartz-crystal microbalance data on the gold dissolution rate qualitatively agree with the results of voltammetric measurements of a renewable gold electrode. A possible version of explanation of the catalytic effect of sulfide ion adsorption on the gold dissolution is put forward.

A reduced mechanism for methane and one-step rate expressions for fuel-lean catalytic combustion of small alkanes on noble metals

A reduced mechanism and a one-step rate expression for fuel-lean methane/air catalytic combustion on an Rh catalyst are proposed. These are developed from a detailed microkinetic model using a computer-aided model reduction strategy that employs reaction path analysis, sensitivity analysis, partial equilibrium analysis, and simple algebra to deduce the most abundant reaction intermediate and the rate-determining step. The mechanism and the one-step rate expression are then tested on Pt catalyst. It is found that the reaction proceeds effectively via the same mechanistic pathway on both noble metals, but the effective reaction orders differ due to the difference in the adsorption strength of oxygen. Based on the homologous series idea, the rate expression is extended to small alkanes (ethane and propane; butane is also briefly discussed) and is found to reasonably describe experimental data. Estimation of the relevant parameters in the rate expression for various fuels and catalysts using the semiempirical bond-order conservation theory, quantum mechanical density functional theory, and/or simple experiments is discussed. Finally, it is proposed that detailed microkinetic models with coverage-dependent parameters can assist in rationalizing the apparent discrepancies between experimental data from various research groups.

Polycrystalline-silicon LPCVD by silane pyrolysis: The effect of hydrogen injection

Experiments are conducted to investigate the effect of hydrogen injection on polycrystalline-silicon (poly-Si) deposition by silane (SiH4) pyrolysis at 640–700°C in an industrial hot-wall LPCVD reactor, the hydrogen concentrations being comparable with the silane concentration. The main results are as follows: (1) Hydrogen injection raises the activation energy of silane pyrolysis from 171 to 247 kJ/mol. (2) Hydrogen injection considerably retards silane pyrolysis by affecting the molecular mechanism of the reaction in the gas phase, the effective reaction order in hydrogen being −2 ± 0.02. (3) The expended silane percentage is a complicated function of surface-area-to-volume ratio. (4) Process conditions at 700°C are identified that provide a uniform profile of deposition rate along the heated zone of the reactor; having doped with phosphorus, poly-Si films thus obtained are found to have conductivities 1.5 to 2 times as high as those of films deposited under standard conditions at 640°C and doped in the same fashion.

Mechanism of metals’ electrodeposition from complex electrolytes allowing for the anions’ adsorption: Effect of the position of ligands and complex ions in the compact layer on the reaction orders

Effect of the relative position in the compact layer of specifically adsorbed electroactive complex ion and complexing ligand on the magnitude of current is considered. Effect of parameters that characterize the adsorption of such ions and the electrical double layer structure on the dependence of current on the solution composition and effective reaction orders is analyzed.

Fractal adsorption characteristics of complex molecules on particles—A case study of dyes onto granular activated carbon (GAC)

The isothermic and kinetic adsorption processes of three dyes onto granular activated carbon (GAC) were studied in this paper. Langmuir fractal equation was chosen to fit the adsorption isotherm data. The exponent m in the equation is a function of surface fractal dimension ( D s) of particles and cross-section area ( α 0) or radius ( r 0) of dye molecules, which can be approximately estimated according to the relation between the first order term of molecular connective index 1 χ and the total surface molecular area (TSA). Based on the modeling results, D s of the GAC was estimated to be 2.59, indicating the fractal adsorption characteristics of GAC. To elucidate the adsorption kinetics of complex molecules onto fractal surfaces, the relationships of m, kinetic parameter h and fractal spectral dimension ( d s) were developed, and the derivative relation of d s and surface fractal dimension D s was achieved. Adsorption kinetic results showed that the kinetic process had fractal-like characteristics. The instantaneous rate coefficient exhibited power-law relation with reaction time. Although h was larger than one in this study, both the fractal spectral dimension ( d s) and the order of reaction could be obtained, and the corresponding d s values were less than two. The effective reaction order x was related to d s, D s and adsorbate concentration, and it was found that the order of diffusion-controlled adsorption reaction was fractional in most cases. The h or d s values calculated here implied that the reciprocating shake was not always effective to eliminate the dimensional constraints.

Is the water–gas shift reaction on Pt simple?: Computer-aided microkinetic model reduction, lumped rate expression, and rate-determining step

The water–gas shift (WGS) is an essential process in hydrogen production from hydrocarbon and biomass fuel processing. Recently, it was shown that the chemistry of the WGS reaction on Pt is complex and depends critically on the oxidation of CO by adsorbed OH and H2O, mainly via the carboxyl intermediate [A.B. Mhadeshwar, D.G. Vlachos, J. Phys. Chem. B 108 (2004) 15246]. On the other hand, previous one-step rate expressions in the literature have described experimental data reasonably well. Here, starting from a comprehensive microkinetic model, we derive a reduced microkinetic model consisting of elementary reaction steps using principal component analysis and then develop a one-step rate expression for WGS on Pt using a posteriori analysis. It is shown that the rate-determining step of WGS on Pt is the oxidation of CO by H2O, but the effective reaction rate constant and reaction orders are concentration dependent. Finally, the effect of uncertainty in reaction rate constants on the rate-determining step is discussed.