Van Krevelen Model Research Articles

Reaction Kinetics of Ethylene Combustion in a Carbon Dioxide Stream over a Cu–Mn–O Hopcalite Catalyst in Low Temperature Range

The intrinsic kinetics of the catalytic combustion of a trace amount of ethylene in a CO2 stream over a Cu–Mn–O catalyst prepared with a coprecipitation method is investigated. The experiments are carried out in a fixed-bed reactor with 0.3 g of catalyst in a low temperature range (470 to 620 K) and varying the concentration of C2H4 and O2 in the feed stream. The power rate law, Langmuir–Hinshelwood (LH), Eley–Rideal (ER), and Mars–van Krevelen (MVK) models are compared. The residual error distribution of the ethylene conversion is employed to optimize the model equations. The extended MVK model containing desorption terms of the combustion products fit the data well. The pilot test with a fixed-bed reactor and a commercial feed stream is carried out, and the macro kinetic equations are obtained. Combined with the extended MVK model equations of the intrinsic kinetics, the effectiveness factor is calculated, which gives further prediction of the performance of the extruded catalyst under commercial condit...

Mechanism of complete n-hexane oxidation on silica supported cobalt and manganese catalysts

Mono- and bi-component cobalt and manganese samples were prepared by impregnation of silica with aqueous solutions of Co(NO 3) 2·6H 2O and/or Mn(NO 3) 2·6H 2O. The bi-component samples were obtained by a common solution of Co- and Mn nitrates (CoMn-MS) or by deposition of cobalt on calcined Mn sample (Co + Mn). The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed reduction (TPR), Fourier transformed infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), elemental analysis and tested in reaction of complete n-hexane oxidation. It was observed that the well crystalline cobalt oxide partially covers poorly crystalline manganese oxide in the Co + Mn catalysts, while finely divided oxides (MnO 2 and Mn 2O 3, Co 3O 4) are present on the surface of the (CoMn-MS) sample. Four Langmuir–Hinshelwood and two Mars–van Krevelen models were fitted with the experimental data from the catalytic tests. According to the model calculations and results from instrumental methods, the reaction pathway over single component manganese and bi-component Co-Mn catalysts proceeds through Mars–van Krevelen mechanism (the oxidation of the catalyst surface being the rate determining step), while Langmuir–Hinshelwood mechanism is more probable for the Co sample. A considerable increase in activity for the sample obtained from a mixed solution is explained by low crystallinity, simultaneous presence of Mn 4+–Mn 3+ and enrichment of the surface in oxygen species.

Catalytic oxidation of acetic acid over sodium titanate synthesized hydrothermally in supercritical water

Sodium titanate was hydrothermally synthesized on the surface of titanium particles in supercritical water. Sodium titanate showed catalytic activity on supercritical water oxidation (SCWO) of acetic acid and promoted oxidation rate greatly comparing to uncatalyzed oxidation. Global rate law based on power-law model and Langmuir–Hinshelwood model could describe experimental results well. In addition, Mars–Van Krevelen model expresses catalytic process well. This suggested that redox reaction over sodium titanate might be a dominant oxidation mechanism, at least, above 450 °C. Deactivated sodium titanate was easily re-synthesized and it recovered catalytic activity. Sodium titanate showed a great potential of a long-term available catalyst in SCWO processes through its cyclic re-activation.

Kinetic Studies on Pd/CexZr1−xO2 Catalyst for Methane Combustion

In recent years, catalytic combustion of CH4 has been extensively studied as an alternative option to conventional thermal combustion for the production of heat and energy in view of its capability to achieve effective combustion at much lower temperatures than in conventional oxidation processes, with high efficiency and reduced pollutants such as CO and NOx. In this paper a kinetic study on 2% Pd over CexZr1-xO2 catalyst is presented. The catalyst was prepared by solution combustion synthesis, fully characterized and tested for CH4 catalytic combustion. A kinetic study was carried out in a continuous recycle reactor, operated in differential conditions. Twelve different possible rate equations belonging to Eley−Rideal, Langmuir−Hinshelwood, and Mars van Krevelen models were fitted to the experimental data. The best fitting belonged to a Mars van Krevelen mechanism, considering the concentration of the molecular O2 adsorbed on a single active site, which successively dissociated creating an active site with atomic O2. The obtained kinetic parameters were inside the range reported in literature for similar catalysts (Eact,1 = 23.8 kJ mol−1; Eact,2 = 31.9 kJ mol−1; Eact,1* = 5.4 kJ mol−1). For comparison, to better assess the catalytic role of Pd in the CH4 combustion, also the kinetic parameters of the sole carrier, CexZr1-xO2, were calculated. The best rate equation for the carrier was based on a different Mars van Krevelen mechanism, a simpler one, involving the surface reaction between adsorbed molecular oxygen and methane from the gas phase (Eact,1 = 79.6 kJ mol−1, Eact,2 = 114.6 kJ mol−1).

The total oxidation of propane over supported Cu and Ce oxides: A comparison of single and binary metal oxides

The total oxidation of propane is studied by means of steady-state experiments over a set of Cu- and Ce-based catalysts at a propane inlet partial pressure of 0.6 kPa, an oxygen partial pressure of 3.5 kPa and temperatures from 595 to 648 K. The catalysts were characterized by inductively coupled plasma (ICP), Brunauer–Emmett–Teller specific surface area (BET), temperature-programmed desorption (TPD), X-ray diffraction (XRD), pulse reduction–reoxidation, and H 2- and C 3H 8-temperature-programmed reduction (TPR). A synergistic effect between cupric oxide, CuO, and ceria, CeO 2, is observed using a Mars–van Krevelen model to describe the kinetic data: the activation energies for reduction and reoxidation, obtained on the binary metal oxide catalyst with both cupric oxide and ceria, are 20 kJ mol −1 lower than those obtained on the single Cu- or Ce-based catalysts. The corresponding turnover frequencies are the highest.

Structure sensitivity of dimethylamine deep oxidation over Pt/Al2O3 catalysts

The deep oxidation of dimethylamine (DMA) was studied over Pt/Al2O3 catalysts with small (1nm) and large (7.8–15.5nm) Pt crystallite sizes. The turnover frequency (TOF) was higher for the large than for the small Pt crystallites, indicating that the reaction is structure sensitive. Two kinetic models were used to interpret the obtained results, i.e., the Mars van Krevelen and a mechanism based on the adsorption of oxygen and adsorption of dimethylamine on different active sites were employed. Both models showed that the activation energy for the oxygen chemisorption rate constant (ko) decreased with increasing of Pt crystallite size and that the activation energy for the surface reaction rate constant (ki) was independent of the Pt crystallite size. The structure sensitivity may be explained by differences in the reactivity of the oxygen adsorbed on these Pt crystallites.The Mars van Krevelen model fits the TOF values very well at concentrations of DMA higher than 1500ppm, while in the lower concentrations region, the model under predicts the experimental data. The model based on the adsorption of oxygen and DMA on different active sites fits the experimental data quite well over the whole temperature and concentration range. The fitted values of the Henry adsorption constant are independent of the Pt crystallite size.

Innovative PCDD/F-containing gas stream generating system applied in catalytic decomposition of gaseous dioxins over V 2O 5–WO 3/TiO 2-based catalysts

Development of effective PCDD/F (polychlorinated dibenzo- p-dioxin and dibenzofuran) control technologies is essential for environmental engineers and researchers. In this study, a PCDD/F-containing gas stream generating system was developed to investigate the efficiency and effectiveness of innovative PCDD/F control technologies. The system designed and constructed can stably generate the gas stream with the PCDD/F concentration ranging from 1.0 to 100 ng TEQ N m −3 while reproducibility test indicates that the PCDD/F recovery efficiencies are between 93% and 112%. This new PCDD/F-containing gas stream generating device is first applied in the investigation of the catalytic PCDD/F control technology. The catalytic decomposition of PCDD/Fs was evaluated with two types of commercial V 2O 5–WO 3/TiO 2-based catalysts (catalyst A and catalyst B) at controlled temperature, water vapor content, and space velocity. 84% and 91% PCDD/F destruction efficiencies are achieved with catalysts A and B, respectively, at 280 °C with the space velocity of 5000 h −1. The results also indicate that the presence of water vapor inhibits PCDD/F decomposition due to its competition with PCDD/F molecules for adsorption on the active vanadia sites for both catalysts. In addition, this study combined integral reaction and Mars–Van Krevelen model to calculate the activation energies of OCDD and OCDF decomposition. The activation energies of OCDD and OCDF decomposition via catalysis are calculated as 24.8 kJ mol −1 and 25.2 kJ mol −1, respectively.

COMPLETE OXIDATION OF VOLATILE ORGANIC COMPOUNDS OVER Ce/Cu/γ‐Al2O3 CATALYST

The effect of cerium (Ce) addition into Cu (5, 10 or 15 wt%)/γ‐Al2O3 catalysts on the catalyst properties and catalytic activity was investigated for the complete oxidation of volatile organic compounds (VOCs). X‐ray diffraction (XRD), the Brunauer Emmett Teller method (BET), temperature programmed reduction (TPR) by H2, and N2O pulse titration were used to characterize a series of supported copper catalysts modified with cerium. Cerium was observed to be an inhibitor for 5 wt% and promoter for 10 or 15 wt% Cu/γ‐Al2O3 catalyst. The results of TPR, average crystallite size and dispersion indicated that even though Ce loadings on 10 and 15 wt% Cu/γ‐Al2O3 caused a reduction in BET surface area of the catalysts, the loaded amounts of Ce enhanced the catalytic activity through the formation of highly dispersed copper clusters. Kinetic parameters were developed for individual benzene, toluene and o‐xylene (BTX) for 5 wt% Ce/10 wt% Cu/γ‐Al2O3 catalyst at temperatures ranging from 210 to 240°C. The Mars and Van Krevelen model was found to be an adequate description of the catalytic oxidation of BTX for this study. The activity sequence with respect to the BTX molecules was found to be benzene > toluene > o‐xylene under the surface‐reaction‐controlled region.

Ethane and Propane Oxidation over Supported V2O5/TiO2 Catalysts: Analysis of Kinetic Parameters

The effect of surface vanadia loading on the alkane (ethane and propane) oxidation reaction kinetics for supported V2O5/TiO2 catalysts is investigated by employing a parallel−sequential Mars−van Krevelen model. Analysis of the kinetic parameters reveals that the preexponential factors for alkene formation and alkene combustion reactions increase with vanadia loading, except for CO2 formation from ethene. Furthermore, the activation energy for ethene formation appears to be greater than that for propene. The values of the rate constant ratios k1/k2 and k1/(k3 + k4), which represent the amount of alkane available and the net alkene formed, are functions of vanadia loading and alkanes oxidized, and are closely related to the ratio of the respective preexponential factors. The highest k1/(k3 + k4) value, which corresponds to the highest alkene yield at isoconversion, is observed for the monolayer catalyst and is higher for propene formation than for ethene formation.

Combustion of chlorinated volatile organic compounds (VOCs) using bimetallic chromium-copper supported on modified H-ZSM-5 catalyst

The paper reports on the performance of chromium or/and copper supported on H-ZSM-5 (Si/Al = 240) modified with silicon tetrachloride (Cr 1.5/SiCl 4-Z, Cu 1.5/SiCl 4-Z and Cr 1.0Cu 0.5/SiCl 4-Z) as catalysts in the combustion of chlorinated VOCs (Cl-VOCs). A reactor operated at a gas hourly space velocity (GHSV) of 32,000 h −1, a temperature between 100 and 500 °C with 2500 ppm of dichloromethane (DCM), trichloromethane (TCM) and trichloroethylene (TCE) is used for activity studies. The deactivation study is conducted at a GHSV of 3800 h −1, at 400 °C for up to 12 h with a feed concentration of 35,000 ppm. Treatment with silicon tetrachloride improves the chemical resistance of H-ZSM-5 against hydrogen chloride. TCM is more reactive compared to DCM but it produces more by-products due to its high chlorine content. The stabilization of TCE is attributed to resonance effects. Water vapor increases the carbon dioxide yield through its role as hydrolysis agent forming reactive carbocations and acting as hydrogen-supplying agent to suppress chlorine-transfer reactions. The deactivation of Cr 1.0Cu 0.5/SiCl 4-Z is mainly due to the chlorination of its metal species, especially with higher Cl/H feed. Coking is limited, particularly with DCM and TCM. In accordance with the Mars–van Krevelen model, the weakening of overall metal reducibility due to chlorination leads to a loss of catalytic activity.

Effect of phosphorous modifier on V 2O 5/TiO 2 catalyst: ODH of propane

The effect of the P/V ratio on the structure, properties, and reactivity of titania-supported vanadia is evaluated. The Raman spectra and temperature-programmed reduction studies reveal a progressive P V interaction with increasing P/V ratio until nanoscaled α I -VOPO 4 forms above a monolayer. Propane oxidative dehydrogenation (ODH) studies reveal that the conversion and selectivities of the catalysts strongly depend on the P V interaction. For a Mars–van Krevelen model, kinetic parameters are successfully estimated by minimization of an objective function with the use of a genetic algorithm. With an increase in the P/V ratio the kinetic parameters reveal a progressive decrease in the primary ODH reaction rate constant and in the rate constant ratio for propene degradation to propene formation. The decrease in the rate constant ratio for propene degradation and propene formation with increasing P/V ratio leads to a more efficient catalyst. Thus, with the proper design of the catalyst the propene yield can be increased.

Performance of H‐ZSM‐5‐supported bimetallic catalysts for the combustion of polluting volatile organic compounds in air

AbstractThe performance of H‐ZSM‐5‐supported bimetallic catalysts with chromium as the base metal in the combustion of ethyl acetate and benzene is reported. A reactor operated from 100 to 500 °C at a gas hourly space velocity (GHSV) of 32 000 h−1 was used for study of the activity. A combination of 1.0 wt% chromium and 0.5 wt% copper yielded a catalyst (Cr1.0Cu0.5/Z) with improved conversion and carbon dioxide yield. Cr2O3 (Cr3+) and CuO (Cu2+) were the predominant metal species in the catalyst. In agreement with the Mars–van Krevelen model, improved reducibility of Cr3+ in the presence of Cu2+ led to an improvement in activity. The copper content in Cr1.0Cu0.5/Z also favored the formation of deep combustion products. Condensation and subsequent growth of coke precursors in the catalyst pores led to the formation of a softer and less aromatic coke fraction while dehydrogenation activity on acid sites formed a harder and more aromatic coke fraction. The use of Cr1.0Cu0.5/Z favored the formation of lower molecular weight intermediates, leading to reduction in formation of softer coke. Copyright © 2005 Society of Chemical Industry

The kinetic of the catalytic decomposition of methyl isobutyl ketone over a Pt/γ-Al 2O 3 catalyst

Environmental catalysis also can refer to catalytic technologies for reducing emission of environmentally unacceptable compounds. Catalytic decomposition also is one of the cost-effective technologies to solve the troublesome volatile organic compounds. This study treated methyl isobutyl ketone (MIBK) by a commercial catalyst, Pt/γ-Al 2O 3, in an isothermal fixed bed differential reactor. The effects of O 2 and MIBK content in carrier gas on the catalysis’s reaction rate are also observed. Three kinetic models, i.e. the Mars and van Krevelen model, Langmuir–Hinshelwood model and power-rate law were applied to best fit the experimental results. The results indicate that the kinetic behavior of MIBK oxidation with catalysis can be accounted for by using the rate expression of the Mars and van Krevelen model and Langmuir–Hinshelwood model. Kinetic parameters are also determined on the basis of the differential reactor data. The experimental results are compared with those of the model predicted.

Kinetics of the Complete Combustion of Dilute Propane and Toluene over Iron-Doped ZrO2 Catalyst

The kinetics of the complete combustion of propane and toluene at very low concentrations in air (0.45 and 0.3 mol % in air, respectively) over iron-doped ZrO2 (cubic) catalyst (Fe/Zr = 0.25) at different temperatures (598−723 K) in kinetic control regime have been investigated. The combustion rate data could be fitted very well to both the power-law and redox (Mars−Van Krevelen) models. However, the redox model provided a better fit to the kinetic data for the propane combustion. It also showed a better fit to the toluene combustion data at the lower temperatures (623 K). The reaction order (with respect to the hydrocarbon), apparent activation energy, and frequency factor (from the power-law model) for the propane combustion were 0.96 (average), 21.16 kcal/mol, and 4.67 × 105 mol g-1 h-1 kPa-n, respectively, and those for the toluene combustion were 0.77 (average), 26.08 kcal/mol, and 1.48 × 107 mol g-1 h-1 kPa-n, respectively.

Development of a kinetic model for the oxidation of methane over Pd/Al2O3 at dry and wet conditions

The kinetics of the oxidation of methane over a commercial 0.5% Pd on γ-Al2O3 catalyst has been studied in a lab-scale fixed-bed reactor, the effect of temperature, and methane, oxygen and water partial pressures being investigated, in a range of interest for environmental applications. Different Eley–Rideal, Langmuir–Hinshelwood and Mars–van Krevelen models were fitted to the experimental results, the best fitting being obtained for a Mars–van Krevelen model that considers slow desorption of the reaction products. The model parameters obtained both from differential and integral treatment of the experimental data are in good agreement with each other. A modification of the proposed model, taking into account that water is adsorbed over oxidised sites, is also able to model the inhibition produced by steam.

Oxidative dehydrogenation of propane on V 2O 5/Al 2O 3 and V 2O 5/TiO 2 catalysts: understanding the effect of support by parameter estimation

In this paper, the effect of the oxide support for supporting the vanadium oxide phase is studied by estimating the reaction parameters for the oxidative dehydrogenation of propane. To achieve this objective, several V 2O 5/Al 2O 3 and V 2O 5/TiO 2 catalysts were synthesized by an incipient-wetness-impregnation technique. The supported vanadium oxide catalysts were characterized and the surface area, monolayer coverage and reducibility were determined. The surface area of the catalyst samples was not significantly affected with supported vanadium oxide loading. It was observed that the catalysts contain only molecularly dispersed vanadium oxide species below monolayer coverage, and molecularly dispersed and crystalline V 2O 5 above monolayer coverage. TPR studies revealed that the V 2O 5/Al 2O 3 samples were more difficult to reduce relative to the V 2O 5/TiO 2 samples. The monolayer or near-monolayer catalysts, 10% V 2O 5/Al 2O 3 and 4% V 2O 5/TiO 2, were selected for detailed kinetic analysis. A Mars–van Krevelen (MVK) model containing eight parameters was chosen for this purpose. The parameters were estimated using a genetic algorithm (GA), which optimizes a suitable objective function for a non-linear multi-response system. From the parameters estimated, it was determined that a similar catalytic cycle occurs independent of the oxide support. However, the rate at which the catalytic cycle occurs appears to be much faster on the more reducible titania support compared to the rate on the less reducible alumina support. The degree of reduction varies along the length of the reactor and depends on the support. Thus, the support has a significant effect on the reaction parameters for the oxidative dehydrogenation of propane over supported vanadium oxide catalysts.

Evaluation and design of heteropolycompound catalysts for the selective oxidation of isobutane into methacrylic acid

Chemical modifications of the 12-phosphomolybdic acid by substitution of a Mo atom by a V in the so-called Keggin primary structure and/or by introduction of counter-cations (NH 4 +, Cs +) in the secondary structure were studied. The effects of the proportion of ammonium salt in cesium–ammonium salts mixtures and of different preparation methods were also investigated. Concurrently to characterization analyses, the kinetic parameters of the Mars and Van Krevelen (MVK) model, representing well the rate of consumption of isobutane over the solids tested were determined by varying partial pressures of isobutane and oxygen. The results were used for catalysts evaluation and as a tool for the development of a new, more active and stable formulation. On the basis of the results, the role of the constituents of the catalysts are better understood. It is proposed that vanadium atoms are easily removed from the primary structure and participate in the reduction of the catalyst but have no effect on the activation of isobutane. Moreover it is shown that ammonium ions are essential to reach a high selectivity to methacrylic acid (MAA) by reducing the active phase whereas the role of cesium is to form an alkaline salt acting as a support over which the active phase is dispersed and stabilized.

A Kinetic Study of Catalytic Combustion of Ethyl Acetate and Benzene in Air Stream over Cr-ZSM-5 Catalyst

Kinetic modeling of catalytic combustion of ethyl acetate and benzene over Cr-ZSM-5 (Si/Al = 240) is reported. Ethyl acetate was more reactive than benzene and could suppress benzene conversion in the binary mixture while water vapor retarded the conversion of both organics. Kinetic data obtained in the differential reactor at GHSV 78 900 h-1 were best fitted by the Mars−van Krevelen model. In binary mixtures, a competitive model based on the Mars−van Krevelen mechanism satisfactorily predicted the ethyl acetate reaction rate. Ethyl acetate was more favorably adsorbed on the oxidized catalytic sites and could stabilize benzene by inducing an electron cloud from the aromatic ring. The inhibition effect by water was modeled by a competitive adsorption mechanism. Deviation in reaction rates at a feedwater concentration of 9000 ppm was attributed to the formation of water clusters that blocked the internal diffusion of VOC molecules. The oxidation/reoxidation steps of the active sites appeared to be the rate-...

Effect of hydrogen treatment on the performance of Cr–ZSM-5 in deep oxidative decomposition of ethyl acetate and benzene in air

The effect of hydrogen reduction on the activity of chromium exchanged ZSM-5 (Cr–ZSM-5) in the decomposition of ethyl acetate and benzene is reported. Reduction was performed at 400 °C for 1 h while activity study was done using a reactor operated between 100 and 500 °C and at GHSV of 32,000 h −1. Reduction caused the migration of chromium to subsequently form chromia nanoparticles during VOC decomposition process leading to lower conversions. In line with the Mars–van Krevelen model, yield of carbon dioxide increased after reduction. Less coking tendency of reduced Cr–ZSM-5 was ascribed to lower conversion and higher yield of carbon dioxide.

Kinetics study of the oxidative dehydrodimerisation and aromatisation of isobutene on Bi 2O 3–SnO 2

The kinetics of the oxidative dehydrodimerisation and aromatisation of isobutene has been studied on a Bi 2Sn 2O 7 system for a mechanistic understanding of the reaction. The rate of disappearance of isobutene has been found to be first order with respect to oxygen and second order with respect to isobutene. A good fit of the kinetic data with the Mars and van Krevelen model has been established. The k red and k ox have been found to be 2.3×10 6 and 32×10 6 mol min −1 m −2 kPa −1 , respectively, proving that the reduction of the catalyst is the rate-determining step. Studies were also done to verify the mechanistic pathway of the reaction. Previous workers have suggested that the reaction is essentially a consecutive reaction proceeding from isobutene to p-xylene through the intermediate dimer (2,5-dimethyl-1,5-hexadiene). Studies were done at increasing contact time to verify the mechanistic pathway of the reaction and the intermediate dimer was used as the reactant. Our results showed that the intermediate dimer is mainly oxidised to CO x while p-xylene is formed through some direct parallel pathway from isobutene.