W.A. De Jong

Kinetic parameters in Avrami—Erofeev type reactions from isothermal and non-isothermal experiments

Abstract The differential rate equations for some simple frequently occurring Avrami—Erofeev type transformations were solved for isothermal as well as non-isothermal reactions. It is shown that the expressions commonly used to extract kinetic parameters from non-isothermal experiments are obtained via an incorrect procedure. However, the correct kinetic parameters will result from application of these equations to certain types of transformation.

Kinetics of the methanation of CO and CO2 on a nickel catalyst

Abstract The kinetics of the methanation of CO 2 in H 2 on a supported nickel catalyst have been measured at partial pressures of CO 2 below 0.02 atm and at atmospheric pressure and at temperatures between 200 and 230 °C. The methanation of CO was studied in the same concentration range, between 170 and 210 °C. The results can be described by rather simple Langmuir-type rate equations: r CO 2 = 1.36 × 10 12 · exp ( −25 300 RT ) · p CO 2 (1+1270 · p CO 2 ) mol hr −1 g −1 and r CO = 2.09 × 10 5 · exp ( −10 100 RT ) · p CO (1+4.56 × 10 −4 · exp ( +12 400 RT ) · p CO ) 2 mol hr −1 g −1 Experiments at 200 °C show that CO poisons the methanation of CO 2 in concentrations larger than 200 ppm. Water and methane in small concentrations have no effect on the reaction rate. The results of this study are in good agreement with data published previously. Some mechanistic implications of the kinetic data are discussed.

Kinetics and mechanism of the CO shift on CuZnO: 1. Kinetics of the forward and reverse CO shift reactions

The CO shift reaction over a CuZnO catalyst, CO + H2O and CO2 + H2, has been studied to obtain kinetic evidence for the supposition that it proceeds via a formate-type intermediate on the catalyst surface. The results can be described with Langmuir-type kinetic equations and indicate that both the forward and reverse CO shift are characterized by a relatively stable intermediate formed from one molecule of each of the reactants, CO · H2O and CO2 · H2, respectively. The decomposition of this complex is rate determining; in both cases the copper surface coverage by the complex is relatively high and varies little in the temperature range of commercial application of the shift reaction.

Brass formation in a copper/zinc oxide CO shift catalyst

Abstract It is shown that thermal treatment of a copper/zinc oxide CO-shift catalyst in a reducing gas causes a decrease in BET surface area as well as partial reduction of zinc oxide followed by the formation of α-brass. The presence of zinc in the copper-rich phase has a very strong negative effect on catalyst activity. The amount of brass formed is in agreement with concentrations predicted from thermodynamic calculations and the rate of diffusion of zinc in the copper crystallites.

Kinetics and mechanism of the CO shift on CuZnO: II. Kinetics of the decomposition of formic acid

Abstract The decomposition of formic acid over the Cu ZnO catalyst used in earlier work on CO shift kinetics has been examined. The reaction proceeds through a formate-type surface intermediate, the decomposition of which is rate determining. Surface coverage is rather high. The rate of formic acid decomposition is similar to that of the forward shift reaction and shows about the same temperature dependence. Both reactions as well as the reverse CO shift proceed via a surface intermediate of the same overall atomic composition. The high selectivity for formic acid dehydrogenation is in line with the value of about 50 found for the ratio between the rates of forward and reverse shift reactions. It is concluded that the kinetics of the CO shift conversion over Cu ZnO catalyst are consistent with a mechanism in which the decomposition of a stable formate-type surface intermediate is rate determining.

The disproportionation of toluene over a HY/β-AlF3/Cu catalyst: 1. Preparation and characterization

The preparation, characterization, and testing of a catalyst consisting of 72 wt% HY zeolite, 18 wt% β -AlF 3 , and 10 wt% Cu for the conversion of toluene into benzene and xylenes are described. The effect of β -AlF 3 , copper, and the activation temperature on the activity, selectivity, and stability of the catalyst were investigated. The results demonstrate that the catalyst shows satisfactory performance and reveal that 500°C is its optimum activation temperature. Texture determinations and activity measurements suggest that disproportionation activity is localized in the transitional pores of the catalyst and that the micropores only serve to collect heavy reaction products which would otherwise lead to deactivation. The results of ammonia adsorption combined with the effect of activation temperature on activity indicate that Bronsted acid sites formed during activation are responsible for the activity. It appears that only about 10% of the surface sites on freshly activated catalyst are acidic.

Transient behaviour of an adiabatic fixed-bed methanator—II: Methanation of mixtures of carbon monoxide and carbon dioxide

Abstract The transient behaviour of an adiabatic fixed-bed methanator has been studied using the hydrogenation of mixtures of CO and CO2 at concentrations up to 2·7 vol.% carbon oxide in hydrogen as the test reactions. Responses to disturbances in feed conditions were studied by measuring the axial temperature profile as a function of time. The results show that the dynamic behaviour of the reactor is complicated by the inhibition by CO of the methanation of CO2. The agreement between theory and experiment was again quite satisfactory: the quasi-homogeneous plug flow model which applied to experiments using binary mixtures of hydrogen and a carbon oxide applies to the data obtained with mixtures of CO, CO2 and hydrogen, provided that the successive hydrogenation of CO and CO2 is taken into account. However, it is improbable that the quasi-homogeneous model can be applied to industrial methanation, when the higher temperatures and consequent faster rates of methanation are likely to cause heat and mass transfer limitations. Nevertheless, there is no doubt that response times of but a few seconds must be expected in industrial methanation.

Thermal decomposition of aqueous manganese nitrate solutions and anhydrous manganese nitrate. Part 1. Mechanism

Abstract The thermal decomposition of aqueous manganese nitrate solutions in air or nitrogen proceeds in three steps: 1. (i) partial evaporation of water to a concentrated solution containing equimolar amounts of water and Mn(NO3)2, 2. (ii) a first decomposition step in which the residual water evolves and part of the Mn(NO3)2 decomposes to MnO2, and 3. (iii) a second decomposition step in which the remaining Mn(NO3)2 decomposes to MnO2. Decomposition of part of the Mn(NO3)2 in the first step is caused by the presence of water (vapour) which accelerates the decomposition of anhydrous Mn(NO3)2 and lowers the temperature at which this reaction starts. Without water [anhydrous Mn(NO3)2] only one decomposition step occurs. Off-gas analysis with mass spectrometry, IR spectroscopy and chemiluminescence showed NO2 to be the main gaseous product, NO being formed in much smaller amounts. amounts.

Thermal decomposition of aqueous manganese nitrate solutions and anhydrous manganese nitrate. Part 2. Heats of reaction

Abstract Differential thermal analysis of the thermal decomposition of aqueous manganese nitrate solutions in air and nitrogen shows three distinct endothermic heat effects. The first effect results from partial evaporation of water to a solution containing equimolar amounts of H2O and Mn(NO3)2. The two subsequent decomposition steps produce MnO2. The heat effect of each of the latter two steps depends on the sample weight but their sum remains constant at about 170 ± 14 kJ mole−1. Decomposition under vacuum of anhydrous Mn(NO3)2 shows only one heat effect of 155 ± 12 kJ mole−1. DTA and X-ray analysis indicate that decomposition of a Mn(NO3)2 solution results in γ-MnO2 and decomposition under vacuum of anhydrous Mn(NO3)2 results in γ-MnO2 with large amounts of Mn(NO3)2. A comparison between measured and calculated heats of reaction shows fair agreement.

Thermal decomposition of aqueous manganese nitrate solutions and anhydrous manganese nitrate. Part 4. Non-isothermal kinetics

Abstract Kinetic parameters for the decomposition of aqueous manganese nitrate solutions were derived from non-isothermal experiments in air. First, most of the water evaporates to a solution containing approximately equimolar quantities of water and manganese nitrate which then decomposes in two steps to MnO 2 . The first step can be described by a model valid for two-dimensional growth of a constant number of nuclei, viz. g(α) = [−ln-(1 − α)] 1 2 , and the second by a model based on a surface reaction, viz. g(α) = 1 − (1 − α) 1 3 . The decomposition of anhydrous manganese nitrate most probably occurs via nuclei formation with a decreasing rate and one-dimensional growth of the nuclei formed. The model g(α) = [ −ln(1 − α)] 0.6 described the measurements satisfactorily. The parameters in the above models closely agree with results from isothermal experiments.