Contracting Sphere Model Research Articles

A study of reduction and adsorption on lanthanum rhodium oxide (LaRhO3)

The reduction of LaRhO/sub 3/ and also the adsorption of H/sub 2/, CO, and NO on this perovskite have been studied. Reduction starts at 475 K and occurs in a single step with formation of La/sub 2/O/sub 3/, metallic Rh, and small amounts of Rh/sub 2/O/sub 3/. By reduction and reoxidation of LaRhO/sub 3/ at 1173 K a perovskite of particle size lower than that of the starting sample (preparation temperature, 973 K) was obtained. The kinetic data suggest that the reduction process occurs according to the contracting sphere model. An activation energy of reduction of 128 kJ mol/sup -1/ was calculated. Coverages (theta) of H/sub 2/ and CO adsorbed on LaRhO/sub 3/ at room temperature were lower than 1%. Coadsorption data suggest that the irreversible parts of H/sub 2/ and CO absorption different centers (noncompetitive adsorption). However, the reversible parts of H/sub 2/ and CO seem to compete with the irreversible parts of CO and H/sub 2/, respectively, for the same adsorption centers. A 1:1 correspondence between surface Rh/sup 3 +/ ions and molecules of adsorbed NO per surface unit was not found. A method for oxides characterization based on NO adsorption is suggested. 52 references, 8 figures, 3more » tables.« less

Surface properties of LaNiO 3: Kinetic studies of reduction and of oxygen adsorption

The kinetics of reduction and the kinetics of oxygen adsorption on LaNiO 3 were studied in a wide range of temperatures. Temperature-programmed reduction showed two reduction steps: (a) of 1 e − per molecule between 425 and 675 K and (b) of 3 e − per molecule between 675 and 900 K. In reduction step (a) a distorted LaNiO 3 (because of the presence of anion vacancies) was identified by X-ray diffraction; after sintering in He at 1073 K the perovskite structure was destroyed and La 2NiO 4 and NiO were formed. In reduction step (b) La 2O 3 and Ni were formed, these species being better crystallized after sintering at 1173 K. Steps (a) and (b), in the nonsintered samples, were reversible, i.e., after oxidation at 973 K the perovskite structure was newly formed. Reduction in step (a) takes place according to the contracting sphere model while reduction in step (b) is controlled by formation and growth of reduction nuclei. Experimental data in step (b) fit Avrami-Erofeev′s equation. Activation energies for reduction steps (a) and (b) were 108 and 221 kJ mol −1, respectively. The kinetics of oxygen adsorption obeyed Elovich′s equation, t 0 being calculated by extrapolation to Z( t) = 0 of t vs Z( t) plots. The activation energy of adsorption was 38 kJ mol −1. Within the LaMeO 3 series, the more easily reducible oxides (LaNiO 3, LaCoO 3, LaMnO 3) were found to be better adsorbents of oxygen and better catalysts for oxidation than the less reducible One (LaCrO 3).

The thermal decomposition of oxalates. Part 20. The decomposition kinetics of acid barium oxalates

The thermal decomposition of barium oxalate is followed using isothermal and rising temperature techniques. The results obtained from the two methods are in good agreement for both loss of oxalic acid and carbon monoxide. The values of the activation energies for the loss of oxalic acid and carbon monoxide are: isothermally, 109 and 162 kJ mol −1; and by the rising temperature method, 114 and 186 kJ mol −1. The result also indicate that the loss of oxalic acid from the salt follows an Avrami—Erofeev type expression and the loss of carbon monoxide follows a contracting-sphere model.

On the kinetics of the thermal decomposition of sulfates related with hydrogen water splitting cycles

The purpose of this paper is to present a set of kinetic results on the decomposition of MgSO 4, NiSO 4, ZnSO 4 and to discuss these in relation to the theoretical models. Isothermal and non-isothermal thermogravimetric studies as functions of temperature, gas flowrate, shape and solid additives have been performed. The gas effluent analysis in the three cases show that the products are SO 2 and O 2. For NiSO 4 and ZnSO 4 the contracting sphere model, chemically controlled, gives a good representation of the results. In the case of MgSO 4 a diffusion shrinking-core model had been derived. The reaction is controlled by the mass transfer in the product layer.

A Study of the Kinetics of the Basic Zinc Carbonate Formation Reaction

A study of the kinetic parameters involved in the formation of basic zinc carbonate by the action of carbon dioxide and water vapour on zinc oxide has been undertaken. A contracting sphere model is used to explain the reaction but its applicability is dependent upon the carbon dioxide pressure used. Water vapour pressures close to saturated water vapour pressure are necessary for the reaction to proceed and the role of the necessary near-liquid surface water layer in the absorption reaction is discussed.

A kinetic study of the reduction of molybdenum trioxide by hydrogen

A study has been made of the kinetics of the reduction of molybdenum trioxide to molybdenum dioxide by hydrogen in the temperature range 480–600 °C, and the subsequent reduction of the dioxide to molybdenum metal in the range 650–800 °C. The activation energies, E, and frequency factors, A, found for a sample of sublimed molybdenum trioxide were: MoO 3 + H 2 → MoO 2 + H 2O E = 113 kJ mol −1, A = 4.2 × 10 −1 s −1, MoO 2 + 2H 2 → Mo + 2H 2O E = 98.6 kJ mol −1, A = 2.2 × 10 −2 s −1, The reduction of the trioxide follows a linear rate equation and an explanation of this somewhat unusual result is presented. However, reduction of the dioxide to the metal followed the more usual deceleratory path characteristic of the contracting sphere model. The results obtained in the present work are compared with the results of previous investigations.

Sublimation of ammonium perchlorate

The sublimation of ammonium perchlorate has been investigated using simultaneous differential thermal analysis and thermogravimetric analysis. Critical evaluation of isothermal kinetics of data obtained by thermobarogravimetric technique indicate that the 'kinetics are consistent with an expression for a contracting cylinder rather than a contracting sphere model. The activation energy for sublimation obtained experimentally is in good agreement with that derived from the Schultz—Dekker theory (18±2kcal·mole −1). An analogy of the sublimation process in ammonium perchlorate to that in ammonium halides is also presented. It is shown that a previously reported value of 30 kcal·mole −1 for the activation energy was obtained under conditions where decomposition dominated.

Studies on metal hydroxy compounds. XII. Thermal analyses, decomposition kinetics, and infrared spectra of copper basic oxysalts

The thermal analyses of copper basic oxysalts of the type Cu2(OH)2XOy, where X = S, C, Cl, Br, I, N, HC and y = 4, 3, or 2 reveal that decomposition occurs, in most cases, by dehydroxylation with concomitant disproportionation of the oxysalt. An outstanding exception to this pattern of decomposition is the basic sulfate. Calorimetric measurements along with related enthalpic values for the decomposition reaction are given.The kinetics of thermal decomposition of the compounds are classified into three main categories: (i) three-dimensional contracting sphere model, (ii) first-order rate, and (iii) nucleation controlled rate processes.The infrared spectral data of nine copper basic oxycompounds in the frequency region 4000–250 cm−1 are presented with their assignments. The fundamental infrared inactive modes of the anionic species SO42−, NO3−, etc. become active by the presence of the copper hydroxide ligand indicating a lowering of symmetry in the anion.

Effect of processing parameters on the kinetics of decomposition of magnesium sulphate

Thermogravimetric analysis was used to study the kinetics of decomposition of MgSO 4 between 920°C and 1080°C as a function of sample mass and processing pressure. The kinetics of decomposition of MgSO 4 can be represented either by the contracting sphere (phase boundary controlled) rate equation or by a nuclei growth equation with ( m) equal to 1.10. The contracting sphere model gives a slightly better mathematical representation. The experimental activation energy of 74.5 ± 3 kcal/mole is close to the heat of decomposition. The measured kinetics are influenced by the reversible nature of the reaction. Pelletization, or increasing the sample mass, retards the reaction by increasing the partial pressure of SO 3 developed within the powder compact.