R.M. Gabr

Effect of spinel (ZnCr2O4) formation on the texture, electrical conduction and catalytic behaviour of the ZnOCr2O3 system

Abstract Structural and phase changes accompanying the calcination in air, up to 1000 °C, of the ZnOCr 2 O 3 system were monitored using differential thermal analysis, differential scanning calorimetry, infrared spectrophotometry, X-ray diffractometry and chromium ion estimation. The texture was assessed by analyzing nitrogen sorption isotherms measured at −196 °C. The results indicate that ZnOCr 2 O 3 solid solution becomes a major product at 500 °C, and the ZnCr 2 O 4 spinel phase starts to form at 400 °C. The electrical conductance properties of the samples were investigated before and after admission of 2-propanol in the temperature range 100–400 °C. The electrical conduction is attributed to the existence of a surface mobile-electron Zener phase that maximizes the conductivity. The obtained interstitial Zn 2+ cations and free electrons (charge carriers) are considered to be localized at the ions or vacant sites, and the conduction occurs via a hopping-type process, which implies a thermally activated electronic mobility. The catalytic activity data of the vapor-phase decomposition of 2-propanol were obtained in the temperature range 200–400 °C, using a flow system technique. It was found that propylene is the main reaction product, with a minor yield of acetone. In the solid solution of ZnO in Cr 2 O 3 , the Cr ion sites will be electronically more isolated than either disordered or ordered spinel phases. The neutralization of these sites will not occur by bulk electron transfer, but they tend to trap electrons from the oxygen end of the 2-propanol molecule, leading to a high dehydration activity. Correlations were attempted between the structure of the catalysts and their catalytic activity.

Formation, conductivity and activity of zinc chromite catalyst

Abstract The kinetics of ZnCr2O4 formation was followed by calcining a powder mixture of ZnCrO4 and CrO3 (1:1 molar ratio) in the temperature range 150–1000 °C in air. The structural changes were identified using DTA, XRD and IR analysis. Surface Cr6+ concentration and electrical conductivity measurements (between 100 and 400 °C) were performed. It was found that ZnCr2O4 formation started at 275 °C, became a major phase at 400 °C and reached its maximum formation at 500 °C. The fraction of reaction completed (α) vs. calcination temperature (Tc) curve was constructed. The kinetic analysis was performed using the Coats-Redfern equation. The reaction order follows first order kinetics and the activation energy was determined as 19.65 kJ mol−1. The lowest conduction activation energy (Eσ = 0.172 eV) was observed at Tc = 600 °C. The sharp decrease of the built-in positive hole concentration with annealing the ZnCr2O4 spinel phase at Tc > 600 °C is reflected in a corresponding sharp lowering of the Eσ values. The catalytic activity of the calcination products was investigated kinetically using the decomposition of H2O2 as a model catalytic process. The study revealed that the early stages of the reaction are heterogeneous, especially at Tc ⩽ 500 °C. Whenever surface Cr6+ ions go into solution they catalyse the reaction homogeneously. A mobile electron zener phase on the surface (coupled Cr3+-Cr6+ ions) optimizes heterogeneous activity. The parameter which best characterises the intrinsic order of catalytic activity is the activation energy of the reaction.

Influence of metal oxide additives on the kinetics of isothermal decomposition of ammonium metavanadate

The kinetics of isothermal decomposition of ammonium metavanadate (AMV) and its mixtures with MgO, CaO, CuO and NiO — in the molar ratio of 2∶1 — were investigated. The computer — oriented kinetic analysis of theα-t data reveals the validity of Ginstling-Brounshtein equation to describe these reactions. The accelerating effect accompanying the MgO addition can be attributed to its affinity to abstract protons. In case of CaO, the formation of the relatively stable Ca(OH)2 during the proceeding of the decomposition process results in a retarding effect. On the other hand, the higher calculatedEa, value for the AMV + CuO mixture was explained in terms of a solid-solid interaction between CuO and the solid decomposition product, leading to the formation of Cu3(VO4)2. According to electronic factors, the effect of NiO addition on the decomposition process was discussed. It was found that the decomposition process is less favourable in presence ofp-type semiconducting additive.ZusammenfassungEs wurde die Kinetik der isothermen Zersetzung von Ammoniummetavanadat (AMV) und von Gemischen aus AMV mit MgO, CaO, CuO und NiO im Molverhältnis 2∶1 untersucht. Eine computergestützte kinetische Analyse dert Daten erwies die Gültigkeit der Ginstling-Brounshtein-Gleichung zur Beschreibung dieser Reaktionen. Der beschleunigende Effekt bei Zugabe von MgO kann dessen Neigung zum Entzug von Protonen zugeschrieben werden. Im Falle von CaO führt die Bildung von relativ stabilem Ca(OH)2 während des Zersetzungsvorganges eher zu einer Verlangsamung. Die hohen berechneten Ea2-Werte für das AMV + CuO-Gemisch können mit einer Feststoff-Feststoff-Wechselwirkung zwischen CuO und dem festen Zersetzungsprodukt erklärt werden, wobei Cu3(VO4)2 gebildet wird. Der Einfluß von NiO-Zugabe auf den Zersetzungsprozeß wurde hinsichtlich elektronischer Faktoren diskutiert. Man fand, daß die Gegenwart vonp-Halbleiterzusätzen für den Zersetzungsvorgang weniger günstig ist.

Role of metal vanadate additives on the decomposition reaction kinetics of nickel tetrahydroxy carbonate as a precursor during the thermal genesis of nickel(II) oxide catalyst

Abstract The catalytic effect of copper and magnesium metavanadate additives on the thermal decomposition of nickel tetrahydroxy carbonate (NTHC) was studied using the TG and DTA methods. Mathematical analysis of the TG data showed that first order kinetics are applicable in all cases using the Coats-Redfem equation. Thermodynamic kinetic parameters (i.e. energy and entropy of activation and preexponential factor) are reported. Additionally, the kinetic parameters of the decomposition reaction were calculated adopting a computer oriented kinetic analysis of the α - t data, obtained in isothermal conditions in the temperature range 200–450°C. Comparative studies of the kinetics of the thermal decomposition of NTHC salt in isothermal and non-isothermal conditions were performed. These indicate that the overall rate of salt decomposition is controlled by the rate at which the gaseous products can diffuse away from the reaction centres. It was also concluded that the participation of oxygen in interface reactions yielded highly oxidized Ni 3+ phases, which are necessary participants in the sequence of the anion (CO 2− 3 ) breakdown. Accordingly, advance of the interface proceeded by a “chain type” mechanism involving regeneration of the active intermediate Ni 3+ . The effect of CuV 2 O 6 additive was discussed on the basis of the redox couple Cu 2+ + e ∡ Cu + , which accelerates the decomposition reaction via a charge-transfer mechanism. Mixing the salt with MgV 2 O 6 resulted in a retardation effect, for which a tentative explanation has been made in terms of the ability of such an additive to occupy a grain boundary, which hinders the decomposition process. A decomposition reaction mechanism involving the participation of Ni 3+ ions was proposed.

Kinetics and mechanism of thermal decomposition of magnesium perchlorate catalysed by metal metavanadate additives

Abstract The thermal decomposition of magnesium perchlorate, Mg(ClO 4 ) 2 , was studied non-isothermally and isothermally. Under non-isothermal conditions it was found that Mg(ClO 4 ) 2 decomposes through a stepwise release of oxygen forming unstable intermediates which produce MgO, not MgCl 2 , as a final decomposition product. The kinetic parameters of the decomposition reaction were deduced employing a computer-oriented kinetic analysis of the α- t data obtained under isothermal conditions in the temperature range 300–500° C. This indicates that the decomposition process is governed by the Ginstling-Brounshtein equation. In order to evaluate the effect of additives on the catalytic thermal decomposition reaction, the decomposition of Mg(ClO 4 ) 2 was followed in the presence of CuV 2 O 6 and MgV 2 O 6 catalysts. The results show that CuV 2 O 6 had a remarkable catalytic acceleration effect while addition of MgV 2 O 6 retarded the decomposition of Mg(ClO 4 ) 2 . These effects are discussed on the basis of the redox couple, Cu 2+ + e − ⇌ Cu + , which promotes the decomposition reaction via a charge-transfer mechanism. In the case of MgV 2 O 6 addition, the retarding effect seems to arise from the hindrance of gaseous oxygen evolution. Finally, the effect of additives was followed kinetically and a modified catalytic mechanism is proposed.

Role of the structural and electronic properties of Cr1−xFexVO4 (0⩽x⩽1) solid solutions on their catalytic activity

Abstract A series of solid solutions having the general formula Cr1−xFexVO4 (0⩽x⩽1) was prepared. X-ray analysis revealed that the structure of the solid solutions formed at 0 Cr 3+ Fe 3+ ratio. According to the ligand field stabilization energy considerations, it was suggested that coordinatively unsaturated Cr3+ ions represent the active sites responsible for the higher dehydration activity of the catalysts. Electrical conductivity measurements proved that the lowering in activity runs parallel to the decreasing of the energy barrier which the bulk electrons are required to overcome. A possible decomposition mechanism was proposed.

Physico-chemical and catalytic studies on the calcination products of BaCrO4-CrO3 mixture

Abstract The phase composition, electrical behaviour, and the catalytic activity towards H2O2 decomposition were studied for the products of the solid state reaction of BaCrO4-CrO3 mixture (1:1 molar ratio) in the temperature range 300–1000 °C. Experimental evidence from acid-soluble chromate estimation and X-ray analysis revealed that the presence of the low melting point component CrO3 enhances the chromite spinel formation. On the basis of these experimental data together with those obtained from electrical conductivity measurements, a mechanism of chromite formation was suggested. The decisive role of surface Cr6+ ions in the catalytic decomposition reaction of H2O2 indicated the existence of these surface active centers in different energy states depending on the calcination temperature. This was also verified from the electrical conductivity measurements.