A. M. El-Awad

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.

CATALYTIC EFFECT OF SOME CHROMITES ON THE THERMAL DECOMPOSITION OF KClO4 Mechanistic and non-isothermal kinetic studies

The catalytic effect of two different groups of chromites on the non-isothermal decomposition of KClO4was studied. TG and DSC curves of the thermal behaviour obtained for KClO4-alkaline earth chromites (Series 1) mixtures indicate the formation of K2Cr2O7, through a solid-solid interaction, before accelerating the decomposition stage of KClO4. Such an accelerating effect becomes more pronounced in the case of admixing KClO4 with some transition metal chromites (Series 2). The results were discussed taking into consideration the electronic configuration of cations and the electrical properties of the chromite catalysts. The presence of coordinatively active cations, able to form surface complex with the oxygen of the perchlorate anion, was proved to be necessary for obtaining an active catalyst. The kinetic parameters and models describing the catalyzed thermal decomposition process of KClO4 were evaluated by using a computer program that allows the analysis using five different methods. It was found that the adopted kinetic model for pure KClO4 and that mixed with catalysts from Series 1 is one-dimensional movement of phase boundary. On the other hand, random nucleation mechanism was achieved in the presence of catalysts from Series 2. Finally, a tentative reaction mechanism consistent with the obtained results was suggested.

Thermal decomposition of KClO4 catalyzed by MnO2-CuO mixed oxides systems

Abstract The catalytic effect of a mixed oxides system, consisting of MnO 2 -CuO in different atomic ratios (Cu/(Cu+Mn) = x , where x = 0-1), on the non-isothermal decomposition of KClO 4 has been studied. The results are discussed according to an electron transfer mechanism. The enhancement effect of trace Cu(II) added to the host oxide MnO 2 was attributed to an increase of the exchange capacity of manganese cations between the different oxidation states, while the retarding effect associated with Cu(II)-rich samples was attributed to the trapping of the electrons required to initiate the decomposition reaction by the holes created. On the basis of the applicability of a non-isothermal kinetic equation, it was demonstrated that two main stages are involved in the decomposition process.

Role of the Structural and Electronic Properties of Molybdenum Trioxide Catalysts on the Structure-Sensitive Oxidation of Methanol to Formaldehyde

SummaryAs catalysts for selective oxidation, molybdenum trioxide catalysts—prepared by two different methods—have been investigated. The results of vapor-phase oxidation of methanol are discussed on the basis of the acidic property, oxidizing functions, and the crystallographic structure of the catalysts. It was found that the acidic property is not the sole factor deciding the catalytic activity. Electrical conductivity measurements proved that molecular oxygen in the gas feed is necessary for maintaining the catalysts active. The difference in the catalytic activity and selectivity were reasonably interpreted in view of the structure sensitivity of those catalysts. A redox mechanism is also discussed in this paper.ZusammenfassungAuf zwei verschiedene Arten hergestellte Molybdäntrioxidkatalysatoren wurden für selektive Oxidationen untersucht. Die Ergebnisse der Dampfphasenoxidation von Methanol werden auf Basis der sauren Eigenschaften, der oxidierenden Funktionen und der kristallographischen Struktur der Katalysatoren diskutiert. Es wurde festgestellt, daß die sauren Eigenschaften zur Erklärung der entscheidenden Wirksamkeit der Katalysatoren nicht alleine verantwortlich sind. Elektrische Leitfähigkeitsmessungen zeigten, daß molekularer Sauerstoff im zugeleiteten Gas zur Erhaltung der katalytischen Aktivität notwendig ist. Die Unterschiede in der Katalysatoraktivität und der Selektivität sind aus der Struktur der Katalysatoren zu erklären. Es wird ein adäquater Redox-Mechanismus diskutiert.

A study on the thermal decomposition of iron-cobalt mixed hydroxides

Thermal decomposition of pure Fe(OH)3 and mixed with Co(OH)2 were studied using TG, DTA, kinetics of isothermal decomposition and electrical conductivity measurements. The thermal products were characterized by X-ray diffraction and IR spectroscopy. The TG and DTA analysis revealed the presence of Co2+ retards the decomposition of ferric hydroxide and the formation of α-Fe2O3. The kinetics of decomposition showed that the mixed samples need higher energy to achieve thermolysis. The investigation of thermal products of mixed samples indicated the formation of cobalt ferrite on addition ofx=1 or 1.5 cobalt hydroxide. The electrical conductivity accompanying the thermal decomposition decreases in presence of low ratio of Co2+ (x=0.2) via the consumption of holes created during thermal analysis. The continuous increase in σ values on increasing of Co2+ concentration corresponded to the electron hopping between Fe2+ and Co3+.ZusammenfassungMittels TG, DTA und der Kinetik von Messungen der isothermen Zersetzung und der elektrischen Leitfähigkeit wurde die Zersetzung von Fe(OH)3 in reinem Zustand und vermengt mit Co(OH)2 untersucht. Die thermischen Produkte wurden mittels Röntgendiffraktion und IR-Spektroskopie charakterisiert. TG und DTA zeigen, daß die Zersetzung von Eisen(III)-hydroxid und die Bildung von -Fe2O3 durch Gegenwart von Co2+ verzögert wird. Die Zersetzungskinetik zeigt, daß die Mischproben mehr Energie für die Thermolyse benötigen. Die Untersuchung der thermischen Produkte zeigt die Bildung von Cobaltferrit bei Zusatz vonx=1 oder 1,5 Cobalthydroxid. Die elektrische Leitfähigkeit nimmt bei der thermischen Zersetzung in Gegenwart von niedrigen Co2+-Konzentrationen (x=0.2) durch Verbrauch der bei der Thermoanalyse geschaffenen Löcher ab. Das monotone Ansteigen der -Werte bei steigender Co2+-Konzentration stimmt mit dem Überspringen von Elektronen zwischen Fe2+ und Co3+ überein.

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.

Action of Salt Formation on the Bifunctional Nature of 12- Molybdophosphoric Acid and Their Relationship to the Catalytic Activity

The acidic properties and the catalytic activity of 12-molybdophosphates (Mx/nn+ H3-x PMo12O40;M = Cu2+, Bi3+, Cr3+ andX=1−3) have been studied. The results are discussed as the effect of these catalyst components on the partial charge on oxygen atom which is in a relation with the acidity. It is shown that the oxygen-hydrogen bond is weakened as the value of partial charge on oxygen (− δ0) becomes more negative, while the vapor-phase dehydration activity of 2-propanol was explained on the basis of the reacting zone wideness taking into account the pseudo liquid phase nature of the heteropoly compounds. A correlation of the percentage conversion of 2-propanol with the fractional charge on the molybdenum atom was successful to interpret the effect of the redox properties of these catalysts on their catalytic activity.ZusammenfassungEs wurden die sauren Eigenschaften und die katalytische Aktivität von 12-Molybdänphosphaten (Mx/nn+ H3-x PMo12O40;M = Cu2+, Bi3+, Cr3+ andX=1−3) untersucht. Die Ergebnisse werden bezüglich der Partialladung am Sauerstoffatom diskutiert, wobei gezeigt wird, daß die Sauerstoff—Wasserstoff-Bindung mit negativerem Sauerstoff schwächer wird, während die katalytische Dampfphasen-Dehydrierungsaktivität an 2-Propanol mit der Reaktionszone der pseudoflüssigen Struktur der Heteropolyverbindungen in Zusammenhang gebracht werden kann. Die Redox-Eigenschaften der Katalysatoren beim prozentuellen Umsatz der Reaktion von 2-Propanol sind direkt mit der fraktionellen Ladung am Molybdänatom zu korrelieren.

Texture studies on chromium oxide doped catalysts

Physisorption of N2 gas at 77 K has been used to determine the surface area (SBET) and porosity of Cr2O3 impregnated with Li2O in molar ratios extending from 0.25 to 50 mol %. Calcination of catalysts was affected in air flow in temperature range of 473-773 K. Variation of the SBET-calcination temperature relationship was interpreted according to the possible lattice rearrangements via a solid-state reaction leading to Li2CrO4 phase formation for Li2O rich samples and by creation of cation vacancies in case of Li2O trace samples. The cumulative calculations together with Va-t plots showed a mesoporous surface nature of samples.