A comparative study of the thermal reactivities of some transition metal oxalates in selected atmospheres

A comparative investigation has been made of the nonisothermal, solid-state thermal decompositions of the oxalates of six divalent transition metals (cations: manganese, iron, cobalt, nickel, copper and zinc) in alternative flowing atmospheres, inert (N₂, CO₂), reducing (H₂) and oxidizing (air). Derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) response peak maxima, providing a measure of reaction temperatures, have been used to determine salt reactivities and thus to characterize the factors that control the relative stabilities of this set of chemically related reactants. Two trends were identified. Trend (1): in the inert and reducing atmospheres, the decomposition temperature (salt stability) increased with rise in enthalpy of formation of the divalent transition metal oxide, MO. It is concluded that the rupture of the cation-oxygen (oxalate) bond is the parameter that determines the stability of salts within this set. Trend (2): the diminution of decomposition temperatures from values for reactions in inert/reducing atmosphere to those for reactions in an oxidizing atmosphere increased with the difference in formation enthalpy between MO and the other participating oxide (MO_3/2 or MO_1/2). The change of cation valence tended to promote reaction, most decompositions in O₂ occurred at lower temperatures, but the magnitude of the effect varied considerably within this set of reactants. Observed variations in stoichiometric and kinetic characteristics with reaction conditions are discussed, together with the mechanisms of thermal decompositions of these solid oxalates.This approach to the elucidation of crystolysis reaction mechanisms emphasizes the value of comparative investigations within the group of chemically related reactants. Previous isothermal kinetic studies had been made for each of the reactants selected here. From these, much has been learned about the form of the (isothermal) solid-state yield-time curves, often interpreted to provide information about the geometry of interface development for the individual rate processes. However, identification of the controls of reactivity, reaction initiation (nucleation) and advance (nucleus growth), is much more difficult to characterize and less progress has been made towards elucidation of the interface chemistry. The trends of reactivity changes with salt compositions, identified here, offer a complementary approach to that provided by the study of single salts. Much of the recent literature on thermal decompositions of solids has been concerned with individual reactants, but many results and conclusions are not presented in the widest possible perspective. Comparisons between systematically related reactants are identified here as providing a chemical context for the elucidation of the chemical steps that participate in interface reactions. The article advocates the use of a more chemical approach in investigations of crystolysis (solid-state chemical) reactions.     

Sandwich-type silicotungstates: structure and magnetic properties of the dimeric polyoxoanions[[SiM2W9O34(H2O)]2]12- (M = Mn2+, Cu2+, Zn2+)

The novel dimeric silicotungstates [[SiM2W9O34(H2O)]2]12- (M = Mn2+, Cu2+, Zn2+) have been synthesized and characterized by IR spectroscopy, elemental analysis, and magnetic measurements. X-ray single-crystal analyses were carried out on K4Na6Mn[[SiMn2W9O34(H2O)]2].33H2O (1), which crystallizes in the triclinic system, space group P1, with a = 12.2376(7) A, b = 13.6764(8) A, c = 15.6177(9) A, alpha = 70.2860(10) degrees, beta = 79.9150(10) degrees, gamma = 70.2760(10) degrees, and Z = 1; K3Na5[[SiCu2W9O34(H2O)]2].26H2O (2) crystallizes in the triclinic system, space group P1, with a = 11.4271(12) A, b = 12.5956(13) A, c = 15.3223(16) A, alpha = 80.456(2)degrees, beta = 76.383(2) degrees, gamma = 76.968(2) degrees, and Z = 1; K4Na6[[SiZn2W9O34(H2O)]2].34H2O (3) crystallizes also in the triclinic system, space group P1, with a = 12.2596(14) A, b = 13.2555(15) A, c = 16.2892(18) A, alpha = 96.431(2) degrees, beta = 100.944(2) degrees, gamma = 110.404(2) degrees, and Z = 1. The polyanions consist of two lacunary B-alpha-[SiW9O34]10- Keggin moieties linked via a rhomblike M4O16 (M = Mn, Cu, Zn) group leading to a sandwich-type structure. Magnetic measurements show that the central Mn4 unit in 1 exhibits antiferromagnetic (J = -1.77(5) cm(-1)) as well as weak ferromagnetic (J' = 0.08(2) cm(-1)) Mn-Mn exchange interactions. In 2 the Cu-Cu exchange interactions are antiferromagnetic (J = -0.10(2) cm(-1), J' = -0.29(2) cm(-1)).     

Assessment of Lewis-Acidic Surface Sites Using Tetrahydrofuran as a Suitable and Smart Probe Molecule

Measuring the Lewis-acidic surface sites in catalysis is problematic when the material‘s surface area is very low (S_BET ≤1 m² ⋅ g⁻¹). For the first time, a quantitative assessment of total acidic surface sites of very small surface area catalysts (MoO₃ as pure and mixed with 5–30 % CdO (wt/wt), as well as CdO for comparison) was performed using a smart new probe molecule, tetrahydrofuran (THF). The results were nearly identical compared to using another commonly used probe molecule, pyridine. This audition is based on the limited values of the surface area of these samples that likely require a relatively moderate basic molecule as THF with pK_b=16.08, rather than strong basic molecules such as NH₃ (pK_b=4.75) or pyridine (pK_b=8.77). We propose mechanisms for the interaction of vapour phase molecules of THF with the Lewis-cationic Mo and Cd atoms of these catalysts. Besides, dehydration of isopropyl alcohol was used as a probe reaction to investigate the catalytic activity of these catalysts to further support our findings in the case of THF in a temperature range of 175–300 °C. A good agreement between the obtained data of sample MoO₃-10 % CdO, which is characterised by the highest surface area value, the population of Lewis-acidic sites and % selectivity of propylene at all the applied reaction temperatures was found.     

Electron paramagnetic resonance of Er doped in YVO4: hyperfine parameters

X-band (9.60 GHz) electron paramagnetic resonance (EPR) spectra of the Er³⁺ Kramers ion substituting for the Y³⁺ ion in yttrium vanadate (YVO₄) single crystal were recorded at liquid-helium temperatures. Fine and hyperfine EPR transitions were observed for the ¹⁶⁶Er (zero nuclear magnetic moment) and ¹⁶⁷Er (I=7/2) isotopes, respectively. The values of the elements of the anisotropic ²- and ò-tensors of the ¹⁶⁷Er³⁺ ion, and those of the ² tensor for the ¹⁶⁶Er ion, were estimated. The admixture of crystal-field wave functions in the ground-state wave function of Er³⁺ were determined using the experimental g-values.     

Superconductivity in non-stoichiometric and tin-substituted La2CuO4: preparation, characterization, Mössbauer and microwave-absorption studies

Two samples of non-stoichiometric La₂CuO₄ were synthesized, one with La/Cu<2, and the other with 10% Sn substituting Cu. They were investigated by X-ray diffraction, Mössbauer spectroscopy, and microwave-absorption techniques. The microwave-absorption data indicated that they were both superconducting, with the transition temperatures T_c of 40.5 and 41.5 K, the one doped with Sn possessing the higher T_c. The Mössbauer spectra revealed that there exist two kinds of Sn(IV) atoms disordered with Cu. Their isomer shift, δ=−0.244(4) mm/s, is in agreement with Sn(IV) coordinated by oxygen. One site was characterized by a single Mössbauer line, being associated with a weakly distorted environment, wherein the Sn, coordinated more symmetrically, is surrounded by four Cu²⁺ ions. On the other hand, the other site, characterized by a Mössbauer doublet exhibited a quadrupole splitting Δ=1.07(2) mm/s, being associated with a highly distorted coordination, explained to be due to Sn occupying two adjacent cationic sites. To our knowledge, such a substitution for copper ions not resulting in a decrease of T_c has not been reported previously.     

Kinetic and thermodynamic studies of the nonisothermal decomposition of anhydrous copper(II) formate in different gas atmospheres

The thermal decomposition of anhydrous (orthorhombic) copper(II) formate was studied by programmed rising-temperature methods (TG, DTG, DTA and DSC) to about 250 °C in flowing gas atmospheres of nitrogen (inert), hydrogen (reducing) and air (oxidizing). The degradation reaction, anion breakdown, proceeded to completion in two distinct, but partially overlapping, rate processes and apparent Arrhenius parameters, calculated by the Ozawa nonisothermal kinetic method, agreed satisfactorily with the literature results. It was concluded that the two consecutive processes, contributing to the overall reaction, involved stepwise cation reduction: Cu²⁺→Cu⁺→Cu⁰, with copper(I) formate as intermediate. This mechanism is similar to that proposed in previous studies of the decompositions of copper(II) oxalate, malonate, maleate, fumarate, mellitate and squarate. For all of these reactants, the Cu⁺ salt has been identified as an intermediate, exhibiting a (slightly) lower relative reactivity than the corresponding Cu²⁺ salt. For copper(II) formate the response curves in the three different gaseous atmospheres were generally similar, showing that neither oxidizing nor reducing conditions caused a marked change in reactivity. The temperature of reaction initiation in H₂ was slightly diminished and the temperature of the second stage of reaction in O₂ was raised appreciably. It is believed that electron transfer contributed to the control of reactivity and that the gases present appreciably influence the rates of the contributory reactions occurring.     

Magnetic susceptibility study of Ce in PbCeA (A=Te,Se, S)

The magnetic susceptibility of Pb_1-xCe_xA (A=S, Se and Te) crystals with Ce³⁺ concentrations 0.006≤x≤0.036 was investigated in the temperature range from 2 K to 300 K. The magnetic susceptibility data was found to be consistent with a ²F_5/2 lowest manifold for Ce³⁺ ions with a crystal-field splitting Δ=E(Γ₈)−E(Γ₇) of about 340 K, 440 K and 540 K for Pb_1-xCe_xTe, Pb_1-xCe_xSe, and Pb_1-xCe_xS, respectively. For all the three compounds the doublet Γ₇ lies below the Γ₈ quadruplet which confirms the substitution of Pb²⁺ by Ce³⁺ ions in the host crystals. The observed values for the crystal-field splitting are in good agreement with the calculated ones based on the point-charge model. Moreover, the effective Landé factors were determined by X-band (∼9.5 GHz), electron paramagnetic measurements (EPR) to be g=1.333, 1.364, and 1.402 for Ce ions in PbA, A = S, Se and Te, respectively. The small difference with the predicted Landé factor g of 10/7 for the Γ₇ (J=5/2) ground state was attributed to crystal-field admixture.     

Effect of the Preparation Methodof Al–Mg–O Catalysts on the Selective Decomposition of Ethanol

Summary.  Selective decomposition of ethanol was used as a test reaction at 350°C to evaluate the catalytic activity of two Al–Mg–O mixed oxides prepared by two different methods (wet impregnation and coprecipitation). The catalyst precursors were examined by TG and DTA and were calcined between 500–900°C for 5 h in air. The surface area of all catalysts was measured by N₂ sorption using the BET method. The total acidity and basicity were determined by TPD using pyridine and formic acid. The catalysts were characterized by XRD analysis. It was found that the preparation method of Al–Mg–O catalyst has a great effect on the selective decomposition of ethanol. Al–Mg–O (I) catalysts, prepared by wet impregnation, were more selective towards ethene formation during dehydration of ethanol. This is ascribed to their high total surface acidity. On the other hand, Al–Mg–O (II) catalysts, prepared by coprecipitation, were highly selective in the oxidative dehydrogenation of ethanol to yield acetaldehyde. This could be attributed to their high concentration of basic sites. In addition, the production of traces of diethyl ether was also observed (three times more for Al–Mg–O (II) than for Al–Mg–O (I)). Corresponding author. E-mail: shalawy99@yahoo.com Received October 12, 2001. Accepted (revised) January 7, 2002     

Unpromoted and K2O-Promoted Cobalt Molybdate as Catalysts for the Decomposition of Acetic Acid

Summary.  Unpromoted cobalt molybdate was prepared from Co(NO₃)₂·6H₂O and (NH₄)₆Mo₇O₂₄·4H₂O, then calcined between 350 and 600°C for 5 h. K₂O (10 w%), as a promoter, was added to the calcined sample at 350°C from two different sources (i.e. KOH and KNO₃) and was subjected to further calcination at 350°C for 5 h. The catalytic activity of unpromoted catalysts towards the vapour phase decomposition of CH₃COOH was greatly influenced by the increase in the calcination temperature. This is attributed to the diminution of both S _BET and their dual acidic–basic characters. The promoted sample from the KOH source was found to be the most active of the catalysts studied. This is due to its high population of both acidic–basic surface sites and the formation of two new phases. XRD and FTIR analyses of the used catalysts, after the decomposition reaction of acetic acid, showed a remarkable change in its structure compared with the parent samples. E-mail: shalawy99@yahoo.com Received May 8, 2002; accepted (revised) July 9, 2002