Gamal A.M. Hussein

Rare earth metal oxides: formation, characterization and catalytic activity Thermoanalytical and applied pyrolysis review

This review provides a guide to the recent literature on the characterization of the decomposition routes of different precursors, especially carboxylates of rare earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Th), and the formation of the final oxide products. It also discusses the effects of 1. (1) the nature of precursors, i.e. acetate, oxalate, carbonates, etc., 2. (2) the nature and stability of the intermediates formed during decomposition, 3. (3) the calcination temperature and 4. (4) the composition of the gas atmosphere, i.e. N2, H2, O2, air and vacuum, on the chemical composition, structure, texture and catalytic activity of the generated oxides.

Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 2.—Methanol over TiO2, ZrO2 and HfO2

Infrared spectroscopy has been used to analyse the gas-phase reaction products and the related adsorbed species obtained between room temperature and 400 °C from the dehydrogenation/dehydration reactions of propan-2-ol over a series of differently calcined catalysts of TiO2, ZrO2 and HfO2. The ZrO2 and HfO2 results were independent of the calcination pretreatment, and the surfaces of these oxides, like that from a TiO2 sample calcined at 800 °C, were dehydroxylated. Different results were obtained from a TiO2 sample calcined at 300 °C which had a hydroxylated surface. The acidic sites and reactivities of the surfaces of TiO2(300 °C) and TiO2(800 °C) were explored by pyridine adsorption and infrared spectroscopy. Only Lewis-acid sites were detected by pyridine.On raising the reaction temperature, in all cases the dehydrogenation reaction to give acetone occurred either before or simultaneously to the onset of the dehydration reaction to give propene. Acetone production was most pronounced over ZrO2 and HfO2 but also occurred more with TiO2(800 °C) than with TiO2(300 °C). The dehydrogenation reaction was largely quenched by pre-adsorbed pyridine on both TiO2 samples. The TiO2(300 °C) catalyst showed the presence of adsorbed propan-2-ol and 2-propoxide groups at room temperature. The dehydroxylated ZrO2, HfO2 and TiO2(800 °C) samples only showed appreciable amounts of 2-propoxide groups. In each case the 2-propoxide ions occurred in two different forms, presumably formed by adsorption on different types of sites.Both the acetone and propene products appeared as absorptions from 2-propoxide surface species decreased in intensity, so the latter are clearly reactive species. Gas-phase acetone production was followed by the chemisorption of acetone at a higher temperature. This subsequently decomposed to give surface acetate species, and finally at 400 °C to give CO2 and methane in the gas phase. Propene did not give rise to adsorbed species, or to further products in the gas phase.At the higher temperatures, above 300 °C, the reaction was always selective in favour of the dehydration reaction. However, each of the dehydroxylated catalysts showed some selectivity in favour of the dehydrogenation reaction over the earliest temperature range for alcohol decomposition, between 200 and 250 °C.A discussion is given of possible mechanistic pathways for the production of surface 2-propoxide species and the two types of products, based on the infrared-supported assumption that the different adsorbed forms of 2-propoxide [and possibly adsorbed propan-2-ol on TiO2(300 °C)] are reactive intermediates.

Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 3.—Ethanol over TiO2, ZrO2 and HfO2, and general conclusions from parts 1 to 3

The dehydrogenation and dehydration reactions of ethanol over TiO2, ZrO2 or HfO2 catalysts has been monitored in the gas phase and on the surfaces by infrared spectroscopy. The reaction pathways closely parallel those of methanol reported in the prevous paper (Part 2) with the addition of the direct dehydration reaction C2H5OH → C2H4+ H2O and the production of benzene as a minor product. The infrared spectroscopic analysis of the decomposition of diethyl ether as initial reagent over the TiO2(500) catalyst confirms that the ethane product is derived from the ether precursor. As with methane from methanol, it is probably produced by reduction of the ether to the alkane plus water by hydrogen derived from the parallel dehydrogenation reaction.A summary is given of probable mechanisms for the catalysed reactions of the three alcohols, methanol, ethanol and propan-2-ol based on the gas-phase products and surface species identified by infrared spectroscopy. The general importance of alkoxide surface intermediates is emphasised. Alkoxides of a given formula occur on different spectroscopically distinguishable sites with different reactivities.

Ceria on silica and alumina catalysts: dispersion and surface acid-base properties as probed by X-ray diffractometry, UV-Vis diffuse reflectance and in situ IR absorption studies

Abstract X-ray diffractometry and UV-Vis diffuse reflectance spectroscopy revealed that fluorite-structured CeO 2 crystallites (mean size 22.3 nm) are dispersed on silica surfaces (CeSi) into microcrystallites (11.2-8.1 nm) and dispersed further on alumina surfaces (CeAl) into nanocrystallites ( x monolayers. Consequently, IR spectroscopy of adsorbed pyridine found Lewis acid sites to be far more strengthened on CeAl. Bronsted acid sites (proton-donors) were probed exclusively on CeAl. On the other hand, IR spectroscopy of adsorbed deuterated chloroform (CDCl 3 ) showed the originally moderate Lewis base sites (low-coordinated OH − and O 2− ) to be weakened on CeSi, but markedly strengthened on CeAl. Lewis base sites exposed on ceria surfaces assume a strong nucleophilic reactivity.

Thermal and chemical events in the decomposition course of manganese compounds

Abstract Thermogravimetry and differential thermal analysis, infrared spectroscopy (solid phase and gas phase), and X-ray diffraction were used to characterize the pathways of the decomposition of the hydrated acetate, oxalate and nitrate of manganese in air. The non-isothermal activation energy ( ΔE , kJ mol −1 ) was determined for the thermal processes monitored throughout the decomposition course. The results showed that Mn ( CH 3 COO ) 2 ·4 H 2 O dehydrates in three steps up to 130 °C, and then decomposes to a mixture of manganese oxides Mn 3 O 4 (major) and Mn 2 O 3 (minor) through the intermediates Mn(OH)CH 3 COO and/or MnOCH 3 COO and MnCO 3 . MnC 2 O 4 ·2 H 2 O dehydrates in one step at 150 °C, and then decomposes via MnCO 3 into Mn 3 O 4 (hausmannite structure) at 350 °C. Mn ( NO 3 ) 2 ·4 H 2 O undergoes stepwise dehydration up to 175 °C, and decomposes above 200 °C via an unstable oxynitrate intermediate yielding MnO 2 ; this decomposes at about 550 °C to the α-Mn 2 O 3 phase.

Formation of high surface-area yttrium oxide by the thermal decomposition of different inorganic precursors

Abstract Y(CH 3 COO) 3 · 4H 2 O, Y(NO 3 ) 3 · 5H 2 O and Y 2 (C 2 O 4 ) 3 · 8H 2 O were used as precursor compounds for the formation of Y 2 O 3 at 100–700°C. Thermal events occurring during the decomposition courses were monitored by means of TG and DTA. Intermediates and final solid products were characterized using infrared spectroscopy and X-ray diffractometry. The Y 2 O 3 residues thus formed were subjected to surface and texture investigations. The results indicate that Y(NO 3 ) 3 · 5H 2 O is completely decomposed at 450°C via different unstable intermediates to give high surface area (58 m 2 g −1 ) Y 2 O 3 . Both Y(CH 3 COO) 3 · 4H 2 O and Y 2 (C 2 O 4 ) 3 · 8H 2 O are completely decomposed at 650°C via Y 2 O 2 CO 3 intermediate. However, Y(CH 3 COO) 3 · 4H 2 O yields a higher surface-area Y 2 O 3 product (55 m 2 g −1 ) than Y 2 (C 2 O 4 ) 3 · 8H 2 O (12 m 2 g −1 ).

Characterization of lanthanum oxide formed as a final decomposition product of lanthanum acetylacetonate: thermoanalytical, spectroscopic and microscopic studies

Abstract The porous and high surface area of lanthanum oxide was obtained as a final decomposition product of lanthanum acetylacetonate tetrahydrate. The decomposition course in dry nitrogen was thoroughly studied. Thermal processes occurring throughout the decomposition range (100–800 °C) were monitored by thermogravity; differential thermal analysis and infrared spectroscopy gaseous products. These processes were characterized on the basis of the solid products analyses, using X-ray diffractometry and IR-spectrometry. The results showed La(C5H7O2)3 completely decomposed to La2O3 at 730 °C, through amorphous and unstable intermediates La(CH3COO)(C5H7O2)2 at 190 °C, La(CH3COO)2(C5H7O2) at 225 °C, La(CH3COO)3 at 285 °C, La2(CO3)3 at 390 °C and crystalline and stable La2O2(CO3) at 430 °C. Gas phase decomposition products included propyne, acetone, carbon oxides, methane and isobutene. Methane and isobutene resulted from interfacial reactions involving the initial product (acetone). The ultimate products, La2O3 at 700 °C and 800 °C, are crystalline porous solids and having a surface area of 21 and 45 m2/g respectively.

Formation of praseodymium oxide from the thermal decomposition of hydrated praseodymium acetate and oxalate

Abstract Thermogravimetry, differential thermal analysis, infrared spectroscopy, X-ray diffractometry and surface area measurements were used to characterize the thermal decomposition of the hydrated acetate and oxalate of praseodymium in air. Non-isothermal kinetic parameters ( A , Δ E ) were determined for the thermal events monitored throughout the decomposition course. The results showed that Pr(CH 3 COO) 2 ·H 2 O became dehydrated at 180°C, melted at 270°C and decomposed to PrO 1.833 at 575°C through intermediate oxy-carbonate compounds. In contrast, Pr 2 (C 2 O 4 ) 3 ·10H 2 O dehydrates completely in three steps by 390°C, and then decomposes similarly to PrO 1.833 . The oxide formed from the acetate precursor has a higher surface area (18 M 2 /g) than that obtained from the oxalate precursor (8 m 2 /g). The gas-phase decomposition products identified by infrared spectroscopy are acetone, acetic acid, carbon oxides, methane and isobutene. Some of these were initial decomposition products, whereas others (methane and isobutene) were interfacial reaction products involving initial ones.

Thermal genesis course and characterization of praseodymium oxide from praseodymium nitrate hydrate

Pr(NO3)36H2O was used as a precursor to produce PrO1.833 at 6008C in an atmosphere of static air. Thermal processes occurred were monitored by means of thermogravimetry, differential thermal analysis and mass spectrometry. IR-spectroscopy and X-ray characterized the intermediates and final solid products. The results showed that, Pr(NO3)36H2O decomposes through 11 endothermic weight loss processes. Five dehydration steps occurred at 130, 180, 200, 230 and 2508C, leading to the formation of crystalline nitrate monohydrate, Which decomposes to Pr(NO3)2 at 3408C. The latter, decomposes to PrO1.833 at 4658C, via four different intermediates PrO(NO3) at 4308C, and nonstoichiometric unstable, PrO0.25(NO3)2.5 at 3628C; Pr(O)0.5(NO3)2 at 3828C and Pr(O)0.75(NO3)1.5 at 4008C. The gaseous decomposition products identified by mass spectroscopy were water vapor and nitrogen oxides (NO, NO2 and N2O5). The activation energy was determined nonisothermally for the thermal processes monitored throughout the decomposition course. The final product PrO1.833 has a surface area of 46.3 m 2 /g. # 2001 Elsevier Science B.V. All rights reserved.

Texture properties of yttrium oxides generated from different inorganic precursors

Abstract The texture properties of four samples of Y 2 O 3 (cubic structure) generated from different inorganic precursors (hydrated Yttrium acetate, Y(CH 3 COO) 3 ·4H 2 O; nitrate, Y(NO 3 ) 3 ·5H 2 O and oxalate, Y 2 (C 2 O 4 ) 3 ·8H 2 O) at different temperatures have been studied using BET, pore size distribution, cumulative surface area, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that samples obtained from both acetate at 700°C (55 m 2 /gm) and nitrate at 500°C (58 m 2 /gm) are very similar in surface area and pore structure (a wider spectrum of mesoporosity); i.e., good precursors for catalyst and catalyst support. The third sample obtained from oxalate at 700°C exhibits a lower surface area (12 m 2 /gm) and different in pore structure, i.e., a good precursors for ceramics and superconductor manufacture. For Y 2 O 3 generated from nitrate, the increase in calcination temperature from 500 to 700 °C caused a decrease in the surface area from 58 to 20 m 2 /g, and in the microporosity development. The SEM and particle size calculations have been used to assess the surface properties of Y 2 O 3 samples.