Mohamed I. Zaki

In situ FTIR spectra of pyridine adsorbed on SiO2–Al2O3, TiO2, ZrO2 and CeO2: general considerations for the identification of acid sites on surfaces of finely divided metal oxides

Abstract Exposure of strong Lewis (coordinatively unsaturated metal atoms) and Bronsted (proton donor OH-groups) acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ FTIR spectroscopy of small adsorbed base molecules, often NH 3 , pyridine, CH 3 CN, NO or CO molecules, has been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine (Py) has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room (RT) and higher temperature regimes, since it is (i) more selective and stable than NH 3 ; (ii) much more strongly adsorbed than CO and CH 3 CN; and (iii) relatively more sensitive to the strength of Lewis acid sites than NO. In the present work, in situ IR spectra of Py adsorbed at ≥RT on characterized alumina, silica, silica–alumina, titania, zirconia and ceria were measured, and compared with RT-spectra of liquid and gas phase Py obtained under identical spectroscopic conditions, in order to characterize spectral consequences of mutual Py–Py interactions in the adsorbed phase. It has been concluded that the availability of Lewis acid sites can be unequivocally monitored by formation of coordinated Py molecules giving rise to IR-absorption(s) due to the ν 8a mode of ν CCN vibrations at 1630–1600 cm −1 , where the higher the frequency assumed, the stronger the acidity of the site. Formation of pyridinium surface species (PyH + ) is identifiable by (i) an ν 8a -absorption at ≥1630 cm −1 ; (ii) an ν 19b -absorption at 1550–1530 cm −1 ; as well as (iii) ν N + H and δ N + H absorptions occurring, respectively, near 2450 and 1580 cm −1 , and, thus, the availability of Bronsted acid sites. Moreover, products and IR-characteristics of Py surface reactions at >RT have been identified, and used to imply nature of surface base sites (OH − and O 2− ) involved in formation of acid–base site pairs.

Thermochemistry of manganese oxides in reactive gas atmospheres : Probing redox compositions in the decomposition course MnO2 → MnO

Abstract The thermal decomposition course MnO2 → MnO was examined in various gas atmospheres (O2, air, N2 and H2) by temperature-programmed studies employing thermogravimetry and differential thermal analysis. Weight-invariant thermal events encountered were subjected to non-isothermal and isothermal kinetic analysis. Product analysis was carried out using infrared spectroscopy and X-ray diffractometry. Cyclic TG experiments carried out in air have revealed that, of the intermediate decomposition products characterized, viz. Mn5O8, Mn2O3 Mn3O4, the mixed-valence Mn3O4 (= Mn(II)Mn2(III)O4) can tolerate reversible oxygenation-deoxygenation processes at (500–1050°C). Moreover, the presence of Mn(II) in the mixed-valence Mn5O8 (= Mn2(II)Mn3(IV)O8) is seen to sustain a synproportionation of Mn(II) Mn(IV) during the oxide deoxygenation, giving rise to Mn(III) species (= Mn2O3). The electron-mobile environment thus established in such mixed-valence oxides is seen to promise a catalytic potential in oxidation/reduction reactions.

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.

Monopropellant decomposition catalysts V. Thermal decomposition and reduction of permanganates as models for the preparation of supported MnOx catalysts

Abstract The need for highly concentrated hydrogen peroxide solutions to fuel spacecraft propulsion systems necessitates the development of new catalytic formulations mainly based on supported manganese oxides. In order to improve the preparation of such catalysts, the thermal decomposition and the hydrothermal reduction of different permanganate precursors were studied, as well as the effect of washing on the products. The test samples were characterized by X-ray power diffraction (XRD) and surface area determination, and compared for their catalytic activity towards H2O2 decomposition using a constant pressure reactor (for 1.71% H2O2) and a constant volume reactor (for 50% H2O2). The catalysts prepared by thermal decomposition of KMnO4 or NaMnO4·H2O have shown the intermediate formation of potassium (or sodium) manganate, and its partial dissolution during washing with water; higher catalytic activity and larger surface area were observed for the decomposition products of KMnO4. The hydrothermal reduction yielded products exhibiting larger surface area and higher specific catalytic activity when carried out under stoichiometric conditions. Comparable kinetic parameters were determined using both catalytic reactors. Unsupported and alumina supported catalysts displayed comparable activity per gram of manganese.

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.

Synthesis of high surface area titania powders via basic hydrolysis of titanium(IV) isopropoxide

Abstract TiO2 powders were obtained by calcination of base hydrolysis products of titanium(IV) isopropoxide, Ti(OPri)4 at 400°C for 3 h. The hydrolysis was carried out at room temperature in two different solvents, namely isopropanol and n-heptane, and in the presence of low and high ammonia contents. X-ray powder diffractometry showed the resulting TiO2 powders to consist of anatase crystallites, irrespective of the hydrolysis conditions applied. In contrast, N2 adsorption isotherms (determined at liquid nitrogen temperature) probed notably different surface textures for the test powders, depending on the amount of base added and the solvent used. These results were confirmed by transmission electron microscopy, and attributed to the solvent-permitted ammonia interactions with the titania precursors formed during the course of the alkoxide hydrolysis. Accordingly, preparative conditions can be resolved for producing titania powders of higher specific surface area (67–73 m2/g) than the commercial titanias described by manufacturers as being high surface area powders (∼ 50 m2/g).

Low-temperature Synthesis of Hausmannite Mn3O4

[15, 16] resulted in a major product thatcontained varied amounts of â-MnOOH (Feitknech-tite), depending in whether the oxidant was merelyair or pure oxygen atmosphere and on the exposureperiod to the oxidizing atmosphere [15, 16]. Employ-ing X-ray diffractometry, the major product wassometimes identified as being Mn

Characterization of the thermal genesis course of manganese oxides from inorganic precursors

Abstract NH4MnO4, Mn3O4 and Mn(NO3)2·6H2O were used as precursor compounds for the thermal genesis (at 150–600°C) of manganese oxides. Thermal events occurring during the genesis course were monitored by means of thermogravimetry and differential thermal analysis, in oxidizing and non-oxidizing atmospheres. Intermediate and final solid-phase products were characterized using X-ray diffractometry and infrared spectroscopy. Model manganese oxides were subjected to similar examinations for reference purposes. The results indicated that NH4MnO4 is almost completely decomposed near 120°C, giving rise to predominantly α-Mn2O3. The presence of K+ contaminant supports an oxidative conversion of α-Mn2O3 into KMn8O16+ at ⩾300°C. In contrast, the genesis of pure α-Mn2O3 from Mn(NO3)2·6H2O is not achieved unless the calcination temperature exceeds 500°C; β-MnO2 was the only detectable intermediate. Mn3O4, obtained at room temperature by the addition of aqueous Mn2+ to ammonia solution, was converted into α-Mn2O3 via the formation and subsequent decomposition of Mn5O8 at ⩾300°C.

ADSORPTION AND SURFACE-REACTIONS OF PYRIDINE ON PURE AND DOPED CERIA CATALYSTS AS STUDIED BY INFRARED-SPECTROSCOPY

Abstract Pyridine (Py) adsorption on ceria surfaces, prepared by thermal decomposition of diammonium hexanitratocerate at 400 °C, was studied by infrared spectroscopy and gravimetric techniques. At room temperature, Py was irreversibly adsorbed via coordination to Lewis sites of different acid strengths. High temperature calcination (at 800 °C) of ceria greatly reduced the Py adsorption capacity of the surface. Upon thermoevacuation at 100–400 °C, Py cracking occurred with formation of various surface species (among these caboxylates and nitrites) deriving from cleavage of the ring via attack of surface oxygen on adsorbed Py. Doping of ceria with Na + , Al 3+ and Cr 3+ ions (at 1 atom dopant per Ce atom) largely modified the Py adsorption capacity and surface reactions, thus revealing disparate impacts on acid/base properties of the surface.