Nasr E. Fouad

Temperature-programmed reduction of calcined chromia-coated alumina and silica catalysts" probing chromium (VI)-oxygen species

Abstract Chromia-coated alumina and silica, containing 0.5, 5 and 10 wt%Cr 2 O 3 , have been prepared by adding Cr(NO 3 ) 3 solution to a suspension of the support in NH 4 OH followed by calcination at 600°C. Temperature-programmed reduction (TPR) of a corresponding unsupported chromia showed peaks at 370 and 490°C, due to reduction of surface chromate groups; commercial α-Cr 2 O 3 showed no hydrogen uptake. TPR measurements on the chromia-coated supports, together with information from the literature, lead to the conclusion that reducible chromate groups are present on the surface in a variety of forms having different degrees of ease of reduction and hydrolysis, their numbers and structures depending on Cr content and the nature of the support. H 2 uptakes are markedly less than expected on the basis of hydrazine titration of surface excess oxygen. The reactivities of surface chromate groups may be understood on the basis of difference in the polarizability of metal-oxygen bonds in the support.

Surface to bulk characterization of phosphate modified aluminas

Abstract γ-Al2O3, dried alumina gel, as well as their phosphated forms using (NH4)2HPO4 were prepared by wet impregnation and calcined at 870 K. Resulted samples were subjected to investigate the consequent bulk [X-ray powder diffractometry (XRD), diffuse reflectance spectroscopy (DRS) and infrared spectroscopy (IR)] and surface (texture, N2-adsorption and surface acid properties, pyridine adsorption). Results indicated no detectable bulk phase changes due to phosphation. However, the phosphated gel sample reveals the highest SBET. Surface stabilization of phosphate species by γ-Al2O3 or gel is indicated, leading to modifications on surface hydroxyl and hence surface acidity. The phosphated gel sample exhibits the strongest acidity (both Bronsted and Lewis).

CO and CH4 total oxidation over manganese oxide supported on ZrO2, TiO2, TiO2–Al2O3 and SiO2–Al2O3 catalysts

Zirconia, titania, titania–alumina and silica–alumina supported and unsupported MnOx catalysts were prepared and characterized by X-ray diffractometry and photoelectron spectroscopy, infrared spectroscopy and nitrogen sorptometry. Their catalytic activities were tested towards total oxidation of carbon monoxide and methane. The results show the unsupported MnOx (exposed on an α-Mn2O3 bulk structure) to catalyze CO oxidation at ⩽250°C and CH4 oxidation at 250°C, and to remain chemically and structurally stable. The CO oxidation activity of MnOx is improved when dispersed on zirconia, whereas its CH4 oxidation activity is improved on silica–alumina. These results may help in concluding that (i) CO oxidation is not the rate determining step of the oxidation of CH4, (ii) availability of strong acid sites (as those exposed on silica–alumina) is important for CH4 oxidation and (iii) the difference in the catalytic activity towards the oxidation of CO and CH4 resides in the need for different catalytic functions for each reaction, which are, therefore, not related in terms of kinetic control.

Chromia on Silica and Alumina Catalysts: Temperature-Programmed Reduction and Structure of Surface Chromates

• 9H20 and Cr03 followed by calcination at 573 and 873 K. For comparison purposes unsupported materials were also prepared by decomposition of the parent compounds at the same temperatures, namely 573 and 873 K. The materials were structurally characterized by UV-VIS diffuse reflectance spectroscopy (DRS) and XRD, and their stoichiometry was determined by temperature programmed reduction (TPR). Decomposition of the nitrate at 573 K yielded dominantly poorly crystallized a-Cr203 with surface Cr4+ and Cr6+ minority species being also present, while Cr205 was preferentially formed from Cr03. In contrast, high temperature decomposition of both parent compounds gave well crystallized 0t-Cr2O3 containing also surface chromates. These bulk and surface oxides were irreducible in H2 at temperatures < 970 K. Chromia-support interactions turned out to be very weak with Si02, leading to essentially the same structural characteristics as found for the unsupported oxides. The CrO,/Si02 samples calcined at 573 K were more easily reduced than the unsupported analogues due to higher dispersion of the chromia. Due to strong support interactions in the Al203-supported systems, highly dispersed Cr-0 species are anchored to the alumina surface with Cr4+ and Cr6+ being the dominant oxidation states.

Thermal decomposition and creation of reactive solid surfaces: IV. effect of NH4NO3 inclusion on the thermal genesis of chromia catalyst from a parent gel

The effect of NO; contaminant on the thermal decomposition of chromia gel has been thoroughly investigated. NOT-free gel, prepared by treating a dilute solution of chromium nitrate with aqueous ammonia, and a simulated NOT-contaminated one, obtained by mechanically mixing calculated amounts of the pure gel and NH,NO, (4.7-33.3% by weight), have been subjected to thermogravimetric and differential thermal analyses. The thermal behaviour thus monitored has been physicochemically characterized by means of infrared and X-ray diffraction techniques. The results obtained have revealed a notable disparity between the reaction pathways conceded by the pure and the contaminated gel yielding chromias of different properties.

Spectro-thermal investigation of the decomposition intermediates developed throughout reduction of ammonium paratungstate

Abstract The thermal decomposition course of ammonium paratungstate (APT) in hydrogen was studied using thermogravimetric and differential thermal analyses. X-ray diffractometry, infrared spectroscopy and diffuse reflectance spectroscopy were used to characterize and identify the intermediate solid products. It was found that ammonium paratungstate decomposes to tungsten metal W 0 through five main steps encompassing different tungsten intermediate compounds. Ammonium tungsten bronze (NH 4 ) 0.33 WO 3 , which precedes the formation of WO 3 , is relatively the most stable intermediate (250–550°C) encountered through reduction of APT to tungsten metal W 0 . The influence of hydrogen spillover on the reduction behavior commences to be effective just after formation of the bronze intermediate.

Stability of surface chromate – A physicochemical investigation in relevance to environmental reservations about calcined chromia catalysts

Abstract The present investigation presents and correlates observed and reported characterization results of calcined, supported and unsupported chromias obtained via various precursor compounds. Studies performed employed a range of surface and bulk analytical techniques, in hopes of a proper assessment of the stability of surface chromate (Cr(VI)–O) species thus generated to thermal decomposition, hydrolysis and reduction. Impacts on the redox catalytic activity were probed towards the decomposition of H 2 O 2 solutions and the CO oxidation in the gas phase. The results have revealed that chromate species established on bulk and dispersed α-Cr 2 O 3 particles enjoy high stability against thermal and chemical treatments, and yet contribute to the formation of surface sites that are catalytically active in redox reactions. This should help lessening environmental reservations about the industrial application of calcined chromia catalysts.

Thermogravimetry of WO3 reduction in hydrogen: Kinetic characterization of autocatalytic effects

Abstract Chemical and autocatalytic events occurring during the hydrogen-reduction of WO 3 were observed and characterized by thermogravimetry, X-ray diffractometry and a kinetic analysis. The results indicated that WO 3 is completely reduced to W° near 800 °C, via the formation of WO 2.72 (at 520 °C) and WO 2 (at 600 °C) as intermediate products. Kinetically, the initial stage of the reduction was found to be dominated by the formation of WO 2 , and to be influenced by strong autocatalytic effects associated with the commencement of formation of W°. These results fulfil the requirements of a consecutive autocatalytic kinetic model, whereby the dissociative adsorption of H 2 on freshly generated W° surfaces is considered the rate determining step. Arrhenius plot based on the initial rate as a function of temperature data resulted in an activation energy value of 118±5 kj mole −1 .

Chromia on Silica and Alumina Catalysts: Chromia Dispersion as Determined by N2-Adsorption Measurements

Silicaand alumina-supported chromia catalysts at loadings varying between 0.5 and 10 mole % Cr203 were prepared from different parent compounds and characterized by N2-adsorption at 77 K. The adsorption isotherms were evaluated on the basis of the BETand as-methods. The results suggest that the chromia exists on the surface of silica with rather low dispersion probably in the form of small Cr203 crystallites which lead to pore blocking. In contrast, on alumina narrowing of pores is observed, this being suggestive of formation of a highly dispersed chromia layer. These conclusions can be rationalized on the basis of the different surface chemical properties of the oxide supports, and they are consistent with previously reported spectroscopic characterizations of the same materials. It is inferred that in fact N2-adsorption data do provide valuable qualitative information on the dispersion of supported oxide materials.

Characterization of ammonium tungsten bronze [(NH4)0.33WO3] in the thermal decomposition course of ammonium paratungstate

Abstract Ammonium tungsten bronze, (NH 4 ) 0.33 WO 3 , was characterized in the calcination product of ammonium paratungstate at 400°C for 2 h in a static atmosphere of air. X-ray diffractometry, thermogravimetry, and infrared and UV–vis diffuse reflectance spectroscopies found the yellowish orange product to also contain hexagonal tungsten trioxide, WO 3 . On heating up to 500°C, the bronze together with the hexagonal WO 3 were transformed completely into the yellowish green monoclinic tungsten trioxide.