Gamal A.H. Mekhemer

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 genesis course and characterization of lanthanum oxide

Abstract La(NO3)3·6H2O was used as a precursor to produce La2O3 at 650°C in an atmosphere of air. Thermal processes occurred were monitored by means of thermogravimetry, differential thermal analysis, and gas-mass spectrometry. Infrared (IR)-spectroscopy, X-ray diffractometry and scanning electron microscopy characterized the intermediates and final solid products. The results showed that, La(NO3)3·6H2O decomposes through nine endothermic weight loss processes. Five dehydration steps occurred at 90, 105, 150, 175 and 215°C, leading to the formation of crystalline nitrate monohydrate, which decomposes to La(OH)(NO3)2 at 410°C. The latter, decomposes to La2O3 at 640°C, via two different intermediates; LaO(NO3) at 440°C, and non-stoichiometric unstable, La(O)1.5(NO3)0.5 at 570°C. The gaseous decomposition products as identified by gas-mass spectroscopy were water vapor, nitric acid and nitrogen oxides (NO, NO2 and N2O5). The final product La2O3 has a large crystalline containing pores, voids and cracks, with a surface area of 23 m2 g−1. Also it possessed Lewis acidic and basic sites, as indicated by Pyridine adsorption.

Surface characterization of silica-supported cobalt oxide catalysts

Silica supported cobalt oxides were prepared by the impregnation method, using an aqueous solution of cobalt nitrate hexahydrate (Co(NO3)2 · 6H2O), then calcined at different temperatures (510, 620 and 870 K). Characterization of the samples was carried out by X-ray diffraction, N2-adsorption at −196°C, UV–Vis diffuse reflectance spectroscopy and KBr-IR spectroscopy of the calcination products. The surface acidity was studied by IR spectroscopy of adsorbed pyridine at different temperatures (300, 370, 470 and 570 K). Results indicated that Co3O4 is the stable phase on silica, however, dispersion of minor amount of cobalt oxide could not be ruled out. Results also indicated that the crystallinity of the formed Co3O4 increased by increasing the loading level and/or the calcination temperature. Furthermore, the surface area of the support was decreased by increasing the loading level and the calcination temperatures. It has been also found that the surface of the supported catalysts exposed strong different Lewis acid sites.

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).

Surface structure and acid–base properties of lanthanum oxide dispersed on silica and alumina catalysts

Supported lanthanum oxide catalysts were prepared by the impregnation method, using an aqueous solution of lanthanum nitrate hexahydrate (La(NO3)3·6H2O), then calcined at 920 K for 2 h. Characterization of the samples was carried out by X-ray diffraction, N2-adsorption at −196 °C and IR spectroscopy of the calcination products. The surface acidity was studied by IR spectroscopy of adsorbed pyridine at different temperatures. GC was used to investigate the decomposition of propan-2-ol on the surface of the catalysts. X-ray diffractometry revealed that La2O3 crystallites are dispersed on the silica and alumina surfaces. The surface area of the support was decreased by increasing the loading level of La2O3. It was also found that the surface of the supported catalysts had strong different Lewis acid sites and basic sites. Propan-2-ol was found to decompose to acetone (dehydrogenation). The dehydration reaction gave propene as the sole product.

Chemical and morphological consequences of acidification of pure, phosphated, and phosphonated CaO: Influence of CO₂ adsorption

In situ Fourier transform infrared (FTIR) spectroscopy was employed to characterize the adsorption behavior (as a function of pressure or time) and surface species of CO2 molecules on pure, phosphated, and phosphonated CaO. Carbonate and bicarbonate species were found to form on the pure oxide, whereas on the phosphated and phosphonated oxide samples the carbonate species were found to substitute favorably some of the OH(-) and PO4(3-) groups thereon exposed, respectively. Before and after carbonation, the test samples were further examined by in situ FTIR spectroscopy of adsorbed pyridine species, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Then they were in situ acidified by exposure to a wet atmosphere of HCl vapor at 673 K for 10 min and re-examined similarly to reveal the influence of CO2 adsorption on the chemical and morphological consequences of acidification. The results obtained show the carbonate substitution of PO4(3-) groups to enhance agglomeration of the otherwise fine, longitudinal material particles into much bulkier ones and to render the otherwise more stable phosphonate groups less stable to acid treatment than the phosphate groups. Moreover, the bulky particle agglomerates of the carbonated test samples were detectably eroded following the acid treatment.

Formation and surface characterization of thulium oxide catalysts

Thulium oxide, Tm2O3, was obtained as a final product of the thermal decomposition of Tm(CH3COO)3·4H2O. The decomposition steps up to 800°C were characterized by TG, DTA, XRD, SEM, GC-MS and gas- and solid-phase IR spectroscopy. The results showed that Tm(CH3COO)3 ·4H2O dehydrates completely in two overlapping steps at 90 and 110°C and decomposes to Tm2O3 at 540°C through a non-crystalline intermediate Tm(OH)(CH3COO)2 at 350°C, Tm(O)(CH3COO) at 375°C and Tm2O2CO3 at 400°C. The oxides obtained at 600 and 800°C were subjected to texture analysis and pyridine adsorption. The results revealed that the oxide obtained at 600°C has a higher surface area of 49.7 m2 g−1 with larger pores than that obtained at 800°C (SBET=32.8 m2 g−1). On the other hand, the oxide obtained at 600°C has a basic surface character and contains at least two different Lewis acid sites as indicated from pyridine adsorption. The gaseous decomposition products as identified by gas-phase IR and GC-MS are water vapour, acetic acid, ketene, acetone and methane.

Structure analysis of phosphated zirconia catalysts using XRD and nitrogen adsorption methods

Abstract Phosphated zirconia catalysts were prepared by impregnating two different precursors (zirconium hydroxide and crystalline ZrO2) with an aqueous solution of diammonium hydrogen phosphate. X-ray powder diffractograms and nitrogen adsorption isotherms at −196°C on pure and phosphated zirconia catalysts were investigated. The pore volume was calculated from the adsorption branch to explore the modification of porosity for pure and phosphated zirconia. Pore structure analysis shows that phosphated zirconia, resulted from calcination of zirconium hydroxide at low temperature, exhibited high surface area and accessible porosity. Whereas for those calcined at higher temperatures, inaccessible porosity is shown to predominate. Phosphation of crystalline zirconia did not modify either surface area or the porosity, as compared to pure zirconia. The influence of phosphation on the structure of zirconium hydroxide and zirconia during the calcination course was studied.

Holmium oxide from holmium acetate, formation and characterization: thermoanalytical studies

Abstract Thermal processes involved during the decomposition course of hydrated holmium acetate (Ho(CH 3 COO) 3 ·3.5H 2 O) up to 800°C, in an atmosphere of air, were monitored by thermogravimetry and differential thermal analysis. The gaseous decomposition products were identified by IR- spectroscopy. X-ray diffraction and IR-spectroscopy characterized intermediates and final solid products. The results showed that, Ho-acetate, dehydrates completely in two steps then decomposes to Ho 2 O 3 at 570°C, through three noncrystalline unstable intermediates. The oxide obtained at 600 and 800°C possesses a surface area of 31 and 15.0 m 2 g −1 , respectively. The volatile decomposition products from the acetate were water vapor, acetic acid, ketene, acetone, methane and isobutene.

Low-Temperature IR Spectroscopy of CO Adsorption on Calcined Supported CeO2: Probing Adsorbed Species and Adsorbing Sites

Carbon monoxide adsorption on ceria dispersed on silica (CeSi) and alumina (CeAl) at 300–80 K was observed by in-situ IR spectroscopy. For control purposes, CO adsorption was also observed on unsupported CeO2 and the individual support materials (SiO2 and Al2O3). The adsorbents were prepared ex situ by heating at 770 K (3 h) in air, and pretreated in situ by heating at 720 K (1 h) in oxygen and then in vacuum. The results, as disclosed by v(OH) (3900–3300 cm−1) and v(CO) (2250–2050 cm−1) spectra taken before and after CO adsorption, reveal that CO adsorbs on all of the test adsorbents at temperatures < 300 K only. The resulting adsorbed species include CO coordinated to Lewis acid sites (on all of the adsorbents, but not SiO2), hydrogen-bonded CO (on all of the adsorbents, but not CeO2) and CO bound to electron-rich defect sites (only on unsupported CeO2). It is concluded that the dispersion of ceria, particularly on alumina, is associated with a considerable development of the Lewis acidity of Ce4+ sites.