132,161 research outputs found
Critical Aspects of Energetic Transition Technologies and the Roles of Materials Chemistry and Engineering
No full textThe perspectives of technological advances needed for short term energetic transition are briefly reviewed and discussed critically. In particular, the technologies for the greenhouse gas emission-free production of electrical energy, its storage and transport, the production, transport, storage and use of hydrogen, and the use of biomass derived technologies are shortly and critically reviewed. Critical aspects are emphasized. The role of chemistry, and in particular materials chemistry and engineering, in short-term developments are underlined
Catalytic materials based on silica and alumina: Structural features and generation of surface acidity
No full textThe structural, surface chemical and catalytic properties of the materials belonging to the SiO2-Al2O3 system are reviewed critically. In particular, amorphous silicas, transitional aluminas, different silica-aluminas (silica-rich and alumina-rich) and protonic zeolites are taken into consideration. The nature of the acid sites, of the Lewis and of the Bronsted type, over these surfaces is discussed and rationalized, based on the fundamental chemistry and structural chemistry of silicon and aluminum compounds
Silica-alumina catalytic materials: A critical review
No full textThe applications of silica-aluminas in industrial catalytic processes are briefly reviewed. The literature data on the surface chemistry and structural chemistry of silica-aluminas are summarized. The Al-stuffed amorphous silica model is proposed for fully amorphous materials, and described. The reasons for the versatility of these materials as acid or acido-basic catalysts as well as tunable supports for metal and sulphide catalysts are also discussed
High-Quartz solid solution phases from xerogels with conposition 2MgO·2Al2O3·5SiO2 (cordierite) and Li2O·Al2O3·nSiO2 (n=2 to 4) (β-eucriptite) characterization by XRD, FTIR and surface measurements
No full text[100714.EG "beta". SAPERI.ALTRE_INFORMAZIONI:] Si riferisce sulla preparazione di xerogeli amorfi di composizioni pari a quella della cordierite (Mg2Al4Si5O18) e della eucriptite (Li2O·Al2O3·n SiO2 dove n= da2 a 4). Questi interessanti materiali, ad elevatissimo sviluppo superficiale (fino a 900 m²/g) furono caratterizzati mediante diffrazione RX, FTIR e misure di superficie. Caratteristica peculiare di questi materiali è anche lo distribuzione monomodale del diametro dei pori, centrata attorno a 40
Surface Characterization of Ba-beta-alumina
No full textThe surface properties of a Ba-beta-alumina sample (Ba : Al ratio 1 : 12) very stable to sintering (15 m(2)/g after calcination at 1670 K for 10 h) have been investigated by FT-IR spectroscopy of adsorbed probe molecules (water, pyridine and CO2) and by measuring the isoelectric point. The surface has a medium-strong basic character,justified by the preferential exposition of Ba-O-containing ''mirror planes'', parallel to 0001 planes, on the surface of plate-like particles. This behavior has been associated to the resistance to sintering, typical of this material, possibly originated by the difficulty of the very big Ba2+ ions to migrate into the spinel-type alumina blocks. This should imply the blocking of the crystal growth along the crystallographic c axis
Surface Dynamics of Adsorbed Species On Heterogeneous Oxidation Catalysts - Evidence From the Oxidation of C-4 and C5 Alkanes On Vanadyl Pyrophosphate
No full textMullite-type structures in the systems Al2O3-Me2O (Me=Na,K) and Al2O3-B2O3
No full textAmorphous solids belonging to the systems Al2O3-Me2O (Me = Na,K) and Al2O3-B2O3 were prepared by nitrate decomposition, introducing boron in the form of boric acid. Crystalline metastable solids with pseudotetragonal symmetry were obtained from thermal treatment at 850-degrees to 900-degrees-C for the compositions Al6MexO(9+0.5x) (x almost-equal-to 1; Me = Na, K) and Al(6-x)BxO9 (1 less-than-or-equal-to x less-than-or-equal-to 3). The resultant solids were stable only within a definite temperature range and transformed, with further treatment increases, into stable equilibrium phases. The structures of the metastable phases were examined by X-ray diffraction and Fourier transform infrared spectroscopy, and both analyses showed a mullite type of framework, inside of which the atomic coordinates were refined in the Pbam (no. 55) space group. The present results indicate that these silica-free mullite structures are stabilized by two different mechanisms: (1) interstitial occupation of bulky cations (Na+,K+) or (2) substitution of B for Al in some of the tetrahedral positions
Characterization of microporous amorphous Alumina-Boria
No full textAlumina-boria samples with B : (B + Al) 0-50 atom% have been prepared by an unconventional procedure consisting of the reductive decomposition of the mixture of nitrates in the presence of glycerol X-ray diffraction (XRD)-amorphous samples having surface areas of 380-470 m2 g-1 were obtained after calcination at 673 K. Their thermal stability has been studied by differential thermal analysis (DTA) and XRD analysis, showing that crystallization occurred in all cases above 1150 K. FTIR data show the predominance of octahedral Al3+ and triangular B3+ both in the bulk and on the surface. Correspondingly, all samples show lower catalytic activity in methanol dehydration compared with gamma-alumina. It was concluded that boron tends to hinder crystallization of amorphous alumina, to increase its surface area and to further decrease it surface acidity
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