Showing papers on "BET theory published in 2000"

Catalytic combustion of volatile organic compounds on gold/cerium oxide catalysts

Abstract: Catalytic combustion of some representative volatile organic compounds (VOCs) (2-propanol, methanol and toluene) was investigated on gold/cerium oxide catalysts prepared by coprecipitation (CP) and deposition–precipitation (DP). The presence of gold has been found to enhance the activity of cerium oxide towards the oxidation of the selected volatile organic compounds, the extent of this effect depending on the preparation method of gold catalysts. On the basis of characterisation data (H2-TPR, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), BET surface area) it has been suggested that the catalytic activity of the Au/CeO2 system is related to the capacity of gold nanoparticles to weaken the surface CeO bonds adjacent to Au atoms, thus enhancing the reactivity of the CeO2 surface capping oxygen which is involved in the volatile organic compounds oxidation through a Mars-van Krevelen reaction mechanism.

Tungsten carbide (WC) synthesis from novel precursors

Abstract: This paper deals with the production and properties of tungsten carbide (WC) powders from novel carbon coated precursors. The process has two steps in which the oxide powders were first coated with carbon by cracking of a hydrocarbon gas, propylene (C 3 H 6 ), secondly mixed with a substantial amount of carbon black, and finally treated at temperatures in the range of 600-1400°C for 2 h in flowing Ar or 10% H 2 -Ar atmosphere to synthesize WC. The produced powders were characterized using TEM, BET surface area analyzer, X-ray diffraction and chemical analysis (oxygen and carbon). The results obtained for various types of precursors treated in different atmospheres indicated that the coated precursors produced high quality powders. Single phase, submicron WC powders were synthesized at temperatures as low as 1100°C. WC powders produced at 1400°C for 2 h in flowing 10%H 2 -Ar gas mixture were submicron (3–5 m 2 /g), single phase, and had low oxygen content (0.2–0.5 wt%). The sintering tests demonstrated that these powders can be densified to near theoretical density using 20 wt% Co binder at 1500°C for 2 h in flowing 10%H 2 -Ar atmosphere.

Hydrothermal synthesis of nanosized titania powders : Influence of peptization and peptizing agents on the crystalline phases and phase transitions

Abstract: Nanocrystalline TiO 2 particles were prepared by hydrothermally treating three different titanium alkoxide species, which were derived from (i) the hydrolysis of Ti(OPr) 4 and Ti(OBu) 4 4 (PT and BT, respectively) in the presence of HNO 3 , tetraethylammonium hydroxide (TENOH), or NH 3 .H 2 O; (ii) water-washed precipitates; or (iii) sols that were obtained from the precipitates by peptizing with HNO 3 or TENOH. X-ray diffractometry, Brunauer-Emmitt-Teller (BET) surface area analysis, differential thermal analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy were used to characterize the powders. The results showed that peptization of the precipitate favored formation of the rutile phase and highly crystalline anatase under hydrothermal treatment. Particles of 100% rutile phase with high specific surface areas were obtained at a relatively low temperatures: room temperature, for HNO 3 -peptized ([H + ]/[Ti] = 4) PT sample (BET surface area of 96 m 2 /g); 200°C, for the HNO 3 -peptized ([H + ]/[Ti] = 1) PT sample (BET surface area of 49 m 2 /g); and 240°C for the HNO 3 -peptized ([H + ]/[Ti] = 1) BT sample (BET surface area of 42 m 2 /g). Particles that contained highly crystalline anatase, together with a small portion of rutile, were formed in both TENOH-peptized samples. However, rutile was difficult to form, both in the water-washed precipitates and in the species that was hydrolyzed in the presence of HNO 3 , TENOH, and NH 3 .H 2 O. The peptization effect of the H + ion mainly lies in the breakage of oxolation by attacking the electrophilic O atoms among Ti atoms, thus creating conditions for the formation of rutile or anatase nuclei, after structural rearrangement.

Reduction of High Surface Area CeO2−ZrO2 Mixed Oxides

Abstract: The objective of this study was to examine the mechanism of the reduction by hydrogen of ceria−zirconia (CZ) mixed oxides having a high BET surface area (100 m2 g-1). Three methods were used in parallel to assess the Ce3+ content, the surface and bulk oxygen vacancy concentrations, and the resulting oxygen storage capacity (OSC): temperature programmed reduction, Fourier transform infrared (FT-IR) measurements of methanol adsorbed on the reduced surfaces, and a Faraday microbalance to determine the magnetic susceptibility of the reduced oxides. The three methods conclude that the introduction of zirconium into the ceria lattice has a positive influence on the OSC. Compared to pure ceria, the CZ mixed oxides exhibit better redox properties, with a lower temperature of initial reduction and a higher reduction percentage for all compositions. The reducibility increases with the zirconium content, however the OSC per gram of solid is practically the same for Zr contents between 20% and 50%. The reduction pro...

Liquid-Phase Adsorption of Phenol onto Activated Carbons Prepared with Different Activation Levels.

Abstract: The influence of the pore size distribution of activated carbon on the adsorption of phenol from aqueous solutions was explored. Activated carbons with different porous structures were prepared by gasifying a bituminous coal char to different extents of burn-off. The results of adsorption experiments show that the phenol capacity of these carbons does not proportionally increase with their BET surface area. This reflects the heterogeneity of the carbon surface for adsorption. The pore size distributions of these carbons, determined according to the Dubinin-Stoeckli equation, were found to vary with the burn-off level. By incorporating the distribution with the Dubinin-Radushkevich equation using an inverse proportionality between the micropore size and the adsorption energy, the isotherms for the adsorption of phenol onto these carbons can be well predicted. The present study has demonstrated that the heterogeneity of carbon surface for the phenol adsorption can be attributed to the different energies required for adsorption in different-size micropores. Copyright 2000 Academic Press.

Effect of hydrolysis conditions on morphology and crystallization of nanosized TiO2 powder

Abstract: Nanosized titania powders were prepared by controlling the hydrolysis of TiCl4 in aqueous solution. The powders were characterised by TEM, HREM, XRD, ED, and BET techniques. In the presence of a small amount sulphate ions, when TiCl4 solution hydrolysed at 70°C, the obtained powder was pure anatase and its primary particle size was 3.5 nm, which is finer than that of alkoxide-derived powders, moreover, its anatase-rutile transformation was retarded. However, at the same temperature, in the absence of SO42− the synthetic powder was a mixture of the anatase and rutile, the primary particle size in the rutile phase was 4.3 nm. When TiCl4 solution hydrolysed at 20°C, the prepared TiO2 powder was amorphous and its BET surface area was as high as 501 m2/g. The results of UV–Vis absorption spectra indicate that the presence of sulphate ions accelerated the growth of TiO2 clusters to anatase.

A monolayer dispersion study of titania-supported copper oxide

Abstract: A monolayer dispersion of copper oxide on the surface of anatase and its effect on the anatase properties have been studied by BET surface area measurements, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and X-ray diffraction (XRD). XPS results give a monolayer dispersion threshold of 5.1 mg CuO (g TiO2)−1. Strong interactions between CuO and anatase can be seen from the TPR results. When the loading of CuO is below its monolayer dispersion threshold, the onset temperature of the anatase–rutile transformation and the transition speed strongly depend on the dispersion of CuO. Crystalline CuO appears when the CuO loading is higher than the monolayer dispersion threshold, but it has little effect on the phase transformation of the samples.

Characterization of rice husk ash-supported nickel catalysts prepared by ion exchange

Abstract: The rice husk ash-supported nickel catalysts prepared by ion exchange technique and the support itself have been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). In addition to BET surface area, total pore volume and average pore diameter were also measured. The metal surface area and the crystal size of nickel were measured by the selective chemisorption of hydrogen. The effect of calcination temperature on the catalyst reduction was investigated by the temperature-programmed reduction (TPR) technique. The results show that nickel silicate with a layer structure formed after drying step. The BET surface area increases with the nickel loading of the catalyst. The nickel surface area increases with nickel loading up to 16.7 wt.% Ni and then decreases with further increase in nickel loading. The dispersion of nickel gradually decreases with nickel loading. Furthermore, the mean size of nickel crystallites increases with nickel loading. The reduction temperature increases with the increase of calcination temperature. Reduction of NiO from the thermal decomposition of the layered nickel silicates is found to be unusually difficult.

Effects of Added Organosilanes on the Formation and Adsorption Properties of Silicates Surface-Imprinted with an Organophosphonate

Abstract: Silica particles surface-imprinted with the soman hydrolysis product pinacolyl methylphosphonate (PMP) demonstrated binding selectivity for the hydrolysis product over other phosphonate mono- and diesters. Particle surfaces were imprinted during particle formation by adding pinacolyl methylphosphonate to the microemulsion formed from polyoxyethylene(5) nonylphenyl ether (NP-5), cyclohexane, ammoniated ethanol, and water. A number of trialkoxysilanes with terminal functional groups such as primary amine, quaternary amine, dihydroimidazole, and ethylpyridine were mixed with tetraethoxysilane (TEOS) during particle formation to enhance the binding and selectivity of the resulting surface-imprinted particles. The BET surface area of the silicates was dependent on the specific organosilane added during particle formation. The addition of primary amine- and dihydroimidazole-terminated organosilanes produced high-density, low-pore-volumes colloids while the addition of 2-ethylpyridine- and quatermnary amine-term...

Effect of ceria content on the sintering of ZrO2 based ceramics synthesized from a polymeric precursor

Abstract: Zirconia-ceria powders with ceria concentration varying from 0 to 12 mol% were synthesized using a polymeric precursor route based on the Pechini process. Powder characteristics were evaluated with regard to the crystallite size, BET surface area, phase distribution, nitrogen adsorption/desorption behavior, and agglomeration state. Sintering was studied considering the shrinkage rate, densification, grain size, and phase evolution. It was demonstrated that the synthesis method is effective to prepare nanosized powders of tetragonal zirconia single-phase. Sinterability mainly depended on the agglomeration state of powders and the monoclinic phase content. Fully tetragonal zirconia ceramic, with grain size of 2.4 μm, was obtained after addition of at least 9 mol% ceria and sintering at 1500°C for 4 h.

Effects of alumina incorporation in coprecipitated NiO–Al2O3 catalysts

Abstract: The effect of Al2O3 levels on the properties of NiO in coprecipitated NiO–Al2O3 samples were investigated, using samples with up to 60.7 wt.% Al2O3 that had been calcined in the range 300–700°C. Characterization techniques included BET surface area of fresh and reduced catalysts, X-ray diffraction analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H2. Only NiO was detected in samples with up to 4.1 wt.% Al2O3 for all sample calcination temperatures. Surface areas were similar for all fresh samples but decreased rapidly after calcination at high temperatures. The surface area loss was less for the higher Al2O3-containing samples. Nickel oxide crystallite sizes increased at higher calcination temperatures, but remained approximately the same for each Al2O3 level. The NiO was nonstoichiometric (NiO1+x), with x decreasing at higher calcination temperatures and increasing with small amounts of added Al2O3 through a maximum at about 3 wt.% Al2O3. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al2O3 addition. At higher levels of Al2O3 (13.6 wt.% and above), surface areas increased with higher Al2O3 loadings, but NiO crystallite sizes remained approximately the same, independent of both Al2O3 content and calcination temperature. X-ray diffraction patterns were very diffuse, and it was not possible to rule out the presence of pseudo-spinel combinations of NiO and Al2O3. Reducibility was more difficult than with low Al2O3 levels, and nonstoichiometry was low and independent of Al2O3 content. Reducibilities of all samples calcined at 300°C correlated well with the final BET surface area of the reduced samples, indicating that more dispersed NiO crystallites are more difficult to reduce, a conclusion that supports a model for reduction proposed previously.