Cooling specific surface area

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

Micro heat exchangers and also any kind of micro channel devices, heated or cooled externally, offer considerably improved heat transfer owing to their large internal specific surface areas. Hence they offer unique possibilities to steer oxidations to increased selectivity of the partial-oxidation products. 

At later times after the laser pulse heat conduction becomes the dominating heat loss mechanism (Will et al., 1995), and therefore, particles with different specific surface areas cool down differently, which is shown schematically in Figure 2 (Roth and Filippov, 1996 Will et al., 1995,1998). The local gas temperature adjacent to particles turns out to be the most critical parameter for the accuracy of size determination (Will et al., 1998), which however can be derived from the temperatures of the soot particles themselves (Schraml et al., 2000). 

To produce the substance having a high specific surface area So = 10 to 20 X lO cm /g suitable for detonators a shock crystallization was used. Pure PI T was dissolved in acetone (1 5) and the solution dropped into aqueous ethanol at ca. —30 0. The resulting crystals are much different in sizes (0.5 -16 tx and irregular shapes from spheres to needles. A modification of the method can be used by introducing a 10% solution of pure PE IN in aceione-ethanol into ethanol-acetone (1 2) cooled to —20 C. 

XRD patterns were recorded on a Philips PW 1710 instrument using Cu K i radiation (A. = 0.15418 A, 40 kW, 25 mA). N2 sorption experiments at 77 K were carried out with a Sorpty 1750 instrument (Carlo Erba Strumentazione) and specific surface areas (SS) calculated using the BET method (Table 1). Absorption/transmission IR spectra were run at RT on a Perkin-Elmer FT-IR 1760-X spectrophotometer equipped with a Hg-Cd-Te cryodetector. For IR analysis powders were pelletted in self-supporting discs (10-15 mg cm ), activated in vacuo at increasing temperature up to 823 K, heated in dry O2 at the same temperature and cooled down in oxygen or subsequently reduced in H2 at 623 K and outgassed at the same temperature. CO and NO (Matheson C.P.) were used as probes. 

Specific surface area was measured by the N2-BET (Brunner-Emmett-Teller) method on a Micromeritics ASAP2400 automatie adsorption meter. Samples were pretreated in a muffle furnace at 573 K for 1 h. The sample in system was outgassed at 523 K for 4 h, then, cooled down in liquid nitrogen. Nitrogen adsorption isotherms were recorded for BET calculation. 

In another research report by Yoshikaw and his group, mesoporous anatase Ti02 nanopowder was synthesized at 130°C for 12 h (see Table 2) [106]. Titanium (IV) butoxide (Ci6H3604Ti) was mixed in a 1 1 molar ratio with acetylacetone (CH3-CO-CH2-CO-CH3) to slowdown the hydrolysis and the condensation reactions. 40 ruL of distilled water was added in the solution and stirred at room temperature for 5 min. The solution was put into a Teflon-hned stainless steel autoclave while stirring and heated at 130°C for 12 h. The final product was naturally cooled to room temperature and washed with 2-propanol and distilled water. This was then dried followed by drying at 100°C for 12 h. The synthesized sample had a narrow pore size distribution with an average pore diameter of about 3 nm. The specific surface area of the sample was about 193 m /g. Mesoporous anatase TiOz nanopowders showed higher photocatalytic activity than the nanorods, nanofibers and commercial 7702 nanoparticles. 

Due to the large specific surface area, nanopowders are also more reactive than micrometer-sized materials and some of the nano-sized nitrides and carbides can be completely oxidized if directly exposed to air. For example, M02N with specific surface area of 120 m g can be rapidly oxidized upon exposure to air (powder starts to glow red), while low specific surface area material (< 1 m g ) will show no reaction to the same treatment. In order to prevent rapid oxidation, nanoparticulate materials can be slowly passivated in inert gas (N2, He) containing low amounts of oxygen (0.5% to 1%). In such conditions only a thin layer of oxide is formed and nitrogen cools down the system in order to slow down the reaction. It is known that... 

Fly ash. Fly ash (also called pulverised fuel ash, PFA) is a by-product of the combustion of coal powder in thermoelectric power plants. It consists of very fine and spherical particles (dimensions from 1 to 100 pm and specific surface area of 300 to 600 m /kg) that are collected from exhaust gases with electrostatic or mechanical filters. Its composition depends on the type of coal it derives from the most common PFA is mainly siliceous. Because of the high temperature at which it is formed, it subsequently undergoes rapid cooling so that its structure is mainly amorphous (glassy) and thus reactive. 

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