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leeying至尊木虫 (职业作家)
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[交流]
问关于活性炭的一个问题
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大家都知道很多催化剂载体都有等电点，这也是我们制备负载型催化剂所要考虑的，那么活性炭有没有等电点那，有是多少？没有，有是为什么那？ [ Last edited by daiqiguang on 2007-5-11 at 09:30 ] |
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penfee
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6楼2006-08-10 19:10:07
2楼2006-08-09 16:37:28
leeying
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关于等电点
★
paracyclophane(金币+1):3x!
paracyclophane(金币+1):3x!
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关于等电点 Uniform filling of the pores can be achieved using the incipient wetness technique, using concentrated solutions at pH values that minimize adsorption. The pH is adjusted to take account of the isoelectric point of the support (Table 7). To overcome the problem of aggregation during the drying step and nonuniformity of the adsorption using the incipient wetness technique, greater quantities of liquid can be used and, in the extreme, this involves the use of a large excess of dilute solution, which has been used by Wang and Hall (21) to prepare monolayer molybdenum/alumina catalysts. Table 7. Isoelectric Points of Various Oxides Oxide IEPS Adsorption Sb2O5 <0.4 Cations WO3 hydrous <0.5 SiO2 1.0–2.0 MnO2 3.9–4.5 Cations or anions SnO2 ~5.5 TiO2 (anatase, rutile) ~6 -Fe2O3 6.5–6.9 ZrO2 hydrous ~6.7 CeO2 hydrous ~6.75 Cr2O3 hydrous 6.5–7.5 , -Al2O3 7.0–9.0 -Fe2O3 ~8.9 Anions ZnO 8.4–9.0 La2O3 hydrous 8.7–9.7 MgO ~10.4 12.1–12.7 Controlled adsorption from solution uses highly diluted solutions of the additive and the pH is adjusted to control the rate of adsorption. The method has been optimized for MoO3/ -Al2O3 and WO3/ -Al2O3 catalysts (20-22). The pH is used to control two factors: (1) the surface charge of the support and (2) the size of oxyanions of molybdenum and tungsten. It must be recognized that the surface of the oxide support changes its polarization according to the value of the pH of the solution and the isoelectronic point, which is also known as the zero charge point (Table 7) (23). At a pH below the isoelectronic point, the surface is positively charged and, therefore, attracts anions while, at a pH above the isoelectronic point, the surface becomes negatively charged and, hence, attracts cations. An example of the control of surface charge affecting adsorption is shown in Fig. 4 for the adsorption of cobalt and tungsten species onto -Al2O3. The isoelectric point of alumina is ca pH 8–9 and the lack of adsorption of cobalt at pH <6 is due to the inability of the positively charged cobalt cations to adsorb in the positively charged alumina surfaces. Additionally, at pH >8, cobalt precipitates from the solution as hydroxides or basic carbonates. Hence, for alumina, there is a very narrow pH window that can be used for the controlled adsorption of positively charged cations (22, 24). The reverse situation is observed for the tungsten, and also molybdenum, species because of the negative charge of the tungstate and polytungstate anions, which are readily adsorbed onto the positively charged alumina surface below pH 8. [ Last edited by leeying on 2006-8-9 at 16:51 ] |
3楼2006-08-09 16:50:25
leeying
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4楼2006-08-10 08:26:16
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