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Surface Chemistry of Titania (Anatase) and Titania-supported Catalysts

机译:二氧化钛(锐钛矿)和二氧化钛负载的催化剂的表面化学

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The anatase surface is highly heterogeneous. Three kinds of Lewis acid sites (differently coordinated Ti~(4+) ions) and at least two kinds of hydroxy groups are present on the surface. This high heterogeneity is due to the exposure of different planes on the real crystallites. The local arrangement of the anionic and cationic vacancies on the anatase surface determines the lack of centres for dissociative adsorption of water and alcohols. This particular anatase property is relevant to (ⅰ) the low hydroxyl coverage of anatase, and (ⅱ) its (photo)catalytic behaviour. Adsorbed anions strongly affect the surface properties of anatase. Surface sulfates increase the Lewis acidity, whereas phosphates block the Lewis acid sites. Both anions induce a Br0nsted acidity. The design of titania-supported oxide catalysts is very important since a bare titania surface often leads to parasitic reactions and decreases the selectivity of the catalysts. Titania-supported metals reduced above 300℃ are characterized by a strong metal-support interaction (SMSI). This effect strongly decreases the chemisorption capacity of the supported metals and is due to their coverage by Ti-suboxide moieties. The TOF of some metals in CO hydrogenation is enhanced when supported on titania. This effect is also due to covering of the metal particles with a Ti-containing phase but the catalysts are not in the SMSI state during the reaction.
机译:锐钛矿表面高度异质。表面上存在三种路易斯酸位点(不同配位的Ti〜(4+)离子)和至少两种羟基。这种高的异质性是由于真实微晶上不同平面的暴露所致。锐钛矿表面上阴离子和阳离子空位的局部排列决定了缺少水和醇的解离吸附中心。该特定的锐钛矿性质与(ⅰ)锐钛矿的低羟基覆盖度和(ⅱ)其(光)催化行为有关。吸附的阴离子强烈影响锐钛矿的表面性质。表面硫酸盐增加了路易斯酸度,而磷酸盐则阻止了路易斯酸位。两种阴离子均产生布朗斯台德酸度。二氧化钛负载的氧化物催化剂的设计非常重要,因为二氧化钛裸露的表面通常会导致寄生反应并降低催化剂的选择性。还原温度高于300℃的二氧化钛负载金属的特征是强烈的金属负载相互作用(SMSI)。这种作用强烈降低了负载金属的化学吸附能力,这是由于它们被Ti-次氧化物部分覆盖了。当负载在二氧化钛上时,某些金属在CO加氢中的TOF会增加。该效果也归因于用含Ti的相覆盖金属颗粒,但是催化剂在反应过程中不处于SMSI状态。

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