Bhattacharyya K, Varma S, Tripathi A K, Bharadwaj S R, Tyagi A K
Chemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India.
J Phys Chem B. 2009 Apr 30;113(17):5917-28. doi: 10.1021/jp8103529.
Vanadium-doped titania is found to be a better photocatalyst for gas phase photo-oxidation of ethylene than nano titania. In situ FTIR studies were undertaken to elucidate the mechanistic pathway for ethylene oxidation on these two catalyst surfaces. Vanadium doping leads to formation of more chemisorbed hydroxyl species, which makes it a better photocatalyst. The labile hydroxyls which were responsible for the reduction of V(5+) to V(4+) during the process of calcination were also ascertained. The ethylene decomposition occurs via formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates/formates, and then to CO(2). The enolates were more stabilized on the TiO(2) surface, leading to formation of formates along with the acetates. On vanadium-doped TiO(2), acetaldehyde was more stabilized than its enol tautomer, leading to the formation of labile acetic acid and acetates. The formation of the labile acetic acid, adsorbed acetates, and the adsorbed acetate -M salts led to easier oxidation of them to provide higher yield of CO(2). The higher positive charge density over Ti in Ti(0.95)V(0.05)O(2) with respect to nano TiO(2) makes the acetate (stronger nucleophile) a more stable intermediate on it.
钒掺杂二氧化钛被发现是一种比纳米二氧化钛更好的用于乙烯气相光氧化的光催化剂。进行了原位傅里叶变换红外光谱研究以阐明乙烯在这两种催化剂表面上氧化的机理途径。钒掺杂导致形成更多的化学吸附羟基物种，这使其成为更好的光催化剂。还确定了在煅烧过程中负责将V(5+)还原为V(4+)的不稳定羟基。乙烯分解通过形成乙氧基发生，乙氧基转化为乙醛或烯醇盐，随后转化为乙酸盐/甲酸盐，然后转化为CO(2)。烯醇盐在TiO(2)表面上更稳定，导致与乙酸盐一起形成甲酸盐。在钒掺杂的TiO(2)上，乙醛比其烯醇互变异构体更稳定，导致形成不稳定的乙酸和乙酸盐。不稳定的乙酸、吸附的乙酸盐以及吸附的乙酸盐-M盐的形成导致它们更容易被氧化，从而提供更高的CO(2)产率。相对于纳米TiO(2)，Ti(0.95)V(0.05)O(2)中Ti上更高的正电荷密度使乙酸盐（更强的亲核试剂）在其上成为更稳定的中间体。
