Radical versus Nucleophilic Mechanism of Formaldehyde Polymerization Catalyzed by (WO3)3 Clusters on Reduced or Stoichiometric TiO2(110)

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• 作者：Cristiana Di Valentin ; Massimo Rosa ; Gianfranco Pacchioni
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：August 29, 2012
• 年：2012
• 卷：134
• 期：34
• 页码：14086-14098
• 全文大小：699K
• 年卷期：v.134,no.34(August 29, 2012)
• ISSN：1520-5126

文摘

(WO₃)₃ clusters deposited on the (110) rutile TiO₂ surface are excellent catalysts for the formaldehyde (CH₂O) polymerization reaction (J. Phys. Chem. C2010, 114, 17017). The present B3LYP study unravels the possible paths of this catalyzed reaction. According to the stoichiometry of the r-TiO₂ surface, the (WO₃)₃ clusters can be neutral, singly charged, or doubly charged. We find that only neutral (WO₃)₃ and anionic (WO₃)₃^{鈥?/sup> clusters are reactive toward CH₂O molecules. In both cases it is possible to determine more than one mechanism on the basis of a nucleophilic attack of the formaldehyde O atom to the W ions of the cluster. The reaction proceeds through successive attacks of other CH₂O molecules and the formation of acetal and polyacetal intermediates, which inhibits the chain propagation. Only in the case of the anionic (WO₃)₃鈥?/sup> catalyst is a totally different reaction path possible at low temperatures. This path involves the formation of radical species where the unpaired electron is localized on the organic moiety bound to the cluster. The polymer chain propagation follows a radical mechanism with low activation barriers. Thus, a cluster鈥檚 electron charging speeds up the formaldehyde polymerization at low temperatures. On the basis of these unexpected results, we conclude that electron-rich supports and low working temperatures are the keys to kinetic control of the reaction favoring a fast radical chain propagation mechanism.}