J. Phys. Chem. C 2010, 114, 17017–17022
17017
Formaldehyde Polymerization on (WO3)3/TiO2(110) Model Catalyst† Jooho Kim,‡ Bruce D. Kay,* and Zdenek Dohna´lek* Chemical and Materials Sciences DiVision, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352 ReceiVed: March 25, 2010; ReVised Manuscript ReceiVed: May 13, 2010
Polymerization of formaldehyde, H2CO, was studied under ultrahigh vacuum conditions on a model catalyst consisting of monodispersed (WO3)3 clusters anchored on TiO2(110). Formaldehyde oligomers, (H2CO)n, desorbing from the polymer that formed on the catalyst surface are detected between 250 and 325 K in temperature-programmed desorption experiments. At least two monolayers (ML) of H2CO are required on the surface to observe (H2CO)n, desorption and the amount saturates for H2CO coverages in excess of ∼40 ML. The presence of H2CO multilayers is required for the polymerization to take place indicating that it had to occur below 100 K. The saturation amount increases with increasing coverage of (WO3)3 clusters with the highest amount of ∼13 ML observed on 1.2 (WO3)3/nm2. No (H2CO)n desorption was observed on the bare TiO2(110) surface. 1. Introduction Studies of catalytic processes on well-defined systems under UHV conditions can provide an unparalleled understanding of the underlying reaction mechanisms. While the catalytic properties of single crystalline surfaces have been studied in great detail, the chemistry of well-defined supported nanoclusters remains largely unexplored.1-4 This is primarily due to the difficulties in preparation of such systems. In our recent studies, we have developed a procedure to carry out model reactivity studies on monodispersed cyclic (WO3)3 clusters supported on TiO2(110).5-8 Our studies of the partial oxidation of alcohols have revealed the dominant role Lewis acid/base chemistry of WdO groups plays in the competitive reactions leading to alkenes, adehydes, ketons, and ethers.7,8 In this study, we focus on the interactions of formaldehyde, H2CO, one of the simplest products of methanol partial oxidation. Formaldehyde is one of the most reactive organic compounds, and as such, it is a very important reagent in numerous catalytic reactions with other organics such as carboxylic acids and longer chain aldehydes.9 In addition, formaldehyde is important in a number of polymerization reactions with urea, melamine, and phenol.9 Hence the reactivity of formaldehyde has been extensively studied on many metal and metal oxide surfaces.10-30 On a number of surfaces, formaldehyde undergoes reactions to form other products such as methanol, formic acid, CO, CO2, H2, and H2O.10-13,15,19,22,26,30 In many cases, when bound molecularly, it undergoes polymerization,15-17,20,21,23-25,28,29 which is the main topic of this study. We show that on the (WO3)3/TiO2(110) model catalyst H2CO polymerizes at very low temperatures (
