Acrolein coupling on reduced TiO 2 (1 1 0): The effect of surface oxidation and the role of subsurface defects Lauren Benz a , Jan Haubrich a , Ryan G. Quiller b , Cynthia M. Friend a,b, * a Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, Massachusetts 02138, United States b School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States article info Article history: Received 30 November 2008 Accepted for publication 10 February 2009 Available online 20 February 2009 Keywords: Temperature programmed reaction Titanium dioxide Acrolein Coupling reaction Interstitials Point defects Subsurface defects Oxygen vacancies abstract Reactions of acrolein, water, and oxygen with the vacuum-reduced surface of TiO 2 (1 1 0) are reported in a temperature programmed reaction study of the interaction of an aldehydic pollutant with a reducible metal oxide. A total of 25% of the acrolein that binds to the surface is converted to products. Notably, car- bon–carbon coupling occurs with 86% selectivity for formation of C 6 products: C 6 H 8 , identified as 1,3- cyclohexadiene, in a peak at 500 K and benzene immediately thereafter at 530 K. Acrolein is evolved from the surface in three peaks: a peak independent of coverage at 495 K, attributed to decomposition of an intermediate that is partly converted to C 6 H 8 ; a coverage-dependent peak that shifts from 370 K (low coverage) to 260 K (high coverage), which is attributed to adsorption at 5-fold coordinated Ti sites; and a multilayer state at 160 K. Water and acrolein compete for 5-fold coordinated titanium sites when dosed sequentially. The addition of water also opens a new reaction pathway, leading to the hydrogena- tion of acrolein to form propanal. Water has no effect on the yield of 1,3-cyclohexadiene. Exposure of the surface to oxygen prior to acrolein dosing quenches the evolution of acrolein at 495 K and concurrently eliminates the coupling. From these results, we propose that reduced subsurface defects such as titanium ion interstitials play a role in the reactions observed here. The notion that subsurface defects may con- tribute to the reactivity of organic molecules over reducible oxide substrates may prove to be general. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The chemistry of organic compounds on transition metal oxide surfaces, especially TiO 2 , is relevant to both heterogeneous cataly- sis and atmospheric chemistry. For example, TiO 2 is a component in mixed vanadia/titania catalysts used for selective oxidation reactions [1]. Titania is also commonly used as a support for Au- based catalysts used in the partial oxidation of olefins, for example [2]. Since products can diffuse or ‘‘spillover” onto the oxide sup- port, it is important to understand potential secondary chemistry. The interaction of natural and engineered oxide surfaces with or- ganic molecules is also relevant to atmospheric chemistry because volatile organic contaminants in the air can adsorb and react on the surfaces of metal oxide aerosol particles [3]. Organic–metal oxide interactions may specifically play a role in the formation of second- ary organic aerosol (SOA), which can constitute up to 80% of the or- ganic aerosol present in urban areas [4–6]. SOAs form when oxidation products form in the atmosphere that have lower volatil- ity than their precursors, so they agglomerate or condense with existing particles to form new aerosols that pose a significant health hazard to humans [7]. In this work, we study the chemistry of acrolein, an atmospheric contaminant and unsaturated aldehyde, on TiO 2 (1 1 0). Acrolein, typically employed in the synthesis of acrylic acid [8], is also a haz- ardous by-product of burning tobacco and gasoline [9]. We also investigate the effects of water and O 2 on the chemistry of acrolein on TiO 2 (1 1 0) because these species are present in the atmosphere and may, thus, have a significant impact on the remediation of pol- lutants. Water and O 2 also may alter selectivity and reactivity in catalytic processes. Titania was selected for these studies because it is used in sev- eral applications in addition to heterogeneous catalysis as men- tioned above, e.g. in solar cells, as an air purifier, in self-cleaning coatings on glass, in wastewater treatment, and as a brightener in paints [1,10–12]. The rutile TiO 2 (1 1 0) surface was specifically studied because it is well characterized and it is the most stable surface [10,11]. The interaction of water with vacuum-annealed TiO 2 (1 1 0) has been extensively studied, providing a basis for understanding how water affects acrolein chemistry [11,13,14]. We are particularly interested in the possible role of defects in determining the chemical behavior of acrolein. Vacuum-annealed TiO 2 is known to contain defects at and near the surface. In fact, Ti interstitials in the near surface region have been reported to re- 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.02.016 * Corresponding author. Address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, Massachusetts 02138, United States. Tel.: +1 617 495 4052; fax: +1 617 496 8410. E-mail address: cfriend@deas.harvard.edu (C.M. Friend). Surface Science 603 (2009) 1010–1017 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc