J. Phys. Chem. C 2007, 111, 3021-3027
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Direct Visualization of 2-Butanol Adsorption and Dissociation on TiO2(110) Zhenrong Zhang,† Oleksandr Bondarchuk,‡ Bruce D. Kay,† J. M. White,*,†,‡ and Zdenek Dohna´ lek*,† Pacific Northwest National Laboratory, Fundamental Sciences Directorate and Institute for Interfacial Catalysis, Richland, Washington 99352, and Department of Chemistry and Biochemistry, Center for Materials Chemistry, UniVersity of Texas at Austin, Texas 78712 ReceiVed: NoVember 10, 2006; In Final Form: December 21, 2006
High-resolution scanning tunneling microscopy (STM) images of identical regions of a TiO2(110) surface were gathered before and after controlled doses of 2-butanol (CH3CH2CH(OH)CH3) at ambient temperature (∼300 K). When dosing is initiated, 2-butanol preferentially adsorbs at bridge-bonded oxygen vacancy (BBOV) sites and dissociates via O-H, not C-O, bond scission to form paired 2-butoxy and hydroxyl species evidenced by two local maxima in STM line profiles. As the dose increases, but before all the BBOV’s are occupied, there is direct STM evidence of hydrogen hopping of the hydroxyl to an adjacent oxygen anion row. This process is facilitated by species bound to 5-coordinate Ti4+ rows, presumably undissociated 2-butanol, that hop slowly on the STM imaging time scale. The carbon backbones of these mobile species are centered over the Ti4+ rows with a preference for lying parallel to these rows. On the other hand, the carbon backbones of the 2-butoxy species that fill BBOV’s are centered over the O2- rows and prefer an orientation perpendicular to these rows. As the oxygen vacancy concentration increases from 0.4 to 11% and 2-butanol is dosed, the ratio of mobile Ti4+-bound 2-butanol to stationary BBOV-bound 2-butoxy species decreases for doses that do not fill all the BBOV’s.
I. Introduction Catalysis on TiO2 has attracted widespread interest due to a variety of applications, such as wastewater treatment, air purification, self-cleaning, and solar cells.1-3 Surface properties play a significant role in these applications, and the decomposition of alcohols has been used to ascertain the reactive sites for heterogeneously catalyzed reactions on titania and other oxides.4-8 The basic chemistry undergirding the thermal and photocatalytic oxidation of alcohols has been extensively studied on TiO2 surfaces as a prototype for the catalytic oxidation of organic species that model environmental contaminants.6-16 Studies of primary, secondary, and tertiary aliphatic alcohols on TiO2, both rutile and anatase single-crystal surfaces as well as on polycrystalline samples, show that both molecular and dissociative adsorption occur, the latter forming alkoxide and hydroxyl groups.6-16 In CH3OH thermal chemistry on TiO2, there is evidence for a variety of reaction products (methanol, methane, dimethyl ether, formaldehyde, and CO) on the (001) surfaces of rutile TiO2,9 but only CH3OH desorbs from the (110) surface, even though bridge-bonded oxygen vacancy (BBOV) defect sites are present.16 At the atomic level, dissociation on TiO2(110) has been attributed to reaction at BBOV sites.12,14,15 For longer-chain alcohols, for example, ethanol, other reaction paths are available due to the presence of C-C bonds that can be altered.7,10,12,14 For example, ethoxy derived from ethanol, is removed by two main pathways on the (110) surface.12,14 In one pathway, similar to methoxy, alkoxy recombines with hydrogen to form and desorb the parent alcohol below 400 K. The second, unlike methoxy, involves forming and desorbing * Corresponding authors. E-mails:
[email protected],
[email protected]. † Pacific Northwest National Laboratory, Richland, Washington. ‡ University of Texas, Austin, Texas.
an alkene, for example, ethylene, at temperatures between 500 and 600 K. Other minor reaction paths are also observed; for example, for ethoxy, dehydrogenation to produce acetaldehyde has been observed on rutile and anatase.7,10 The secondary and tertiary aliphatic alcohols also undergo both dehydration and dehydrogenation.7,8,11,13,14 The surface structure, defect concentration, and the nature of the reactive intermediates all influence the reactivity of oxide surfaces.2 A key issue in developing catalytic materials and systems is observing and understanding site-specific surface processes of adsorbed molecules and reaction intermediates on an atomic scale.17 Because there are various kinds and extents of heterogeneity, ensemble average studies are not able to clarify many aspects of oxygenate chemistry, for example, alcohols, on oxides, for example, TiO2. One very productive way forward, makes use of atomically resolved scanning tunneling microscopy (STM) images of identical surface regions before and after dosing with a small molecule. Comparing images of the same area before and after dosing (and as a function of time after dosing) gives unprecedented detail for chemical processes that occur slowly on the time scale of STM image acquisition. In recent previously published work,15 we took this approach in a study of the smallest alcohol, CH3OH, on TiO2(110)-(1 × 1) at 300 K. The BBOV concentration was fixed at 10%. For low exposures, new and uniform bright features appear at some, but not all, the BBOV’s without otherwise altering the images recorded before dosing. These are ascribed to alkoxy-hydroxy pairs formed by dissociation at BBOV’s. At higher exposures, methanol adsorbed on Ti4+ rows is inferred, but not directly imaged. Under these conditions, there are two distinguishable features, one brighter than the other. The brighter ones are located at BBOV positions and the less bright ones on a bridgebonded oxygen (BBO) row but not at a BBOV position. While
10.1021/jp067461c CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007
3022 J. Phys. Chem. C, Vol. 111, No. 7, 2007
Zhang et al.
Figure 1. STM images of the same area before and after adsorption of 2-butanol on reduced TiO2(110) at 300 K (bias voltage: +1.1 ( 0.3 V, tunneling current: 0.07 nA): (a) bare surface; (b) after 50-s exposure to 2-butanol; (c) after 80-s exposure to 2-butanol; (d) after 110-s exposure to 2-butanol. The assignment of different features appears in the legend.
the former are stationary from image to image, the latter move. Furthermore, the former are not altered, but the latter can be removed by scanning with a positive sample bias of +3 V. From this evidence, we assigned the brighter features as methoxy and the less-bright features as hydroxyl. As in other literature,15 we proposed that mobile CH3OH adsorbed on Ti4+ rows assists hydrogen hopping by transiently acquiring the hydroxyl hydrogen to form CH3OH2+ as it passes an alkoxy-hydroxy pair. The CH3OH2+ deposits the hydrogen on the same or a nearby BBO, and the latter forms a hydroxyl separated from the alkoxy group and resolvable in STM images. Extending this previous work on CH3OH, we report here analogous imaging using a small asymmetric alcohol, 2-butanol, as the adsorbate. Generally, the chemical phenomena observed at room temperature are the same, that is, RO-H dissociates at vacancies to form alkoxide and hydroxyl and alcohol-assisted hydrogen hopping. However, unlike the case for CH3OH, the nascent alkoxy-hydroxyl pairs formed by dissociation at BBOV’s are resolved in the STM images, hydrogen hopping away from the nascent pair to an adjacent anion row is tracked directly and alcohol adsorbed on Ti4+ rows is imaged. In addition, images were gathered after dosing on surfaces with selected vacancy concentrations (e11%). For doses sufficient to fill half the initial vacancies, the ratio of 2-butanol bound to Ti4+ rows to 2-butoxy bound at BBOV’s decreased as the initial vacancy concentration increased. II. Experimental Section Experiments were performed in an ultrahigh vacuum chamber (base pressure
