Dynamic interaction of CO/H 2 O mixtures with gold nanocrystals: Real-time imaging and local chemical probing Thierry Visart de Bocarme ´ * , Thoi-Dai Chau, Norbert Kruse Chimie Physique des Mate ´riaux (Catalyse - Tribologie), Universite ´ Libre de Bruxelles, Campus Plaine, CP 243, B-1050 Bruxelles, Belgium Available online 19 May 2006 Abstract The dynamic interaction of pure gold nanocrystals (‘‘tips’’) with H 2 O/CO gas mixtures was studied by means of video-field ion microscopy (FIM). While imaging with nano-scale resolution selected areas of the equivalent of 200 atomic Au sites were analysed for their chemical composition using short field pulses and injecting respective ions into a time-of-flight mass spectrometer (pulsed field desorption mass spectrometry, PFDMS). At room temperature the exposure of a clean Au sample to water gas at 10 4 Pa, in the pres- ence of an electric field of 10 V/nm, led to water adsorption and formation of bright patterns in FIM. Additional exposure to CO gas at 5 · 10 3 Pa led to the removal of the water layer. This was associated with the occurrence of bright wave fronts which ignited simulta- neously in several regions of the Au surface with no preference for a certain crystallographic surface plane. In some cases wave fronts were seen to collide resulting in more complicated patterns such as concentric rings. Surface areas free of water appeared with low bright- ness. The phenomena were completely reversible. PFDMS demonstrated water ions to be responsible for image formation. Surface hydroxyl was also detected mass spectrometrically and respective ion intensities decreased during the titration with CO. The results sug- gest that gold nanocrystals, in the absence of oxidic support materials, may be active in the reaction between water and CO at temper- atures as low as 300 K and in the presence of an electric field of 10 V/nm. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Field ion microscopy; Atom-probe; Gold; Water; Carbon monoxide 1. Introduction For centuries gold has been regarded too noble for being useful in catalytic applications. However, the pioneering research of Haruta et al. has dramatically changed this perception [1]. Accordingly, ultrafine gold particles may become highly active in a variety of reactions at low temperatures. This finding has triggered a number of studies on the catalysis by gold, which have been reviewed in detail recently [2–4]. Despite a multitude of experimental and theoretical evidence the viewpoints on the nature of the active sites and microscopic reaction mechanisms remain partly conflicting and are still a matter of considerable debate. Although the CO + O 2 reaction dominates the research on catalysis by gold, other reactions seem to gain interest as well. For example, the low-temperature water gas shift reaction (WGSR), CO + H 2 O ! CO 2 +H 2 , may provide an economic route to manufacture pure hydrogen. Particu- larly attractive applications may result from the use of H 2 as a deNO x reductant in automobile exhaust converters or in context with fuel cell power system developments. In comparison with commercial Ni- or Cu-based catalysts which are operated at 900 or 600 K, respectively, supported Au-based catalysts are active at much lower temperatures. For example, Andreeva et al. reported high activity at 120 °C on Au/a-Fe 2 O 3 and suggested the occurrence of synergetic effects, i.e. the dissociative adsorption of H 2 O on Au followed by spillover of OH to the Fe 2 O 3 support [5,6]. OH surface groups were also considered important in the promotion of the CO oxidation by H 2 O [7,8]. Recently, Daniells et al. reported [9] that the increased 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.01.145 * Corresponding author. Tel.: +32 2 6505720; fax: +32 2 6505708. E-mail address: tvisart@ulb.ac.be (T. Visart de Bocarme ´). www.elsevier.com/locate/susc Surface Science 600 (2006) 4205–4210