Dynamic interaction of CO/H2O mixtures with gold nanocrystals:: Real-time imaging and local chemical probing

The dynamic interaction of pure gold nanocrystals (tips) with H2O/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 similar to 200 atomic An 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 An sample to water gas at 10(-4) Pa, in the presence of an electric field of similar to 10 V/nm, led to water adsorption and formation of bright patterns in FIM. Additional exposure to CO gas at 5 x 10(-3) Pa led to the removal of the water layer. This was associated with the occurrence of bright wave fronts which ignited simultaneously 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 brightness. 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 suggest that gold nanocrystals, in the absence of oxidic support materials, may be active in the reaction between water and CO at temperatures as low as 300 K and in the presence of an electric field of similar to 10 V/nm. (c) 2006 Elsevier B.V. All rights reserved.