Surface reaction kinetics studied with atomic-scale lateral resolution

Norbert Kruse ,  Matthieu Moors ,  Thierry Visart de Bocarm√© 

Université Libre de Bruxelles, Chemical Physics of Materials, Campus Plaine, CP 243, Brussels B-1050, Belgium

Abstract

This presentation reviews the recent progress made in imaging catalytic and non-catalytic surface reactions and in providing information on the local chemical composition of reactive layers. The methodical approach is based on video-Field Ion Microscopy (FIM) and atom-probe Pulsed Field Desorption Mass Spectrometry (PFDMS). The usefulness of the approach will be demonstrated in several case studies.
First, the carbonylation of Nickel to Ni(CO)4 will be presented. This reaction involves subcarbonyls, Ni(CO)2,3 as intermediates. Time-resolved PFDMS studies - in this method short field pulses of variable repetition frequency are used to field-desorb reactive species during the ongoing reaction - reveal Ni(CO)2 formation to be the slowest step in the overall reaction. In FIM this process is seen to coincide with Ni kink sites detaching in a repetitive manner so that an originally hemispherical Ni crystal ("field emitter") is transformed into a polyhedral one. Second, we show that strong morphological changes do also occur during the reaction of CO and mixtures of CO/H2 with Co crystals. In this latter case video-FIM allows mapping of the catalytic activity and revealing the competition between morphological reshaping and chemical restructuring due to carbon deposition. CxHy species are detected by PFDMS during the ongoing CO/hydrogen reaction. The ion intensities of these species depend on the reaction time (ms time scale at 450 K and 10-3 mbar, H2/CO=2) and methane is the first product of this hydrogenation. The data will be directly compared to transient kinetic build-up measurements using Co supported model catalysts.
Finally, video-FIM data on the interaction of ethylene with Ni and Co crystals will be shown. In both cases step sites cause a strong promotion of ethylene decomposition. In the Ni case, graphitic overlayers seem to form at intermediate temperatures (~600 K). These layers undergo an explosive clean-off reaction (with hydrogen) in which chemical fronts ignite in (001) planes before travelling in a self-accelerating manner toward the central (001) plane of the crystal. In a similar study, a Ni crystal is heated to ~1000 K in the presence of 10-2 Pa ethylene. Subsequent in-situ quenching and FIM imaging at 500 K reveals string-like structures arranged in a concentric manner in the outskirts of the crystal. These structures point away from the crystal centre. A tentative explanation of this observation is based on the formation of carbon nanofibers and their repulsive interaction with the positive electric field present during imaging.

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Presentation: Invited oral at E-MRS Fall Meeting 2006, Symposium B, by Norbert Kruse
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-06-23 15:07
Revised:   2006-07-25 13:54
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