Abstract
In-situ imaging of catalytic reactions has provided insights into reaction front propagation, pattern formation and other spatio-temporal effects for decades. Most recently, analysis of the local image intensity opened a way towards evaluation of local reaction kinetics. Herein, our recent studies of catalytic CO oxidation on Pt(hkl) and Rh(hkl) via the kinetics by imaging approach, both on the meso- and nano-scale, are reviewed. Polycrystalline Pt and Rh foils and nanotips were used as µm- and nm-sized surface structure libraries as model systems for reactions in the 10–5–10–6 mbar pressure range. Isobaric light-off and isothermal kinetic transitions were visualized in-situ at µm-resolution by photoemission electron microscopy (PEEM), and at nm-resolution by field emission microscopy (FEM) and field ion microscopy (FIM). The local reaction kinetics of individual Pt(hkl) and Rh(hkl) domains and nanofacets of Pt and Rh nanotips were deduced from the local image intensity analysis. This revealed the structure-sensitivity of CO oxidation, both in the light-off and in the kinetic bistability: for different low-index Pt surfaces, differences of up to 60 K in the critical light-off temperatures and remarkable differences in the bistability ranges of differently oriented stepped Rh surfaces were observed. To prove the spatial coherence of light-off on nanotips, proper orthogonal decomposition (POD) as a spatial correlation analysis was applied to the FIM video-data. The influence of particular configurations of steps and kinks on kinetic transitions were analysed by using the average nearest neighbour number as a common descriptor. Perspectives of nanosized surface structure libraries for future model studies are discussed.
Highlights
A mechanistic understanding of catalytic processes on solid surfaces requires to reveal the correlative relationship between the catalyst structure and the reaction kinetics over a wide range of the length scales
We focus on the advantages of both model systems, illustrated by recent photoemission electron microscopy (PEEM) and field emission microscopy (FEM)/ field ion microscopy (FIM) studies of CO oxidation on Pt and Rh
Catalysis on solid surfaces is a prime example for the manifold correlations between microscopic properties and spatiotemporal effects on the meso-scale, such as reaction front propagation and pattern formation
Summary
A mechanistic understanding of catalytic processes on solid surfaces requires to reveal the correlative relationship between the catalyst structure and the reaction kinetics over a wide range of the length scales. We have used the domains of polycrystalline PGM foils, i.e. μm-sized single crystal surfaces combined in one sample, as scale-bridging model systems (Fig. 2) The prerequisites of such an application are the ability to crystallographically characterise each surface domain and to monitor the local reaction kinetics on the μm-scale [29]. The collection of different, but well defined μm-sized surfaces in “one-sample” serves as a surface structure library, consisting e.g. for a 10 × 10 mm sample of hundreds of domains, often exposing high-Miller-index orientations interesting for catalysis [19, 29] Under certain conditions, such domains can behave independently in a surface reaction as was observed already in the 90ies [31], detecting oscillations in CO oxidation on Pt(110) and Pt(100) domains, while the (111)-type grains remained in a steady state, i.e. structure-dependent behaviour was directly observed. We focus on the advantages of both model systems, illustrated by recent PEEM and FEM/ FIM studies of CO oxidation on Pt and Rh
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