Situ STM Studies of Model Catalysts

Like most surface science techniques, conventional in situ STM studies have been carried out in UHV on model catalysts consisting of extended planar surfaces. When extrapolating the information obtained in UHV surface science studies to real-world catalysis, two issues have generally concerned the catalysis community, namely, the pressure and material gaps. 

To visualize the fundamental steps of chemisorptions and reactions that occur at surfaces, in situ STM investigations typically monitor the diffusion or transformation of adsorbed molecules. A series of snapshots of preselected surface regions, compiled into a STM movie, can reveal the evolution of surface phenomena. On metal surfaces, the surface diffusion of adsorbates is usually so rapid that the 

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This chapter reviews the recent progress in in situ STM studies of model catalysts. From revealing reaction pathways to delineating active sites, in situ STM studies in UHV and on extended surfaces have demonstrated their power to solve fundamental questions in catalysis and enhance our understanding of the elementary steps of... 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

The objective of this chapter is to show that particles in the mesoscopic regime have very different properties to the bulk phase and, specifically, to demonstrate how in-situ STM and FTIR spectroscopy have been successfully employed to determine information on the structure of model catalysts based on modification of substrate electrodes with metal particles of mesoscopic dimensions, and the effect of this structure on reactivity. It will be shown that studying these model electrodes helps provide a link between single-crystal electrodes, which have provided a wealth of useful information, and electrodes for real application. FTIR has long been invaluable as a probe for localized particle reaction on surfaces in electrochemical processes, and the present work will show how it can complement STM in providing excellent characterization of mesoscopic properties. 

In situ CO titration experiments have also been conducted on multicomposition systems, that is, inverse model catalyst. Schoiswohl et al. [68] in their studies compared the CO titration reaction on three surfaces clean Rh(l 1 1) surface, Rh (111) surface covered with large 2D V309 islands (mean size >50 nm), and Rh(l 11) surface covered with small 2D V309 islands (meansize<15 nm). Prior to CO titration, the three surfaces were exposed to 10-7 mbar 02 to form a (2 x l)-0 phase at room temperature. In situ STM was used to follow the titration reaction in the presence of 10 x-10 7 m liar CO. CO titration on the clean Rh(l 1 1) surface or the Rh(l 1 1) surface with large V309 islands exhibits similar reaction kinetics. Figure 3.19 shows... 

The main modification that enables the system to analyze in situ reactions is the custom built chamber for the STM stage with indirect heating via an electron beam. Therefore, the sample can be brought to the desired temperature and pressure without disturbing surface interactions. While this technique is primarily used for model catalysts to be studied, it provides very good insight into the mechanisms present over a range of pressures. 

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