Platinum surface defects

Fig. VIII-2. Scanning tunneling microscopy images illustrating the capabilities of the technique (a) a 10-nm-square scan of a silicon(lll) crystal showing defects and terraces from Ref. 21 (b) the surface of an Ag-Au alloy electrode being electrochemically roughened at 0.2 V and 2 and 42 min after reaching 0.70 V (from Ref. 22) (c) an island of CO molecules on a platinum surface formed by sliding the molecules along the surface with the STM tip (from Ref. 41).

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

In the first part of this chapter, we investigate the desorption of NO and CO molecules from a platinum surface to leam about energy flow between the substrate and the adsorbate. By using a stepped surface we unravel how the chemical dynamics are affected by surface defects [2], and demonstrate that the chemical dynamics depend critically on the precise adsorption site. In the second part we will follow diffusion of CO over the surface in real-time by using the adsorbate vibration-sensitive SFG method in a pump-probe manner. The stepped surface allows us to distinguish between molecules on the step and the terrace sites and therefore makes the diffusion from step to terraces sites visible [1]. 

The electro-catalytic oxidation of hydrogen, and reduction of oxygen, at carbon supported platinum based catalysts remain essential surface processes on which the hydrogen PEM fuel cell relies. The particle size (surface structure) and promoting component (as adsorbate or alloy phases) influence the activity and tolerance of the catalyst. The surface chemical behavior of platinum for hydrogen, oxygen, and CO adsorption is considered, in particular with respect to the influence of metal adsorbate and alloy components on close packed and stepped (defect) platinum surfaces. Dynamical measurements (employing supersonic molecular beams) of the... 

It is shown below that a number of molecular species are important in the interaction of oxygen with platinum surfaces, and these are precursors to the dissociative state. The interception of these adsorbed states in the electrochemical process are important in determining the catalytic mechanism of electroreduction, and we also show below that surface defects play an important role in their reactivity and surface lifetimes. 

In the case of platinum and gold, some incipient progress has already been achieved in finding the optimum alloy combination in the nanoscale domain to enhance the glucose oxidation, which is the anodic reaction in a biofuel cell system, concomitant with surface defects control.25,82,83... 

For many years it has been well known that CO electrooxidation on platinum is a structure-sensitive reaction. Studies with singlecrystal electrodes have shown that the kinetic parameters depend not only on the surface composition of the catalyst but also on the symmetry of the surface and that the presence of steps and defects alters significandy the reaction rate. As a consequence, the surface structure of the nanoparticles should also affect the performance for the oxidation of CO. Understanding how the different variables affect CO oxidation on Pt nanoparticles dispersed on carbon requires the control of the platinum surface in a similar way as has been achieved for single-crystal electrodes. In this sense, the influence of the surface site distribution on CO oxidation using nanoparticles of well-defined... 

An interesting example of the potential of STM measurements is illustrated in Figure 21-29. The image was produced from the surface of a sample of iodine atoms absorbed on platinum. The hexagonal pattern of absorbed iodine atoms is interrupted by a defect where an iodine atom is absent, which appears as a divot in the bottom center of the image. The scan that is shown represents a 3 nm X 3 nm area of the platinum surface. This example clearly shows how an STM may be used to reveal structures of solid surfaces at the atomic level. 

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