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Fig. 18.2. Pt (111) unit cell containing 1-top, 3-bridge and 2-hollow adsorption sites

It is well established that oxygen adsorption on platinum occurs through different adsorption states, which have been thoroughly investigated using various spectroscopic techniques. Four adsorption states have been found for O2/Pt (111) at various substrate temperatures using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission (UPS), near edge X-ray absorption fine structure (NEXAFS), auto ionisation, and Auger electron spectroscopy (Puglia et al. 1995), and are summarised in Fig. 18.3.

Oxygen molecules were found to physisorb on the surface at about 25°K, and two molecular chemisorbed states were identified as precursors for the thermally activated dissociation process (Puglia et al. 1995). The first of these molecular states is weakly chemisorbed at 90°K and it is adsorbed in a bridge site with a saturation coverage of 0.23 (molecules per Pt surface atom). The second state, chemisorbed at 135°K is more strongly bonded to the Pt substrate and seems to be adsorbed in hollow or hollow-bridge sites at fractional coverages of 0.15 (Puglia et al. 1995). Moreover, adsorbed atomic oxygen has been detected on platinum threefold adsorption sites with saturation coverage of 0.25 prevailing at temperatures above 150°K.

Other authors studied oxygen adsorption and desorption on the Pt (111) and on stepped Pt(S)-12(111)x(111) surfaces over the 100 to 1300°K temperature range using thermal desorption (TD), Auger electron spectroscopy, XPS, and low energy electron diffraction (LEED) (Gland 1980). These authors have reported little or no dissociation of adsorbed molecular oxygen occurring at 100°K, and adsorption of oxygen above 170°K proceeding rapidly to a (2x2) structure (Gland 1980). Studying the kinetics and energetics of oxygen adsorption on Pt (111), the maximum O2 desorption rate was found at ~140°K using TPD, isothermal desorption, Auger spectroscopy, LEED and isotopic measurements (Winkler et al. 1988). Similar findings regarding the existence of adsorbed oxygen states of oxygen was reported on the basis of EELS, UPS, and TD spectroscopy techniques over the temperature range 100°K to 1400°K (Gland et al. 1980).

160 140 120 100

H 80

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Fig. 18.3. O2 adsorption states found on Pt (111) surfaces at various substrate temperatures

Microscopic details of the oxygen adsorption process on platinum surfaces have been obtained through ab initio studies from which two distinct, but energetically almost degenerate chemisorbed precursors were identified on Pt (111) (Fig. 18.3): a superoxo-like paramagnetic precursor formed at the bridge site (Eichler et al. 1997), with the molecule parallel to the surface, and a second peroxo-like non-magnetic precursor formed in the threefold hollow, with the atom slightly canted in a top-hollow-bridge geometry (Eichler et al. 1997). Cluster models provide the advantage that the local chemistry can be analysed in geometries that most closely resemble active sites. Studies on Pt21O2 indicated that the O2 lying-down orientations are favored over the upright orientations, and chemisorption on the bridge site with the O-O axis along the bridge is the most stable configuration (Zhou et al. 1992). Density Functional Theory calculations on Pt clusters also showed the existence of peroxo and superoxo states and provided insights into the effect of an applied electric field on the geometries and energies of the chemisorption states (Li et al. 2001).

Experiments and ab initio calculations thus provide microscopic details about possible states of oxygen adsorption and dissociation. However, in order to understand the oxygen adsorption and reduction reactions in fuel cell cathode catalysts, studies on the time evolution of the rates of oxygen adsorption and desorption on platinum surfaces, and their dependence on pressure, temperature, and applied voltage are needed.

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Physisorption Chemisorption I Chemisorption II Atomic

Oxygen Adsorption Phase

Physisorption Chemisorption I Chemisorption II Atomic

Oxygen Adsorption Phase

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