Oxygen O2 dissociation on single crystal platinum surfaces

Oxygen (O2) dissociation on single crystal platinum surfaces 

Motivated by the application to catalytic oxidation processes [1], much research has been conducted to determine and elucidate the mechanism(s) of the dissociative chemisorption of 02 on metal surfaces, in particular, Pt single crystal surfaces. 

Studies of oxygen dissociation on the Pt(l 1 1) surface have been conducted by many investigators, including Nolan et al. [75, 76], Rettner and Mullins [77], and Luntz et al. [78, 79], producing results which have led these investigators to propose that 02 dissociation proceeds by some type of precursor-mediated mechanism to dissociation. 

Using evidence obtained from XPS and TPD experiments, Luntz and co-workers [78, 79] propose that 02 dissociation at low kinetic energies on the Pt(l 11) surface occurs via a series of two sequential precursors a physisorbed 02 state and a more strongly held, molecularly chemisorbed state. TPD spectra from a Pt(l 11) crystal exposed to oxygen at Ts = 25 K produced desorption peaks at Ts = 30, 45, 130, and 750 K, characteristic of multilayer desorption, physisorbed 02 desorption, molecularly chemisorbed 02 desorption, and associative 02 desorption, respectively. The existence of these 02 states was also confirmed by Luntz et al. by XPS spectra of the Pt(l 11) surface after exposure to 02 at different surface temperature ranges. 

Shown in Fig. 21 are plots of the integrated O Is intensities from XPS spectra for physisorbed 02 and molecularly chemisorbed 02 in the temperature range in which both species coexist (Ts 38 K). This plot, argues Luntz et al., shows a quantitative thermal conversion of physisorbed 02 to molecularly chemisorbed 02 upon a temperature ramp to Ts = 80 K. 

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