Surface structure sensitivity of high-pressure CO dissociation on Pt(5 5 7), Pt(1 0 0) and Pt(1 1 1) using sum frequency generation surface vibrational spectroscopy

Using sum frequency generation (SFG) surface vibrational spectroscopy, the interaction of CO and platinum single crystals was investigated at high pressure and high temperatures. Under 40 Torr of CO, the molecule was found to dissociate on Pt(1 1 1), Pt(5 5 7) and Pt(1 0 0) at 673, 548 and 500 K, respectively. The CO top site frequency was observed to shift to lower frequencies as a function of temperature. At a particular temperature, dependent on the surface structure, the SFG spectra evolved with time, indicating the surface was being modified. The observed frequency shift before CO dissociation is attributed to anharmonic coupling to the frustrated translational mode. At the temperature at which dissociation occurs, the frequency shift is attributed to surface roughening. The surface roughening is believed to result from the formation of platinum carbonyl species, which would be a driving force to extract platinum atoms from the surface lattice. For both the (1 1 1) and (1 0 0) surfaces of platinum, the crystal must be heated to a temperature at which platinum carbonyls are formed to produce step and kink sites, which are needed for dissociation. The Pt(1 1 1) surface exhibits a much higher CO dissociation temperature as compared to Pt(1 0 0) because it is the most stable surface for platinum. Pt(5 5 7) is essentially a (1 1 1) surface with steps already present in the structure, and so the crystal does not need to be heated to a temperature to produce step and kink sites necessary for CO dissociation through the production of platinum carbonyls. CO can dissociate on the existing steps of Pt(5 5 7).