Sum frequency generation (SFQ) vibrational spectroscopy studies of combustion reactions on platinum single crystal surfaces

Abstract

We have studied the dissociation of CO catalyzed by platinum single crystals. At 40 torr of CO, the Pt(111) crystal dissociates CO at 673 K. Under the same conditions, Pt(100) dissociates CO at 500 K, and Pt(557) dissociates CO at 548 K. Hence, the CO dissociation reaction is a structure sensitive reaction. SFG was used to monitor the CO top site resonance as the platinum crystals were heated to the dissociation temperature when exposed to 40 torr of CO. In all three systems, the CO resonance shifts to lower frequency as the platinum crystal is heated. However, the frequency of the CO resonance at the dissociation frequency is lower on the (100) and (111) crystal faces than on the Pt(557) crystal. We believe that the (111) and (100) crystal faces must undergo roughening to expose step or kink sites in order to facilitate the dissociation reaction. This is supported by UHV studies of CO dissociation catalyzed by platinum crystals. These studies observe dissociation only when step or kink sites are present. Since the Pt(111) surface is very stable, it needs to be heated to 673 K to produce the low coordination number sites needed for CO dissociation. Since the Pt(100) surface easily reconstructs, it is able to form the active sites for CO dissociation at relatively low temperatures. The SFG spectra support our conclusion that the CO molecules are sitting on low coordination number platinum atoms at the dissociation temperature. Since the Pt(557) surface already has step sites, the dissociation reaction can take place without further roughening of the surface. The CO resonance on the (557) crystal face at the dissociation temperature is at a very similar frequency to CO molecules adsorbed on only the step sites of the crystal. Further studies showed that the dissociation reaction takes place on the (557) surface at CO pressures as low as 1 torr. At 1 torr of CO, the carbon deposition rate is 1.0 x 10-2 ML minute-1. A series of experiments at CO pressures ranging from 5 to 20 torr leads to a 0.8 order dependence of the dissociation reaction on CO pressure.