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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) …