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

Important insights into the nature of combustion reactions catalyzed by a Pt(557) model catalyst can be gained by studying the surface composition of the catalyst both during and after high pressure catalytic reactions. During the reactions, the catalyst gas interface was elucidated using SFG vibrational spectroscopy. The SFG spectra were correlated with reaction rates obtained using gas chromatography and post reaction surface science studies to gain an understanding of the active surface during the combustion reactions.; The dissociation of 40 torr of CO catalyzed by Pt(557) was studied and compared to earlier results obtained on Pt(111) and Pt(100). In all three systems, the CO resonance shifts to lower frequency as the platinum crystal is heated. The CO resonance on the (557) crystal face at the dissociation temperature is at a very similar frequency to CO molecules adsorbed only on the step sites of the crystal. Further studies show the dissociation reaction takes place on the (557) surface at 548 K at CO pressures as low as 1 torr with a carbon deposition rate of 1.0 x 10-2 ML minute-1.; The production of CO2 catalyzed by both initially clean and initially carbon covered Pt(557) catalysts was monitored using gas chromatography. Above ignition, our results indicate at least two reaction channels exist. The predominant reaction channel is most likely mass transport limited. We also found the reaction rate below ignition is higher for the initially carbon covered surface in the presence of equal pressures of CO and O2 and when exposed to 100 torr of CO and 40 torr of O2. This indicates the initially carbon covered metal surface is a very good catalyst for the dissociation of molecular oxygen.; In our studies of methane dissociation catalyzed by the Pt(557) single crystal in the presence of 40 torr of methane, the platinum catalyst was able to facilitate both methane dissociation and carbon deposition at temperatures as low as 373 K. SFG spectra show that CxHy fragments are present on the platinum surface during the high temperature reaction. Small amounts of both hydrogen and CO inhibit the deposition of carbon on the platinum catalyst. The hydrogen most likely rehydrogenates the C xHy fragments, inhibiting carbon deposition. The CO most likely inhibits methane dissociation by blocking the sites needed for the reaction to proceed.; We also studied fuel rich methane combustion catalyzed by the Pt(557) single crystal. We found both CO and CO2 are formed as products during the combustion reaction at temperatures at and above 823 K. Based upon the rate of product formation as a function of temperature and oxygen pressure, we have concluded that the CO present in the reaction chamber is most likely due to a temperature induced decomposition of CO2, not the direct result of the methane combustion reaction. (Abstract shortened by UMI.)...