Analys av platinaytor och platinatennytor under katalytisk etanoloxidation med röntgenfotoelektronspectroskopi

Sammanfattning: Fuel cells are more efficient and cleaner than combustion engines. Ethanol as a fuel has a high energy density and is safer and easier to handle than hydrogen which is normally used in fuel cells. If efficient fuel cells on alcohol were available, they could be used for engines and power sources for electronics. Platinum-tin surfaces have proven to be good catalysts for ethanol and an improvement over pure platinum. The mechanism and the structure during catalysis are not well known. An experiment was performed at the Hippie beam line at Max IV to improve the knowledge in this area. The (111) surface of Pt and Pt3Sn alloy and the (223) surface of Pt, was exposed to ethanol and oxygen. Pt and Pt3Sn both have face-centered-cubic (FCC) crystal structures. The (111) surface is the most close-packed in an FCC crystal. A (223) surface is a (111) surface cut at a low angle. So it has the appearance of a stepped (111) surface. The edges on the (223) surface should increase the activity compared to the (111) surface. The surfaces and the gas phases were measured in situ with ambient pressure x-ray photoelectron spectroscopy and a quadrupole mass spectrometer was used to analyze the gas composition. The hypothesis that increasing the number of edges as with the Pt(223) surface should increase the activity is accurate. Pt(223) was more active than Pt(111). Pt(223) and Pt3Sn(111) have similar ethanol conversion rate. Increasing the oxygen-to-ethanol ratio increased the activity both with Pt(111) and Pt(223), Pt3Sn(111) was not tested with increased oxygen-to-ethanol ratio. The gas phases were analyzed, and the existing compounds were identified. Acetaldehyde shows up in the C1s gas spectrum in all of the sequences. When ethanol decreases acetaldehyde increase. The difference between these two compounds is only two hydrogen atoms. This reaction is the start of the catalytic process and it is the same for all tested crystals. Ethylene (CH2CH2) shows up as a vague peak in the gas phase. It is only present at higher temperatures and with a low oxygen rate. Compared to the other crystals the Pt3Sn(111) sample doesn't produce CO2, at least not to a detectable degree. In the gas phases of the other crystals, the CO2 peak was visible. Pt(223) creates CO2 but to a lesser degree than Pt(111). The goal of the experiment was to investigate which Sn phases are present during ethanol oxidation. This turned out to be difficult. The Pt3Sn crystal was carbon poisoned during the first test sequence and the graphite layer was not possible to remove during the beam time. Curve fitting of the Sn3d peak resulted in two components. The components were Pt3Sn alloy and Sn with adsorbed molecules. The expected SnO2 and SnO peaks notably absent. The oxygen probably bonds with carbon instead of tin. Carbon was present on the surface due to insufficient cleaning. In the oxygen spectrum, chemically bonded oxygen seems to be present from 100 °C, as SnO2 or SnO. This peak is most likely from some other component containing oxygen. If oxygen is bonded to Sn, it should be visible in the Sn3d peak, unless it is hiding underneath one of the present peaks. According to Batzill et al. a quasimetalic state consisting of oxidized Sn alloyed with Pt has a similar binding energy as Pt3Sn alloy. So it could be that the oxygen is hiding underneath the Pt3Sn alloy component. The experiment has improved the knowledge of ethanol oxidation on platinum and platinum-tin surfaces. The knowledge gained here is a good start for further experiments and simulations.