Catalysis on a Crowded Surface: The Role of Surface Oxygen on NO Oxidation of Pt
Ribeiro, Fabio H.
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The reaction NO + 1/2O₂ â†’ NO₂ is a key step in a new technology for exhaust emissions called NOx catalytic traps. The reaction proceeds on a Pt catalyst only when a high coverage of oxygen lowers the binding energy of NO allowing the reaction to proceed without NO dissociation. Based on the reaction kinetics, we proposed that oxygen is the most abundant surface intermediate and that the adsorption of O₂ in one site is the rate determining step. The high coverage of oxygen was confirmed by in situ XPS measurements. The oxygen should erase any special arrangement of Pt atoms on the surface and thus the reaction should not be sensitive to the structure of the catalyst. However, the turnover rate of reaction normalized using the total Pt exposed area varies by a factor of about 100 as the particle size changes from 2 to 9 nm suggesting that the structure of the catalyst is important. The apparent discrepancy is explained by the observation that small Pt particles over-oxidize and become inactive, as demonstrated by in situ x-ray absorption measurements. The structure insensitivity is confirmed by rate measurements on large Pt single crystals exposing the (111), (110) and (100) facets which show that the rates are similar to the one on 9 nm particles. The oxidation of NO is an example of a common occurrence in catalysis where the reaction proceeds on a surface mostly covered by a fragment generated during reaction. The consequence is that understanding the catalytic chemistry can be made only on a surface of adequate coverage.