from first principles PP-I-1

OP-III-8Does the Bulk Matter? Methane and Ethylene Oxidation on PalladiumZemlyanov D.Y. 1 , Klötzer B. 21 Purdue University, Birck Nanotechnology Center, West Lafayette, IN 47907-2057, USA2 Institut für Physikalische Chemie, Universität Innsbruck, A-6020, Innsbruck, Austriadzemlian@purdue.eduAccording to the traditional understanding, heterogeneous catalytic chemical reactions occuron the surface and the reaction rate mainly depends on a surface composition and a surfacestructure. However, dissolution of small molecules such as hydrogen, oxygen, carbon mighttake place and this might result in the changes of electronic and structure property of thesurface. As the consequence, catalyst behavior could change. This was demonstrated formethane and ethylene oxidation over palladium. It was found that dissolution of carbon andoxygen could change catalytic activity of palladium, leading to passivation or activation of thecatalyst.We have shown that the reaction of combustion of methane over Pd is not sensitive to thestructure. This finding implies that industrial catalysts should be prepared to maximize the Pdsurface area without consideration of the cluster shape. On the other hand, the reaction rateshows hysteresis behavior, which relates to palladium oxidation. It was found that thepalladium oxidation is controlled by oxygen dissolution and the oxidation is triggered whensuper saturated solid solution of oxygen is achieved. The in situ XPS highlighted thatappearing PdO nucleus is an active phase for methane oxidation.A carbidic-like phase forms during ethylene oxidation through carbon dissolution in thepalladium bulk. The reaction is catalyzed by both the pure metallic and the carbon modifiedphase, but the temperature/selectivity performance of the catalyst changes significantly. Atlow temperatures, the clean Pd (111) surface mainly runs the reaction of complete oxidationwith a low activation barrier. However, dissolution of carbon in the bulk occurs during thisstage and once the reaction is catalyzed by the new carbon-containing phase, the selectivityshifts towards CO and the activation energy increases.Dissolution of small molecules/atoms such as carbon and oxygen can change catalytic activityof palladium, leading to passivation or activation of the catalyst. Dissolution is the importantstep of the reaction mechanism.51