abstracts - Институт катализа им. Г.К. Борескова

KS-I-6pressure study. For this investigation we exploited in situ XPS in combination with ex situTPD, STM, LEED, etc. along with kinetic measurements. The in situ high-pressure XPS canoperate up to a few mbar [5, 6].Palladium Oxidation: Palladium oxidation was found to be complex process, which consistsof several steps. In the case of Pd(111) single crystal, at 431 K in 0.4 mbar O 2 the surface iscovered by the 2D oxide phase and by the supersaturated O ads layer. The supersaturated O adslayer transforms into the Pd 5 O 4 phase upon heating. The supersaturated O ads layer disappearscompletely at approximately 470 K. Above 653 K the bulk PdO phase appears and this phasedecomposes completely at 815 K. It was supposed that the 2D oxide phase acts as a precursorfor bulk PdO. Decomposition of the bulk oxide is followed by oxygen dissolution in the bulkand formation of the O γ species along with O β : the oxygen species dissolved deeply in thebulk. The formation of the O γ oxygen results in the expansion of the lattice. The O β oxygen ismore favourable at high temperatures because the O γ oxygen could not accumulate in thenear-surface region and diffusion shifts the equilibrium towards O β . The saturation of the bulk“reservoir” with O β results in increasing of the O γ uptake confirming the transformation of O γto O β at high temperatures. The bulk PdO phase does not form during cooling in 0.4 mbar O 2but below 747 K the Pd 5 O 4 phase appeared. Dissolution of oxygen in the palladium bulk isinvolved in the oxidation mechanism as an important step.Ethylene Oxidation: The main finding is a carbidic-like phase, which forms during ethyleneoxidation through carbon dissolution in the palladium bulk. The reaction is catalyzed by boththe pure metallic and the carbon modified phase but the temperature/selectivity performanceof the catalyst changes significantly. At low temperatures, the clean Pd (111) surface mainlyruns the reaction of complete oxidation with a low the activation barrier. However,dissolution of carbon in the bulk occurs during this stage and once the reaction is catalyzed bythe new carbon-containing phase, the selectivity shifts towards CO and the activation energyincreases. During cooling ramp the activity of the catalyst decreases slowly in the hightemperature range and very fast once the new carbidic phase reappears at approximately600 K. Carbon modification of the surface/bulk causes the changes of the stability ofadsorbed species. Although the chemical potential of the oxygen in the gas phase remainsconstant during the cooling cycle, carbonaceous adsorbats, CO and likely ethylidyne, appearon the surface at 506 K and poison the reaction.Methane Oxidation: Using large single crystal model catalysts, we have shown that thereaction of combustion of methane over Pd is not sensitive to the structure of the catalyst.35