The interaction of oxygen with high loaded Ru/γ-Al2O3 catalyst

The interaction of oxygen with the Ru/gamma Al2O3 catalyst comprising metal particle with sizes of 1-16 nm, was examined over a temperature range 20-400 degrees C. The catalyst loaded with 10.8 wt.% Ru was prepared by incipient wetness from RuCl3 precursor. The structure of the Cl-containing catalyst and the catalyst after elimination Cl- ions was characterized using H-2 and O-2 chemisorption, O-2 uptake, BET, XRD and TEM. The Cl- ions in the catalyst decreased the H-2 and O-2 chemisorption capacity of Ru and caused large discrepancies between the mean particle size calculated from gas chemisorption and from TEM. Exposure to O-2 at 100-200 degrees C caused oxidation of small Ru particles, while larger particles were covered with very thin RuxOy skin (undetected by XRD and TEM). The O/Ru ratio increased up to 200 degrees C implying high affinity of the small Ru particles to oxygen. Oxidation at 250 degrees C led to the formation of poorly crystalline RuO2 particles with a mean size of 4 nm, and coverage of large Ru particles with 1.6 nm thick oxide layer. At 300 and 400 degrees C crystallization of the RuO2 phase, as well as significant agglomeration of oxide particles was observed. However, even at 400 degrees C, metallic Ru was detected by XRD. TEM and SAED suggesting that large metal particles were not fully oxidized under the used conditions. Also, the O-2 uptake at 400 degrees C was lower than expected for oxidation of Ru metal to RuO2. For the catalyst after elimination Cl ions the O-2 uptake (O/Ru ratio = 1.50) was higher, than for sample with large amount of Cl ions (O/Ru ratio = 1.34), indicating that the presence of Cl inhibits ruthenium oxidation in the Ru/Al2O3 catalyst. (C) 2008 Elsevier Ltd. All rights reserved.