Stable bimetallic Ru-Mo/Al2O3 catalysts for the light alkane combustion: Effect of the Mo addition

摘要

A series of Ru-Mo/gamma-Al2O3 catalysts containing fixed Ru loading (5 wt.%) and variable Mo contents (1.6, 4.8 and 9.5 wt.%), were synthesized by co-impregnation method using Ru(NO)(NO3)(3) and (NH3)(6)Mo7O24 x 4H(2)O as metal precursors and their catalytic characteristics were evaluated for the first time in combustion of propane used as a model compound. The effect of the Ru/Mo atomic ratio (3:1, 1:1 and 1:2) on the structure of the reduced samples was investigated by ICP-AES, BET, XRD, HRTEM, SEM-EDX, H-2-TPR and chemisorption of hydrogen and oxygen. The surface properties of the catalysts were detailed studied by XPS spectroscopy. Structural characterization data lead to conclusion that a synergy effect of ruthenium and molybdenum is observed in the Ru-Mo/gamma-Al2O3 catalysts. An increase of Mo loading caused the formation of smaller nanoparticles (d(av) = 1.3, 1.0 and 0.8 nm, respectively) as compared to the monometallic Ru system (d(av) = 1.4 nm). For low Mo loading (Ru:Mo atomic ratio of 3:1) the H-2 uptake was higher (H/Ru = 0.55) than in the Ru/gamma-Al2O3 sample (H/Ru = 0.52). This leads to the high activity of the 5%Ru-1.6%Mo catalyst in the propane combustion (TOF = 0.034 (s(-1))). Moreover, the catalytic stability was much higher for the bimetallic Ru-Mo systems than for the monometallic Ru catalyst. Despite their high thermal stability under oxygen-rich atmosphere, bimetallic catalyst containing 4.8%Mo (Ru:Mo atomic ratio of 1:1) was less active in propane oxidation (TOF = 0.016 (s-1)) than the Ru system (TOF = 0.029 (s(-1))). The lower activity results from partial blocking the active centers on the ruthenium by covering them with an inactive MoOx layer. At the highest Mo content (Ru:Mo atomic ratio of 1:2) the blocking of the active Ru centres increases, much lower H-2 chemisorption capacity and a significant increase of the metallic Ru (74%) and metallic Mo (25%) species, resistant to oxidation at low temperature, were observed. This leads to very low activity of the 5%Ru-9.5%Mo catalyst in the propane combustion. Importantly, our studies also revealed that the molybdenum modifies interaction of hydrogen and oxygen with ruthenium in the Ru-Mo catalysts. For hydrogen, the adsorption process becomes activated and Mo species weaken the adsorption strength.