Methanation of CO2 on Ni/gamma-Al2O3: Influence of Pt, Pd or Rh promotion

Mthanation of carbon dioxide was carried out in a plug flow reactor (CO2:H-2 = 1:4, GHSV = 5700 h(-1)) over a series of noble metal promoted Ni/gamma-Al(2)O(3)catalysts, prepared by the co-impregnation method: Ni(10 wt.%)-Pt(0.5 wt.%)/gamma-Al2O3, Ni(10 wt.%)-Pd(0.5 wt.%)/gamma-Al2O3, and Ni(10 wt.%)-Rh(0.5 wt.%)/gamma Al2O3. Characterization of catalysts was performed by N-2 adsorption-desorption isotherms at -196 degrees C(BET), X-ray powder diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of H-2(H-2-TPD), and CO2(CO2-TPD). Promotion by noble metals resulted in better reducibility of NiO (Ni-Rh > Ni-Pt > Ni-Pd > Ni), enhanced metal dispersion and H-2 chemisorption capacity(Ni-Pt > Ni-Rh > Ni-Pd > Ni), with no significant influence upon the support intrinsic activity given by the CO2 adsorption capacity. Catalytic activity of the promoted catalysts compared to the unpromoted one was performed by temperature programmed reactions (TPRea, 30-300 degrees C) and stability tests at 250 degrees C, under atmospheric pressure, and was evaluated in relation with the structural and functional properties of the catalysts. Promotion by Pt and Pd of Ni/gamma-Al2O3 resulted in enhanced CO2 conversion and CH4 selectivity values in the 180-270 degrees C domain, while Ni-Rh/gamma-Al2O3 was inferior to Ni/gamma-Al2O3. Considering the stability, all catalysts show stable catalytic performance over the investigated time range, with CH4 selectivity values above 90%. Ni-Pd/gamma-Al2O3 is the most efficient catalyst, with stable CO2 conversion of 74.6% and CH4 selectivity of 96.6% at 250 degrees C, in addition to the maximum values of 90.5% and 98.7%, respectively, at 300 degrees C, obtained during temperature programmed reaction runs. (c) 2016 Elsevier B.V. All rights reserved.