Sintering of nickel catalysts: Effects of time, atmosphere, temperature, nickel-carrier interactions, and dopants
Supported nickel catalysts are widely used in the steam-reforming process for industrial scale production of hydrogen and synthesis gas. This paper provides a study of sintering in nickel-based catalysts (Ni/Al2O3 and Ni/MgAl2O4). Specifically the influence of time, temperature, atmosphere, nickel-carrier interactions and dopants on the rate of sintering is considered. To probe the sintering kinetics, all catalysts were analyzed by sulfur chemisorption to determine the Ni surface area. Furthermore selected samples were further analyzed using X-ray diffraction (XRD), mercury porosimetry, BET area measurements, and electron microscopy (EM). The observed sintering rates as a function of time, temperature, and PH2O/PH2PH2O/PH2 ratio were consistent with recent model predictions [J. Sehested, J.A.P. Gelten, I.N. Remediakis, H. Bengaard, J.K. Nørskov, J. Catal. 223 (2004) 432] over a broad range of environmental conditions. However, exposing the catalysts to severe sintering conditions the loss of nickel surface area is faster than model predictions and the deviation is attributed to a change in the sintering mechanism and nickel removal by nickel-carrier interactions. Surprisingly, alumina-supported Ni particles grow to sizes larger than the particle size of the carrier indicating that the pore diameter does not represent an upper limit for Ni particle growth. The effects of potassium promotion and sulfur poisoning on the rates of sintering were also investigated. No significant effects of the dopants were observed after ageing at ambient pressure. However, at high pressures of steam and hydrogen (31 bar and H2O:H2 = 10:1) potassium promotion increased the sintering rate relative to that of the unpromoted catalyst. Sulfur also enhances the rate of sintering at high pressures, but the effect of sulfur is less than for potassium.
Journal: Applied Catalysis A: General - Volume 309, Issue 2, 1 August 2006, Pages 237–246