Sintering of nickel steam-reforming catalysts: effects of temperature and steam and hydrogen pressures

Steam reforming over nickel catalysts is widely used for industrial-scale production of hydrogen and synthesis gas. This work is a study of the effects of atmosphere and temperature on the rate of sintering of nickel steam-reforming catalysts. The relative nickel areas of Ni/MgAl2O4 and Ni/Al2O3 catalysts after sintering in H2O:H2 atmospheres at high (40 bar) and low (1 bar) pressures are reported. The data are discussed in terms of the recently proposed model for the sintering rate of supported nickel catalysts [J. Sehested, J. Catal. 217 (2003) 417] and density functional theory (DFT) calculations of the stability and diffusivity of transport species at the surface of nickel particles. OH-bonded nickel dimers are found to have a much lower energy of formation than nickel adatoms (ΔE=58 kJ mol−1). It is therefore concluded that in steam/hydrogen mixtures, OH-bonded nickel dimers are dominating the surface transport on nickel particles and consequently sintering via particle migration and coalescence. Expressions connecting the diffusion constant for nickel particles to the diffusion constant and energy of formation of nickel adatoms and of OH-bonded nickel dimers are given. These equations are used in a sintering model [J. Sehested, J. Catal. 217 (2003) 417] and good agreement between the model and the experimental data is obtained at moderate temperatures. Above temperatures of ca. 600 °C at 40 bar and approximately 700 °C at 1 bar total pressure, the rate and the activation energy of sintering increase considerably. The reason for this observation may be that sintering via Ostwald ripening dominates the sintering rate under these conditions.