Criteria for stable Ni particle size under methanation reaction conditions: Nickel transport and particle size growth via nickel carbonyl

The loss of methanation activity of alumina-supported nickel catalysts was investigated at pressures up to ca. 600 kPa and temperatures from 400 to 740 K. Deactivation is attributed primarily to growth in nickel particle size. A mechanism involving Ni(CO)4 formation, diffusion and subsequent decomposition is proposed to explain the observed growth in nickel particle size. A region of “safe” operating conditions for achieving stable catalytic activity for methanation was identified. These safe operating conditions were sensitive to temperature and carbon monoxide partial pressure. A criterion based on the equilibrium partial pressure of Ni(CO)4 is proposed for assessing the safety of a set of operating conditions. Specifically, conditions for which the equilibrium Ni(CO)4 pressure is less than ca. 1 × 10−6 Pa result in stable methanation activity.