Sustainability of Ni loaded Mg-Al mixed oxide catalyst in daily startup and shutdown operations of CH₄ steam reforming

Daily startup and shutdown (DSS) operations using several purge gases were applied for Ni-loaded Mg(Al)O periclase catalyst to test the sustainability in steam reforming of CH₄. Thirteen wt% Ni loaded Mg(Al)O catalysts were prepared by using Mg(Ni)-Al hydrotalcite as the precursor with varying Mg/Al ratios. H₂O/N₂ (100/25 ml min^-1), O₂/N₂ (40/10 ml min^-1) and CO₂/H₂O/N₂ (40/15/25 ml min^-1) were tested as the purge gas in imitation of steam, air and spent gas, respectively. Use of air as the purge gas resulted in a quick deactivation of all Ni loaded catalysts due to the oxidation of Ni metal on the catalyst surface. Spent gas was the most inert for the DSS operation causing no significant deactivation of Ni loaded catalysts. Steam, the most convenient purge gas for DSS operation of PEFC, revealed evident deactivation depending on the (Mg + Ni)/Al ratio in Ni/Mg(Al)O catalysts; use of the (Mg + Ni)/Al ratio of 3/1 resulted in a quick deactivation with steam purge, although this ratio was the most profitable for the steady state operation with the Ni/Mg(Al)O catalysts. The most stable operation was achieved with the ratio of 6/1 in steam as the purge gas. The deactivation took place mainly by the oxidation of Ni metal by steam as the purge gas during the DSS operation between 200 and 700 °C. It is likely that Mg(Al)O on the Ni/Mg(Al)O catalysts was hydrated by steam to form Mg(OH)₂ on the catalyst surface, resulting in the oxidation of Ni metal finely dispersed on Mg(Al)O periclase.