Self-activation and self-regenerative activity of trace Rh-doped Ni/Mg(Al)O catalysts in steam reforming of methane

摘要

Ni/Mg(Al)O catalyst doped with trace amounts of Rh was tested in steam reforming of methane and the catalytic behavior was compared with the Ru-doped catalyst.Rh-doped Ni/Mg(Al)O catalyst showed a self-activation without any reduction treatment due to the ability for hydrogen production by C-H bond cleavage.Such production led to the reduction of lattice Ni2+ in Mg(Al,Ni)O periclase to metallic Ni by hydrogen-spillover.The Rh-doped Ni/Mg(Al)O catalyst showed also a self-regenerative activity during a daily start-up and shut-down (DSS) operation of steam reforming of methane.NiRh alloy was formed in the surface layer of metallic Ni particles and Rh was located more profoundly in the particles than Ru in NiRu alloy on the Ru-doped catalyst.The Rh-doping exhibited a prominent performance,whilst the Ru-doping resulted in neither self-activation nor stable activity in the DSS operation.Although the catalyst deactivation took place by the Ni° oxidation into the lattice Ni2+ in Mg(Al,Ni)O periclase,trace Rh assisted the regeneration of the active metallic Ni from the lattice Ni2+ by hydrogen-spillover.Even metallic Ni particles that had been strongly sintered on the Rh-Ni/Mg(Al)O catalyst by steaming at 900 °C were re-dispersed during the DSS operation,resulting in high and stable activity.The self-regeneration of the Rh-doped Ni/Mg(Al)O catalyst has been achieved by continuous rebirth of active Ni metal particles due to reversible reduction-oxidation between the metallic Ni on the surface and the lattice Ni2+ in Mg(Ni,Al)O periclase,i.e.,oxidative incorporation of surface Ni~0 into the lattice Ni2+ of Mg(Al,Ni)O and reductive migration of the lattice Ni2+ to the surface Ni~0 by hydrogen-spillover.