Insight into deactivation of the carbon-/sintering-resistant Ni@Silicalite-1 for catalytic partial oxidation of methane to syngas

Ni-based catalysts show good activity and selectivity for the catalytic partial oxidation of methane (CPOM) to syngas, but still suffer from carbon deposition and Ni-sintering. Herein, Ni nanoparticles of 3–7 nm are successfully embedded inside the crystal of silicalite-1 (S-1) zeolite via one-pot hydrothermal synthesis. The Ni@S-1 catalysts, with high activity/selectivity, show high carbon-/sintering-resistance but still deactivates fast. The surface of Ni nanoparticles involved in the reaction is oxidized to form intermediate NiO species, which may react with the circumambient SiO₂ (from S-1) in the presence of water vapor (by-product and/or intermediate-product of CPOM reaction) to form a dense Ni₃(Si₂O₅)₂(OH)₂ layer around Ni nanoparticles. The formed Ni₃(Si₂O₅)₂(OH)₂ compounds are hard to be reduced (even in H₂ at 800 °C; thus losing capacity to involve catalytic reaction cycle) while completely blocking the Ni⁰ sites from contacting with reactants, which is the main cause for the Ni@S-1 deactivation. Not surprisingly, such deactivation problem can be solved easily by replacing Ni with highly anti-oxidative metals such as Rh.