(234a) Coking Resistant, High-Temperature Stable Ni@SiO2 Core-Shell Catalysts

Nickel-based catalysts are good candidates for catalytic partial oxidation of methane to synthesis gas (CPOM) because of their reasonable activity and low cost compared to noble metal catalysts. However, low thermal stability and severe coking currently prevent the use of Ni for this reaction. Reports in the literature indicate that the coking resistance of Ni is increasing with decreasing particle size. However, smaller Ni (nano)particles are even more prone to sintering. In order to develop a robust Ni catalyst, it is hence necessary to develop methods to stabilize very small Ni nanoparticles in a way that reconciles stabilization with high activity and potential coking resistance.

We are presenting results in which a range of Ni-silica core-shell materials were synthesized in a fairly straightforward one-pot synthesis approach. The synthesis approach allows for a broad tailoring of the critical dimensions of both components of the catayst: The size, shape, and thickness of the silica shell can be tailored over a broad range, and the size of the Ni nanoparticles can be adjusted from sub-nanometer to ~10 nm size. Furthermore, we were able to make core-shell as well as "yolk-shell" nanocomposite Ni@SiO2 materials. The latter are characterized by the presence of a pronounced cavity in the core of the nanoparticles with the inside wall decorated with Ni nanoparticles.

These materials were investigated in CPOM and showed good activity (well in excess of a conventionally prepared Ni catalyst) and excellent stability with no detectable deactivation via sintering or coking. At 900oC, thermodynamic equilibrium conversion and selectivity is obtained (i.e. essentially complete methane conversion and 100% selectivity to syngas for a stoichiometric feed of CH4 and air) with stable operation for at least 10 hours time-on-stream and over multiple ignition-extinction cycles. Finally, the excellent coking resistance of the materials is reflected in stable operation of these catalyst even at very fuel rich conditions (CH4/O2=3).

Overall, these nanostructured Ni@SiO2 catalysts show great promise for application in CPOM and related fuel processing reactions. We are currently working on extending the synthesis approach to other metal@SiO2 nanostructured materials.