(532bq) Importance of Precise Nickel Siting and Dispersion in Supported Nickel Dry Reforming Catalysts

The dry reforming of methane (DRM) is a potential route to convert greenhouse gases (CO₂ and CH₄) to syngas (H₂ and CO). Supported Ni catalysts are commonly employed to take advantage of the chemical activity and earth abundance. Still, most supported Ni catalysts, especially those using the rare-earth oxides, will eventually deactivate either due to coking at lower temperatures (<700°C) or sintering at higher ones. Such deactivation becomes more of a problem at total pressures higher than atmospheric. At the same time, the reducible nature of the CeO_x or other rare earth oxide facilitates the reverse water gas shift reaction, lowering the H₂:CO ratio. In this project, we are using hierarchical catalysts to try to limit deactivation mechanisms and inhibit the RWGS reaction by coupling reducible (CeO_x) supports and more non-reducible (Al₂O₃, MgO, SiO₂) thin overlayers.

We have used a commercial Ce_0.5Zr_0.5O₂ support (PIDC CZA-40) with 7 mol% Ni deposited on the surface. Next, atomic layer deposition (ALD) was used to deposit thin (<2 nm) porous layers of Al₂O₃ over the CZA-40 catalyst. The coating greatly improved the stability of the sample, with high activities present after over 100 h, whereas the uncoated catalyst quickly coked over 12 h. The hierarchical catalyst activity was dependent on the thickness of the films, with optimum performance achieved at between 5-7 layers, i.e., 70-80% methane conversion over 80 h. HRTEM and TPO analyses of the uncoated and coated samples indicated similar amounts of coking. However, Ni aggregation and filamentous coke were severely inhibited by the thin shell layers. Interestingly, the thin layers were also able to completely inhibit the RWGS reaction for some time before eventually H₂:CO ratios above 1 and high coking rates were observed. Additionally, we will also discuss the significant induction period often observed with these catalysts.