(449f) Improved Selectivity and Stability in Methane Dry Reforming By Atomic Layer Deposition Onto Ni-CeO2-ZrO2/Al2O3 Catalysts

The dry reforming of methane (DRM) is a potential route to convert greenhouse gases (CO₂ and CH₄) to syngas (H₂ and CO) used as building blocks for petrochemicals. 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, by multiple mechanisms. At the same time, the reducible nature of the CeO_x, or other rare earth oxides, facilitates the reverse water gas shift reaction, lowering the H₂:CO ratio. In this project, we are using hierarchical catalysts to attempt to limit deactivation mechanisms and inhibit the RWGS reaction by coupling reducible (CeO_x) supports and more non-reducible (Al₂O₃, MgO, SiO₂) thin overlayers.

In this work, Ni on CZA (CeO₂-ZrO₂) was coated with an ultrathin Al₂O_3­ overlayer using atomic layer deposition (ALD) to study the effects of the overlayer on catalyst activity, stability, and H₂/CO ratio. A low conversion screening method revealed improved DRM activity and lower coking rate upon addition of the Al₂O₃ ALD overcoat, improvements subsequently confirmed in a high conversion reactor at long times onstream. The overcoated samples gave ~1 H₂/CO ratio at high conversion, greatly superior to uncoated catalysts, and no evidence of deactivation. Characterization of used (but still active) catalysts using several techniques suggest active Ni is in a formal oxidation state >0 under reaction conditions, that Ni-Ce-Al is present as a mixed oxide at the surface, and that a nominal initial thickness of 0.5 nm for the Al₂O₃ overcoat is optimal.

We will also present work documenting the effects of water feed staging in DRM catalysis.