Improved Selectivity and Stability in Methane Dry Reforming by Atomic Layer Deposition on Ni-CeO₂-ZrO₂/Al₂O₃ Catalysts

Ni can be used as a catalyst for dry reforming of methane (DRM), replacing more expensive and less abundant noble metal catalysts (Pt, Pd, and Rh) with little sacrifice in activity. Ni catalysts deactivate quickly under realistic DRM conditions. Rare earth oxides such as CeO₂, or as CeO₂-ZrO₂-Al₂O₃ (CZA), are supports that improve both the activity and stability of Ni DRM systems due to their redox activity. However, redox-active supports can also enhance the undesired reverse water gas shift (RWGS) reaction, reducing the hydrogen selectivity. In this work, Ni on CZA was coated with an ultrathin Al₂O₃ 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 the addition of the Al₂O₃ ALD overcoat, and improvements were subsequently confirmed in a high-conversion reactor at long times onstream. The overcoated samples gave an H₂/CO ratio of ∼1 at high conversion, much greater than uncoated catalysts, and no evidence of deactivation. Characterization of used (but still active) catalysts using several techniques suggests that active Ni is in formal oxidation state >0, Ni-Ce-Al is most likely present as a mixed oxide at the surface, and a nominal thickness of 0.5 nm for the Al₂O₃ overcoat is optimal.