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

Abstract

The use of Ni in the dry reforming of methane (DRM) has been widely studied as a replacement for noble metal catalysts (Pt, Pd, Rh) due to Ni’s low cost compared to noble metals, its abundance, and the fact that its DRM activity is near that of noble metal catalysts. However, Ni has been shown to deactivate quickly under DRM conditions. Rare earth oxides such as CeO2, or as CeO2-ZrO2 (CZO) are supports that improve both the activity and stability of Ni DRM systems due to their redox activity. However, this same activity is thought to enhance the undesired reverse water gas shift (RWGS) reaction, reducing the hydrogen selectivity. In this work, Ni:CZA nanoparticles were coated with an Al2O3­ overlayer using an atomic layer deposition (ALD) method to study the effect an Al2O­3 overlayer would have on the catalysts: their activity, stability, and H2/CO ratio, when compared to an uncoated reference. A low conversion screening method showed an improvement in DRM activity and coking rate upon the addition of an Al2O3 ALD overcoat, and this improvement was also seen in a higher conversion lab scale reactor. The overcoated sample displayed an ~1 H2/CO ratio in the high conversion reactor. Analysis of X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) showed that the active Ni oxidation state was +2 and that Ni-Ce-Al were most likely present as mixed oxides in the overlayer level of the structure.