Gold and Ceria Modified NiAl Hydrotalcite Materials as Catalyst Precursors for Dry Reforming of Methane

Abstract

Structured hydrotalcite NiAl-HT material with Ni/Al atomic ratio of 2.5 was prepared by co-precipitation of Ni and Al nitrate precursors and then modified by the addition of 1 wt% Ce and/or 3 wt% Au species. The obtained materials, after calcination at 600 °C, were characterized by XRD, XPS and TPR. Their catalytic performance was tested through dry reforming of methane (DRM) and by the temperature-programmed surface reaction of methane (TPSR-CH4). Thermal gravimetry analysis (TGA) of the spent catalysts was performed to determine the amount of carbon accumulated during the reaction. The effects of the addition of cerium as a support promoter and gold as nickel promoter and the sequential addition of cerium and gold on the structural properties and on the catalytic efficiency were investigated. Under the severe condition of high space velocity (600,000 mL g−1 h−1), all the catalysts were quite active, with values of CH4 conversion between 67% and 74% at 700 °C. In particular, the combination of cerium and gold enhanced the CH4 conversion up to 74%. Both additives, individually and simultaneously, enhanced the nickel dispersion with respect to the unpromoted NiAl and favored the reducibility of the nickel. During DRM all the catalysts formed graphitic carbon, contributing to their deactivation. The lower carbon gasification temperature of the promoted catalysts confirmed a positive effect played by Ce and Au in assisting the formation of an easier-to-remove carbon. The positive effect was testified by the better stability of the Ce/NiAl with respect to the other catalysts. In the gold-containing samples, this effect was neutralized by Au diffusing towards the catalyst surface during DRM, masking the nickel active sites. TPSR-CH4 test highlighted different CH4 activation capability of the catalysts. Furthermore, the comparison of the deposited carbon features (amount and removal temperature) of the DRM and TPSR spent catalysts indicated a superior activation of CO2 by the Au/Ce/NiAl, to be related to the close interaction of gold and ceria enhancing the oxygen mobility in the catalyst lattice.