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

Structured hydrotalcite NiAl-HT material with Ni/Al atomic ratio of 2.5 was preparedby co-precipitation of Ni and Al nitrate precursors and then modified by the addition of 1 wt% Ceand/or 3 wt% Au species. The obtained materials, after calcination at 600 C, were characterizedby 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). Thermalgravimetry analysis (TGA) of the spent catalysts was performed to determine the amount of carbonaccumulated during the reaction. The effects of the addition of cerium as a support promoter andgold as nickel promoter and the sequential addition of cerium and gold on the structural propertiesand 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 conversionup to 74%. Both additives, individually and simultaneously, enhanced the nickel dispersion withrespect to the unpromoted NiAl and favored the reducibility of the nickel. During DRM all thecatalysts formed graphitic carbon, contributing to their deactivation. The lower carbon gasificationtemperature of the promoted catalysts confirmed a positive effect played by Ce and Au in assistingthe formation of an easier-to-remove carbon. The positive effect was testified by the better stabilityof the Ce/NiAl with respect to the other catalysts. In the gold-containing samples, this effect wasneutralized by Au diffusing towards the catalyst surface during DRM, masking the nickel activesites. 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 andTPSR spent catalysts indicated a superior activation of CO2 by the Au/Ce/NiAl, to be related to theclose interaction of gold and ceria enhancing the oxygen mobility in the catalyst lattice.