Effect of la promotion on Ni/Mg-Al hydrotalcite derived catalysts for glycerol steam reforming

Abstract

The influence of lanthanum promotion on the physico-chemical properties and catalytic activity of Ni/Mg-Al catalysts in the glycerol steam reforming (GSR) reaction was studied. An Mg-Al and different Mg-Al-La supports with various molar ratios were synthesized using a co-precipitation technique. The calcined supports were then impregnated with 5 wt% Ni. After calcination at 600 °C, physico-chemical properties of the Ni-based catalysts were investigated by XRD, BET, H2-TPR, and CO2-TPD techniques. H2-TPR analyses showed that the quantity of lanthanum in the support composition influenced the reducibility of Ni species as well as their dispersion. CO2-TPD analyses showed that lanthanum addition improved catalyst basicity. The activity of all catalysts in the GSR reaction was tested (T = 400−700 °C, water/glycerol feed ratio 9:1, flow rate of 0.025 mL/min). The composition of the Ni/MAL0.4 (Ni/Mg6Al1.6La0.4; subscripted numbers indicate the molar ratio of each metal) catalyst provided a good metal-support interaction and Ni dispersion which translated to an overall superior performance in terms of hydrogen yields, glycerol conversion to gaseous products, and total glycerol conversions. TG-DSC analyses of the spent catalysts showed that the lanthanum quantity also influenced the quantity of coke deposition. The formation of encapsulating coke over all catalysts was linked to high concentrations of dehydration products observed in liquid product analysis. A stability test conducted at 600 °C for Ni/MAL0.4 revealed its deactivation due to encapsulating coke formation which resulted in active metal site blockage. With a reduced flow rate (0.008 mL/min), a different coke formation mechanism allowed the catalyst to last for 24 h.