Thermal decomposition of Cu-based hydroxycarbonate catalytic precursors for the low-temperature CO-shift reaction

The thermal decomposition of Cu-Zn-Al hydroxycarbonate precursors to obtain water-gas shift catalysts was studied by employing a variety of experimental techniques. A set of six samples containing 34 wt% of Cu and different Al/Zn ratios were prepared by coprecipitation. Depending on the cation ratio, the ternary precursors contained hydrotalcite, aurichalcite and/or rosasite phases. Malachite and hydrozincite were determined in binary Cu/Al and Cu/Zn samples, respectively. The precipitates decomposed in three endothermic transformations in the temperature ranges 363-453 K, 453-673 K and 673-923 K. In the first step (Delta W=0-9%), the hydrotalcite-containing samples lost the crystallization water of the hydrotalcite phase. In the middle-temperature transition (Delta W=18-30%), the samples were completely dehydroxylated and simultaneously eliminated a proportion of the carbonate ions through a two-step dehydroxylation/decarbonation process. The high-temperature transformation (Delta W=3-7%) corresponded to the final decarbonation of the samples. Mixed oxides with a high dispersion of copper were obtained from hydrotalcite-containing precursors: the higher the amount of hydrotalcite in the precursor, the lower the CuO crystallite size in the resulting mixed oxide.