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 o btain 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, a urichalcite and/or rosasite phases. Malachite and hydrozincite were de termined in binary Cu/Al and Cu/Zn samples, respectively. The precipit ates decomposed in three endothermic transformations in the temperatur e 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 simul taneously eliminated a proportion of the carbonate ions through a two- step dehydroxylation/decarbonation process. The high-temperature trans formation (Delta W=3-7%) corresponded to the final decarbonation of th e samples. Mixed oxides with a high dispersion of copper were obtained from hydrotalcite-containing precursors: the higher the amount of hyd rotalcite in the precursor, the lower the CuO crystallite size in the resulting mixed oxide.