The preparation and properties of lanthanum-promoted nickel-alumina catalysts:Structure of the precipitates

Precursors of La-promoted Ni-alumina catalysts have been prepared by precipitation from their nitrate solutions at pH 7 using solutions of NH4HCO3, Na2CO3 or K2CO3. The preparation was carried out either by coprecipitation from a mixed salt solution or by sequential precipitation of Al3+, La3+ and Ni2+ in succession. In the absence of promoter, the precipitate with Ni/Al ratio of 2.5 is of the pyroaurite structure and has the composition Ni5Al2(OH)14CO3.4H2O. Two types of lanthanum-containing precipitate were made in which either extra La was added (Ni/Al kept constant at 2.5) or the proportion Ni/(Al+La) was kept constant at 2.5. The majority of these precipitates were single compounds which also had the pyroaurite structure. At high La contents, the series in which La is added gives separation of the compounds La2O(CO3)2 and LaONO3 in addition to the layer structure; with the series in which the La is substituted for Al, all the samples appeared to have the pyroaurite structure, even one in which no Al was present. The sequential precipitation route yields smaller crystallites than does coprecipitation. Materials precipitated with NH4HCO3 in all cases contained NH4NO3 while those precipitated with Na2CO3 gave inclusion of NaNO3. In both cases, the presence of the nitrates causes a decrease of crystallinity of the layer compound. Potassium is not included in the precipitate in any of the samples examined. A model is presented for the structure of the lanthanum-containing precipitates