The Effect of Potassium Addition on the Surface Chemical Structure and Activity of Supported Iron

Abstract

The surface activity of a Fe–K/ZrO2sample prepared from iron citrate and potassium carbonate has been investigated by the FTIR spectroscopy of adsorbed CO and NO and compared with the behavior of a Fe/ZrO2sample. The addition of potassium produces a strong decrease of the BET area and of the amounts of the adsorbed molecules. At the same time the spectrum of the sample reduced with H2at 573–773 K shows, under anhydrous conditions, the presence of small patches of reduced Fe0, not easily formed on Fe/ZrO2under the same thermal and reducing conditions. The formation of new phases containing iron and potassium in close combination, such as mixed iron–potassium oxalates and potassium ferrites that fully cover the zirconia surface, was inferred from FTIR data. CO adsorption favors the partial surface reduction of these phases producing small Fe-carbonyl clusters in tight contact with potassium and Fe,K-carboxylate surface species. The adsorption of NO, producing nitrosyl species on the reduced iron particles and nitrite groups on the potassium phase is in good agreement with the results of adsorbed CO. These results are confirmed by quantitative determinations of adsorbed gases and by TPR experiments.