Studies of the activation process over Pd perovskite-type oxides used for catalytic oxidation of toluene

Calcined and reduced catalysts Pd/LaBO3 (B = Co, Fe, Mn, Ni) were used for the total oxidation of toluene. Easiness of toluene destruction was found to follow the sequence based on the T-50 values (temperature at which 50% of toluene is converted): Pd/LaFeO3 > Pd/LaMnO3+delta > Pd/LaCoO3 > Pd/LaNiO3. In order to investigate the activation process (calcination and reduction) in detail, the reducibility of the samples was evaluated by H-2-TPR on the calcined catalysts. Additionally, characterization of the Pd/LaBO3 (B = Co, Fe) surface was carried out by X-ray photoelectron spectroscopy (XPS) at each stage of the global process, namely after calcination, reduction and under catalytic reaction at either 150 or 200 degrees C for Pd/LaFeO3 and either 200 or 250 degrees C for LaCoO3. The different results showed that palladium oxidized entities were totally reduced after pre-reduction at 200 degrees C for 2 h (2 L/h, 1 degrees C/min). As LaFeO3 was unaffected by such a treatment, for the other perovskites, the cations B are partially reduced as B3+ (B = Mn) or B2+ even to B-0 (B = Co, Ni). In the reactive stream (0. 1 % toluene in air), Pd-0 reoxidized partially, more rapidly over Co than Fe based catalysts, to give a Pd2+/Pd4+ and Pd-0/Pd2+/Pd4+ surface redox states, respectively. Noticeably, reduced cobalt species are progressively oxidized on stream into Co3+ in a distorted environment. By contrast, only the lines characteristic of the initial perovskite lattice were detected by XRD studies on the used catalysts. The higher activity performance of Pd/LaFeO3 for the total oxidation of toluene was attributed here to a low temperature of calcination and to a remarkable high stability of the perovskite lattice whatever the nature of the stream which allowed to keep a same palladium dispersion at the different stages of the process and to resist to the oxidizing experimental conditions. On the contrary, phase transformations for the other perovskite lattices along the process were believed to increase the palladium particle size responsible of a lower activity. (c) 2007 Elsevier B.V. All rights reserved.