Catalytic oxidation of chlorobenzene over Pd/perovskites

The catalytic performances of pre-reduced palladium catalysts supported on lanthanum based perovskites LaBO3 (B = Co, Mn, Fe, Ni) were investigated for the total oxidation of chlorobenzene (PhCl; 1000 ppmv) in air. The catalysts were prepared using a wet impregnation technique and Pd-nitrate was used as a palladium precursor. The catalytic performances were compared to those of a reference palladium catalyst supported on a conventional support, namely γ-Al2O3. Easiness of chlorobenzene destruction was found to follow the sequence based on the T50 values (temperature at which 50% of chlorobenzene was converted into products): Pd/LaMnO3+δ (243 °C) > Pd/LaFeO3 (270 °C) > Pd/Al2O3 (348 °C) > Pd/LaCoO3 (360 °C) > Pd/LaNiO3 (408 °C). Complete conversion of chlorobenzene is reached at ca. 320–500 °C, but at those temperatures substantial amounts of polychlorinated benzenes are formed. Quasi in situ XPS studies were monitored on Pd/LaCoO3 and Pd/LaFeO3 after each stage of the global process, namely after calcination, reduction and exposure to the flowing reactive mixture (1000 ppmv PhCl in air) from room temperature to 230 and 310 °C (Pd/LaFeO3) and to 280 °C (Pd/LaCoO3). It was shown that the calcination treatment leads to a palladium which a BE higher than that of PdO and to a (B/La)XPS < 1 which attests of a lanthanum enrichment at the XPS surface. After H2 treatment it was shown that palladium is totally reduced while the B cation is either unreduced (Fe3+) or reduced (Co3+ into Co2+ and Co0). In the reactive atmosphere, Pd0 is progressively (oxi)chlorined while the perovskite network is reconstructed with productions of LaOCl and Co3O4. The pre-reduced Pd/LaBO3 are more active than the perovskite alone for PhCl transformation but substantially increase the chlorination rate of PhCl. Among the different catalysts Pd/LaFeO3 shows the best compromise between PhCl oxidation and chlorination rates.