III International Conference

PP-II-138adsorbed linearly on Pd 1+ and Pd 2+ were identified on the oxidized catalysts. Being in thesamples with 1.6 and 8 wt % ZrO 2 where more Pd 1+ could be stabilized.Figure 1. IR spectra of Pd/Al 2 O 3 -ZrO 2 samples pretreated in O 2at 550 o C. CO adsorbed at -120 o C.CO oxidation was used as a model reaction. The comparison of the different catalysts wasdone by ramping the temperature up and down. A typical set of curves is shown in thefigure 2 for the catalyst 0, 8 and 44 wt % ZrO 2 . It is seen that activity increase for the 8 wt %sample, compared to the 0 % sample. Then for high zirconia contents, 44 wt %, the activitygoes down significantly. The above results can be explained by the combination of chemicaland physical changes induced by the addition of zirconia to support, changes includingaverage Pd particle size and its oxidation state.1.0 T 50=175 o C0.80.60.4Pd/Al-Zr(0)0.20.00 50 100 150 200 250Temperature, °C1.00.80.60.40.20.0T 50=102-119 °CPd/Al-Zr(8)0 50 100 150 200 250Temperature, °CPd/Al-Zr(44)0.00 50 100 150 200 250Figure 2. CO oxidation curves for Pd/Al 2 O 3 -ZrO 2 (0, 8 and 44).1.00.80.60.40.2T 50= 210-200 °CTemperature, °CReferences1. J. C. Summers and W. B. Williamson, Palladium-only Catalysts for closed-loop control,American Chemical Society, 1994.2. A. Barrera et al, Appl Catal B-Environ 34 2 (2001) 97.3. Tiznado et al, Langmuir 20 (2004) 10490.4. Perez-Osorio et al, Applied Catalysis B: Environmental 69 (2006) 219.416