Showing papers by "Mika Valden published in 2001"

Effect of Ce-Zr mixed oxides on the chemical state of Rh in alumina supported automotive exhaust catalysts studied by XPS and XRD

Abstract: Several γ-Al2O3 supported Rh catalysts containing Ce–Zr mixed oxides in various proportions were prepared by different manufacturing methods. Both fresh and aged catalysts were investigated, as well as Rh/Ce–Zr mixed oxide powders. Aging was performed in air at 1000°C for 3 h. Disposition of the Rh-oxide formed in the aging towards reduction was investigated by exposing the catalysts to 400 mbar of H2 at 300°C for 30 min. Rhodium supported on pure alumina was found to sinter easily and react irreversibly with the alumina support at elevated temperatures to form an irreducible oxide phase. On the contrary, rhodium supported on Ce–Zr mixed oxide was observed to remain in a reducible form also in high temperature aging. It was also found that Ce–Zr mixed oxide is capable of stabilizing Rh particles against sintering at elevated temperatures. However, incorporation of Ce–Zr mixed oxide to γ-Al2O3 supported catalysts was found to be ineffective in preventing the reaction between rhodium and alumina. After 3 h aging in air at 1000°C an irreducible rhodium oxide, which is likely RhO2 in intimate contact with alumina surface, was formed regardless of the type of Ce–Zr mixed oxide additive and manufacturing method of the catalyst.

Characterization of alumina supported Pd catalysts modified by rare earth oxides using X-ray photoelectron spectroscopy and X-ray diffraction: enhanced thermal stability of PdO in Nd/Pd catalysts

Abstract: Thermal stability of palladium oxides on industrially manufactured alumina supported catalysts modified by Nd, La and Ba oxides was investigated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Catalysts were prepared by mechanical mixing of washcoat material with Pd precursor compound. A stepwise thermal reduction in high vacuum was performed in order to investigate the effect of noble metal loading, stabilizers and aging on the thermal stability of PdO. One of the pseudo-binary compounds of Nd2O3/PdO, Nd4PdO7, was successfully manufactured and characterized by XRD and XPS. The presence of Nd4PdO7 as a surface compound was identified by its unusually high binding energy of the Pd 3d5/2 photoelectron peak (337.6 eV). PdO formed as a result of the decomposition of the alumina supported Nd4PdO7 upon aging in air at 950°C exhibited clearly higher thermal stability than pure PdO supported on alumina. La, Ba and Nd oxides were also found to enhance the thermal stability of PdO.

Adsorption and thermal behavior of CO and NO on Pd{110} and Pd{320}

Abstract: Chemisorption and thermal behavior of CO and NO on Pd{110} and its vicinal surface, Pd{320} [Pd(S)[3(110)×(100)]], have been investigated using molecular beam surface scattering (MBSS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). CO is shown to adsorb molecularly on Pd{320} in the temperature range of 250–500 K. No evidence for long-range ordering of CO adlayers on Pd{320} was observed. Saturation coverage for CO/Pd{320} at 300 K is 0.61 ML. The initial sticking probability of CO is 0.91 and is shown to depend only weakly on the surface temperature. It is also found that the initial sticking probability of CO on Pd{110} exhibits a strong dependence on the kinetic energy of CO molecules and the CO adsorption on Pd{320} may also exhibit this same behavior. CO desorbs molecularly from Pd{320} with desorption features near 390 and 500 K. NO is shown to adsorb molecularly below 420 K with a high initial sticking probability ...