Peiyan Lin

A Study on the Properties and Mechanisms for NOx Storage Over Pt/BaAl2O4-Al2O3 Catalyst

The NOx storage catalyst Pt/BaAl2O4-Al2O3 was prepared by a coprecipitation--impregnation method. For fresh sample, the barium mainly exists as the BaAl2O4 phase except for some BaCO3 phase. The BaAl2O4 phase is the primary NOx storage phase of the sample. EXAFS and TPD were used for investigating the mechanism of NOx storage. It is found that two kinds of Pt sites are likely to operate. Site 1 is responsible for NO chemisorption and site 2 for oxidizing NO to nitrates and nitrites. When NO adsorbs on the sample below 200 °C, it mainly chemisorbs in the form of molecular states. Such adsorption results in an increase of the coordination magnitude of Pt-O, and a decrease of that of Pt-Pt and Pt-Cl. The coordination distance of Pt-Pt, Pt-Cl and Pt-O also increases. When the adsorption occurs above 200 °C, NO can be easily oxidized by O2, and stored as nitrites or nitrates at the basic BaAl2O4. Site 2 is regenerated quickly. A high adsorption temperature is favorable for nitrate formation.

The catalytic removal of CO and NO over Co-Pt(Pd, Rh)/γ-Al2O3 catalysts and their structural characterizations

A series of Co-Pt(Pd, Rh)/γ- Al2O3 catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C2H4 over the samples calcined at 500°C and reduced at 450°C were determined. The activities of the samples calcined at 750°C and reduced at 450°C for NO selective catalytic reduction (SCR) by C2H4 were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/γ-Al2O3 for CO or C2H4 oxidation. For NO decomposition, the H2-reduced Co-Pt(Pd, Rh)/γ- Al2O3 catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C2H4, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/γ-Al2O3 has been reduced to zero valence during reduction by H2 at 450°C, but in Co/γ-Al2O3 only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl2O4-like spinel which is difficult to reduce. For the samples calcined at 750°C, the cobalt exists as CoAl2O4 which cannot be reduced by H2 at 450°C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/γ- Al2O3 has lower coordination symmetry than that in Co-Pt(Pd, Rh)/γ-Al2O3. This difference mainly results from the distorting tetrahedrally- coordinated Co2+ ions which have lower coordination symmetry than Co0 in the catalysts. The coordination number for the Co-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/γ- Al2O3 catalysts. The TPR results indicate that the addition of noble metals to Co/γ- Al2O3 makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co0 is the main active phase. For NO decomposition and selective reduction, Co0 is also catalytically active, but it can be oxidized into Co3O4 by oxygen at high reaction temperature.

Redox properties and catalytic behavior of praseodymium-modified (Ce-Zr)O2 solid solutions in three-way catalysts

The three-way catalyst promoters (Ce-Zr)O2, (Pr-Ce-Zr)O2 and (Pr-Zr)O2 were prepared by the sol-gel method. The reduction/oxidation behavior of these mixed oxides was compared. It is shown that the formation of (Pr-Zr)O2 cubic solid solution at high temperature up to 800 °C makes it more reducible, and that the ternary solid solution that formed in (Pr-Ce-Zr)O2 mixed oxides plays an important role in the reduction process. The catalytic performance tests reveal that the introduction of a small amount of praseodymium into (Ce-Zr)O2 favors the light-off temperature of C3H6 and NO and the effectiveness for NO conversion at the lean region.

A Novel Preparation Route of Three-Way Catalysts

A novel preparation route of TWC, which is combined with the synthesis of nanosized (Ce–Zr–M)O2 (M = La, Pr) solid solution by modified sol–gel method, is introduced. The TWC precursor shows high thermal stability and possesses high OSC. High thermally stable TWCs with this kind of precursors are also exhibited. The main reason is discussed.

Spillover effect and CO oxidation activity on the supported noble metals-metal oxides catalysts

The enhacement of CO oxidation activity on cobalt, manganese, copper(metal or oxide) supported on γ-Al 2 O 3 by a small amount of noble metals (Pt, Pd or Rh) has been observed. TPR and surface phases determination suggest that hydrogen spillover may occur on Co-Pt(Pd, Rh) and Mn-Pt in the process of H 2 reduction O 2 -TPD and TPO-MS results show that introducing a small amount of Pt, Pd or Rh into Co/ γ-Al 2 O 3 is beneficial to oxygen adsorption and oxidation reaction. Oxygen spillover may improve the CO oxidation process on these catalysts.