Yilu Fu

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.

EXAFS study of γ-Mo2N and Mo nitrides supported on zeolites

Abstract By analysis of the extended X-ray absorption fine structure (EXAFS) of the Mo K-absorption edge, the structural information of both oxidic and nitrided Mo catalysts supported on zeolites was obtained. The supported MoO 3 samples show three peaks in the radial structure function (RSF). The first two peaks represent the nearest Mo–O coordination shell and the ratio of the first peak to the second one is much larger than that of the MoO 3 crystallites, which suggests that the MoO 3 supported on zeolites has a compact structure than the unsupported one. Three peaks on the RSF of γ-Mo 2 N represent one Mo–N and two Mo–Mo coordination shells, respectively. This is in good agreement with the face-centered cubic model. Calculation from X-ray diffraction data and EXAFS spectra reveals that the N atoms in γ-Mo 2 N lengthen the distance between Mo atoms and weaken the Mo–Mo bond. Supported Mo 2 N samples give out nearly the same RSFs as the γ-Mo 2 N and only the peak corresponding to Mo–N shell is weak. This suggests the supported Mo 2 N has more configurational disorder.

A highly active K-Co-Mo/C catalyst for mixed alcohol synthesis from CO + H2

A highly homogeneous and dispersed K-Co-Mo/C catalyst which prepared by a sol-gel method exhibits high alcohol yield, especially high C2+ OH selectivity for mixed alcohol synthesis from CO + H2.

Effect of Steam in CO2 Reforming of CH4over a Ni/CeO2–ZrO2–Al2O3 Catalyst

CO2 reforming of CH4 was carried out with and without steam over a Ni/CeO2–ZrO2–Al2O3 catalyst. The catalytic performance, amount of carbon deposit and the EXAFS of the Ni K-edge of samples were measured. The results show that when the catalyst is used for CO2 reforming of CH4 without the addition of steam, the catalyst gradually deactivates, however, addition of a small amount of steam to the feed gas can significantly inhibits the deactivation, which is due to the great suppression of coke formation on the catalyst during the reaction. The EXAFS result shows that, maybe due to the penetration of more carbon atoms into the Ni lattice, the coordination number of the nearest Ni–Ni of the sample after the reaction without steam reduces more than that of samples after the reaction with a small amount of steam in the feed gas.

The adsorption characteristics of two Mo sites on a sulfided MoO3/γ-Al2O3 surface

The surface structure of the oxidized and ammonia dissolved MoO3/γ-Al2O3 samples and the adsorption characteristics of the sulfided samples were investigated by Laser Raman Spectroscopy (LRS) and Low Temperature Infrared Spectroscopy (LTIR) techniques.It was verified that there were two kinds of coordinated unsaturated Mo sites (denoted as MoA(CUS) and MoB(CUS)) on the surface and the precursors of MoA(CUS) and MoB(CUS) were Mo(O) and Mo(T), respectively. It was also concluded that MoA(CUS) sites could adsorb NO and CO, while MoB(CUS) could adsorb CO only. The surface concentration of MoA(CUS) might be far smaller than that of MoB(CUS).

Effect of CeO2 on supported Pd catalyst in the SCR of NO: a DRIFT study

The activity of Pd/Al2O3 and Pd/Al2O3–CeO2 samples has been tested in the selective catalytic reduction of NO by propene. It is found that the activity of Pd/Al2O3 decreases with calcination temperature, while the activity of Pd/Al2O3–CeO2 increases abnormally with increasing calcination temperature. Surface-area measurement shows both samples suffer a linear decrease in their surface area, so it is reasonable to attribute the activity enhancement to the effect of CeO2. The adsorption behavior and state of surface-active sites have been characterized by diffuse reflectance FTIR spectroscopy using CO and NO as probes and the effect of CeO2 has been revealed. The CeO2 component increases and stabilizes the dispersion of surface Pd species to prevent it from aggregating at high temperature. CeO2 may also act as a buffer during the redox cycle of Pd, lengthen the period of Pd redox procedure and render Pd a property of “inertia” in its redox process, thus increasing the activity of the Pd/Al2O3–CeO2 sample. The essential feature of both effects is the strong interaction between Pd and CeO2. The intensity of interaction increases linearly with calcination temperature and so does the sample activity.

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.