Haidi Xu

Excellent complete conversion activity for methane and CO of Pd/TiO2-Zr0.5Al0.5O1.75 catalyst used in lean-burn natural gas vehicles

Abstract Palladium catalysts are supported on TiO 2 , ZrO 2 , Al 2 O 3 , Zr 0.5 Al 0.5 O 1.75 and TiO 2 -Zr 0.5 Al 0.5 O 1.75 prepared by co-precipitation method, respectively. Catalytic activities for methane and CO oxidation are evaluated in a gas mixture that simulated the exhaust from lean-burn natural gas vehicles (NGVs). Pd/TiO 2 -Zr 0.5 Al 0.5 O 1.75 performs the best catalytic activity among the tested five catalysts. For CH 4 , the light-off temperature ( T 50 ) is 254 °C, and the complete conversion temperature ( T 90 ) is 280 °C; for CO, T 50 is 84 °C, and T 90 was 96 °C. Various techniques, including N 2 adsorption-desorption, X-ray diffraction (XRD), H 2 -temperature-programmed reduction (H 2 -TPR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) are employed to characterize the effect of supports on the physicochemical properties of prepared catalysts. N 2 adsorption-desorption and SEM show that TiO 2 -Zr 0.5 Al 0.5 O 1.75 expresses uniform nano-particles and large meso-pore diameters of 26 nm. H 2 -TPR and XRD indicate that PdO is well dispersed on the supports and strongly interacted with each other. The results of XPS show that the electron density around PdO and the proportion of active oxygen on TiO 2 -Zr 0.5 Al 0.5 O 1.75 are maxima among the five supports.

Effect of the calcination temperature of cerium–zirconium mixed oxides on the structure and catalytic performance of WO3/CeZrO2 monolithic catalyst for selective catalytic reduction of NOx with NH3

A series of WO3/CeZrO2 catalysts, prepared at different calcination temperatures (400, 500, 600 and 700 °C) of cerium–zirconium mixed oxides (CeZrO2) for the selective catalytic reduction of NOx with ammonia (NH3-SCR), were investigated via various characterizations, such as N2 physisorption, XRD, Raman, NH3-TPD, DRIFTS, XPS and H2-TPR. The catalytic performance of NH3-SCR was remarkably promoted by modestly increasing the calcination temperature of CeZrO2: WO3/CeZrO2-500 possessed the lowest light-off temperature (173 °C) and complete conversion temperature (205 °C), while W/CeZrO2-600 could achieve greater than 90% NOx conversion in a broad temperature range of 220–455 °C. The characterization results indicated that modest enhancement of the calcination temperature of CeZrO2 was beneficial to stabilizing the structure of the catalysts. The largest amount of Lewis acid sites, Ce3+ and surface active oxygen species, as well as strong redox properties of WO3/CeZrO2-500 should together contribute to its better low-temperature deNOx activity. Moreover, increasing the calcination temperature of cerium–zirconium mixed oxides resulted in the enhancement of Bronsted acid sites, which was responsible for the widened operation temperature window. Therefore, WO3/CeZrO2 serial catalysts with appropriate calcination treatment of CeZrO2 would be a good choice for the removal of NOx emitted from diesel engines.

Promotional effects of Zr on K + -poisoning resistance of CeTiO x catalyst for selective catalytic reductionof NO x with NH 3

CeTiO x and CeZrTiO x catalysts were prepared by a coprecipitation method and used for selective catalytic reduction of NO x by NH 3 (NH 3 -SCR). Various amounts of KNO 3 were impregnated on the catalyst surface to investigate the effects of Zr addition on the K + -poisoning resistance of the CeTiO x catalyst. The NH 3 -SCR performance of the catalysts showed that the NO x removal activity of the Zr-modified catalyst after poisoning was better than that of the CeTiO x catalyst. Brunauer-Emmett-Teller data indicated that the Zr-containing catalyst had a larger specific surface area and pore volume both before and after K + poisoning. X-ray diffraction, Raman spectroscopy, and transmission electron microscopy showed that Zr doping inhibited anatase TiO 2 crystal grain growth, i.e., the molten salt flux effect caused by the loaded KNO 3 was inhibited. The Ce 3 d X-ray photoelectron spectra showed that the Ce 3+ /Ce 4+ ratio of CeZrTiO x decreased more slowly than that of CeTiO x with increasing K + loading, indicating that Zr addition preserved more crystal defects and oxygen vacancies; this improved the catalytic performance. The acidity was a key factor in the NH 3 -SCR performance; the temperature-programmed desorption of NH 3 results showed that Zr doping inhibited the decrease in the surface acidity. The results suggest that Zr improved the K + -poisoning resistance of the CeTiO x catalyst.

Promotional effect of cobalt addition on catalytic performance of Ce0.5Zr0.5O2 mixed oxide for diesel soot combustion

A series of Co-modified Ce0.5Zr0.5O2 catalysts with different concentrations of Co (mass %: 0, 2, 4, 6, 8, 10) was investigated for diesel soot combustion. Ce0.5Zr0.5O2 was prepared using the coprecipitation method and Co was loaded onto the oxide using the incipient wetness impregnation method. The activities of the catalysts were evaluated by thermogravimetric (TG) analysis and temperature-programmed oxidation (TPO) experiments. The results showed the soot combustion activities of the catalysts to be effectively improved by the addition of Co, 6 % Co/Ce0.5Zr0.5O2 and that the 8 % Co/Ce0.5Zr0.5O2 catalysts exhibited the best catalytic performance in terms of lower soot ignition temperature (Ti at 349°C) and maximal soot oxidation rate temperature (Tm at 358°C). The reasons for the improved activity were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). These results revealed that the presence of Co could lower the reduction temperature due to the synergistic effect between Co and Ce, thereby improving the activity of the catalysts in soot combustion. The 6 % Co catalyst exhibited the best catalytic performance, which could be attributed to the greater amounts of Co3+ and surface oxygen species on the catalyst.

Promotional effect of niobium substitution on the low-temperature activity of a WO3/CeZrOx monolithic catalyst for the selective catalytic reduction of NOx with NH3

A series of Nb-substituted WO3/CeZrOx catalysts were prepared by the co-impregnation method and applied in the selective catalytic reduction of NOx with NH3 (NH3-SCR). NH3 oxidation, N2 sorption, XRD, Raman, UV-vis, XPS, H2-TPR, O2/NH3-TPD and in situ DRIFTS were performed to correlate the redox property and surface acidity to NH3-SCR performance of Nb-substituted catalysts. The catalyst with 5 wt% substitution amount of Nb2O5 presented excellent deNOx activity and N2 selectivity in a broad reaction temperature window of 190–434 °C at a gas space velocity of 30 000 h−1. The characterization results demonstrated that the partial substitution of WO3 by Nb2O5 not only led to strong redox properties arising from abundant surface active oxygen species, but also promoted the adsorption of NH3 and the redistribution of acid sites due to Nb–OH related to Bronsted acid sites and NbO bonded to strong Lewis acid sites. The enhancement of surface active oxygen species and Bronsted acid sites promoted the low-temperature (below 250 °C) deNOx activity. However, the preoxidation of NH3 at high temperatures slightly suppressed the NOx conversion of the catalyst with more strong Lewis acid sites at above 400 °C. Moreover, the catalyst also showed excellent sulfur tolerance and could be a promising candidate for practical applications in NOx abatement.