Lean-burn Natural Gas Vehicles Research Articles

The Impact of CeO2 Loading on the Activity and Stability of PdO/γ-AlOOH/γ-Al2O3 Monolith Catalysts for CH4 Oxidation

This study reports on the activity and stability of PdO/γ-AlOOH/γ-Al2O3 monolith catalysts, promoted with varying amounts of CeO2, for CH4 oxidation. Although the beneficial effects of CeO2 have been reported for powdered catalysts, this study used a cordierite (2MgO.2Al2O3.5SiO2) mini-monolith (400 cells per square inch, 1 cm diameter × 2.5 cm length; ~52 cells), washcoated with a suspension of γ-Al2O3 combined with boehmite (γ-AlOOH), followed by sequential deposition of Ce and Pd (0.5 wt.%) by wetness impregnation. The monolith catalysts’ CH4 oxidation activity and stability were assessed in the presence of CO, CO2, H2O and SO2 at low temperature (≤550 °C), relevant to emission control from lean-burn natural gas vehicles (NGVs). The CeO2 loading (0 to 4 wt.%) did not significantly impact the adhesion and thermal stability of the washcoat, but CeO2 reduced the inhibition of CH4 oxidation by H2O and SO2. The catalyst activity, measured by temperature-programmed methane oxidation (TPO) in a dry feed gas with 0.07 vol.% CH4, showed that adding CeO2 to the γ-AlOOH/γ-Al2O3 washcoat suppressed the activity of the catalysts; whereas, CeO2 improved the catalyst activity when H2O (2 and 5 vol.%) was present in the feed gas. Moreover, adding CeO2 decreased catalyst deactivation that occurred in the presence of 10 vol.% H2O and 5 ppmv SO2 at 500 °C, measured over a 25 h time-on-stream (TOS) period. The highest catalyst activity and stability for CH4 oxidation in the presence of H2O was obtained by adding 2 wt.% CeO2 to the washcoat.

Design of Stable Palladium-Based Zeolite Catalysts for Complete Methane Oxidation by Postsynthesis Zeolite Modification

Catalytic methane oxidation is used in exhaust gas aftertreatment to reduce methane emissions from lean-burn natural gas vehicles as well as in stationary combustion processes. Pd/zeolite catalysts...

The Effect of CH4 on NH3-SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas Vehicles

We present a systematic investigation of the deNOx activity of two commercial metal exchanged zeolite NH3-SCR catalysts, a Cu-SAPO and a Fe-BEA, in view of their application to the exhaust after-treatment systems of lean-burn natural gas vehicles. The catalytic activity data collected under realistic operating conditions, representative of the after-treatment system of lean-burn vehicles, were compared to those obtained adding methane to the gas feed stream in order to assess the impact of this hydrocarbon, which is usually emitted from natural gas engines, on the NH3-SCR catalytic chemistry. Our results indicate a negligible impact of methane on the SCR activity at all conditions, but in the presence of a large excess of NO2 at T > 400 °C due to methane oxidation by NO2. The data collected over the two individual metal-promoted zeolites were also compared with those obtained combining both catalysts in sequential arrangements, in order to take advantage of their complementary high activities in different temperature ranges. The Fe-zeolite + Cu-zeolite sequence outperformed the two individual components in terms of both overall deNOx efficiency and N2O selectivity, and was equally insensitive to methane.

Effect of MOx (M = Ce, Ni, Co, Mg) on activity and hydrothermal stability of Pd supported on ZrO2–Al2O3 composite for methane lean combustion

Zr0.5Al0.5O1.75 (ZA) was modified by metal oxide (Ce, Co, Ni or Mg oxide) via double solvent method. The Pd nanoparticles were supported on the modified ZA and then tested for methane lean combustion. The textural, structural and redox properties, surface Pd oxidation states and stability of the samples were investigated systematically. The modified catalysts were aged in the 750 °C humid air (10 vol.% water vapor) for 10 h, namely aging treatment. It was found that the activity and hydrothermal stability of the Pd/ZA catalyst was enhanced after the addition of NiO, MgO or Co3O4. Particularly, the Pd/NiO/Zr0.5Al0.5O1.75 (Pd/NiO/ZA) catalyst exhibited the best performance for methane combustion in the presence of water vapor, regardless of aging treatment. X-ray diffraction (XRD) patterns showed that the presence of spinel phase in the Pd/NiO/ZA. CO-chemisorption results demonstrated the highest Pd dispersion on the fresh and aged Pd/NiO/ZA. H2-temperature-programmed reduction (H2-TPR) measurements indicated that the Pd/NiO/ZA catalyst possessed the best reducibility regardless of aging treatment. X-ray photoelectron spectroscopy (XPS) analysis and temperature-programmed oxidation (TPO) experiments revealed that the Pd/NiO/ZA catalyst possessed the highest active phase PdO content and stability. Consequently, the Pd/NiO/ZA catalyst would be promisingly applied to exhaust after-treatment for lean-burn natural gas vehicles.

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

Palladium catalysts are supported on TiO2, ZrO2, Al2O3, Zr0.5Al0.5O1.75 and TiO2-Zr0.5Al0.5O1.75 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/TiO2-Zr0.5Al0.5O1.75 performs the best catalytic activity among the tested five catalysts. For CH4, the light-off temperature (T50) is 254 °C, and the complete conversion temperature (T90) is 280 °C; for CO, T50 is 84 °C, and T90 was 96 °C. Various techniques, including N2 adsorption-desorption, X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-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. N2 adsorption-desorption and SEM show that TiO2-Zr0.5Al0.5O1.75 expresses uniform nano-particles and large meso-pore diameters of 26 nm. H2-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 TiO2-Zr0.5Al0.5O1.75 are maxima among the five supports.

Effects of ZnO content on the performance of Pd/Zr0.5Al0.5O1.75 catalysts used in lean-burn natural gas vehicles

Mixed oxides obtained by adding varying amounts of ZnO into Zr0.5Al0.5O1.75 were used as supports to prepare 1.5% Pd catalysts. The catalyst activity and H2O tolerances of the catalysts were evaluated in a gas mixture simulating the exhaust of lean-burn natural gas-fueled vehicles. The catalysts were further characterized by N2 adsorption-desorption, X-ray diffraction, H2 temperature programmed reduction and X-ray photoelectron spectroscopy. The catalytic activity was greatly influenced by ZnO contents in Zr0.5Al0.5O1.75 support. The catalyst containing 15% ZnO exhibited a light-off temperature and complete conversion temperature for CH4 in the absence of H2O of 278 and 314 °C, respectively; while these same values in the presence of H2O were 342 and 371 °C, respectively. These results indicate that this combination of materials offers excellent catalytic activity at low temperature and also exhibits significant H2O tolerance.

Pd catalysts supported on modified Zr05Al05O1.75 used for lean-burn natural gas vehicles exhaust purification

Composite supports Zr0.5Al0.5O1.75 modified by metal oxides, such as La2O3, ZnO, Y2O3 or BaO, were prepared by co-precipitation method, and palladium catalysts supported on the modified composite supports were prepared by impregnation method. Their properties were characterized by X-ray diffraction (XRD), NH3 temperature-programmed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR), N2 adsorption/desorption, and CO-chemisorption. The catalytic activity and the resistance to water poisoning of the prepared Pd catalysts were tested in a simulated exhaust gas from lean-burn natural gas vehicles with and without water vapor. The results demonstrated that the modified supports had an apparent effect on the performance of Pd catalysts, compared with the Pd catalyst supported on the unmodified ZrAl. The addition of ZnO or Y2O3 promoted the conversion of CH4. In the absence of water vapor, Pd/ZnZrAl exhibited the best activity for CH4 conversion with the light-off temperature (T50) of 275 °C and the complete conversion temperature (T90) of 314 °C, respectively. However, in the presence of water vapor, Pd/YZrAl was the best one over which the light-off temperature (T50) of methane was 339 °C and the complete conversion temperature (T90) was 371 °C. These results indicated that Pd catalyst supported on the modified composite ZrAl support showed excellent catalytic activity at low temperature and high resistance to H2O poisoning for the exhaust purification of lean-burn natural gas vehicles.

Influence of Support Calcination Temperature on the Performance of Pd/Zr<SUB>0.5</SUB>Al<SUB>0.5</SUB>O<SUB>1.75</SUB> Catalyst Used in Lean-Burn Natural Gas Vehicles

Influence of Support Calcination Temperature on the Performance of Pd/Zr<SUB>0.5</SUB>Al<SUB>0.5</SUB>O<SUB>1.75</SUB> Catalyst Used in Lean-Burn Natural Gas Vehicles

Effect of cobalt oxide on performance of Pd catalysts for lean-burn natural gas vehicles in the presence and absence of water vapor

Pd-based catalysts modified by cobalt were prepared by co-impregnation and sequential impregnation methods, and characterized by X-ray powder diffraction (XRD), N 2 adsorption/desorption (Brunauer-Emmet-Teller method), CO-chemisorption and X-ray photoelectron spectroscopy (XPS). The activity of Pd catalysts was tested in the simulated exhaust gas from lean-burn natural gas vehicles. The effect of Co on the performance of water poisoning resistance for Pd catalysts was estimated in the simulated exhaust gas with and without the presence of water vapor. It was found that the effect of Co significantly depended on the preparation process. PdCo/La-Al 2O 3 catalyst prepared by co-impregnation exhibited better water-resistant performance. The results of XPS indicated that both CoAl 2O 4 and Co 3O 4 were present in the Pd catalysts modified by Co. For the catalyst prepared by sequential impregnation method, the ratio of CoAl 2O 4/Co 3O 4 was higher than that of the catalyst prepared by co-impregnation method. It could be concluded that Co 3O 4 played an important role in improving water-resistant performance.

The effect of CeO 2 and BaO on Pd catalysts used for lean-burn natural gas vehicles

Palladium catalysts supported on Al 2O 3 doped by BaO and/or CeO 2 were prepared by impregnation method. Various techniques, including N 2 adsorption (Brunauer–Emmet–Teller method; BET), X-ray diffraction (XRD), CO-chemisorption, H 2-temperature-programmed reduction (H 2-TPR) and X-ray photoelectron spectroscopy (XPS), were employed to characterize the effect of BaO and/or CeO 2 on the physicochemical properties of prepared catalysts. Catalytic activities for methane and CO oxidation were evaluated in a gas mixture simulated the exhaust from lean-burn natural gas vehicles (NGVs) . The BET results showed that the supports possessed high specific surface area, suitable pore volume and pore size distribution. The activity results indicated that the catalyst modified by CeO 2 showed the best performance for methane and CO removal, and the one modified by both BaO and CeO 2 the worst. CO-chemisorption and H 2-TPR results indicated that the addition of ceria or barium alone could increase the dispersion of palladium and improve the reducibility of catalysts. The results of XPS showed that BaO increased the electron density around PdO while CeO 2 activated the lattice oxygen and increased the content of oxygen species on the surface of catalysts. When BaO and CeO 2 were added simultaneously, their respective potential effect was restrained by each other resulting in the decrease of catalytic activity.

Palladium catalyst performance for methane emissions abatement from lean burn natural gas vehicles

As little as 1 ppm SO x present in the exhaust of a lean burn natural gas engine strongly inhibits the oxidation of CH 4 over a Pd containing catalyst. Non-methane emissions oxidation, such as C 2H 6, C 3H 8 and CO, are also inhibited by low SO x concentrations, but to a lesser extent than CH 4 emissions. The mechanism for SO x inhibition indicates a 1 : 1 selective adsorption of SO x on PdO for palladium on a non-sulfating support such as SiO 2. Deactivation is therefore very rapid. In contrast, palladium on sulfating supports, that is γ-Al 2O 3, deactivate more slowly and can tolerate more SO x because the SO x is also adsorbed onto the carrier. The activation energy for methane oxidation is dramatically increased after SO x poisoning for all Pd catalysts, while the Arrhenius pre-exponential term is relatively constant, indicating a transformation from very active PdO sites to less active PdO SO x sites. Platinum catalysts are considerably less active than Pd as evidenced by a much lower pre-exponential term, but are more resistant to deactivation by SO x . Non-methane hydrocarbon and particulate emissions standards for lean burn natural gas engines for the United States can be met with Pd catalysts. However, the non-enforced methane emissions standards are not met. For the European truck test cycle, methane emissions standards are met since the test cycle heavily weights the hotter modes where Pd SO x is sufficiently active.