A study of the behaviour of Pt supported on CeO2–ZrO2/Al2O3–BaO as NO storage–reduction catalyst for the treatment of lean burn engine emissions

Shape dependence of support for NOx storage and reduction catalysts

Pt/BaO/Al₂O₃ catalysts with different BaO loadings prepared from Al₂O₃ nanorods (Pt/BaO/Al₂O₃-nr) and irregular Al₂O₃ nanoparticles (Pt/BaO/Al₂O₃-np) were investigated for NO_x storage and reduction (NSR). The Pt/BaO/Al₂O₃ materials derived from Al₂O₃ nanorods always exhibited much higher NO_x storage capacity (NSC) over the whole temperature range of 100-400°C than the corresponding Pt/BaO/Al₂O₃-np samples containing the same BaO loading, giving the maximum NSC value of 966.9 μmol/g_cat at 400°C, 1.4 times higher than that of Pt/BaO/Al₂O₃-np. Higher catalytic performance of nanorod-supported NSR samples was also observed during lean-rich cyclic conditions (90 sec vs. 5 sec), giving more than 98% NO_x conversion at 300-450°C over the Pt/BaO/Al₂O₃-nr sample with 15% BaO loading. To reveal this dependence on the shape of the support during the NSR process, a series of characterization techniques including the Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂ temperature programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were also conducted. It was found that intimate contact of Ba-Al and Ba-Pt sites was achieved over the Pt/BaO/Al₂O₃ surface when using Al₂O₃-nr as a support. This strong interaction among the multi-components of Pt/BaO/Al₂O₃-nr thus triggered the formation of surface nitrite and nitrate during the lean period, and also accelerated the reverse spillover of ad-NO_x species onto the Pt surface, enhancing their reduction and leading to high NSR performance.

Storage and reduction of NOx by combining Sr-based perovskite catalyst with nonthermal plasma

A novel NO_x storage and reduction (NSR) system is developed for NO_x removal by integrating Sr-based perovskite catalyst with nonthermal plasma (NTP)-assisted process. In this hybrid system, Sr-based perovskite catalyst is applied for NO_x adsorption in the lean-burn condition while NTP is used as a desorption-reduction step to convert NO_x into N₂ under rich-burn condition. Innovative Sr-based perovskites including SrKMnCoO₄/BaO/Al₂O₃ (SKMCBA), SrKMnCeO₄/BaO/Al₂O₃ (SKMCeBA), and SrKCoNiO₄/BaO/Al₂O₃ (SKCNBA) are successfully prepared by impregnation method. Results indicate that SKMCBA possesses the highest NO_x trapped (214 μmole NO_x/g_catalyst) at 400 °C among 3 Sr-based perovskites investigated. High performance of SKMCBA for NO_x adsorption is mainly attributed to the addition of Mn and Co which own good oxidation ability. Further, SKMCBA is combined with NTP-assisted process for NO_x reduction. Result indicates that NO_x conversion achieved with NTP-assisted process reaches 83% with the applied voltage of 18 kV and frequency of 10 kHz in the absence of reducing agent. Additionally, various reducing agents including hydrogen (H₂), carbon monoxide (CO), and propene (C₃H₆) are introduced, individually, into the NTP reduction process, and the results indicate that performance of NSR with NTP can be effectively enhanced. Especially, 100% NO_x conversion is achieved with H₂-NTP. This study demonstrates that reduction of NO_x via NTP-assisted process is promising.

LaCoO3 perovskite in Pt/LaCoO3/K/Al2O3 for the improvement of NOx storage and reduction performances

A series of x wt% Pt/LaCoO₃/K/Al₂O₃ (x = 0, 0.3 and 1.0) were synthesized for NOx storage and reduction. 0.3 wt% Pt/LaCoO₃/K/Al₂O₃ shows higher NOx storage capacities, stronger SO₂ tolerance and better regenerability than 1.0 wt% Pt/K/Al₂O₃.

DRIFTS study of the role of alkaline earths in promoting the catalytic activity of HC and NOx conversion over Pd-only three-way catalysts

Modification with alkaline earths, especially Ba, promotes NO_x storage efficiency and NO dissociation, as well as the deep oxidation of HC. Thus doped catalysts present improved catalytic activity.

Insights into the role of a structural promoter (Ba) in three-way catalyst Pd/CeO2–ZrO2 using in situ DRIFTS

Doping with Ba, especially when the content is 5 wt.%, accelerates the dissociation of NO_x and facilitates the formation of intermediates due to the outstanding electron-donating ability of Ba.

Promoting effect of alkaline earth metal doping on catalytic activity of HC and NOx conversion over Pd-only three-way catalyst

The influence of alkaline earth metal (M=Mg, Ca, Sr and Ba) promoter on the structural/textural properties of Ce0.67Zr0.33O2 (designated as CZ) and the catalytic behavior of its supported Pd-only three-way catalyst (Pd/CZM) have been investigated. The results show that the modification with alkaline earth metal obviously improves the catalytic activity for hydrocarbon (HC) and nitrogen oxides (NOx) conversion, especially the introduction of Ba. Furthermore, the operation window of the promoted catalysts has also been widened. The doping of alkaline earth metal leads to the formation of more homogeneous Ce-Zr-M ternary solid solution with higher surface area and smaller crystallite size, and the corresponding Pd/CZM catalysts present improved reducibility of PdO species. The modification with Ca, Sr and Ba improves the thermal aging resistance, especially Ba. DRIFTS results reveal that the doping of alkaline earth metal enhances the oxygen and electron transfer ability and favors the dissociation of NO, which promotes the activation and storage capacity of the acidic atoms like NOx, and leads to enhanced catalytic activity performance.

Promotion effects in the oxidation of CO over zeolite-supported Pt nanoparticles

Well-defined Pt-nanoparticles with an average diameter of 1 nm supported on a series of zeolite Y samples containing different monovalent (H+, Na+, K+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations have been used as model systems to investigate the effect of promotor elements in the oxidation of CO in excess oxygen. Time-resolved infrared spectroscopy measurements allowed us to study the temperature-programmed desorption of CO from supported Pt nanoparticles to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated CO vibration compared to that of gas-phase CO increases with an increasing cation radius-to-charge ratio. In addition, a systematic shift from linear (L) to bridge (B) bonded CO was observed for decreasing Lewis acidity, as expressed by the Kamlet-Taft parameter alpha. A decreasing alpha results in an increasing electron charge on the framework oxygen atoms and therefore an increasing electron charge on the supported Pt nanoparticles. This observation was confirmed with X-ray absorption spectroscopy, and the intensity of the experimental Pt atomic XAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt nanoparticles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T(50%) for CO oxidation decreases with decreasing alpha. In other words, basic promotors facilitate the chemisorption of CO on the Pt particles. The most promoted CO oxidation catalyst is a Pt/K-Y sample, which has a T(50%) of 390 K and a L:B intensity ratio of 2.7. The obtained results provide guidelines to design improved CO oxidation catalysts.

Mesoporous Pt–SiO2 and Pt–SiO2–Ta2O5 Catalysts Prepared Using Pt Colloids as Templates

Sol-gel synthesis of silica and silica-tantalum oxide embedded platinum nanoparticles is carried out using Pt colloids as templates. These colloids are prepared by reduction with Na[AlEt(3)H] and stabilized with different ligands (ammonium halide derivatives, non-ionic surfactants with polyether chains, and 2-hydroxy-propionic acid). The aim of the present study is to prepare mesoporous silica embedded Pt colloids combining the "precursor concept" with the model of catalyst preparation using preformed spheres. Nanoparticles of Pt incorporated in high surface area mesoporous materials are formed after calcination. Further, it is observed that calcination of these catalysts causes partial aggregation and oxidation of the parent colloids, a process that is largely dependent on the nature of the stabilizing ligands. Several methods have been used for characterization of these materials: adsorption-desorption isotherms at 77 K, H(2) chemisorption, X-ray diffraction(XRD), (29)Si and (13)C magic angle spinning (MAS) NMR, ammonia diffuse reflectance Fourier transform infrared spectroscopy (NH(3)-DRIFT), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It is found that both metal oxide systems exhibit Brønsted acidity (weaker for silica and quite strong for silica-tantalum oxide). In addition, NH(3)-DRIFT experiments demonstrate the oxidative properties of the surface. Part of the adsorbed NH(4) (+) species is oxidized to N(2)O. Testing these catalysts in the reduction of NO and NO(2) with isopentane under lean conditions indicate that the activity of these catalysts is indeed dependent on the size of the platinum particles, with those of size 8-10 nm demonstrating the best results. The support likely contributes to this effect, particularly after Ta incorporation into silica.