Hui Xian

De-NOx in alternative lean/rich atmospheres on La1−xSrxCoO3 perovskites

Herein, we introduce a La1−xSrxCoO3 perovskite catalyst, substituting for Pt containing LNT catalysts, to remove efficiently NOx from lean-burn engines. The NOx storage/reduction occurred alternatively on the perovskite in successive lean/rich atmospheres, and the NOx conversion reached 71.4% with 100% selectivity to N2 at 300 °C.

Hydrotalcite-Derived MnxMg3−xAlO Catalysts Used for Soot Combustion, NOx Storage and Simultaneous Soot-NOx Removal

The hydrotalcite-based Mn(x)Mg(3-x)AlO catalysts with different Mn:Mg atomic ratios were synthesized by coprecipitation, and employed for soot combustion, NOx storage and simultaneous soot-NO(x) removal. It is shown that with the increase of Mn content in the hydrotalcite-based Mn(x)Mg(3-x)AlO catalysts the major Mn-related species vary from MnAl(2)O(4) and Mg(2)MnO(4) to Mn(3)O(4) and Mn(2)O(3). The catalyst Mn(1.5)Mg(1.5)AlO displays the highest soot combustion activity with the temperature for maximal soot combustion rate decreased by 210 degrees C, as compared with the Mn-free catalyst. The highly reducible Mn(4+) ions in Mg(2)MnO(4) are identified as the most active species for soot combustion. For NO(x) storage, introduction of Mn greatly influences bulk NO(x) storage, with the adsorbed NO(x) species varying from linear nitrites to ionic and chelating bidentate nitrates gradually. The coexistence of highly oxidative Mn(4+) and highly reductive Mn(2+) in Mn(1.0)Mg(2.0)AlO is favorable to the simultaneous soot-NO(x) removal, giving a NO(x) reduction percentage of 24%. In situ DRIFTS reveals that the ionic nitrate species are more reactive with soot than nitrites and chelating bidentate nitrates, showing higher NO(x) reduction efficiency.

NO adsorption behaviors of the MnOx catalysts in lean-burn atmospheres

NO(x) emission control of lean-burn engines is one of the great challenges in the world. Herein, the MnOx model catalysts with the different calcination temperatures were synthesized to investigate their NO adsorbability for lean-burn exhausts. The transformation from (β-)MnO₂ to (α-)Mn₂O₃ following the increased calcination temperatures was evidenced from the viewpoint of the local atomic level. Among these samples, the one calcined at 550 °C containing the single α-Mn₂O₃ phase displayed the best NO adsorbability: NO was mainly adsorbed in the forms of NO/nitrites and NO₂/nitrates at the low and high temperatures, respectively; the NO oxidation ability displayed the volcano-shape following the increased operating temperatures, and reached the maximum, i.e. 92.4% of the NO-to-NO₂ conversion, at 250 °C. Moreover, this sample presented the efficiently reversible NO adsorption/desorption performance in alternative lean-burn/fuel-rich atmospheres, due to the weakly bonded NO(x) on it. The superficial oxygen species plays a critical role for the NO oxidation over α-Mn₂O₃. The consumed superficial oxygen could be further compensated by the gaseous and lattice oxygen therein. Our findings show that the α-Mn₂O₃ material is a promising NO(x) adsorber for lean-burn exhausts even at low operating temperatures.

Facilely synthesized H-mordenite nanosheet assembly for carbonylation of dimethyl ether.

Hard coke blockage of micropores of acidic zeolites generally causes serious catalytic deactivation for many chemical processes. Herein, we report a facile method to synthesize H-mordenite nanosheet assemblies without using any template agent. The assemblies exhibit the high catalytic activity for carbonylation of dimethyl ether because of their large quantity of framework Brønsted acids. The specific morphology of the nanosheet unites improves mass diffusion for both reactants and products. Consequently, the coke precursor species can readily migrate from the micropores to the external surface of the assemblies, inducing the improved catalytic stability through inhibiting hard coke formation in frameworks.

Tuning interactions between zeolite and supported metal by physical-sputtering to achieve higher catalytic performances

To substitute for petroleum, Fischer-Tropsch synthesis (FTS) is an environmentally benign process to produce synthetic diesel (n-paraffin) from syngas. Industrially, the synthetic gasoline (iso-paraffin) can be produced with a FTS process followed by isomerization and hydrocracking processes over solid-acid catalysts. Herein, we demonstrate a cobalt nano-catalyst synthesized by physical-sputtering method that the metallic cobalt nano-particles homogeneously disperse on the H-ZSM5 zeolite support with weak Metal-Support Interactions (MSI). This catalyst performed the high gasoline-range iso-paraffin productivity through the combined FTS, isomerization and hydrocracking reactions. The weak MSI results in the easy reducibility of the cobalt nano-particles; the high cobalt dispersion accelerates n-paraffin diffusion to the neighboring acidic sites on the H-ZSM5 support for isomerization and hydrocracking. Both factors guarantee its high CO conversion and iso-paraffin selectivity. This physical-sputtering technique to synthesize the supported metallic nano-catalyst is a promising way to solve the critical problems caused by strong MSI for various processes.

Mesoporous SiO2-confined La0.7Sr0.3CoO3 perovskite nanoparticles: an efficient NOx adsorber for lean-burn exhausts

Herein, we successfully synthesized well crystallized La0.7Sr0.3CoO3 perovskite nanoparticles confined in the mesopores of a SiO2 support. This perovskite exhibited extremely high NOx adsorbability for lean-burn exhausts, as well as improved stability in reducing atmospheres.

Engineering surface defects and metal–support interactions on Pt/TiO2(B) nanobelts to boost the catalytic oxidation of CO

Herein, we report the high performance of thermally reduced Pt/TiO2(B) catalysts for the catalytic oxidation of CO. Our findings show that through hydrogen spillover from Pt to TiO2, surface-engineered defects of oxygen vacancies are “constructed” on the TiO2 support during the reduction process, thus generating active surface-adsorbed oxygen species. With an increase of the reduction temperature, the TiO2(B) phase gradually transforms to the anatase phase, which takes place from the bulk to the surface of TiO2, and is eventually completed at 700 °C. Compared with the anatase phase, the oxygen vacancies are more easily formed on the TiO2(B) phase, and the latter has much stronger interactions with Pt, as well. As the reduction temperature increases, the metal–support interaction between Pt and TiO2(B) is strengthened. Meanwhile, we simultaneously observe an increase in the dispersion of Pt, the proportion of Pt0 and the adsorbed oxygen species on the surface. Our findings reveal that for thermally reduced Pt/TiO2 catalysts, surface-adsorbed oxygen and Pt0 are active species for the catalytic oxidation of CO. Among the thermally reduced catalysts, H-600 shows the highest catalytic activity because it has the largest amount of active Pt0 sites and surface-adsorbed oxygen species. In addition, it shows high water vapor resistance.