Xiaoyin Chen

Improvement of Activity and SO2 Tolerance of Sn-Modified MnOx–CeO2 Catalysts for NH3-SCR at Low Temperatures

The performances of fresh and sulfated MnOx-CeO₂ catalysts for selective catalytic reduction of NOx by NH₃ (NH₃-SCR) in a low-temperature range (T < 300 °C) were investigated. Characterization of these catalysts aimed at elucidating the role of additive and the effect of sulfation. The catalyst having a Sn:Mn:Ce = 1:4:5 molar ratio showed the widest SCR activity improvement with near 100% NOx conversion at 110-230 °C. Raman and X-ray photoelectron spectroscopy (XPS) indicated that Sn modification significantly increases the concentration of oxygen vacancies that may facilitate NO oxidation to NO₂. NH₃-TPD characterization showed that the low-temperature NH₃-SCR activity is well correlated with surface acidity for NH3 adsorption, which is also enhanced by Sn modification. Furthermore, as compared to MnOx-CeO₂, Sn-modified MnOx-CeO₂ showed remarkably improved tolerance to SO₂ sulfation and to the combined effect of SO₂ and H₂O. In the presence of SO₂ and H₂O, the Sn-modified MnOx-CeO₂ catalyst gave 62% and 94% NOx conversions as compared to 18% and 56% over MnOx-CeO₂ at temperatures of 110 and 220 °C, respectively. Sulfation of SnO₂-modified MnOx-CeO₂ may form Ce(III) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO₂ during NH₃-SCR at T > 200 °C.

High temperature thermal stabilization of alumina modified by lanthanum species

The effects of precursor pretreatment and addition methods of lanthanum species on stabilization of alumina (surface area loss, phase transformations and high temperature interaction with lanthanum species) have been investigated by BET specific surface area measurements (BET), X-ray powder diffraction (XRD), N2 adsorption–desorption isotherms, thermal analysis and X-ray photoelectron spectroscopy (XPS) in the range of 600–1150°C. Although powder La2O3, which is mechanically mixed with γ-Al2O3 or pseudo boehmite, can effectively retard the α phase transformation by solid phase interaction with Al2O3, it does not show a positive effect on retarding the loss of surface area. Compared with the direct impregnation of γ-Al2O3, the gelation of pseudo boehmite by acidification accelerates phase transformations and weakens the stabilizing influence of lanthanum species. At 600°C and for atomic ratio of La/Al up to 0.1 or at 1150°C and La/Al≤0.02, the lanthanum species is highly dispersed in alumina. With the increase of calcination temperature or lanthanum content, lanthanum species is present as dispersed La2O3, LaAlO3 and crystalline La2O3. At T≤1000°C the surface area loss of alumina is mainly attributed to the sintering of particles. The follow-up loss at T>1000°C results from both sintering and phase transformations. The highly dispersed lanthanum species retard both sintering and phase transformations, and their associated surface area loss. However, the formation of LaAlO3 mainly retards the surface area loss resulting from the α phase transformation. Having considered the purely mechanical mixing effect of additive on the surface area loss of alumina, an influence criterion of lanthanum species on retarding the surface area loss whether resulting from sintering or from α phase transformation at high temperature ≥1000°C has been proposed in this paper.

Characterization and catalytic performance of mesoporous molecular sieves Al-MCM-41 materials

Mesoporous Al-MCM-41 materials of different Si/Al ratios have been synthesized and characterized by X-ray powder diffraction, 27Al and 29Si MAS NMR, differential thermogravimetric analysis, N2 adsorption measurements, FT-IR and catalytic cracking of alkanes. The experimental results show that the incorporation of aluminium into the framework of MCM-41 has a great effect on the degree of long-distance order, the surface acidities and the mesoporous structures of the materials. With increase of the aluminium content, the amounts of tetrahedral framework aluminium and the acid sites on the samples increase, but the acid strength decreases. Al-MCM-41 materials exhibit high activity for n-C160 cracking and good selectivity for producing low carbon alkylenes, particularly for i-C4=.

Synthesis of microporous molecular sieves by surfactant decomposition

Microporous molecular sieves can be synthesized under the high temperatures of 165–180 °C and high pH of 10–12.5, using surfactant cetyltrimethylammonium bromide (CTAB) as a template. It was found that various factors, such as temperature, alkalinity, Si∶Al ratio and the presence of a small amount of organic alkylammonium species (TMA+, TPA+) can affect the nature of the microporous materials obtained. In the wide range of Si∶Al ratios, we can obtain such zeolites as sodalite, analcime, omega, ZSM-35, mordenite, ZSM-5 and silicalite. The experimental results suggest that the decomposition products of CTAB act as templates instead of surfactant CTAB itself. The synthesis condition of high pH (>10) and high temperatures (165 °C) can ensure the decomposition of CTAB, which is quite essential for the formation of microporous molecular sieves.

Comparison of effect of La-modification on the thermostabilities of alumina and alumina-supported Pd catalysts prepared from different alumina sources

Abstract The thermostability of La-free and La-modified aluminas derived from different alumina sources including pseudo-boehmite, aluminum sulfate, aluminum nitrate, and aluminum chloride have been studied by characterizations of BET specific surface area (BET) and X-ray powder diffraction (XRD). Further, the activity and thermal durability of monolith-supported Pd catalysts using above different alumina as washcoats for simultaneously catalytic oxidation of C3H8, C3H6 and CO have been investigated using continuous flowing reaction. The experimental results show that the oxidation activity over the alumina-supported Pd catalysts does not keep in agreement with the surface area of aluminas derived from four aluminum sources. Lanthanum modification improves the activity of all catalysts calcined over 1000°C for simultaneously catalytic oxidation of C3H8, C3H6 and CO. Alumina source plays a role on the activity and thermostability of alumina and alumina-supported Pd catalysts. The experimental results show that pseudo boehmite is the best alumina source used for catalytic oxidation at high temperature over 1000°C.

A thermogravimetric determination of dispersed and bulk-like barium species supported on γ-alumina

A novel, thermogravimetric (TG) method is demonstrated for the quantitative determination of a dispersed active phase on a γ-Al2O3 catalytic support.

Synthesis and evaluation of mesopore structured ZSM-5 and a CuZSM-5 catalyst for NH3-SCR reaction: studies of simulated exhaust and engine bench testing

A modified ZSM-5 zeolite (denoted as ZSM-5-M), which was synthesized using tetrapropylammonium hydroxide (TPAOH) and cetyltrimethylammonium bromide (CTAB) as dual templates, and the commercial ZSM-5 zeolite (denoted as ZSM-5-C), have been used to prepare the corresponding CuZSM-5 (M & C) catalysts containing 3 wt% Cu by ion exchange method. Compared to CuZSM-5-C catalyst, the CuZSM-5-M catalyst demonstrated remarkably higher catalytic activity at low temperatures (<450 °C) for selective catalytic reduction of NOx with NH3 both in the simulated exhaust and engine bench testing. The modified synthesis by the dual template with the product aged for a long time at room temperature, leads to the formation of the ZSM-5-M zeolite with much higher specific surface area (608 m2 g−1) and higher total pore volume (0.8880 cm3 g−1) due to the presence of more mesoporous pores. The X-ray diffraction results showed that ZSM-5-M maintained its typical MFI structure, while its crystallinity (84.1%) was lower than that of the ZSM-5-C zeolite. The characterization results by H2 temperature-programmed reduction and X-ray photoelectron spectra revealed that the higher redox properties of isolated Cu2+ ions combined with the high-dispersion CuO crystallites and Cu+ ions are likely the main cause for the excellent low-temperature activity of the CuZSM-5-M catalysts. The isolated active Cu2+ species and high-dispersion CuO crystallites had a stronger interaction with other atoms in CuZSM-5-M catalysts. The results from thermogravimetric analysis, temperature-programmed desorption of ammonia and in situ diffuse reflectance infrared Fourier transform spectroscopy demonstrated that ZSM-5-M is a strong Bronsted acid site and the CuZSM-5-M catalyst had a relatively higher exchange rate of Cu2+ and Cu+ ions and more Lewis acidic sites, giving it a high NH3 adsorption capacity. The strong Bronsted acid site might be another cause that results in the higher NH3-SCR performance of the CuZSM-5-M catalyst.

Fe2O3@SiTi core–shell catalyst for the selective catalytic reduction of NOx with NH3: activity improvement and HCl tolerance

A core–shell structured Fe2O3@SiTi catalyst with a SiTi shell and Fe2O3 core was prepared and used for the selective catalytic reduction (SCR) of NOx with NH3. The thickness of the shell could be well controlled by the amount of TEOS added during the preparation procedure. The catalyst was characterized by techniques including STEM, XRD, BET, NH3-TPD, XPS, and H2-TPR. In comparison with Fe2O3, the Fe2O3@SiTi catalyst presented higher SCR activity, N2 selectivity, better SO2 resistance and HCl tolerance. The characterization results showed that the addition of the SiTi shell onto the Fe2O3 core enlarged the BET specific surface area, increased the number of surface acidic sites, led to the generation of more Fe2+ species and chemisorbed oxygen species, and influenced the reducibility of Fe2O3. The improvement of HCl tolerance of the Fe2O3@SiTi catalyst can be attributed to the fact that the SiTi shell prevents the Fe2O3 core from reacting with HCl to form FeCl3.