Zhongxian Song

Influence of calcination temperature on selective catalytic reduction of NOx with NH3 over CeO2-ZrO2-WO3 catalyst

Abstract A series of CeO2-ZrO2-WO3 catalysts for the selective catalytic reduction (SCR) of NO with NH3 were prepared by hydrothermal method. The influence of calcination temperature on the catalytic activity, microstructure, surface acidity and redox behavior of CeO2-ZrO2-WO3 catalyst was investigated using various characterization methods. It was found that the CeO2-ZrO2-WO3 catalyst calcined at 600 °C showed the best catalytic performance and excellent N2 selectivity, and yielded more than 90% NO conversion in a wide temperature range of 250–500 °C with a space velocity (GHSV) of 60000 h−1. As the calcination temperature was increased from 400 to 600 °C, the NO conversion obviously increased, but decreased at higher calcination temperature. The results implied that the higher surface area, the strongest synergistic interaction, the superior redox property and the highly dispersed or amorphous WO3 species contributed to the excellent SCR activity of the CeO2-ZrO2-WO3 catalyst calcined at 600 °C.

Enhanced performance in NOx reduction by NH3 over a mesoporous Ce–Ti–MoOx catalyst stabilized by a carbon template

Mesoporous Ce–Ti–MoOx catalysts have been prepared by stabilization with an in situ formed carbon template and used for selective catalytic reduction of NOx with NH3 (NH3-SCR). The Ce–Ti–MoOx catalysts were characterized by N2 adsorption–desorption, XRD, HR-TEM, H2-TPR and XPS analysis. It was found that the stabilization with a carbon template obviously improved the NOx conversion of the Ce–Ti–MoOx catalyst and the pore distribution. The Ce–Ti–MoOx catalyst stabilized with a carbon template at 850 °C in N2 exhibited the best NH3-SCR activity and showed a uniform mesopore distribution, and more than 90% NOx conversions were obtained at 175–425 °C with a GHSV of 60 000 h−1. The high stabilization temperature with a carbon template in N2 not only led to an increase in specific surface area, pore volume and low-temperature SCR activity, but also contributed to the enhanced redox properties, highly dispersed titanium or molybdenum species and narrow mesopore distribution. The treatment of Ce–Ti–MoOx with a high calcination temperature for removal of the carbon template led to a drastic decrease in surface area, an abrupt decrease in surface oxygen and cerium, production of rutile TiO2, and a correspondingly low SCR activity.

Low-temperature catalytic oxidation of CO over highly active mesoporous Pd/CeO2–ZrO2–Al2O3 catalyst

Mesoporous nano Pd/CeO2, Pd/CeO2–ZrO2 and Pd/CeO2–ZrO2–Al2O3 catalysts were prepared via a soft template-assisted hydrothermal method and employed in low-temperature catalytic CO oxidation. Almost 100% of CO conversion could be obtained by the optimal Pd/CeO2–ZrO2–Al2O3 catalyst at 60 °C. Addition of Zr and Al into Pd/CeO2 lead to an increase of the pore volume, average size of mesopores, the surface Ce3+/Ce4+ atomic ratios and Oadsorbed/Olattice, while the activation energy (Ea) decreased in the order Pd/CeO2 (50.4 kJ mol−1) > Pd/CeO2–ZrO2 (40.7 kJ mol−1) > Pd/CeO2–ZrO2–Al2O3 (32.4 kJ mol−1). The low-temperature CO catalytic conversion increased with the decreased activation energy (Ea).

Activity and hydrothermal stability of CeO2–ZrO2–WO3 for the selective catalytic reduction of NOx with NH3

A series of CeO2-ZrO2-WO3 (CZW) catalysts prepared by a hydrothermal synthesis method showed excellent catalytic activity for selective catalytic reduction (SCR) of NO with NH3 over a wide temperature of 150-550°C. The effect of hydrothermal treatment of CZW catalysts on SCR activity was investigated in the presence of 10% H2O. The fresh catalyst showed above 90% NOx conversion at 201-459°C, which is applicable to diesel exhaust NOx purification (200-440°C). The SCR activity results indicated that hydrothermal aging decreased the SCR activity of CZW at low temperatures (below 300°C), while the activity was notably enhanced at high temperature (above 450°C). The aged CZW catalyst (hydrothermal aging at 700°C for 8 hr) showed almost 80% NOx conversion at 229-550°C, while the V2O5-WO3/TiO2 catalyst presented above 80% NOx conversion at 308-370°C. The effect of structural changes, acidity, and redox properties of CZW on the SCR activity was investigated. The results indicated that the excellent hydrothermal stability of CZW was mainly due to the CeO2-ZrO2 solid solution, amorphous WO3 phase and optimal acidity. In addition, the formation of WO3 clusters increased in size as the hydrothermal aging temperature increased, resulting in the collapse of structure, which could further affect the acidity and redox properties.

Enhancement of N2O catalytic decomposition over Ca modified Co3O4 catalyst

A series of Ca modified Co3O4 catalysts with different Ca/Co molar ratios were synthesized by the co-precipitation method and applied to N2O catalytic decomposition. The experimental results showed that the performance of N2O catalytic decomposition was obviously enhanced by the addition of Ca into the Co3O4 catalyst. The Ca modified Co3O4 catalyst with Ca/Co molar ratio of 1:2 exhibited the highest catalytic performance and almost 100% N2O conversion was achieved at 400 °C. The characterization results showed that the addition of suitable calcium composition could promote the growth of the 111 crystal plane of Co3O4 and provide abundant surface oxygen on the surface of the catalyst. The kinetics studies confirmed that the activation energy of the Ca modified Co3O4 catalyst with Ca/Co molar ratio of 1:2 (Ea = 17.84 kJ mol−1) was lower than that of pure Co3O4 (Ea = 43.21 kJ mol−1), implying that the addition of Ca into the Co3O4 was beneficial to the catalytic decomposition of N2O.

Catalytic hydrolysis of HCN on ZSM-5 modified by Fe or Nb for HCN removal: surface species and performance

The catalytic hydrolysis of HCN was systematically investigated using Fe/ZSM-5, Nb/ZSM-5 and Fe–Nb/ZSM-5 catalysts. Fe–Nb/ZSM-5 exhibited the highest HCN hydrolysis activity and the reaction products were NH3 and CO. However, no NH3 was detected due to the large ammonia storage capacity of the catalysts. The interaction between the Fe and Nb species resulted in increased amounts of isolated Fe3+, Nb5+, oligomeric FexOy and NbxOy clusters, which could contribute to improving HCN hydrolysis. Furthermore, the excellent redox properties, favored pore structure and abundance of surface acid sites were responsible for the superior catalytic hydrolysis of HCN. Furthermore, the reaction pathway was speculated as follows: HCN and H2O reacted to produce methanamide. Methanamide further reacted with H2O to generate ammonium formate, which decomposed to formic acid and NH3. Formic acid was then converted into CO and H2O via pyrolysis.

Probing NH3-SCR catalytic activity and SO2 resistance over aqueous-phase synthesized Ce-W@TiO2 catalyst

Abstract A series of mixed Ce, W and Ti oxides catalysts have been prepared and employed to selective catalytic reduction of NO with ammonia (NH 3 -SCR). It was found that Ce-W@TiO 2 exhibited the optimal catalytic activity. Over 90 % of NO conversion could be obtained by the surface TiO 2 -coated Ce-W@TiO 2 catalyst at 205–515°C. The presence of SO 2 leads to a decrease of low-temperature NO catalytic conversion and an increase of NO conversion above 425°C over various catalysts, while Ce-W@TiO 2 exhibits the optimal low-temperature SO 2 resistance. It is noted that introducing Ti into Ce-W mixed oxide via the conventional co-precipitation method attributed to the decrease of surface W atomic abundance and acidic sites as well as inferior low-temperature NO conversion and SO 2 resistance. In contrast, the surface TiO 2 -coated Ce-W@TiO 2 catalyst prepared via a modified aqueous-phase method exhibit increased surface W atomic abundance and acidic sites as well as the superior low-temperature NO conversion and SO 2 resistance.

Introduction manner of sulfate acid for improving the performance of SO42−/CeO2 on selective catalytic reduction of NO by NH3

Abstract A series of sulfated CeO 2 catalysts were synthesized by impregnation and sol-gel methods and used for selective catalytic reduction (SCR) of NO x by NH 3 . The results showed that the sulfated CeO 2 catalysts prepared by sol-gel method showed excellent catalytic activity at 150–450 °C, and more than 90% NO x conversion was obtained at 232–450 °C with a gas hourly space velocity of 60000 h −1 . The catalysts were characterized by X-ray diffraction (XRD), N 2 adsorption, Raman, thermogravimetry (TG), H 2 -temperature-programmed reduction (H 2 -TPR) and Py-infrared spectroscopy (Py-IR). The excellent SCR performance was associated with the surface acidity and the micro-structure. The introduction of sulfate acid into CeO 2 could increase the amount of Bronsted and Lewis acid sites over the catalysts, resulting in the improvement of the low temperature activity. The sulfated CeO 2 catalysts prepared by sol-gel method possessed lower crystallization degree, excellent redox property and larger specific surface areas, which were responsible for the superior SCR performance.

Effect of WO3 content on the catalytic activity of CeO2-ZrO2-WO3 for selective catalytic reduction of NO with NH3

Abstract A series of CeO2-ZrO2-WO3 catalysts (CZW) was prepared by the hydrothermal method. The effect of WO3 content on their catalytic properties for selective reduction of NOx with NH3 was investigated. The catalysts were characterized by X-ray diffraction, N2 sorption, H2 temperature-programmed reduction, NH3 temperature-programmed desorption and NO temperature-programmed desorption techniques. It was shown that WO3 existed as amorphous species in the CZW. Introduction of WO3 in the CZW dramatically enhanced its surface acidity and gave rise to strongly adsorbed NO species, consequently increasing the catalytic activity. In addition, appropriate amounts of WO3 also increased the surface area and improved the reduction behavior of the catalyst. Compared to the CeO2-ZrO2, the CZW with 20% WO3 not only exhibited high resistivity to SO2, but also had a wider reaction temperature window. It showed a NOx conversion of > 90% at the space velocity of 60000 h−1 in the temperature range of 200–463°C.

Performance and characterisation of CeO2–TiO2–WO3 catalysts for selective catalytic reduction of NO with NH3

Ce-Ti-W-Ox catalysts were prepared and applied to the NH3-selective catalytic reduction (SCR) reaction. The experimental results showed that the Ce-Ti-W-Ox catalyst prepared by the hydrothermal method exhibited higher NO conversion than those synthesised via the sol-gel and impregnating methods, while the optimal content of WO3 and molar ratio of Ce/Ti were 20 mass % and 4: 6, respectively. Under these conditions, the catalyst exhibited the highest level of catalytic activity (the NO conversion reached values higher than 90 %) across a wide temperature range of 225–450°C, with a range of gas hourly space velocity (GHSV) of 40000–140000 h−1. The catalyst also exhibited good resistance to H2O and SO2. The influences of morphology, phase structure, and surface properties on the catalytic performance were investigated by N2 adsorption-desorption measurement, XRD, XPS, H2-TPR, and SEM. It was found that the high efficiency of NO removal was due to the large BET surface area, the amorphous surface species, the change to element valence states, and the strong interaction between Ce, Ti, and W.