Qiulin Zhang

Novel HCN sorbents based on layered double hydroxides: sorption mechanism and performance.

Layered double hydroxides (LDHs) and layered double oxides (LDOs) have been prepared and used as sorbents for hydrogen cyanide (HCN). Based on results from sorbent optimization experiments, the optimal performance for HCN removal was found in Ni-Al LDH. As evidenced by fixed-bed sorption studies, the Ni-Al LDO with the greatest surface area showed better performance and outperformed products calcined at 200, 400, or 500 °C, whereas, the Ni-Al LDH showed a more twofold higher sorption capacity than the Ni-Al LDO. Investigation of the mechanisms between HCN and sorbents reveals that the HCN removal by the Ni-Al LDH and Ni-Al LDO leads to the formation of the complex anion, [Ni(CN)4](2-). Nevertheless, the [Ni(CN)4](2-) can enter interlayer region of the Ni-Al LDH due to its anion exchangeability, which endows this LDH with more binding sites, not only on its external surfaces, but also on its internal surfaces located in the interlayer region. In contrast, [Ni(CN)4](2-) were only adsorbed on the external surface of the Ni-Al LDO. As a result, the sorption capacity of the Ni-Al LDH for HCN is twice as high as that of the Ni-Al LDO, which is at 21.55 mg/g.

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.

A stable Ni/SBA-15 catalyst prepared by the ammonia evaporation method for dry reforming of methane

A stable Ni/SBA-15 catalyst was prepared by an ammonia evaporation (Ni-AE) method for dry reforming of methane. The characterization results exhibited that the highly dispersed and uniformly distributed Ni nanoparticles with strong metal–support interactions can be obtained by the ammonia evaporation method. The presence of Ni phyllosilicate was crucial to obtain Ni nanoparticles with a size of 3.2–5.2 nm after reduction by H2, which were smaller and exhibited a narrower size distribution than in the sample prepared by impregnation (Ni-IM). Dry reforming of methane reactivity results showed no observed decrease of activity for Ni-AE even when reacted at 800 °C for 100 min or 700 °C for 100 h, while the Ni-IM presented an obvious decrease under the same conditions. TEM images of spent Ni-AE catalyst further confirmed that the Ni nanoparticles were highly dispersed, and the weight loss of Ni-AE (2.09%) revealed by the TG curves is much lower than that of the Ni-IM sample (24.16%). The strong metal–support interactions derived from Ni phyllosilicate were mainly responsible for resistance to coking and sintering.

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.

Dry reforming of methane over Ni/SBA-15 catalysts prepared by homogeneous precipitation method

Ni/SBA-15 catalyst was prepared by homogeneous precipitation method (Ni-HP) and used for dry reforming of methane (DRM). The related characterization results indicated that the Ni particles were highly dispersed with a size range of 2-5 nm. Compared with Ni/SBA-15 catalyst prepared by impregnation (Ni-IM), the reduction temperature of Ni-HP obtained from H2-TPR was greatly improved, suggesting the stronger metal-support interaction. After reacting at 700 °C for 100 h, the CH4 conversion of DRM over Ni-HP catalyst slightly decreased from 74.5% to 73.8%. While, for the Ni-IM catalyst, the CH4 conversion dropped from 61.7% to 37.3%. Furthermore, the average particle size of Ni-HP was 3.7 nm and 4.7 nm before and after the long-time stability test, respectively, ascribed to the good anti-sintering property. Although a certain amount of coke was produced, mainly with disorder filamentous carbon of base-growth, the Ni/SBA-15 prepared by homogeneous precipitation exhibited excellent catalytic activity and stability.

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.