How to control the selectivity in the reduction of NOx with H2 over Pt-Ba/Al2O3 Lean NOx Trap catalysts

The reduction process of NO_x species stored over Pt-Ba/Al₂O₃ Lean NO_x Trap systems is analysed in this paper when H₂ is used as a reductant. The effect of different experimental conditions (temperature, reductant concentration, adsorption lengths, etc.) is addressed and discussed in relation to the selectivity and the efficiency of the reduction process.     

Reduction of NOx by H2 on WOx-Promoted Pt/Al2O3/SiO2 Catalysts Under O2-Rich Conditions

This work addresses the reduction of NO_x by H₂ under O₂-rich conditions using Al₂O₃/SiO₂-supported Pt catalysts with different loads of WO_x promotor. The samples were thoroughly characterised by N₂ physisorption, temperature-programmed desorption of CO, scanning electron microscopy, X-ray diffraction, laser raman spectroscopy, X-ray photoelectron spectroscopy and diffuse reflectance infrared fourier transform spectroscopy with probe molecule CO. The catalytic studies of the samples without WO_x showed pronounced NO_x conversion below 200 °C, whereas highest efficiency was related to small Pt particles. The introduction of WO_x provided increasing deNO_x activity as well as N₂ selectivity. This promoting effect was referred to an additional reaction path at the Pt-WO_x/Al₂O₃/SiO₂ interface, whereas an electronic activation of Pt by strong metal support interaction was excluded.

Graphic Abstract

     

The Effect of Preparation Variables on the CO Hydrogenation Activity of Coprecipitated Co/Al2O3 Catalysts

Studies have been conducted on the effect of preparation variables on the activity of coprecipitated cobalt–alumina catalysts to be used for the production of C₁–C₄ hydrocarbons by CO hydrogenation. The preparation parameters considered were the precipitation pH, the precipitation agent, the metal loading, the reduction temperature and the reduction period. It was found that changing pH precipitation with a constant final pH between 11·0 and 12·5 and the use of NaOH together with nitrates as precursors yielded better catalysts with maximal metal surface area. The optimum cobalt loading for the selective production of C₁–C₄ hydrocarbons is around 35 wt%. Optimum activity and selectivity are obtained by applying an 8-h reduction scheme at 648 K under 100 cm³ min⁻¹ hydrogen. Calcination prior to reduction has a detrimental effect on metal surface area and hence on catalytic activity. © 1997 SCI.     

Novel Sn–Ce/Al2O3 Catalyst for the Selective Catalytic Reduction of NOx Under Lean Conditions

Effect of additives, Ce and Mn, on the catalytic performance of Sn/Al₂O₃ catalyst prepared by sol–gel method for the selective reduction of NO_x with propene under lean conditions was studied. Sn–Ce/Al₂O₃ catalysts exhibited higher activity than Sn/Al₂O₃ catalyst and the optimum Ce loading is 0.5–1%. The promoting effect of Ce is to enhance the oxidation of NO to NO₂ and facilitate the activation of propene, both of which are important steps for the NO_x reduction. The presence of oxygen contributes to the oxidation of NO and shows a promoting effect.     

载体氧化铝的粒径对对硝基苯酚加氢催化剂Ni/Al2O3的影响

The catalytic hydrogenation ofp-nitrophenol to p-aminophenol was investigated over Ni/Al2O3 catalyst on alumina support with different particle size. It is found that support particle size has significant influences on physiochemical properties and catalytic activity of the resulting Ni/A12O3 catalyst,but little influence on the selectivity. At a comparable amount of Ni loading,the catalytic activity of Ni/Al2O3 prepared with alumina support of smaller particle size is lower. The reduction behavior of the catalyst is a key factor in determining the catalytic activity of Ni/A12O3 catalyst. The supported nickel catalyst 10.3Ni/Al2O3-3 improves the life span of the membrane by reducing fouling on the membrane surface compared to nano-sized nickel.     

CO/FTIR Spectroscopic Characterization of Pd/ZnO/Al2O3 Catalysts for Methanol Steam Reforming

An as-synthesized 8.8wt% Pd/ZnO/Al₂O₃ catalyst was either pretreated under O₂ at 773 K followed by H₂ at 293 K or under H₂ at 773 K to obtain, respectively, a supported metallic Pd° catalyst (Pd°/ZnO/Al₂O₃) or a supported PdZn alloy catalyst (PdZn/ZnO/Al₂O₃). Both catalysts were studied by CO adsorption using FTIR spectroscopy. For the supported PdZn alloy catalyst (PdZn/ZnO/Al₂O₃), exposure to a mixture of methanol and steam, simulating methanol steam reforming reaction conditions, does not change the catalyst surface composition. This implies that the active sites are PdZn alloy like structures. The exposure of the catalyst to an oxidizing environment (O₂ at 623 K) results in the break up of PdZn alloy, forming a readily reducible PdO with its metallic form being known as much less active and selective for methanol steam reforming. However, for the metallic Pd°/ZnO/Al₂O₃ catalyst, FTIR results indicate that metallic Pd° can transform to PdZn alloy under methanol steam reforming conditions. These results suggest that PdZn alloy, even after an accidental exposure to oxygen, can self repair to form the active PdZn alloy phase under methanol steam reforming conditions. Catalytic behavior of the PdZn/ZnO/Al₂O₃ catalyst also correlates well with the surface composition characterizations by FTIR/CO spectroscopy.     

TiO2掺杂对Na2CO3/Al2O3吸收剂CO2捕捉性能的影响

钠基固体吸收剂脱除燃煤烟气CO₂技术具有反应温度低、能耗低等优点,日益受到学术界的关注。该技术的主要不足是吸收剂的活性成分碳酸钠与CO₂的反应（碳酸化反应）活性较低。针对这一问题,本文旨在研制一种新型改性钠基固体吸收剂,采用活性氧化铝作为载体、TiO₂作为掺杂剂进行改性,利用热重分析装置、XRD、SEM和氮吸附仪研究钠基固体吸收剂的CO₂捕捉性能。结果表明：掺杂TiO₂后,钠基固体吸收剂与CO₂的反应速率加快,CO₂捕捉量增加;反应前后除TiO₂外无其他含Ti化合物生成;碳酸化反应产物为NaHCO₃和Na₅H₃（CO₃）₄;然而TiO₂掺杂过多会堵塞吸收剂的微观孔道,不利于甚至阻碍碳酸化反应的进行,因此,TiO₂的掺杂量应控制在一定的范围内。     

Ethylene Hydrogenation over Pt/Ga2O3/Al2O3 Catalysts

The effects of gallia addition on the Pt dispersion and the activity for ethylene hydrogenation at 0 °C were studied for Pt-supported catalysts as a function of the reduction temperature (350, 450, and 550 °C). The catalysts contained 0.5 wt.% Pt and were prepared by successive incipient wetness impregnations with Ga(NO₃)₃ and H₂PtCl₆ aqueous solutions. CO and H₂ chemisorption data indicated that, the addition of small amount of Ga₂O₃ caused an increase of the Pt dispersion and a decrease of ethylene conversion. Both of them decreased appreciably when Ga₂O₃ addition was increased, particularly, for the case where β-Ga₂O₃ was used as a support. The increase in reduction temperature magnified the negative effects of the gallia addition on dispersion and activity, although the addition of small amount of gallia improved the resistance to metal sintering. Results were interpreted in terms of the presence of reduced Ga species, which can encapsulate Pt particles.     

Effect of La2O3 promoter on NiO/Al2O3 catalyst in CO methanation

A series of NiO/Al2O3 catalysts promoted by different La₂O₃ contents were prepared by impregnation method. The physicochemical properties of NiO-La₂O₃/Al₂O₃ were characterized by N₂ adsorption-desorption, X-ray diffraction (XRD), H₂ temperature programmed reduction (H₂-TPR) and H₂ chemisorption. The effect of La₂O₃ on the activity of NiO/Al₂O₃ for CO methanation was investigated in a fixed bed reactor. A lifetime test, as well as thermogravimetric (TG) analysis, was performed to investigate the stability performance and anti-carbon deposition of catalysts. The results showed that the addition of La₂O₃ can restrain the growth of NiO particles, increase the H₂ uptake and Ni dispersion, and therefore enhance the activity of catalysts. When the La₂O₃ content was 3 wt%, a CO conversion of 98% and a selectivity to CH₄ of 96% were obtained at 400 °C. Furthermore, the catalyst NiO-La₂O₃/Al₂O₃ with 3 wt% La₂O₃ content displayed highly stable performance in long-term tests, especially exhibiting good anti-carbon deposition property.     

负载型K2CO3/Al2O3二氧化碳吸收剂的碳酸化反应特性

引言 全球变暖已经成为一个备受关注的环境问题.据预测,如不采取积极的温室气体减排措施,从现在起到2100年,全球近地面平均气温将继续升高1.4～5.8℃[1].CO2是主要的温室气体,而我国燃煤电厂是CO2排放量最大、最集中的化石燃料燃烧场所.因此研究和开发适用于燃煤电厂的CO2减排技术至关重要.     

Effect of SO2 on NOx reduction by ethanol over Ag/Al2O3 catalyst

A new Ag/Al₂O₃ catalyst for removing NO_x in diesel engine exhaust gas was developed. The influence of SO₂ on the reduction of lean NO_x by ethanol over the Ag/Al₂O₃ catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al₂O₃ catalyst was highly active for the reduction of NO_x with ethanol in the presence of SO₂ although the reduction of NO_x is suppressed at lower temperatures. The activity for NO_x reduction is high even on the Ag/Al₂O₃ catalyst exposed to a SO₂ (200 ppm)/O₂ (10%)/H₂O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al₂O₃ catalyst. No crystallized Ag metal and Ag compounds were formed by the SO₂/O₂/H₂O exposure. On the other hand, crystallized Ag₂SO₄ was easily formed when the Ag/Al₂O₃ catalyst was exposed to a SO₂ (200 ppm)/O₂ (10%)/NO (800 ppm)/H₂O (10%) flow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag₂SO₄ results in growing of Ag particles in larger size and lowering the surface content of Ag particles. In addition, the specific surface area of the Ag/Al₂O₃ catalyst decreases from 221 to 193 m²/g. Although the dispersion of Ag particles was decreased by the formation of Ag₂SO₄, the activity for the reduction of lean NO_x was, remarkably, not affected. This suggests that the Ag–alumina sites created by the Ag₂SO₄ formation are still active for the lean catalytic reduction of NO_x. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation of Pd/Al2O3/Al catalysts for CO removal

Aluminum was carefully anodized, then palladium salts were supported on its A1₂O₃ surface layer by ion-exchange. CO 3.76 vol% contained in the air can be eliminated at around 200 °C. Both anodization conditions and the nature of the precursor solutions affect catalyst nature, particularly the pH value of the precursor solutions. The pH value of the solution suitable for supporting active components is 5–6. Palladium, which is present as Pd²⁺ and Pd⁴⁺ after calcination,distributes uniformly on the support surface which had a honeycomb structure.     

Differences Between Al2O3 and Ag/Al2O3 for Lean Reduction of NO x with Dimethyl Ether

Lean reduction of NO _x with DME occurs with high selectivity to N₂ over Al₂O₃ between 300 °C and 550 °C with a maximum of 47% at 380 °C, and with lower selectivity over Ag/Al₂O₃ between 250 °C and 400 °C due to the catalysts’ sensitivity to gas phase radical reactions and activity for NO _x reduction with methanol.     

负载型K2CO3/Al2O3吸收剂吸收CO2反应机理

利用热重分析仪、扫描电镜和氮吸附仪对不同粒径的K₂CO₃颗粒和负载型K₂CO₃/Al₂O₃二氧化碳吸收剂的碳酸化特性进行研究。负载后的吸收剂比表面积和孔隙结构得到较大改善,使得碳酸化反应速率和转化率均提高,吸收剂碳酸化特性得到改善。纯K₂CO₃颗粒吸收剂的反应速率和转化率随着粒径的增加而减小,负载型吸收剂的反应速率和转化率随着粒径的增加略增大。研究了不同粒径和反应时间对K₂CO₃/Al₂O₃颗粒微观结构的影响,结果表明K₂CO₃/Al₂O₃颗粒具有较稳定的微观结构。采用负载型粒子模型对K₂CO₃/Al₂O₃吸收剂吸收CO₂碳酸化过程进行研究,所建立的粒子模型计算结果与试验值吻合较好。利用建立的模型对不同CO₂浓度下K₂CO₃/Al₂O₃吸收剂碳酸化反应特性进行模拟计算,模拟结果具备一定的合理性和准确性,为开展进一步研究提供了基础。     

Ag对Pd/Al2O3催化分解NO反应的促进作用

研究了不同含量的银对Pd/Al₂O₃催化分解NO反应活性的影响。结果表明，低温下，Pd/Al₂O₃催化剂对NO的分解反应活性不高；反应温度达到720 ℃时，NO分解活性迅速上升。O₂-TPD研究表明，Pd/Al₂O₃上氧的脱附速率也是在720 ℃左右达到最大值，因此，在温度低于720 ℃时，氧从催化剂表面的脱附是NO分解反应的速率控制步骤。Pd/Al₂O₃中添加适量的Ag对NO分解反应有促进作用。O₂-TPD研究表明，这种促进作用与Pd表面氧物种的起始脱附温度向低温移动有关。     

Pd/Al2O3催化剂上低浓度CO氧化的研究

采用共沉淀法和浸渍法制备了2%Pd-Al2O3催化剂,在固定反应器中评价不同制备方法(共沉淀和浸渍)、助剂(1%K和1%Mg)、预处理方法(快速氧化和缓慢还原)、空速(726h-1、1638h-1和5274h-1)情况下2%Pd-Al2O3催化剂对CO氧化的催化活性。结果表明,共沉淀法制备的2%Pd50%Zr-Al2O3催化剂有较高的反应活性,该催化剂经缓慢还原预处理活化,在温度为70℃时就可以使低浓度的CO完全氧化。     

Effect of Modifiers on the Reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 Catalysts for the Oxidative Dehydrogenation of Propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (T_max and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.