Effects of WOx modification on the activity, adsorption and redox properties of CeO2 catalyst for NOx reduction with ammonia

A series of WO₃/CeO₂ (WOx/CeO₂) catalysts were synthesized by wet impregnation of ammonium metatungstate on a CeO₂ support. The resulting solid acid catalysts were characterized by X-ray diffraction (XRD), UV-Vis spectroscopy (UV-Vis), Raman spectroscopy (Raman), in-situ Fourier transform infrared spectroscopy (in-situ FT-IR) of ammonia adsorption, NH₃-TPD, H₂ temperature-programmed reduction (H₂-TPR), NH₃/NO oxidation and activity measurements for NOx reduction by NH₃ (NH₃-SCR). The results show that polytungstate (WOx) species are the main species of tungsten oxide on the surface of ceria. The addition of tungsten oxide enhances the Brönsted acidity of ceria catalysts remarkably and decreases the amount of surface oxygen on ceria, with strong interaction between CeO₂ and WOx. As a result, the N₂ selectivity of NH₃ oxidation and NH₃-SCR at high temperatures (> 300℃) is enhanced. Therefore, a wide working temperature window in which NOx conversion exceeds 80% (NOx conversion > 80%) from 200 to 450℃, is achieved over 10 wt.% WOx/CeO₂ catalyst. A tentative model of the NH₃-SCR reaction route on WOx/CeO₂ catalysts is presented.     

Role of NO and SO2 in mercury oxidation over a La2O3/Fe2O3 catalyst with high thermal stability

In this study, the thermal stability of a ferric oxide catalyst for mercury oxidation was found to be considerably promoted by doping with La₂O₃. The catalysts doped with La₂O₃ maintained a higher surface area when subjected to high-temperature calcination, with lower average pore size and a narrower pore size distribution. X-ray diffraction (XRD) results revealed that La₂O₃ doping hinders the growth of catalyst particles and crystallization of the material at high temperatures. Both NO and SO₂ inhibited Hg⁰ oxidation over the La₂O₃/Fe₂O₃ catalyst. Fourier transform infrared (FTIR) spectra revealed that SO₂ reacts with O₂ over the catalysts to form several species that are inert for mercury oxidation, such as SO₄^2?, HSO₄^?, or other related species; these inert species cover the catalyst surface and consequently decrease Hg⁰ oxidation capacity. In addition, NO or SO₂ competed with Hg⁰ for active sites on the La₂O₃/Fe₂O₃ catalyst and hindered the adsorption of mercury, thereby inhibiting subsequent Hg⁰ oxidation. Hg⁰ oxidation on the La₂O₃/Fe₂O₃ catalyst mainly followed the Eley–Rideal mechanism. Moreover, the inhibition effects of NO and SO₂ were at least partially reversible, and the catalytic activity was temporarily restored after eliminating NO or SO₂.     

Effects of Ce on catalytic combustion of methane over Pd-Pt/Al2O3 catalyst

Activity and stability of 1%Pd-0.2%Pt/Al₂O₃ and 1%Pd-0.2%Pt/0.6%Ce/Al₂O₃ catalysts prepared by impregnation method for catalytic combustion of methane in air were investigated. The catalysts before and after reaction were characterized by BET, CO chemisorption, XRD and XPS techniques. Results showed that the presence of Ce significantly increased the activity and thermal stability of the Pd-Pt/Al₂O₃ catalyst towards methane combustion, which could be attributed to more highly-dispersed active PdO particles over the Pd-Pt/Ce/Al₂O₃ catalyst surface as well as the retarded sintering of PdO and the maintained oxidized state of surface Pd during the combustion process in the presence of Ce.     

Effect of CeO2 and Al2O3 on the activity of Pd/Co_3O_4/cordierite catalyst in the three-way catalysis reactions (CO/NO/CnHm)

The present article studies the effect of CeO₂ and Al₂O₃ on the activity of Pd/Co₃O₄/cordierite catalyst in conversion of NO, CO, C_nH_m. The catalysts were characterized by temperature programmed reduction with hydrogen, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. It is shown that the effect of CeO₂ on the properties of Pd/Co₃O₄/cordierite catalyst depends on preparation method. The catalyst obtained by co-deposition of cerium and cobalt oxides has higher activity in CO oxidation (CO + O₂ and CO + NO) and total hexane oxidation (C₆H₁₄ + O₂). Such phenomenon is probably caused by more than stoichiometric amount of formed oxygen vacancies, an increase in both mobility of surface oxygen and dispersity of components in the catalytic composition. It is demonstrated that CeO₂ addition promotes the SO₂ resistance of Pd/Co₃O₄/cordierite. The second support decreases the activity of Pd/Co₃O₄/cordierite catalyst in the reactions of CO and C₆H₁₄ with oxygen because of CoAl₂O₄ formation.     

Ce-promoted Mn/ZSM-5 catalysts for highly efficient decomposition of ozone

Manganese oxides supported by ZSM-5 zeolite (Mn/ZSM-5) as well as their further modified by Ce promoter were achieved by simple impregnation method for ozone catalytic decomposition. The yCe20Mn/ZSM-5–81 catalyst with 8% Ce loading showed the highest catalytic activity at relative humidity of 50% and a space velocity of 360 L/(g × hr), giving 93% conversion of 600 ppm O₃ after 5 hr. Moreover, this sample still maintained highly activity and stability in humid air with 50%–70% relative humidity. Series of physicochemical characterization including X-ray diffraction, temperature-programmed technology (NH₃-TPD and H₂-TPR), X-ray photoelectron spectroscopy and oxygen isotopic exchange were introduced to disclose the structure-performance relationship. The results indicated that moderate Si/Al ratio (81) of zeolite support was beneficial for ozone decomposition owing to the synergies of acidity and hydrophobicity. Furthermore, compared with 20Mn/ZSM-5-81, Ce doping could enhance the amount of low valance manganese (such as Mn²⁺ and Mn³⁺). Besides, the Ce³⁺/Ce⁴⁺ ratio of 8Ce20Mn/ZSM-5-81 sample was higher than that of 4Ce20Mn/ZSM-5-81. Additionally, the synergy between the MnO_x and CeO₂ could easily transfer electron via the redox cycle, thus resulting in an increased reducibility at low temperatures and high concentration of surface oxygen. This study provides important insights to the utilization of porous zeolite with high surface area to disperse active component of manganese for ozone decomposition.     

Engineered FeVO4/CeO2 nanocomposite as a two-way superior electro-Fenton catalyst for model and real wastewater treatment

FeVO₄/CeO₂ was applied in the electro-Fenton (EF) degradation of Methyl Orange (MO) as a model of wastewater pollution. The results of the characterization techniques indicate that FeVO₄ with triclinic structure and face-centered cubic fluorite CeO₂ maintained their structures during the nanocomposite synthesis. The effect of applied current intensity, initial pollutant concentration, initial pH, and catalyst weight was investigated. The MO removal reached 96.31% and chemical oxygen demand (COD) removal 70% for 60 min of the reaction. The presence of CeO₂ in the nanocomposite plays a key role in H₂O₂ electro-generation as a significant factor in the electro-Fenton (EF) system. The metal leaching from FeVO₄/CeO₂ was negligible (cerium 4.1%, iron 4.3%, and vanadium 1.7%), which indicates that the active species in the nanocomposite are strongly interacting with each other and are stable. The performance of the nanocatalyst in real wastewaters, salty, and binary systems was acceptable and the pollutions were removed efficiently. The synergistic effect between V, Fe, and Ce could be account as the reason for the respectable function of FeVO₄/CeO₂. The electron transfer proceeds via Haber-Weiss mechanism. A degradation pathway was proposed through by-products analysis using gas chromatography-mass spectrometry (GC–MS) technique. The pseudo-first-order kinetic model described the obtained experimental results (R² = 0.9906). The electro-Fenton system efficiency was improved by adding persulfate. The nanocomposite preserved almost its efficiency after six cycles. The obtained results demonstrate that the synergistic catalyst (FeVO₄/CeO₂) has the capability to introduce as a promising replacement of conventional catalysts in the electro-Fenton processes with brilliant proficiency.     

Activation of sulfite by metal-organic framework-derived cobalt nanoparticles for organic pollutants removal

Sulfite (SO₃²⁻) activation is one of the most potential sulfate-radical-based advanced oxidation processes, and the catalysts with high efficiency and low-cost are greatly desired. In this study, the cobalt nanoparticles embedded in nitrogen-doped graphite layers (Co@NC), were used to activate SO₃²⁻ for removal of Methyl Orange in aqueous solution. The Co@NC catalysts were synthesized via pyrolysis of Co²⁺-based metal-organic framework (Co-MOF), where CoO was firstly formed at 400℃ and then partially reduced to Co nanoparticles embedded in carbon layers at 800℃. The Co@NC catalysts were more active than other cobalt-based catalysts such as Co²⁺, Co₃O₄ and CoFe₂O₄, due to the synergistic effect of metallic Co and Co_xO_y. A series of chain reaction between Co species and dissolved oxygen was established, with the production and transformation of SO₃^•−, SO₅²⁻, and subsequent active radicals SO₄^•− and HO•. In addition, HCO₃⁻ was found to play a key role in the reaction by complexing with Co species on the surface of the catalysts. The results provide a new promising strategy by using the Co@NC catalyst for SO₃²⁻ oxidation to promote organic pollutants degradation.     

Catalytic liquid-phase oxidation of acetaldehyde to acetic acid over a Pt/CeO2–ZrO2–SnO2/γ-alumina catalyst

Pt/CeO₂–ZrO₂–SnO₂/γ-Al₂O₃ catalysts were prepared by co-precipitation and wet impregnation methods for catalytic oxidation of acetaldehyde to acetic acid in water. In the present catalysts, Pt and CeO₂–ZrO₂–SnO₂ were successfully dispersed on the γ-Al₂O₃ support. Dependences of platinum content and reaction time on the selective oxidation of acetaldehyde to acetic acid were investigated to optimize the reaction conditions for obtaining both high acetaldehyde conversion and highest selectivity to acetic acid. Among the catalysts, a Pt(6.4 wt.%)/Ce_0.68Zr_0.17Sn_0.15O_2.0(16 wt.%)/γ-Al₂O₃ catalyst showed the highest acetaldehyde oxidation activity. On this catalyst, acetaldehyde was completely oxidized after the reaction at 0°C for 8 hr, and the selectivity to acetic acid reached to 95% and higher after the reaction for 4 hr and longer.     

Mn-CeOx/Ti-PILCs for selective catalytic reduction of NO with NH3 at low temperature

Titanium-pillared clays (Ti-PILCs) were obtained by different ways from TiCl₄, Ti(OC₃H₇)₄ and TiOSO₄, respectively. Mn-CeO_x/Ti-PILCs were then prepared and their activities of selective catalytic reduction (SCR) of NO with NH₃ at low-temperature were evaluated. Mn-CeO_x/Ti-PILCs were characterized by X-ray diffraction, N₂ adsorption, Fourier transform infrared spectroscopy, thermal analysis, temperature-programmed desorption of ammonia and H₂-temperature-programmed reduction. It was found that Ti-pillar tend to be helpful for the enlargement of surface area, pore volume, acidity and the enhancement of thermal stability for Mn-CeO_x/Ti-PILCs. Mn-CeO_x/Ti-PILCs catalysts were active for the SCR of NO. Among three resultant Mn-CeO_x/Ti-PILCs, the catalyst from TiOSO₄ showed the highest activity with 98% NO conversion at 220℃, it also exhibited good resistance to H₂O and SO₂ in flue gas. The catalyst from TiCl₄ exhibited the lowest activity due to the unsuccessful pillaring process.     

Enhanced catalytic complete oxidation of 1,2-dichloroethane over mesoporous transition metal-doped γ-Al2O3

High-surface-area mesoprous powders of γ-Al₂O₃ doped with Cu²⁺, Cr³⁺, and V³⁺ ions were prepared via a modified sol-gel method and were investigated as catalysts for the oxidation of chlorinated organic compounds. The composites retained high surface areas and pore volumes comparable with those of undoped γ-Al₂O₃ and the presence of the transition metal ions enhanced their surface acidic properties. The catalytic activity of the prepared catalysts in the oxidation of 1,2-dichloroethane (DCE) was studied in the temperature range of 250-400℃. The catalytic activity and product selectivity were strongly dependent on the presence and the type of dopant ion. While Cu²⁺- and Cr³⁺-containing catalysts showed 100% conversion at 300℃ and 350℃, V³⁺-containing catalyst showed considerably lower conversion. Furthermore, while the major products of the reactions over γ-alumina were vinyl chloride (C₂H₃Cl) and hydrogen chloride (HCl) at all temperatures, Cu- and Cr-doped catalysts showed significantly stronger capability for deep oxidation to CO₂.     

Simultaneous removal of NO and dichloromethane（CH2Cl2） over Nb-loaded cerium nanotubes catalyst

Herein, a series of niobium oxide supported cerium nanotubes（Ce NTs） catalysts with different loading amount of Nb₂O₅（0–10 wt.%） were prepared and used for selective catalytic reduction of NOxwith NH₃（NH₃-SCR） in the presence of CH₂Cl₂. Commercial V₂O₅-WO₃-TiO₂ catalyst was also prepared for comparison. The physcial properties and chemical properties of the Nb₂O₅ lo...     

Unraveling SO2-tolerant mechanism over Fe2（SO4）3/TiO2 catalysts for NOx reduction

Developing low-temperature SO₂-tolerant catalysts for the selective catalytic reduction of NO_x is still a challenging task. The sulfation of active metal oxides and deposition of ammonium bisulfate deactivate catalysts, due to the difficult decomposition of the as-formed sulfate species at low temperatures（<300 °C）. In recent years, metal sulfate catalysts have attracted increasing attention owing to their good catalytic activity and strong SO₂ tolerance at hi...     

Mn/TiO2和Mn-Ce/TiO2低温脱硝催化剂的抗硫性研究

在低温选择性催化还原(SCR)反应条件下考察了烟气残余SO2对Mn/TiO2和Mn-Ce/TiO2催化剂选择性催化还原活性的影响,同时对SO2的影响机理进行了探讨.结果发现,Mn/TiO2催化剂在SO2反应气氛中失活很快,硫铵盐的沉积和活性组分的硫酸化是催化剂失活的重要原因;Ce的加入可以有效地抑制催化剂活性组分的硫酸化,同时还能降低硫酸盐在催化剂表面的稳定性,从而可以提高催化剂的抗硫性.     

NH3-SCR performance and the resistance to SO2 for Nb doped vanadium based catalyst at low temperatures

Niobium oxide as the promoter was doped in the V/WTi catalyst for the selective catalytic reduction(SCR)of NO.The results showed that the addition of Nb_2O_5could improve the SCR activity at low temperatures and the 6 wt.%additive was an appropriate dosage.The enhanced reaction activity of adsorbed ammonia species and the improved dispersion of vanadium oxide might be the reasons for the elevation of SCR activity at low temperatures.The resistances to SO_2of 3V6Nb/WTi catalyst at different temperatures were investigated.FTIR spectrum and TG-FTIR result indicated that the deposition of ammonium sulfate species was the main deactivation reason at low temperatures,which still exhibited the reactivity with NO above 200°C on the catalyst surface.There was a synergistic effect among NH_3,H_2O and SO_2that NH_3and H_2O both accelerated the catalyst deactivation in the presence of SO_2at 175°C.The thermal treatment at 400°C could regenerate the deactivated catalyst and get SCR activity recovered.The particle and monolith catalysts both kept stable NO_xconversion at 225°C with high concentration of H_2O and SO_2during the long time tests.     

Iron-doped Mn-Ce/TiO2 catalyst for low temperature selective catalytic reduction of NO with NH3

The catalysts of iron-doped Mn-Ce/TiO 2(Fe-Mn-Ce/TiO 2) prepared by sol-gel method were investigated for low temperature selective catalytic reduction(SCR) of NO with NH 3.It was found that the NO conversion over Fe-Mn-Ce/TiO 2 was obviously improved after iron doping compared with that over Mn-Ce/TiO 2.Fe-Mn-Ce/TiO 2 with the molar ratio of Fe/Ti = 0.1 exhibited the highest activity.The results showed that 96.8% NO conversion was obtained over Fe(0.1)-Mn-Ce/TiO 2 at 180°C at a space velocity of 50,000 hr 1.Fe-Mn-Ce/TiO 2 exhibited much higher resistance to H 2 O and SO 2 than that of Mn-Ce/TiO 2.The properties of the catalysts were characterized using X-ray diffraction(XRD),N 2 adsorption,temperature programmed desorption(NH 3-TPD and NOx-TPD),and Xray photoelectron spectroscopy(XPS) techniques.BET,NH3-TPD and NOx-TPD results showed that the specific surface area and NH3 and NOx adsorption capacity of the catalysts increased with iron doping.It was known from XPS analysis that iron valence state on the surface of the catalysts were in Fe3+ state.The doping of iron enhanced the dispersion and oxidation state of Mn and Ce on the surface of the catalysts.The oxygen concentrations on the surface of the catalysts were found to increase after iron doping.Fe-Mn-Ce/TiO2 represented a promising catalyst for low temperature SCR of NO with NH3 in the presence of H2 O and SO2.     

Na2SO4中毒SCR催化剂对SO3生成特性的影响

为研究Na₂SO₄中毒SCR催化剂（V₂O₅-WO₃/TiO₂催化剂）对SO₃生成特性的影响,采用湿式浸渍法制备w（Na）为3%的Na₂SO₄中毒SCR催化剂,并通过N₂物理吸附/脱附、XRD（X射线衍射）技术、SEM（扫描电镜）、XPS（X射线光电子能谱）分析技术对催化剂的物理化学特性进行表征.结果表明:①随着反应温度的升高,所有催化剂上的SO₃生成率逐渐增加.当温度升至490℃时,SCR催化剂上的SO₃生成率为0.85%,而3% Na₂SO₄中毒SCR催化剂上的SO₃生成率高达1.36%.SO₄2-的存在导致V-O-S增多,从而促进SO₃的生成.②随入口ρ（SO₂）的增加,SO₃生成率呈下降的趋势.当入口ρ（SO₂）为1 000 mg/m3时,3% Na₂SO₄中毒SCR催化剂上的SO₃生成率为1.02%,而SCR催化剂上仅为0.60%.ρ（SO₂）对SO₃生成率的影响主要依赖于温度和催化剂活性位点数等.③N₂物理吸附/脱附、XRD和SEM表征结果表明,与SCR催化剂相比,Na₂SO₄中毒SCR催化剂表面有Na₂SO₄的积聚,出现了裂纹和大孔隙,催化剂的比表面积和孔容下降,这些变化均不利于催化剂的催化性能;XPS结果表明,Na₂SO₄的加入提高了表面化学吸附氧含量,降低了活性组分中w（V4+）/w（V5+）的值.研究显示,相比于SCR催化剂,Na₂SO₄中毒SCR催化剂上的SO₃生成率大幅增加.      

Fe2O3添加对钛基SCR催化剂表面硫酸氢铵分解行为的影响规律

采用共沉淀法合成了TiO_2及TiO_2-Fe_2O_3载体,并对硫酸氢铵与上述载体之间的相互作用及硫酸氢铵的具体分解行为进行了研究.结果表明,催化剂载体表面含硫官能团主要以双齿硫酸盐的形式存在,含氮官能团以铵根离子的形式存在.当硫酸氢铵沉积于催化剂载体表面时,由于硫酸根离子具有较强的电负性,Ti原子及Fe原子处于电子缺失状态.对于TiO_2载体,硫酸根离子主要与Ti原子相连;而对于TiO_2-Fe_2O_3载体,Ti原子及Fe原子均为硫酸根离子主要的附着位点.采用热分析方法及原位红外对硫酸氢铵在TiO_2及TiO_2-Fe_2O_3载体表面的分解行为进行了研究,发现铁氧化物的添加显著促进了硫酸氢铵在低温区间内的分解行为;与铵根离子相比,硫酸根离子具有更高的热稳定性.催化剂稳定性测试结果表明,铁氧化物的添加显著提高了低温抗硫抗水性能,为实现低温SCR技术的工业应用提供了理论基础.     

钒和钨负载量对V2O5-WO3/TiO2表面形态及催化性能的影响

采用浸渍法制备了一系列不同钒和钨负载量的V₂O₅-WO₃/TiO₂催化剂样品,对样品NH₃选择性催化还原NO性能进行了评价,并用BET、XRD、XPS等手段对催化剂样品的表面形态进行了表征.研究发现,钒的负载量对催化剂的比表面积和催化活性有显著影响,当钒负载量从1％升高到8％时,催化剂比表面积下降了16 m²/g,最高活性温度降低了约100℃.钨起到稳定剂和助剂的双重作用,当钒负载量为1％时,钨负载量从0升高到6％,催化剂比表面积仅下降了3 m²/g,而活性温度窗口向高温和低温各拓宽了约50℃.研究表明钒和钨负载量都能影响催化剂表面的VO_x物种,但对催化剂的表面晶型没有明显影响.     

SO2对MnOx/PG催化剂低温脱硝的影响机理研究

采用等体积浸渍法制备了锰氧化物负载凹凸棒石(MnO_x/PG)低温SCR催化剂,通过SO₂暂态响应、程序升温表面反应(TPSR)等实验技术研究了烟气中SO₂对催化剂SCR脱硝活性的影响行为.采用程序升温脱附(TPD)、BET比表面及孔径分布测定、XPS等表征技术对催化剂硫中毒的机理及化学本质进行了深入分析.结果表明,低温下烟气中SO₂对MnO_x/PG催化剂的SCR脱硝活性存在显著的抑制作用,催化剂中毒主要由烟气中SO₂的催化氧化引起.一方面SO₂氧化为SO₃后与NH₃及H₂O竞争反应形成复杂的硫酸铵盐堵塞催化剂孔道,另一方面与活性组分MnO₂结合形成MnSO₄使得部分活性组分形态发生变迁.其中硫酸铵盐的形成可通过适当的热处理得以去除,而MnSO₄则不可恢复,但催化剂SCR活性却显著增加,表明MnSO₄的形成不是催化剂失活的主要因素.吸附态的硫可显著增加催化剂表面酸性,因此对SCR活性有促进作用.催化剂失活主要机理为:由气相SO₂的连续氧化并与NH₃相结合形成硫酸铵盐,并且在低温下难以分解,以致堵塞催化剂活性中心.     

尖晶石型MnCo_2O_4催化剂的制备及SCR性能研究

用柠檬酸配位燃烧法制备了尖晶石复合金属氧化物MnCo2O4,使用X射线衍射仪、傅立叶红外分析仪对其进行了表征,并进一步在固定床反应器中考查了该催化剂的催化氧化、还原NO的性能以及SO2的影响。结果表明,所制备的MnCo2O4催化剂呈现典型的尖晶石结构,在300℃,空速20000h-1时,NO氧化率可以达到30%,而NH3催化还原NO的效率可达99.5%,但SO2对其还原性能有明显影响。