Studies on structural properties, surface acidity and benzene isopropylation activity of sulphated ZrO2–TiO2 mixed oxide catalysts

A series of sulphated ZrO₂–TiO₂ mixed oxide with different nominal sulphate loadings in the range of 2–15 wt.% was prepared and characterized for their structural properties, surface acidity and benzene isopropylation activity. The catalyst with 10 wt.% nominal sulphate loading showed highest surface area and uniform pore size distributions. Surface acidity, measured by NH₃–TPD method, showed increase in acidity with sulphate loading and the 10 wt.% sulphate loaded catalyst showed highest acidity. The activities of these catalysts were tested for isopropylation of benzene to cumene using 2-propanol as the alkylating agent. The 10 wt.% sulphate-loaded catalyst also showed highest activity for this reaction with 97% cumene selectivity. The higher activity of this catalyst was attributed to its higher acidity.     

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution: Part II. Characterization study of the effect of alkali and alkaline earth metals

A characterization study on a practice-oriented V₂O₅/WO₃–TiO₂ SCR catalyst deactivated by Ca and K, respectively, was carried out using NH₃-TPD, DRIFT spectroscopy, and XPS as well as theoretical DFT calculations. It was found from NH₃-TPD experiments that strongly basic elements like K or Ca drastically affect the acidity of the catalysts. Detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V₂O₅ active phase, thus affecting the NH₃ adsorption. Moreover, these experiments also indicated that the V⁵⁺ = O sites are much less reactive on the poisoned catalysts. XPS investigations of the O 1s binding energies showed that the oxygen atoms of the V⁵⁺ = O sites are affected by the presence of the poisoning agents. Based on these results and on DFT calculations with model clusters of the vanadia surface, the poisoning mechanism is explained by the stabilization of the non atomic holes of the (0 1 0) V₂O₅ phase as a result of the deactivation element. Consequently, V–OH Brønsted acid sites and V⁵⁺ = O sites are inhibited, which are both of crucial importance in the SCR process. The deactivation model also gives an explanation to the very low concentrations of potassium needed to deactivate the SCR catalyst, since one metal atom sitting on such a non-atomic hole site deactivates up to four active vanadium centers.     

工业V2O5-WO3/TiO2催化剂的碱土中毒研究

以某燃煤电厂应用的V₂O₅-WO₃/TiO₂蜂窝型SCR脱硝催化剂为研究对象,在实验室条件下模拟该催化剂Ca、Mg及二者复合中毒,研究了不同浓度的Ca和Mg对催化剂活性的影响,并用XRD、H₂-TPR及NH₃吸附FT-IR对催化剂进行表征。结果表明,Ca和Mg对工业SCR脱硝催化剂均具有毒性作用,其中Mg毒性相对较强,复合中毒在一定程度上弱化单组分对催化剂活性的影响;催化剂组分分散度和结晶度不受Ca和Mg的影响,但CaO和MgO的掺入抑制V₂O₅的还原能力;CaO和MgO对催化剂表面的酸性位均存在影响,尤其是B酸位,二者复合中毒对酸性位的影响小于单一组分。     

SCR脱硝催化剂抗砷中毒改性优化与再生研究进展

选择性催化还原（SCR）是目前应用最为广泛的烟气脱硝技术,催化剂是整个SCR脱硝系统的核心。在实际应用过程中,催化剂存在各种失活问题,其中砷中毒是催化剂失活的重要原因之一。本文详细阐述了SCR脱硝催化剂砷中毒的物理和化学失活机理,其中物理失活是由于As₂O₃在催化剂表面沉积、氧化造成催化剂孔道堵塞所致,而化学失活是由于砷氧化物破坏催化剂酸位点、改变活性基团形态、降低催化剂氨吸附及氧化还原能力所致。然后,系统介绍了抗砷中毒SCR脱硝催化剂的研发路线以及现有抗砷中毒催化剂优化改进的主要技术手段,主要包括调整催化剂孔隙结构、优化催化剂化学配方和烟气侧砷氧化物吸附固化等,其中MoO₃是优选的催化剂活性助剂,金属元素（如Bi、In、Sn、Mg）是主要的抗砷助剂,钙基物质是典型的烟气侧砷氧化物吸附添加剂。最后,对砷中毒废弃催化剂的再生技术进行了简要介绍,包括湿法清洗、热还原法、复合再生等,在实际工业应用中,主要以物理清扫、湿法清洗配合活性组分添加的复合再生方式实现中毒催化剂再生。本文可对未来抗砷中毒SCR脱硝催化剂的研发与优化提供重要支撑。     

典型SCR脱硝催化剂钾中毒特征研究

以钾为代表的碱金属元素是引发SCR催化剂中毒的主要来源,通过浸渍法制备钾中毒催化剂,比较钾盐种类和钾盐浓度对催化剂脱硝活性的影响。结果表明,随着钾盐浓度的增加,催化剂比表面积下降,并出现板结现象;在KNO3、K2SO4和KCl钾盐中,同浓度下KCl造成的催化剂失活最严重。钾中毒对催化剂孔径结构和V2O5在载体TiO2上的分散度影响不显著;催化剂吸附表面因中毒会有粒子积聚成团,从而增大催化剂表面粗糙度;同时,钾中毒导致催化剂表面酸性位点的毒害,与新鲜催化剂相比,中毒后催化剂表面总酸量下降约62%。     

Effect of phosphate ion on the textural and catalytic activity of titania–silica mixed oxide

Phosphate-modified TiO₂-SiO₂ mixed oxide catalysts have been prepared by varying the method of preparation, source and concentration of phosphate ion. The prepared catalysts were compared for their catalytic activity/selectivity in nitration of toluene. The characterisation of the catalysts was performed using X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermal analysis (TG–DTA), nitrogen adsorption–desorption methods, surface acid strength measured by Hammett indicator method, surface acid sites measured by amine titration method, and phosphate content measured by UV–VIS spectrophotometry. The XRD patterns revealed that phosphate ion stabilises the anatase phase up to 1173 K activation. FT-IR results show that phosphate species strongly bound bidentately, and that both the internal weakly H-bonded hydroxyl groups and free hydroxyl groups are present on TiO₂–SiO₂ mixed oxide support. Surface area and surface acidity are found to increase with the increase in phosphate loading up to 7.5 wt.% and thereafter the values decrease drastically. However, average pore radius and total pore volume shows the reverse order. Phosphated samples prepared using H₃PO₄ as the source of phosphate ion exhibit higher acidity, and surface area but lower porosity than the samples prepared from (NH₄)₃PO₄, though both the samples contain the same amount of phosphate (7.5 wt.%). Similar results were also observed when varying the method of preparation. TiO₂–SiO₂ samples prepared at pH=3 exhibit higher acidity and surface area but lower porosity than the samples prepared at pH=7. The acid strength of 7.5P/TiO₂–SiO₂ (H) is found to be stronger than that of 100% concentrated H₂SO₄. The material modified with phosphate ion was found to be an efficient and selective catalyst for solvent-free mono-nitration of toluene. Selectivity to the para-product is correlated with the porosity of the material.     

Transition metal oxides supported on active carbons as low temperature catalysts for the selective catalytic reduction (SCR) of NO with NH3

The selective catalytic reduction of nitric oxide with ammonia was studied using Fe₂O₃, Cr₂O₃ and CuO loaded active carbons as catalysts in the presence and absence of oxygen at reaction temperatures between 100 and 500°C. Active carbons pretreated with concentrated nitric acid showed a higher catalytic activity in SCR than catalysts which were oxidized in air. The ash content (from 0.2 to 7.1 wt.%) of different unloaded active carbons had no effect on the catalytic activity below 300°C in the absence of oxygen. However, in the presence of oxygen an increasing ash content resulted in an increase in activity. Transition-metal oxide loading led to an increase in SCR activity, especially in the absence of oxygen. An increasing transition-metal content from 1 to 10 wt.% improved the activity as well. The presence of oxygen in the reaction mixture enhanced the conversion of nitric oxide especially in the low-temperature range between 100 and 200°C. Activity and selectivity of the respective catalysts were influenced by the type of metal oxide: in the presence of oxygen, catalysts with 10 wt.% Fe were the most active and selective.     

NO reduction by CH4 in the presence of excess O2 over Mn/sulfated zirconia catalysts

Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3 wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH₃–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N₂ adsorption and the H₂–TPR of the catalysts demonstrated that the presence of the SO₄²⁻ species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH₄ by O₂ and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO₄²⁻. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO₂ poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO₂.     

WO3–TiO2 monolithic catalysts for high temperature SCR of NO by NH3: Influence of preparation method on structural and physico-chemical properties, activity and durability

The WO₃–TiO₂ catalysts with different WO₃ loadings prepared by the coprecipitation method were investigated in comparison with those prepared by the conventional impregnation method for the activity and durability in the high temperature SCR of NO by NH₃ and the structural and physico-chemical properties which were characterized by BET and XRD measurements, IR, Raman and XPS spectroscopies. The catalyst prepared by coprecipitation, as compared with that prepared by impregnation, was found to exhibit a higher SCR activity at high temperatures and also to possess a larger surface area, higher Brønsted acidity and larger monolayer capacity of the support with WO₃. Increasing the WO₃ loading of the catalysts enhances the SCR activity and simultaneously increases the Brønsted acidity. The observed improvement of SCR activity for the catalyst prepared by coprecipitation is mainly attributed to the higher Brønsted acidity and the presence of the more highly dispersed WO₃ species which is suggested by the larger monolayer capacity of ca. 13 μmol(W)/m² and no crystalline WO₃ on TiO₂ detected with XRD at the high WO₃ loading up to 40 wt.%. The catalyst with 20 wt.% WO₃, as compared with that prepared by impregnation, was found to exhibit a better thermal durability at high temperatures from 550 to 600 °C. The better durability is attributed to that the reduction of the surface area and the formation and subsequent growth of crystalline WO₃ upon aging are more remarkably inhibited.     

Alkane conversion over Pd/SAPO molecular sieves: influence of acidity, metal concentration and structure

The activity and selectivity for the isomerization of heptanes over Pd-loaded SAPO molecular sieves with AEL and AFI structure were investigated. By synthesis of the AEL molecular sieves from Al-isopropoxide, the acid site concentration could be controlled by the amount of silicon in the synthesis gel. The resulting concentration of SiAl–OH groups ranged from 0.07 to 0.3 mmol/g, exhibiting the same acid strength, independent from the acid site concentration. Catalysts were impregnated with 0.1–2 wt.% Pd. The activity increased with increasing Pd content up to a Pd to acid site ratio of 0.02–0.03. Above this value, the metal loading did not influence the activity. A further enhancement of the activity was possible by increasing the number of SiAl–OH acid sites. The selectivity to isomerization was constant for the catalysts with an acid site concentration >0.19 mmol/g. The catalysts synthesized from Al-isopropoxide showed a lower activity compared to the catalysts synthesized from aluminum oxide, probably due to diffusional restrictions caused by the formation of polycrystalline agglomerates.     

钒钛基SCR脱硝催化剂碱土金属中毒

分别制备了钒钨体系和钒钼体系的新鲜催化剂,并通过浸渍法、固态扩散法与干混法分别制备了碱土金属中毒的催化剂,比较了中毒方法、碱土金属以及催化助剂种类等因素对催化剂脱硝活性的影响,并利用程序升温脱附（NH₃-TPD）、程序升温还原（H₂-TPR）和X射线光电子能谱分析（XPS）等表征手段进行分析。结果表明,3种中毒方法中,浸渍法碱土金属中毒对催化剂毒害作用最大;Ca对催化剂的毒性大于Mg,且钒钼体系催化剂比钒钨体系催化剂具有更强的抗碱土金属中毒性能。碱土金属的存在会降低催化剂表面的总酸量与酸位点的强度,提高催化剂的还原温度,改变活性组分的价态并降低催化剂上表面活性氧的比例。     

Effect of preparation methods on the hydrocracking performance of NiMo/Al2O3 catalysts

In this work, NiMo catalysts with various contents of MoO_3 were prepared through incipient wetness impregnation by a two-step method(NM-x A) and one-pot method(NM-xB). The catalysts were then characterized by XRD, XPS, NH_3-TPD, H_2-TPR, HR-TEM, and N2 adsorption–desorption technologies.The performance of the NiMo/Al_2O_3 catalysts was investigated by hydrocracking low-temperature coal tar. When the MoO_3 content was 15 wt%, the interaction between Ni species and Al_2O_3 on the NM-15 B catalyst was stronger than that on the NM-15 A catalyst, resulting in the poor performance of the former.When the MoO_3 content was 20 wt%, MoO_3 agglomerated on the surface of the NM-20 A catalyst, leading to decreased number of active sites and specific surface area and reduced catalytic performance. The increase in the number of MoS_2 stack layers strengthened the interaction between Ni and Mo species of the NM-20 B catalyst and consequently improved its catalytic performance. When the MoO_3 content reached 25 wt%, the active metals agglomerated on the surface of the NiMo catalysts, thereby directly decreasing the number of active sites. In conclusion, the two-step method is suitable for preparing catalysts with large pore diameter and low MoO_3 content loading, and the one-pot method is more appropriate for preparing catalysts with large specific surface area and high MoO_3 content. Moreover, the NMx A catalysts had larger average pore diameter than the NM-xB catalysts and exhibited improved desulfurization performance.     

Comparison study of Na poisoning effect on copper-based chabazite micropore catalysts for NH3-SCR reaction

Using wet impregnation method to mimic alkali metal poison of ammonia-selective catalytic reduction (NH₃-SCR) catalysts, Cu-based micropore zeolites Cu/SSZ-13 and Cu/SAPO-34 with different Na contents(mass fraction) were prepared, and the Na poisoning mechanism on them was studied. The results show that the externally introduced Na ions can severely affect the NH₃-SCR catalytic activity of the two catalysts, resulting in the collapse of the crystal structure of the catalyst, the decrease of acidity and the reduction of active species. In detail, when Na content was less than 1.82%, Cu/SAPO-34 has higher resistance of Na ions than Cu/SZZ-13; while when Na> 3.48%, Cu/SAPO-34 catalysts almost deactivated thoroughly. By structural characterization (BET, XRD and SEM) and acidity characterization (DRIFTS, NH₃-TPD and H₂-TPR), it was found that with Na poisoning deeper and deeper, Cu/SSZ-13 took a gradual style of the structural destruction, but Cu/SAPO-34 adopted a sudden way. Studies on the mechanism of Na poisoning show that the decrease of acid sites is the main reason for the decrease of SCR activity of Cu/SSZ-13, and the structural collapse is the main reason for the decrease of SCR activity of Cu/SAPO-34.     

Effect of the addition of alkali metals on the metallic phase of Pt/Al2O3 catalysts

The effect of the addition of alkali metals to Pt/Al₂O₃ catalysts for dehydrogenation of paraffins was studied. Results can be interpreted in terms of different effects. Thus, the addition of alkali metals produces a poisoning of the acid sites of the alumina support and a modification of the characteristics of the metallic phase. The latter effect involves not only an increase of the Pt particles size, but also an electronic modification of the metallic phase. These two effects are more marked with alkali metals with higher ionic radius, such as K.     

Deactivation of a Fe-ZSM-5 catalyst during the selective catalytic reduction of NO by n-decane: An operando DRIFT study

The selective catalytic reduction (SCR) of NO by n-decane was investigated on a Fe-ZSM-5 prepared by the FeCl₃ sublimation method. NO conversion profiles versus temperature were followed in both temperature programmed surface reaction (TPSR, 10 °C min⁻¹) and steady state experiments. A higher NO conversion with a maximum of ca. 80% at 400 °C is observed in the course of the TPSR tests. This phenomenon has been attributed to strong adsorption of n-decane which protects the active sites against the poisoning. Indeed, in steady state experiments at 390 °C the strong decrease of activity as a function of time on stream is due to the polymerisation of conjugated nitriles. This study indicates that long chain alkanes are not the most adequate reductants of NO for high temperature SCR applications. Moreover, due to an easier polymerization of conjugated nitriles on iron zeolites (stronger Fe Lewis sites), this type of catalyst seems less attractive than Cu-zeolite catalysts for the SCR of NO by hydrocarbons in this respect.     

基于NH3-SCR反应铜基小孔分子筛催化剂Na中毒对比研究

以浸渍法模拟碱金属中毒NH₃-SCR催化剂过程,制备不同Na含量（质量分数）的铜基小孔分子筛Cu/SSZ-13和Cu/SAPO-34,对比研究了二者的碱金属中毒机理。结果表明,外引Na离子均可严重影响两种催化剂的NH₃-SCR催化活性,造成催化剂的晶相结构坍塌,酸性量减少,活性物种减少。不同的是,Na引入量较低（<1.82%）时,Cu/SAPO-34比Cu/SSZ-13具有更强的Na离子耐受性,而当Na含量高于3.48%时,Cu/SAPO-34几乎完全丧失NH₃-SCR催化活性。通过催化剂的结构表征（BET、XRD和SEM）和酸性位表征（DRIFTS、NH₃-TPD和H₂-TPR）,研究表明随着Na中毒程度的加深,Cu/SSZ-13的结构破坏是渐变式的,而Cu/SAPO-34的结构破坏是突变式的;Na中毒的机理研究表明,酸性位的减少是Cu/SSZ-13的SCR活性下降的主导原因,结构坍塌是Cu/SAPO-34的SCR活性下降的主导原因。     

Research progress in the SO2resistance of the catalysts for selective catalytic reduction of NOx

The selective catalytic reduction(SCR)of NOxwith NH3has been proven to be an efficient technology for NOx conversion to N2.However,the catalysts used for SCR usually suffer from the problem of sulfur poisoning which seriously limits their practical application.This review summarized sulfur poisoning mechanisms of various SCR deNOxcatalysts and strategies to reduce deactivation caused by SO2such as doping metals,control-ling the structures and morphologies of the catalysts,and selecting appropriate supports.The methods and procedures of catalysts preparation and the reaction conditions also have effect on SO2-resistance of the catalysts. Several novel catalyst systems that exhibited good SO2resistance are also introduced.This paper could provide guidance for the development of highly efficient sulfur-tolerant deNOxcatalysts.     

Isobutane/1-butene alkylation on LaNaY catalysts modified by alkali and alkaline-earth cations

The effect of incorporating alkali and alkaline-earth cations into La exchanged NaY zeolites on the alkylation of isobutane with 1-butene has been investigated. The strength of the acid sites of the LaMNaY (M = Group I A and Group II A elements) catalysts do not change significantly, while the density of acidic sites decrease in the order of Na > K > Rb > Cs in the alkali Group I A elements and Ba > Mg > Ca > Sr in the alkaline-earth Group II A elements. The LaBaNaY catalyst has a higher density of acid sites than expected based on the electronegativity of Ba among the Group II A elements . A higher yield of alkylation and oligomerization products and also a higher amount of carbonaceous deposit were observed on the LaMNaY catalysts with a lower density of acidic sites except for LaBaNaY . Thermal gravimetric and differential thermal analysis indicate that the carbonaceous material could be located on the pore mouth and inside the pores of the zeolite. The carbonaceous material on the pore mouth can be desorbed before 300°C, while that in the zeolite pore is more difficult to be removed and a series of reactions such as dehydrogenation, aromatization and polymerization may take place during a temperature-programmed oxidation process. The diffusion of oligomerization products from the zeolite pore appears to be a slow step. The accumulation of these products in the zeolite pores is believed to be the main factor for the catalyst deactivation.     

Enhanced alkali resistance of CeO2/SO42 −–ZrO2 catalyst in selective catalytic reduction of NOx by ammonia

Ceria catalysts supported on sulfated zirconia showed a remarkable resistance towards alkali metal poisoning in selective catalytic reduction of NO by NH₃. TPD results revealed sulfation treatment of supports produced massive acid sites on the surface of catalysts. Adequate acidity was of importance for the enhanced resistance to alkali metal ions. The introduction of potassium did not affect the crystalline phases and morphology of catalysts, but it could lead to higher surface areas of samples.     

Fischer–Tropsch synthesis over Co–SiO2 catalysts prepared by the sol–gel method

Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO₂ catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO₂ catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO₂ was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N,N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO₂, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO₂ prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO₂.