Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

The adsorption and transformation of ammonia over V₂O₅, V₂O₅/TiO₂, V₂O₅-WO₃/TiO₂ and CuO/TiO₂ systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH₂ amide species or to its dimeric form N₂H₄, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N₂⁻ and azide anions N₃⁻ are further observed over CuO/TiO₂. The comparison of the infrared spectra of the species arising from both NH₃ and N₂H₄ adsorbed over CuO/TiO₂ strongly suggest that N₂H₄ is an intermediate in NH₃ oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH₂ species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V₂O₅-based systems, but not over CuO/TiO₂, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity.     

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

TiO₂-SiO₂ with various compositions prepared by the coprecipitation method and vanadia loaded on TiO₂-SiO₂ were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH₃ and in the undesired oxidation of SO₂ to SO₃, using BET, XRD, XPS, NH₃-TPD, acidity measurement by the titration method and activity test. TiO₂-SiO₂, compared with pure TiO₂, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V₂O₅. The SCR activity and N₂ selectivity are found to be more excellent over vanadia loaded on TiO₂-SiO₂ with 10–20 mol% of SiO₂ than over that on pure TiO₂, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH₃ adsorbed on the catalysts. With increasing SiO₂ content, the remarkable activity decrease in the oxidation of SO₂ to SO₃, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V⁴⁺ on TiO₂-SiO₂, while V⁵⁺ exists on TiO₂, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO₂-SiO₂ with low SiO₂ content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO_x activity but also the low SO₂ oxidation performance.     

Combined effect of H2O and SO2 on V2O5/AC catalysts for NO reduction with ammonia at lower temperatures

Combined effect of H₂O and SO₂ on V₂O₅/AC the activity of catalyst for selective catalytic reduction (SCR) of NO with NH₃ at lower temperatures was studied. In the absence of SO₂, H₂O inhibits the catalytic activity, which may be attributed to competitive adsorption of H₂O and reactants (NO and/or NH₃). Although SO₂ promotes the SCR activity of the V₂O₅/AC catalyst in the absence of H₂O, it speeds the deactivation of the catalyst in the presence of H₂O. The dual effect of SO₂ is attributed to the SO₄²⁻ formed on the catalyst surface, which stays as ammonium-sulfate salts on the catalyst surface. In the absence of H₂O, a small amount of ammonium-sulfate salts deposits on the surface of the catalyst, which promote the SCR activity; in the presence of H₂O, however, the deposition rate of ammonium-sulfate salts is much greater, which results in blocking of the catalyst pores and deactivates the catalyst. Decreasing V₂O₅ loading decreases the deactivation rate of the catalyst. The catalyst can be used stably at a space velocity of 9000 h⁻¹ and temperature of 250 °C.     

The role of acidity in the decomposition of 1,2-dichlorobenzene over TiO2-based V2O5/WO3 catalysts

The influence of Lewis and Brønsted acid sites on the performance of V₂O₅/TiO₂ and V₂O₅–WO₃/TiO₂ catalysts in the total oxidation of o-dichlorobenzene was investigated. Catalytic activity of these materials resulted strongly affected by their acidic properties. The presence of Brønsted acid sites significantly increases the o-DCB conversion but also leads to the uncompleted degradation of chlorinated compounds, promoting the formation of partial oxidation products, as dichloromaleic anhydride. On the contrary, Lewis acid sites, acting as absorbing sites, promote the further oxidation of intermediates to CO and CO₂, without any by-products desorption.

Furthermore, the presence of water in the feed-stream was proven to decrease o-DCB conversion but also to play a positive role on process selectivity, increasing COx production. Plausible reasons for this effect are the reduction of Brønsted acid sites and the hydrolysis of anhydride during wet tests.     

DRIFTS investigation of V=O behavior and its relations with the reactivity of ammonia oxidation and selective catalytic reduction of NO over V2O5 catalyst

The behavior of V=O band over V₂O₅ crystallite during NH₃ adsorption and SCR reaction was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the results are correlated with the reactivity in NH₃ oxidation and SCR reaction. It is found that the decrease of V=O band intensity is due either to the reduction of V₂O₅ surface and/or to the adsorption of ammonia. The 70% intensity of original V=O band is preserved up to 573 K under the conditions of SCR reaction. The vanadium oxidation state is about +4.4. When the temperature reached 673 K, almost all the V=O band was recovered. From these results, it can be suggested that the decrease of the apparent SCR activity due to the increase of NO amount through NH₃ oxidation above 673 K be attributed to the increase of two neighboring V=O sites, which favor the NO formation in ammonia oxidation.     

Low temperature SCR of NO with NH3 over carbon nanotubes supported vanadium oxides

A novel multiwalled carbon nanotube (CNTs) supported vanadium catalyst was prepared. The structure of catalyst prepared was characterized by TEM, BET, FTIR, XRD and temperature-programmed desorption (TPD) methods. The results indicated that vanadium particles were highly dispersed on the wall of carbon nanotubes. The V₂O₅/CNT catalysts showed good activities in the SCR of NO with a temperature range of 373–523 K. The Lewis acid sites on the surface of V₂O₅/CNT are the active sites for the selective catalytic reduction (SCR) of NO with NH₃ at low temperatures. It was suggested that the reaction path might involve the adsorbed NH₃ species reacted with NO from gaseous phase and as well as the adsorbed NO₂ species. The diameter of CNTs showed positive effect on the activities of the catalysts. Under the reaction conditions of 463 K, 0.1 Mpa, NH₃/NO = 1, GHSV = 35,000 h⁻¹, and V₂O₅ loading of 2.35 wt%, the outer diameter of CNTs of 60–100 nm, the NO conversion was 92%.     

Deactivation of V2O5-WO3-TiO2 SCR catalyst at a biomass-fired combined heat and power plant

The deactivation of a commercial type V₂O₅-WO₃-TiO₂ monolith catalyst under biomass combustion was studied at a full-scale grate-fired power plant burning straw/wood using a slip stream pilot scale reactor. The aerosols in the flue gas consisted of a mixture of potassium chloride and sulphate. Three catalyst elements were exposed at 350 °C, and one element was exposed at 250 °C for comparison. The catalyst activity was measured in the reactor at the exposure temperature by addition of NH₃ and extra NO. The activity, in terms of a first-order rate constant, dropped by 52% after about 1140 h indicating a very fast deactivation compared to coal firing. It was also found that the reactor temperature was not of importance for the deactivation rate. SEM-EDX analysis showed that particle deposition and pore blocking contributed to the deactivation by decreasing the diffusion rate of NO and NH₃ into the catalyst. However, potassium also penetrated into the catalyst wall and the resulting average K/V ratio in the catalyst structure was high enough (about 0.3–0.5) for a significant chemical deactivation. Chemisorption studies carried out in situ showed that the amount of chemisorbed NH₃ on the catalyst decreased as a function of exposure time, which reveals that Brøndsted acid sites had reacted with potassium compounds and thereby rendered inactive. When washed by 0.5 M H₂SO₄ the regenerated catalyst regains a higher activity than that of the fresh catalyst at temperatures higher than 300 °C, but even though reactivation is possible, the deactivation rate appears too high for practical use of the SCR process in straw combustion.     

SO2 resistant antimony promoted V2O5/TiO2 catalyst for NH3-SCR of NOx at low temperatures

To get the low temperature sulfur resistant V₂O₅/TiO₂ catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V₂O₅/TiO₂ catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD) and Brunner Emmett Teller surface area (BET-SA). Se, Sb, Cu, S promoted V₂O₅/TiO₂ catalysts showed high catalytic activity for NH₃ selective catalytic reduction (NH₃-SCR) of NO_x carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V₂O₅/TiO₂ > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V₂O₅/TiO₂ for maximum NO_x conversion, which also showed high resistance to SO₂ in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V₂O₅/TiO₂ and compared with commercial W(10%)V₂O₅/TiO₂ catalyst. High electrical conductivity difference (ΔG) for Sb(2%)/V₂O₅/TiO₂ catalyst with temperature was observed. SO₂ deactivation experiments were carried out for Sb(2%)/V₂O₅/TiO₂ and W(10%)/V₂O₅/TiO₂ at a temperature of 230 °C for 90 h, resulted Sb(2%)/V₂O₅/TiO₂ was efficient catalyst. BET-SA, X-ray photoelectron spectroscopy (XPS) and carbon, hydrogen, nitrogen and sulfur (CHNS) elemental analysis of spent catalysts well proved the presence of high ammonium sulfate salts over W(10%)/V₂O₅/TiO₂ than Sb(2%)/V₂O₅/TiO₂ catalyst.     

Factors controlling the selectivity of V2O5 supported catalysts in the oxidative dehydrogenation of propane

The surface properties of a series of V₂O₅ catalysts supported on different oxides (Al₂O₃, H–Na/Y zeolite, MgO, SiO₂, TiO₂ and ZrO₂) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH₃ adsorption. In the case of the V₂O₅/SiO₂ system TEM images evidenced the presence of V₂O₅ crystallites, whereas such segregated phase was not observed for the other samples. VO_x species resulted widely spread on the surface of Al₂O₃, H–Na/Y zeolite, MgO and SiO₂, whereas on TiO₂ and ZrO₂ they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Brønsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V₂O₅/TiO₂ and V₂O₅/ZrO₂ catalysts.     

The importance of Fe loading on the N2O reduction with NH3 over Fe-MFI: Effect of acid site formation on Fe species

Effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts on the catalytic performance of N₂O reduction with NH₃ was investigated, and the results indicated that the turnover frequency (TOF) was almost constant in the Fe/Al range between 0.05 and 0.40. The activity of N₂O + NH₃ reaction was much lower than that of N₂O + CH₄ reaction over Fe-MFI (Fe/Al = 0.40), and the preadsorption of NH₃ decreased drastically the activity of N₂O + CH₄ reaction. The temperature-programmed desorption (TPD) of NH₃ showed the formation of stronger acid sites on Fe-MFI (Fe/Al = 0.24 and 0.40), and the amount of the acid sites agrees well with the desorption amount O₂ in O₂-TPD in the low temperature range. The acid sites gave a 3610 cm⁻¹ peak (Brønsted acid) in FTIR observation. These results suggest that the acid sites were formed on the bridge oxide ions in binuclear Fe species. Adsorbed NH₃ on the strong acid sites inhibited N₂O dissociation, which can be related to the low activity of N₂O + NH₃ reaction over Fe-MFI with high Fe loading.     

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.     

水热处理和钨添加对低钒催化剂高温脱硝性能的影响

利用真空过量浸渍法在TiO₂和WO₃-TiO₂载体上负载0.5%钒（质量分数）制备了V₂O₅-TiO₂ （VTi）和V₂O₅/WO₃-TiO₂（VWTi）催化剂,并在含有10%（体积分数）水蒸气的空气氛下750℃水热处理24h得到处理态的VTi-A和VWTi-A催化剂。研究了水热老化和钨掺杂对催化剂脱硝效率的影响,并通过XRD、TEM、NH₃-TPD、H₂-TPR、XPS和Raman等表征方法对催化剂的结构和表面特性进行了表征分析。研究表明,水热处理和钨的协同作用使催化剂中产生更多的聚合态VO_x和低价态钒（V⁴⁺+V³⁺）。这两类活性物质所含有的氧物种具有较强SCR反应活性,可改善VWTi催化剂的高温脱硝性能,即使在酸性位数量减少的情况下仍有提升,这是低钒含量催化剂可满足燃气机组高温脱硝需求的主要原因。     

Effect of promoters including tungsten and barium on the thermal stability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3

The effect of tungsten and barium on the thermal stability of V₂O₅/TiO₂ catalyst for NO reduction by NH₃ was examined over a fixed bed flow reactor system. The activity of V₂O₅/sulfated TiO₂ catalyst gradually decreased with respect to the thermal aging time at 600 °C. The addition of tungsten to the catalyst surface significantly enhanced the thermal stability of V₂O₅ catalyst supported on sulfated TiO₂. On the basis of Raman and XRD measurements, the tungsten on the catalyst surface was identified as suppressing the progressive transformation of monomeric vanadyl species into crystalline V₂O₅ and of anatase into rutile phase of TiO₂. However, the NO removal activity of V₂O₅/sulfated TiO₂ catalyst including barium markedly decreased after a short aging time, 6 h at 600 °C. This may be due to the transformation of vanadium species to inactive V–O–Ba compound by the interaction with BaO which was formed by the decomposition of BaSO₄ on the catalyst surface at high reaction temperature of 600 °C. The addition of SO₂ to the feed gas stream could partly restore the NO removal activity of thermally aged V₂O₅/sulfated TiO₂ catalyst containing barium.     

WO_3负载量对V_2O_5/WO_3-TiO_2催化剂脱硝性能的影响

采用V_2O_5/WO_3-TiO_2作为脱硝催化剂,考察活性组分V_2O_5和助剂WO_3负载量对催化剂脱硝活性和抗硫抗水性能的影响。结果表明,3%V_2O_5/x WO_3-TiO_2催化剂(x=3%、4%、5%、6%、7%、8%、9%、10%)上NOx转化率随着WO_3负载量增加而升高,催化剂反应温度窗口不断拓宽。单独通水蒸汽及同时通SO2和水蒸汽对催化剂的毒害作用均较强,表明H2O和NH3的竞争吸附是催化剂抗硫抗水性能较差的重要原因。SO_2与H_2O和NH_3反应生成亚硫酸铵盐和硫酸铵盐,导致催化剂孔隙堵塞,催化活性降低。     

Catalytic activity of Pd loaded on WO3/Al2O3 for NO–CH4–O2 in the presence of water vapor

Pd loaded on various kinds of monolayer supports was applied for selective reduction of NO by methane in the presence of O₂ and water vapor. Pd/WO₃/Al₂O₃ exhibited the highest conversion of NO to N₂ among Pd loaded monolayer supports. The catalyst was relatively tolerant and reversible upon the exposure of water vapor. This is due to the enhanced amount of Brønsted acid sites under the moisture as evidenced by the IR measurement of adsorbed pyridine. The Brønsted acid sites generated on WO₃/Al₂O₃ support were required to give the dispersed Pd species, similar to on the zeolite.     

Characterization and catalytic activity of Pd/V2O5/Al2O3 catalysts on benzene total oxidation

The role of vanadium oxide and palladium on the benzene oxidation reaction over Pd/V₂O₅/Al₂O₃ catalysts was investigated. The Pd/V₂O₅/Al₂O₃ catalysts were more active than V₂O₅/Al₂O₃ and Pd/Al₂O₃ catalysts. The increase of vanadium oxide content decreased the Pd dispersion and increased the benzene conversion. A strong Pd particle size effect on benzene oxidation reaction was observed. Although the catalysts containing high amount of V⁴⁺ species were more active, the Pd particle size effect was responsible for the higher activity.     

Activity and acidity of Nb2O5-MoO3 and Nb2O5-WO3 in the Friedel-Crafts alkylation

Nb₂O₅ loaded on the supports and mixed with oxides was studied to investigate the activity and acidity for Friedel-Crafts benzylation of anisole. From the study on the loaded catalysts, a preliminary conclusion for the selection of metal oxide was obtained; namely, such an acidic oxide as silica was suitable for the support of Nb₂O₅. Then, MoO₃ and WO₃ were mixed with Nb₂O₅, and prominent high catalytic activity and acidities were observed. Both oxides of Nb₂O₅-MoO₃ and Nb₂O₅-WO₃ showed almost similar behavior with respect to characterization and catalytic activity. Surface area increased, X-ray diffraction (XRD) and Raman bands were lost, acid sites, both Brønsted and Lewis characters generated, and surface acid site density was as high as 2–4 nm⁻². The acid sites were generated on the amorphous metal oxides consisting of Nb and Mo or W oxides, different in nature from those of Nb₂O₅ calcined and un-calcined, and active for Friedel-Crafts benzylation.     

溶胶-凝胶原位合成宽活性温度V2O5/TiO2脱硝催化剂

利用溶胶-凝胶技术原位合成一系列不同V₂O₅担载量的V₂O₅/TiO₂催化剂,通过BET、XRD、NH₃-TPD及紫外-可见光等手段对催化剂进行表征。结果表明：制备的催化剂均具有介孔结构,V₂O₅在TiO₂表面高度分散,且存在3种典型的酸性位。通过选择性催化还原反应对V₂O₅/TiO₂催化剂进行活性评价,结果显示随着V₂O₅含量的增加,NO转化率大于75%的温度窗口向低温方向偏移,含10% （质量分数）V₂O₅的催化剂的NO转化率为80%的温度窗口最宽为200～450℃,240℃时20 h连续实验表现出稳定的抗硫抗水性能。结合紫外-可见光谱分析,揭示了钒掺杂所形成的单聚和低聚钒酸盐为催化剂的活性组分。     

In-containing H-ZSM5 zeolites with various Si/Al ratios for the NO SCR in the presence of CH4 and O2. PAC, TPAD and FTIR studies

A series of In-loaded ZSM5 catalysts with Si/Al ratios of 17, 27 and 50 was studied. The catalytic activity for the NO reduction followed the 27 > 17 = 50 order. The acid properties were investigated by FTIR of pyridine as probe molecule desorbed in vacuum at different temperatures and by Temperature-Programmed Ammonium Desorption (TPAD) of NH₄–In–ZSM5. The nature of the In-species was determined by the Perturbed Angular Correlation (PAC) technique using ¹¹¹In as a probe. The contribution of indium to the acidity nature of the samples seems to be important taking into account that the number of Brönsted acid sites was reduced after the In exchange. In the same way new and strong electron-donor acceptor sites were generated. The PAC results indicate that there exists an important fraction of the indium present in the active sample coordinated as in the In₂O₃ case, while another one corresponding to a non-well defined near-In-neighborhood, is present too in a small fraction.