Low temperature selective catalytic reduction of NOx with NH3 over amorphous MnOx catalysts prepared by three methods

Unsupported manganese oxide catalysts with amorphous phase were prepared by three methods, and their activities for SCR of NO_x with ammonia were investigated in the presence of O₂. The results showed the catalysts have superior low temperature activity, and the NO_x conversion is about 98% at 80 °C, and nearly 100% NO_x conversion between 100 and 150 °C. Due to competing adsorption with the reactant, H₂O has slight impact on the activity. The activity was suppressed with coexisting of SO₂, however the deactivation of SO₂ is reversible. The excellent low temperature catalytic activity of amorphous MnO_x catalysts is mainly due to their amorphous phase and high specific areas.     

Redox features in the catalytic mechanism of the “standard” and “fast” NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods

The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO₂ by ammonia at low temperature was investigated by transient reactive experiments over a commercial V₂O₅/WO₃/TiO₂ catalyst for diesel exhaust aftertreatment. NO + NH₃ temperature-programmed reaction runs over reduced catalyst samples pretreated with various oxidizing species showed that both NO₂ and HNO₃ were able to reoxidize the V catalyst at much lower temperature than gaseous O₂: furthermore, they significantly enhanced the NO + NH₃ reactivity below 250 °C via the buildup of adsorbed nitrates, which act as a surface pool of oxidizing agents but are decomposed above that temperature. Both such features, which were not observed in comparative experiments over a V-free WO₃/TiO₂ catalyst, point out a key catalytic role of the vanadium redox properties and can explain the greater deNO_x efficiency of the “fast” SCR (NO + NH₃ + NO₂) compared with the “standard” SCR (NO + NH₃ + O₂) reaction. A unifying redox approach is proposed to interpret the overall NO/NO₂–NH₃ SCR chemistry over V-based catalysts, in which vanadium sites are reduced by the reaction between NO and NH₃ and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the latter formed via NO₂ disproportion over other nonreducible oxide catalyst components.     

Regeneration of sulfur-poisoned CeO2 catalyst for NH3-SCR of NOx

The effects of regeneration on the activities and structure of CeO₂ catalysts for NH₃-SCR of NO_x have been studied in this article. CeO₂ catalyst is deactivated by SO₂ for NH₃-SCR of NO_x in a 200 h long-term operation at 350 °C due to the formation of sulfates, and its NO_x conversion decreases from 100% to 83% gradually. However, sulfates can be removed from sulfur-poisoned CeO₂ catalysts under high temperature thermal treatment in air. After regeneration, NO_x conversion of sulfur-poisoned CeO₂ catalyst is recovered to about 100% at 350 °C. Moreover, the regeneration temperature is related to the nature of the sulfates formed on the sulfur-poisoned CeO₂ catalysts.     

Evaluation of amines for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust

With a view to developing onboard generation of selective reductants for NO_x removal from diesel engine exhaust we compared the performance of a primary, secondary and tertiary amine to NH₃ using a typical mini core NH₃-SCR catalyst. Primary amines with short hydrocarbon chains, e.g. CH₃NH₂ (maximum NO_x conversion, 50%) approached the NO_x conversion obtained using NH₃ (maximum NO_x conversion, 70%). Increasing the amine to NO_x ratio greater than 1 results in NO_x conversions closer to those of NH₃ (maximum NO_x conversion increased to 60%). Secondary and tertiary amines had smaller NO_x conversions as a function of temperature and the drop in NO and NO_x conversion decreased with increasing amine hydrogen substitution. Also, the maximum NO_x conversion for each reductant tends to move to a lower temperature as the degree of substitution increases.Unlike NH₃, the amines can react in the gas phase at temperatures within the range of diesel engine exhaust. Due to this gas phase reactivity the NO_x conversions measured using the mini core SCR catalyst also contain a gas phase conversion component. Gas phase conversions were investigated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. Subtraction of the two results highlighted the differences between the mini core catalytic and gas phase conversions measured in this manner over the temperature range investigated. These differential NO_x conversions for the three amines had maxima at about 375 °C.     

Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

In this study, new Fe₂O₃ based materials are developed for the selective catalytic reduction (SCR) of NO_x by NH₃ in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO₂ is considered to be the most effective carrier for Fe₂O₃. The modification of the Fe₂O₃/ZrO₂ system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO₂ that is coated with 1.4 mol% Fe₂O₃ and 7.0 mol% WO₃ (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO_x is obtained between 285 and 430 °C with selective formation of N₂. Here, the turnover frequency of NO_x per Fe atom is found to be 35 × 10⁻⁵ s⁻¹ that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H₂O and CO₂, whereas it is increased by NO₂.Temperature programmed reduction by H₂ (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe₂O₃, whereby relatively small oxide entities are also present. The strongly aggregated Fe₂O₃ species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH₃ that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N₂ and H₂O. In the final step, the reduced Fe sites be regenerated by oxidation with O₂. As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO₂ or O₂ into NO.     

Screening of doped MnOx–CeO2 catalysts for low-temperature NO-SCR

The effect of different dopants including niobium, iron, tungsten and zirconium oxide on the low-temperature activity of MnO_x–CeO₂ catalysts for the selective catalytic reduction (SCR) of NO_x with ammonia has been studied with coated cordierite monoliths in model gas experiments. A clearly higher activity and particularly superior nitrogen selectivity was obtained with the niobium-doped catalyst in comparison with the MnO_x–CeO₂ reference system. At 200 °C, the DeNO_x was 80% while the N₂ selectivity reached more than 96%. In contrast, a decrease of the SCR activity was observed when iron, zirconium or tungsten oxides were added to MnO_x–CeO₂. However, the addition of niobium oxide did not improve the resistance of the catalyst against SO₂ poisoning. A strong and irreversible deactivation occurred after exposure to SO₂.     

Characterization of CeO2–WO3 catalysts prepared by different methods for selective catalytic reduction of NOx with NH3

In this work, cerium–tungsten oxide catalysts were prepared by three methods: single step sol–gel (SG), impregnation (IM), and solid processing (SP). The catalysts were used for selective catalytic reduction (SCR) of NO_x with ammonia over a wide temperature range. The results indicated that the catalysts prepared by the SP and IM methods exhibited better SCR activity than that prepared via the SG method in 175–500 °C. The excellent activity can be attributed to larger surface area, higher surface concentrations of Ce and Ce^3 +, enhanced NO oxidization ability, and greater number of surface acid sites.     

Application of MnOx/HNTs catalysts in low-temperature NO reduction with NH3

Halloysite (HNTs) was firstly adopted as a carrier to develop manganese based SCR catalysts. Supporting of manganese oxides (MnO_x) on halloysite was carried out via a simple in-situ reaction of Mn(AC)₂ and KMnO₄ on the surface of halloysite. The obtained MnO_x/HNTs catalyst was estimated for selected reduction of NO by NH₃. It was found that the SCR activity of the catalyst is perfectly high with a nearly 100% NO conversion at temperatures from 60 to 250 °C. Results showed that the adoption of Halloysite as carrier and the in-situ preparation method jointly play key function for the performance of the catalyst.     

Loss of NOx storage capacity of Pt–Ba/Al2O3 catalysts due to incorporation of phosphorous

This work aims at determining the effect of the incorporation of P on NO_x storage capacity by NO_x storage-reduction (NSR) catalysts. Different amounts of phosphorous were deposited on a Pt–Ba/Al₂O₃ catalyst (1 wt% Pt and 17 wt% Ba) by impregnation of a phosphate salt. Samples were calcined at 723 K and characterized using X-ray diffraction (XRD), N₂ adsorption isotherms and X-ray photoelectron spectroscopy (XPS). Their NO_x storage capacity was also measured. It was observed that storage capacity decreased almost linearly with the P/Ba ratio and besides at a phosphorous concentration P/Ba ratio of <0.1 deterioration was low. At higher P concentrations (P/Ba ratio = 0.7) the NO_x storage was significantly reduced. It is proposed that the cause of the decline in NO_x retention capacity would be the formation of Ba–P phases (very likely Ba phosphates) at the expense of Ba carbonate or Ba oxide. These new phases would be unable to exchange NO_x.     

Monolithic Cr–V/TiO2/cordierite catalysts prepared by in-situ precipitation and impregnation for low-temperature NH3-SCR reactions

Cr–V/TiO₂/cordierite monolithic catalysts for low-temperature NH₃-SCR reactions were prepared by in-situ precipitation and impregnation. The adhesion and surface characteristics were examined using ultrasonic oscillation and scanning electron microscopy, respectively. The results showed that the TiO₂ coating adhered well to cordierite by in-situ precipitation. If the loading of the active component, Cr–V, was at least 5.5% by weight, the catalyst achieved excellent SCR activity, with an NO_x conversion above 90%, in the temperature range 160–300 °C. The Cr–V/TiO₂/cordierite monolithic catalysts had relatively high stability in the presence of SO₂.     

Resistance to sulfidation of the catalytic reduction of NOx over Ag/Al2O3 by controlling the loadings of Ag and Ag+

SO₂ strongly decreased the catalytic activities of low loading Ag/Al₂O₃ below 500 °C in selective catalytic reduction (SCR) of NO_x by propene with or without the assistance of non-thermal plasma (NTP), which was mainly attributed to the competition between SO₂ and NO. By controlling the loadings of Ag and Ag⁺ over alumina, the resistance of SO₂ was remarkably enhanced between 400 °C and 500 °C in thermal SCR. In the NTP-assisted SCR, most of the NO_x conversions were also apparently recovered from 250 °C to 500 °C.     

Selective catalytic reduction of NOx by NH3 on Fe/HBEA zeolite catalysts in oxygen-rich exhaust

This paper deals with the systematic study of Fe/HBEA zeolites for the selective catalytic reduction (SCR) of NO_x by NH₃ in diesel exhaust. The catalysts are prepared by incipient wetness impregnation of H-BEA zeolite (Si/Al = 12.5). The SCR examinations performed under stationary conditions show that the pattern with a Fe load of 0.25 wt.% (0.25Fe/HBEA) reveals pronounced performance. The turnover frequency at 200 °C indicates superior SCR activity of 0.25Fe/HBEA (8.5 × 10⁻³ s⁻¹) as compared to commercial Fe-exchanged BEA (0.99 × 10⁻³ s⁻¹) and V₂O₅/WO₃/TiO₂ (1.0 × 10⁻³ s⁻¹). Based upon powder X-ray diffraction (PXRD), temperature programmed reduction by H₂ (HTPR), diffuse reflectance UV–vis spectroscopy (DRUV–VIS) and catalytic data it is concluded that the pronounced performance of 0.25Fe/HBEA is substantiated by its high proportion of isolated Fe oxo sites. Furthermore, isotopic studies show that no association mechanism of NH₃ takes place on 0.25Fe/HBEA, i.e. N₂ is mainly formed from NO and NH₃.The evaluation of 0.25Fe/HBEA under more practical conditions shows that H₂O decreases the SCR performance, while CO and CO₂ do not affect the activity. Contrary, SCR is markedly accelerated in presence of NO₂ referring to fast SCR. Moreover, hydrothermal treatment at 550 °C does not change SCR drastically, whereas a clear decline is observed after 800 °C aging.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Impact of Biodiesel-Based Na on the Selective Catalytic Reduction of NOx by NH3 Over Cu–Zeolite Catalysts

A single-cylinder diesel engine was used to investigate the impact of Na on Cu–zeolite SCR catalysts using 20 % bio- and petrol-diesel fuel blend (B20) with elevated levels of Na. The Na exposure was performed on light-duty (DOC–SCR–DPF) and heavy-duty (DOC–DPF–SCR) configurations of the diesel emissions control devices. The accelerated Na aging is achieved by exposing the system to elevated levels of Na that represent full useful life exposure (700,000 km) and periodically increasing the exhaust temperature to replicate DPF regeneration. After aging, the NO_x performance and relevant chemistry of the SCR catalysts were evaluated in a bench flow reactor. The SCR in the DOC–SCR–DPF configuration was found to be severely affected by Na contamination, especially when NO was the only NO_x species in the simulated exhaust gases. In the DOC–DPF–SCR configuration, no impact is observed in the SCR NO_x reduction activity. Electron microprobe analysis (EPMA) reveals that Na contamination on the SCR samples in the DOC–SCR–DPF configuration is present throughout the length of the catalysts.     

Selective catalytic reduction of NO by ammonia using mesoporous Fe-containing HZSM-5 and HZSM-12 zeolite catalysts: An option for automotive applications

Mesoporous and conventional Fe-containing ZSM-5 and ZSM-12 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in the selective catalytic reduction (SCR) of NO with NH₃. It was found that for both Fe/HZSM-5 and Fe/HZSM-12 catalysts with similar Fe contents, the activity of the mesoporous samples in NO SCR with NH₃ is significantly higher than for conventional samples. Such a difference in the activity is probably related with the better diffusion of reactants and products in the mesopores and better dispersion of the iron particles in the mesoporous zeolite as was confirmed by SEM analysis. Moreover, the maximum activity for the mesoporous zeolites is found at higher Fe concentrations than for the conventional zeolites. This also illustrates that the mesoporous zeolites allow a better dispersion of the metal component than the conventional zeolites. Finally, the influence of different pretreatment conditions on the catalytic activity was studied and interestingly, it was found that it is possible to increase the SCR performance significantly by preactivation of the catalysts in a 1% NH₃/N₂ mixture at 500 °C for 5 h. After preactivation, the activity of mesoporous 6 wt% Fe/HZSM-5 and 6 wt% Fe/HZSM-12 catalyst is comparable with that of traditional 3 wt% V₂O₅/TiO₂ catalyst used as a reference at temperatures below 400 °C and even more active at higher temperatures.     

Thermodynamic calculation for the activity and mechanism of Mn/TiO2 catalyst doped transition metals for SCR at low temperature

The introduction of transition metals in Mn/TiO₂ catalysts played significant roles in oxidative abstraction of hydrogen from adsorbed ammonia during the selective catalytic reduction (SCR). Thermodynamic calculation studies showed that the SCR performance was in accordance with the ammonia oxidation with transition metals, and the reaction tendency for the ammonia oxidation was decreased in the following order: CuO > Co₃O₄ > NiO > Fe₂O₃ > Cr₂O₃ > ZnO > La₂O₃ > CeO₂ > ZrO₂. In addition, Mn/TiO₂ catalyst doped metal (Fe and Cu) oxides enhanced performance for NO_x conversion, being approximately 100% at 453 K.     

A new NOx storage-reduction electrochemical catalyst

A new NO_x storage-reduction electrochemical catalyst has been prepared from a polycrystalline Pt film deposited on 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ) solid electrolyte. BaO has been added onto the Pt film by impregnation method. The NO_x storage capacity of Pt-BaO/YSZ system was investigated at 350 °C and 400 °C under lean conditions. Results have shown that the electrochemical catalyst was effective for NO_x storage. When nitric oxides are fully stored, the catalyst potential is high and reaches its maximum. On the other hand, when a part of NO and also NO₂ desorb to the gas phase, the catalyst potential remarkably drops and finally stabilizes when no more NO_x storage occurs but only the reaction of NO oxidation into NO₂. Furthermore, the investigation has clearly demonstrated that the catalyst potential variation versus temperature or chemical composition is an effective indicator for in situ following the NO_x storage-reduction process, i.e. the storage as well as the regeneration phase. The catalyst potential variations during NO_x storage process was explained in terms of oxygen coverage modifications on the Pt.     

Selective catalytic reduction of NOx with methane over indium supported on tungstated zirconia

Selective catalytic reduction (SCR) of NO_x with methane was investigated over a series of non-noble metal (cobalt, manganese, nickel, tin, silver, indium) catalysts supported on tungstated zirconia (WZr). A great improvement of catalytic activity was found over the indium-loaded WZr catalysts upon the other WZr. The highest NO_x conversion of 70% was achieved over an 1%In/WZr catalyst at 450 °C and 12,000 h⁻¹. InO⁺ was proposed to be the active site in NO reduction. H₂O and SO₂ significantly inhibited NO reduction by competing with reactants to adsorb on InO⁺ site and forming inactive $\mathrm{In}(\mathrm{OH}{)}_2^{+}$ and In(SO₃)⁺ species.     

SO42−–Mn–Co–Ce supported on TiO2/SiO2 with high sulfur durability for low-temperature SCR of NO with NH3

The catalysts SO₄^2 − Mn–Co–Ce/TiO₂/SiO₂ were investigated for the low-temperature SCR of NO with NH₃ in the presence of SO₂. An excellent SO₂ durability at low temperature was obtained with the catalyst used TiO₂/SiO₂ as support and modified with SO₄^2 −. The catalyst sulfated with 0.1 mol/L H₂SO₄ solution and then calcined at 300 °C exhibited the best NO_x conversion efficiency of 99.5% at 250 °C in the presence of 50 ppm SO₂. The conversion efficiency did not decrease after repeatedly used for 8 times.     

Tungstophosphoric acid supported on titania: A solid acid catalyst in benzylation of phenol with benzylalcohol

Benzylation of phenol with benzylalcohol was carried out in liquid phase over tungstophosphoric acid (TPA) supported on titania. The catalysts were prepared with different TPA (10–25%) loading by wet impregnation method, were calcined at 700 °C and characterized by XRD, surface area, FTIR and acidity of the catalysts was measured by temperature programmed desorption of NH₃–TPD, FTIR pyridine adsorption. The catalysts have been represented by a general formula as xPTiO₂₋y (where x = wt%, P = TPA, and y = calcination temperature in °C). The 20PTiO₂ catalyst calcined at various temperatures to know the effect of calcination temperature on activity of the catalyst and the 20PTiO₂-700 showed highest activity in benzylation of phenol with benzylalcohol because it had highest acidity. The effects of temperature, catalyst weight, mole ratio of the reactants on conversion of phenol and product selectivities have been optimized. 20PTiO₂-700 catalyst gave conversion of benzylalcohol (BA) 98% and the selectivity to benzyl phenol (BP) 83.6%, phenyl benzyl ether (PBE) 9.4%, benzylether (BE) 7% at 130 °C, phenol to benzylalcohol molar ratio 2 and in 1 h.