Effects of WO3 doping on stability and N2O escape of MnOx–CeO2 mixed oxides as a low-temperature SCR catalyst

MnO_x–WO_x–CeO₂ catalysts synthesized using a sol–gel method were investigated for the low-temperature NH₃-SCR reaction. Among them, W_0.1Mn_0.4Ce_0.5 mixed oxides exhibited above 80% NO_x conversion from 140 to 300 °C. In addition, this catalyst exhibited high stability and CO₂ tolerance in a 50 h activity test at 150 °C. Substantially reduced N₂O production and enhanced N₂ selectivity were achieved by WO₃ doping, which was due to the weakened reducibility and increased number of acid sites. The decreased SO₂ oxidation activity as well as the reduced formation of ammonium and manganese sulfates resulted in a high SO₂ resistance of this catalyst.     

NH3-SCR activity,hydrothermal stability,sulfur resistance and regeneration of Ce0.75Zr0.25O2–PO43 −catalyst

Ce_0.75Zr_0.25O₂–PO₄^3 − catalyst with high NH₃-SCR performance was prepared by impregnating phosphates on Ce_0.75Zr_0.25O₂ synthesized by a sol–gel method. The catalysts were characterized by IR, H₂-TPR, NH₃ adsorption and activity measurements. Ce_0.75Zr_0.25O₂–PO₄^3 − catalyst shows over 80% NO_x conversion at 250–450 °C. The hydrothermal-aged catalyst at 760 °C for 48 h still presents high NH₃-SCR performance at 300–400 °C. The sulfur-aged Ce_0.75Zr_0.25O₂–PO₄^3 − catalyst can be regenerated completely by treating catalyst in air at 650 °C. Phosphates improve the ammonia adsorption and decrease the ammonia oxidation on catalyst, leading to a high NH₃-SCR activity and a high N₂ selectivity of Ce_0.75Zr_0.25O₂–PO₄^3 − catalyst.     

Selective reduction of NO by NH3 over Fe-zeolite-promoted V2O5-WO3/TiO2-based catalysts: Great suppression of N2O formation and origin of NO removal activity loss

Great depression of the formation N₂O in the selective catalytic reduction of NO by NH₃ (NH₃-SCR) has been studied by combining a V₂O₅-WO₃/TiO₂ (VWT) catalyst with a Fe-exchanged ZSM-5 zeolite (FeZ). At temperatures > 400 °C, N₂O formation was significant over VWT but < 5 ppm over FeZ/VWT catalysts with the FeZ ≥ 8%. Unfortunately, all these FeZ-promoted catalysts disclosed a decrease in deNO_xing performances, due to an enhanced NH₃ oxidation into NO. At temperatures > 350 °C, the chemically-combined VWT-based FeZ systems could facilitate both N₂O reduction with NH₃ and N₂O decomposition, thereby suppressing N₂O emissions in NH₃-SCR reaction.     

A Ce–Sn–Ox catalyst for the selective catalytic reduction of NOx with NH3

A series of Ce–Sn–O_x catalysts prepared by the facile coprecipitation method exhibited good catalytic activity in a broad temperature range from 100 °C to 400 °C for the selective catalytic reduction of NO_x with NH₃ at the space velocity of 20,000 h^− 1. The Ce₄Sn₄O_x catalyst calcined at 400 °C showed high resistance to H₂O, SO₂, K₂O and PbO under our test conditions. The better catalytic performance was associated with the synergistic effect between CeO₂ and SnO₂, which strengthened the NH₃ and NO_x adsorption capacity on the surface of the catalyst.     

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.     

Zr/HZSM-5 catalyst for NO reduction by C2H2 in lean-burn conditions

Zr-based zeolite catalysts were investigated for the first time in selective catalytic reduction of NO by hydrocarbon (HC-SCR). Highly dispersed zirconium species, especially the amorphous ultrafine zirconium oxide in the catalyst, considerably enhanced the activity for selective catalytic reduction of NO by acetylene (C₂H₂-SCR), both by accelerating the NO oxidation to NO₂ and enlarging the NO₂ adsorption capacity of the catalyst under the reaction conditions. Thus a durable and active Zr/HZSM-5 catalyst giving 89% of NO conversion to N₂ at 350 °C in 1600 ppm NO, 800 ppm C₂H₂, and 9.95% O₂ in helium was obtained. For the C₂H₂-SCR of NO, it was suggested that acidic sites with strong acidity on the Zr-based HZSM-5 catalysts are indispensable to initiate the aimed reaction via the route of NO oxidation to NO₂, which explains the higher activity for the reaction obtained over the Zr/HZSM-5 catalyst sample with lower SiO₂/Al₂O₃ ratio. The zirconium species could only functioned in the presence of protons in the C₂H₂-SCR of NO, so a synergistic effect between the zirconium species and protons of the Zr/HZSM-5 catalyst was proposed.     

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.     

Mn/Mg/Al-spinels as catalysts for SOx abatement: Influence of CeO2 incorporation and catalytic stability

Mg,Mn,Al-oxides with spinel structure, Al/(M²⁺ + Al) molar ratio of 0.25 and 0.50 and an Mn/Mg molar ratio of 0.30 have been evaluated as catalysts for SO_x removal under conditions similar to those found in FCC units. The best performance was that of the sample with the higher aluminium content. The incorporation of CeO₂ in this sample favored SO_x uptake for short reaction times as well as the reduction of the sulfated catalysts. When the regeneration was started at 530 °C, only H₂S was observed as reaction product, but when this step started at 650 °C, the release of SO₂ preceded that of H₂S, regardless of the chemical composition of the sample. As to the additive performance for successive reaction–regeneration cycles, the incorporation of CeO₂ produced a less efficient catalyst with regard to the removal of the SO₂ along the process, but with a higher regeneration efficiency and a lower formation of SO₂ as regeneration product.     

Oxy-methane reforming over high temperature stable NiCoMgCeOx and NiCoMgOx supported on zirconia–haffnia catalysts: Accelerated sulfur deactivation and regeneration

NiCoMgO_x and NiCoMgCeO_x on commercial low surface area zirconia–haffnia catalysts have unusually high thermal stability (⩾2000 °C) for syngas generation via the methane partial oxidation process (J. Catal., 233, 36, 2005). Herein we report the results on accelerated sulfur deactivation (0.74 mol% sulfur in feed) and corresponding regeneration (at 800 °C in 1:1 O₂ + N₂ flow) over these catalysts. The NiCoMgCeO_x catalyst, due to a larger mobility of lattice oxygen, showed a considerably higher resistance to sulfur poisoning; the higher mobility of the lattice oxygen in case of the NiCoMgCeO_x catalyst may be related to the presence of CeO₂. During the deactivation process, the selectivity for H₂ was decreased to a much greater extent than that for CO. Regeneration studies showed that even after complete deactivation of the catalysts, the original activity/selectivity of both the catalysts could be completely restored after a simple regeneration process. Based on their exceptionally high thermal stability, high activity/selectivity and easily regenerability, the NiCoMgO_x and NiCoMgCeO_x catalysts appear to be very promising candidates for the CPOM process.     

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.     

Promoting catalytic performances of Ni-Mn spinel for NH3-SCR by treatment with SO2 and H2O

Ni(0.4)-MnO_x catalyst was prepared by citrate combustion, which showed high catalytic performance for NH₃-SCR reaction. After the resistance tests of SO₂ and H₂O, Ni(0.4)-MnO_x-SH showed better NH₃-SCR activity than that of Ni(0.4)-MnO_x, when the temperature was > 240 °C. The characterizations suggest that Ni(0.4)-MnO_x-SH has more acid sites for ammonia adsorption and far weaker oxidation capacity for NH₃, which resulted in the high catalytic activity at middle-temperature.     

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.     

Soot oxidation on a catalytic NOx trap: Beneficial effect of the Ba–K interaction on the sulfated Ba,K/CeO2 catalyst

The Ba,K/CeO₂ catalyst is active both for NO_x trapping and soot combustion. In this work we report a Ba–K interaction that prevents K sulfation when NO_x is present, thus preserving the activity of K towards soot combustion during the working period of the trap. This effect is originated in the K₂SO₄(s) + Ba(NO₃)₂(s) → 2KNO₃(s) + BaSO₄(s) reaction, which is thermodynamically favored. In the absence of NO_x, the soot combustion reaction is strongly depressed by SO₂ whereas when NO_x is present both the sulfated and the non-sulfated catalysts present similar TPO patterns.     

An excellent support of Pd–Fe–Ox catalyst for low temperature CO oxidation: CeO2 with rich (200) facets

Pd–Fe–O_x catalysts for low temperature CO oxidation were supported on SBA-15, CeO₂ nano-particles with rich (111) facets and CeO₂ nano-rod with rich (200) facets, and characterized by X-ray diffraction, low-temperature nitrogen adsorption, transmission electron microscopy and temperature-programmed reduction. The results showed that when CeO₂ nano-rod was used as a support, Pd–Fe–O_x catalyst exhibits higher activity (T₁₀₀ = 10 °C), resulting from the rich (200) facets of CeO₂ nano-rod, which leads to a formation of large numbers of the oxygen vacancies on the surface of Pd–Fe–O_x catalysts.     

Size controlled ZSM-5 on the structure and performance of Fe catalyst in the selective catalytic reduction of NOx with NH3

Fe/ZSM-5 catalysts with various morphologies and sizes were prepared and the catalytic properties in NH₃-SCR were also investigated. The different ZSM-5 morphologies and sizes indeed influence the dispersion of Fe species. The Fe/ZSM-5 catalyst, which was cauliflower-like morphology of ZSM-5 support aggregated by small nano-crystal zeolite with crystallite size of about 50 nm, exhibited the best NH₃-SCR activity (T_≥ 90% = 280–650 °C). This specific morphology and size of ZSM-5 support were considered to benefit the distribution of isolated Fe^3 + species, which was proved to be the main active sites in SCR reaction.     

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.     

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.     

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.     

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.     

Enhanced activity of MnxW0.05Ti0.95 − xO2 − δ for selective catalytic reduction of NOx with ammonia by self-propagating high-temperature synthesis

A series of TiO₂ supported MnWO_x catalysts Mn_xW_0.05Ti_0.95 − xO_2 − δ (x = 0.05, 0.1, 0.15) were synthesized by solution combustion method. The Mn_0.10W_0.05Ti_0.85O_2 − δ catalyst showed highest activity in NH₃-SCR reaction within a broad temperature range of 200 °C–400 °C. XRD and TEM results indicate that the active Mn and W species are highly dispersed over TiO₂ support in the form of nanoparticles (4–7 nm). The TEM and H₂-TPR results also suggest that a MnWO_x phase has been formed on the TiO₂, which is beneficial for the activity of the Mn_xW_0.05Ti_0.95 − xO_2 − δ catalysts in the high temperature range of 280 °C–400 °C.