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.     

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.     

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.     

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.     

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.     

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.     

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.     

Synergetic catalytic removal of Hg0 and NO over CeO2(ZrO2)/TiO2

A series of CeO₂(ZrO₂)/TiO₂ monolith catalysts were investigated for catalytic oxidation of Hg⁰ and NH₃-SCR of NO. Effect of flue gases components on catalytic oxidation of Hg⁰ was mainly studied. Results showed that the CeO₂(ZrO₂)/TiO₂ catalyst exhibited high efficiency for catalytic oxidation of Hg⁰ at 240–400 °C without adding other oxidant, and its catalytic performance for NH₃-SCR of NO was not affected. NH₃ had slight inhibitory effect while SO₂ and NO had no influence on catalytic oxidation of Hg⁰, but O₂ obviously improved catalytic oxidation of Hg⁰ for its oxidation susceptibility.     

MnOx-CeO2 shell-in-shell microspheres for NH3-SCR de-NOx at low temperature

A novel MnO_x-CeO₂ catalyst with shell-in-shell microsphere structure was successfully prepared by a one-step hydrothermal method for the first time. The obtained catalyst was characterized by SEM, TEM, XRD, N₂ adsorption and desorption, XPS, H₂-TPR and NH₃-TPD in detail, and its catalytic activity was investigated by selective catalytic reduction of NO_x with NH₃. The results showed that the MnO_x-CeO₂ microsphere catalyst presented high catalytic activity at low temperatures (lower than 210 °C), which was much superior to the counterpart MnO_x-CeO₂ catalyst without shell-in-shell microsphere structure prepared by co-precipitation method.     

Mercury oxidation by hydrochloric acid over TiO2 supported metal oxide catalysts in coal combustion flue gas

Mercury oxidation by hydrochloric acid over the metal oxides supported by anatase type TiO₂ catalysts, 1 wt.% MO_x/TiO₂ where M = V, Cr, Mn, Fe, Ni, Cu, and Mo, was investigated by the Hg⁰ oxidation and the NO reduction measurements both in the presence and absence of NH₃. The catalysts were characterized by BET surface area measurement and Raman spectroscopy. The metal oxides added to the catalyst were observed to disperse well on the TiO₂ surface. For all catalysts studied, the Hg⁰ oxidation by hydrochloric acid was confirmed to proceed. The activity of the catalysts was found to follow the trend MoO₃ ~ V₂O₅ > Cr₂O₃ > Mn₂O₃ > Fe₂O₃ > CuO > NiO. The Hg⁰ oxidation activity of all catalysts was depressed considerably by adding NH₃ to the reactant stream. This suggests that the metal oxide catalysts undergo the inhibition effect by NH₃. The activity trend of the Hg⁰ oxidation in the presence of NH₃ was different from that observed in its absence. A good correlation was found between the NO reduction and the Hg⁰ oxidation activities in the NH₃ present condition. The catalyst having high NO reduction activity such as V₂O₅/TiO₂ showed high Hg⁰ oxidation activity. The result obtained in this study suggests that the oxidation of Hg⁰ proceeds through the reaction mechanism, in which HCl competes for the active catalyst sites against NH₃. NH₃ adsorption may predominate over the adsorption of HCl in the presence of NH₃.     

Mn-substituted Ca–La–hexaaluminate nanoparticles for catalytic combustion of methane

Mn-substituted Ca–La–hexaaluminate rod-like nanoparticles (Ca_1−xLa_xMnAl₁₁O_19−α) with high surface area ranging from 47 to 80 m²/g for catalytic combustion of methane have been prepared using alumina sol as the (NH₄)₂CO₃ coprecipitation precursor and a supercritical drying (SCD) method. Ca substitution gave rise to the maximum combustion activity (T_10% = 459 °C) at x = 0 owing to the highest surface area. Meanwhile, Ca substitution affects the oxygen sorption property and the oxidation state of Mn ions in the hexaaluminate lattice. Ca_0.6La_0.4MnAl₁₁O_19−α catalyst with high catalytic activities was obtained owing to the excellent performance of activating oxygen.     

Highly dispersed Mn–Ce mixed oxides supported on carbon nanotubes for low-temperature NO reduction with NH3

A series of Mn–Ce mixed-oxide catalysts supported on carbon nanotubes (CNTs) were prepared for the first time and used for the selective catalytic reduction of NO with NH₃. Mn(0.4)-Ce/CNTs catalysts with loading from 0.6% to 1.8% (molar ratio) in our tests showed more than 90% NO conversion at 120–180 °C at a high space velocity of 42,000 h^− 1. Transmission electron microscopy confirmed that the particle size of Mn–Ce mixed oxides supported on CNTs was 2 to 4 nm. BET result indicated Mn–Ce mixed-oxide catalysts obtained enlarged surface area and pore volume which was beneficial to the catalytic activity.     

Preparation of Pd/(Ce1 − xYx)O2/γ-Al2O3/cordierite catalysts and its catalytic combustion activity for methane

A series of (Ce_1 − xY_x)O₂ (x = 0,0.15,0.35,0.5) coatings on γ-Al₂O₃ pre-coated cordierite honeycomb were prepared by sol–gel method, and then palladium was loaded by aqueous solution impregnation deposition with Pd(NO₃)₂ as precursor. The structure and morphology of samples were evaluated and the catalytic combustion activity for methane was also discussed. (Ce_1 − xY_x)O₂ synthesized by sol–gel has a single-phase cubic fluorite structure. Increasing the Y/Ce ratio can significantly improve the inner surface morphology of the honeycomb channels and also the coating mechanical stability, and leads to a considerable improvement in the catalytic activity of the prepared catalysts for methane.     

Role of ceria in the improvement of SO2 resistance of LaxCe1 − xFeO3 catalysts for catalytic reduction of NO with CO

Perovskite-type catalysts with LaFeO₃ and substituted La_xCe_1 − xFeO₃ compositions were prepared by sol–gel method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), and SO₂ temperature-programmed desorption (SO₂-TPD). Catalytic reaction for NO reduction with CO in the presence of SO₂ has been investigated in this study. LaFeO₃ exhibited an excellent catalytic activity without SO₂, but decreased sharply when SO₂ gas was added to the CO + NO reaction system. In order to inhibit the effect of SO₂, substitution of Ce in the structure of LaFeO₃ perovskite has been investigated. It was found that La_0.6Ce_0.4FeO₃ showed the maximum SO₂ resistance among a series of La_xCe_1 − xFeO3 composite oxides.     

Catalytic properties in N2O decomposition of mixed cobalt–iron spinels

The effect of introduction of iron in the Co_3 − xFe_xO₄ on catalytic activity in N₂O decomposition was investigated. The spinel catalysts were characterized by XRD, SEM, RS, BET methods, work function measurements and Mössbauer spectroscopy. Introduction of iron in the Co_3 − xFe_xO₄ spinel catalysts at the level of x < 1 leads to preferential substitution of Fe³⁺ in tetrahedral sites, whereas for x > 1 also octahedral ones are substituted. A strong correlation between deN₂O activity (T_50%) and work function was observed showing that electronic factor controls the catalytic reactivity of Co–Fe spinels. The results revealed that the active centers for N₂O decomposition are cobalt ions and thus even a low level of their substitution by iron leads to substantial decrease of the deN₂O activity of the cobalt spinel.     

Highly efficient non-noble metal based nanostructured catalysts for selective CO methanation

Highly efficient non-noble metal based catalysts (Zr_1 − xNi_xO₂) showed outstanding catalytic performance at lower temperature for selective CO methanation. The characterization techniques revealed that Zr_1 − xNi_xO₂ catalyst contained uniformly dispersed Ni particles with strong interaction between Ni and ZrO₂.     

Fractional-hydrolysis-driven formation of non-uniform dopant concentration catalyst nanoparticles of Ni/CexZr1 − xO2 and its catalysis in methanation of CO2

Ni/ceria–zirconia solid solution (Ni/Ce_xZr_1 − xO₂) with Ce and Ni enriched on the surface of the catalysts are prepared via a simple, low-cost method. The aqueous phase reactions simplify the preparation process, save the cost and lead the Ce to enriching on the surface of products. The impregnation method makes the Ni enriching on the surface of products which enhances the efficiency of active component. These catalysts exhibit significantly high catalytic performance. The formation mechanism is also investigated.     

Etherification of n-butanol to di-n-butyl ether over H3PMo12 − xWxO40 (x = 0, 3, 6, 9, 12) Keggin and H6P2Mo18 − xWxO62 (x = 0, 3, 9, 15, 18) Wells–Dawson heteropolyacid catalysts

Etherification of n-butanol to di-n-butyl ether was carried out over H₃PMo_12 − xW_xO₄₀ (x = 0, 3, 6, 9, 12) Keggin and H₆P₂Mo_18 − xW_xO₆₂ (x = 0, 3, 9, 15, 18) Wells–Dawson heteropolyacid (HPA) catalysts. Acid strength of H₃PMo_12 − xW_xO₄₀ Keggin and H₆P₂Mo_18 − xW_xO₆₂ Wells–Dawson HPA catalysts was determined by NH₃-TPD (temperature-programmed desorption) measurements. The correlations between desorption peak temperature (acid strength) of the HPA catalysts and catalytic activity revealed that conversion of n-butanol and yield for di-n-butyl ether increased with increasing acid strength of the catalysts, regardless of the identity of HPA catalysts (without HPA structural sensitivity).     

Catalytic gasification of lignin with Ni/Al2O3–SiO2 in sub/supercritical water

Lignin has been gasified with a Ni/Al₂O₃–SiO₂ catalyst in sub/supercritical water (SCW) to produce gaseous fuels. XRD pattern at 6θ angle shows characteristic peaks of crystalline NiO, NiSi, and AlNi₃, suggesting that Al₂O₃–SiO₂ not only offers high surface area (122 m² g) for Ni, but also changes the crystal morphology of the metal. 9 mmol/g of H₂ and 3.5 mmol/g of CH₄ were produced at the conditions that 5.0 wt% alkaline lignin plus 1 g/g Ni/Al₂O₃–SiO₂ operating for 30 min at 550 °C. A kinetic model was also developed, and the activation energies of gas and char formation were calculated to be 36.68 ± 0.22 and 9.0 ± 2.4 kJ/mol, respectively. Although the loss of activity surface area during reuse caused slight activity reduction in Ni/Al₂O₃–SiO₂, the catalyst system still possessed high catalytic activity in generating H₂ and CH₄. It is noted that sulfur linkage could be hydrolyzed to hydrogen sulfide in the gasification process of alkaline lignin. The stable chemical states of Ni/Al₂O₃–SiO₂ grants its insensitivity to sulfur, suggesting that Ni/Al₂O₃–SiO₂ should be economically promising for sub/supercritical water gasification of biomass in the presence of sulfur.     

A novel one-pot synthesized CuCe-SAPO-34 catalyst with high NH3-SCR activity and H2O resistance

CuCe-SAPO-34 catalysts based on the one-pot hydrothermal synthesis method were prepared for the first time. The addition of Ce suppressed the formation of CuO and increased the amount of active Cu^2 +, resulting in better NH₃-SCR activity than Cu-SAPO-34. Ce greatly improved the H₂O resistance during the SCR process by stabilizing the zeolite structure and obstructing the transformation of active Cu^2 + into inactive forms.