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.     

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.     

Effect of Cu doping on the SCR activity of CeO2 catalyst prepared by citric acid method

CeO₂–CuO catalyst prepared by citric acid method was investigated for selective catalytic reduction of NO with NH₃. The activity of the CeO₂ catalyst was enhanced about 8–27% in the temperature range of 125–225 °C at a space velocity of 28,000 h⁻¹ by the addition of Cu. It was found that the state of Cu species had great impact on the SCR performance of CeO₂–CuO catalyst. Cu²⁺ can enhance the low temperature activity of SCR reaction, while CuO would promote NH₃ oxidation before SCR reaction at high temperature, which would cause the decrease of its high temperature SCR activity.     

Catalytic performance of CuCl<Subscript>2</Subscript>-modified V<Subscript>2</Subscript>O<Subscript>5</Subscript>-WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> catalyst for Hg<Superscript>0</Superscript> oxidation in simulated flue gas

CuCl₂-SCR catalysts prepared by an improved impregnation method were studied to evaluate the catalytic performance for gaseous elemental mercury (Hg⁰) oxidation in simulated flue gas. Hg⁰ oxidation activity of commercial SCR catalyst was significantly improved by the introduction of CuCl₂. Nitrogen adsorption, XRD, XRF and XPS were used to characterize the catalysts. The results indicated that CuCl₂ was well loaded and highly dispersed on the catalyst surface, and that CuCl₂ played an important role for Hg⁰ catalytic oxidation. The effects of individual flue gas components on Hg⁰ oxidation were also investigated over CuCl₂-SCR catalyst at 350 oC. The co-presence of NO and NH₃ remarkably inhibited Hg⁰ oxidation, while this inhibiting effect was gradually scavenged with the decrease of GHSV. Further study revealed the possibility of simultaneous removal of Hg⁰ and NO over CuCl₂-SCR catalyst in simulated flue gas. The mechanism of Hg⁰ oxidation was also investigated.     

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.     

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.     

Chemical hydrogen storage and release properties using redox reaction over the Cu-added Fe/Ce/Zr mixed oxide medium

The chemical hydrogen storage (hydrogen reduction) and release (water-splitting oxidation) properties of the Cu-added Fe/Ce/Zr mixed oxide medium were investigated. The media with Cu content ranging from 0 to 5 wt% were prepared by a co-precipitation method using urea as a precipitant. The hydrogen reduction and the water-splitting oxidation on the medium were tested by temperature programmed reduction/oxidation (TPR/TPO) and repeated isothermal redox cycles at 550 °C for reduction and 350 °C for oxidation. The initial reduction rates and oxidation rates of the media increased with increasing the amount of the Cu additive. In addition, the reactivity of the medium for water-splitting oxidation was enhanced as the CeO₂/ZrO₂ ratio increased. Especially, the Fe-based mixed oxide medium with Cu/CeO₂/ZrO₂ contents of 3/30/10 wt% (Cu(3%)-Fe-CeO₂/ZrO₂(3/1)) showed superior performance in chemical hydrogen storage and release. As the results of isothermal redox cycles using the medium, the total amount of hydrogen evolved in water-splitting oxidation was maintained at ca. 8.5 mmol g^?1-medium (ca. 1.8 wt% hydrogen storage amounts on the basis of the total medium) over 15 repeated redox cycles.     

Effects of mercury oxidation on V2O5–WO3/TiO2 catalyst properties in NH3-SCR process

The effects of mercury oxidation on V₂O₅–WO₃/TiO₂ SCR catalyst's physical and chemical properties have been investigated. Both fresh catalyst and mercury exposed catalyst have been examined by BET, XRD, XPS and catalytic activity measurements. Mercury oxidation promoted the V^5 + species transforming to the V^4 + species and consumed the lattice oxygen on the surface of catalyst. In addition, the NO conversion of mercury exposed catalyst decreased in the range of 200 °C to 300 °C. It suggested a competitive relationship between gaseous NH₃ and adsorbed mercury on the catalyst surface in that temperature range.     

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.     

Raman spectra and extended X-ray absorption fine structure characterization of La(2−x)/3NaxTiO3 and Nd(2−x)/3LixTiO3 microwave ceramics

A-site deficient perovskite compounds, La_(2?x)/3Na_xTiO₃ (0.02 ≤ x ≤ 0.5) and Nd_(2?x)/3Li_xTiO₃ (0.1 ≤ x ≤ 0.5) microwave ceramics, were investigated by Raman scattering. Nd_(2?x)/3Li_xTiO₃ (0.1 ≤ x ≤ 0.5) was also investigated by extended X-ray absorption fine structure (EXAFS) measurement. The Raman shifts of the E (239 cm^?1) and A₁ (322 cm^?1) modes of La_(2?x)/3Na_xTiO₃ were found to decrease with x. However, the E (254 cm^?1) and A₁ (338 cm^?1) of Nd_(2?x)/3Li_xTiO₃ were found to blueshift with x, which was caused by Li substitution. The redshift of the A₁ (471 cm^?1) phonon of Nd_(2?x)/3Li_xTiO₃ (0.1 ≤ x ≤ 0.3) indicates that O–Ti–O bonding forces lessen with Li concentration, which is consistent with the EXAFS result that Ti–O bond lengths increase for 0.1 ≤ x ≤ 0.3. For x > 0.3, the EXAFS result shows that Ti–O bond lengths decrease. Moreover, Ti–O bond lengths show strong correlation with the microwave dielectric constants of Nd_(2?x)/3Li_xTiO₃.     

Influence of SBA-15 support on CeO2–ZrO2 catalyst for the dehydrogenation of ethylbenzene to styrene with CO2

Pure oxides of ceria (CeO₂) and zirconia (ZrO₂) were prepared by precipitation method and a catalyst comprising of 25 mol% of CeO₂ and 75 mol% of ZrO₂ (25CZ) mixed metal oxide was prepared by co-precipitation method and also a catalyst with 25 wt% of 25CZ (25 mol% of CeO₂ and 75 mol% of ZrO₂) and 75 wt% SBA-15(25/25CZS) was prepared by precipitation–deposition method. Aqueous NH₃ solution was used as a hydrolyzing agent for all the precipitation reactions. These catalysts were characterized by X-ray diffraction and nitrogen adsorption–desorption techniques for the confirmation of SBA-15 structural intactness. All these catalysts were found to be effective for the oxidative dehydrogenation of ethylbenzene (ODHEB) to styrene in the presence of CO₂ and also it was observed that there was a sequential enhancement in the catalytic activity from individual oxides to mixed oxides followed by supported mixed oxide catalysts. Of the catalysts studied in this work, the supported 25/25CZS catalyst exhibited the superior activity, which was about 10–20 times higher than the activity of bulk single oxides in terms of turn over frequency.     

Preparation,characterization and catalytic activity studies of CeO2–K diesel soot oxidation catalysts loaded on porous Al2O3 substrate using water-immiscible solvent

CeO₂–K catalysts supported on porous alumina substrate have been prepared by using a novel water-immiscible solvent. The advantage of this method is to load the catalyst onto the filter surface by one-time coating and prevent depositing the catalyst into the porous structure of support materials. The catalytic activities of the supported catalysts were evaluated by TPR system and the results showed that the pure CeO₂ displayed a poor catalytic activity for soot oxidation, while the addition of K element into CeO₂ would result in the formation of CeO₂–K solid solution and significant enhancement of catalytic activity. Nevertheless, the variation of K content had a limited effect on soot ignition temperature. The catalyst with a Ce:K molar ratio of 1:2 exhibited an ignition temperature of about 330 °C and the oxidation rates of about 0.16 and 0.28 mg min⁻¹ cm⁻² at temperatures of 370 and 390 °C, respectively.     

Dehydration of 2,3-butanediol into 3-buten-2-ol catalyzed by ZrO2

Vapor-phase catalytic dehydration of 2,3-butanediol was investigated over metal oxides such as CeO₂, La₂O₃, Yb₂O₃, ZrO₂, Al₂O₃, TiO₂, ZnO, Fe₂O₃, NiO, and Cr₂O₃. In the dehydration of 2,3-butanediol, 3-buten-2-ol was preferentially produced over monoclinic ZrO₂ along with major by-products such as butanone and 3-hydroxy-2-butanone. Over ZrO₂ calcined at 900 °C, 3-buten-2-ol was produced with a maximum selectivity of 59.0% at 300 °C without producing 1,3-butadiene.     

Dispersion and valence state of MnO2/Ce(1 − x)ZrxO2–TiO2 for low temperature NH3-SCR

Zr-doped MnO_x/CeO₂–TiO₂ for high temperature stability was investigated in terms of its dispersion and oxidation state. Aggregation of cerium oxide was observed in MnO_x/CeO₂–TiO₂, but the Zr-doped catalyst was well dispersed. An increase of the Mn^4 +/Mn^3 + ratio was confirmed with a Zr addition through valence state analysis. De-NOx efficiency of the catalyst was increased to 40–45% by Zr addition at low temperature (150–200 °C). The substitution of Zr led the catalyst to improve the de-NOx efficiency, with a high dispersion and MnO₂ ratio.     

Catalytic role of vanadium(V) sulfate on activated carbon for SO2 oxidation and NH3-SCR of NO at low temperatures

Carbon-supported catalyst with vanadium(V) sulfate (V₂O₃(SO₄)₂) as active component was prepared, characterized and tested for SO₂ and NO catalytic removal. Result shows that this catalyst is very active towards SO₂ oxidation and selective catalytic reduction of NO with NH₃ in the low temperature range of 100–250 °C.     

Ru/CexZr1−xO2, a novel and effective catalyst for the catalytic wet air oxidation of 2-chlorophenol

We report for the first time the use of Ce_xZr_1−xO₂ solid solutions as supports for Ru catalysts to be implemented in the catalytic wet air oxidation of 2-chlorophenol. Such catalytic systems appeared to be very efficient, even at low temperature (393 K) and low pressure (3 MPa), and exhibited a great advantage over Ru/CeO₂ or Ru/ZrO₂.     

Role of CeO2–ZrO2 in diesel soot oxidation and thermal stability of potassium catalyst

Potassium nitrate catalysts supported on different oxides (CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂) were prepared for diesel soot combustion. The ageing treatment was performed at 800 °C for 24 h and the catalytic activity was evaluated by a temperature-programmed oxidation technique. The results demonstrated that, compared with CeO₂ and ZrO₂, Ce_0.5Zr_0.5O₂ presented good redox properties, a high surface area and available potassium-holding capacity at an elevated temperature. For aged K/Ce_0.5Zr_0.5O₂, the combustion temperature of soot particle was 359 °C under tight contact conditions and 455 °C under loose contact conditions. Thus, ceria–zirconia mixed oxides were considered as good candidate supports for diesel soot oxidation catalysis.     

Catalytic performance of Ag–Co/CeO2 catalyst in NO–CO and NO–CO–O2 system

A new bimetallic catalyst (Ag–Co/CeO₂) was studied for simultaneously catalytic removal of NO and CO in the absence or presence of O₂. CeO₂ prepared by homogeneous precipitation method was optimized as supports for the active components. The addition of Ag on CeO₂ greatly improved the catalytic activities in the lower temperature regions (⩽300 °C), and the introduction of Co on CeO₂ increased the activities at higher temperatures (⩾250 °C). The bimetallic Ag–Co/CeO₂ catalyst combined the advantages of the corresponding individual metal supported catalysts and showed superior activity due to the synergetic effect. The effect of support, temperature, loading amount, GHSV and oxygen on catalysis was investigated. NO and CO could be completely removed in the temperature range of 200–600 °C at a very high space velocity of 120 000 h⁻¹. No deactivation was observed over 4% Ag–0.4% Co/CeO₂ catalyst even after 50 h test.     

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.     

Novel MoO3/CeO2–ZrO2 catalyst for the selective catalytic reduction of NOx by NH3

A series of MoO₃-doped CeO₂–ZrO₂ catalysts were investigated for the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). It was found that the added MoO₃ significantly enhanced the activity of CeO₂–ZrO₂ catalyst for NH₃-SCR of NO_x in a wide temperature range and the optimum MoO₃ loading is 5%. The highly dispersed MoO₃ not only resulted in more Lewis acid and Brønsted acid sites formed on the catalyst surface, but also increased the redox property of the catalyst, all of which account for the enhanced SCR activity.