NOx Adsorption/Desorption Processes Over Ce0.76Zr0.24O2 and Their Influence on DeSoot Activity: Effect of the Catalyst Calcination Temperature

The catalytic activity of a Ce_0.76Zr_0.24O₂ mixed oxide, calcined at different temperatures, for soot oxidation under NO _x /O₂ was correlated with the catalytic activity for NO₂ production. The Ce_0.76Zr_0.24O₂ mixed oxide samples were prepared by co-precipitation and calcination at different temperatures: 500–1000 °C. A satisfactory correlation between the total amount of NO₂ desorbed after NO + O₂ adsorption at 60 °C and the T50% for soot combustion was found. The NO _x adsorption process was also explored by in situ DRIFTS.     

Novel Silver Loaded Hydroxyapatite Catalyst for the Selective Catalytic Reduction of NOx by Propene

A novel and high performance silver loaded hydroxyapatite (HAp) catalyst for the selective catalytic reduction (SCR) of NO_x by propene is reported for the first time. The catalysts with variable silver contents have been prepared and characterized extensively by different techniques such as XRD, XPS, BET-surface area, TPR, TPD and ICP analyses. The DeNO_x activities of these catalysts are measured at reaction temperature ranged from 250 to 500 °C. The 1.5 wt.% Ag/HAp is found to be best among all the catalysts studied showing about 70% conversion and 60% selectivity towards N₂ formation at 375 °C in oxygen rich atmosphere.     

Preparation of Co3O4/ZrO2(Y2O3) Catalyst and its Enhanced Catalytic Oxidation of CO

Yttria-stabilized zirconia powders were prepared by the sol–gel method coupled with supercritical CO₂ fluid-drying technology, using ZrOCl₂·8H₂O as the precursor, urea as the precipitant, and yttria as the stabilizer. The particles were characterized by X-ray diffraction, TEM and BET. The Co₃O₄/ZrO₂(Y₂O₃) catalysts were prepared by the impregnation method. The content of cobalt was varied from 5 to 12 wt%. The prepared catalysts were calcined at 200–500 °C and the pretreating temperature was varied from 200–400 °C. The performance of CO catalytic oxidation was tested and the catalyst with 8% Co loading, calcined at 200 °C, and with a pretreating temperature of 300 °C, showed the highest catalytic activity. The temperature for 95% CO conversion was as low as 113 °C; and, the catalyst showed both good cycling stability and excellent long-term stability.     

Pt-V2O5-WO3/TiO2 catalysts supported on SiC filter for NO reduction at low temperature

The catalytic filter, V₂O₅-WO₃-TiO₂ supported on a ceramic filter, is known as a promising material for treating particulates and NO _x simultaneously at optimum temperatures around 320°C. In order to improve its catalytic activity at low temperatures, the effect of Pt addition on the catalytic filter has been investigated. Catalytic filters, Pt-V₂O₅-WO₃-TiO₂/SiC, were prepared by co-impregnation of Pt, V, and W precursors on TiO₂ coated-SiC filter by vacuum aided-dip coating. The Pt-added catalytic filter shifted the optimum working temperature from 280–330°C (for the non Pt-impregnated filter) to 180–230°C, providing N _x slip concentration less than 20 ppm for the treatment of 700 ppm NO at a face velocity of 2 cm/s with the same value over the non Pt-added catalytic filters. The promotional effect following the addition of Pt is believed to result from electrical modification of the catalyst maintaining a high electron transfer state. Ammonia oxidation was also observed to be dominant above the optimal temperature for SCR.     

Effect of Silver Loading on the Lean NOx Reduction with Methanol Over Ag–Al2O3

The influence of silver loading on the lean NO_x reduction activity using methanol as reductant has been studied for alumina supported silver catalysts. In general, increasing the silver loading (0–3 wt%), in Ag–Al₂O₃, shifts or extends the activity window, for lean NO_x reduction towards lower temperatures. In particular Ag–Al₂O₃ with 3 wt% silver is active for NO_x reduction under methanol-SCR conditions in a broad temperature interval (200–500 °C), with high activity in the low temperature range (maximum around 300 °C) typical for exhaust gases from diesel and other lean burn engines. Furthermore, increasing the C/N molar ratio enhances the reduction of NO_x. However, too high C/N ratios results in poor selectivity to N₂.     

WO3 and MoO3 addition effect on V2O5/TiO2 as promoters for removal of NOx and SOx from stationary sources

As an attempt to improve the catalytic activity at higher reaction temperatures between 300-450°C, various mole ratios of WO₃ were added to V₂O₅/TiO₂ catalytic systems. And also, in order to suggest a new mixed oxide catalyst system for simultaneous removal of NO_x and SO_x, from stationary sources, MoO₃-V₂O₅/TiO₂catalysts were prepared by a conventional impregnation method together with a newly introduced method of surface fixation (non-aqueous solution method). In case of WO₃ addition, at higher reaction temperature range (300–450°C), WO₃ and WO₃-V₂O₅/TiO₂ catalysts showed significant high conversion in NO reduction with NH₃ while V₂O₅/TiO₂ catalyst showed a significant change in selectivity mainly due to the excess side reaction of NH₃ oxidation. This difference in selectivity due to NH₃ oxidation at high temperature is supposed to be associated with the difference in values of surface excess oxygen between WO₃ and V₂O₅ on titania. The surface acidities of tested catalysts were relatively well correlated with the % conversion of NO at 400°C. In case of MoO₃ addition, the catalytic activity for the simultaneous removal of NO_x and SO_x were quite enhanced by the addition of MoO₃ into V₂O₅/TiO₂ catalysts. The enhanced activities were responsible for the formation of Mo=O bond on the intermediate species produced by solid solutions on MoO₃-V₂O₅/TiO₂ (aqueous). However, in the case of MoO₃-V₂O₅/TiO₂ (non-aqueous), the exact source of active site was not able to detect in IR spectra in spite of more enhanced activity was obtained in this study. After SO₂ contact, VOSO₄ is newly formed on the surface of catalyst, which supposed to be associated with the activity enhancement.     

Hydrothermally Stable WO3/ZrO2–Ce0.6Zr0.4O2 Catalyst for the Selective Catalytic Reduction of NO with NH3

A new catalyst WO₃/ZrO₂–Ce_0.6Zr_0.4O₂ (15 wt % WO₃/ZrO₂:Ce_0.6Zr_0.4O₂ = 50:50) has been developed for the selective catalytic reduction of NO with NH₃. The redox component Ce_0.6Zr_0.4O₂ was dispersed on the surface of acidic WO₃/ZrO₂ by the solution combustion method showing the best NO _x reduction efficiency among the catalysts prepared by various modes of mixing of the components. The catalyst has been characterized by XRD, Raman spectroscopy and NH₃-TPD. A NO _x reduction efficiency of more than 90 % was obtained between 300 and 500 °C at α = NH_3,in/NO _x,in = 1. The catalyst showed stable NO _x reduction efficiency after hydrothermal ageing at 700 °C. Sulfur poisoning promoted the NO _x reduction efficiency at high temperatures at the expense of a reduced activity at lower temperatures, but the catalyst could be fully regenerated by heating in O₂ at 650 °C.     

Novel Y2O3 Doped MnO x Binary Metal Oxides for NO x Storage at Low Temperature in Lean Burn Condition

MnO _x -Y₂O₃ binary metal oxide catalysts are synthesized by a constant-pH co-precipitation method, their ability of NO _x storage capacity and absorbing process were investigated. The pure MnO _x and Y₂O₃ calcined at 500 °C for 4 h in static air are both of body-centre structure, while the binary metal oxides containing Mn and Y are mainly of amorphous phase. The adulteration of Y₂O₃ can remarkably improve the specific surface areas of the catalysts, which probably result of the enhancement on NO storage capacity and catalytic oxidation ability of NO at 100 °C. The XPS results indicate that both Mn and Y have 3+ chemical states in the binary oxides. FT-IR spectra could be beneficial to explain the NO storage process on the binary metal oxide: NO can be adsorbed on the MnO _x and Y₂O₃ sites as nitrates and nitrites, respectively, and then the nitrites on Y₂O₃ site are shifted to Mn₂O₃ site and then is oxidized to nitrates. As a result, the NO storage capacity is enhanced due to the adulteration of Y₂O₃, finally the NO _x are adsorbed on the Mn₂O₃ site as nitrate species.     

Simultaneous catalytic removal of NOx and soot particulate over Co–Al mixed oxide catalysts derived from hydrotalcites

Co–Al mixed oxides (CAO) was prepared by co-precipitation method from hydrotalcites (HT) as precursors, and their catalytic activity was investigated for the simultaneously catalytic removal of NO_x and diesel soot particulates by the temperature-programmed reaction (TPR) technique. All HT samples present well crystallized, layered structures, no excess phases were detected. A nonstoichiometric spinel phase was formed by calcining the CAO at 500 °C and 800 °C, irrespective of the Co/Al ratio. Both the activity of soot oxidation and the selectivity to N₂ formation of CAO catalysts calcined at 800 °C were higher than that at 500 °C. The observed difference in the catalytic performance was related to the redox properties of the catalysts and the crystallite size of HT precursors. The active species might come from Co₃O₄, which acted for redox-type mechanism for soot oxidation in the NO_x-soot reaction.     

Selective Oxidation and Oxidative Dehydrogenation of Isobutane over Hydrothermally Synthesized Mo–V–O Mixed Oxide Catalysts

A series of Mo–V–O catalysts were prepared by calcining the orthorhombic (M1) Mo–V–O phase containing precursors under different conditions (T = 500 or 600 °C in atmosphere of N₂ or air) and tested for the oxidation of isobutane and isobutene. Characterization results (BET, XRD, XPS, FTIR, and TPR) showed that their structure and properties depend on the composition of the calcined samples and the calcined conditions. Catalytic tests showed that relatively high isobutane conversion and desired product selectivity can be achieved over MoV_0.3-500-N and MoV_0.3-600-A catalysts. It is also found that both orthorhombic M1 phase and (V_0.07M_0.93)₅O₁₄ phase are active and selective for the selective oxidation of isobutane to methacrolein, whereas higher selectivity toward methacrolein (40.4%) can be achieved over the former phase at a moderate isobutane conversion (6.4%). Moreover, (Mo_0.3V_0.7)₂O₅ phase may be propitious to complete oxidation for the selective oxidation of isobutane. On the other hand, the presence of V affects the activity and selectivity, and a low surface V⁴⁺ concentration prefers selective oxidation products. In addition, specific surface areas of the catalysts appear to be little important in determining the catalytic activation.     

Differences Between Al2O3 and Ag/Al2O3 for Lean Reduction of NO x with Dimethyl Ether

Lean reduction of NO _x with DME occurs with high selectivity to N₂ over Al₂O₃ between 300 °C and 550 °C with a maximum of 47% at 380 °C, and with lower selectivity over Ag/Al₂O₃ between 250 °C and 400 °C due to the catalysts’ sensitivity to gas phase radical reactions and activity for NO _x reduction with methanol.     

Performance Improvement of NO x -Storage BaO/Al2O3 by Using Barium Peroxide as the Precursor of BaO

Comparison of barium peroxide, Ba(OH)₂ and Ba(NO₃)₂ as the precursor of BaO for the preparation of NO _x -storage BaO/Al₂O₃ material was carried out. The as prepared materials were calcined at 550 and 800 °C and characterized by N₂ physisorption, XRD, Raman and FT-IR spectroscopy. Measurements of the NO _x storage performances of these BaO/Al₂O₃ materials by NO₂ adsorption and NO _x -TPD experiments showed that the use of barium peroxide as the precursor of BaO inhibited the formation of BaAl₂O₄ and led to remarkable improvements in the thermal stability as well as NO _x storage capacity of the final BaO/Al₂O₃ material calcined at 800 °C.     

Selective Oxidation of n-Pentane Over V2O5 Supported on Hydroxyapatite

The catalytic efficiency of V₂O₅ supported on hydroxyapatite in controlled oxidation of n-pentane to phthalic anhydride and maleic anhydride is investigated as function of flux rates of the reactants and temperature in the gas- phase in a fixed- bed stainless steel microreactor under steady state conditions. The hydroxyapatite was prepared by co-precipitation and the loaded catalysts by wet impregnation using NH₄VO₃ solution. Selectivity towards the products was influenced by the total flow rate, reaction temperature and V₂O₅ loadings. Good selectivities towards the anhydrides (MA 40% and PA 25%) is obtained with 5.0 and 7.5 wt.% of V₂O₅ at 360 °C.     

Designed Highly Effective Photocatalyst of Anatase TiO2 Codoped with Nitrogen and Vanadium Under Visible-light Irradiation Using First-principles

Fe/ZSM-5 and Cu-ZSM-5 were investigated for selective catalytic reduction of NO_x with ammonia. The Fe/ZSM-5 catalyst showed over 90% NO conversion from 350 to 500 °C and Cu-ZSM-5 showed over 90% NO conversion from 250 to 350 °C. After pretreatment in 10% H₂O at 700 °C, the Fe/ZSM-5 still exhibited high activities and stability. On the basis of the experiments, Fe/ZSM-5 and Cu-ZSM-5 seem to be promising candidates for diesel-engine NO_x emission control.     

The promotional effect of MoO3 doped V2O5/TiO2 for chlorobenzene oxidation

A series of V₂O₅–MoO₃/TiO₂ catalysts prepared by an impregnation method was investigated on their catalytic performance of chlorobenzene oxidation. This study demonstrated that MoO₃ is a dopant, which can improve catalyst redox properties and enhance chlorobenzene oxidation catalytic activity at low temperature. Compared to V₂O₅–TiO₂ and V₂O₅–WO₃/TiO₂ catalysts, V₂O₅–MoO₃/TiO₂ catalysts showed outstanding activity on chlorobenzene oxidation, which is due to the promoting influence of MoO₃. For excellent catalytic performance and good N₂ selectivity with water, the V₂O₅–MoO₃/TiO₂ catalysts may have promising commercial potential for the synergetic control of dioxins and NO_x emitted from stationary sources.     

Synthesis and Dispersion of Dendrimer-Encapsulated Pt Nanoparticles on γ-Al2O3 for the Reduction of NO x by Methane

Dendrimer encapsulated Pt nanoparticles were prepared by using hydroxyl terminated generation four (G4OH) PAMAM dendrimers (DEN) as the templating agents. The encapsulated Pt nanoparticles were dispersed on γ-Al₂O₃ at room temperature by impregnation. Pt/Al₂O₃ (DEN) catalysts were then subjected to thermal treatments in oxidizing and reducing atmospheres at different temperatures. These catalysts were characterized by Transmission Electron microscopy (TEM) and In situ Fourier-Transform Infrared (FTIR) spectroscopy. The TEM analysis of the as synthesized catalysts revealed that the Pt nanoparticles were found to be 2–4 nm in size. It is observed that the Pt particle size in 0.5% Pt/Al₂O₃ (DEN) catalyst increased upon thermal decomposition of the dendrimer. The in situ FTIR results suggested that the presence of oxygen and the Pt nanoparticles in the Pt-dendrimer nanocomposite accelerate the dendrimer decomposition at low temperatures. All the catalysts were tested for the reduction of NO _x with CH₄ in the temperature range of 250–500 °C. NO _x reduction efficiency of Pt/Al₂O₃ (DEN) catalysts were compared with the Pt/Al₂O₃ (CON; conventional) catalyst. The conversion of NO _x was started from the low temperatures over Pt/Al₂O₃ (DEN) catalysts. The high selectivity of NO _x to N₂ of 74% was obtained over 0.5% Pt/Al₂O₃ (DEN) catalyst at low temperatures around 350 °C.     

Synthesis, characterization, and catalytic phenol hydroxylation of a novel complex oxide HxV2Zr2O9.H2O

A novel type of complex oxide H_xV₂Zr₂O_9.H _2O with V⁴⁺ and V⁵⁺ mixed valence has been hydrothermally synthesized in a V₂O₅-ZrO₂-H₂O system at 240°C for 5 days in presence of NaF. The catalytic data over these complex oxides show that these complex oxides are catalytically very active in phenol hydroxylation by 30% aqueous hydrogen peroxide, and their catalytic activity is dependent on crystal size of the catalysts. The phenol conversion over the catalyst with crystal size of 7 μm is twice that over the catalyst with crystal size of 35 μm. The V⁵⁺ species are suggested to be the catalytic active sites. Some other factors which influence the catalytic activity were also investigated. This revised version was published online in July 2006 with corrections to the Cover Date.     

The reactivity of V2O5-WO3-TiO2 catalyst supported on a ceramic filter candle for selective reduction of NO

For realizing the environmental issues and constituting an economical treatment system, a catalytic filter based on V₂O₅/TiO₂ supported on tubular filter elements has many advantages by removing NO_x and particulate simultaneously from flue gas. In order to improve the activity of a catalytic filter based on V₂O₅/TiO₂ supported on a commercial high temperature filter element (PRD-66), the promoting effects of WO₃ were investigated in an experimental unit. PRD-66 presented very good properties for SCR catalyst carrier since it contains much active material such as A1₂O₃ SiO{om2}, and MgO whose contributions were remarkable. For additional catalyst carrier, TiO₂ particles were coated in the pores of PRD-66 with relatively good distribution of the particle size less than 1 μm, by a coating process applying centrifugal force. WO₃, in the V₂O₅-WO₃-TiO₂/PRD-66 catalytic filter system, increased the SCR activity significantly and broadened the optimum temperature window. The catalytic filter shows the maximum NO conversion of more than 95% for NO concentration of 700 ppmv at face velocity of 0.02 m/sec, which is comparable to the current commercial catalytic filters of plate form.     

An In Situ DRIFTS Study on SCR of NO with NH3 Over V2O5/AC Surface

Selective catalytic reduction (SCR) of NO with NH₃ on an activated coke supported V₂O₅ (V₂O₅/AC) catalyst was studied at 200 °C by in-situ diffuse reflectance infrared Fourier transform spectroscopy. Results indicate that NH₃ is adsorbed in the forms of coordinated NH₃, NH₄ ⁺ and –NH₂. V₂O₅ promotes formation of –NH₂. NO is oxidized to NO₂ in the presence of O₂. –NH₂ and NO₂ are the intermediates of the 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.