The role of AgOAl species in silver–alumina catalysts for the selective catalytic reduction of NOx with methane

We examined the role of silver and alumina in Ag–alumina catalysts for the selective catalytic reduction (SCR) of NO_x by methane in gas streams containing excess oxygen. A cogelation technique was used to prepare Ag–alumina materials with high dispersion of silver even at high metal loadings (>10 wt%) and after air calcination at 650 °C. Typically, a part of silver is present as fine nanoparticles on the alumina, whereas another part is ionic, bound with the alumina as [AgOAl] species. Dilute nitric acid leaching was used to remove the silver particles and all weakly bound silver from the surface of these materials. Complementary structural characterization was performed by HRTEM, XPS, XRD, and UV–vis DRS. We found that the higher the initial silver content, the higher the amount of the residual [AgOAl] species after leaching. NO–O₂-TPD tests identified that silver does not modify the surface properties of the alumina. The SCR reaction-relevant NO_x adsorption takes place on alumina. Temperature-programmed surface reaction (TPSR) and kinetic measurements at steady state were used to check the reactivity of the adsorbed NO_x species with methane and oxygen to form dinitrogen. Only the alumina-adsorbed nitrates react with CH₄ to produce N₂ in the presence of oxygen, beginning at ∼300 °C as found by TPSR. Moreover, the SCR reaction rates and apparent activation energies are the same for the leached and parent Ag–alumina catalysts. Thus, metallic silver nanoparticles are spectator species in CH₄-SCR of NO_x. These catalyze the direct oxidation of methane at temperatures as low as 300 °C, which explains the lower methane selectivity for the SCR reaction measured over the parent samples.     

Bifunctional mesoporous Cu–Al–MCM-41 materials for the simultaneous catalytic abatement of NOx and VOCs

This study explored the possibility of using waste organic solvent as the source of volatile organic compound (VOC) and it served as a reducing agent of selective catalytic reduction (SCR) deNO_x process, in which the VOC itself can be catalytically oxidized on the mesoporous Cu and/or Al substituted MCM-41 catalysts. The synthesized Cu–Al–MCM-41 catalysts were extensively characterized by powder low-angle X-ray diffraction (XRD), N₂ adsorption–desorption measurements, transmission electron microscopy (TEM), UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS), ²⁷Al magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR), electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma–mass spectrometer (ICP–MS) analysis. The XRD, TEM and N₂ adsorption–desorption studies clearly demonstrated the presence of a well ordered long range hexagonal array with uniform mesostructures. The Cu–Al–MCM-41 materials showed a better long-term-stability than that of copper ion-exchanged H–ZSM-5 (Cu–ZSM-5) zeolite. The Cu–Al–MCM-41 material was found to be an efficient catalyst than that of Cu–MCM-41 without aluminum for the simultaneous catalytic abatement of NO_x and VOCs, which was attributed to the presence of well dispersed and isolated Cu²⁺ ions on the Cu–Al–MCM-41 catalyst as observed by UV–Vis DRS and EPR spectroscopic studies. And the presence of aluminum (Al³⁺ ions) within the framework of Cu–Al–MCM-41 stabilized the isolated Cu²⁺ ions thus it led to higher and stabilized activity in terms of NO_x reduction.     

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.     

Bimetallic PtAu cluster-derived catalysts for the selective catalytic reduction of NO by propylene

The selective catalytic reduction of nitric oxide by propylene in the presence of excess oxygen has been investigated over a series of silica supported Pt and PtAu catalysts. Cluster-derived catalysts were prepared by wet impregnation using a Pt₂Au₄(CC^tBu)₈ organometallic cluster precursor, and compared to catalysts obtained by incipient wetness impregnation using individual Pt and Au salts. The addition of Au by co-impregnation resulted only to a small shift (20–25 °C) in the light-off of propylene towards higher temperatures. A markedly different catalytic behavior was observed in the case of cluster-derived catalysts, where a 150 °C delay was observed both in the temperature of maximum NO reduction and the light-off of propylene in the presence and in the absence of NO. Most importantly, the selectivity towards N₂ increased by 50% compared to the monometallic Pt catalysts. The results of characterization studies indicate that the monometallic Pt/SiO₂ and the co-impregnated PtAu/SiO₂ samples have similar metal particle sizes. A more narrow particle size distribution was observed with the cluster-derived bimetallic sample.     

Direct synthesis,characterization and catalytic performance of Al–Fe-SBA-15 materials in selective catalytic reduction of NO with NH3

A series of Al–Fe-SBA-15 catalysts were synthesized by microwave methods with various iron and aluminum contents. The samples were characterized by XRD, SEM, TEM, BET, NH₃-TPD, FT-IR. Results indicated that all the samples that exhibited well-ordered hexagonal arrays of mesopores, disk-like and petal-like morphologies were observed for different catalysts. NH₃-TPD indicated that the introduction of Al results in the generation of Brönsted acid originated from Al–OH. Brönsted acid played important roles in the selective catalytic reduction of NO by NH₃, NO_x conversion over Al–Fe-SBA-15 samples increased rapidly as temperature increased and achieved 95% at about 360 °C.     

Effects of Rh content on catalytic behavior in CO hydrogenation with Rh–silica catalysts prepared using microemulsion

Rh–silica catalysts having the same physical parameters despite their different Rh contents were prepared using water‐in‐oil (w/o) microemulsions, and the effects of Rh content on the catalytic behavior were investigated in the hydrogenation of carbon monoxide. It was found that both the product selectivity and the turnover frequency changed dramatically with the Rh content. The selectivity to C₂₊ oxygenated compounds increased with increasing Rh content, while the selectivity to hydrocarbons and the turnover frequency decreased. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The kinetic analysis of optimization and selective transportation of Cu(Ⅱ) ions with TNOA as carrier by MDLM system

This study covers the transportation of Cu(Ⅱ) ions by multi-dropped liquid membrane(MDLM) system and tri-noctylamine(TNOA) as carrier in kerosene. Batch experiments are held to obtain optimum conditions for the transportation of Cu(Ⅱ) ions such as volume of donor, organic, and acceptor phase 100 ml, p H of donor phase9.00, temperature 298.15 K, concentration of H_2SO_4 in acceptor phase 1.00 mol·L~(-1), concentration of TNOA in organic phase 5.00 × 10~(-3)mol·L~(-1)and rate of peristaltic pump 50 ml·min~(-1). Optimum circumstances of this extraction are as follows: p H of donor phase is 9.00, concentration of TNOA is 5.00 × 10~(-3)mol·L~(-1),1.00 mol·L~(-1)H_2SO_4 as acceptor phase, and flux rate is 50 ml·min~(-1). Cu(Ⅱ) ion transportation is consecutive first order irreversible reaction. Activation energy is found as 5.22 kcal·mol-1(21.82 k J·mol~(-1), this process is called as diffusion controlled system. Selective transportation of Cu(Ⅱ) ions with alkaline, alkaline earth, and different heavy metal ions at optimum conditions of single Cu(Ⅱ) extraction was conducted. According to the selective transportation Cu(Ⅱ) ions with alkaline and alkaline earth metal ions, Na~+, K~+, and Ba~(2+)ions are not detected in the acceptor phase, but 12.00% of Ca~(2+)ions is transported from donor phase to acceptor phase. At the end of the simultaneous extraction of Zn(Ⅱ), Fe(ⅡI), and Mo(VI) with Cu(Ⅱ) ions, 2.20% of Mo(VI), 0.80% of Fe(Ⅲ) and 3.60% of Zn(Ⅱ) are detected in the acceptor phase.     

An unprecedented acid/base stable Cu-based metal–organic framework with good catalytic activity for the reduction of 4-nitrophenol

A new 3D metal–organic framework with pcu topology {[Cu₂(TPOM)(ddpa)₂]}_n (1) was synthesized under solvothermal condition by employing flexible tetrakis(4-pyridyloxymethylene) methane (TPOM) and semirigid 4,4′-dicarboxydiphenylamine (H₂ddpa) as well as Cu(II) ion. Complex 1 exhibits good endurance in acid/base aqueous solutions and a high catalytic activity for the reduction of 4-nitrophenol in excess NaBH₄ solution.     

The multifunctional mesoporous Sn–Cu–Ti catalysts for the B–V oxidation of cyclohexanone by molecular oxygen

The multifunctional three-component mesoporous Sn–Cu–Ti samples were successfully prepared by a facile one-pot AcHE method and characterized by XRD, N₂ sorption, ICP, FT-IR, UV–vis, SEM and TEM techniques, and their catalytic performances were carried out in the B–V oxidation of cyclohexanone by molecular oxygen. The results show that both tin and copper species can homogeneously incorporated in the crystalline framework of mesoporous anatase TiO₂, where tin species as the active sites would increase the Lewis acidity and the oxidation of benzaldehyde as pro-oxygenic agent would be promoted by the introduction of little copper species resulting in the improving catalytic performance. The prepared 15Sn–3Cu–Ti catalyst shows higher yield of ε-caprolactone than other catalysts and exhibits excellent catalytic stability even after repeated reaction for five times without any further treatment. The outstanding catalytic performance for Sn–Cu–Ti catalysts could offer a valuable reference for the industrial development of B–V oxidation of cyclohexanone.     

Thermal stability of Pt particles of Pt/Al2O3 catalysts prepared using microemulsion and catalytic activity in NO–CO reaction

Sintering behaviors of the Pt particles of Pt/Al₂O₃ catalyst prepared using different preparation methods (microemulsion, sol–gel, and impregnation methods) were investigated. It was found that the catalyst prepared by microemulsion had a higher resistance to sintering than did the sol–gel and impregnation catalysts. To limit the sintering even more, the catalysts were pressed. The resistance to sintering in all the catalysts was improved by pressing. The pressed microemulsion catalyst was little deactivated in the NO–CO reaction by thermal treatment at 700 °C for 12 h, and had a high activity relative to that of the sol–gel and impregnation catalysts.     

The synthesis of new chiral salen complexes immobilized on MCM‐41 by grafting and their catalytic activity in the asymmetric borohydride reduction of ketones

The new chiral salen complexes were synthesized and supported on mesoporous MCM‐41 through the condensation of 3‐aminopropyltrimethoxysilane and 2,6‐diformyl‐4‐tert‐butylphenol by the multi‐grafting method. The immobilized optically active Co(II) salen complexes showed a very high enantioselectivity in the asymmetric borohydride reduction of aromatic ketones. The chiral salen Co(II) complexes immobilized over MCM‐41 were stable during the reaction and exhibited a relatively high enantioselectivity for the reduction of ketones as compared with the homogeneous salen catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of copper valence on catalytic combustion of styrene over the copper based catalysts in the absence and presence of water vapor☆

Catalysts Cu Ox/γ-Al_2O_3-IH and Cu Ox/γ-Al_2O_3-IM were prepared, characterized, and tested for styrene combustion in the absence and presence of water vapor. The effect of copper valence of the catalysts on the catalytic activity for styrene combustion was discussed using the theory of hard soft acids and bases(HSAB).The results showed that the existence of water vapor in feed stream inhibited the catalytic activity for styrene combustion due to the competition adsorption of water molecule. HSAB theory confirmed that the local soft acidity of the catalyst Cu Ox/γ-Al_2O_3-IH was much stronger than that of the catalyst Cu Ox/γ-Al_2O_3-IM because of the higher content of soft acid Cu+on its surface, which increased the adsorption ability toward soft base of styrene and reduced the adsorption toward hard base of water vapor, and thus increased the catalytic activity for styrene combustion and weakened the negative influence of water vapor.     

Ag–Cu–BTC prepared by postsynthetic exchange as effective catalyst for selective oxidation of toluene to benzaldehyde

A mixed-node MOF catalyst Ag–Cu–BTC was prepared by postsynthetic exchange (PSE) method. It is believed that PSE method can realize isomorphous replacement of Ag ion to framework Cu ion in Cu–BTC successfully. The catalytic performance of Ag–Cu–BTC was investigated via selective oxidation of toluene to benzaldehyde by molecular oxygen in the absence of solvent and initiator. This catalyst exhibits good catalytic performance: on the premise of keeping highly selective catalysis of Cu–BTC for toluene oxidizing to benzaldehyde, the introduction of Ag (Ag content is 2.76 wt.%) can promote toluene conversion from 6.5% to 12.7%.     

Effect of activation temperature on the surface copper particles and catalytic properties of Cu–Ni–Mg–Al oxides from hydrotalcite-like precursors

The Cu–Ni–Mg–Al oxides catalysts for furfural hydrogenation were prepared from the hydrotalcite-like precursors, and the effect of activation temperature on the Cu⁰ particles and catalytic properties of the catalyst was thoroughly investigated. The catalyst activated by H₂ at 300 °C was found to exhibit the best catalytic activity, due to the presence of the smallest Cu⁰ particles with a high dispersion. Moreover, the bigger Cu⁰ particles were active for furfuralcohol hydrogenolysis to 2-methylfuran in the liquid-phase (ethanolic solution), and the hydrogenation of the furan ring of furfuralcohol and 2-methylfuran on Cu⁰ particles was easily achieved in the vapour-phase.     

Effects of Fe addition on the structure and catalytic performance of mesoporous Mn/Al–SBA-15 catalysts for the reduction of NO with ammonia

Mesoporous Al–SBA-15 has been synthesized by a hydrothermal method and used as a support for Mn/Al–SBA-15, Fe/Al–SBA-15, and Mn–Fe/Al–SBA-15 catalysts. XRD, N₂ sorption, XPS, H₂-TPR and activity tests have been used to assess the properties of catalysts. The Mn–Fe/Al–SBA-15 catalyst exhibited a higher SCR activity than Mn/Al–SBA-15 or Fe/Al–SBA-15 due to a synergistic effect between Mn and Fe. After the addition of Fe, the binding energy of Mn 2p_3/2 on Mn–Fe/Al–SBA-15(573) decreased by about 0.4 eV and the Mn^4 +/Mn^3 + ratio decreased to 1.10. The appropriate Mn^4 +/Mn^3 + ratio may have a great effect on the reduction of NO over Mn–Fe/Al–SBA-15(573) catalyst.     

Novel Pd–Au/TiO2 catalyst for the selective catalytic reduction of NOx by H2

Novel Pd–Au/TiO₂ catalyst exhibited high catalytic activity with a wide temperature window for the selective catalytic reduction of NO_x by H₂ in the presence of oxygen. The synergetic effect between Pd and Au contributes to the formation of Pd⁰ and Pd–Au alloy, thus promoting the NO_x reduction to proceed.     

Synthesis and catalytic activity of organic–inorganic hybrid catalysts coordinated with cobalt(II) ions for aerobic epoxidation of styrene

New organic–inorganic hybrid materials (HM) containing 3-mercaptopropyl groups (–(CH₂)₃–SH) have been synthesized through a dry gel conversion (DGC) route. The complex catalyst Co–HM was prepared through a simple coordination of –SH with cobalt(II) ions, which was firstly applied in the aerobic epoxidation of alkenes to obtain good results. Co–HM-50 exhibited the highest activity for the epoxidation of styrene with air to achieve 95.8 mol% of conversion with the epoxide selectivity of 89.2%. Recycling and control tests showed high durability and heterogeneity of Co–HM-50 as a heterogeneous catalyst.