The Effect of Copper Loading on the Selective Catalytic Reduction of Nitric Oxide by Ammonia Over Cu-SSZ-13

Abstract  

The effect of Cu loading on the selective catalytic reduction of NO_x by NH₃ was examined over a series of Cu ion-exchanged (20–80%) SSZ-13 zeolite catalysts. High NO reduction efficiencies (80–95%) were obtained over all catalyst samples between 250 and 500 °C, and at the gas hourly space velocity of 200,000 h⁻¹. Both NO reduction and NH₃ oxidation activities under these conditions were found to increase slightly with increasing Cu loading at low temperatures. However, NO reduction activity was suppressed with increasing Cu loadings at high temperatures (>500 °C) due to excess NH₃ oxidation. The optimum Cu ion exchange level appears to be ~40–60% since higher than 80% NO reduction efficiency was obtained over 50% Cu ion-exchanged SSZ-13 up to 600 °C. The NO oxidation activity of Cu-SSZ-13 was found to be low regardless of Cu loading, although it was somewhat improved with increasing Cu ion exchange level at high temperatures. During the “fast” SCR (i.e., NO/NO₂ = 1), only a slight improvement in NO_x reduction activity was obtained for Cu-SSZ-13. Regardless of Cu loading, near 100% selectivity to N₂ was observed; only a very small amount of N₂O was produced even in the presence of NO₂. The apparent activation energies for NO oxidation and NO SCR were estimated to be ~58 and ~41 kJ/mol, respectively.     

Oxidative desulfurization of dibenzothiophene based on molecular oxygen and iron phthalocyanine

Direct oxidation of dibenzothiophene (DBT) based on molecular oxygen and iron tetranitrophthalocyanine (FePc(NO₂)₄) catalyst was performed in hydrocarbon solvent under water-free condition for deep desulfurization. Conversion of DBT in decalin reached 98.7 wt.% at 100 °C and 0.3 MPa of initial pressure with 1 wt.% of FePc(NO₂)₄ over the whole solution for 2 h. In addition to FePc(NO₂)₄, another two catalysts, FePc(NO₂)₃NH₂ and FePc(NH₂)₄, were synthesized to investigate the effect of substituents of iron phthalocyanines on their catalytic activities. The results show that the catalytic activity of these phthalocyanines decreases in the order of FePc(NO₂)₄ > FePc(NO₂)₃NH₂ > FePc(NH₂)₄, indicating that the electron-donating group has negative effect on the catalytic properties. Activity of FePc(NO₂)₄ was kept unchanged after 5 runs of oxidation; whereas, activity of FePc(NH₂)₄ decreased because of its decomposition. Moreover, FePc(NO₂)₃NH₂ was supported on a polyacrylic cationic exchange resin and its activity was remarkably enhanced to the level of FePc(NO₂)₄. Oxidative desulfurization of a model fuel, 500 μg/g solution of DBT in decalin, was performed based on the catalytic oxidation using molecular oxygen and FePc(NO₂)₄ catalyst. The lowest sulfur content in the model fuel could be decreased to less than 4 μg/g after the treatment of this oxidation and a combined adsorption.     

Novel Sn–Ce/Al2O3 Catalyst for the Selective Catalytic Reduction of NOx Under Lean Conditions

Effect of additives, Ce and Mn, on the catalytic performance of Sn/Al₂O₃ catalyst prepared by sol–gel method for the selective reduction of NO_x with propene under lean conditions was studied. Sn–Ce/Al₂O₃ catalysts exhibited higher activity than Sn/Al₂O₃ catalyst and the optimum Ce loading is 0.5–1%. The promoting effect of Ce is to enhance the oxidation of NO to NO₂ and facilitate the activation of propene, both of which are important steps for the NO_x reduction. The presence of oxygen contributes to the oxidation of NO and shows a promoting effect.     

An environmentally friendly FeTiSOx catalyst with a broad operation-temperature window for the NH3-SCR of NOx

A novel FeTiSO_x catalyst prepared by a simple hydrolysis coprecipitation method was used for the selective catalytic reduction (SCR) of NO_x with NH₃, which exhibited high catalytic activity (NO_x conversion of >97% and N₂ selectivity of >95%) and tolerance for both H₂O and SO₂ at a broad temperature window of ~325–475°C. The characterization results showed that the formation of Fe–O–Ti and Fe–O–S species could significantly enhance the acidic sites of the catalyst, which play an important role in NH₃ absorption and their catalytic activity. The Fe³⁺ ions in the bulk anatase TiO₂ could significantly enhance the redox properties for the SCR reaction and suppress the side reaction of NH₃ oxidation to NO or N₂O. In addition, the reaction mechanism was discussed based on in situ diffuse reflectance infrared Fourier transform spectroscopy measurements and kinetic investigation, indicating that the reaction was dominated by the Eley–Rideal mechanism over the FeTiSO_x catalyst.     

Cobalt Porphyrins Immobilized on Polymer Microspheres as Catalysts for the Oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoquinone by Molecular Oxygen

Abstract  

Three types of porous polymer microspheres immobilized with cobalt porphyrins appending p–H, p-Cl and p-NO₂ phenyl substituents (designated as CoPP-GMA/MMA, CoCPP-GMA/MMA and CoNPP-GMA/MMA, respectively) were prepared. Their catalytic activities on the oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoquinone by molecular oxygen were investigated in alkaline methanol. The experimental results showed that the porous microsphere supported cobalt porphyrin catalysts could effectively activate molecular oxygen, and 2-naphthol was selectively oxidized to 2-hydroxy-1,4-naphthoquinone with high conversion in alkaline methanol. A phenomenon of distance-dependent catalytic activity was observed and a critical distance of 3.8 nm between porphyrins was determined for the porous polymer microsphere supported catalyst. More interestingly, the activity of the recycled catalyst increased gradually with the increased times of reuse. These results may be helpful in designing highly efficient metalloporphyrin catalysts.     

Efficient Oxidation of 2,3,6-Trimethyl Phenol using Non-Exchanged and H<Superscript>+</Superscript> Exchanged Manganese Oxide Octahedral Molecular Sieves (K-OMS-2 and H–K-OMS-2) as Catalysts

Abstract  

A new and efficient oxidation process of 2,3,6-trimethyl phenol to 2,3,6-trimethyl benzoquinone (TMQ) is reported forthwith using non-exchanged and H⁺-exchanged manganese oxide octahedral molecular sieves (K-OMS-2 and H–K-OMS-2) as benign catalysts. The oxidation reaction is efficiently carried out using TBHP as oxidant and with catalytic amounts of OMS-2 achieving >95% conversion with excellent selectivity (~99%) to TMQ in 30 min.     

Effect of sintering on the catalytic activity of a Pt based catalyst for CO oxidation: Experiments and modeling

The goal of this paper was to make the link between sintering of a 1.6% Pt/Al₂O₃ catalyst and its activity for CO oxidation reaction. Thermal aging of this catalyst for different durations ranging from 15 min to 16 h, at 600 and 700 °C, under 7% O₂, led to a shift of the platinum particle size distributions towards larger diameters, due to sintering. These distributions were studied by transmission electron microscopy. The number and the surface average diameters of platinum particles increase from 1.3 to 8.9 nm and 2.1 to 12.8 nm, respectively, after 16 h aging at 600 °C. The catalytic activity for CO oxidation under different CO and O₂ inlet concentrations decreases after aging the catalyst. The light-off temperature increased by 48 °C when the catalyst was aged for 16 h at 600 °C. The CO oxidation reaction is structure sensitive with a catalytic activity increasing with the platinum particle size. To account for this size effect, two intrinsic kinetic constants, related either to platinum atoms on planar faces or atoms on edges and corners were defined. A platinum site located on a planar face was found to be 2.5 more active than a platinum site on edges or corners, whatever the temperature. The global kinetic law {r (mol m⁻² s⁻¹) = 103 × exp(−64,500/RT)[O₂]^0.74[CO]^−0.5)} related to a reaction occurring on a platinum atom located on planar faces allows a simulation of the CO conversion curves during a temperature ramp. Modeling of the catalytic CO conversion during a temperature ramp, using the different aged catalysts, allows prediction of the CO conversion curves over a wide range of experimental conditions.     

CeO<Subscript>2</Subscript>–WO<Subscript>3</Subscript> Mixed Oxides for the Selective Catalytic Reduction of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> by NH<Subscript>3</Subscript> Over a Wide Temperature Range

Abstract  

A series of cerium-tungsten oxide catalysts was prepared by the co-precipitation method and was evaluated for the selective catalytic reduction of NO _x by ammonia (NH₃-SCR) over a wide temperature range. These catalysts were characterized by BET, XRD, XPS and H₂-TPR analyses. The experimental studies demonstrated that, among cerium-tungsten oxides, CeO₂–WO₃ with a Ce/W molar ratio of 3/2 exhibited the best activity toward NH₃-SCR reactions, N₂ selectivity and SO₂ durability over a broad temperature range of 175–500 °C at a space velocity of 47,000 h⁻¹. The strong interaction between Ce and W could be the main factor leading to the high activity of the CeO₂–WO₃ mixed oxide catalyst.     

A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5

Kinetic modeling, in combination with flow reactor experiments, was used in this study for simulating NH₃ selective catalytic reduction (SCR) of NO_x over Cu-ZSM-5. First the mass-transfer in the wash-coat was examined experimentally, by using two monoliths: one with 11 wt.% wash-coat and the other sample with 23 wt.% wash-coat. When the ratio between the total flow rate and the wash-coat amount was kept constant similar results for NO_x conversion and NH₃ slip were obtained, indicating no significant mass-transfer limitations in the wash-coat layer. A broad range of experimental conditions was used when developing the model: ammonia temperature programmed desorption (TPD), NH₃ oxidation, NO oxidation, and NH₃ SCR experiments with different NO-to-NO₂ ratios. 5% water was used in all experiments, since water affects the amount of ammonia stored and also the activity of the catalyst. The kinetic model contains seven reaction steps including these for: ammonia adsorption and desorption, NH₃ oxidation, NO oxidation, standard SCR (NO + O₂ + NH₃), rapid SCR (NO + NO₂ + NH₃), NO₂ SCR (NO₂ + NH₃) and N₂O formation. The model describes all experiments well. The kinetic parameters and 95% linearized confidence regions are given in the paper. The model was validated with six experiments not included in the kinetic parameter estimation. The ammonia concentration was varied from 200 up to 800 ppm using NO only as a NO_x source in the first experiment and 50% NO and 50% NO₂ in the second experiment. The model was also validated with transient experiments at 175 and 350 °C where the NO and NH₃ concentrations were varied stepwise with a duration of 2 min for each step. In addition, two short transient experiments were simulated where the NO₂ and NO levels as well as NO₂-to-NO_x ratio were varied. The model could describe all validation experiments very well.     

Catalytic oxidation for carbon-black simulating diesel particulate matter over promoted Pt/Al2O3 catalysts

Catalytic oxidation activity of carbon-black (CB) simulating the soot of diesel particulate matters to CO₂ over 3Pt/Al₂O₃, 3Pt5Mn/Al₂O₃ and 3Pt/30Ba–Al₂O₃ catalysts is investigated with model gases of diesel emission. In case of the large amount of CB compared to the amount of catalyst (3/1, w/w) in the mixture sample, insufficient oxygen at the point of sudden increase in the amount of CO₂ is leaded to the partial oxidation using the lattice oxygen of the catalyst. And the peaks of CO₂ after the first peak were attributed to the regional combustion of the CB, which was not in contact with catalyst particles. The fresh 3Pt5Mn was estimated to the oxidation states on the catalyst surface by XPS. For used sample at 700 °C, the BEs of Pt 4d5 was revealed to metallic state Pt(0) (314.4 eV) in a predominant levels compared with Pt(II) (317.3 eV). While BEs of Mn 2p were similar to that obtained from the fresh 3Pt5Mn. It is suggested that Pt is in charge of the roles in CB-oxidation, using the lattice oxygen of the catalyst. Two-stage catalytic system with the strategies of promoting the soot oxidation and NO_x reduction, simultaneously, were composed of the CB oxidation catalyst and the diesel oxidation catalyst. The catalytic oxidation of CB was accelerated by activated oxidants and exothermic reaction resulted from the diesel oxidation catalyst, which lies in upstream of two-stage. But the system with the CB oxidation catalyst sited in the upstream showed the initiation of CB oxidation at a lower temperature than the other case. Two-stage catalytic system composed of 3Pt5Mn with CB in the upstream and DOC in the downstream showed high oxidation activity with 95% consumption rate of CB to the total loaded CB in the range of 100–500 °C during the TPR process.     

Perovskite-Based Lean-Burn NOx Trap Catalysts Without Using Platinum Group Metals: K/LaCoO3/Ce1?xZrxO2

Abstract  

A series of Ce_1−x Zr _x O₂ (x = 0, 0.1, 0.2, 0.3) solid solution supported lean-burn NO _x trap (LNT) catalysts K/LaCoO₃/Ce_1−x Zr _x O₂ were prepared by successive impregnation. After sulfation the supported perovsikte LaCoO₃ was well maintained; reducing treatment partly destroyed the perovsikte, but it can be well recovered by re-oxidation treatment. Based on NO _x storage and sulfur-resisting performance of the catalysts, the optimal atomic ratio of Zr in Ce_1−x Zr _x O₂ support is x = 0.2. The catalyst K/LaCoO₃/Ce_0.8Zr_0.2O₂ exhibits much better NO _x storage capacity than the Pt-based catalyst Pt/K/Ce_0.8Zr_0.2O₂, which is highly related to its stronger capability for NO to NO₂ oxidation. During NO _x storage much larger amounts of nitrate and nitrite species were identified by in situ DRIFTS over perovskite-based catalysts than over Pt-based one. The H₂-TPR results reveal that after deep sulfation little sulfur species were deposited on the catalyst K/LaCoO₃/Ce_1−x Zr _x O₂, showing strong sulfur-resisting ability. As a result, it is thought that the full replacement of Pt by perovskite LaCoO₃ in the corresponding LNT catalysts is feasible.     

Methanol Partial Oxidation on MoO3/SiO2 Catalysts: Application of Vibrational Spectroscopic Imaging Techniques in a High Throughput Operando Reactor

A home-built, high-throughput operando (HTO) reactor was applied to study methanol partial oxidation reaction over MoO₃/SiO₂ catalysts. This HTO reactor combines Fourier transform infrared (FT-IR) imaging and Raman spectroscopy for high throughput catalyst evaluation and simultaneously for catalyst characterization under operando conditions. The catalyst activity and selectivity of all parallel reaction channels were followed at a time resolution of 2–20 s by the FT-IR imaging system that offers a spatial resolution of 16,384 pixels over a 2 × 2 inches illuminated cross-section area. Six specialized Raman probes were used to simultaneously collect Raman spectra of the catalyst surfaces and reaction intermediates under operando conditions. The structural variation of the MoO₃/SiO₂ catalysts with different molybdenum loadings and their catalytic performance at various temperatures were determined. The HTO reactor with the integrated imaging techniques allowed us to track the catalytic activities and the surface morphologies for multiple samples under various operando conditions.     

Pd/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts on cellular supports for VOC vapor neutralization

The results from investigating the influence of temperature, concentration, and flow rate on the catalytic oxidation of vapors of volatile organic compounds (VOCs) in the presence of Pd/γ-Al₂O₃ catalyst on cellular supports are presented. The activity of Pd/γ-Al₂O₃ catalysts on ceramic and metal monolith supports with a cellular structure during the catalytic neutralization of VOC (ethanol, ethyl acetate) vapors under laboratory conditions was determined, and the most stable catalyst for the preliminary study of a large batch was chosen. A pilot unit was created to test a large batch of cellular monolith catalyst in neutralizing VOC vapors under conditions of flexographic production. It was established that a high rate of conversion (> 99 %) was achieved for VOC concentrations of 0.5 g/m³ at space velocities of up to ∼10⁴ h⁻¹, and for VOC concentrations of 5.0 g/m³ at space velocities of up to ∼5 × 10⁵ h⁻¹. The change in the activity of the catalysts on metal (nickel alloyed by aluminum) and ceramic cellular supports in service was investigated. After 300–500 min of operation, virtually complete deactivation of catalyst on a metal support was observed, accompanied by the formation of nickel oxide and acetate. Pilot unit tests with catalyst on cellular supports having a volume of 14.5 l in neutralizing the ventilation exhausts of flexographic production confirmed the possibility of more than 90% conversion at VOC concentrations of ∼0.1 g/m³ and more than 97% at VOC concentrations of over 1 g/m³. A consistently high conversion of VOC was observed during a 100 h test of the pilot unit. A system for recovering the heat released during VOC oxidation lowers the operating costs of the pilot unit.     

Detailed kinetic modeling of NOx adsorption and NO oxidation over Cu-ZSM-5

Detailed kinetic modeling was used in combination with flow reactor experiments to investigate the NO_x adsorption/desorption and NO oxidation over Cu-ZSM-5. NO oxidation is likely an important step for selective catalytic reduction (SCR) using urea and hydrocarbons, and thus was investigated separately. First the NO₂ adsorption on Brönstedt acid sites in H-ZSM-5 was modeled using an NO₂ temperature programmed desorption (TPD) experiment. These results, together with the results of the NO₂ TPD and NO oxidation experiments, were used in developing the model for Cu-ZSM-5. A substantial amount of NO₂ was adsorbed on the catalyst. However, the results from a corresponding NO TPD experiment showed that only very small amounts of NO were adsorbed on the catalyst and therefore this step was not included in the model. The model consists of reversible steps for NO₂ and O₂ adsorption, O₂ dissociation, NO oxidation and two steps for nitrate formation. The first nitrate formation step was disproportionation of NO₂ to form NO and nitrates. This step enabled us to describe the NO production during NO₂ adsorption. Further, in the reverse step the NO reacts with the nitrates and decreased their stability. Without this step the nitrates blocked the surface resulting in to low NO oxidation activity. However, we observe that nitrates can be decomposed also without the presence of NO and in the second reversible step were the nitrates decomposed to form NO₂ and oxygen on the copper. These steps enabled us to describe both the TPD and activity measurement results. NO oxidation was observed even at room temperature. Interestingly, the NO₂ decreased when increasing the temperature up to 100 °C and then increased as the temperature increased further. We suggest that this low-temperature NO oxidation occurs with species loosely bound on the surface and that is included in the detailed mechanism. An additional NO₂ TPD at 30 °C was also modeled to describe the loosely bound NO₂ on the surface. The detailed model correctly describes NO₂ storage, NO oxidation and low-temperature NO oxidation.     

A Thermodynamic Investigation of the Redox Properties of VPO Catalysts

Abstract  

The redox properties of a vanadium phosphorus oxide (VPO) catalyst with a V:P ratio of one were investigated using Coulometric Titration at 873 K. Equilibrium between (VO)₂P₂O₇ and VOPO₄ exists at a P(O₂) of 3 × 10⁻⁴ atm, corresponding to ΔG of −60 kJ/mol O₂. This value for VPO is significantly lower than that measured with other vanadium-containing catalysts that have been studied. Furthermore, compared to other vanadium catalysts, V⁺⁴ was stabilized against further reduction at lower P(O₂). These redox thermodynamics may help to explain the unique catalytic properties of VPO catalysts for partial oxidation of butane to maleic anhydride.     

Catalytic wet air oxidation of dye pollutants by polyoxomolybdate nanotubes under room condition

In order to develop a catalyst with high activity and stability for catalytic wet air oxidation of pollutant dyes at room condition, a new polyoxometalate Zn_1.5PMo₁₂O₄₀ with nanotube structure was prepared using biological template. The structure and morphology were characterized using infrared (IR) spectra, UV–vis diffuse reflectance spectra (DR-UV–vis), elemental analyses, X-ray powder diffraction (XRD), and transmission electron microscopy (TEM). And the degradation of Safranin-T (ST), a hazardous textile dye, under air at room temperature and atmospheric pressure was studied as a model experiment to evaluate the catalytic activity of this polyoxomolybdate catalyst. The results show that the catalyst has an excellent catalytic activity in treatment of wastewater containing 10 mg/L ST, and 98% of color and 95% of chemical oxygen demand (COD) can be removed within 40 min. And the organic pollutant of ST was totally mineralized to simple inorganic species such as HCO₃⁻, Cl⁻ and NO₃⁻ during this time (total organic carbon (TOC) decreased 92%). The structure and morphology of the catalyst under different cycling runs show that the catalyst are stable under such operating conditions and the leaching tests show negligible leaching effect owning to the lesser dissolution. So this polyoxomolybdate nanotube is proved to be a heterogeneous catalyst in catalytic wet air oxidation of organic dye.     

Catalytic effect of platinum on the kinetics of carbon oxidation by NO2 and O2

The effect of a commercial Pt/Al₂O₃ catalyst on the oxidation by NO₂ and O₂ of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O₂ and NO₂ is observed. However, in presence of NO₂–O₂ mixture, it is found that besides the well established catalytic reoxidation of NO into NO₂, Pt also exerts a catalytic effect on the cooperative carbon–NO₂–O₂ oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO₂ leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications.     

Ethanol electrooxidation on platinum particles dispersed on poly(neutral red) film

The conductive polymer poly(neutral red) polymerized on a graphite electrode (PNR/graphite) as a support material was used for catalytic oxidation of ethanol in acidic solution and investigated by electrochemical methods. Pt particles loaded on the surface of PNR/graphite electrode exhibited higher electrocatalytic activity for ethanol oxidation in comparison with Pt particles supported directly on graphite. With the equivalent loading mass of Pt catalyst, the special activity (S _A ) at peak a of the Pt/PNR/graphite electrode polymerized for 10 cycles in 5 × 10⁻⁴ M NR + 0.5 M H₂SO₄ solution is 3,478 A C⁻¹ and about 2.20 times higher than that of the Pt/graphite electrode (1,582 A C⁻¹). The results show that the electrochemical performance of Pt catalyst for ethanol oxidation is improved by the addition of PNR     

Selective Allylic oxidation of Cyclohexene by a Magnetically Recoverable Cobalt Oxide Catalyst

Abstract  

In this work, we prepared a new magnetically recoverable CoO catalyst through the deposition of the catalytic active metal nanoparticles of 2–3 nm on silica-coated magnetite nanoparticles to facilitate the solid separation from liquid media. The catalyst was fully characterized and presented interesting properties in the oxidation of cyclohexene, as for example, selectivity to the allylic oxidation product. It was also observed that CoO is the most active species when compared to Co²⁺, Co₃O₄ and Fe₃O₄ in the catalytic conditions studied.     

Efficient synthesis of glycerol carbonate from glycerol and urea with lanthanum oxide as a solid base catalyst

Lanthanum oxide catalyst prepared by precipitation method and calcined at 600 °C exhibited better catalytic activity in the catalytic synthesis of glycerol carbonate from glycerol and urea with TOF up to 1506 mmol/g·h. It was proposed that the lanthanum oxide catalyst with more strong basic sites (T_d > 400 °C) exhibited higher catalytic activity. Accordingly, the catalyst containing appropriate amount of La₂O₂CO₃ phase exhibited higher catalytic activity. Moreover, the recycling experiments demonstrated that the catalytic activity can be essentially preserved during the recycling tests investigated.