Showing papers on "Selective catalytic reduction published in 2013"

Synthesis of ammonia directly from air and water at ambient temperature and pressure

Abstract: The N≡N bond (225 kcal mol−1) in dinitrogen is one of the strongest bonds in chemistry therefore artificial synthesis of ammonia under mild conditions is a significant challenge Based on current knowledge, only bacteria and some plants can synthesise ammonia from air and water at ambient temperature and pressure Here, for the first time, we report artificial ammonia synthesis bypassing N2 separation and H2 production stages A maximum ammonia production rate of 114 × 10−5 mol m−2 s−1 has been achieved when a voltage of 16 V was applied Potentially this can provide an alternative route for the mass production of the basic chemical ammonia under mild conditions Considering climate change and the depletion of fossil fuels used for synthesis of ammonia by conventional methods, this is a renewable and sustainable chemical synthesis process for future

In Situ-DRIFTS Study of Selective Catalytic Reduction of NOx by NH3 over Cu-Exchanged SAPO-34

Abstract: The intrinsic mechanism of the selective catalytic reduction (SCR) reaction over a Cu-exchanged SAPO-34 catalyst at low temperature was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), coupled with mass spectrometry to measure inlet and outlet gas concentrations. The evolution of the surface intermediates, as well as the reactivity of NH3 with surface NOx species and NOx with surface NH3 species, was evaluated. In terms of NOx adsorption, surface nitrates and nitrites are the main NOx adsorption species at low temperature. When NO was exposed to the sample with NH3 preadsorbed, surface NH3 was not reactive because of the low surface coverage of nitrates and nitrites. However, the reactivity is significantly enhanced by the inclusion of O2 in the feed, which promotes an increase in the concentration of surface nitrates and nitrites. DRIFTS results also reveal that the low temperature SCR reaction involves the formation of an NH4NO3 intermediate and its subsequent red...

Current Understanding of Cu-Exchanged Chabazite Molecular Sieves for Use as Commercial Diesel Engine DeNOx Catalysts

Abstract: Selective catalytic reduction (SCR) of NOx with ammonia using metal-exchanged molecular sieves with a chabazite structure has recently been commercialized on diesel vehicles. One of the commercialized catalysts, i.e., Cu-SSZ-13, has received much attention for both practical and fundamental studies. For the latter, the particularly well-defined structure of this zeolite is allowing long-standing issues of the catalytically active site for SCR in metal-exchanged zeolites to be addressed. In this review, recent progress is summarized with a focus on two areas. First, the technical significance of Cu-SSZ-13 as compared to other Cu ion-exchanged zeolites (e.g., Cu-ZSM-5 and Cu-beta) is highlighted. Specifically, the much enhanced hydrothermal stability for Cu-SSZ-13 compared to other zeolite catalysts is addressed via performance measurements and catalyst characterization using several techniques. The enhanced stability of Cu-SSZ-13 is rationalized in terms of the unique small pore structure of this zeolite catalyst. Second, the fundamentals of the catalytically active center; i.e., the chemical nature and locations within the SSZ-13 framework are presented with an emphasis on understanding structure–function relationships. For the SCR reaction, traditional kinetic studies are complicated by intra-crystalline diffusion limitations. However, a major side reaction, nonselective ammonia oxidation by oxygen, does not suffer from mass-transfer limitations at relatively low temperatures due to significantly lower reaction rates. This allows structure–function relationships that are rather well understood in terms of Cu ion locations and redox properties. Finally, some aspects of the SCR reaction mechanism are addressed on the basis of in situ spectroscopic studies.

In situ supported MnO(x)-CeO(x) on carbon nanotubes for the low-temperature selective catalytic reduction of NO with NH3.

Abstract: The MnOx and CeOx were in situ supported on carbon nanotubes (CNTs) by a poly(sodium 4-styrenesulfonate) assisted reflux route for the low-temperature selective catalytic reduction (SCR) of NO with NH3. X-Ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR) and NH3 temperature-programmed desorption (NH3-TPD) have been used to elucidate the structure and surface properties of the obtained catalysts. It was found that the in situ prepared catalyst exhibited the highest activity and the most extensive operating-temperature window, compared to the catalysts prepared by impregnation or mechanically mixed methods. The XRD and TEM results indicated that the manganese oxide and cerium oxide species had a good dispersion on the CNT surface. The XPS results demonstrated that the higher atomic concentration of Mn existed on the surface of CNTs and the more chemisorbed oxygen species exist. The H2-TPR results suggested that there was a strong interaction between the manganese oxide and cerium oxide on the surface of CNTs. The NH3-TPD results demonstrated that the catalysts presented a larger acid amount and stronger acid strength. In addition, the obtained catalysts exhibited much higher SO2-tolerance and improved the water-resistance as compared to that prepared by impregnation or mechanically mixed methods.

Relationship between structure and performance of a novel cerium-niobium binary oxide catalyst for selective catalytic reduction of NO with NH3

Abstract: A series of Ce-Nb binary oxide catalysts for the selective catalytic reduction (SCR) of NO with ammonia were synthesized using co-precipitation method and showed good catalytic activity and selectivity. The best catalyst Ce1Nb1 presented over 80% NO conversion in a wide temperature range of 200–450 °C at a gas hourly space velocity (GHSV) of 120,000 mL g−1 h−1. The correlations among the structures, acid properties and redox behaviors of the catalysts were tried to establish. The BET specific surface areas of the binary oxides were larger than those of the monadic ones. The sample with Ce/Nb (1:1) possessed special NbO4 tetrahedral coordination. The surface acid sites were mainly provided by niobium oxide species. The Nb OH bond was related to the Bronsted acid site and the Nb O bond to the Lewis acid site. The abundance of surface adsorbed oxygen, the key factors for the catalytic behaviors, might arise from the short-range activation effect of niobium oxide species to cerium oxide species. In addition, water vapor only had a slight and reversible inhibition effect on the catalytic performance of the catalyst, and it might be a promising SCR catalyst for the practical application.

Low-temperature selective catalytic reduction of NO with NH₃ over nanoflaky MnOx on carbon nanotubes in situ prepared via a chemical bath deposition route.

Abstract: Nanoflaky MnOx on carbon nanotubes (nf-MnOx@CNTs) was in situ synthesized by a facile chemical bath deposition route for low-temperature selective catalytic reduction (SCR) of NO with NH3. This catalyst was mainly characterized by the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2 adsorption–desorption analysis, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR) and NH3 temperature-programmed desorption (NH3-TPD). The SEM, TEM, XRD results and N2 adsorption–desorption analysis indicated that the CNTs were surrounded by nanoflaky MnOx and the obtained catalyst exhibited a large surface area as well. Compared with the MnOx/CNT and MnOx/TiO2 catalysts prepared by an impregnation method, the nf-MnOx@CNTs presented better NH3-SCR activity at low temperature and a more extensive operating temperature window. The XPS results showed that a higher atomic concentration of Mn4+ and more chemisorbed oxygen species existed on the surface of CNTs for nf-MnOx@CNTs. The H2-TPR and NH3-TPD results demonstrated that the nf-MnOx@CNTs possessed stronger reducing ability, more acid sites and stronger acid strength than the other two catalysts. Based on the above mentioned favourable properties, the nf-MnOx@CNT catalyst has an excellent performance in the low-temperature SCR of NO to N2 with NH3. In addition, the nf-MnOx@CNT catalyst also presented favourable stability and H2O resistance.

Ceria added Sb-V2O5/TiO2 catalysts for low temperature NH3 SCR: Physico-chemical properties and catalytic activity

Abstract: A systematic investigation of the effect of ceria loading over Sb-V2O5/TiO2 catalysts was carried out for the selective catalytic reduction (SCR) of NOx by NH3. The various ceria loaded Sb-V2O5/TiO2 catalysts were prepared by deposition precipitation and impregnation methods. Addition of 10% ceria to Sb-V2O5/TiO2 catalyst significantly enhanced the NOx conversion at wide temperature range of 220–500 °C. The 10% ceria loaded Sb-V2O5/TiO2 catalyst showed superior N2 selectivity (>95%) throughout the reaction temperatures. The physicochemical characteristics of the obtained catalysts were thoroughly characterized by BET surface area, X-ray diffractometry (XRD), temperature programmed desorption (TPD) of NO, SO2 and NH3, H2- temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The XRD results indicated the active components of antimony and vanadia were homogeneously dispersed over CeO2/TiO2. It was found that the addition of 10% ceria to Sb-V2O5/TiO2 could enhance the total acidity and redox properties of the catalyst, which lead to show higher NOx conversions at wide temperature window. In XPS studies, increase in intensity of the chemisorbed mobile oxygen peak was observed for ceria loaded catalysts. In particular, the DRIFT spectra of ceria loaded Sb-V2O5/TiO2 catalysts showed abundant Bronsted acid sites at 1436 and 1673 cm−1 band, which are responsible for high NOx conversion. Furthermore, the results of NO and SO2 TPD of 10% ceria loaded Sb-V2O5/TiO2 catalyst showed enhancement of NO adsorption and SO2 inhibition properties, which is thought to play a significant role in long term stability of the catalyst during SO2 on–off study for 38 h at 240 °C.

Synthesis and Evaluation of Cu-SAPO-34 Catalysts for Ammonia Selective Catalytic Reduction. 1. Aqueous Solution Ion Exchange

Abstract: SAPO-34 molecular sieves are synthesized using various structure directing agents (SDAs). Cu-SAPO-34 catalysts are prepared via aqueous solution ion exchange (IE). Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies. Catalytic properties are examined using standard ammonia selective catalytic reduction (NH3–SCR) and ammonia oxidation reactions. During solution IE, different SAPO-34 samples undergo different extent of structural damage via irreversible hydrolysis. Si content within the samples (i.e., Al–O–Si bond density) and framework stress are key factors that affect irreversible hydrolysis. Even using very dilute Cu acetate solutions, it is not possible to generate Cu-SAPO-34 samples with only isolated Cu2+ ions. Small amounts of CuOx species always coexist with isolated Cu2+ ions. Highly active and selective Cu-SAPO-34 catalysts for NH3-SCR are readily ...

Efficient One‐Pot Preparation of Cu‐SSZ‐13 Materials using Cooperative OSDAs for their Catalytic Application in the SCR of NOx

Abstract: The cooperative use of the Cu-tetraethylenepentamine complex and N,N,N-trimethyl-1-adamantammonium as organic structure-directing agents (OSDAs) enabled the rationalized “one-pot” preparation of Cu-containing SSZ-13 zeolites. A detailed study of different synthetic variables permitted the control of the Si/Al and Cu/(Si+Al) ratios in the final solids. Cu-SSZ-13 molecular sieves synthesized in alkaline media demonstrate excellent catalytic activities and good hydrothermal stabilities for the selective catalytic reduction of NOx. Finally, the effect of synthesis media on the catalytic active sites is also demonstrated and a remarkable activity loss for samples synthesized in fluoride media is observed.

Characterization of Ceria’s Interaction with NOx and NH3

Abstract: Ceria is a common component of engine aftertreatment catalysts due to its oxygen storage ability, its redox properties, and its role in stabilizing Pt against sintering. The interactions between ceria and NH3 or NOx were investigated to better understand the role of ceria in oxidation reactions occurring over a diesel oxidation catalyst, in the reduction of NOx on lean NOx traps, and in the selective catalytic oxidation (SCO) of NH3. Ceria proved active in NO oxidation, selective catalytic reduction of NO by NH3 (NH3–SCR), and NH3–SCO reactions. Between 100 and 450 °C, both NH3 and NOx adsorbed on ceria simultaneously. In the absence of NOx, NH3 was oxidized over CeO2 forming N2 via a two-step selective catalytic reduction mechanism at low temperature and NOx at high temperature. In the presence of NOx, NH3 reacted with adsorbed NOx species, again forming N2 at lower temperatures (250–450 °C), while at higher temperature, a significant portion of the NH3 was oxidized, with product NO formed.

Catalytic reduction of nitrate in water over Pd–Cu/TiO2 catalyst: Effect of the strong metal-support interaction (SMSI) on the catalytic activity

Abstract: In this study, we prepared TiO2-supported Pd–Cu catalysts of different anatase/rutile phase compositions, which were used in a nitrate reduction in water. It was shown that the catalysts containing a greater anatase phase composition had higher catalytic performance. Through characterization studies using H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS), the observed trend of the catalytic activity was correlated to the degree of the strong metal-support interaction (SMSI) over the catalysts. The SMSI occurred through pre-treatment of the catalysts by H2 reduction (at 200 °C), which resulted in increased partially reduced TiO2−x and electron-rich active metal (Pd and Cu) states at the catalyst of higher composition of anatase phase. The relationships between the changes in the properties and activities of the catalysts as a result of the anatase phase composition are discussed.

Improvement of the Activity of γ-Fe2O3 for the Selective Catalytic Reduction of NO with NH3 at High Temperatures: NO Reduction versus NH3 Oxidization

Abstract: Lignite is widely used as the fuel for coal-fired power plants, and its flue gas temperature is about 50–100 °C higher than others. V2O5–WO3/TiO2 is extremely restricted in the selective catalytic reduction (SCR) of NO from the coal-fired power plants burning lignite due to the drop of NOx conversion, low N2 selectivity, and volatility of vanadium pentoxide at high temperatures. Therefore, a more environmental-friendly SCR catalyst with excellent SCR activity and better N2 selectivity at 350–450 °C should be developed for this application. In this work, sulfated Fe–Ti spinel catalyst was developed for the SCR of NO from the coal-fired power plants burning lignite. The drop of NOx conversion at high temperatures was mainly related to the simultaneous occurrence of the catalytic oxidization of NH3 to NO during the SCR reaction. Ti was incorporated into γ-Fe2O3 to decease the oxidization ability of Fe3+ on the surface, and the sites for −NH2 adsorption and the active components for −NH2 oxidization were sepa...

Catalytic reduction of p-nitrophenol over precious metals/highly ordered mesoporous silica

Abstract: Precious metals, Au, Pt, and Pd, were successfully deposited on highly ordered mesoporous SBA-15. Two different reduction routes were employed to deposit precious metals: (1) under H2 and (2) using sodium citrate. Samples prepared using sodium citrate exhibit a uniform particle size of 10 nm while samples synthesized under H2 show high dispersion with a particle size of 8 and 20 nm for Pt and Pd, respectively. The surface area and pore volume of the mesoporous SBA-15 were significantly reduced due to the impregnation of the precious metals. Detailed TEM and XPS analyses reveal a uniform distribution of particles with a metallic valence state and no evidence of metallic oxides. The prepared catalysts were used to reduce p-nitrophenol (PNP) into p-aminophenol (PAP) where two trends were observed. The catalytic reduction efficiency for PNP reduction using the catalyst prepared with sodium citrate as a reducing agent is in the order of Au > Pt > Pd with a rate constant of 3.24 × 10−1 s−1 for Au/SBA-15. On the other hand, the catalyst prepared with H2 as a reducing agent showed a reverse trend Pd > Pt > Au with a rate constant of 7.15 × 10−1 s−1 for Pd/SBA-15. The highest catalyst efficiency was observed for the case of Pd/SBA-15 synthesized via the H2 route with a rate constant of 7.15 × 10−1 s−1. Also the reaction rate of Pd/SBA-15 synthesized via the H2 route was 2.2 times higher than that of Au/SBA-15 prepared using the sodium citrate route.

What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning Car Exhaust Gases?

Abstract: Recently, the outstanding properties of Cu-SSZ-13 (a zeolite in the chabazite structure) for the selective catalytic reduction of nitrous oxides were discovered. However, the true nature of the active site is still not answered satisfactorily. In this work, we identify the active site for the given reaction from first-principles simulations of the total energy of Cu(II) ions in various positions in combination with previously published catalytic activity as a function of the copper exchange level. This attribution is confirmed by the simulation of vibrational properties of CO adsorbed to the reduced Cu(I) species. The relation between energetic considerations, vibrational calculations, and experiment allows a clear statement about the distribution of active sites in the catalyst. We furthermore discuss the structural properties of the active site leading to the high stability under reaction conditions over a large temperature range. The insights from this work allow a more targeted catalyst design and rep...

The effect of SiO2 on a novel CeO2–WO3/TiO2 catalyst for the selective catalytic reduction of NO with NH3

Abstract: A series of novel catalyst complexes for the selective catalytic reduction of NOx to NH3 were prepared by doping CeO2–WO3/TiO2 with different loadings of SiO2. The complexes were synthesized by impregnating P25 with colloidal silica to form a complex support. The NOx conversion values and the calculated reactive rate constants confirm that the presence of SiO2 increased the reaction activity at low temperatures. This increase in activity may be directly correlated to the increase in the presence of unstable Bronsted acid sites as well as active nitrite, monodentate nitrates and adsorbed NO2, as opposed to an increase in the BET surface area and a change in the redox properties. Furthermore, the surface bridging and bidentate nitrate species that originated from the adsorption of NOx were quite stable and inactive below 300 °C. Finally, both Ti(1)Si(0) and Ti(3)Si(1) catalysts were employed to study the reaction mechanism by in situ IR spectroscopy at 200 °C. The two catalysts exhibited similar reaction mechanisms, wherein the Lewis and Bronsted acid sites reacted with active nitrite, monodentate nitrates and adsorbed NO2 species.

Selective catalytic reduction of nitrogen oxides by ammonia over Co3O4 nanocrystals with different shapes

Abstract: In this work, single-crystalline one-dimensional Co 3 O 4 nanorods with well-defined crystal planes as catalysts in the selective catalytic reduction of NO by ammonia are investigated. The Co 3 O 4 nanorods synthesized by ethylene glycol-mediated precipitation at 160 °C predominantly expose {1 1 0} planes which are rich in Co 3+ species, while the traditional Co 3 O 4 nanoparticles expose the {0 0 1} and {1 1 1} planes which contain mainly Co 2+ species. Compared with Co 3 O 4 nanoparticles, Co 3 O 4 nanorods show much higher NO x conversion in NH 3 -SCR reaction. TPD of various reactant gases, including NH 3 , NO, NO/NH 3 , NO/O 2 , NO/NH 3 /O 2 , and transient experiment reveals Co 3 O 4 nanorods adsorb a large amount of ammonia on their surface. Slight surface H 2 reduction significantly decreases the activity of Co 3 O 4 nanocrystals, while O 2 re-oxidation partially recovers their catalytic activity, demonstrating the presence of Co 3+ cations on the surface of Co 3 O 4 nanocatalysts actually acts as the active sites in NH 3 -SCR reaction. The present results indicate that novel catalyst with high activity can be designed by morphology control at nanoscale.

NH3-SCR Performance of Fresh and Hydrothermally Aged Fe-ZSM-5 in Standard and Fast Selective Catalytic Reduction Reactions

Abstract: Hydrothermal stability is one of the challenges for the practical application of Fe-ZSM-5 catalysts in the selective catalytic reduction (SCR) of NO with NH3 (NH3-SCR) for diesel engines. The presence of NO2 in the exhaust gases can enhance the deNOx activity because of the fast SCR reaction. In this work, a Fe- ZSM-5 catalyst was prepared by a solid-state ion-exchange method and was hydrothermally deactivated at 800 °C in the presence of 10% H2O. The activity of fresh and hydrothermal aged Fe-ZSM-5 catalysts was investigated in standard SCR (NO2/NOx = 0) and in fast SCR with NO2/NOx = 0.3 and 0.5. In standard SCR, hydrothermal aging of Fe-ZSM-5 resulted in a significant decrease of low-temperature activity and a slight increase in high- temperature activity. In fast SCR, NOx conversion over aged Fe- ZSM-5 was significantly increased but was still lower than that over fresh catalyst. Additionally, production of N2O in fast SCR was much more apparent over aged Fe-ZSM-5 than over fresh catalyst. We propose that, in fast SCR, the rate of key reactions related to NO is slower over aged Fe-ZSM-5 than over fresh catalyst, thus increasing the probabilities of side reactions involving the formation of N2O.

H2O and SO2 deactivation mechanism of MnOx/MWCNTs for low-temperature SCR of NOx with NH3

Abstract: The effect of H 2 O and SO 2 on the catalytic activity of manganese oxides supported on multi-walled carbon nanotubes (MnO x /MWCNTs) for low-temperature selective catalytic reduction (SCR) of NO x with NH 3 was studied. Also, N 2 adsorption, transient response experiments, X-ray powder diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and in situ FT-IR spectroscopy were performed to investigate the deactivation mechanism of MnO x /MWCNTs catalyst in the presence of H 2 O or SO 2 . Experimental results showed that H 2 O had a reversible negative effect on the catalytic activity of the catalyst. When the temperature was higher than 270 °C, the effect of H 2 O could be negligible. The competitive adsorption of H 2 O and NH 3 on the Lewis acid sites contributed to the deactivation of the catalyst. The integrity increase of MWCNTs in the presence of H 2 O might be another reason for the deactivation of the catalyst. However, SO 2 led to the irreversible deactivation of the catalyst. The higher the reaction temperature, the more dramatically the catalystic activity decreased. The sulfation of the active center atoms was the main poisoning route. Also, formation of ammonium sulfates on the catalyst surface and the competitive adsorption between SO 2 and NO were responsible for the partial deactivation of the catalyst to some extent.

A study of the mechanism of low-temperature SCR of NO with NH3 on MnOx/CeO2

Abstract: A MnOx/CeO2 supported catalyst was prepared using the deposition–precipitation (DP) method for low-temperature selective catalytic reduction (SCR) of NO with NH3. At short reaction time, nearly 100% NO conversion was obtained across the whole evaluated temperature range of 80–150 °C at a space velocity of 50,000 h−1. The catalyst rapidly lost its activity at 80 °C, while stable activity was achieved at 120 °C at a space velocity of 120,000 h−1 in a 36 h test. A possible reaction pathway is proposed based on the results of an in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study coupled with mass spectrometric (MS) data. It is suggested that the NH3-SCR reaction on the MnOx/CeO2 catalyst involves a [NH3⋯NO−] complex as an intermediate, the decomposition of which into N2 and H2O is the rate-limiting step. The role of oxygen in the SCR reaction is to reoxidize the reduced catalyst surface, thereby completing the catalytic cycle.