Catalysts For Low-temperature Selective Catalytic Reduction Research Articles

Comparison of titania nanotubes and titanium dioxide as supports of low-temperature selective catalytic reduction catalysts under sulfur dioxide poisoning

ABSTRACTA series of iron–manganese oxide catalysts supported on TiO2 and titanium nanotubes (TNTs) were studied for low temperature selective catalytic reduction (SCR) of NO with NH3 in the presence of SO2. The results showed that the specific surface area and the amount of Brønsted acid sites were highly correlated. The results also demonstrated that higher Mn4+/Mn3+ ratios and larger specific surface areas might be the main reasons for the excellent performance of MnFe-TNTs catalyst after SO2 poisoning. The SO2 poisoning effect could be minimized by reducing the GHSV, increasing the reaction temperature, or increasing the [NH3]/[NO] molar ratio. The results also indicated that the formation of ammonium sulfate had a stronger effect on the NO conversion efficiency as compared to the formation of metal sulfate. Thus operating the low temperature SCR at above 230 oC to avoid the formation of ammonium sulfate would be the priority choice when SO2 poisoning is a concerned issue. Implications: Low-temperature selective catalytic reduction (SCR) has attracted increasing attention due to that it can reduce the energy consumption for the SCR process employed in industries such as steel plants and glass manufacturing plants. However, it also suffers from the sulfur dioxide (SO2) poisoning problem. This study investigates the possibility of using titania nanotubes (TNTs) as the support of Mn/Fe bimetal oxide catalysts for low-temperature SCR to reduce the SO2 poisoning. The results indicated that the MnFe-TNT catalyst can tolerate SO2 for a longer time as compared with the MnFe-TiO2 catalyst.

Co-catalytic effect of Al-Cr pillared montmorillonite as a new SCR catalytic support

BACKGROUND It is of great interest to use pillared clay (PILC) as a cheap support for a selective catalytic reduction (SCR) catalyst. Al-Cr pillared clay (AlCr-PILC) has high thermal stability due to the introduction of polycations of Al. As a support, AlCr-PILC may show a co-catalytic effect in SCR processes because chromium oxide itself is an active SCR catalyst. The aim of this work is to investigate the potential of AlCr-PILC as a new low temperature SCR catalyst support. RESULTS A series of AlCr-PILC with Al:Cr molar ratio varying from 0:4 to 4:0 were successfully synthesized to support manganese and cerium active phase. MnCe/AlCr(2:2)-PILC exhibited the highest NOx conversion close to 100% at 200–300 °C, and more than 90% N2 selectively over a wide temperature window of 140–300 °C. The catalytic activity of MnCe/AlCr-PILC in terms of both NOx conversion and N2 selectivity is much higher than those of MnCe/Al-PILC, MnCe/Cr-PILC, AlCr-PILC, and the simple mixed MnCeAlCr, indicating the co-catalytic effect of AlCr-PILC support. CONCLUSION AlCr-PILC is a very promising low-temperature SCR catalyst support. The XRD, XPS, and H2-TPR measurements prove that the co-catalytic effect of AlCr-PILC occurred via the strong interaction between MnCe and the AlCr-PILC support. © 2016 Society of Chemical Industry

Low temperature selective catalytic reduction of NO<sub>x</sub> with NH<sub>3</sub> over Mn-based catalyst: A review

The removals of NOx by catalytic technology at low temperatures (100–300 °C) for industrial flue gas treatment have received increasing attention. However, the development of low temperature catalysts for selective catalytic reduction (SCR) of NOx with ammonia is still a challenge especially in the presence of SO2. The current status of using Mn-based catalysts for low temperature SCR of NOx with ammonia (NH3-SCR) is reviewed. Reaction mechanisms and effects of operating factors on low temperature NH3-SCR are addressed, and the SCR efficiencies of Mn-based metal oxides with and without SO2 poisoning have also been discussed with different supports and co-metals. The key factors for enhancing low temperature NH3-SCR efficiency and SO2 resistance with Mn-based catalysts are identified to be (1) high specific surface area; (2) high surface acidity; (3) oxidation states of manganese; (4) well dispersion of manganese oxide metals; (5) more surface adsorbed oxygen; (6) more absorbed NO3− on the catalyst surface; (7) easier decomposition of ammonium sulfates. Moreover, the regenerative methods such as water washing, acid and/or alkali washing and heat treatment to the poisoned catalysts could help to recover the low temperature SCR efficiency to its initial level.

Low Temperature de-NOx Technology-a Challenge for Vehicular Exhaust and its Remedation: An Overview

Over the past few decades, nitrogen oxides (NOx) from mobile and stationary sources have become a major environmental concern. Environmental regulations about NOx reduction are forced to become more stringent due to escalating global issues like respiratory diseases, photochemical smog, acid rain and stratospheric ozone depletion. Worldwide stringent legislations Euro 6/VI has been implemented recently in September 2014 to control the NOx emission from light and heavy duty vehicles respectively. Selective catalytic reduction (SCR) technology acquires its wide acceptance in mobile and stationary sources in catalytic NOx reduction with aqueous urea/ammonia. Most of the NOx abatement catalysts are able to reduce NOx under intermediate and high temperature ranges. At the low temperature (<523K), during the engine start-up, active NOx abatement catalysts are the challenging area in this regard due to the presence of excess H2O and SO2. A low temperature noble metal free SCR catalyst has the benefit of lesser energy expenditure and low cost advantage. The aim of the present paper is to compile the various parameters like choice of precursor, oxidation states of active components, effect of different dopants, and structural stability affects. The catalyst efficiency verified by various surface characterization studies in structure-activity relationship is also reviewed. A low temperature SCR catalyst with broad operating temperature range in engine exhausts are the viable solutions to the current situation.

MnOx supported on Fe–Ti spinel: A novel Mn based low temperature SCR catalyst with a high N2 selectivity

In this work, a novel 10% Mn/Fe–Ti spinel catalyst with an excellent N2 selectivity was developed for the selective catalytic reduction (SCR) of NO with NH3 at low temperatures. The mechanism of NO reduction and N2O formation over Fe–Ti spinel, 10% Mn/Fe–Ti spinel and 5% Mn–10% Fe/TiO2 was investigated using in situ DRIFT study and the transient reaction study. Meanwhile, the reaction kinetic constant of the SCR reaction (i.e., N2 formation) through the Eley–Rideal mechanism, the reaction kinetic constant of the SCR reaction through the Langmuir–Hinshelwood mechanism and the reaction kinetic constant of the non selective catalytic reduction (NSCR) reaction (i.e., N2O formation) were obtained according to the steady state kinetic study. They all indicate that the NSCR reaction over 10% Mn/Fe–Ti spinel through the Eley–Rideal mechanism was cut off. NH2 adsorbed on 10% Mn/Fe–Ti spinel can be hardly oxidized to NH as NH2 mainly adsorbed on the support (i.e., Fe–Ti spinel) of 10% Mn/Fe–Ti spinel, which was far away from Mn4+ cations on 10% Mn/Fe–Ti spinel. Therefore, the NSCR reaction over 10% Mn/Fe–Ti spinel through the Eley–Rideal mechanism was suppressed. However, the regeneration of Fe3+ on Fe–Ti spinel was accelerated due to the rapid electron transfer between Mn4+ and Fe2+ on 10% Mn/Fe–Ti spinel resulting in a remarkable promotion on NH3 activation although NH3 adsorbed on 10% Mn/Fe–Ti spinel cannot be directly activated by Mn4+ on 10% Mn/Fe–Ti spinel. Therefore, the SCR reaction over Fe–Ti spinel was promoted remarkably after the load of MnOx. As a result, 10% Mn/Fe–Ti spinel showed an excellent SCR performance especially N2 selectivity at low temperatures, which was much better than 5% Mn-10% Fe/TiO2 with the same chemical composition.

Identification of MnOx species and Mn valence states in MnOx/TiO2 catalysts for low temperature SCR

As the selective catalytic reduction (SCR) catalysts for NOx removal, various Mn-based catalysts have been widely investigated in the academic world. However, few studies focus on the identification of MnOx species and Mn valence states in Mn-based catalysts. In this study, Mn-based catalysts, which were prepared by using different precursors or pure MnOx species, were investigated for their activities for SCR of NO by NH3. Meanwhile, according to XRD, TG and H2-TPR measurement, a mass of crystalline and/or amorphous MnOx species were detected, while a very small amount of amorphous MnOx species could be observed with the TG–DSC-MS by detecting the oxygen coming from manganese oxides because of the decomposition. In addition, the contents and distributions of Mn2+, Mn3+ and Mn4+ as well as the average valences of manganese in catalysts were tested with the chemical titration method, while the XPS results could not identify the MnOx species, showing a semi-quantitative assessment of Mn2+, Mn3+ and Mn4+ contents. Furthermore, based on the SCR performance, Mn2O3 and Mn3O4 species were active for NOx reduction.

Hollow MnOx–CeO2 Nanospheres Prepared by a Green Route: A Novel Low-Temperature NH3-SCR Catalyst

A series of manganese–cerium oxide (MnOx–CeO2) catalysts were successfully prepared by supercritical antisolvent process and used for low temperature selective catalytic reduction (SCR) of NOx with NH3. The physicochemical properties of the catalysts were studied by using high-resolution transmission electron microscope, N2 adsorption, X-ray diffraction, X-ray photoelectron spectra, Raman spectra and temperature programmed reduction etc. Hollow spherical morphologies, nanocrystalline structures and solid solution structures were detected for these catalysts. The most active hollow nanospheres were obtained with a molar Mn/(Mn+Ce) ratio of 0.32 and a calcination temperature of 500 °C. Higher surface area, better oxygen mobility and richer surface active oxygen species are responsible for the good performance. The hollow MnOx–CeO2 nanospheres can be a new alternative for the low-temperature SCR catalysts. .

Study on Noble Metal Catalyst for Selective Catalytic Reduction of NOx at Low Temperature

The method of selective catalytic reduction (SCR) to removal NOx is very mature. However, its initial investment and operation cost are still high, which limits the development of SCR technology. Low temperature SCR catalysts can significantly reduce the cost. Catalyst active component and its carrier is the key to the efficiency of denitration. This paper summarizes the research progress of noble metal catalyst of low temperature SCR.

DRIFT Studies on the Selectivity Promotion Mechanism of Ca-Modified Ce-Mn/TiO2 Catalysts for Low-Temperature NO Reduction with NH3

Manganese-based catalysts have shown excellent low-temperature selective catalytic reduction (SCR) activity for NOx removal. However, they would always produce a high amount of more toxic byproduct, N2O. Ca modification has been reported to be able to promote N2 selectivity for SCR catalysts, but the mechanism is still not clear. In this study, a series of Ca-modified Ce0.02Mn0.4/TiO2 SCR catalysts were utilized for mechanism study. In terms of DRIFT analysis, the addition of Ca had significantly reduced the formation of NH on catalyst surface, which limited its reaction with NO to form N2O. Furthermore, Ca addition had also caused a deceased formation of NO2, reducing its reaction with NH3 to form N2O, further increasing the N2 selectivity of the catalysts. The results present herein could be beneficial to the development of efficient low-temperature SCR catalysts, particularly for Mn-based SCR catalysts.

Iron-doped Mn-Ce/TiO 2 catalyst for low temperature selective catalytic reduction of NO with NH 3

Abstract The catalysts of iron-doped Mn-Ce/TiO 2 (Fe-Mn-Ce/TiO 2) prepared by sol-gel method were investigated for low temperature selective catalytic reduction (SCR) of NO with NH 3. It was found that the NO conversion over Fe-Mn-Ce/TiO 2 was obviously improved after iron doping compared with that over Mn-Ce/TiO 2. Fe-Mn-Ce/TiO 2 with the molar ratio of Fe/Ti = 0.1 exhibited the highest activity. The results showed that 96.8% NO conversion was obtained over Fe(0.1)-Mn-Ce/TiO 2 at 180°C at a space velocity of 50,000 hr −1. Fe-Mn-Ce/TiO 2 exhibited much higher resistance to H 2O and SO 2 than that of Mn-Ce/TiO 2. The properties of the catalysts were characterized using X-ray diffraction (XRD), N 2 adsorption, temperature programmed desorption (NH 3-TPD and NO x-TPD), and X-ray photoelectron spectroscopy (XPS) techniques. BET, NH 3-TPD and NO x-TPD results showed that the specific surface area and NH 3 and NO x adsorption capacity of the catalysts increased with iron doping. It was known from XPS analysis that iron valence state on the surface of the catalysts were in Fe 3+ state. The doping of iron enhanced the dispersion and oxidation state of Mn and Ce on the surface of the catalysts. The oxygen concentrations on the surface of the catalysts were found to increase after iron doping. Fe-Mn-Ce/TiO 2 represented a promising catalyst for low temperature SCR of NO with NH 3 in the presence of H 2O and SO 2.

Characterization and Ammonia Adsorption-Desorption of Palygor-skite-Supported Manganese Oxide as a Low-Temperature Selective Catalytic Reduction Catalyst

Characterization and Ammonia Adsorption-Desorption of Palygor-skite-Supported Manganese Oxide as a Low-Temperature Selective Catalytic Reduction Catalyst

DRIFT Study of the SO2 Effect on Low-Temperature SCR Reaction over Fe−Mn/TiO2

SO2 would deactivate the low-temperature SCR (selective catalytic reduction) catalysts and reduce NO removal. In this study, Fe(0.1)−Mn(0.4)/TiO2 prepared by sol−gel method was selected to carry out the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) investigation for revealing the mechanism of the SO2 effect on the SCR reaction. The DRIFT spectroscopy showed that SO2 could be adsorbed on the surface of the catalyst as the bidentate mononuclear sulfate. This type of sulfate would retard the formation of NO complex on the surface of catalyst, resulting in the decrease of NO adsorption. For NH3 adsorption, the adsorption of SO2 had little effect on the coordinated NH3, but would increase the amount of NH4+ because of the formation of new Bronsted acid sites. Therefore, besides the deposition of ammonium sulfates, the competitive adsorption between SO2 and NO on the active sites of the catalysts also contributed to the poisoning effect of SO2 on the SCR reaction. When sulfate was ...

Sulfur poisoning resistant mesoporous Mn-base catalyst for low-temperature SCR of NO with NH3

Mesoporous MnO2-Fe2O3-CeO2-TiO2 was prepared with sol–gel method and demonstrated to have good low-temperature activity and sulfur-poisoning resistance for selective catalytic reduction (SCR) of NO with NH3 in SO2-containing gases. In comparison with this, the catalyst with the same composition but made according to the conventional impregnation method exhibited obviously lower SO2-poisoning resistance and selectivity to the formation of N2 in the SCR reactions. FTIR analysis of the spent catalysts after SCR reactions for 16h and 60h in a SO2-contaning gas demonstrated that there was little difference in the amount of deposited ammonium sulfate over the mesoporous catalyst between the two cases. The mesopore channels existing in the mesoporous catalyst enabled probably a dynamic balance between the formation and decomposition of ammonium sulfate in SCR reactions. This concern was justified through comparing the N2 adsorptions and XPS spectra for the catalysts made with the impregnation and sol–gel methods. The article clarified as well the facilitation effects of introducing Ce and Fe into the mesoporous catalyst on activity, selectivity and SO2-poisoning resistance.

Study of low-temperature selective catalytic reduction of NO by ammonia over carbon-nanotube-supported vanadium

In this article, low-temperature selective catalytic reduction (SCR) reaction of NO on carbon nanotube-supported vanadium (V 2O 5/CNT) catalysts was carried out. When compared with activated carbon as a support catalyst, carbon nanotubes with low load of V 2O 5 show excellent catalytic activity than that of activated carbon, and the catalytic activity of V 2O 5/CNT catalysts increases markedly in the presence of SO 2. The transient reaction experiment shows that the reaction of V 2O 5/CNT catalyst for low-temperature SCR of NO follows the mechanism of Eley–Rideal, namely by the reaction between the adsorbed NH 3 and NO in the gaseous or a weakly adsorbed state.

Surface characterization studies of TiO2 supported manganese oxide catalysts for low temperature SCR of NO with NH3

A series of TiO2 supported manganese oxide catalysts were prepared by wet-impregnation method for the low temperature selective catalytic reduction (SCR) of NO with ammonia as a reductant. A combination of various physico-chemical techniques such as N2 physisorption, O2 chemisorption, TPR, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman were used to characterize the chemical environment of these catalysts. O2 chemisorption and XRD results suggest that Mn exist in a well-dispersed state at below 16.7wt.% of Mn on TiO2 anatase (Hombikat), 7.5wt.% on TiO2 rutile (Kemira) and P-25 (80% anatase+20% rutile), and in microcrystalline phase above these loading levels on respective support materials. These results also reveal that Mn interacts very well with pure anatase phase compared to rutile. XPS results of Mn/TiO2 anatase (Hombikat) catalysts illustrated the presence of MnO2 as a major phase (peak at 642.0eV) along with Mn2O3 as the minor phase at lower loadings. The presence of Mn2O3 disappears at higher loadings. The characterization results indicated that the manganese oxide exists as an isolated species at very low loadings, highly dispersed state probably as two dimensional monolayer species at intermediate loadings, polymeric or microcrystalline form of manganese oxide at higher (above monolayer capacity) loadings was envisaged. The catalytic performance of various amounts of Mn loaded on different TiO2 supported catalysts for low temperature SCR reaction at catalyst bed temperature 175°C under power plant conditions using GHSV=50,000h−1 was studied. The catalyst with 16.7wt.% Mn/TiO2 anatase (Hombikat) was found to be highly active and selective catalyst for this reaction. The Raman studies acted as complimentary tool to XPS in order to characterize the manganese oxides (MnO, Mn2O3, Mn3O4, MnO2). Raman data show that there is a strong interaction between the Mn oxides and the support, which is responsible for the impressive catalytic performance in comparison with other systems we investigated.

Experimental study on a low-temperature SCR catalyst based on MnO x/TiO 2 prepared by sol–gel method

A catalyst based on MnO x /TiO 2 was prepared by sol–gel method for low-temperature selective catalytic reduction (SCR) of NO with NH 3. Focusing on the effects of the operating parameters, the SCR reaction was investigated at temperatures from 353 to 523 K under steady and transient states. Under the optimal conditions, the efficiency of NO removal could exceed 90% at temperature of 423 K. Furthermore, within the range investigated, the reaction order of NO, NH 3, O 2 was determined to be 1, 0, and 0.5, respectively. Apparent activation energy was also calculated to be 38 kJ/mol, lower than that for most of the catalysts reported by previous investigations.