NH3 Selective Catalytic Reduction Research Articles

Influence of Cu-speciation in mordenite on direct methane to methanol conversion: Multi-Technique characterization and comparison with NH3 selective catalytic reduction of NOx

The direct conversion of methane to methanol has the potential of substantially reducing methane emissions and flaring, as such a process might provide an alternative for remote natural gas locations. In this report, we investigate the performance of a range of Cu-exchanged mordenite zeolites as active materials for such a reaction, employing a stepwise protocol comprising activation in oxygen, methane loading, and methanol extraction with steam. We employ in situ HERFD XANES, FT-IR spectroscopy with CO as probe molecule, and XPS to investigate the Cu species in the zeolites during the process. The activity of the materials is investigated both for methane to methanol conversion and NH3 Selective Catalytic Reduction of NOx. It is demonstrated that, despite the fact that the same zeolite materials are active both for NH3-SCR and direct methane to methanol conversion, the active site requirements for these two reactions are different.

Tailored Alkali Resistance of DeNOx Catalysts by Improving Redox Properties and Activating Adsorbed Reactive Species

SummaryIt is still challenging to develop strongly alkali-resistant catalysts for selective catalytic reduction of NOx with NH3. It is generally believed that the maintenance of acidity is the most important factor because of neutral effects of alkali. This work discovers that the redox properties rather than acidity play decisive roles in improving alkali resistance of some specific catalyst systems. K-poisoned Fe-decorated SO42−-modified CeZr oxide (Fe/SO42−/CeZr) catalysts show decreased acidity but reserve the high redox properties. The higher reactivity of NHx species induced by K poisoning compensates for the decreased amount of adsorbed NHx, leading to a desired reaction efficiency between adsorbed NHx and nitrate species. This study provides a unique perspective in designing an alkali-resistant deNOx catalyst via improving redox properties and activating the reactivities of NHx species rather than routinely increasing acidic sites for NHx adsorption, which is of significance for academic interests and practical applications.

Mechanistic Aspects of Highly Efficient FeaSbTiOx Catalysts for the NH3-SCR Reaction: Insight into the Synergistic Effect of Fe and S Species

A series of highly efficient, environmentally friendly, and low-cost FeaSbTiOx catalysts were prepared by a simple thermal hydrolysis method for the NH3-selective catalytic reduction (SCR) reaction...

An Aging Model of NH3 Storage Sites for Predicting Kinetics of NH3 Adsorption, Desorption and Oxidation over Hydrothermally Aged Cu-Chabazite

A unified transient kinetic model which can predict the adsorption, desorption and oxidation kinetics of NH3 over hydrothermally aged Cu-chabazite was developed. The model takes into account the variation of fractional coverages of NH3 storage sites due to hydrothermal aging. In order to determine the fractional coverage of these sites, the catalyst was aged for various times at a certain temperature followed by NH3 adsorption, desorption and temperature-programmed desorption (TPD) experiments. TPD profiles were deconvoluted mainly into three peaks with centres at 317, 456 and 526 °C, respectively. Hydrothermal aging resulted in the progressive increase in the intensity of the peak at 317 °C and decrease in the intensity of the peaks at 456 and 526 °C, along with decreased NH3 oxidation at high temperatures. A model for hydrothermal aging kinetics of the fractional coverage of storage sites was developed using three reactions with appropriate rate expressions with parameters regressed from experimental data. The model was then incorporated into a multi-site kinetic model for the degreened Cu-Chabazite by the addition of aging reactions on each storage site. The effects of both aging time and temperature on the kinetics NH3 adsorption, desorption and oxidation were successfully predicted in the 155-540 °C range. This study is the first step towards the development of a hydrothermal aging-unified kinetic model of NH3-Selective Catalytic Reduction over Cu-chabazite.

Two-dimensional MnFeCo layered double oxide as catalyst for enhanced selective catalytic reduction of NOx with NH3 at low temperature (25–150 °C)

Low temperature catalysts for selective catalytic reduction of NOx with NH3 plays an important role in flue gas treatments. Here, a two-dimensional MnFeCo layered double oxide were synthetized, characterized and employed as NH3 selective catalytic reduction catalyst, showing excellent low temperature de-NOx performance. MnFeCo layered double oxide exhibited a specific surface area of 92 m2/g with average pore size and volume of 2 nm and 1.5 cm3/g. At 100 °C, the MnFeCo layered double oxide performed an efficient NO conversion of 100 %. Even at 25 and 50 °C, NO conversions reached values of 49 and 86 %, respectively. Furthermore, the NO conversion reached an efficiency of 89 % in the presence of H2O vapor (5 %) and SO2 (100 ppm). Based on that, it can be said that this NH3 selective catalytic reduction catalyst shows great potential in NOx denitration of stationary industrial installations at a low temperature range.

Preparation of Mesoporous Mn-Ce-Ti-O Aerogels by a One-Pot Sol-Gel Method for Selective Catalytic Reduction of NO with NH3.

Novel Mn–Ce–Ti–O composite aerogels with large mesopore size were prepared via a one-pot sol–gel method by using propylene oxide as a network gel inducer and ethyl acetoacetate as a complexing agent. The effect of calcination temperature (400, 500, 600, and 700 °C) on the NH3–selective catalytic reduction (SCR) performance of the obtained Mn–Ce–Ti–O composite aerogels was investigated. The results show that the Mn–Ce–Ti–O catalyst calcined at 600 °C exhibits the highest NH3–SCR activity and lowest apparent activation energy due to its most abundant Lewis acid sites and best reducibility. The NO conversion of the MCTO-600 catalyst maintains 100% at 200 °C in the presence of 100 ppm SO2, showing the superior resistance to SO2 poisoning as compared with the MnOx–CeO2–TiO2 catalysts reported the literature. This should be mainly attributed to its large mesopore sizes with an average pore size of 32 nm and abundant Lewis acid sites. The former fact facilitates the decomposition of NH4HSO4, and the latter fact reduces vapor pressure of NH3. The NH3–SCR process on the MCTO-600 catalyst follows both the Eley–Rideal (E–R) mechanism and the Langmuir–Hinshelwood (L–H) mechanism.

Insights on the mechanism of enhanced selective catalytic reduction of NO with NH3 over Zr-doped MnCr2O4: A combination of in situ DRIFTS and DFT

The NH3 selective catalytic reduction (SCR) performance of MnCr2O4 was shown to be greatly enhanced by doping zirconium to form Zr0.05Mn0.95Cr2O4 in our previous study. In this work, the active sites and reaction mechanisms of the catalysts were further investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations in this work. The total quantities of Lewis acid and Brønsted acid sites of Zr0.05Mn0.95Cr2O4 became 3.5 times higher than that of MnCr2O4, and various nitrate species were detected on the surface, which are beneficial for the promotion of SCR activity below 250 °C. The catalytic reactions over MnCr2O4 mainly occur between NH3 adsorbed on Cr cations and monodentate nitrates/adsorbed NO2 on Mn cations below 250 °C, and NH3 on Cr cations and gaseous NO above 250 °C, while the nitrites and N2O22− species are not reactive. The reactions between NH3 adsorbed on Mn and Zr cations and nitrates/adsorbed NO2 are the dominant pathways over Zr0.05Mn0.95Cr2O4 at all temperatures, and NH4+ species enrichment (4.3 times higher) contributed to the enhanced catalytic activity below 250 °C. DFT calculation show that when species including NH3, NH2, NO, and NO2 adsorb on Zr0.05Mn0.95Cr2O4, they yield lower adsorption energies than on MnCr2O4, and the energy barrier for NH2 and NO2 formation is also remarkably decreased. These two energy advantages facilitate conducive intermediate formation and the catalytic reactions.

In situ DRIFT study on NH3 selective catalytic reduction of NOx over HBEA zeolite doped with CeO2

The reaction mechanism of selective catalytic reduction of NO with NH3 on Ce-doped HBEA catalysts was investigated using in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results illustrated that the amide species adsorbed on Lewis acid sites (AlO)+ and NH4+ species adsorbed on Bro¨nsted acid sites (Al-OH-Si) were main intermediates on HBEA. Only Eley-Rideal (E-R) mechanism was presented over HBEA catalyst. By contrast, both E-R and Langmuir-Hinshelwood (L-H) mechanism were presented over Ce7.0HBEA and the E-R mechanism was the dominated reaction mechanism. NH4+ adspecies and nitrate adspecies were primary intermediates on Ce7.0HBEA. Meanwhile, the presence of Ce3+ could lead to the increase of surface oxygen concentration, which could actively take part in the activation process of ammonia adspecies and NOx adspecies and then facilitated the NH3-SCR reaction. Hence, the NOx conversion of Ce7.0HBEA was much higher than that of HBEA. In addition, the surface oxygen vacancy derived from ceria on Ce7.0HBEA could inhibit the formation of N2O. As a result, the N2 selectivity of Ce7.0HBEA is higher than that of HBEA. The above results are useful for guiding the way to develop Ce-containing SCR catalysts.

Effect of manganese and/or ceria loading on V2O5–MoO3/TiO2 NH3 selective catalytic reduction catalyst

The effect of manganese and/or ceria loading of V2O5–MoO3/TiO2 catalysts was investigated for selective catalytic reduction (SCR) of NOx by NH3. The manganese and/or ceria loaded V2O5–MoO3/TiO2 catalysts were prepared by the wetness impregnation method. The physicochemical characteristics of the catalysts were thoroughly characterized. The catalytic performance of 1.5 wt% V2O5–3 wt% MoO3/TiO2 (V1.5Mo3/Ti) is greatly enhanced by addition of 2.5 wt% MnOx and 3.0 wt% CeO2 (V1.5Mo3Mn2.5Ce3/Ti) below 450 °C. Compared with the V1.5Mo3/Ti catalyst with NOx conversion of 75% at 275 °C, V1.5Mo3Mn2.5Ce3/Ti exhibits higher NOx conversion of 84% with good resistance to SO2 and H2O at a gas hourly space velocity value of 150000 h−1. The active manganese, cerium, molybdenum, and vanadium oxide species are highly dispersed on the catalyst surface and some synergistic effects exist among these species. Addition of MnOx significantly enhances the redox ability of the cerium, vanadium, and molybdenum species. Addition of Ce increases the acidity of the catalyst. More active oxygen species, including surface chemisorbed oxygen, form with addition of Mn and/or Ce. Because of the synergistic effects, appropriate proportions of manganese in different valence states exist in the catalysts. In summary, the good redox ability and the strong acidity contribute to the high NH3-SCR activity and N2 selectivity of the V1.5Mo3Mn2.5Ce3/Ti catalyst in a wide temperature range. And the V1.5Mo3Mn2.5Ce3/Ti catalyst shows good resistance to H2O and SO2 in long-time catalytic testing, which can be ascribed to the highly sulfated species adsorbed on the catalyst.

Mechanism of the H2 Effect on NH3-Selective Catalytic Reduction over Ag/Al2O3: Kinetic and Diffuse Reflectance Infrared Fourier Transform Spectroscopy Studies

The mechanism of H2-assisted NH3-selective catalytic reduction (SCR) over Ag/Al2O3 was systematically investigated by kinetic measurements and in situ diffuse reflectance infrared Fourier transform...

In situ DRIFTS investigation on CeO2/TiO2–ZrO2–SO2−4 catalyst for NH3–SCR: the influence of surface acidity and reducibility

A series of CeO2-modified TiO2–ZrO2–SO42− catalysts were employed to the selective catalytic reduction (SCR) of NOx by NH3. The obtained results indicated that the interaction between CeO2 and TiO2–ZrO2–SO42− contributed to the increased reducibility and decreased surface acidity with the augment of CeO2. The in situ DRIFTS results suggested that suitable surface acidity and reducibility could be offered by increasing the ceria loadings from 24 to 40 wt%. The amount of adsorbed NH3 species, amide species (–NH2) and activated nitrate was increased with the augment of ceria. Excessive addition of CeO2 (80 wt%) resulted in the lacking of surface acid sites and induced the inert nitrate species generation on the catalysts surface.

Understanding the deposition and reaction mechanism of ammonium bisulfate on a vanadia SCR catalyst: A combined DFT and experimental study

The deactivation of NH3-selective catalytic reduction (SCR) catalysts due to NH4HSO4 deposition at low temperatures (< 300 °C) is still a significant challenge. In this work, we present a comprehensive mechanism describing the formation, deposition, and reaction of NH4HSO4 on a V2O5/TiO2 catalyst using a combination of theoretical and experimental methods. The results show that NH4HSO4 is mainly formed in the gas phase through the nucleation of SO3, H2O, and NH3 and then deposits onto the catalyst surface. The decomposition of NH4HSO4 on the surface of the V2O5/TiO2 catalyst consists of two steps: NO is reduced by the NH4+ of NH4HSO4 forming N2 and H2O by transferring an electron to the adjacent vanadium site, followed by a reoxidation of the reduced vanadium site by either O2 or NO2. At low temperatures, due to the weak reoxidizing ability of O2, the reaction of NH4HSO4 with NO in the NO/O2 mixture is rather slow. Adding NO2 can remarkably enhance the decomposition of NH4HSO4 on the catalyst surface. Our results reveal that the rate-determining step of the reaction between NH4HSO4 and NO/O2 is the reoxidation of the reduced vanadium site and that NO2 is a better reoxidizing agent than O2, which has been confirmed by X-ray photoelectron spectroscopy analysis and the designed transient response method experiments. Finally, the catalyst sulfur tolerance test has proven that the commercial V2O5-WO3/TiO2 catalyst can successfully maintain its long-term activity for NOx reduction in SO2-contained flue gas at 250 °C due to the rapid decomposition of deposited NH4HSO4 on the catalyst surface by the NO/NO2 mixture.

Superior activity of iron–manganese supported on kaolin for NO abatement at low temperature

A series of Fe–Mn catalysts was prepared using different supports (kaolin, diatomite, and alumina) and used for NO abatement via low-temperature NH3-selective catalytic reduction (SCR). The results showed that 12Fe–10Mn/Kaolin (with the concentration of Fe and Mn 12 and 10wt.%, respectively) exhibited the highest activity, and more than 95.8% NO conversion could be obtained within the wide temperature range of 120–300°C. The properties of the catalysts were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES), thermogravimetry (TG), Brunner–Emmet–Teller (BET) measurements, X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR), NH3-temperature programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS), scanning electron microprobe (SEM) and energy dispersive spectroscopy (EDS) techniques. The support effects resulted in significant differences in the components and structures of catalysts. The 12Fe–10Mn/Kaolin catalyst exhibited better dispersion of active species, optimum low-temperature reduction behavior, the largest amount of normalized Brønsted acid sites, and the highest Mn4+/Mn and Fe3+/(Fe3++Fe2+), all of which may be major reasons for its superior catalytic activity.

Efficient Fe-ZSM-5 catalyst with wide active temperature window for NH3 selective catalytic reduction of NO: Synergistic effect of isolated Fe3+ and Fe2O3

The influence of preparation conditions (urea as precipitant, H2 reduction, and air passivation) on the catalytic performance and mechanism of Fe-ZSM-5 for NH3 selective catalytic reduction of NO was systematically investigated to understand the effect of iron species in depth. An appropriate molar ratio of urea addition to Fe-ZSM-5 prepared in a mixed solvent formed a large amount of Fe2O3, which increased the low-temperature activity. H2 reduction transformed some Fe2O3 into elemental Fe, which weakened the inhibiting effect of NH3. It showed more than 90% NO conversion at 125 °C, but the loss of active components and Brønsted acid sites caused deteriorating high-temperature performance. Air passivation generated some isolated Fe3+ and adsorbed oxygen species on the surface, which resulted in similar low-temperature activity as a hydrogen-reduced catalyst. Moreover, the regenerated Fe2O3 caused more NH4+ to combine with Brønsted acid sites, which maintained excellent high-temperature performance.

Speciation of Cu Cations in Cu-CHA Catalysts for NH3-SCR: Effects of SiO2/AlO3 Ratio and Cu-Loading Investigated by Transient Response Methods

We develop an experimental protocol to quantify the speciation of Cu ions (ZCuIIOH and Z2CuII) in Cu-CHA catalysts for the NH3 Selective Catalytic Reduction of NOx (NH3-SCR). Toward this end, we performed four transient tests, namely H2-TPR, NO + NH3 TPR, NO2 adsorption + TPD, and NH3 adsorption + TPD, over two sets of Cu-CHA research catalysts characterized by different Cu contents (0–2.1% w/w) and SiO2/Al2O3 (SAR) ratios (10–25). Preliminary H2-TPR tests on the samples with the extreme SAR and Cu loading values were used to identify the variability range of the fractions of ZCuIIOH and Z2CuII species in these catalysts. The ZCuIIOH fraction was found to vary between 0.55 (at Cu/Al = 0.11) and 0.79 (at Cu/Al = 0.29). NO+NH3 TPR runs demonstrated that the NO + NH3 mixture is a much stronger reducing agent than H2: full reduction of all the Cu was obtained already at lower temperature, and differences in the reducibility of ZCuIIOH and Z2CuII were strongly attenuated. Both the integral NO consumption and t...

Enhanced hydrothermal stability of a Cu-SSZ-13 catalyst for the selective reduction of NOx by NH3 synthesized with SAPO-34 micro-crystallite as seed

The distribution of framework Si and Al in the Cu-SSZ-13 catalyst, for NH3 selective catalytic reduction of NOx, is manipulated via a new strategy, using SAPO-34 as a seed in hydrothermal synthesis instead of the conventional SSZ-13 micro-crystallite. The SAPO-34 seed has a strong impact on the hydrothermal stability of the catalyst, moderating the dealumination as well as the agglomeration of the active Cu2+ ions in the Cu-SSZ-13 with hydrothermal treatment at 800 °C.

Surface lattice oxygen activation via Zr4+ cations substituting on A2+ sites of MnCr2O4 forming ZrxMn1−xCr2O4 catalysts for enhanced NH3-SCR performance

Zr4+ cations were incorporated into the lattice of MnCr2O4 spinels, forming a series of novel ternary ZrxMn1−xCr2O4 (x = 0, 0.05, 0.1 and 0.2) catalysts. The as-prepared Zr-doped catalysts all exhibited enhanced performance selective catalytic reduction of NOx by NH3 than undoped MnCr2O4 catalyst. Among all the prepared catalysts, the Zr0.05Mn0.95Cr2O4 exhibited the best catalytic performance, with good stability in the presence of H2O, the lowest T50 and T90 (149 °C and 201 °C, respectively) and the broadest T80 operation window (183–354 °C) under a gas hourly space velocity (GHSV) of 112,000 h−1. The characterization results revealed that Zr4+ cations were successfully incorporated, and the resultant lattice deformation changed the physical and chemical properties of the catalysts remarkably. The introduction of zirconium produced higher levels of beneficial Mn3+, Mn4+ and Cr5+ species and increased the acidity and redox ability, and electrons transfer effects might be established between Mn, Cr and Zr cations. Moreover, surface lattice oxygen species are activated by Zr doping, and play an important role in NH3-selective catalytic reduction (NH3-SCR). The promoted electrons transfer effects and surface lattice oxygen activation are further substantiated by density functional theory (DFT) results.

Modeling intraphase and interphase mass transfer limitations for NH3–SCR over Cu–ZSM–5

A model describing NH3 selective catalytic reduction (NH3-SCR) over Cu-ZSM-5 to investigate the effects of intraphase and interphase mass transfer on standard SCR reaction is developed and further verified with the synthetic gas bench. The correlation between tortuosity and porosity has been analyzed by least squares method and applied to the sinuousoidal capillary bundle model. The effects of washcoat thicknesses, lengths, cell densities and channel widths on the standard SCR reaction are analyzed in temperature range of 150–500 °C. It is found that deNOx efficiency is affected by intraphase diffusion limitations at 200–500 °C and by interphase mass transfer limitation at intermediate (250–400 °C) and high temperatures (400–500 °C). As the temperature increases, the reaction is firstly controlled by kinetics, with the kinetics resistance decreasing and then increasing until the maximum among three processes. Furthermore, intake flow rates (GHSVs) play an important role impacting NO conversion on NH3–SCR.

Improving high-temperature NOx conversion in the combined NSR-SCR system with an SCR catalyst mixed with an NH3 adsorbent

The growing worldwide interest in environmental protection and the strong demand to decrease the emission of pollutants from automotive exhaust require the development of catalysts and post-cleaning systems that are very efficient and stable. To that end, we investigated ways of improving the high-temperature NH3 selective catalytic reduction (SCR) activity in the NOx storage reduction (NSR) and SCR combined system, and found that NH3 oxidation is promoted under high-temperature NSR-SCR reaction conditions. To suppress NH3 oxidation and improve the NH3-SCR reaction in the NSR-SCR system, we examined mixing an NH3 adsorbent with the SCR catalyst (NSR-SCR + NH3 adsorbent system). The H-type mordenite zeolite (H-MOR) adsorbent demonstrated superior performance for retaining NH3 without oxidation under high-temperature transient conditions than the other NH3 adsorbents examined. NOx conversion under high-temperature transient conditions was improved in going from the single NSR system (71% at 450 °C) to NSR-SCR (75% at 450 °C), and NSR-SCR+H-MOR (85% at 450 °C). We confirmed that N2 formed over long times, from the commencement to the end of the lean phase, in the NSR-SCR+H-MOR system. This phenomenon, in which the NH3 adsorbed on H-MOR is gradually released to the SCR up to the end of the lean phase, enhances NOx reduction in the NSR-SCR system.

Tourmaline-Modified FeMnTiO x Catalysts for Improved Low-Temperature NH3-SCR Performance.

Low temperature NH3 selective catalytic reduction (NH3-SCR) technology is an efficient and economical strategy for cutting NO x emissions from power-generating equipment. In this study, a novel and highly efficient NH3-SCR catalyst, tourmaline-modified FeMnTiO x is presented, which was synthesized by a simple one-step sol-gel method. We found that the amount of tourmaline has an important impact on the catalytic performance of the modified FeMnTiO x-based catalysts, and the NO x conversion exceeded 80% from 160 to 380 °C with the addition of 5 wt % tourmaline. Compared with the pure FeMnTiO x, the catalytic efficiency at a temperature below 100 °C was increased by nearly 18.9%, and the operation temperature window was broadened significantly. The enhanced catalytic performance of the FeMnTiO x catalyst was mainly attributed to the small spherical nanoparticles structure around the tourmaline powders, resulting in the increased content of Mn3+, Mn4+, and chemical oxygen on the catalytic surface. These as-developed tourmaline-modified FeMnTiO x materials have been demonstrated to be promising as a new type highly efficient low temperature NH3-SCR catalyst.