Study of SO2 effect on selective catalytic reduction of NO with NH3 over Fe/CNTs: The change of reaction route

Overview of mechanisms of Fe-based catalysts for the selective catalytic reduction of NOx with NH3 at low temperature

With the increasingly stringent control of NOx emissions, NH3-SCR, one of the most effective de-NOx technologies for removing NOx, has been widely employed to eliminate NOx from automobile exhaust and industrial production. Researchers have favored iron-based catalysts for their low cost, high activity, and excellent de-NOx performance. This paper takes a new perspective to review the research progress of iron-based catalysts. The influence of the chemical form of single iron-based catalysts on their performance was investigated. In the section on composite iron-based catalysts, detailed reviews were conducted on the effects of synergistic interactions between iron and other elements on catalytic performance. Regarding loaded iron-based catalysts, the catalytic performance of iron-based catalysts on different carriers was systematically examined. In the section on iron-based catalysts with novel structures, the effects of the morphology and crystallinity of nanomaterials on catalytic performance were analyzed. Additionally, the reaction mechanism and poisoning mechanism of iron-based catalysts were elucidated. In conclusion, the paper delved into the prospects and future directions of iron-based catalysts, aiming to provide ideas for the development of iron-based catalysts with better application prospects. The comprehensive review underscores the significance of iron-based catalysts in the realm of de-NOx technologies, shedding light on their diverse forms and applications. The hope is that this paper will serve as a valuable resource, guiding future endeavors in the development of advanced iron-based catalysts.

Denitrification activity test of a V modified Mn-based ceramic filter

In view of the characteristics of high temperature denitrification and low water and sulfur resistance of single manganese-based catalysts, a vanadium-manganese-based ceramic filter (VMA(14)-CCF) was prepared by the impregnation method modified with V. The results showed that the NO conversion of VMA(14)-CCF was more than 80% at 175-400 °C. At 225-300 °C, the conversion of NO can reach 100%. High NO conversion and low pressure drop can be maintained at all face velocities. The resistance of VMA(14)-CCF to water, sulfur and alkali metal poisoning is better than that of a single manganese-based ceramic filter. XRD, SEM, XPS and BET were further used for characterization analysis. The introduction of V protects the MnO_x center, promotes the conversion of Mn³⁺ to Mn⁴⁺, and provides abundant surface adsorbed oxygen. The development of VMA(14)-CCF greatly broadens the application range of ceramic filters in denitrification.

Enriching SO42− Immobilization on α-Fe2O3 via Spatial Confinement for Robust NH3-SCR Denitration

The application of iron oxide to NH3-SCR is attractive but largely hindered by its poor acid properties, and surface sulfation is proven to be a prominent way of enhancing the acidity. As such, the method of enriching the sulfate species on iron oxide is crucial for improving the NH3-SCR performance. In the present study, by employing ammonium bisulfate (ABS) as the source of gaseous SO2 for the purpose of trapping, we reported an effective strategy for enhancing the SO42− immobilization on α-Fe2O3 catalyst via spatial confinement in a mesoporous SBA-15 framework. Interestingly, although the presence of the mesopore channel had an adverse effect on the ABS decomposition, which was expected to produce less available SO2, the measured SO42− immobilized on α-Fe2O3 in the mesoporous SBA-15 system was significantly greater than that of the regular SiO2, demonstrating the promoting effect of the spatial confinement on the SO42− enrichment. Further characterizations of the NH3-TPD, NO oxidation, and NH3-SCR performance tests proved that, as a result of the enhanced acidity, the enrichment of SO42− on α-Fe2O3 displayed a clear correlation with the SCR activity. The results of the present study provide an effective strategy for boosting the catalytic performance of iron oxide in NH3-SCR via SO42− enrichment.

An investigation on the promoting effect of Pr modification on SO2 resistance over MnOx catalysts for selective reduction of NO with NH3

Pr-modified MnOx catalyst was synthesized through a facile co-precipitation process, and the results showed that MnPrOx catalyst exhibited much better selective catalytic reduction (SCR) activity and SO2 resistance performance than pristine MnOx catalyst. The addition of Pr in MnOx catalyst led to a complete NO conversion efficiency in 120-220 °C. Moreover, Pr-modified MnOx catalyst exhibited a superior resistance to H2O and SO2 compared with MnOx catalyst. After exposing to SO2 and H2O for 4 h, the NO conversion efficiency of MnPrOx catalyst could remain to 87.6%. The characterization techniques of XRD, BET, hydrogen-temperature programmed reduction (H2-TPR), ammonia-temperature programmed desorption (NH3-TPD), XPS, TG and in situ diffuse reflectance infrared spectroscopy (DRIFTS) were adopted to further explore the promoting effect of Pr doping in MnOx catalyst on SO2 resistance performance. The results showed that MnPrOx catalyst had larger specific surface area, stronger reducibility, and more L acid sites compared with MnOx catalyst. The relative percentage of Mn4+/Mnn+ on the MnPrOx-S catalyst surface was also much higher than those of MnOx catalyst. Importantly, when SO2 exists in feed gas, PrOx species in MnPrOx catalyst would preferentially react with SO2, thus protecting the Mn active sites. In addition, the introduction of Pr might promote the reaction between SO2 and NH3 rather than between SO2 and Mn active sites, which was also conductive to protect the Mn active sites to a great extent. Since the presence of SO2 in feed gas had little effect on NH3 adsorption on the MnPrOx catalyst surface, and the inhibiting effect of SO2 on NO adsorption was alleviated, SCR reactions could still proceed in a near-normal way through the Eley-Rideal (E-R) mechanism on Pr-modified MnOx catalyst, while SCR reactions through the Langmuir-Hinshelwood (L-H) mechanism were suppressed slightly.

Insight into the enhancing activity and stability of Ce modified V2O5/AC during cyclic desulfurization-regeneration-denitrification

Cyclic desulfurization-regeneration-denitrification over carbon-based catalysts is a promising technology for SO₂ and NOx simultaneous elimination in steel industry. Regeneration is imperative to the long-term operation of the process, while the research is limited. In this work, Ce modified V₂O₅/AC catalyst (CeVOx/AC) with higher desulfurization and denitrification activity was prepared and the effect of cyclic regeneration was investigated. Results illustrated that the desulfurization and denitrification activity of CeVOx/AC gradually improved with increasing the regeneration cycles at the optimum regeneration temperature of 470 °C in N₂. The increasing Ce³⁺, V⁵⁺ and oxygen vacancies, enhanced surface acidity and improved redox ability contributed to the catalytic activity of regenerated catalysts. For desulfurization, more SO₂ transformed into H₂SO₄ rather than to metal sulfates after cyclic regeneration. For denitrification, the improved redox ability accelerated the oxidation of NO to active NO₂, bridged nitrites and nitrates, and the enhanced acidity facilitated the NH₃ adsorption, further generating more -NH₂ and promoting the SCR activity of regenerated samples. The CeVOx/AC with good activity and regenerative stability shows great application potential in steel industry for the simultaneous SO₂ and NOx removal.

Understand the role of redox property and NO adsorption over MnFeOx for NH3-SCR

Widening the operation temperature window of selective catalytic reduction NO by NH3 (NH3-SCR) is a challenge to meet the increasingly stringent emission control regulations of NOx. Hence, MnFeOx with different...

Modification of CrCeOx with Mo: improved SO2 resistance and N2 selectivity for NH3-SCR at medium–low temperatures

Mo doping effectively changed the reaction mechanism and surface acidity of CrCeOx catalysts from E–R to L–H, enhancing the sulfur resistance and N2 selectivity.

Confinement of MnOx@Fe2O3 core-shell catalyst with titania nanotubes: Enhanced N2 selectivity and SO2 tolerance in NH3- SCR process

Surface interface regulation is an important research content in the field of heterogeneous catalysis. To improve the interface interaction between the active component and matrix, tremendous efforts have been dedicated to tailoring the morphology, size, and structure of composite catalysts. In this work, we report a confinement strategy to synthesize a series of core-shell catalysts loaded with metal oxides on titania nanotubes (TNTs), which were applied to the selective catalytic reduction of NO_x with ammonia. Interestingly, the core-shell catalyst with confinement of TNTs exhibited the remarkable activity at low temperature region, N₂ selectivity and sulfur tolerance. Benefiting from the superior interfacial confinement characteristic of TNTs and Fe₂O₃, strong component interactions, the surface acid sites and strong oxidizability of MnO_x were properly regulated, thus obtained the outstanding activity, N₂ selectivity and provide chemical protection to effectively prevent SO₂ poisoning. As far as the reaction mechanism, we found that the adsorption and reactivity of Lewis acid sites were the dominant factors affecting the activity in the NH₃-SCR process by in situ DRIFT spectra. In general, our work provides an innovative strategy for constructing an TNTs-enwrapped nanocomposite with nano-confinement and core-shell structure to improve the low temperature SCR process.

Novel Methods for Assessing the SO2 Poisoning Effect and Thermal Regeneration Possibility of MOx-WO3/TiO2 (M = Fe, Mn, Cu, and V) Catalysts for NH3-SCR

In this study, the sulfur resistance and thermal regeneration of a series of MO_x-WO₃/TiO₂ (denoted as MW/Ti, M = Fe, Mn, Cu, V) catalysts were investigated. After in situ sulfur poisoning, the selective catalytic reduction (SCR) activity of the poisoned catalysts was inhibited at low temperatures but was promoted at high temperatures. After thermal regeneration, the FeW/Ti catalyst was more thoroughly regenerated among nonvanadium-based catalysts. To investigate the impacts of sulfur poisoning, characterizations including X-ray diffraction, thermogravimetric analysis, H₂ temperature-programmed reduction, and SO₂ temperature-programmed desorption were applied. It was discovered that different sulfur-containing species blocked the adsorption of NH₃/NO to a distinct extent over all of the catalysts, thus affecting the catalytic activity. The effect depends on which are dominant (NO or NH₃) during the reaction at different temperatures. The difference in regeneration depends on the formation of sulfate species. The ratio of M_x(SO₄)_y to NH₄HSO₄ generated on the catalysts was adopted to assess the possibility of regeneration. The ratios were 0.5, 1.4, 1.5, and 1.7 for VW/Ti, FeW/Ti, CuW/Ti, and MnW/Ti catalysts, respectively. The lower the ratio was, the easier the catalyst could be regenerated. Meanwhile, the sulfate species could be decomposed more easily on the poisoned FeW/Ti catalyst. FeW/Ti is an excellent candidate for low- and medium-temperature NH₃-SCR among nonvanadium-based catalysts.

SO2-Tolerant Selective Catalytic Reduction of NO x over Meso-TiO2@Fe2O3@Al2O3 Metal-Based Monolith Catalysts

It is an intractable issue to improve the low-temperature SO₂-tolerant selective catalytic reduction (SCR) of NO _x with NH₃ because deposited sulfates are difficult to decompose below 300 °C. Herein, we established a low-temperature self-prevention mechanism of mesoporous-TiO₂@Fe₂O₃ core-shell composites against sulfate deposition using experiments and density functional theory. The mesoporous TiO₂-shell effectively restrained the deposition of FeSO₄ and NH₄HSO₄ because of weak SO₂ adsorption and promoted NH₄HSO₄ decomposition on the mesoporous-TiO₂. The electron transfer at the Fe₂O₃ (core)-TiO₂ (shell) interface accelerated the redox cycle, launching the "Fast SCR" reaction, which broadened the low-temperature window. Engineered from the nano- to macro-scale, we achieved one-pot self-installation of mesoporous-TiO₂@Fe₂O₃ composites on the self-tailored AlOOH@Al-mesh monoliths. After the thermal treatment, the mesoporous-TiO₂@Fe₂O₃@Al₂O₃ monolith catalyst delivered a broad window of 220-420 °C with NO conversion above 90% and had superior SO₂ tolerance at 260 °C. The effective heat removal of Al-mesh monolithcatalysts restrained NH₃ oxidation to NO and N₂O while suppressing the decomposition of NH₄NO₃ to N₂O, and this led to much better high-temperature activity and N₂ selectivity. This work supplies a new point for the development of low-temperature SO₂-tolerant monolithic SCR catalysts with high N₂ selectivity, which is of great significance for both academic interests and practical applications.

Investigation of the selective catalytic reduction of NO with NH3 over the WO3/Ce0.68Zr0.32O2 catalyst: the role of H2O in SO2 inhibition

The effect of H₂O and SO₂ on the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) over WO₃/Ce_0.68Zr_0.32O₂ at 250 °C was systematically investigated using various characterization techniques.

Reduction of NO with NH3 over ferric oxide nanocrystals: the crystallographic facet-induced catalytic enhancement

The {001} surface of the Fe₂O₃ hexagon-shaped catalyst is particularly active for NO removal, which is of major environmental interest for air pollution control.