Ni-Ce-Ti as a superior catalyst for the selective catalytic reduction of NOx with NH3

Knowledge-Driven Experimental Discovery of Ce-Based Metal Oxide Composites for Selective Catalytic Reduction of NOx with NH3 through Interpretable Machine Learning

Mining the scientific literature, combined with data-driven methods, may assist in the identification of optimized catalysts. In this paper, we employed interpretable machine learning to discover ternary metal oxides capable of selective catalytic reduction of nitrogen oxides with ammonia (NH3-SCR). Specifically, we devised a machine learning framework utilizing extreme gradient boosting (XGB), identified for its optimal performance, and SHapley Additive exPlanations (SHAP) to evaluate a curated database of 5654 distinct metal oxide composite catalytic systems containing cerium (Ce) element, with records of catalyst composition and preparation and reaction conditions. By virtual screening, this framework precisely pinpointed a CeO2-MoO3-Fe2O3 catalyst with superior NOx conversion, N2 selectivity, and resistance to H2O and SO2, as confirmed by empirical evaluations. Subsequent characterization affirmed its favorable structural, chemical bulk properties and reaction mechanism. Demonstrating the efficacy of combining knowledge-driven techniques with experimental validation and analysis, our strategy charts a course for analogous catalyst discoveries.

Selective Synergistic Catalytic Elimination of NOx and CH3SH via Engineering Deep Oxidation Sites against Toxic Byproducts Formation

NOx and CH3SH as two typical air pollutants widely coexist in various energy and industrial processes; hence, it is urgent to develop highly efficient catalysts to synergistically eliminate NOx and CH3SH. However, the catalytic system for synergistically eliminating NOx and CH3SH is seldom investigated to date. Meanwhile, the deactivation effects of CH3SH on catalysts and the formation mechanism of toxic byproducts emitted from the synergistic catalytic elimination reaction are still vague. Herein, selective synergistic catalytic elimination (SSCE) of NOx and CH3SH via engineering deep oxidation sites over Cu-modified Nb-Fe composite oxides supported on TiO2 catalyst against toxic CO and HCN byproducts formation has been originally demonstrated. Various spectroscopic and microscopic characterizations demonstrate that the sufficient chemisorbed oxygen species induced by the persistent electron transfer from Nb-Fe composite oxides to copper oxides can deeply oxidize HCOOH to CO2 for avoiding highly toxic byproducts formation. This work is of significance in designing superior catalysts employed in more complex working conditions and sheds light on the progress in the SSCE of NOx and sulfur-containing volatile organic compounds.

Promotional effects of cerium and titanium on NiMn2O4 for selective catalytic reduction of NO by NH3

In this work NiMn2O4, NiMn2O4/TiO2 and NiMn2O4/CeO2 nanocomposites were prepared by co-precipitation method and evaluated for the selective catalytic reduction of NOx with NH3. Various characterization methods such as X-ray diffraction, field emission scanning electron microscope, specific surface area, average pore diameter, temperature programmed desorption (NH3-TPD), temperature-programmed reduction (H2-TPR) and inductively coupled plasma optical emission spectrometer were conducted to probe the physical and chemical properties of these catalysts. The catalytic activity tests showed that in the temperature window of 200–400 °C and the space velocity of 10,000–40,000 h−1, NiMn2O4/CeO2 demonstrated the best performance among the synthesized catalysts.

An overview of the deactivation mechanism and modification methods of the SCR catalysts for denitration from marine engine exhaust

Selective catalytic reduction (SCR) technology is currently the most effective deNO_x technology and has broad application prospects. Moreover, there is a large NO_x content in marine engine exhaust. However, the marine engine SCR catalyst will be affected by heavy metals, SO₂, H₂O_(g), hydrocarbons (HC) and particulate matter (PM) in the exhaust, which will hinder the removal of NO_x via SCR. Furthermore, due to the high loading operation of the marine engine and the regeneration of the diesel particulate filter (DPF), the exhaust temperature of the engine may exceed 600 °C, which leads to sintering of the SCR catalysts. Therefore, the development of new catalysts with good tolerances to the above emissions and process parameters is of great significance for further reducing NO_x from marine engines. In this work, we first elaborate on the mechanism of the SCR catalyst poisoning caused by marine engine emissions, as well as the working mechanism of SCR catalysts affected by the engine exhaust temperature. Second, we also summarize the current technologies for improving the properties of SCR catalysts with the aim of enhancing the resistance and stability under complex working conditions. Finally, the challenges and perspectives associated with the performance optimization and technology popularization of marine SCR systems are discussed and proposed further. Consequently, this review may provide a valuable reference and inspiration for the development of catalysts and improvement in the denitration ability of SCR systems matched with marine engines.

Progress of selective catalytic reduction denitrification catalysts at wide temperature in carbon neutralization

With the looming goal of carbon neutrality and increasingly stringent environmental protection policies, gas purification in coal-fired power plants is becoming more and more intense. To achieve the NOx emission standard when coal-fired power plants are operating at full load, wide-temperature denitrification catalysts that can operate for a long time in the range of 260–420°C are worthy of study. This review focuses on the research progress and deactivation mechanism of selective catalytic reduction (SCR) denitration catalysts applied to a wide temperature range. With the increasing application of SCR catalysts, it also means that a large amount of spent catalysts is generated every year due to deactivation. Therefore, it is necessary to recycle the wide temperature SCR denitration catalyst. The challenges faced by wide-temperature SCR denitration catalysts are summarized by comparing their regeneration processes. Finally, its future development is prospected.

NOx Reduction over Smart Catalysts with Self-Created Targeted Antipoisoning Sites

Selective catalytic reduction of NO_x in the presence of alkali (earth) metals and heavy metals is still a challenge due to the easy deactivation of catalysts. Herein, NO_x reduction over smart catalysts with self-created targeted antipoisoning sites is originally demonstrated. The smart catalyst consisted of TiO₂ pillared montmorillonite with abundant cation exchange sites to anchor poisoning substances and active components to catalyze NO_x into N₂. It was not deactivated during the NO_x reduction process in the presence of alkali (earth) metals and heavy metals. The enhanced surface acidity, reducible active species, and active chemisorbed oxygen species of the smart catalyst accounted for the remarkable NO_x reduction efficiency. More importantly, the self-created targeted antipoisoning sites expressed specific anchoring effects on poisoning substances and protected the active components from poisoning. It was demonstrated that the tetrahedrally coordinated aluminum species of the smart catalyst mainly acted as self-created targeted antipoisoning sites to stabilize the poisoning substances into the interlayers of montmorillonite. This work paves a new way for efficient reduction of NO_x from the complex flue gas in practical applications.

DeNO x performance enhancement of Cu-based oxides via employing a TiO2 phase to modify LDH precursors

CuAl-LDO, CuAl-LDO/TiO2 and CuAl-LDO/TiO2NTs catalysts were obtained from TiO2 modified LDHs precursor which were prepared by in situ assembly method. Then catalysts were evaluated in the selective catalytic reduction of NO x with NH3(NH3-SCR), and the results showed that the CuAl-LDO/TiO2NTs catalyst exhibited preferable deNO x performance (more than 80% NO x conversion and higher than 90% N2 selectivity at a temperature range of 210-330 °C) as well as good SO2 resistance. With the aid of series of characterizations such as XRD, N2 adsorption/desorption, XPS, NH3-TPD, H2-TPR, and in situ DRIFTS, it could be concluded that, doping TiO2NTs afforded the catalyst larger specific surface area, more abundant surface chemisorption oxygen species and more excellent redox performance. Meanwhile, In situ DRIFTS evidenced that CuAl-LDO/TiO2NTs catalyst has a strong adsorption capacity for the reaction gas, which is more conducive to the progress of the SCR reaction.

Unraveling SO2-tolerant mechanism over Fe2(SO4)3/TiO2 catalysts for NOx reduction

Developing low-temperature SO₂-tolerant catalysts for the selective catalytic reduction of NO_x is still a challenging task. The sulfation of active metal oxides and deposition of ammonium bisulfate deactivate catalysts, due to the difficult decomposition of the as-formed sulfate species at low temperatures (<300 °C). In recent years, metal sulfate catalysts have attracted increasing attention owing to their good catalytic activity and strong SO₂ tolerance at higher temperatures (>300°C); however, the SO₂-tolerant mechanism of metal sulfate catalysts is still ambiguous. In this study, Fe₂(SO₄)₃/TiO₂ and Ce₂(SO₄)₃/TiO₂ catalysts were prepared using the corresponding metal sulfate salt as the precursor. These catalysts were tested for their low-temperature activity and SO₂ tolerance activity. Compared to Ce₂(SO₄)₃/TiO₂, Fe₂(SO₄)₃/TiO₂ showed significantly better low-temperature activity and SO₂ tolerance. It was demonstrated that less surface sulfate species formed on Fe₂(SO₄)₃/TiO₂ and Ce₂(SO₄)₃/TiO₂. However, the presence of NO and O₂ could assist the decomposition of NH₄HSO₄ over Fe₂(SO₄)₃/TiO₂ at a lower temperature, endowing Fe₂(SO₄)₃/TiO₂ with better low-temperature SO₂ tolerance than Ce₂(SO₄)₃/TiO₂. This study unraveled the SO₂-tolerant mechanism of Fe₂(SO₄)₃/TiO₂ at lower temperatures (<300 °C), and a potential strategy is proposed for improving the low-temperature SO₂-tolerance of catalysts with Fe₂(SO₄)₃ as the main active component or functional promoter.

Enhanced Ni–Ce interactions to enable efficient low-temperature catalytic CO2 methanation

The increase of Ni–O–Ce interfacial sites for H2-assisted CO2 activation leads to an improved CO2 methanation performance.

Insight on the anti-poisoning mechanism of in situ coupled sulfate over iron oxide catalysts in NOx reduction

In situ coupled sulfate uniquely migrated to the surface of iron oxide catalysts to capture metal poisons and thus maintained efficient adsorption and activation of NH3 and NOx reactants.

Mutual inhibition mechanism of simultaneous catalytic removal of NOx and toluene on Mn-based catalysts

NO_x and toluene have been identified as the dominant air pollutants in solid wasted combustion, and it is of great importance to remove these two pollutants simultaneously. Here, we found that Mn/CeO₂ and Mn/TiO₂ exhibited a bifunctional property in both NO reduction and toluene oxidation, and both of which could achieve 80% of conversion rate in NO reduction and toluene oxidation processes. However, the activity of both Mn/CeO₂ and Mn/TiO₂ decreased in simultaneous removal of NO_x and toluene compared with separate NO_x reduction and toluene oxidation. This indicates that there is a mutual inhibition between NO_x reduction and toluene oxidation in simultaneous removal process over Mn-based catalysts, attributing to the competitive adsorption and utilization of active oxygen. In detail, the adsorption of toluene occupied the Lewis acid sites and restrained the NH₃ adsorption. Meanwhile, the competitive utilization of active oxygen by NH₃/NO_x inhibited toluene oxidation to CO₂ by active oxygen species as the reaction between NH₃/NO_x and active oxygen species would occur more easily.

MOF-74-M (M = Mn, Co, Ni, Zn, MnCo, MnNi, and MnZn) for Low-Temperature NH3-SCR and In Situ DRIFTS Study Reaction Mechanism

Monometallic and bimetallic MOF-74-M (M = Mn, Co, Ni, Zn, MnCo, MnNi, and MnZn) catalysts were prepared by the solvothermal method for NH₃-SCR. XRD, BET, SEM, and EDS-mapping tests indicate the successful synthesis of the MOF-74-M catalyst with uniform distribution of metal elements and large specific surface area, and the morphology is almost hexagonal. Adding Mn element to a single-metal catalyst can enhance activity, which is mainly because of the existence of various valence states of Mn so that it has excellent redox properties; the catalytic activity of water and sulfur resistance tests showed that the catalytic activity of MOF-74-M increases after adding a proper amount of SO₂, mainly because of the increase in acidic sites. In situ DRIFTS results indicate that the low-temperature range of MOF-74-MnCo and MOF-74-Mn is dominated by the E-R mechanism and the high-temperature range is dominated by the L-H mechanism. The entire temperature range of MOF-74-Zn is dominated by the L-H mechanism.

Alkali-Resistant NOx Reduction over SCR Catalysts via Boosting NH3 Adsorption Rates by In Situ Constructing the Sacrificed Sites

Currently, improving the alkali resistance of vanadium-based catalysts still remains as an intractable issue for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). It is generally believed that the decrease in adsorbed NH_x species deriving from the declined acidic sites is the chief culprit for the deactivation of alkali-poisoned catalysts. Herein, alkali-resistant NO_x reduction over SCR catalysts via boosting NH₃ adsorption rates was originally demonstrated by in situ constructing the sacrificed sites. It is interesting that the adsorbed NH_x species largely decrease while the NH₃ adsorption rate is well kept over the V₂O₅/CeO₂ catalyst by in situ constructing the sacrificed sites. The SCR activity could be maintained after alkali poisoning because in situ constructed SO₄^2- groups would prefer to be combined with K⁺ so that the specific V═O species can endow K-poisoned V₂O₅/CeO₂ with high adsorption rate of NH₃ and high reactivity of NH_x species. This work provides a new viewpoint that NH₃ adsorption rate plays more decisive roles in the performance of alkali-poisoned catalysts than the amount of NH₃ adsorption and enlightens an alternative strategy to improve the alkali-resistance of catalysts, which is significant to both the academic and industrial fields.

Conversion of lanthanum and cerium recovered from hazardous waste polishing powders to hazardous ammonia decomposition catalysts

The polishing process in high-tech industries produces a large amount of waste polishing slurry, which is harmful to the environment, and the solid powders in the slurry contain a lot of La and Ce, which have been widely used as catalyst materials. The aim of this study was to convert this hazardous material into hazardous material decomposition catalyst. A novel and simple approach was successfully developed for the recovery of La and Ce from hazardous waste polishing powders to synthesize a composite metal oxide catalyst for decomposition of harmful ammonia. Here, La and Ce were leached from waste polishing powder by using nitric acid, and the Ce, La, and total REE recovery rates were approximately 100%, 83.3%, and 96.4%, respectively. The elemental concentrations of leached acidic solution was analyzed, after which stoichiometry was performed to replenish elements with insufficient mole numbers. Finally, the catalyst material was prepared using the glycine-nitrate combustion process. The catalyst prepared from the recovered polishing powders achieved a 100% ammonia conversion rate at the relatively low temperature of 250 °C. The proposed environmentally friendly method does not require complex purification and separation procedures and can be used for the synthesis of catalysts for decomposing other harmful pollutants.

Fe-modified Ce-MnOx/ACFN catalysts for selective catalytic reduction of NOx by NH3 at low-middle temperature

A series of MnO_x/ACF_N, Ce-MnO_x/ACF_N, and Fe-Ce-MnO_x/ACF_N catalysts on selective catalytic reduction (SCR) of NO_x with NH₃ at low-middle temperature had been successfully prepared through ultrasonic impregnation method, and the catalysts were characterized by SEM, XRD, BET, H₂-TPR, NH₃-TPD, XPS, and FT-IR spectroscopy, respectively. The results demonstrated that the 15 wt% Fe₍₁₎-Ce₍₃₎-MnO_x(7)/ACF_N catalyst achieved 90% NO_x conversion (100~300 °C), good water resistance, and stability (175 °C). The excellent catalytic performance of the Fe₍₁₎-Ce₍₃₎-MnO_x(7)/ACF_N catalyst was mainly attributed to the interaction among Mn, Ce, and Fe. The doping of Fe promoted the dispersion of Ce and Mn and the formation of more Mn⁴⁺ and chemisorbed oxygen on the surface of a catalyst. This work laid a foundation for the successful application of active carbon fiber in the field of industrial denitrification, especially in the aspect of denitrification moving bed. Graphic abstract.

Development of cerium-based catalysts for selective catalytic reduction of nitrogen oxides: a review

Nitrogen oxides (NO_X) are major pollutants of the atmosphere, and selective catalytic reduction of nitrogen oxides using ammonia as a reductant (NH₃-SCR) is an effective method to remove nitrogen oxides.

NOx removal by selective catalytic reduction with ammonia over hydrotalcite-derived NiTi mixed oxide

The speculated mechanism of the SCR reaction over the NiTi-LDO catalyst and the synergetic catalytic effect between Ni and Ti.

NOx Removal by Selective Catalytic Reduction with Ammonia over a Hydrotalcite-Derived NiFe Mixed Oxide

A series of NiFe mixed oxide catalysts were prepared via calcining hydrotalcite-like precursors for the selective catalytic reduction of nitrogen oxides (NOx) with NH3 (NH3-SCR). Multiple characterizations revealed that catalytic performance was highly dependent on the phase composition, which was vulnerable to the calcination temperature. The MOx phase (M = Ni or Fe) formed at a lower calcination temperature would induce more favorable contents of Fe2+ and Ni3+ and as a result contribute to the better redox capacity and low-temperature activity. In comparison, NiFe2O4 phase emerged at a higher calcination temperature, which was expected to generate more Fe species on the surface and lead to a stable structure, better high-temperature activity, preferable SO2 resistance, and catalytic stability. The optimum NiFe-500 catalyst incorporated the above virtues and afforded excellent denitration (DeNOx) activity (over 85% NOx conversion with nearly 98% N2 selectivity in the region of 210–360 °C), superior SO2 resistance, and catalytic stability.

Active Site of O2 and Its Improvement Mechanism over Ce-Ti Catalyst for NH3-SCR Reaction

The current study on Ce-Ti catalyst was mainly focused on the function of NH3 and NO adsorption sites. In our study, by comparing Ce-Ti (doped catalyst) to Ce/Ti (supported catalyst), the active site of O2 and its improvement mechanism over Ce-Ti catalyst for NH3-Selective catalytic reduction (SCR) reactions were investigated. For Ce-Ti catalyst, a cerium atom was confirmed entering a TiO2 crystal lattice by X-ray diffraction (XRD) and Raman; the structure of Ce-□-Ti (□ represents oxygen vacancy) in Ce-Ti catalyst was confirmed by X-ray photoelectron spectroscopy (XPS) and Photoluminescence spectra (PL spectra). The nature of this structure was characterized by electron paramagnetic resonance (EPR), Ammonia temperature-programmed desorption (NH3-TPD), hydrogen temperature-programmed reduction (H2-TPR), Nitric oxide temperature-programmed desorption (NO-TPD) and In situ DRIFT. The results indicated that oxygen vacancies had a promotive effect on the adsorption and activation of oxygen, and oxygen was converted to superoxide ions in large quantities. Also, because of adsorption and activation of NO and NH3, electrons were transferred to adsorbed oxygen via oxygen vacancies, which also promoted the formation of superoxide ions. We expected that our study could promote understanding of the active site of O2 and its improvement mechanism for doped catalyst.