A comparative study of MOx (M = Mn, Co and Cu) modifications over CePO4 catalysts for selective catalytic reduction of NO with NH3

Enhanced NOx reduction on CePO4 catalysts: Cu-loading, phosphotungstic acid, and insights from In-situ DRIFTs and DFT

This study investigates the effects of varying Cu/Ce doping ratios on the NH3-SCR denitrification efficiency using Cu-HPW/CePO4 catalysts, where CePO4 serves as the support and copper-doped phosphotungstic acid (HPW) acts as the active phase. The NH3-SCR reaction mechanism was studied by In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTs) and Density Functional Theory (DFT). In-situ DRIFTs were employed to delve into the intricacies of adsorption and transformation dynamics at the surface sites of catalysts. This approach furnished a robust theoretical foundation aimed at augmenting the efficacy of low-temperature denitrification catalysts. DFT calculations were used to systematically investigate the reaction pathways, intermediates, transition states, and energy barriers over the HPW structure model to complete the NH3-SCR reaction. Empirical evidence suggests that modifying the catalysts with copper substantially enhances their denitrification efficacy and extends their operational temperature spectrum. A notable initial increase in denitrification efficiency was observed with increasing levels of copper modification, followed by a decline. Within the HPW-O15H site, the NH3-SCR reaction advances through both the E-R and L-H mechanisms, encompassing processes such as NH3 adsorption, intermediate formation and transformation, and product release.

Improved N2 Selectivity of MnOx Catalysts for NOx Reduction by Engineering Bridged Mn3+ Sites

Mn-based catalysts are promising for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures due to their excellent redox capacity. However, the N₂ selectivity of Mn-based catalysts is an urgent problem for practical application owing to excessive oxidizability. To solve this issue, we report a Mn-based catalyst using amorphous ZrTiO_x as the support (Mn/ZrTi-A) with both excellent low-temperature NO_x conversion and N₂ selectivity. It is found that the amorphous structure of ZrTiO_x modulates the metal-support interaction for anchoring the highly dispersed active MnO_x species and constructs a uniquely bridged Mn³⁺ bonded with the support through oxygen linked to Ti⁴⁺ and Zr⁴⁺, respectively, which regulates the optimal oxidizability of the MnO_x species. As a result, Mn/ZrTi-A is not conducive to the formation of ammonium nitrate that readily decomposes to N₂O, thus further increasing N₂ selectivity. This work investigates the role of an amorphous support in promoting the N₂ selectivity of a manganese-based catalyst and sheds light on the design of efficient low-temperature deNO_x catalysts.

Study of the denitration performance of a ceramic filter using a manganese-based catalyst

A MnO x /γ-Al2O3 catalyst was prepared by impregnation of manganese acetate and alumina. After optimizing the composition, it was loaded into a ceramic filter (CF) by a one-step coating method. The results show that MnO x /γ-Al2O3 had the best denitration activity when the Mn loading was 4 wt% with a calcination temperature of 400 °C. The MnO x /γ-Al2O3 catalyst ceramic filter (MA-CCF) was made by loading the CF twice with MnO x /γ-Al2O3. When face velocity (FV) was 1 m min-1, MA-CCF displayed more than 80% NO conversion at 125-375 °C and possessed a good resistance of H2O and SO2. The abundant surface adsorbed oxygen, dense membrane and high-density fiber structure on the outer layer of CF effectively protected the catalyst and could improve MA-CCF denitration activity. The multiple advantages of MA-CCF made it possible for good application prospects.

Phosphate on ceria with controlled active sites distribution for wide temperature NH3-SCR

Practical catalysts that work well at a wide operation window for selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) are essential for the purification of non-isothermal emission such as vehicle exhaust. However, NH₃-SCR catalyst with high low-temperature performance has excellent NO activation and oxidation ability, leading inevitably to NH₃-intermediates over-oxidation and N₂ selectivity deterioration at high operation temperatures. By far the best performance ceria-based catalyst with a super-wide temperature window of 175-400 ^oC for 90% NO_x conversion in ideal environment and 225-475 ^oC for 90% NO_x conversion by addition of 50 ppm SO₂ and 5% H₂O is obtained via distributing phosphate over the outer of ceria. NH₃ protection strategy is the key for keeping high-temperature activity. Brønsted acidity surged as the formation of P-OH network via a charge compensatory mechanism of phosphate. NH₃ was prone to be captured by the surface P-OH network, forming NH₄⁺ species, avoiding being oxidized and contributing to both low and high temperature activity. NO can also be readily absorbed and oxidized to the absorbed NO₂(ad) species over phosphate as reflected by in situ DRIFTS and DFT calculation, providing a facile pathway for 'fast SCR' by reacting with NH₄⁺ species to form N₂ and H₂O. The reaction followed the L-H mechanism and contributed to catalytic activity under 300 ^oC. This directional structure fabricate strategy helps to increases the NO_x conversion and N₂ selectivity under a broaden temperature window. The enriched Brønsted acid sites over phosphate treated ceria were also demonstrated to have largely suppressed SO₂ adsorption, which significantly slowed down the catalyst poisoning. A dynamic equilibrium between the poisoning and regeneration process can be achieved according to the shrinking-core model for each nanosphere, leading to the excellent resistance.

LaOx modified MnOx loaded biomass activated carbon and its enhanced performance for simultaneous abatement of NO and Hg0

A battery of agricultural straw derived biomass activated carbons supported LaOx modified MnOx (LaMn/BACs) was prepared by a facile impregnation method and then tested for simultaneous abatement of NO and Hg0. 15%LaMn/BAC manifested excellent removal efficiency of Hg0 (100%) and NO (86.7%) at 180 °C, which also exhibited splendid resistance to SO2 and H2O. The interaction between Hg0 removal and NO removal was explored; thereinto, Hg0 removal had no influence on NO removal, while NO removal preponderated over Hg0 removal. The inhibitory effect of NH3 was greater than the accelerative effect of NO and O2 on Hg0 removal. The physicochemical characterization of related samples was characterized by SEM, XRD, BET, H2-TPR, NH3-TPD, and XPS. After incorporating suitable LaOx into 15%Mn/BAC, the synergistic effect between LaOx and MnOx contributed to the improvement of BET surface area and total pore volume, the promotion of redox ability, surface active oxygen species, and acid sites, inhibiting the crystallization of MnOx. 15%LaMn/BAC has the best catalytic oxidation activity at low temperature. That might be answerable for superior performance and preferable tolerance to SO2 and H2O. The results indicated that 15%LaMn/BAC was a promising catalyst for simultaneous abatement of Hg0 and NO at low temperature.

Vacancy-triggered and dopant-assisted NO electrocatalytic reduction over MoS2

Nitric oxide electroreduction reaction (NOER) is an efficient method for NH₃ synthesis and NO_x-related pollutant treatment. However, current research on NOER catalysts mainly focuses on noble metals and single atom catalysts, while low-cost transition metal dichalcogenides (TMDCs) are rarely considered. Herein, by applying density functional theory (DFT) calculations, we study the catalytic performance of NOER over 2H-MoS₂ monolayers with the most common S vacancies and some Mo atoms substituted by transition metal atoms (denoted as TM-MoS₂@V_S). Our results show that an S vacancy and a heteroatom substitution tend to form a first nearest neighbour (1NN) pair, which greatly improves the NOER catalytic performance of 2H-MoS₂. The S vacancy site can trigger NOER by strongly adsorbing a NO molecule and elongating the NO bond, while the heteroatom dopant can assist NOER by tuning the electron donating capability of 2H-MoS₂ which breaks the linear scaling relations among key reaction intermediates. At low NO coverage, NH₃ can be correspondingly yielded at -0.06 and -0.38 V onset potentials over the Pt- and Au-doped MoS₂ catalysts with S vacancies (Pt-MoS₂@V_S and Au-MoS₂@V_S). At high NO coverage, N₂O/N₂ is thermodynamically favored. Meanwhile, the competing hydrogen evolution reaction (HER) is suppressed. Thus, the Pt-MoS₂@V_S catalysts are promising candidates for NOER. In addition, coupling the substitutional doping of Mo atoms to S vacancies presents great potential in improving the catalytic activity and selectivity of MoS₂ for other reactions. In general, the strategy of coupling hetero-metal doping and chalcogen vacancy can be extended to enhance the catalytic activity of other TMDCs.

Improved NOx Reduction over Phosphate-Modified Fe2O3/TiO2 Catalysts Via Tailoring Reaction Paths by In Situ Creating Alkali-Poisoning Sites

The deactivation issue arising from alkali poisoning over catalysts is still a challenge for the selective catalytic reduction of NO_x by NH₃. Herein, improved NO_x reduction in the presence of alkaline metals over phosphate-modified Fe₂O₃/TiO₂ catalysts has been originally demonstrated via tailoring the reaction paths by in situ creating alkali-poisoning sites. The introduction of phosphate results in the partial formation of iron phosphate species and makes the catalyst to mainly exhibit the characteristics of FePO₄, which is responsible for the widened temperature window and enhanced alkali resistance. The tetrahedral [FeO₄]/[PO₄] structures in iron phosphate act as the Brønsted acid sites to increase the catalyst surface acidity. In addition, the formation of an Fe-O-P structure enhances the redox ability and increases surface adsorbed oxygen. Furthermore, the created phosphate groups (PO₄^3-) serving as alkali-poisoning sites preferentially combine with potassium so that iron species on the active sites are protected. Therefore, the enhanced NH₃ species adsorption capacity, improved redox ability, and active nitrate species remaining in the phosphate-modified Fe₂O₃/TiO₂ catalyst ensure the de-NO_x activity after being poisoned by alkali metals through the Langmuir-Hinshelwood reaction pathway. Hopefully, this novel strategy could provide an inspiration to design novel catalysts to control NO_x emission with extraordinary resistance to alkaline metals.

Boosting the sintering resistance of platinum–alumina catalyst via a morphology-confined phosphate-doping strategy

The size-dependent metal–support interaction, high surface area, and, above all, the support morphology-confined effect contribute to a good sintering-resistant catalyst.

Environmental benign synthesis of Nano-SSZ-13 via FAU trans-crystallization: Enhanced NH3-SCR performance on Cu-SSZ-13 with nano-size effect

Small pore zeolites with chabazite structure have been commercialized for selective catalytic reduction (SCR) of nitrogen oxides (NO_x) with ammonium (NH₃) from diesel exhaust. However, conventional zeolite synthesis processes detrimental effects on the environment due to the consumption of large amount of water, organic templates. Thus, this study proposed a green synthesis process with addition of minimal amount of water, structure directing agent and shortened steps to prepare nano-sized SSZ-13 (0.12 μm) using trans-crystallization strategy and exhibited enhanced performance for NO_x removal after copper ion-exchange. The operation temperature window (NO_x conversion >90 %) as well as the SO₂ and H₂O resistance over the green-route prepared nano-sized SSZ-13 (178-480 °C) outperformed the conventional SSZ-13 (29.8 μm, 211-438 °C) mainly due to the much shorter diffusion path. This clearly implied that the mass transportation was key for NH₃-SCR of NO_x on such small pore zeolite catalysts, which was further confirmed via an in-depth mass transportation calculation process. These results demonstrate that the Cu-nano-sized SSZ-13 prepared by the environmental benign route has great potential to act as a new generation of deNO_x catalyst for diesel exhaust and provided a guideline for researchers to develop new methods to synthesize nano-catalysts for air pollution control.

MnOx-CuOx cordierite catalyst for selective catalytic oxidation of the NO at low temperature

Low-value solid waste cordierite honeycomb ceramics were used as carrier of SCO denitration catalyst, and the active component was supported by the impregnation method to improve the performance of the catalyst. Firstly, the effect of calcination conditions on the denitration performance of the Mn-loaded cordierite catalyst was studied for the cordierite-loaded active component MnO_X. Secondly, the preferred catalyst was reloaded with another active component to further improve its denitration performance; the bimetal ratios were affected by the denitration performance, which was, finally, characterized by XRD, XPS, and SEM. The result shows the following: (1) Mn-loaded cordierite prepared at 450 °C for 3 h has a good denitration effect; (2) the MnO_X-CuO_X/CR catalyst is superior to MnO_X-FeO_X/CR, MnO_X-CoO_X/CR, and MnO_X-CeO_X/CR; (3) the MnO₂ crystal form in the single metal-supported catalyst plays a major role, and Cu₂Mn₃O₈ in the bimetallic catalyst affects the performance and activity of the catalyst. Graphical abstract.

Technologies for the nitrogen oxides reduction from flue gas: A review

The required energy of the global industry is mostly generated from fossil fuel sources, such as natural gas, gasoline, diesel, oil, and coal. Nitrogen oxides are one of the main air pollutants that are produced from the combustion of fossil fuels in stationary and mobile sources. Development of new technologies to decrease the NO_x emission from exhaust gases is essential due to the harmful effect of NO_x on the environment and human health. Compared with pre-combustion and combustion methods (with <50% NO_x removal efficiency), the post-combustion methods with higher efficiency (above 80%) have attracted more attention in NO_x elimination. This review describes the currently used technologies of NO_x abatement. Different available post-combustion methods of NO_x removal, including selective catalytic reduction (using different types of reducing reagents, including ammonia, hydrogen, hydrocarbons, and carbon monoxide), selective noncatalytic reduction, wet scrubbing, adsorption, electron beam, nonthermal plasma, and electrochemical reduction of NO_x, are discussed.

Highly efficient WO3-FeOx catalysts synthesized using a novel solvent-free method for NH3-SCR

WO₃-FeO_x catalysts with various WO₃ contents were synthesized through a facile solvent-free method, satisfying the selective catalytic reduction of NO (NH₃-SCR). Strikingly, the optimum 30 %WO₃-FeO_x catalyst with the largest surface area exhibited the most outstanding catalytic activity, achieving the nearly 100 % NO_x removal efficiency in a wide temperature window between 225-500 °C, which was better than that of Fe-W series catalysts reported in other studies. In addition, Raman and XPS results proved that the introduction of WO₃ altered the electronic environment of Fe₂O₃, inducing the formation of Fe₃O₄ (Fe²⁺) and surface adsorbed oxygen. In situ DRIFTS demonstrated that the interaction between WO₃ and Fe₂O₃ not only promoted the adsorption capacity of NH₃ on the catalyst, but also contributed to the formation of adsorbed NO_x species. NO_x reduction reaction on WO₃-FeO_x catalyst proceeded via the Eley-Rideal and Langmuir-Hinshelwood mechanism synchronously. All of these factors, jointly, accounted for the superior catalytic activity and N₂ selectivity of WO₃-FeO_x catalysts.

Full spectrum driven SCR removal of NO over hierarchical CeVO4/attapulgite nanocomposite with high resistance to SO2 and H2O

Removal of hazardous NO at low temperature via photo-assisted selective catalytic reduction (photo-SCR) strategy is promising, however fully harvesting of solar energy and achieving high SO₂/H₂O tolerance still remain a challenge. Herein, the phosphoric acid modified natural attapulgite(P-ATP) was employed as a matrix to immobilize CeVO₄ by microwave hydrothermal method. Results show that P-ATP provides abundant active sites facilitating the in situ grow of CeVO₄ nanorods on its surface which hierarchically construct a dendritic-like photocatalyst. The near-infrared (NIR) light is upconverted to visible and UV light through CeVO₄ which not only broaden the absorption range of solar light, but also build Z-scheme heterostructure with P-ATP enhancing the redox potential of charge carriers. The CeVO₄/P-ATP nanocomposite can reach as high as 92 % for NO conversion under full-spectrum solar irradiation, while retaining nearly 60 % conversion under NIR light. Moreover, the catalyst exhibits outstanding tolerance with SO₂ and H₂O due to the presence of Ce species which can prevent NH₃ from being sulfated, while ATP prevent catalyst from being corroded by H₂O. This work may open up a new window for full-spectrum driven SCR of NO based on cost-effective mineral catalyst.

Excellent low-temperature NH3-SCR NO removal performance and enhanced H2O resistance by Ce addition over the Cu0.02Fe0.2CeyTi1-yOx (y = 0.1, 0.2, 0.3) catalysts

In the present work, a serial of Cu_0.02Fe_0.2Ce_yTi_1-yO_x catalysts are prepared by sol-gel method and applied for NH₃-SCR of NO, meanwhile Cu_0.02Fe_0.2Ce_0.2Ti_0.8O_x shows good low-temperature NH₃-SCR performance with/without water and an outstanding water resistance. The bulk structure, redox ability, surface acidity and surface species of Cu_0.02Fe_0.2Ce_yTi_1-yO_x are measured and discussed by series of characterization in details to illuminate the reasons for the good low-temperature activity and water resistance. The Ce modification can tune the surface acidic distribution, improve the surface oxygen content and surface oxidation reduction cycle (Ce⁴⁺ + Fe²⁺ ↔ Ce³⁺ + Fe³⁺), which contribute the good activity. In addition, the effect of water on NH₃-SCR performance over Cu_0.02Fe_0.2TiO_x and Cu_0.02Fe_0·2Ce_0·2Ti_0.8O_x are investigated emphatically by in situ DRIFTS.

Synthesis of self-assembled spindle-like CePO4 with electrochemical sensing performance

Three different morphologies of CePO₄ nanocrystals (rods, columns, and spindle-like assembled nanosheets), spindle-like LaPO₄, spindle-like PrPO₄, and TbPO₄ microspheres were successfully synthesized using a hydrothermal method.

MnOx–CeO2@TiO2 core–shell composites for low temperature SCR of NOx

The MnO_x–CeO₂@TiO₂ catalyst presents excellent NH₃-SCR activity and the TiO₂ shell is responsible for the good SO₂ tolerance.