Effect of Ceria Precursor on the Physicochemical and Catalytic Properties of Mn–W/CeO2 Nanocatalysts for NH3 SCR at Low Temperature

The inhibition mechanism of N2O generation in NH3-SCR process by water vapor

N₂O is a typical by-product in the NH3-SCR process, which requires urgent resolution due to its negative economic and environmental impacts. This study investigates in detail the mechanism of N2O generation on the surface of the Mn-Ce/TiO2 catalyst (Mn-Ce/TiO2-ZS) with anatase {001} facets preferentially exposed. The deep oxidation of NH3 and *NH2 capture of NO via O2 were proved to be the dominant N2O generation pathways. The production of N2O was remarkably reduced by the introduction of a low percentage of water vapor (H2O). The results revealed that low percentage of H2O was capable of enhancing the acid sites on the catalyst surface and facilitating the generation of active hydroxyl species. These active species inhibited the deep dehydrogenation of ammonia and the disintegration of nitrate species on the catalyst surface, as well as suppressing the generation of N2O.

Understanding structure-performance relationships of CoOx/CeO2 catalysts for NO catalytic oxidation: Facet tailoring and bimetallic interface designing

Crystalline structure and bimetallic interaction of metal oxides are essential factors to determine the catalytic activity. Herein, three different CoO_x/CeO₂ catalysts, employing CeO₂ nanorods (predominately exposed {110 facet), CeO₂ nanopolyhedra ({111} facet) and CeO₂ nanocubes ({100} facet) as the supports, are successfully prepared for investigating the effect of exposed crystal facets and bimetallic interface interaction on NO oxidation. In comparison with the {111} and {100} facets, the exposed crystal facet {110} exists the best superiority to anchor and stabilize Co species. Moreover, ultra-small CoO_x clusters composed of strong Co-O coordination shells with minor Co-O-Ce interaction are formed and uniformly dispersed on the CeO₂ nanorods. Structural characterizations reveal that the active exposed crystal facet {110} and the strong bimetallic interface interaction in CoO_x/CeO₂-nanorods (R-CC) result in more structural defect, endowing it with abundant oxygen vacancies, excellent reducibility and strong adsorption capacity. The DRIFTs spectra further indicate that the exposed crystal facet {110} has a significant promoting effect on the strength of nitrates compared with {111} and {100} facets. The bimetallic interface interaction not only significantly facilitates the formation of nitrate species at lower temperature, but also effectively suppresses the generation of sulfate and lower the sulphation rate. Therefore, R-CC catalyst exhibits the maximum NO oxidation activity with the conversion of 86.4 % at 300 °C and still sustains its high activity under cyclic condition or 50 ppm SO₂. The provided crystalline structure and interaction-enhanced strategy sheds light on the design of high-activity NO oxidation catalysts.

Nitric Oxide Decomposition via Selective Catalytic Reduction by Ammonia on a Transition-Metal Cluster of W2TcO6

Decomposition of nitric oxide (NO) gas on a reactive transition-metal cluster of W₂TcO₆ has been examined and investigated via selective catalytic reduction by ammonia (NH₃-SCR) using the M06-L density functional method. The transition-metal cluster of W₂TcO₆ can be employed to transform NO to N₂ gas efficiently over an active site of tungsten (W). A reaction mechanism of NO conversion based on the NH₃-SCR process has been elucidated by a potential energy surface along the reaction pathways. The reaction pathways of this NH₃-SCR process begin with adsorption of NH₃, adsorption of NO to the cluster, formation of nitrosamine (NH₂NO) and NHNO/NHNOH intermediates, and rearrangement of NHNO/NHNOH to obtain N₂ and H₂O, respectively. Notably, a significant NH₂NO as a key intermediate, namely, "nitrosamine", must be formed before further steps can take place in the generation of N₂ from NO, followed by the involvement of the NHNO or NHNOH intermediate. From our calculated results, the NHNO intermediate via TS3a is found in pathway a, while NHNOH is found in pathway b via TS3b. Pathway b has a lower energy barrier of 35.1 kcal/mol than pathway a with an energy barrier of 41.8 kcal/mol, indicating that pathway b should be more energetically favorable. The step for NHNO intermediate rearrangement is a rate-determining step for the reaction occurring through pathway a, which is found to be more difficult in accordance with a difficult N-H bond cleavage to form the NNOH intermediate before N₂ formation. The overall reaction is an exothermic process with thermodynamic and kinetic favors. Thus, this bimetallic W₂TcO₆ cluster could be used as a promising and active catalyst for NO decomposition via the NH₃-SCR process to an eco-friendly gas, that is, N₂.

Manganese oxide nanorod catalysts for low-temperature selective catalytic reduction of NO with NH3

MnO_x nanorod catalysts were successfully synthesized by two different preparation methods using porous SiO₂ nanorods as the template and investigated for the low-temperature selective catalytic reduction (SCR) of NO with NH₃. The catalysts were characterized by scanning electron microscopy, transmission electron microscopy, nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, and NH₃ temperature-programmed desorption. The results show that the obtained MnO_x-P nanorod catalyst prepared by redox precipitation method exhibits higher NO removal activity than that prepared by the solvent evaporation method in the low temperature range of 100–180 °C, where about 98% NO conversion is achieved over MnO_x(0.36)-P nanorods. The reason is mainly attributed to MnO_x(0.36)-P nanorods possessing unique flower-like morphology and mesoporous structures with high pore volume, which facilitates the exposure of more active sites of MnO_x and the adsorption of reactant gas molecules. Furthermore, there is a lower crystallinity of MnO_x, higher percentage of Mn⁴⁺ species and a large amount of strong acid sites on the surface. These factors contribute to the excellent low-temperature SCR activity of MnO_x(0.36)-P nanorods.

Compared with MnO_x(0.36)-E nanorods, MnO_x(0.36)-P nanorods possess unique flower-like morphology and mesoporous structures with high pore volume, contributing to the excellent low-temperature SCR activity of MnO_x(0.36)-P nanorods.

Modified red mud catalyst for the selective catalytic reduction of nitrogen oxides: Impact mechanism of cerium precursors on surface physicochemical properties

Red mud, as industrial solid waste, causes severe environmental problems such as soil alkalization and groundwater pollution. In this work, we researched and developed the red mud as a selective catalytic reduction catalyst for NO_x removal with NH₃ (NH₃-SCR). After selective dissolution and specific heat treatment, different Ce precursors were used to modifying its physical and chemical properties. The results showed that Ce(NO₃)₃ and Ce(NH₄)₂(NO₃)₆ modified red mud (RMcn and RMcan) had excellent SCR performance below 300 °C. Ce(SO₄)₂ modified red mud (RMcs) showed relatively low NO_x conversions at 200-300 °C. The redox property was improved with the Ce(NO₃)₃ and Ce(NH₄)₂(NO₃)₆, while depressed with the Ce(SO₄)₂. Agglomerates generated on the RMcs and blocked the accumulated pores due to the formation of Ce₂(SO₄)₃. The surface acidity of RMcs enhanced with increased adsorption for ammonia. However, these new adsorbed ammonia species, highly related to the sulfate from the Ce₂(SO₄)₃, were inert and did not react with the adsorbed or gaseous NO species at 200-300 °C. The abundant surface lattice oxygen from CeO₂ microcrystals improved the catalytic oxidation capacity of the RMcn and RMcan.

Significantly enhanced Pb resistance of a Co-modified Mn–Ce–Ox/TiO2 catalyst for low-temperature NH3-SCR of NOx

Co modification can significantly improve the performance for low-temperature NH₃-SCR of NO_x and the Pb resistance of the Mn–Ce–O_x/TiO₂ catalyst.

Improved NO x Reduction in the Presence of SO2 by Using Fe2O3-Promoted Halloysite-Supported CeO2-WO3 Catalysts

Currently, selective catalytic reduction (SCR) of NO _x with NH₃ in the presence of SO₂ by using vanadium-free catalysts is still an important issue for the removal of NO _x for stationary sources. Developing high-performance catalysts for NO _x reduction in the presence of SO₂ is a significant challenge. In this work, a series of Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalysts were synthesized by a molten salt treatment followed by the impregnation method and demonstrated improved NO _x reduction in the presence of SO₂. The obtained catalyst exhibits superior catalytic activity, high N₂ selectivity over a wide temperature range from 270 to 420 °C, and excellent sulfur-poisoning resistance. It has been demonstrated that the Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalyst increased the ratio of Ce³⁺ and the amount of surface oxygen vacancies and enhanced the interaction between active components. Moreover, the SCR reaction mechanism of the obtained catalyst was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy. It can be inferred that the number of Brønsted acid sites is significantly increased, and more active species could be produced by Fe₂O₃ promotion. Furthermore, in the presence of SO₂, the Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalyst can effectively prevent the irreversible bonding of SO₂ with the active components, making the catalyst exhibit desirable sulfur resistance. The work paves the way for the development of high-performance SCR catalysts with improved NO _x reduction in the presence of SO₂.

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.