Different morphologies on Cu-Ce/TiO2 catalysts for the selective catalytic reduction of NOx with NH3 and DRIFTS study on sol-gel nanoparticles

The copper-cerium binary oxide catalysts supported by titanium dioxide with nanosphere core-shell structures, nanotube (TNT) core-shell structures, impregnation (imp) nanoparticles and sol-gel nanoparticles were prepared for NH3-SCR of NOx under medium-low temperature conditions. The effect of different morphologies on the Cu-Ce/TiO2 catalysts was comprehensively studied through physicochemical characterization. The results showed that the sol-gel nanoparticles exhibited 100% NOx reduction efficiency in the temperature range of 180-400 °C. Compared with the other catalysts, the sol-gel nanoparticle catalyst had the highest dispersion and lowest crystallinity, indicating that morphology played an important role in the NH3-SCR of the catalyst. The in situ DRIFTS study on the sol-gel nanoparticle catalyst shows that cerium could promote Cu2+ to produce abundant Lewis acid sites, which would significantly increase the adsorption reaction of ammonia on the catalyst surface, thereby promoting the occurrence of the Eley-Rideal (E-R) mechanism. With the Ce-Ti interaction on the atomic scale, the Ce-O-Ti structure enhanced the redox properties at a medium temperature. In addition, cerium oxide enhances the strong interaction between the catalyst matrix and CuO particles. Therefore, the reducibility of the CuO species was enhanced.

Recent advances in core-shell structured catalysts for low-temperature NH3-SCR of NOx

Ammonia selective catalytic reduction (NH₃-SCR) of nitrogen oxides is an effective and well-established technology for NO_x removal, but current commercial denitrification catalysts based on V₂O₅-WO₃/TiO₂ have some obvious disadvantages, including narrow operating temperature windows, toxicity, poor hydrothermal stability, and unsatisfied SO₂/H₂O tolerance. To overcome these drawbacks, it is imperative to investigate new types of highly efficient catalysts. In order to design catalysts with outstanding selectivity, activity, and anti-poisoning ability, core-shell structured materials have been widely applied in the NH₃-SCR reaction, which exhibits numerous advantages including the large surface area, the strong synergy interaction of core-shell materials, the confinement effect, and the shielding effect from the shell layer to protect the core. This review summarizes recent developments of core-shell structured catalysts for NH₃-SCR, including basic classification, synthesis methods, and a detailed description of the performance and mechanisms of each type of catalyst. It is hoped that the review will stimulate future developments in NH₃-SCR technology, leading to novel catalyst designs with improved denitrification performance.

Engineering yolk-shell MnFe@CeOx@TiOx nanocages as a highly efficient catalyst for selective catalytic reduction of NO with NH3 at low temperatures

To broaden the reaction temperature range and improve the H₂O-resistance of manganese-based catalysts, yolk-shell structured MnFe@CeO_x@TiO_x nanocages were prepared. The CeO₂ shell could effectively increase the oxygen vacancy defect sites, and the TiO₂ shell could remarkably improve the surface acid sites. Combining the advantages of the two shells could effectively solve the above questions. The catalytic efficiency of the yolk-shell MnFe@CeO_x@TiO_x-40 nanocages could reach above 90% in the range of 120-240 °C, and the water resistance could reach 90% at 240 °C. On the one hand, the construction of double shells could significantly increase the proportion of active species (Mn⁴⁺, Fe³⁺, Ce³⁺ and O_ads) and the interface effect between the shell layers could effectively enhance the interaction between metal oxides. On the other hand, the construction of double shells could achieve an appropriate balance between the redox capacity of the catalyst and surface acidity. Simultaneously, in situ DRIFT spectroscopy indicated that the yolk-shell MnFe@CeO_x@TiO_x-40 nanocages mainly followed the L-H mechanism during the NH₃-SCR reaction. Finally, this double-shell structure strategy provided a new idea for constructing a Mn-based catalyst with a wide temperature window and better low-temperature water resistance.

Recent advances and perspectives in the resistance of SO2 and H2O of cerium-based catalysts for NOx selective catalytic reduction with ammonia

This review emphasizes the aspects related to cerium-based catalysts at different levels: metal modification, preparation methods, structures, and reaction mechanisms.

Synthesis of metal–organic frameworks (MOFs) and their application in the selective catalytic reduction of NOx with NH3

This review summarizes the synthesis, applications for the NH3-SCR and methods for strengthening the water resistance and thermal stability of MOF catalysts.

Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review

Recently, manganese oxides (MnOx)/cerium(IV) oxide (CeO2) composites have attracted widespread attention for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), which exhibit outstanding catalytic performance owing to unique features, such as a large oxygen storage capacity, excellent low-temperature activity, and strong mechanical strength. The intimate contact between the components can effectively accelerate the charge transfer to enhance the electron–hole separation efficiency. Nevertheless, MnOx/CeO2 still reveals some deficiencies in the practical application process because of poor thermal stability, and a low reduction efficiency. Constructing MnOx/CeO2 with other semiconductors is the most effective strategy to further improve catalytic performance. In this article, we discuss progress in the field of MnOx/CeO2-based ternary composites with an emphasis on the SCR of NOx by NH3. Recent progress in their fabrication and application, including suitable examples from the relevant literature, are analyzed and summarized. In addition, the interaction mechanisms between MnOx/CeO2 catalysts and NOx pollutants are comprehensively dissected. Finally, the review provides basic insights into prospects and challenges for the advancement of MnOx/CeO2-based ternary catalysts.

Theoretical study on NO x adsorption properties over the α-MnO2(110) surface

Herein, α-MnO2 was studied as an adsorbent for the removal of NO x (NO, NO2) derived from flue gas. First-principles calculations based on the density functional theory (DFT) were performed to investigate the NO x adsorption properties over the α-MnO2(110) surface. NO strongly adsorbed over the α-MnO2(110) surface via chemisorption spontaneously under 550 K. The NO2 molecules adsorbed over the surface via chemisorption and physisorption when the terminal N- and O atoms approached the surface, respectively. The joint adsorption of NO x was more stable than the isolated adsorption system. Furthermore, the net charge was transferred from the molecule to the surface. The surface and temperature affected the entropy, enthalpy, NO adsorption and NO2 desorption in the temperature range of 300-550 K. The equilibrium constants decreased with an increase in temperature, which reduced the conversion rate.

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.

Selective catalytic reduction of NOx with NH3 over TiO2 supported metal sulfate catalysts prepared via a sol–gel protocol

Metal sulfate catalysts exhibited high SO₂ tolerance in the NH₃-SCR reaction. The NH₃-SCR reaction mechanism on metal sulfate catalysts should follow the Eley–Rideal mechanism.