Ce-Si Mixed Oxide: A High Sulfur Resistant Catalyst in the NH3-SCR Reaction through the Mechanism-Enhanced Process

Water-Driven Surface Lattice Oxygen Activation in MnO2 for Promoted Low-Temperature NH3-SCR

Water is ubiquitous in various heterogeneous catalytic reactions, where it can be easily adsorbed, chemically dissociated, and diffused on catalyst surfaces, inevitably influencing the catalytic process. However, the specific role of water in these reactions remains unclear. In this study, we innovatively propose that H2O-driven surface lattice oxygen activation in γ-MnO2 significantly enhances low-temperature NH3-SCR. The proton from water dissociation activates the surface lattice oxygen in γ-MnO2, giving rise to a doubling of catalytic activity (achieving 90% NO conversion at 100 °C) and remarkable stability. Comprehensive in situ characterizations and calculations reveal that spontaneous proton diffusion to the surface lattice oxygen reduces the orbital overlap between the protonated oxygen atom and its neighboring Mn atom. Consequently, the Mn-O bond is weakened and the surface lattice oxygen is effectively activated to provide excess oxygen vacancies available for converting O2 into O2-. Therefore, the redox property of Mn-H is improved, leading to enhanced NH3 oxidation-dehydrogenation and NO oxidation processes, which are crucial for low-temperature NH3-SCR. This work provides a deeper understanding and fresh perspectives on the water promotion mechanism in low-temperature NOx elimination.

Modulating the Microstructure and Surface Acidity of MnO2 by Doping-Induced Phase Transition for Simultaneous Removal of Toluene and NOx at Low Temperature

It is a great challenge to remove VOCs and NOx simultaneously from flue gas in nonelectric industries. This study focuses on the construction of Fe-MnO2 catalysts that perform well in the simultaneous removal of toluene and NOx at low temperatures. Utilizing the Fe-induced phase transition of MnO2, Fe-MnO2-F&R catalysts with a composite morphology of nanoflowers and nanorods were successfully prepared that provided an abundant microporous structure to facilitate the diffusion of molecules of different sizes. Through in-depth investigation of the active sites and reaction mechanism, we discovered that Fe-induced phase transition could modulate the surface acidity of Fe-MnO2-F&R. The higher concentration of surface Mn4+ provided numerous Brønsted acid sites, which effectively promoted the activation of toluene to reactive intermediates, such as benzyl alcohol/benzoate/maleic acid. Simultaneously, Fe provided a large number of Lewis acid sites that anchor and activate NH3 species, thereby inhibiting NH3 nonselective oxidation. Furthermore, additional Brønsted acid sites were generated during the simultaneous reaction process, enhancing toluene activation. Consequently, the simultaneous removal of toluene and NOx was achieved through regulation of the physical structure and the concentration of acidic sites. The present work provides new insights into the rational design of bifunctional catalysts for the synergistic control of VOCs and NOx emissions.

Role of the In-Situ-Formed Surface (Pt-S-O)-Ti Active Structure in SO2-Promoted C3H8 Combustion over a Pt/TiO2 Catalyst

Typically, SO2 unavoidably deactivates catalysts in most heterogeneous catalytic oxidations. However, for Pt-based catalysts, SO2 exhibits an extraordinary boosting effect in propane catalytic oxidation, but the promotive mechanism remains contentious. In this study, an in situ-formed tactful (Pt-S-O)-Ti structure was concluded to be a key factor for Pt/TiO2 catalysts with a substantial SO2 tolerance ability. The experiments and theoretical calculations confirm that the high degree of hybridization and orbital coupling between Pt 5d and S 3p orbitals enable more charge transfer from Pt to S species, thus forming the (Pt-S-O)-Ti structure with the oxygen atom dissociated from the chemisorbed O2 adsorbed on oxygen vacancies. The active oxygen atom in the (Pt-S-O)-Ti active structure is a robust site for C3H8 adsorption, leading to a better C3H8 combustion performance. This work can provide insights into the rational design of chemical bonds for high SO2 tolerance catalysts, thereby improving economic and environmental benefits.

Fine-Tuning of Pt Dispersion on Al2O3 and Understanding the Nature of Active Pt Sites for Efficient CO and NH3 Oxidation Reactions

Fine-tuning the dispersion of active metal species on widely used supports is a research hotspot in the catalysis community, which is vital for achieving a balance between the atomic utilization efficiency and the intrinsic activity of active sites. In this work, using bayerite Al(OH)3 as support directly or after precalcination at 200 or 550 °C, Pt/Al2O3 catalysts with distinct Pt dispersions from single atoms to clusters (ca. 2 nm) were prepared and evaluated for CO and NH3 removal. Richer surface hydroxyl groups on AlOx(OH)y support were proved to better facilitate the dispersion of Pt. However, Pt/Al2O3 with relatively lower Pt dispersion could exhibit better activity in CO/NH3 oxidation reactions. Further reaction mechanism study revealed that the Pt sites on Pt/Al2O3 with lower Pt dispersion could be activated to Pt0 species much easier under the CO oxidation condition, on which a higher CO adsorption capacity and more efficient O2 activation were achieved simultaneously. Compared to Pt single atoms, PtOx clusters could also better activate NH3 into -NH2 and -HNO species. The higher CO adsorption capacity and the more efficient NH3/O2 activation ability on Pt/Al2O3 with relatively lower Pt dispersion well explained its higher CO/NH3 oxidation activity. This study emphasizes the importance of avoiding a singular pursuit of single-atom catalyst synthesis and instead focusing on achieving the most effective Pt species on Al2O3 support for targeted reactions. This approach avoids unnecessary limitations and enables a more practical and efficient strategy for Pt catalyst fabrication in emission control applications.

Insight into the mechanisms of activity promotion and SO2 resistance over Fe-doped Ce-W oxide catalyst for NOx reduction

Ceria-based catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR) are always subject to deactivation by sulfur poisoning. In this study, Fe-doped Ce-W mixed oxides, which were synthesized by the co-precipitation method, improved the SCR activity and SO2 durability at low temperatures of undoped Ce-W oxides. The improved low-temperature activity was mainly due to the enhancement of redox properties at low temperatures and more active oxygen species, together with the adsorption and activation of more abundant NOx species, facilitating the "fast SCR" reaction. In the presence of SO2, doping with Fe species effectively prevented sulfate deposition on the CeW catalyst, due to the interaction between Fe, Ce, and W species inducing electron transfer among different metal sites and altering the electron distribution. The competitive adsorption behavior between NO and SO2 was changed by Fe doping, in which the adsorption and oxidation of SO2 were restrained. Besides, the elevated NO oxidation accelerated the decomposition of ammonium bisulfate, causing the SCR reaction to not be greatly suppressed. Hence, Fe-doped Ce-W oxides catalysts showed excellent sulfur resistance. This study provides an in-depth understanding of efficient Ce-based catalysts for SO2-tolerance strategies.

Facile H2O-Contributed O2 Activation Strategy over Mn-Based SCR Catalysts to Counteract SO2 Poisoning

Mn-based catalysts preferred in low-temperature selective catalytic reduction (SCR) are susceptible to SO2 poisoning. The stubborn sulfates make insufficient O2 activation and result in deficient reactive oxygen species (ROS) for activating reaction molecules. H2O has long been regarded as an accomplice to SO2, hastening catalyst deactivation. However, such a negative impression of the SCR reaction was reversed by our recent research. Here, we reported a H2O contribution over Mn-based SCR catalysts to counteract SO2 poisoning through accessible O2 activation, in which O2 was synergistically activated with H2O to generate ROS for less deactivation and more expected regeneration. The resulting ROS benefited from the energetically favorable route supported by water-induced Ea reduction and was actively involved in the NH3 activation and NO oxidation process. Besides, ROS maintained high stability over the SO2 + H2O-deactivated γ-MnO2 catalyst throughout the mild thermal treatment, achieving complete regeneration of its own NO disposal ability. This strategy was proven to be universally applicable to other Mn-based catalysts.

Fabricating Robust Pt Clusters on Sn-Doped CeO2 for CO Oxidation: A Deep Insight into Support Engineering and Surface Structural Evolution

The size effect on nanoparticles, which affects the catalysis performance in a significant way, is crucial. The tuning of oxygen vacancies on metal-oxide support can help reduce the size of the particles in active clusters of Pt, thus improving catalysis performance of the supported catalyst. Herein, Ce-Sn solid solutions (CSO) with abundant oxygen vacancies have been synthesized. Activated by simple CO reduction after loading Pt species, the catalytic CO oxidation performance of Pt/CSO was significantly better than that of Pt/CeO₂ . The reasons for the elevated activity were further explored regarding ionic Pt single sites being transformed into active Pt clusters after CO reduction. Due to more exposed oxygen vacancies, much smaller Pt clusters were created on CSO (ca. 1.2 nm) than on CeO₂ (ca. 1.8 nm). Consequently, more exposed active Pt clusters significantly improved the ability to activate oxygen and directly translated to the higher catalytic oxidation performance of activated Pt/CSO catalysts in vehicle emission control applications.

Formic Acid-Mediated Regeneration Strategy for As-Poisoned V2O5-WO3/TiO2 Catalysts with Lossless Catalytic Activity and Simultaneous As Recycling

Regeneration of spent V₂O₅-WO₃/TiO₂ catalysts is highly desirable, especially for those containing hypertoxic As, which is categorized as hazardous waste. However, common solution-leaching methods suffer from the trade-off between As removal and V₂O₅ retention, and it would be necessary to introduce extra proceedings like ingredients reimplantation and As-bearing waste treatment after regeneration. Herein, a formic acid-mediated regeneration strategy has been developed to achieve superior catalytic activity, short timescale regeneration, and nontoxic metallic As recycling with controllable and safe conduction. The specific activity of the optimal regenerated catalyst reaches 98.3% of the fresh catalyst with 99.1% As removal and less than 1.8% V loss within 15 min. Structure characterizations reveal that the distorted VO_x molecular structure, surface acidity, and redox property recover to the fresh level after regeneration. In situ investigation of the regeneration process indicates that As-OH removal together with V-OH generation occurs at the first regeneration stage, followed by the active center V═O sites over-reduction at the second stage. The retained V═O species by suitable regeneration temperature and time are essential for NH₃-selective catalytic reduction (SCR) since As existence and VO_x over-reduction will separately cause unstable and excessive NH₃ adsorption to further suppress the reaction cycle. The developed strategy and improved understanding of active site protection would exert benefits on the development of efficient and time-saving regeneration methods for spent catalysts.

Cu-VWT Catalysts for Synergistic Elimination of NOx and Volatile Organic Compounds from Coal-Fired Flue Gas

A dual-function catalyst, designated as Cu5-VWT, has been constructed for the synergistic removal of NO_x and volatile organic compounds under complex coal-fired flue gas conditions. The removal of toluene, propylene, dichloromethane, and naphthalene all exceeded 99% (350 °C), and the catalyst could effectively block the generation of polycyclic aromatic hydrocarbons. Mechanistic studies have shown that Cu sites on the Cu5-VWT catalyst facilitate catalytic oxidation, while V sites facilitate NO_x reduction. Thus, toluene oxidation and NO_x reduction can proceed simultaneously. The removal of total hydrocarbons and nonmethane total hydrocarbons from 1200 m³·h^-1 real coal-fired flue gas by a monolithic catalyst were determined as 92 and 96%, respectively, much higher than those of 54 and 72% over a commercial VWT catalyst, indicating great promise for industrial application.

SO2- and H2O-Tolerant Catalytic Reduction of NOx at a Low Temperature via Engineering Polymeric VOx Species by CeO2

Selective catalytic reduction (SCR) of NO_x over V₂O₅-based oxide catalysts has been widely used, but it is still a challenge to efficiently reduce NO_x at low temperatures under SO₂ and H₂O co-existence. Herein, SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature has been originally demonstrated via engineering polymeric VO_x species by CeO₂. The polymeric VO_x species were tactfully engineered on Ce-V₂O₅ composite active sites via the surface occupation effect of Ce, and the obtained catalysts exhibited remarkable low-temperature activity and strong SO₂ and H₂O tolerance at 250 °C. The strong interaction between Ce and V species induced the electron transfer from V to Ce and tuned the SCR reaction via the E-R pathway between the NH₄⁺/NH₃ species and gaseous NO. In the presence of SO₂ and H₂O, the polymeric VO_x species had not been hardly influenced, while the formation of sulfate species on Ce sites not only promoted the adsorption of NH₄⁺ species and the reaction between gaseous NO and NH₄⁺ but also facilitated the decomposition of ammonium bisulfate through weakening the strong bond between HSO₄^- and NH₄⁺. This work provided a new strategy for SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature.

Interaction Mechanism for Simultaneous Elimination of Nitrogen Oxides and Toluene over the Bifunctional CeO2-TiO2 Mixed Oxide Catalyst

Simultaneous catalytic elimination of nitrogen oxides (NO_x, x = 1 and 2) and volatile organic compounds (VOCs) is of great importance for environmental preservation in China. In this work, the interactions of simultaneous removal of NO_x and methylbenzene (PhCH₃) were investigated on a CeO₂-TiO₂ mixed oxide catalyst, which demonstrated excellent bifunctional removal efficiencies for the two pollutants. The results indicated that NO_x positively promotes PhCH₃ oxidation, while NH₃ negatively inhibits through competitive adsorption with PhCH₃. The underlying mechanism is that a pseudo PhCH₃-SCR reaction happened in this process is parallel to NH₃-SCR. Combined with in situ diffuse reflectance infrared Fourier transform spectroscopy and quasi in situ X-ray photoelectron spectroscopy, the interaction mechanism between NO_x and PhCH₃ is proposed. Specifically, NO_x is adsorbed on the catalyst surface to produce nitrate species, which reacts with the carboxylate generated during PhCH₃ oxidation to form organic nitrogen intermediates that create N₂ and CO₂ in the following reactions. In the reaction process, the superoxide (O₂^-) generated by O₂ activation on the catalyst surface is an important species for the propelling of oxidation reaction. This work could provide guidelines for the design of state-of-the-art catalysts for simultaneous catalytic removal of NO_x and VOCs.

Low-temperature Fe–MnO2 nanotube catalysts for the selective catalytic reduction of NOx with NH3

Fe improved the reducibility of Mn sites and reduced the oxidizing properties of bridging oxygen.