Mechanistic insights into NH3-assisted selective reduction of NO on CeO2: a first-principles microkinetic study on selectivity and activity

To understand the activity- and selectivity-limiting factors of selective catalytic reduction of NO with NH3 (NH3-SCR) catalyzed by CeO2-based oxides, a molecular-level mechanistic exploration was performed on CeO2(110) using a first-principles microkinetic study. Herein, the favored reaction pathway for N2 formation on CeO2(110) is unveiled, which includes three key subprocesses. (i) NH3 adsorbs on the Cecus site and dissociates into *NH2 assisted by Olat; (ii) *NH2 preferentially couples with NO adsorbed on Olat (ONO#), forming *NH2NO on the Cecus site; (iii) *NH2NO undergoes dehydrogenation into *NHNO, which can be easily anchored by Ovac and can then decompose into N2. The quantitative microkinetic results show that the transfer of NHNO from Cecus to Ovac, rather than the further conversion of N2O to N2 on Ovac, emerges as the N2 selectivity-determining step on CeO2, in which Ovac plays a key role. The number of Ovac is an important factor determining the N2 selectivity of CeO2-based catalysts. The sensitivity analysis reveals that NH2NO formation, i.e., *NH2 + ONO# → *NH2NO + O#, is the rate-determining step for NH3-SCR on the CeO2 catalyst; accordingly, enhancing NH3 adsorption could be an effective strategy to boost the catalytic activity of CeO2 for NH3-SCR. In general, creating Ovac on CeO2 and introducing components (e.g., WO3) with strong NH3 adsorption would be efficient for designing CeO2-based catalysts with superior N2 selectivity and activity. These results could provide a consolidated theoretical basis for understanding and optimizing CeO2-based catalysts for NH3-SCR.

Hydrothermal Synthesis of a Ce-Zr-Ti Mixed Oxide Catalyst with Enhanced Catalytic Performance for a NH3-SCR Reaction

A series of mesoporous CeZrTiO_x catalysts were prepared by a facile hydrothermal method. Compared with CeTiO_x catalysts synthesized under the same conditions, the catalytic activity and anti-SO₂ performance of the Ce1Zr1TiO_x catalyst are greatly improved, and at the gas hourly space velocity (GHSV) of 60 000 h^-1, the NO_x removal efficiency is maintained at 90% in the temperature range of 290-500 °C. The catalytic effect of ZrO₂ on the Ce-Ti catalyst NH₃-SCR activity was elucidated through a series of characterizations. The results revealed that the doping of Zr could significantly improve and optimize the structure of Ce-Ti catalysts. At the same time, due to the doping of Zr, the synergistic effect between Ce and Zr in the CeZrTiO_x catalyst can effectively increase oxygen mobility, total acid content, and surface adsorbed oxygen species and lead to a larger pore volume. In addition, the introduction of ZrO₂ made the transformation of Ce⁴⁺ into Ce³⁺ more obvious, and the 2Ce⁴⁺ + Zr²⁺ ↔ 2Ce³⁺ + Zr⁴⁺ reaction greatly improved the reducibility of Ce1Zr1TiO_x. Among them, the improvement of SCR performance and H₂O/SO₂ tolerance is due to the electronic interaction between Zr and Ce.

Tungsten-Based Catalysts for Environmental Applications

This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).

A Study on the Effect of Different Ball Milling Methods on the NH3-SCR Activity of Aluminum-Laden Bayan Obo Tailings

Rich in Fe, Ce, Mn, Si and other elements which have good catalytic activity, Bayan Obo rare-earth tailings are naturally advantaged as the carrier of denitrification catalysts. In this paper, pseudo boehmite (γ-Al2O3) was mixed with Bayan Obo tailings using different ball milling methods for modification to prepare NH3-SCR catalysts. The effect of different mixing methods on the SCR denitrification activity at a low temperature was investigated and the prepared catalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), temperature programed desorption (NH3-TPD), temperature programed reduction (H2-TPR) and other means. The conversion rate of NOx at 250–350 °C was above 80% and the highest conversion rate of NOx of 90% was achieved at 300 °C. SEM and XRD revealed that the tailings modified by pseudo boehmite (γ-Al2O3) using the ordinary ball milling method have loose structure and good dispersion of active substances, and specific surface area (BET) analysis shows that the tailings have the maximum specific surface area and pore volume. However, over grinding and secondary spheronization were observed in the tailings modified by pseudo boehmite (γ-Al2O3) using high energy ball milling method, leading to the decrease of specific surface area and pore volume, poor dispersion of active substances, and ultimately low denitrification rate.

Preparation and Performance of Cerium-Based Catalysts for Selective Catalytic Reduction of Nitrogen Oxides: A Critical Review

Selective catalytic reduction of nitrogen oxides with NH3 (NH3-SCR) is still the most commonly used control technology for nitrogen oxides emission. Specifically, the application of rare earth materials has become more and more extensive. CeO2 was widely developed in NH3-SCR reaction due to its good redox performance, proper surface acidity and abundant resource reserves. Therefore, a large number of papers in the literature have described the research of cerium-based catalysts. This review critically summarized the development of the different components of cerium-based catalysts, and characterized the preparation methods, the catalytic performance and reaction mechanisms of the cerium-based catalysts for NH3-SCR. The purpose of this review is to highlight: (1) the modification effect of the various metal elements for cerium-based catalysts; (2) various synthesis methods of the cerium-based catalysts; and (3) the physicochemical properties of the various catalysts and clarify their relations to catalytic performances, particularly in the presence of SO2 and H2O. Finally, we hope that this work can give timely technical guidance and valuable insights for the applications of NH3-SCR in the field of NOx control.

A DFT-D study on the reaction mechanism of selective catalytic reduction of NO by NH3 over the Fe2O3/Ni(111) surface

The adsorption and SCR reaction mechanism of NH₃, NO, and O₂ molecules on the Fe₂O₃/Ni(111) catalyst surface was revealed.

CeO2 grafted with different heteropoly acids for selective catalytic reduction of NOx with NH3

The CeO₂ catalysts grafted with heteropoly acid (i.e., HPA) could enhance their catalytic performance for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). In comparison to HSiW/CeO₂, HPMo/CeO₂, and commercial V₂O₅-WO₃/TiO_x catalysts, HPW/CeO₂ catalysts showed the best SCR performance. XPS and DRIFTS demonstrated that the amount of HPA on HPW/CeO₂ was more than those on HSiW/CeO₂ and HPMo/CeO₂. H₂-TPR results indicated that reducibility of HPMo/CeO₂ was stronger than those of HSiW/CeO₂ and HPW/CeO₂, resulting in the high-temperature performance loss. According to kinetic results, below 250 °C, k_SCR-ER and k_SCR-LH of HPW/CeO₂ were higher than those of HSiW/CeO₂, meanwhile k_side of both HSiW/CeO₂ and HPW/CeO₂ were low. Therefore, HPW/CeO₂ had the better SCR performance than HSiW/CeO₂. As NH₃ was completely consumed, SCR activity depended on the ratio of SCR reaction in the consumption of NH₃. The selectivity of SCR reaction, NSCR reaction, and C-O reaction of HSiW/CeO₂ were almost the same as those of HPW/CeO₂ above 250 °C, resulting in the NO_x conversion of HPW/CeO₂ was basically the same as that of HSiW/CeO₂ above 250 °C. Due to the lowest k_SCR-ER and k_SCR-LH, and highest k_side, NO_x conversion of HPMo/CeO₂ was the worst compared to HSiW/CeO₂ and HPW/CeO₂ catalysts.

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh as novel monolithic catalysts for NOx reduction

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh were developed as novel monolithic catalysts for NO_x reduction.

Boosting the low-temperature activity and sulfur tolerance of CeZr2Ox catalysts by antimony addition for the selective catalytic reduction of NO with ammonia

In this paper, a series of Sb modified CeZr₂O_x mixed oxides (Sb_yCZ) were synthesized by citrate method for the selective catalytic reduction of NO with ammonia (NH₃-SCR). Experimental results exhibited that the Sb addition could bring a great improvement of SCR activity at 200-360 °C owing to the enhancement in surface area, redox ability and surface acidity. More importantly, the sulfur tolerance of the catalyst with proper Sb loading contents was dramatically improved. For instance, above 85% deNO_x efficiency was retained over Sb_0.5CZ catalyst after 24 h reaction in the presence of 100 ppm SO₂ and 5 vol.% H₂O. As for pure CeZr₂O_x and the catalysts with low Sb loading contents, the serious accumulation of ammonium sulfates resulted in the deactivation after SO₂ exposure. However, with excessive Sb addition, more labile oxygen readily reacted with SO₂ and the redox cycle was then disrupted, leading to the decrease of SCR activity. With an appropriate Sb loading contents, the sulfate species preferentially formed around Sb cations could restrain the further consumption of oxygen species in Ce-O-Ce or Ce-O-Zr mode by SO₂ via a space confinement effect. Thus, a certain amount of labile oxygen was preserved to drive the SCR reaction, thereby enhancing the sulfur tolerance of the catalyst.

Dual Promotional Effects of TiO2-Decorated Acid-Treated MnO x Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NO x

Alkali metals generated during waste incineration in power stations are not conducive to the control of nitrogen oxide (NO _x) emission. Hence, improved selective catalytic reduction of NO _x with ammonia (NH₃-SCR) in the presence of alkali metals is a major issue for practical NO _x removal. In this work, we developed a novel TiO₂-decorated acid-treated MnO _x octahedral molecular sieve (OMS-5(H)@TiO₂) catalyst with improved alkali-resistant NO _x reduction at low temperature, and the dual promotional effects of OMS-5(H)@TiO₂ catalysts were clarified. It was found that the special structure of the acid-treated MnO _x octahedral molecular sieve (OMS-5(H)) was responsible for the trapping of alkali metals and high deNO _x activity at low temperature. Subsequently, the decoration by TiO₂ further improved the redox properties by accelerating the high ratio of Mn⁴⁺ and O_α on the surface of the highly active (OMS-5(H)@TiO₂) catalyst. Moreover, a thorough mechanism study via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs) demonstrated that the acid treatment led to remarkable increment of acid sites, which enabled the catalyst to resist alkali metals in the form of ion exchange. Meanwhile, the decoration of TiO₂ further increased the strength of the Lewis acid sites, assisting more active intermediate species to effectively take part in the deNO _x reaction. Besides, a "fast SCR" process was observed to certify that the decoration of TiO₂ promoted the improvement of low-temperature activity in the presence of alkali metals. The dual effects combining OMS-5(H) with TiO₂ decoration in terms of alkali metal resistance and high catalytic activity at low temperature proved that the high-performance deNO _x catalyst was successfully developed in this work. The work paves a way for the development of superior low-temperature SCR catalysts with improved NO _x reduction efficiency in the presence of alkali metals.

Mechanistic insight into the selective catalytic reduction of NO by NH3 over α-Fe2O3 (001): a density functional theory study

The adsorption properties and the selective catalytic reduction mechanism of NO, NH₃ and O₂ molecules over the α-Fe₂O₃ (001) surface were studied by density functional theory.

Novel CeMo x O y -clay hybrid catalysts with layered structure for selective catalytic reduction of NO x by NH3

A facile method is to prepare novel CeMo_xO_y-clay hybrid catalysts with layered structures by using organic cation modified clay as support. During the preparation process, cerium cations and molybdate anions are easily adsorbed and impregnated into the interlamellar space of the organoclay, and after calcination they undergo transformation to highly dispersed CeMo_xO_y nanoparticles within the interlamellar space of the clay. As expected, the prepared CeMo_0.15O_x-OC-T catalysts with layered structures had high selective catalytic reduction (SCR) activity such as high NO_x conversion of >90% in the wide temperature range of 220–420 °C. Meanwhile, they also exhibit high stability and tolerance to water vapor (5 vol%) and SO₂ (200 ppm), demonstrating that these novel catalysts could serve as a good alternative for NH₃-SCR in practical application.

A facile method is to prepare novel CeMo_xO_y-clay hybrid catalysts with layered structures by using organic cation modified clay as support.

The effect of Ce on a high-efficiency CeO2/WO3–TiO2 catalyst for the selective catalytic reduction of NOx with NH3

Characterizations were used to investigate the effect of Ce on a high-efficiency CeO₂/WO₃–TiO₂ catalyst for the selective catalytic reduction of NO_x with NH₃.