Morphology and Crystal-Plane Effects of CeO2 on TiO2/CeO2 Catalysts during NH3-SCR Reaction

How Do the Morphology and Crystal Facet of CeO2 Determine the Catalytic Activity toward NO Removal?

Cerium oxide (CeO2) exhibits application potential for the selective catalytic reduction of nitrogen oxides (NOx) with NH3 (NH3-SCR). The crystal facets and morphology of CeO2 have a vital impact on the catalytic performance of NH3-SCR. However, the precise influence mechanisms on SCR activity remain elusive. In this work, CeO2 is successfully synthesized with three distinct crystal facets and nine diverse morphologies. This investigation involves a comprehensive blend of theoretical analysis and experiments, to gain profound insights into the underlying mechanisms governing the SCR catalytic activity concerning morphology and crystal facets. By closely integrating density functional theory (DFT) calculations, Ab initio thermodynamic analysis, SCR catalytic activity experiments, and X-ray photoelectron spectroscopy experiments, it is discovered that the concentration of surface-active oxygen (O*) plays a pivotal role in determining the catalytic activity of CeO2 in SCR reactions, as opposed to factors like specific surface area or oxygen defect concentration. This experimental-theoretical joint study provides design principles of CeO2 catalysts for NO removal.

The modification of surface basicity and its role in naphthalene oxidation: The effect of the basic sites introduced by Ce

A series of V-xCe/Ti catalysts was prepared by a step impregnation method with gradual increased Ce amount. Compared to the commercial V-W/Ti catalysts, the V-xCe/Ti catalysts exhibited considerably higher COx selectivity during the oxidation of naphthalene (Nap), and less intermediates or by-products were detected both in gas phase and on the surface of the catalysts. Through a series of characterizations, it was found that abundance of weak basic sites in the form of OH was introduced by Ce, as well as the oxygen vacancies caused by the redox cycle of V4++Ce4+↔V5++Ce3+. The weak basic sites introduced by Ce could greatly enhance the Nap adsorption, and the Nap adsorbed was quickly converted to naphthol on Ce-OH. Furthermore, V existed at a high valence with the interaction of V and Ce, and the oxygen vacancies also increased the Oads and OOH. It improved the redox ability and the regeneration of Ce-OH on V-xCe/Ti catalysts. The intermediates could be further oxidized, and the Ce-OH consumed in the reaction could recover quickly. Therefore, almost 100% Nap conversion and a high COx selectivity was observed in the V-xCe/Ti catalysts system.

Enhanced formaldehyde oxidation over MnO2 and doped manganese-based catalysts: Experimental and theoretical Insights into mechanism and performance

Thermal catalytic degradation of formaldehyde (HCHO) over manganese-based catalysts is garnering significant attention. In this study, both theoretical simulations and experimental methods were employed to elucidate the primary reaction pathways of HCHO on the MnO2(110) surface. Specifically, the effects of doping MnO2 with elements such as Fe, Ce, Ni, Co, and Cu on the HCHO oxidation properties were evaluated. Advanced characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS), were employed to discern the physical properties and chemical states of the active components on the catalyst surface. The comprehensive oxidation pathway of HCHO on the MnO2(110) surface includes O2 adsorption and dissociation, HCHO adsorption and dehydrogenation, CO2 desorption, H2O formation and desorption, oxygen vacancy supplementation, and other elementary reactions. The pivotal rate-determining step was identified as the hydrogen migration process, characterized by an energy barrier of 234.19 kJ mol-1. Notably, HCHOO and *CHOO emerged as crucial intermediates during the reaction. Among the doped catalysts, Fe-doped MnO2 outperformed its counterparts doped with Ce, Ni, Co, and Cu. The optimal degradation rate and selectivity were achieved at a molar ratio of Fe: Mn = 0.1. The superior performance of the Fe-doped MnO2 can be ascribed to its large specific surface area, conducive pore structure for HCHO molecular transport, rich surface-adsorbed oxygen species, and a significant presence of oxygen vacancies.

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.

Catalytic-CO2-Desorption Studies of BZA-AEP Mixed Absorbent by the Lewis Acid Catalyst CeO2-γ-Al2O3

Traditional organic amines exhibit inferior desorption performance and high regeneration energy consumption. The implementation of solid acid catalysts presents an efficacious approach to mitigate regeneration energy consumption. Thus, investigating high-performance solid acid catalysts holds paramount importance for the advancement and implementation of carbon capture technology. This study synthesized two Lewis acid catalysts via an ultrasonic-assisted precipitation method. A comparative analysis of the catalytic desorption properties was conducted, encompassing these two Lewis acid catalysts and three precursor catalysts. The results demonstrated that the CeO₂-γ-Al₂O₃ catalyst demonstrated superior catalytic desorption performance. Within the desorption temperature range of 90 to 110 °C, the average desorption rate of BZA-AEP catalyzed by the CeO₂-γ-Al₂O₃ catalyst was 87 to 354% greater compared to the desorption rate in the absence of the catalyst, and the desorption temperature can be reduced by approximately 10 °C. A comprehensive analysis of the catalytic desorption mechanism of the CeO₂-γ-Al₂O₃ catalyst was conducted, and indicated that the synergistic effect of CeO₂-γ-Al₂O₃ conferred a potent catalytic influence throughout the entire desorption process, spanning from the rich solution to the lean solution.

Elucidating the facet-dependent reactivity of CrMn catalyst for selective catalytic reduction of NOx with NH3

The facet-dependent reactivity of CrMn catalysts was still unclear, hindering the further enhancement of their low-temperature SCR performance. Herein, the facet-dependent reactivity of CrMn_1.5O₄ catalyst for NH₃-SCR of NO_x was innovatively illustrated by numerous characterizations and density functional theory (DFT) calculations. Exposed (100) facet of CrMn_1.5O₄ catalyst exhibited best low-temperature SCR activity with ≥90 % NO conversion within 148-296 °C and 2.86 × 10^-3 mol/(g·s) reaction rate within 160-240 °C. The characterizations revealed that (100) facet could induce the increase of BET specific area, electron transfer, concentration of Mn⁴⁺ and O_α, surface acidity, redox ability, NH₃ and NO_x adsorption/activation capacity. Subsequently, DFT calculations demonstrated that (100) facet exhibited the strongest affinity for NH₃ and NO due to its unique 3O_3c-Mn_5c-2O_4c bond and abundant charges transfer near the active adsorption sites, and Brønsted acid and oxygen vacancies were most easily formed on (100) facet. Furthermore, H₂O formation as the rate determining step easily occurred on (100) facet. Eventually, we successfully improved the low-temperature SCR activity of CrMn_1.5O₄ catalyst by further tailoring highly active (100) facet from 0.754 to 0.865. This work provides the deeper understanding of facet-dependent reactivity and a good strategy to improve the catalytic activity of the catalysts.

Preparation of high temperature NH3-SCR catalysts with carbonate as precursors by ball milling method

High-temperature 10Ce-2La/TiO2 catalysts for selective catalytic reduction of NO with NH3 were prepared by the ball milling, impregnation and co-precipitation methods and their catalytic performance was compared. The effects of different starting materials of the ball milling method on the catalytic activity were investigated. The results showed that the 10Ce-2La/TiO2 catalyst prepared by the ball milling method using carbonates as starting materials exhibited the highest NO conversion, which was more than 80% in the temperature range of 330-550 °C. The as-prepared catalysts were characterized by XRD, TEM, and XPS. Results showed that the ball milling prepared 10Ce-2La/TiO2 had the advantages of uniform active site distribution, high oxygen storage capacity, and high Ce3+ and Oα ratio. The results of NH3-TPD and H2-TPR showed that the ball milling method not only improved the redox ability but also increased the quantities and concentration of the acidic sites. The green production and economically viable concept of this process provides a new solution for the production application of industrial catalysts.

NO Reduction Reaction by Kiwi Biochar-Modified MnO2 Denitrification Catalyst: Redox Cycle and Reaction Process

NO is a major environmental pollutant. MnO2 is often used as a denitrification catalyst with poor N2 selectivity and weak SO2 resistance. Kiwi twig biochar was chosen to modify MnO2 samples by using the hydrothermal method. The NO conversion rates of the biochar-modified samples were >90% at 125–225 °C. Kiwi twig biochar made the C2MnO2 sample with a larger specific surface area, a higher number of acidic sites and Oβ/Oα molar ratio, leading to more favorable activity at high temperatures and better SO2 resistance. Moreover, the inhibition of the NH3 oxidation reaction and the Mn3+ → Mn4+ process played a crucial role in the redox cycle. What was more, Brønsted acidic sites present on the C1MnO2 sample participate in the reaction more rapidly. This study identified the role of biochar in the reaction process and provides a reference for the wide application of biochar.

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.

Effects of different exposed crystal surfaces of CeO2 loaded on an MnO2/X catalyst for the NH3-SCR reaction

To study the effects of the loading of different exposed crystal surfaces of CeO2 on an MnO2/X catalyst for the NH3-selective catalytic reduction (SCR) reaction, Mn/X, Mn–CeNP/X, Mn–CeNC/X and Mn–CeNR/X catalysts were synthesized via a solid-state diffusion method.

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.

Effect of surface properties of TiO2 on the performance of Pt/TiO2 catalysts for furfural hydrogenation

Hydrogenation of biomass-derived furfural is an important process in biofuel production. Herein, different Pt-supported TiO₂ morphologies: nanorod (NR), nanoparticle (NP), and hollow microsphere (HMS) were prepared by the impregnation–chemical reduction method. The furfural conversion increased with an increase of Pt dispersion. However, cyclopentanone selectivity was affected by TiO₂ properties, the strong metal–support interaction (SMSI) effect, and the reaction conditions. The Pt/TiO₂ NR catalyst exhibited the highest cyclopentanone selectivity of 50.4%. Based on the H₂-temperature programmed desorption (H₂-TPD) and X-ray photoelectron spectroscopy (XPS) results, the Pt/TiO₂ NR catalyst showed a SMSI effect, which was introduced by the chemical reduction method. We suggest that electron charge transfer from Ti species to Pt in the Pt/TiO₂ NR catalyst affects the cyclopentanone selectivity by controlling the adsorption strength between the reactant and the Pt surface, thus retarding the formation of byproducts.

Hydrogenation of biomass-derived furfural is an important process in biofuel production.

Insight into the Morphology-Dependent Catalytic Performance of CuO/CeO2 Produced by Tannic Acid for Efficient Hydrogenation of 4-Nitrophenol

The construction of a heterogeneous nanocatalyst with outstanding catalytic performance via an environmentally benign and cost-effective synthetic category has long been one of the challenges in nanotechnology. Herein, we synthesized highly efficient and low-cost mesoporous morphology-dependent CuO/CeO₂ -Rods and CuO/CeO₂ -Cubes catalysts by employing a green and multifunctional polyphenolic compound (tannic acid) as the stabilizer and chelating agent for 4-nitrophenol (4-NP) reduction reaction. The CuO/CeO₂ -Rods exhibited excellent performance, of which the activity was 3.2 times higher than that of CuO/CeO₂ -Cubes. This can be connected with the higher density of oxygen vacancy on CeO₂ -Rods (110) than CeO₂ -Cubes (100), the oxygen vacancy favors anchoring CuO species on the CeO₂ support, which promotes the strong interaction between finely dispersed CuO and CeO₂ -Rods at the interfacial positions and facilitates the electron transfer from BH₄ ^- to 4-NP. The synergistic catalytic mechanism illustrated that 4-NP molecules preferentially adsorbed on the CeO₂ , while H₂ from BH₄ ^- dissociated over CuO to form highly active H* species, contributing to achieving efficient hydrogenation of 4-NP. This study is expected to shed light on designing and synthesizing cost-effective and high-performance nanocatalysts through a greener synthetic method for the areas of catalysis, nanomaterial science and engineering, and chemical synthesis.

Revealing the effect of paired redox-acid sites on metal oxide catalysts for efficient NOx removal by NH3-SCR

Understanding the nature of active sites on metal oxide catalysts in the selective catalytic reduction (SCR) of NO by NH₃ (NH₃-SCR) is a crucial prerequisite for the development of novel efficient NH₃-SCR catalysts. In this work, two CeO₂-based SCR catalyst systems with diverse acidic metal oxides-CeO₂ interfaces, i.e., Nb₂O₅-CeO₂ (Nb₂O₅/CeO₂ and CeO₂/Nb₂O₅) and WO₃-CeO₂ (WO₃/CeO₂ and CeO₂/WO₃), were prepared and used to reveal the relationship between NH₃-SCR activity and surface acidity/redox properties. In combination with the results of the NH₃-SCR activity test and various characterizations, it was found that the NH₃-SCR performance of Nb₂O₅-CeO₂ and WO₃-CeO₂ catalysts was highly dependent on the strong interactions between the redox component (CeO₂) and acidic component (Nb₂O₅ or WO₃), as well as the amount of paired redox-acid sites. From a quantitative perspective, an activity-surface acidity/redox property relationship was proposed. For both Nb₂O₅-CeO₂ and WO₃-CeO₂ catalysts systems operated at the more concerned low-temperature range (200 °C), the NH₃-SCR activity in low NO_x conversion region (< 40%) was mainly dominated by the surface acidity of catalysts, while the NH₃-SCR activity in high NO_x conversion region (> 40%) was more determined by redox properties.

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

Investigating catalytic reaction mechanisms could help guide the design of catalysts. Here, aimed at improving both the catalytic performance and SO₂ resistance ability of catalysts in the selective reduction of NO by NH₃ (NH₃-SCR), an innovative CeO₂-SiO₂ mixed oxide catalyst (CeSi2) was developed based on our understanding of both the sulfur poisoning and reaction mechanisms, which exhibited excellent SO₂/H₂O resistance ability even in the harsh working conditions (containing 500 ppm of SO₂ and 5% H₂O). The strong interaction between Ce and Si (Ce-O-Si) and the abundant surface hydroxyl groups on CeSi2 not only provided fruitful surface acid sites but also significantly inhibited SO₂ adsorption. The NH₃-SCR performance of CeSi2 was promoted by an enhanced Eley-Rideal (E-R) mechanism in which more active acid sites were preserved under the reaction conditions and gaseous NO could directly react with adsorbed NH₃. This mechanism-enhanced process was even further promoted on sulfated CeSi2. This work provides a reaction mechanism-enhanced strategy to develop an environmentally friendly NH₃-SCR catalyst with superior SO₂ resistance.

Enhancing the K resistance of CeTiOx catalyst in NH3-SCR reaction by CuO modification

It is generally accepted that CeTiO_x catalyst owns outstanding catalytic activity for ammonia-selective catalytic reduction (NH₃-SCR), but the tolerance to alkali metals is still dissatisfactory. Thus, it is of great importance to further elevate the catalytic activity and resistance to alkali metals of CeTiO_x catalyst. In our work, a series of CeTiO_x, CuO/CeTiO_x, K-CeTiO_x and K-CuO/CeTiO_x catalysts were prepared to comprehensively analyze the influence of CuO modification on the physicochemical features, catalytic activity and anti-K ability of CeTiO_x catalyst. The results manifest that CuO modification effectively enhances low-temperature catalytic activity and anti-K poisoning ability of CeTiO_x catalyst by protecting the reduction ability and the surface acidity as well as weakening the adsorption strength of NO_x.

Application of ReOx/TiO2 catalysts with excellent SO2 tolerance for the selective catalytic reduction of NOx by NH3

The adsorption of SO₂ on ReO_x/TiO₂ catalysts was rather weak; thus, ReO_x/TiO₂ catalysts exhibited excellent SO₂ tolerance in the NH₃-SCR reaction.

Nanosized V-Ce Oxides Supported on TiO2 as a Superior Catalyst for the Selective Catalytic Reduction of NO

Nanosized V-Ce oxides supported on TiO2 (VCT) were prepared and utilized in the low-temperature selective catalytic reduction (SCR) of NO with NH3. Compared with the other V-Ce oxides-based catalysts supported on Al2O3, ZrO2, and ZSM-5, VCT showed the best SCR activity in a low-temperature range. The NOx conversion of 90% could be achieved at 220 °C. Characterizations including X-ray diffraction (XRD), scanning election micrograph (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption with NH3 (NH3-TPD), and temperature-programmed reduction with H2 (H2-TPR) showed that V1.05Ce1/TiO2 exhibited a good dispersion of V2O5, enrichment of surface Ce3+ and chemical-absorbed oxygen, and excellent redox capacity and acidity, which resulted in the best SCR performance at low temperature.

In Situ DRIFTS Investigation on CeOx Catalyst Supported by Fly-Ash-Made Porous Cordierite Ceramics for Low-Temperature NH3-SCR of NOX

A series of CeOx catalysts supported by commercial porous cordierite ceramics (CPCC) and synthesized porous cordierite ceramics (SPCC) from fly ash were prepared for selective catalytic reduction of NOx with ammonia (NH3-SCR). A greater than 90% NOx conversion rate was achieved by the SPCC supported catalyst at 250–300 °C when the concentration of loading precursor was 0.6 mol/L (denoted as 0.6Ce/SPCC), which is more advantageous than the CPCC supported ones. The EDS mapping results reveal the existence of evenly distributed impurities on the surface of SPCC, which hence might be able to provide more attachment sites for CeOx particles. Further measurements with temperature programmed reduction by hydrogen (H2-TPR) demonstrate more reducible species on the surface of 0.6Ce/SPCC, thus giving rise to better NH3-SCR performance at a low-temperature range. The X-ray photoelectron spectroscopy (XPS) analyses reveal that the Ce atom ratio is higher in 0.6Ce/SPCC, indicating that a higher concentration of catalytic active sites could be found on the surface of 0.6Ce/SPCC. The in situ diffused reflectance infrared fourier transform spectroscopy (DRIFTS) results indicate that the SCR reactions over 0.6Ce/SPCC follow both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. Hence, the SPCC might be a promising candidate to provide support for NH3-SCR catalysts, which also provide a valuable approach to recycling the fly ash.

Development of cerium-based catalysts for selective catalytic reduction of nitrogen oxides: a review

Nitrogen oxides (NO_X) are major pollutants of the atmosphere, and selective catalytic reduction of nitrogen oxides using ammonia as a reductant (NH₃-SCR) is an effective method to remove nitrogen oxides.

Doping effect of Sm on the TiO2/CeSmOx catalyst in the NH3-SCR reaction: structure–activity relationship, reaction mechanism and SO2 tolerance

A good balance between the redox properties and surface acidity induces the high activity of the Sm doped TiO₂/CeO₂ catalyst.