Selective Catalytic Reduction of NOx over Perovskite-Based Catalysts Using CxHy(Oz), H2 and CO as Reducing Agents-A Review of the Latest Developments

Selective catalytic reduction (SCR) is probably the most widespread process for limiting NO_x emissions under lean conditions (O₂ excess) and, in addition to the currently used NH₃ or urea as a reducing agent, many other alternative reductants could be more promising, such as C_xH_y/C_xH_yO_z, H₂ and CO. Different catalysts have been used thus far for NO_x abatement from mobile (automotive) and stationary (fossil fuel combustion plants) sources, however, perovskites demand considerable attention, partly due to their versatility to combine and incorporate various chemical elements in their lattice that favor deNO_x catalysis. In this work, the C_xH_y/C_xH_yO_z⁻, H₂⁻, and CO-SCR of NO_x on perovskite-based catalysts is reviewed, with particular emphasis on the role of the reducing agent nature and perovskite composition. An effort has also been made to further discuss the correlation between the physicochemical properties of the perovskite-based catalysts and their deNO_x activity. Proposed kinetic models are presented as well, that delve deeper into deNO_x mechanisms over perovskite-based catalysts and potentially pave the way for further improving their deNO_x efficiency.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Oxidation of soot over supported RuRe nanoparticles prepared by the microwave-polyol method

The oxidation of soot over RuRe bimetallic nanoparticles (NPs) supported on γ-Al2O3 has been investigated. The catalysts were synthesized by a microwave-polyol method and characterized by ICP, BET, TEM, STEM-EDS, XRD and XPS techniques. The study revealed that the proper choice of the Re loading (0.4–2.0 wt%) is crucial for the catalytic behavior of the 2% Ru–Re/Al2O3 nano-catalysts.The best catalytic properties, in terms of overall activity and stability, were observed for the 2%Ru-0.8%Re/γ-Al2O3 nano-catalyst. The stability of all bimetallic 2% Ru–Re nano-catalysts in catalytic soot oxidation in the presence of oxygen is very high in contrast to the 2% Ru/γ-Al2O3 sample. The presence of rhenium in the catalytic system hinder the formation of large RuO2 agglomerates leading to a better dispersion of active ruthenium phase and a better catalytic performance. The relationship between the catalytic activity of Ru–Re/γ-Al2O3 and the synergetic roles of Ru and Re is discussed.

Catalytic Converters for Vehicle Exhaust: Fundamental Aspects and Technology Overview for Newcomers to the Field

This works aims to provide an understanding on basic chemical kinetics pertaining to three-way catalytic (TWC) converters from an educational perspective, aimed at those novel readers in this field. Rate of reactions and its factors are explained, showcasing that the chosen catalyst is the main factor affecting the overall rate of reaction. Furthermore, this overview revisit insights of the catalytic converter structure and the environmental issues that come along with it. Lastly, the chemical and physical properties of the reactants and products-pollutant and less-toxic gases—are discussed, in order to gather a better understanding of the reactants and products that enters a catalytic converter.

The pure and representative types of disordered platinum nanoparticles from machine learning

The development of interpretable structure/property relationships is a cornerstone of nanoscience, but can be challenging when the structural diversity and complexity exceeds our ability to characterise it. This is often the case for imperfect, disordered and amorphous nanoparticles, where even the nomenclature can be unspecific. Disordered platinum nanoparticles have exhibited superior performance for some reactions, which makes a systematic way of describing them highly desirable. In this study we have used a diverse set of disorder platinum nanoparticles and machine learning to identify the pure and representative structures based on their similarity in 121 dimensions. We identify two prototypes that are representative of separable classes, and seven archetypes that are the pure structures on the convex hull with which all other possibilities can be described. Together these nine nanoparticles can explain all of the variance in the set, and can be described as either single crystal, twinned, spherical or branched; with or without roughened surfaces. This forms a robust sub-set of platinum nanoparticle upon which to base further work, and provides a theoretical basis for discussing structure/property relationships of platinum nanoparticles that are not geometrically ideal.

Reactive Fe-O-Ce Sites in Ceria Catalysts for Soot Oxidation

This study investigates the role of oxygen vacancy on Fe-doped CeO2 catalyst activity for soot oxidation. The oxygen vacancy was assessed through Ce3+ content. The Fe content was varied between 0 and 30% for two catalyst preparation methods, co-precipitation (CP) and solution combustion synthesis (SCS). X-ray photoelectron spectroscopy indicates that ceria exists as both Ce4+ and Ce3+, while iron is present only as Fe3+. The catalyst’s activity was evaluated by ignition (T10) and combustion (T50) temperatures using thermogravimetric analysis. Optimum Fe contents yielding the highest activity were found to be 10% and 5% for CP and SCS catalysts, respectively. The surface area and morphology showed a moderate effect on catalyst activity, because catalytic soot oxidation involves solid–solid contact. More importantly, regardless of the fabrication method, it was found that Ce3+ content, which is closely related to oxygen vacancies, plays the most important role in affecting the catalyst activity.

NO Removal by Plasma-Enhanced NH3-SCR Using Methane as an Assistant Reduction Agent at Low Temperature

The effects of using CH4 as an assistant reduction agent in plasma-assisted NH3–SCR were investigated. The new hybrid reaction system performed better than DBD–NH3–SCR when the O2 concentration varied from 2% to 12%. Compared with DBD–NH3–SCR, DBD–NH3–CH4–SCR (NH3:CH4 = 1:1) showed a more significant promotion effect on the performance and N2 selectivity for NOX abatement. When the O2 concentration was 6% and the SIE was 512 J/L, the NO removal efficiency of the new hybrid system reached 84.5%. The outlet gas components were observed via FTIR to reveal the decomposition process and its mechanism. This work indicated that CH4, as an assistant agent, enhances DBD–NH3–SCR in excess oxygen to achieve a new process with significantly higher activity at a low temperature (≤348 K) for NOX removal.

Shape dependence of support for NOx storage and reduction catalysts

Pt/BaO/Al₂O₃ catalysts with different BaO loadings prepared from Al₂O₃ nanorods (Pt/BaO/Al₂O₃-nr) and irregular Al₂O₃ nanoparticles (Pt/BaO/Al₂O₃-np) were investigated for NO_x storage and reduction (NSR). The Pt/BaO/Al₂O₃ materials derived from Al₂O₃ nanorods always exhibited much higher NO_x storage capacity (NSC) over the whole temperature range of 100-400°C than the corresponding Pt/BaO/Al₂O₃-np samples containing the same BaO loading, giving the maximum NSC value of 966.9 μmol/g_cat at 400°C, 1.4 times higher than that of Pt/BaO/Al₂O₃-np. Higher catalytic performance of nanorod-supported NSR samples was also observed during lean-rich cyclic conditions (90 sec vs. 5 sec), giving more than 98% NO_x conversion at 300-450°C over the Pt/BaO/Al₂O₃-nr sample with 15% BaO loading. To reveal this dependence on the shape of the support during the NSR process, a series of characterization techniques including the Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H₂ temperature programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were also conducted. It was found that intimate contact of Ba-Al and Ba-Pt sites was achieved over the Pt/BaO/Al₂O₃ surface when using Al₂O₃-nr as a support. This strong interaction among the multi-components of Pt/BaO/Al₂O₃-nr thus triggered the formation of surface nitrite and nitrate during the lean period, and also accelerated the reverse spillover of ad-NO_x species onto the Pt surface, enhancing their reduction and leading to high NSR performance.

Direct observation of atomically-resolved silver species on a silver alumina catalyst active for selective catalytic reduction of nitrogen oxides

We characterize the size and state of the silver species in a 2 wt% silver alumina catalyst, which is highly active for the selective catalytic reduction of nitrogen oxides with ammonia or hydrocarbons as reductant.

Correlating anisotropy and disorder with the surface structure of platinum nanoparticles

Due to the competition between numerous physicochemical variables during formation and processing, platinum nanocatalysts typically contain a mixture of shapes, distributions of sizes, and a considerable degree of surface imperfection. Structural imperfection and sample polydispersivity are inevitable at scale, but accepting bulk and surface diversity as legitimate design features provides new opportunities for nanoparticle design. In recent years disorder and anisotropy have been embraced as useful design parameters but predicting the impact of uncontrollable imperfection a priori is challenging. In the present work we have created an ensemble of uniquely imperfect nanoparticles extracted from classical molecular dynamics trajectories and applied statistical filters to restrict the ensemble in ways that reflect common industrial design principles. We find that targeting different sizes and size distributions may be an effective way of promoting or suppressing internal disorder or crystallinity (as required), but the degree of anisotropy of the particle as a whole has a greater impact on the population of different types of surface ordering and active sites. These results indicate that tuning of disordered and anisotropic Pt nanoparticles is possible, but it is not as straightforward as geometrically ideal nanoparticles with a high degree of crystallinity. It is unlikely that a synthesis strategy could eliminate this diversity entirely, or ensure this type of structural complexity does not develop post-synthesis under operational conditions, but it may be possible to bias the formation of specific bulk structures and the surface anisotropy.

Nanostructured Ceria-Based Materials: Effect of the Hydrothermal Synthesis Conditions on the Structural Properties and Catalytic Activity

In this work, several nanostructured ceria catalysts were prepared by means of a hydrothermal procedure, in which the synthesis conditions (i.e., temperature and pH values) were varied. CeO2 samples of different shapes and structural properties were obtained, namely cubes, rods, cube and nanorod mixtures, and other polyhedra. The prepared materials were tested using four probe catalytic reactions: CO oxidation, NO oxidation, NOx-free soot oxidation, and NOx-assisted soot oxidation. The physicochemical properties of the prepared catalysts were studied by means of complementary techniques (i.e., XRD, N2-physisorption at −196 °C, CO-TPR (temperature-programmed reduction), field emission scanning electron microscopy (FESEM), micro-Raman spectroscopy). The abundance of defects of the catalysts, measured through in-situ Raman spectroscopy at the typical temperatures of each catalytic process, was correlated to the CO and NO oxidation activity of the prepared catalysts, while the soot oxidation reaction (performed in loose conditions), which was hindered by a poor soot-catalyst contact, was found to be less sensitive to the observed structural defects.

The mechanism of selective catalytic reduction of NOx on Cu-SSZ-13 - a computational study

The copper-exchanged aluminosilicate zeolite SSZ-13 is a leading catalyst for the selective catalytic reduction of NO. Density functional theory calculations are used to construct a complete catalytic cycle of this process paying special attention to the coordination geometries and redox states of copper. N₂ can be produced in the reduction half-cycle via a nitrosamine intermediate generated from the reaction of the additive reductant NH₃ with a NO⁺ intermediate stabilized by the zeolite lattice. The decomposition of this nitrosamine species can be assisted by incipient Brønsted acid sites generated during catalysis. Our calculations also suggest that the reoxidation of Cu(i) to Cu(ii) requires the addition of both NO and O₂. The production of a second equivalent of N₂ during the oxidation half-cycle proceeds through a peroxynitrite intermediate to form a Cu-nitrite intermediate, which may react with an acid, either HNO₂ or NH₄⁺ to close the catalytic cycle. Models of copper neutralized by an external hydroxide ligand are also examined. These calculations form a key basis for understanding the mechanism of NO reduction in Cu-SSZ-13 in order to develop strategies for rationally optimizing the performance in future experiments.

Catalytic oxidation of nitric oxide (NO) over different catalysts: an overview

Nitrogen oxides (mainly NO) are one of the major air pollutants that lead to a number of environmental problems such as photochemical smog, acid rain and haze.

Enhanced activity of CuO/K2CO3/MgAl2O4catalyst for lean NOxstorage and reduction at high temperatures

The CuO/K₂CO₃/MgAl₂O₄catalyst can store and reduce NO_xefficiently in alternative lean/rich atmospheres at high operating temperatures.

It is no laughing matter: nitrous oxide formation in diesel engines and advances in its abatement over rhodium-based catalysts

N₂O appears as one of the undesired by-products in exhaust gases emitted from diesel engine aftertreatment systems, such as diesel oxidation catalysts (DOC), lean NO_x trap (LNT, also known as NO_x storage and reduction (NSR)) or selective catalytic reduction (NH₃-SCR and HC-SCR) and ammonia slip catalysts (ASC, AMOX, guard catalyst).

Control of cerium oxidation state through metal complex secondary structures

A series of alkali metal cerium diphenylhydrazido complexes, M _x (py) _y [Ce(PhNNPh)₄], M = Li, Na, and K, x = 4 (Li and Na) or 5 (K), and y = 4 (Li), 8 (Na), or 7 (K), were synthesized to probe how a secondary coordination sphere would modulate electronic structures at a cerium cation. The resulting electronic structures of the heterobimetallic cerium diphenylhydrazido complexes were found to be strongly dependent on the identity of the alkali metal cations. When M = Li⁺ or Na⁺, the cerium(iii) starting material was oxidized with concomitant reduction of 1,2-diphenylhydrazine to aniline. Reduction of 1,2-diphenylhydrazine was not observed when M = K⁺, and the complex remained in the cerium(iii) oxidation state. Oxidation of the cerium(iii) diphenylhydrazido complex to the Ce(iv) diphenylhydrazido one was achieved through a simple cation exchange reaction of the alkali metals. UV-Vis spectroscopy, FTIR spectroscopy, electrochemistry, magnetic susceptibility, and DFT studies were used to probe the oxidation state and the electronic changes that occurred at the metal centre.