Kinetics and H2O Influence on NOx Trapping and Selective Catalytic Reduction over Ce/Pd Doping Catalyst

In this paper, the removal effects and activation energy of Ce and Pd doping on pollutants (CO, C3H6, and NO) were comparatively analyzed by using characterization methods and constructed kinetic equations. Furthermore, the problems of the water influence mechanism on the NSR process were also discussed. The results show the following: (1) Pd doping effectively improves the removal of CO (80%) and C3H6 (71%) in the low-temperature section of the catalyst (150-250 °C) compared to Ce doping, while Ce doping exhibits excellent low-temperature conversion of NO. (2) The reaction activation energy of the LaKMnPdO3 catalyst was 9784 kJ/mol, which was significantly lower than that of the LaKMnCeO3 catalyst. (3) The presence of H2O has an important enhancement effect in the storage performance of the LaKMnPdO3 catalyst for NOx but decreases the catalytic reduction of NO. It provides a solution for the effective treatment of the increasing problems of particulate matter and ozone pollution.

Electrification-Enhanced Low-Temperature NOx Storage-Reduction on Pt and K Co-Supported Antimony-Doped Tin Oxides

NOx storage-reduction (NSR), a promising approach for removing NOx pollutants from diesel vehicles, remains elusive to cope with the increasingly lower exhaust temperatures (especially below 250 °C). Here, we develop a conceptual electrified NSR strategy, where electricity with a low input power (0.5-4 W) is applied to conductive Pt and K co-supported antimony-doped tin oxides (Pt-K/ATO), with C3H6 as a reductant. The ignition temperature for 10% NOx conversion is nearly 100 °C lower than that of the traditional thermal counterpart. Furthermore, reducing the power in the fuel-lean period relative to that in the fuel-rich period increases the maximum energy efficiency by 23%. Electrically driven release of lattice oxygen is revealed to play vital roles in multiple steps in NSR, including NO adsorption, desorption, and reduction, for improved NSR activity. This work provides an electrification strategy for developing high-activity NSR catalysis utilizing electricity onboard hybrid vehicles.

Protection of NOx Sensors from Sulfur Poisoning in Glass Furnaces by the Optimization of a "SO2 Trap"

Electrochemical NOx sensors based on yttria-stabilized zirconia (YSZ) provide a reliable onboard way to control NOx emissions from glass-melting furnaces. The main limitation is the poisoning of this sensor by sulfur oxides (SOx) contained in the stream. To overcome this drawback, an "SO2 trap" with high SOx storage capacity and low affinity to NOx is required. Two CuO/BaO/SBA-15 traps with the same CuO loading (6.5 wt.%) and different BaO loadings (5 and 24.5 wt.%, respectively) were synthetized, thoroughly characterized and evaluated as SO2 traps. The results show that the 6.5%CuO/5%BaO/SBA-15 trap displays the highest SO2 adsorption capacity and can fully adsorb SO2 for a specific period of time, while additionally displaying a very low NO adsorption capacity. A suitable quantity of this material located upstream of the sensor could provide total protection of the NOx sensor against sulfur poisoning in glass-furnace exhausts.

CuCeOx/CuO Catalyst Derived from the Layered Double Hydroxide Precursor: Catalytic Performance in NO Reduction with CO in the Presence of Water and Oxygen

Valencies of metal species and lattice defects, such as oxygen vacancies, play a pivotal role in metal oxide-catalyzed reactions. Herein, we report a promising synthetic strategy for preparing CuO-supported CuCeO_x catalysts (CuCeO_x/CuO) by calcination of a hydrotalcite precursor [Cu₆Ce₂(OH)₁₆]CO₃·nH₂O. The structural and chemical properties of catalysts were characterized by XRD, ICP-AES, TEM, TPR, NH₃-TPD, XPS, Raman spectroscopy, and N₂ adsorption, which revealed that the thermal pretreatment in an oxidative atmosphere caused segregation and reconstitution processes of the precursor, resulting in a mesoporous catalyst consisting of well-dispersed CuO-supported CuCeO_x clusters of 1.8-3.2 nm in size with a high population of oxygen vacancies. The as-prepared catalyst shows excellent catalytic performance in the reduction of NO by CO in the absence as well as in the presence of water and oxygen. This behavior is attributed to its high oxygen defect concentration facilitating the interplay of the redox equilibria between Cu²⁺ and reduced copper species (Cu⁺/Cu⁰) and (Ce⁴⁺/Ce³⁺). The high surface population of oxygen vacancies and in situ-generated metallic copper species have been evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. The layered double hydroxide-derived CuCeO_x/CuO also showed good water tolerance and long-term stability. In situ infrared spectroscopy investigations indicated that adsorbed hyponitrite species are the main reaction intermediates of the NO conversion as also corroborated by theoretical simulations.

CuO decorated vacancy-rich CeO2 nanopencils for highly efficient catalytic NO reduction by CO at low temperature

With the rapid development of transportation and vehicles, the elimination of NO_x and CO has highly attracted public attention. In this work, vacancy-rich CeO₂ nanopencil supported CuO catalysts (CuO/CeO₂-NPC) were successfully prepared for NO reduction by CO. Importantly, CeO₂ with nanopencil-like shape (CeO₂-NPC) have been synthesis by solvothermal method for the first time. The physicochemical properties of all samples were studied in detail by combining the means of X-ray diffraction (XRD), Raman spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (H₂-TPR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N₂ physisorption (Brunauer-Emmett-Teller), and NO and CO temperature-programmed desorption (NO-TPD and CO-TPD) techniques. Compared with CeO₂ nanorods and nanoparticles supported CuO catalysts (CuO/CeO₂-NR and CuO/CeO₂-NP), the CuO/CeO₂-NPC catalysts showed the highest catalytic activity, affording more than 90% NO conversion at 69 °C as well as excellent H₂O tolerance at 150 °C, which is superior to catalysts previously reported. Characterization results indicated that the synergistic effect between the well-dispersed CuO and the CeO₂ nanopencil support enables a favorable electron transfer between these components and enhances the density of surface oxygen vacancies and Cu⁺ species, which consequently accelerating the redox cycle. The results indicated that the morphology control of CeO₂ support could be an efficient way to evidently enhance the catalytic performance for NO + CO reaction.

Fabrication of Multifunctional Electronic Textiles Using Oxidative Restructuring of Copper into a Cu-Based Metal-Organic Framework

This paper describes a novel synthetic approach for the conversion of zero-valent copper metal into a conductive two-dimensional layered metal-organic framework (MOF) based on 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) to form Cu₃(HHTP)₂. This process enables patterning of Cu₃(HHTP)₂ onto a variety of flexible and porous woven (cotton, silk, nylon, nylon/cotton blend, and polyester) and non-woven (weighing paper and filter paper) substrates with microscale spatial resolution. The method produces conductive textiles with sheet resistances of 0.1-10.1 MΩ/cm², depending on the substrate, and uniform conformal coatings of MOFs on textile swatches with strong interfacial contact capable of withstanding chemical and physical stresses, such as detergent washes and abrasion. These conductive textiles enable simultaneous detection and detoxification of nitric oxide and hydrogen sulfide, achieving part per million limits of detection in dry and humid conditions. The Cu₃(HHTP)₂ MOF also demonstrated filtration capabilities of H₂S, with uptake capacity up to 4.6 mol/kg_MOF. X-ray photoelectron spectroscopy and diffuse reflectance infrared spectroscopy show that the detection of NO and H₂S with Cu₃(HHTP)₂ is accompanied by the transformation of these species to less toxic forms, such as nitrite and/or nitrate and copper sulfide and S_x species, respectively. These results pave the way for using conductive MOFs to construct extremely robust electronic textiles with multifunctional performance characteristics.

Effects of substituting iron for aluminum on the low-temperature catalytic activity and sulfur resistance of hydrotalcite-derived LNT catalysts

The storage and reduction of NOx on a series of Fe-modified hydrotalcite-based lean NOx trap catalysts were assessed, together with the product selectivity. The crystal structures and micromorphologies of these materials were characterized using X-ray diffraction and scanning electron microscopy, while in situ diffuse reflectance Fourier transform infrared spectroscopy was used to evaluate the evolution of transition state species. The introduction of Fe was found to improve the synergistic interaction between the Mg and Fe in the hydrotalcite structure, allowing these catalysts to work efficiently at low temperatures. In addition, both Pt/BaO/MgAlO and Pt/BaO/MgFeO catalysts exhibited better NOx adsorption and reduction performance compared with Pt/BaO/Al₂O₃. The superior performance of the former two materials was attributed to the enhanced adsorption of NOx in the form of nitrates and nitrites by Fe and Mg and to the ready decomposition of these nitrates at low temperatures. A Pt/BaO/MgFeO catalyst showed excellent low temperature activity and high selectivity for N₂ together with superior sulfur resistance compared with Pt/BaO/Al₂O₃.

Contributions of Washcoat Components in Different Configurations to the NOX and Oxygen Storage Performance of LNT Catalysts

In addition to SCR systems, lean NOX traps (LNTs) are also used for exhaust aftertreatment of lean burn internal combustion engines to sustainably reduce NOX emissions. Modern LNTs consist of different functional compounds to maximize the performance during NOX storage and regeneration. Based on the material analysis of a serial production LNT, PGM loaded BaO, Al2O3, MgAl2O4, and CeO2 were identified as the main base materials. In this paper, the NOX storage capacity (NSC) of these compounds is investigated both as single catalysts and as physical mixtures to identify possible synergistic effects. Therefore, commercially available support materials were loaded with Platinum and tested in granular form under realistic conditions. To optimize the performance by reducing the diffusion pathways for NOX molecules during storage, PGM, BaO, and Ceria were combined in a composite by the incipient wetness impregnation of alumina. As a result, the temperature dependent NSC of the commercial LNT could be reached with the Pt/Rh/Ba10Ce25/Al2O3 infiltration composite, while reducing the oxygen storage capacity by about 45%. Without the additional Rhodium coating, the low-temperature NSC was insufficient, highlighting the important contribution of this precious metal to the overall performance of LNTs.

Mesoporous LaCoO3 perovskite oxide with high catalytic performance for NOx storage and reduction

A mesoporous LaCoO₃ perovskite oxide (LaCoO₃-Meso) with three-dimensionally ordered helical interwoven structure was synthesized by a nano-casting method using KIT-6 as the hard template. The obtained LaCoO₃-Meso with high surface area was tested for its catalytic performance in the NO_x storage and reduction (NSR) reaction and compared with a sample synthesized by the conventional sol-gel method. The LaCoO₃-Meso showed a significant advantage for NO_x storage, with a NO_x storage capacity 2 times higher than the regular sample. LaCoO₃-Meso also exhibited improved NSR catalytic performance in the 150-450 °C temperature range, especially within 350-400 °C, where the NO_x conversion was raised for 40%. The results of X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements suggested the presence of a high concentration of oxygen defects on the LaCoO₃-Meso surface. Further results provided by temperature programmed reduction and temperature programmed desorption indicated that the oxygen defects not only increase the amount of trapped NO_x, but also improve the low-temperature redox performance of the catalyst. The lower stability of NO_x species adsorbed on oxygen defects promotes the NO_x release step in the NSR reaction and benefits the regeneration of storage sites.

Preparation of 3DOM ZrTiO4 Support, WxCeMnOδ/3DOM ZrTiO4 Catalysts, and Their Catalytic Performance for the Simultaneous Removal of Soot and NOx

As an efficient and durable engine, a diesel engine has a broad application. However, soot particles (PM) and nitrogen oxides (NO_x) coming from diesel engines are the main causes of air pollution, so it is necessary to design and prepare an effective catalyst for the simultaneous elimination of PM and NO_x. In this work, a novel 3DOM ZrTiO₄ support and a series of W_xCeMnO_δ/3DOM ZrTiO₄ catalysts (where x indicates the wt% of W) were designed and fabricated by the colloidal crystal template technique. Among the as-prepared catalysts, the W₁CeMnO_δ/3DOM ZrTiO₄ catalyst exhibits the highest NO conversion rate (52%) at the temperature of maximum CO₂ concentration (474°C) and achieves 90% NO conversion in the temperature range of 250–396°C. The excellent catalytic performance is associated with the macroporous structure, abundant oxygen vacancies, sufficient acid sites, and the synergistic effect among the active components. The possible reaction mechanisms of W_xCeMnO_δ/3DOM ZrTiO₄ catalysts were also discussed based on the characterization results.

Experimental and numerical investigation of NO oxidation on Pt/Al2O3- and NOx storage on Pt/BaO/Al2O3-catalysts

Effects of temperature and inlet conditions on NO oxidation and NOx storage, as well as reduced NOx storage capacity over time – reflected by changes of measured NO concentration, which are reproduced by CFD using detailed reaction mechanisms.

Effect of the Configuration of Copper Oxide-Ceria Catalysts in NO Reduction with CO: Superior Performance of a Copper-Ceria Solid Solution

Various copper-ceria-based composites have attracted attention as efficient catalysts for the reduction of NO with CO. In this comparative study, we have examined the catalytic potential of different configurations of copper oxide-ceria catalysts, including catalysts based on a copper-ceria solid solution, copper oxide particles supported on ceria, and ball-milled copper oxide-ceria. The structurally different interfaces between the constituents of these catalysts afforded very different catalytic performances. The solid solution catalyst outperformed the corresponding ceria-supported and ball-milled CuO-CeO₂ catalysts. The copper cations incorporated into the ceria lattice strongly improved the activity, N₂ selectivity, and water vapor tolerance compared to the other catalyst configurations. The experimental observations are supported by first-principles density functional theory (DFT) studies of the reaction pathway, which indicate that the incorporation of Cu cations into the ceria matrix lowers the energy required for activating the lattice oxygen, thereby enhancing the formation and healing of oxygen vacancies, and thus promoting NO reduction with CO.

NOx Storage on BaTi0.8Cu0.2O3 Perovskite Catalysts: Addressing a Feasible Mechanism

The NOx storage mechanism on BaTi_0.8Cu_0.2O₃ catalyst were studied using different techniques. The results obtained by XRD, ATR, TGA and XPS under NOx storage-regeneration conditions revealed that BaO generated on the catalyst by decomposition of Ba₂TiO₄ plays a key role in the NOx storage process. In situ DRIFTS experiments under NO/O₂ and NO/N₂ show that nitrites and nitrates are formed on the perovskite during the NOx storage process. Thus, it seems that, as for model NSR catalysts, the NOx storage on BaTi_0.8Cu_0.2O₃ catalyst takes place by both "nitrite" and "nitrate" routes, with the main pathway being highly dependent on the temperature and the time on stream: (i) at T < 350 °C, NO adsorption leads to nitrites formation on the catalyst and (ii) at T > 350 °C, the catalyst activity for NO oxidation promotes NO₂ generation and the nitrate formation.

Reduction of NOx by H2 on WOx-Promoted Pt/Al2O3/SiO2 Catalysts Under O2-Rich Conditions

This work addresses the reduction of NOx by H2 under O2-rich conditions using Al2O3/SiO2-supported Pt catalysts with different loads of WOx promotor. The samples were thoroughly characterised by N2 physisorption, temperature-programmed desorption of CO, scanning electron microscopy, X-ray diffraction, laser raman spectroscopy, X-ray photoelectron spectroscopy and diffuse reflectance infrared fourier transform spectroscopy with probe molecule CO. The catalytic studies of the samples without WOx showed pronounced NOx conversion below 200 °C, whereas highest efficiency was related to small Pt particles. The introduction of WOx provided increasing deNOx activity as well as N2 selectivity. This promoting effect was referred to an additional reaction path at the Pt-WOx/Al2O3/SiO2 interface, whereas an electronic activation of Pt by strong metal support interaction was excluded.

Graphic Abstract

Core-shell PdAu nanocluster catalysts to suppress sulfur poisoning

Reducing sulfur poisoning is significant for maintaining the catalytic efficiency and durability of heterogeneous catalysts. We screened PdAu nanoclusters with specific Pd : Au ratios based on Monte Carlo simulations and then carried out density functional calculations to reveal how to reduce sulfur poisoning via alloying. Among various nanoclusters, the core-shell structure Pd13Au42 (Pd@Au) exhibits a low adsorption energy of SO2 (-0.67 eV), comparable with O2 (-0.45 eV) and lower than CO (-1.25 eV), thus avoiding sulfur poisoning during the CO catalytic oxidation. Fundamentally, the weak adsorption of SO2 originates from the negative d-band center of the shell and delocalized charge distribution near the Fermi level, due to the appropriate charge transfer from the core to shell. Core-shell nanoclusters with a different core (Ni, Cu, Ag, Pt) and a Pd@Au slab model were further constructed to validate and extend the results. These findings provide insights into designing core-shell catalysts to suppress sulfur poisoning while optimizing catalytic behaviors.

NOx Storage Performance at Low Temperature over Platinum Group Metal-Free SrTiO3-Based Material

Pt-based catalysts are commonly employed as NOx-trapping catalysts for automobiles, while perovskite oxides have received attention as Pt-free NOx-trapping catalysts. However, the NOx storage performance of perovskite catalysts is significantly inferior at low temperatures and with coexisting gases such as H2O, CO2, and SO2. This study demonstrates that NOx storage reactions proceed over redox site (Mn, Fe, and Co)-doped SrTiO3 perovskites. Among the examined catalysts, Mn-doped SrTiO3 exhibited the highest NOx storage capacity (NSC) and showed a high NSC even at a low temperature of 323 K. Moreover, the high NOx storage performance of Mn-doped SrTiO3 was retained in the presence of poisoning gases (H2O, CO2, and SO2). NO oxidation experiments revealed that the NSC of Co-doped SrTiO3 was dependent on the NO oxidation activity from NO to NO2 via lattice oxygen, which resulted in an inferior NSC at low temperatures. On the other hand, Mn-doped SrTiO3 successfully adsorbed NO molecules onto its surface at 323 K without the NO oxidation process using lattice oxygens. This unique adsorption behavior of Mn-doped SrTiO3 was concluded to be responsible for the high NSC in the presence of poisoning gases.

A Robust Mesoporous Al2 O3 -Based Nanocomposite Catalyst for Abundant NOx Storage with Rational Design of Pt and Ba Species

The nanostructural design of heterogeneous catalysts has often been demanded for assessing synergetic effects, which should be developed further by using high-surface-area porous metal oxide supports. However, such opportunities have been undermined by the poor stability of ordered mesoporous structures. Herein, rational design is demonstrated to obtain nanocomposite catalysts showing improved NO_x storage properties owing to the presence of Ba species over a well-designed mesoporous alumina (Al₂ O₃ ) support. It is found that Ba species are impregnated successfully only after the stabilization of the mesoporous structure by full crystallization of Al₂ O₃ frameworks to the γ-phase, with the formation of Pt nanoparticles coinciding with complete removal of organic components. All the insights during this synthetic procedure are essential for designing high-performance catalysts to purify and recover NO_x molecules, and are applied for designing a variety of cutting-edge mesoporous nanocomposite catalysts.

Oxidation and Storage Mechanisms for Nitrogen Oxides on Variously Terminated (001) Surfaces of SrFeO3-δ and Sr3Fe2O7-δ Perovskites

The Ruddlesden-Popper (RP)-type layered perovskite is a candidate material for a new nitrogen oxide (NO_x) storage catalyst. Here, we investigate the adsorption and oxidation of NO_x on the (001) surfaces of RP-type oxide Sr₃Fe₂O_7-δ for all of the terminations by comparing to those of simple perovskite SrFeO_3-δ by the density functional theory (DFT) calculations. The possible (001) cleavages of Sr₃Fe₂O₇ generate two FeO₂- and three SrO-terminated surfaces, and the calculated surface energies indicated that the SrO-terminated surface generated by the cleavage at the rock salt layer is the most stable one. The oxygen of the FeO₂-terminated surfaces could be removed with significantly low energy because the process involves the favorable reduction of the Fe⁴⁺ site. Consequently, the surface oxygen at the FeO₂ site could easily oxidize adsorbed NO to NO₂ by the Mars-van Krevelen mechanism. The resulting oxygen vacancy in the surface would be filled easily with lattice oxygen in bulk. The oxidation of NO with adsorbed molecular O₂ was unfavorable by both the Langmuir-Hinshelwood and Eley-Rideal mechanisms because this process does not involve the reduction of the Fe⁴⁺ site. The oxygen of the SrO-terminated surfaces was tightly bound and acted as the adsorption site of NO and NO₂. An electron transfer strengthened the NO_x binding to the surface by forming nitrite (NO₂^-) or nitrate (NO₃^-) species. The DFT calculations revealed that the RP-type structure promoted NO_x oxidation and storage properties by forming active oxygen due to the Jahn-Teller distortion and by exposing SrO-terminated surfaces due to the cleavage at the rock salt layer.

Novel Preparation of Cu and Fe Zirconia Supported Catalysts for Selective Catalytic Reduction of NO with NH3

Copper and iron promoted ZrO2 catalysts were prepared by one-pot synthesis using urea. The studied catalysts were characterized by XRD, N2 physisorption, XPS, NH3-TPD, and tested in the selective catalytic reduction of NO with NH3 (NH3-SCR) in the absence and presence of water vapor under the experimental conditions representative of exhaust gases from stationary sources. The influence of SO2 on catalytic performance was also investigated. Among the studied catalysts, the Fe-Zr sample showed the most promising results in NH3-SCR, being active and highly selective to N2. The addition of SO2 markedly improved NO and NH3 conversions during NH3-SCR in the presence of H2O. The improvement in acidic surface properties is believed to be the cause.

Lithium cuprate, a multifunctional material for NO selective catalytic reduction by CO with subsequent carbon oxide capture at moderate temperatures

Li2CuO2 was evaluated as a possible catalyst for the NO selective catalytic reduction (NO SCR) by CO.

New advances in using Raman spectroscopy for the characterization of catalysts and catalytic reactions

Gaining insight into the mode of operation of heterogeneous catalysts is of great scientific and economic interest. Raman spectroscopy has proven its potential as a powerful vibrational spectroscopic technique for a fundamental and molecular-level characterization of catalysts and catalytic reactions. Raman spectra provide important insight into reaction mechanisms by revealing specific information on the catalysts' (defect) structure in the bulk and at the surface, as well as the presence of adsorbates and reaction intermediates. Modern Raman instrumentation based on single-stage spectrometers allows high throughput and versatility in design of in situ/operando cells to study working catalysts. This review highlights major advances in the use of Raman spectroscopy for the characterization of heterogeneous catalysts made during the past decade, including the development of new methods and potential directions of research for applying Raman spectroscopy to working catalysts. The main focus will be on gas-solid catalytic reactions, but (photo)catalytic reactions in the liquid phase will be touched on if it appears appropriate. The discussion begins with the main instrumentation now available for applying vibrational Raman spectroscopy to catalysis research, including in situ/operando cells for studying gas-solid catalytic processes. The focus then moves to the different types of information available from Raman spectra in the bulk and on the surface of solid catalysts, including adsorbates and surface depositions, as well as the use of theoretical calculations to facilitate band assignments and to describe (resonance) Raman effects. This is followed by a presentation of major developments in enhancing the Raman signal of heterogeneous catalysts by use of UV resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and shell-isolated nanoparticle surface-enhanced Raman spectroscopy (SHINERS). The application of time-resolved Raman studies to structural and kinetic characterization is then discussed. Finally, recent developments in spatially resolved Raman analysis of catalysts and catalytic processes are presented, including the use of coherent anti-Stokes Raman spectroscopy (CARS) and tip-enhanced Raman spectroscopy (TERS). The review concludes with an outlook on potential future developments and applications of Raman spectroscopy in heterogeneous catalysis.

Understanding of NOx storage property of impregnated Ba species after crystallization of mesoporous alumina powders

The regulation of automobile exhaust gas, especially that concerning hazardous nitrogen oxide (called as NOx) becomes stricter year-by-year, which should be urgently corresponded for cleaning the NOx containing emission. According to surface affinity of γ-alumina to metal catalysts and its thermal stability, crystalline γ-alumina has been frequently utilized as catalyst supports showing relatively high specific surface area. From the viewpoint, we consider that highly porous alumina powders prepared using amphiphilic organic molecules are potential as such a catalyst support for improving NOx removing property. In this study, we report surface property of the mesoporous alumina powders against NOx molecules after crystallizing to its γ-phase and NOx storage property after impregnation of barium (Ba) acetate in the mesopores. Adsorption of NO with O₂ on mesoporous γ-alumina powders without Ba species were more likely to be bridging bidentate than chelating bidentate nitrates (NO₃^-) with comparing to commercially available γ-alumina powders. After impregnating the Ba species, admitted NO molecules were oxidized with enough O₂ and stored very strongly as ionic nitrate (NO₃^-) onto the Ba species even after heating at 500 °C. This preliminary study is helpful for designing mesoporous deNOx catalysts combined with unique storage/adsorption property.

A Discussion on the Unique Features of Electrochemical Promotion of Catalysis (EPOC): Are We in the Right Path Towards Commercial Implementation?

The phenomenon of “Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA)” or “Electrochemical Promotion of Catalysis (EPOC)” has been extensively studied for the last decades. Its main strength, with respect to conventionally promoted catalytic systems, is its capability to modify in-situ the activity and/or selectivity of a catalyst by controlling the supply and removal of promoters upon electrical polarization. Previous reviews have summarized the main achievements in this field from both the scientific and technological points of view. However, to this date no commercial application of the EPOC phenomenon has been developed, although numerous advances have been made on the application of EPOC on catalyst nanostructures (closer to those employed in conventional catalytic systems), and on the development of scaled-up reactors suitable for EPOC application. The main bottleneck for EPOC commercialization is likely the choice of the right chemical process. Therefore, from our point of view, future efforts should focus on coupling the latest EPOC advances with the chemical processes where the EPOC phenomenon offers a competitive advantage, either from an environmental, a practical or an economic point of view. In this article, we discuss some of the most promising cases published to date and suggest future improvement strategies. The considered processes are: (i) ethylene epoxidation with environmentally friendly promoters, (ii) NOx storage and reduction under constant reaction atmosphere, (iii) CH4 steam reforming with in-situ catalyst regeneration, (iv) H2 production, storage and release under fixed temperature and pressure, and (v) EPOC-enhanced electrolysers.

Novel investigation of perovskite membrane based electrochemical nitric oxide control phenomenon

The combustion of hydrocarbon fuels within the automotive industry results in harmful and reactive incomplete combustion byproducts. Specifically, nitric oxide emissions (NO) lead to increased smog, acid rain, climate change, and respiratory inflammation within the population [Nitrogen Dioxide | American Lung Association]. Current methods for treating combustion exhaust include the catalytic converter in conjunction with nitrogen oxide traps. However, there is no active, continuous reduction method that does not require restrictions on the combustion environment (Hirata in Catal Surv Asia 18:128-133, 2014). Here, a small voltage potential oscillation across a newly designed electro-chemical catalytic membrane significantly reduces NO emissions. A ceramic membrane consisting of two dissimilar metal electrodes, sandwiching a dielectric layer, is able to achieve an NO reduction in excess of 2X that of a platinum group metal (PGM) three way catalytic converter. An analysis of the exhaust effluent from the membranes indicates N₂O as a precursor to N₂ and O₂ formation, without the introduction of ammonia (NH₃), during the reaction of NO indicating a divergence from current literature. Our results demonstrate how an oscillatory electric potential on a catalytic surface may alter anticipated reaction chemistry and interaction between the catalytic surface and fluid flow.

Low-temperature NO oxidation using lattice oxygen in Fe-site substituted SrFeO3-δ

Improvement of the low-temperature activity for NO oxidation catalysts is a crucial issue to improve the NO_x storage performance in automotive catalysts. We have recently reported that the lattice oxygen species in SrFeO_3-δ (SFO) are reactive in the oxidation of NO to NO₂ at low temperatures. The oxidation of NO using lattice oxygen species is a powerful means to oxidize NO in such kinetically restricted temperature regions. This paper shows that Fe-site substitution of SFO with Mn or Co improves the properties of lattice oxygen such as the temperature and amount of oxygen release/storage, resulting in the enhancement of the activity for NO oxidation in a low-temperature range. In particular, NO oxidation on SrFe_0.8Mn_0.2O_3-δ is found to proceed even at extremely low temperatures <423 K. From oxygen release/storage profiles obtained by temperature-programmed reactions, Co doping into SFO increases the amount of released oxygen owing to the reducibility of the Co species and promotes the phase transformation to the brownmillerite phase. On the other hand, Mn doping does not increase the oxygen release amount and suppresses the phase transformation. However, it significantly decreases the oxygen migration barrier of SFO. Substitution with Mn renders the structure of SFO more robust and maintains the perovskite structure after the release of oxygen. Thus, the oxygen release properties are strongly dependent on the crystal structure change before and after oxygen release from the perovskite structure, which has a significant effect on NO oxidation and the NO_x storage performance.

Non-Volatile Particle Number Emission Measurements with Catalytic Strippers: A Review

Vehicle regulations include limits for non-volatile particle number emissions with sizes larger than 23 nm. The measurements are conducted with systems that remove the volatile particles by means of dilution and heating. Recently, the option of measuring from 10 nm was included in the Global Technical Regulation (GTR 15) as an additional option to the current >23 nm methodology. In order to avoid artefacts, i.e., measuring volatile particles that have nucleated downstream of the evaporation tube, a heated oxidation catalyst (i.e., catalytic stripper) is required. This review summarizes the studies with laboratory aerosols that assessed the volatile removal efficiency of evaporation tube and catalytic stripper-based systems using hydrocarbons, sulfuric acid, mixture of them, and ammonium sulfate. Special emphasis was given to distinguish between artefacts that happened in the 10–23 nm range or below. Furthermore, studies with vehicles’ aerosols that reported artefacts were collected to estimate critical concentration levels of volatiles. Maximum expected levels of volatiles for mopeds, motorcycles, light-duty and heavy-duty vehicles were also summarized. Both laboratory and vehicle studies confirmed the superiority of catalytic strippers in avoiding artefacts. Open issues that need attention are the sulfur storage capacity and the standardization of technical requirements for catalytic strippers.

Activating low-temperature diesel oxidation by single-atom Pt on TiO2 nanowire array

Supported metal single atom catalysts (SACs) present an emerging class of low-temperature catalysts with high reactivity and selectivity, which, however, face challenges on both durability and practicality. Herein, we report a single-atom Pt catalyst that is strongly anchored on a robust nanowire forest of mesoporous rutile titania grown on the channeled walls of full-size cordierite honeycombs. This Pt SAC exhibits remarkable activity for oxidation of CO and hydrocarbons with 90% conversion at temperatures as low as ~160 oC under simulated diesel exhaust conditions while using 5 times less Pt-group metals than a commercial oxidation catalyst. Such an excellent low-temperature performance is sustained over hydrothermal aging and sulfation as a result of highly dispersed and isolated active single Pt ions bonded at the Ti vacancy sites with 5 or 6 oxygen ions on titania nanowire surfaces.

Achieving rational design of alloy catalysts using a descriptor based on a quantitative structure–energy equation

Rational design of high-activity alloy catalysts for NO oxidation.

Insights into the precursor effect on the surface structure of γ-Al2O3 and NO + CO catalytic performance of CO-pretreated CuO/MnOx/γ-Al2O3 catalysts

NO reduction by CO was investigated over CO-pretreated CuO/MnO_x/γ-Al₂O₃ catalysts with different metal precursors (nitrate and acetate). It was found that the catalyst prepared from acetate salts (Cu/Mn/Al-A) exhibited significantly higher activity than counterpart catalyst from nitrate precursors (Cu/Mn/Al-N). XRD, XPS and in situ DRIFT were carried out to approach the nature for the different catalytic performance. For both catalysts, copper mainly existed as CuO, but the status of manganese oxide was markedly different. Mn(IV) was predominant in Cu/Mn/Al-N and Mn(III) was enriched in Cu/Mn/Al-A. As a result, different dispersion behaviors of manganese oxide on γ-Al₂O₃ were displayed, which induced inconsistent Cu-Mn contact. The catalyst obtained from acetate precursor exhibited enriched Cu-Mn contact and thus more Cu⁺-□-Mn^3+/2+ entities would be produced after CO pretreatment, leading to promoted NO dissociation and favorable performance in NO reduction by CO. The present study sheds light on the effective tuning of Cu-O-Mn interfacial sites in CuO/MnO_x/γ-Al₂O₃ via modulating the dispersion behaviors of surface components.

NOx Oxidation and Storage Properties of a Ruddlesden-Popper-Type Sr3Fe2O7-δ-Layered Perovskite Catalyst

The development of NO_x-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. This study demonstrates that NO_x oxidation and storage reactions proceed over Pt-free Sr₃Fe₂O_7-δ with a Ruddlesden-Popper-type layered perovskite structure. Two types of Sr-Fe perovskite with oxygen storage capacity, namely, SrFeO_3-δ and Sr₃Fe₂O_7-δ, are studied as NO_x-trapping catalysts. Sr₃Fe₂O_7-δ shows higher NO_x storage capacity than SrFeO_3-δ; its activity is comparable to that of Pt/Ba/Al₂O₃ calcined at 1273 K. NO_x temperature-programmed desorption and diffuse reflectance infrared Fourier transform experiments confirm the superior NO_x-trapping ability of Sr₃Fe₂O_7-δ over SrFeO_3-δ. In addition, NO temperature-programmed reactions and O₂ temperature-programmed desorption experiments reveal that these catalysts operate through a novel NO oxidation mechanism involving the consumption of their lattice oxygens and topotactic structural changes at a temperature of around 350-400 K. The reduction performance of trapped NO_x on Pd-modified Sr-Fe perovskites is investigated by lean-rich cycle experiments using H₂ as the reductant. Pd/Sr₃Fe₂O_7-δ shows significantly high NO_x removal efficiency over the entirety of each lean-rich period. Modifying Sr₃Fe₂O_7-δ with Pd is also effective for NO_x storage in the presence of H₂O and CO₂ and the regeneration of the catalyst following SO_x sorption. Sr₃Fe₂O_7-δ, with both NO_x adsorption and NO oxidation capabilities, acts as a Pt-free NO_x-trapping catalyst, exhibiting both high NO_x storage capacity and high thermal tolerance.

Morphology and Crystal-Plane Effects of Fe/W-CeO2 for Selective Catalytic Reduction of NO with NH3

The CeO2 ordinary amorphous, nanopolyhedrons, nanorods, and nanocubes were prefabricated by the hydrothermal method, and employed as carriers of Fe/W–CeO2 catalysts to selectively catalyze the reduction of NO with ammonia. Characterization results indicated that the morphology of CeO2 support originated from selectively exposing different crystal surfaces, which has a significant effect on oxygen vacancies, acid sites and the dispersion of Fe2O3. The CeO2 nanopolyhedrons catalyst (Fe/W–CeO2–P) showed most oxygen vacancies, the largest the quantity of acid sites, the largest BET (Brunauer-Emmett-Teller) surface area and the best dispersion of Fe2O3, which was associated with predominately exposing CeO2 (111) planes. Consequently, the Fe/W–CeO2–P catalyst has the highest NO conversion rate in the temperature range of 100–325 °C among the ordinary amorphous, nanorods, and nanocubes Fe/W–CeO2 catalysts.

Synergistic Effect of Cu/CeO2 and Pt-BaO/CeO2 Catalysts for a Low-Temperature Lean NO x Trap

A lean NO _x trap (LNT) catalyst has been widely used for removing NO _x exhaust from lean-burn engines. However, the operation range of LNT has been limited because of the poor activity of LNT catalysts at low temperatures (≤300 °C), especially in urban driving conditions. To increase NO _x removal efficiency during lean-rich cycle operation, a Cu/CeO₂ (CC) catalyst was added to a Pt-BaO/CeO₂ (PBC) catalyst. In comparison to PBC- or CC-only catalysts, the physical mixture of PBC and CC catalysts (PBC + CC) exhibited a significant synergy for both NO _x storage and reduction efficiencies. In particular, low-temperature activity below 200 °C was greatly enhanced. A Pt-BaO-Cu/CeO₂ (PBCC) catalyst, which was synthesized by depositing Pt and Cu together on a ceria support, showed poorer NO _x removal efficiency. The origin of the synergistic effect over PBC + CC was investigated. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NO _x storage on PBC. Under rich conditions, H₂ was generated in situ on the CC by a water-gas shift reaction then accelerated the reduction of NO _x, which had been stored on PBC, with a higher selectivity to N₂. This simple modification in the catalyst can provide an important clue to enhance low-temperature activity of the commercial LNT system.

Metal organic frameworks-assisted fabrication of CuO/Cu2O for enhanced selective catalytic reduction of NOx by NH3 at low temperatures

Porous CuO/Cu₂O heterostructure was successfully synthesized through a metal organic frameworks (MOFs)-assisted template method. Tunable production of pure phase CuO and Cu₂O could be achieved by regulating the coordination environment of copper. The copper oxides inherited the polyhedral morphology from the Cu MOFs and possessed higher surface area and larger pore volume. Compared with pure CuO and Cu₂O, heterostructured CuO/Cu₂O displayed remarkably enhanced NH₃-SCR de-NO_x activity and N₂ selectivity in a low temperature range of 170-220 °C. Systematical in situ DRIFT characterization revealed that the NH₃-SCR of NO_x over CuO/Cu₂O heterostructure followed Eley-Rideal (E-R) mechanism, which was greatly improved by the abundant Lewis acid sites, improved O₂ adsorption and the synergistic effect between Cu⁺ and Cu²⁺. In addition, CuO/Cu₂O heterostructure exhibited excellent H₂O, SO₂, alkali metals, and hydrocarbon durability, indicating its potential use in industrial NH₃-SCR of NO_x.

Perovskite-based catalysts for the control of nitrogen oxide emissions from diesel engines

Nitrogen oxides removal over a wide range of perovskite-based catalysts together with their property-activity relationships.

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.