NO Reduction Activity Research Articles

Promotion of Different Active Phases in MnOX-CeO2 Catalysts for Simultaneous NO Reduction and o-DCB Oxidation

AbstractMnOX-CeO2 catalysts with different Mn and Ce content were prepared to evaluate the effect of metal content on catalytic properties and activity in the simultaneous NO reduction and o-DCB oxidation, in order to elucidate the most active species for the process. Catalytic properties were evaluated by ICP-AES, XRD, skeletal FTIR, STEM-HAADF, XPS, N2-physisorption, H2-TPR, NH3-TPD and pyridine-FTIR. Catalysts with 85%Mn and 15%Ce molar content have been found to be the most active. Their excellent catalytic performance is related to the coexistence of Mn in different phases, i.e., Mn species strongly interacting with Ce and segregated Mn species. The effect of the preparation methods has also been deeply investigated: Co-precipitation method (CP) leads to Mn segregation as Mn2O3, whereas sol-gel preparation method (SG) promotes the formation of an amorphous powder. The synergy between segregated Mn2O3 species and Mn species in high interaction with Ce (resulting in a mixed oxide phase) leads to the presence of Mn with different oxidation states. This effect, together with the high oxygen mobility caused by structural defects, enhances redox, acidic and oxidative properties. The improvement of catalytic properties with Mn content also favors NO reduction side-reactions, with N2O and NO2 being the most important by-products, whereas it limits the production of chlorinated organic by-products in o-DCB oxidation.

Selective Formation of Cu Active Sites with Different Coordination States on Pseudospinel CuAl2O4 and Their NO Reduction Catalysis.

In the spinel framework, copper (Cu) in two distinct coordination states exhibits catalytic activity for NO reduction through different mechanisms. However, detailed exploration of their respective catalytic properties, such as the redox behavior of Cu and substrate molecule adsorption, has been challenging due to difficulties in their separate formation. In this study, we present the controlled formation of pseudospinel CuAl2O4, containing exclusively tetrahedrally or octahedrally coordinated Cu, achieved by manipulating aging temperature and O2 concentration. Through these materials, we observed that in the CO-NO reaction, the step primarily determining the rate differs: NO reduction dominates with octahedrally coordinated Cu, whereas carbon monoxide (CO) oxidation is prominent with tetrahedrally coordinated Cu. The lower coordination number of Cu significantly benefits NO reduction but negatively impacts the CO-NO reaction, albeit positively influencing NO reduction in three-way catalytic reactions.

Light‐off Performance of Three‐Way Catalysts Before and After Accelerated Aging Test with an Engine‐Dynamometer

AbstractThis study demonstrates the reaction behavior during the purification of model automotive exhaust gases over Pd catalysts before and after thermal degradation. In particular, to investigate the relationship between the Pd state and the reaction behavior of Pd/Al2O3 and Pd/CeO2−ZrO2 (CZ), operando X‐ray absorption spectroscopy measurements were performed during purifying exhaust gases over real and model catalysts mimicking the degradation of Pd particles and CZ supports after accelerated aging tests. The NO reduction activity of the aggregated Pd metal species was as high as that of the highly dispersed Pd species, but hydrocarbon (HC) poisoning was significantly enhanced by the aggregation of Pd metal particles caused by thermal aging. The existence of a three‐phase boundary (TPB) between the CZ, the Pd particles, and the gas phase strongly affected the catalytic activity at low temperatures, and the presence of a sufficient TPB facilitated the combustion of unburned HCs owing to the oxygen storage performance of CZ. Thus, the TPB reduced the poisoning of the precious metal surface by HC species at low temperatures. Therefore, the findings of this study will facilitate the development of next‐generation gas purification catalysts with high activity and durability.

NO Reduction with CO on Low‐loaded Platinum‐group Metals (Rh, Ru, Pd, Pt, and Ir) Atomically Dispersed on Ceria

AbstractLow‐loaded platinum‐group single‐atom catalysts on CeO2 (M1/CeO2) were synthesized via high‐temperature atom trapping (AT) and tested for the NO+CO reaction under dry and wet conditions. The activity of these catalysts for NO+CO reaction follows the order Rh>Pd≈Ru>Pt>Ir. For Rh, Ru, and Pd single‐atom catalysts, the N2O byproduct is formed but not clearly observed in Ir and Pt cases, which may result from the higher reaction temperature (>200 °C) required for Pt and Ir catalysts. The presence of water can promote the activity of these M1/CeO2 catalysts for the NO+CO reaction. Under wet conditions, significant NH3 formation occurred during the reaction, which is due to the co‐existence of water‐gas‐shift reaction on these catalysts. Compared with Pt, Pd and Ir, the Rh and Ru single‐atom catalysts show higher selectivity to NH3 species, resulting from the hydride species on the surface. Among all tested catalysts, Ru1/CeO2 shows the highest production of ammonia and highest CO conversion due to excellent water‐gas‐shift activity, whereas Pd1/CeO2 shows lowest ammonia production. Rh1/CeO2 shows the best low temperature NO reduction activity among all tested catalysts.

Carbon doped hexagonal boron nitride as an efficient metal-free catalyst for NO capture and reduction.

The electrochemical NO reduction reaction (NORR) towards NH3 is considered a promising strategy to cope with both NO removal and NH3 production. Currently, the research on NORR electrocatalysts mainly focuses on metal-based catalysts, while metal-free catalysts are quite scarce. In this work, we have systematically investigated the properties of pristine and C/O doped h-BN for efficient NO capture and reduction. Our results reveal that the basal plane of pristine h-BN is inert to the adsorption of NO, while doping C or O can significantly enhance the NO capture abilities of h-BN. Then, we highlight that C-doped h-BN exhibits excellent NORR catalytic performance with a relatively low limiting potential of -0.28 V. Further analysis shows that the suitable adsorption strength of NO on the C-doped h-BN surface is the prime reason for its excellent NO reduction activity, which is shown to be due to appropriate electronic interactions between the active site and NO. Last but not least, the catalytic selectivity of h-BN towards the NORR is confirmed by inhibiting the competing hydrogen evolution reaction. Our findings not only provide deeper insight into the essential effect of element doping on the catalytic activities of h-BN, but also propose general design principles for high-performance metal-free NORR electrocatalysts.

Hydroxyls on CeO2 Support Promoting CuO/CeO2 Catalyst for Efficient CO Oxidation and NO Reduction by CO.

Transition metal catalysts, such as copper oxide, are more attractive alternatives to noble metal catalysts for emission control due to their higher abundance, lower cost, and excellent catalytic activity. In this study, we report the preparation and application of a novel CuO/CeO2 catalyst using a hydroxyl-rich Ce(OH)x support for CO oxidation and NO reduction by CO. Compared to the catalyst prepared from a regular CeO2 support, the new CuO/CeO2 catalyst prepared from the OH-rich Ce(OH)x (CuO/CeO2-OH) showed significantly higher catalytic activity under different testing conditions. The effect of OH species in the CeO2 support on the catalytic performance and physicochemical properties of the CuO/CeO2 catalyst was characterized in detail. It is demonstrated that the abundant OH species enhanced the CuOx dispersion on CeO2, increased the CuOx-CeO2 interfaces and surface defects, promoted the oxygen activation and mobility, and boosted the NO adsorption and dissociation on CuO/CeO2-OH, thus contributing to its superior catalytic activity for both CO oxidation and NO reduction by CO. These results suggest that the OH-rich Ce(OH)x is a superior support for the preparation of highly efficient metal catalysts for different applications.

A comparison of bifunctional MnOx catalysts prepared via different precipitants for simultaneous removal NO and CO

In this work, ammonium carbonate, sodium hydroxide, and ammonium hydroxide were selected as the precipitants to prepare bifunctional MnOx catalysts via the PEG-assisted co-precipitation method. SEM analysis revealed that MnOx catalysts prepared with ammonium carbonate (AC) displayed a spherical morphology with relatively large size, whereas the catalysts synthesized with sodium hydroxide (SH) and ammonium hydroxide (AH) exhibited polyhedron shapes with smaller size. The results demonstrated that Mn-AC catalyst presented superior catalytic activity for both NO reduction and CO oxidation, achieving 89% NO conversion at 100 ºC and over 80% CO conversion at 200 ºC, outperforming Mn-SH and Mn-AH catalysts. Mn-AC catalyst possessed more abundant Mn4+ species and surface chemisorbed oxygen than Mn-AH and Mn-SH catalysts, which served as a significant motivation for the difference in catalytic activity. Additionally, Mn-AC catalyst exhibited stronger surface acidity, which facilitated the adsorption and activation of ammonia species in the NH3-SCR reaction. Furthermore, in situ DRIFTS experiment suggested that the pre-desorption of CO promoted the adsorption and activation of NO. On this basis, the possible mechanism model of three precipitants on bifunctional MnOx catalysts was proposed.

Metal-free C2N catalyst with superior H2O/SO2 tolerance for NO reduction with CO during incomplete combustion process

Designing a highly effective catalyst for purification NOx emissions during the incomplete combustion process in coal-fired power plants is a valuable yet challenging task. The presence of H2O and SO2 during the NOx reduction process for flue gas treatment in coal-fired power plants is a major concern because it can lead to catalyst deactivation due to the deposition of ammonium sulfate. In this work, we propose a novel method for removing NO and CO without the traditional NH3 injection (2NO + 2CO = N2 + 2CO2) by utilizing density functional theory (DFT) calculations. Specifically, we designed metal-free single and double atomic catalysts named MFn/C2N and demonstrated that the Si-DAC (Si2/C2N) system has the highest activity for NO reduction, with outstanding resistance to deactivation from H2O and tolerance to SO2. Our findings show that Si-DAC has the best catalytic performance for NO reduction, with an extremely low energy barrier of 0.28 eV, suggesting that Si-DAC is the most effective among the reported computational results for NO removal. Overall, this work could pave the way towards designing catalytic systems with excellent activity and stability for reactions beyond NOx reduction.

Quantifying the Two-Dimensional Driving Patterns of Chemisorbed Oxygen and Particle Size on NO Reduction Activity and Mechanism.

Quantification in the driving patterns of activity descriptors on structure-activity relationships and reaction mechanisms over heterogeneous catalysts is still a great challenge and needs to be addressed urgently. Herein, with the example of typical Mn-based catalysts, based on the activity regularity and many characterizations, the chemisorbed oxygen density (ρOβ) and particle size (dTEM) have been proposed as the two-dimensional descriptors for selective catalytic reduction of NO, whose role is in quantifying the contents of vacancy defects and the amounts of active sites located on terraces or interfaces, respectively. They can be utilized to construct and quantify the driving patterns for the structure-activity relationships and reaction mechanisms of NO reduction. As a consequence, a complementary modulation for Ea by ρOβ and dTEM is described quantitatively in terms of the fitted functions. Moreover, based on the structure-activity relationships and the quantification laws of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the reaction efficiency (RE) of the specific combined NOx-intermediate is identified as the trigger to drive the Langmuir-Hinshelwood mechanism and modulated by the descriptors complementally and collaboratively following the fitted quantification functions. Either of the two descriptors at its lower values plays a dominant role in regulating Ea and RE, and the dominant factor evolves progressively: dTEM ↔ coupling dTEM with ρOβ ↔ ρOβ, when the dependency of Ea and RE on the descriptors is adopted to identify the dominant factor and domains. Therefore, this work has quantitatively accounted for the essence of activity modulation and may provide insight into the quantitative driving patterns for reaction activity and mechanism.

Effect of alkali addition on a Cu/SmCeO2 @TiO2 catalyst for NO reduction with CO under oxidizing conditions

The accumulation of alkali metals generated during biomass combustion onto the catalysts employed for off-gas treatment can have a detrimental effect on their performance. This work investigates the effect of potassium as a model alkali on an efficient copper-based catalyst supported on SmCeO2 @TiO2. The addition of potassium modifies the redox properties of the Cu species or, more specifically, the metal-ceria interface, providing better catalytic activity for NO reduction in oxidizing conditions. SmCeO2 @TiO2 stabilizes highly dispersed copper species, and the presence of potassium introduces surface distortions that modify the lability of oxygen atoms. The results demonstrate the key role of surface defects and labile oxygen species associated with the CeO2 support. It was found that both Cu/SmCeO2 @TiO2 and K/Cu/SmCeO2 @TiO2 catalysts fully oxidized CO below 350 °C and actively reduced NO with CO in the presence of excess oxygen, reaching a maximum NO conversion of 65% at 316 °C and 83% at 330 °C, respectively, while undesired NO2 release was minimized in the alkaline-loaded sample.

Effect of interaction degree between Mn and Ce of MnOX-CeO2 formulation on NO reduction and o-DCB oxidation performed simultaneously

MnOX-CeO2 catalysts were prepared by different methods in order to assess the influence of Mn and Ce interaction on the catalytic activity of NO reduction and 1,2-dichlorobencene (o-DCB) oxidation carried out simultaneously. Changes on catalytic properties were evaluated by XRD, Raman, N2-physisorption, H2-TPR and NH3-TPD. The samples prepared by co-precipitation and sol-gel exhibited the best catalytic activity over the whole temperature range. These preparation methods provide high interaction between Mn and Ce at surface and bulk, which was evidenced by the formation of mixed oxide phase. According to characterisation, the enhancement of catalytic activity with Mn and Ce interaction is associated to the promotion of structural defects, which improve redox and acid properties. Oxidative capability is also improved as a consequence of the oxygen vacancies generated by the presence of those structural defects. On the other hand, the promotion of Mn and Ce interaction favours the production of N2O in NO reduction at a lower temperature, and also leads to o-DCB total oxidation by decreasing CO production.

Effect of Cu-Doped Co-Mn Spinel for Boosting Low-Temperature NO Reduction by CO: Exploring the Structural Properties, Performance, and Mechanisms.

Cobalt-manganese spinel catalysts performed unsatisfactory activity at low-temperature and narrow reaction temperature window, which greatly limited the application in NO reduction by CO. Herein, we synthesize a series of Cu-doped CoMn2O4 catalysts and apply to NO reduction by CO. The Cu0.3Co0.7Mn2O4 exhibited superior catalytic performance, reaching 100% NO conversion and 80% N2 selectivity at 250 °C. Detailed structural analysis showed that the introduced Cu replaces some Co in tetrahedral coordination to induce a strong synergistic effect between different metals. This endows the catalyst with the promotion of both electron transfer and oxygen vacancy generation on the catalyst surface. Importantly, the reaction mechanism and pathway were further revealed by in situ diffusion Fourier transform infrared spectroscopy (DRIFTS) and density functional theory (DFT) calculations. The results indicated that the cycle of oxygen vacancy mainly determines the catalytic activity of NO reduction by CO. Notably, Cu doping significantly lowered the energy barrier of the rate-determining step (*CO + O → *Ov + CO2), facilitating the desorption of the CO2 and exposing the active sites for efficient NO reduction with CO. This work offers an effective way for designing the catalyst in NO reduction by CO and provides a reference for exploring the catalytic mechanism of the reaction.

Noble Metal Nanoparticles Decorated Boron Nitride Nanotubes for Efficient and Selective Low-Temperature Catalytic Reduction of Nitric Oxide with Carbon Monoxide

Parallel to CO2 emission, NOx emission has become one of the menacing problems that seek a simple, durable, and high-efficiency deNOx catalyst. Herein, we demonstrated simple syntheses of platinum group metal nanoparticle-decorated f-BNNT (PGM = Pd, Pt, and Rh, and f-BNNT stands for -OH-functionalized boron nitride nanotubes) as a catalyst for efficient and selective reduction of NO by CO at low-temperature conditions. PGM/f-BNNT with a low amount of noble metal nanoparticles (0.7-0.8 wt %) presents very efficient catalytic activity for NO reduction as well as CO oxidation during their removal process. The removal efficiencies of NO and CO with Pd/f-BNNT, Pt/f-BNNT, and Rh/f-BNNT catalysts were investigated under various temperatures, flow rates, and reaction times, respectively. For most cases, NO catalytic reduction with CO reaction was >99% at a temperature as low as ∼200 °C. The catalyst robustness and efficiency were also verified by presenting almost 100% conversion of NO using a Rh/f-BNNT catalyst, which was aged under humid air at 600 and 700 °C for 24 h, respectively. The synergic effect of the catalytic efficacy of the well-dispersed noble metal nanoparticles and the excellent surface properties of BNNT are reasons for the high selectivity and catalytic property at a low temperature. On the basis of this investigation, we demonstrated that the noble metal nanoparticle-decorated f-BNNT catalysts are possible to save expensive PGM catalysts, such as Pt, Pd, and Rd, as much as 100 times while presenting similar or better catalytic performance for simultaneous NO and CO removals.

Reduction of nitric oxide adsorbed on iridium cluster cations at high temperatures

Reactions of NO adsorbed on Irn+ and IrnO+ (n = 3–7) at high temperatures were experimentally investigated using gas-phase thermal desorption spectrometry. IrnNmOm+ and IrnNmOm+x+ were prepared by the reaction with NO at 300 K. After heating, NO molecules were released sequentially. For Ir5+ and Ir6+, heating above 800 K resulted in the release of N2 molecules from Ir5N4O4+, Ir5N5O5+, and Ir6N3O3+ after the reduction-decomposition of NO. Similar reactions were also found on Rhn+ clusters. Furthermore, the slightly oxidized Ir7O+ showed high activity for NO reduction, which is considered unique to Ir clusters.

Insights into the promotional effect of alkaline earth metals in Pt-based three-way catalysts for NO reduction

Enhancing NO reduction efficiency of Pt-based three-way catalysts (TWCs) is very challenging due to poor NO activation ability. In this work, NO reduction performance was improved by introducing alkaline earth metals to adjust the electron environment of Pt, while the promoting effect was systematically studied. Structure-activity relationship indicated that NO reduction activity over the Pt-M/LA catalysts (M = Mg, Ca, Sr or Ba, LA = La-Al2O3) increases as the electronegativity of M decreases, and the TOF of Pt-Ba/LA is about 3 times more active than that of Pt/LA, which is attributed to more Pt0 species resulting from Pt-Ba interaction. Moreover, kinetic studies revealed that alkaline earth metals weaken the poisoning effect of CO and improve promoting effect of HCs, leading to an improved NO adsorption and an increased reaction rate of NO with CO and HCs. This study provides an insightful understanding for the promotional effect of alkaline earth metals on NO reduction over Pt-based TWCs.

CsxCo/Na-MOR Coating on Ceramic Monoliths for Co-Adsorption of Hydrocarbons Mixture and Selective Catalytic Reduction of NOx

In this work, ceramic monoliths were coated with powders based on exchanged Cs and/or Co cations in Na-mordenite (MOR) zeolite. SEM images showed that zeolite particles fill the macropores of cordierite walls and form a continuous layer of approximately 40 µm with good adherence. XPS analysis revealed that Co and Cs are present on the film surface solely as Co2+ and Cs+ at exchange positions in zeolite. The monolithic structures were evaluated for the butane-toluene co-adsorption and SCR of NOx with hydrocarbon mixture as the reducing agent. The presence of alkali metal cations in the zeolitic lattice favored the adsorption capacity of both hydrocarbons, while cobalt cations provoked a decrease in the adsorbed amounts due to its weak interaction with the HCs. Breakthrough curves of butane adsorption showed a roll-up phenomenon, associated with a competitive adsorption effect generated from toluene presence. In the desorption process, it was observed that adsorbed toluene hindered the butane diffusion through mordenite channels, which released at higher temperatures (above 250 °C). Cs2CoM and Cs7CoM monoliths were more active than the CoM monolith for NO-SCR. The presence of Cs cations close to Co cations increased the hydrocarbons concentration around active sites at high temperatures, according to TPD results, promoting the reduction activity of NO.

Hexagonal boron nitride nanoribbon as a novel metal-free catalyst for high-efficiency NO reduction to NH3

Electrochemical NO reduction is taken as a promising avenue to eliminate detrimental NO into valuable NH3 for a balanced nitrogen cycle. Current electrocatalysts for NO reduction are mainly focused on metal-bearing catalysts, while metal-free catalysts with low cost, excellent stability, activity and selectivity are strongly desired. Herein, we report hexagonal boron nitride nanoribbons (BNNRs) as novel metal-free catalysts for efficient NO adsorption and reduction into NH3. Our results reveal that bare edge B atoms of armchair and zigzag BNNRs act as active sites for spontaneous chemisorption and activation of NO, originating from significant electron transfer. Notably, in the subsequent NO reduction, NH3 can be spontaneously produced by armchair BNNR with a limiting potential (UL) of 0 V, which is superior to most well-established single-atom, biatom and pure traditional metal catalysts. The awesome catalytic activity for NO reduction is intrinsically introduced by the suitable NO adsorption strength on armchair BNNR, which is illuminated to appropriate electronic interactions between the active site and NO. Moreover, the catalytic selectivity of armchair BNNR for NH3 production at both low and high NO coverage can be well guaranteed due to the higher thermodynamic energy barriers in generating N2O, N2 and H2. Overall, our findings pioneer the catalytic application of BNNR and unveil the veil of metal-free catalysts for direct NO reduction to NH3.

Comparative analysis of NOx reduction on Pt, Pd, and Rh catalysts by DFT calculation and microkinetic modeling

In this study, adsorption energies and reaction energetics on (111) surfaces of Pt, Pd and Rh were established using DFT calculation. Based on these thermodynamic results, reactant conversions and product yields of Pt, Pd and Rh catalysts under various air–fuel ratio (λ) were predicted by microkinetic modeling combined with simulated packed bed reactor. As a result, Pt catalyst efficiently utilizes H2 in assisting NO dissociation and removing surface O* under stoichiometric and fuel-lean conditions. However, it presents high NH3 yield under stoichiometric and fuel-lean conditions. Conversely, Rh catalyst show high NO reduction activity under fuel-rich condition while it hardly reduce NO in presence of O2. In order to take the advantages of both catalysts, we suggest physically-mixed Rh + Pt catalyst is excellent catalyst using the advantages of each catalyst for TWC. Consequently, it is confirmed that Pt sufficiently reduces NO using H2 under stoichiometric and fuel-lean conditions, and Rh easily dissociates NO at low temperature under fuel-rich condition when using the Rh + Pt catalyst. We expect that identifying the reaction characteristics of TWC components under different λ conditions will help to propose future TWC design.

Ordered mesoporous TiO2 framework confined CeSn catalyst exhibiting excellent high activity for selective catalytic reduction of NO with NH3 at low temperature

Synthesis of the cerium-based catalyst with excellent activity, selectivity, and toxicity resistance is one of the difficult problems for the selective reduction of NO by NH3 (NH3-SCR) at low temperature. Herein, we synthesize a novel and rational framework-confined ordered mesoporous CeSnOx/TiO2-E catalyst. Through a classical evaporation-induced self-assembly (EISA) strategy, the active components participate in the formation process of ordered mesoporous, which confines the catalytic active species into the interior of the overall structure. Via an array of microscopic characterizations and spectroscopic analysis, we have discovered that the presence of the ordered mesoporous structure makes the catalyst have a prominent confined effect on the framework. The high dispersion of nanoparticles in the system is realized here. Meanwhile, the cerium-based with Sn-doped catalyst system achieves the change of electronic structure and increases the charge exchange frequency, which enables the catalyst to acquire outstanding redox ability and abundant acidic sites. In addition, the ordered mesoporous structure of the catalyst guided the active components to produce a synergistic effect, and the uniform pore structure and metal doping enhanced the anti-poisoning ability. This strategy not only improves the low-temperature catalytic performance of the catalyst but also protects the nanoparticles from poisoning, which is beneficial to the long-time stability of the NH3-SCR reaction.

Understanding the Catalytic Deactivation upon Hydrothermal Aging at 850 °C of WO3/Fe-Cu-ZSM-5 Catalyst for Selective Catalytic Reduction of NO by NH3

A WO3/Fe-Cu-ZSM-5 catalyst was prepared using the solid state ion exchange method (SSIE) and its performance for the Selective Catalytic Reduction of NO with NH3 (NH3-SCR of NO) was investigated. The study shows that the tungsten addition can slightly improve the high temperature catalytic activity of Fe-Cu-ZSM-5. The influence of hydrothermal aging at 850 °C for 5 h on the structural and textural properties of WO3/Fe-Cu-ZSM-5 was also studied in this paper. The XRD and FE-SEM measurements did not indicate a breakdown of the zeolite structure upon steam treatment for both aged catalysts. The aged W-base catalyst demonstrates a lower deactivation and better catalytic activity for NO reduction than the bimetallic catalyst after hydrothermal aging despite the lower acidic properties as shown by FTIR-Pyr spectroscopy owing to the presence of tungsten oxide crystallites. The “severe” stage of aging occurring in the absence of W led to the formation of copper oxide agglomerates detected using STEM and H2-TPR techniques being responsible for the deterioration of SCR activity of the aged Fe-Cu-ZSM-5.