Catalytic Reduction Of Nitric Oxide Research Articles

Unraveling the Effect of Mo Dopant in Fe2O3 Catalyst for Selective Catalytic Reduction of Nitric Oxide with NH3

Unraveling the Effect of Mo Dopant in Fe₂O₃ Catalyst for Selective Catalytic Reduction of Nitric Oxide with NH₃

Tailoring Iridium Valence States on ZSM-5 for Enhanced Catalytic Performance in CO Selective Catalytic Reduction of NO under Oxygen-Enriched Environments.

Barium and iridium supported on Zeolite Socony Mobil-5 (ZSM-5) are efficient catalysts for the selective catalytic reduction of nitric oxide by carbon monoxide (CO-SCR), with enhanced cyclic stability. The introduction of Ba hindered the oxidation of metallic Ir active species and enabled Ir to maintain an active metallic state, thereby preventing a decrease in catalytic activity in the CO-SCR reaction. Moreover, the Ba modification increased the NO adsorption of the catalyst, further improving the catalytic activity. Owing to the better anti-oxidation ability of Ir0 in IrBa0.2/ZSM-5(27) than in Ir/ZSM-5(27), IrBa0.2/ZSM-5(27) showed better stability than Ir/ZSM-5(27). Considering that all samples in the present study were tested to simulate actual flue gases (such as sintering flue gas and coke oven flue gas), NH3 was introduced into the reaction system to serve as an extra reductant for NOx. The NOx conversion to N2 (77.1%) was substantially improved using the NH3-CO-SCR system. The proposed catalysts and reaction systems are promising alternatives for treating flue gas, which contains considerable amounts of NOx and CO in oxygen-enriched environments.

Selective catalytic reduction of nitric oxide with a novel Mn–Ti–Ce oxide core-shell catalyst having improved low-temperature activity and water tolerance

A novel core-shell-shell Mn–Ti–Ce oxide catalyst (MnOx@TiO2@CeO2) was synthesized by a three-step method and applied for the selective catalytic reduction of NOx with ammonia (NH3-SCR). The catalyst exhibited an excellent low-temperature activity with NOx conversion >80% in a broad temperature range under both dry (120–260 °C) and wet (180–255 °C) conditions with a weight hourly space velocity (WHSV) of 240,000 mL/(g·h). Nitrogen physisorption and X-ray photoelectron spectroscopy (XPS) results showed that the formation of the inner TiO2 shell significantly increased the specific surface area, surface Mn4+/Mn ratio and chemisorbed oxygen, which could provide more active sites and promote the oxidation of NO to NO2. Ammonia temperature-programmed desorption (NH3-TPD) results indicated that the formation of the outer CeO2 shell not only extensively increased the surface acid sites but also enhanced the acid strength, beneficial for the ammonia adsorption and resulting in a good water tolerance.

Ammonia free catalytic reduction of nitric oxide on Ni-embedded graphene nanostructure: A density functional theory investigation

In this study, the catalytic reduction reaction of NO (directly) without the presence of ammonia (NH3) was studied on the Ni-embedded graphene (Ni@GN) layer using periodic Density Functional Theory (DFT) calculations. Ni-embedded graphene surface can be synthesized experimentally and it is predicted that it will cost much less than single crystal surfaces due to the economic usage of the transition metal atoms. First of all, by optimizing the geometric structure of the Ni@GN layer, crucial geometric features and electron density differences (EDD) were obtained. Based on the different adsorption configurations of NO molecule, the reduction reaction was investigated by Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) based mechanisms. Finally, N2O degradation was analyzed in detail. It is shown that the Eley-Rideal model is a more dominant mechanism on the Ni@GN surface than the other model. In addition, all proposed reaction pathways for NO reduction are exothermic. This information can be used for the research and development of graphene-based materials for NO reduction; paves the way for finding new Ni-based catalysts based on active single transition metal atom embedded on different kind of defects.

Density functional theory study on the catalytic reduction of nitric oxide over the bio-char surface during biomass reburning: The influence of inherent potassium

Active sites on char surface have been proven to be effective for nitric oxide (NO) reduction. In this work, density functional theory was used to study the catalytic reduction of NO over the zigzag char surface. Two bio-char models with or without potassium were established and the reaction paths during char-NO interaction were built. The effect of inherent potassium and active sites on NO reduction was taken into consideration. Mulliken charge results showed that the migration of electron from potassium(K) atom to carbon atom would increase the electron density on char surface, which further improved the electron-donating capacity. Orbital electron distribution and ESP results proved that char(K) was more likely to donate electron to NO than char-1 during char-NO interaction. Reaction potential energy results indicated that the bonded K atom on char surface would enhance the chemisorption of NO via electrical charge transfer and reduced energy barriers from 173.7 kJ/mole to 31.6 kJ/mole, which was consistent with the kinetic analysis results. The bonding of –O-K to carbon atoms on char surface greatly activated the NO reduction during char-NO interaction, which enhanced the adsorption and reduction of NO.

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.

An improved reduced model for the evaporation and decomposition of urea-water solution (UWS) droplets

Urea-water-solution (UWS) is often used as a source for the reduction agent ammonia for the selective catalytic reduction of nitric oxides in combustion after-treatments systems. In applications, however, some technical problems, for example the formation of solid residuals or incomplete decomposition of urea, appear. To solve these problems complex phenomena must be taken into account which involve turbulent flow, multiple phases, chemical reactions of products of urea decomposition with exhaust gas etc. Hence, in order to understand and optimize the process of nitric oxides reduction, these very complex processes with different temporal and spatial scales need to be addressed with a reasonable computational effort. In this study the influence of the urea decomposition in the liquid phase onto the evaporation and decomposition of UWS droplets is investigated. It is shown that the liquid chemistry model (13 species and 13 reactions) appropriately describes the decomposition. A detailed parametric and comparative study for typical system parameters of the NOx reduction conditions is performed. Moreover, a simple but robust model reduction concept is suggested and employed to describe the process with only 3 parameters. A self-similar behavior of the droplets during the evaporation process is used for the reduced description and applied to the entire droplet evaporation process. The diameter and the species source terms can be represented with a simple polynomial model for the boundary and initial conditions ambient gas temperature, water concentration and initial droplet diameter, by using a fixed number of tabulation parameters.

Low-temperature NH3-SCR of NO over robust RuNi/Al-SBA-15 catalysts: Effect of Ru loading

Selective catalytic reduction of nitric oxide using ammonia in the presence of excess oxygen remains a major challenge. In the present work, we developed mesoporous Al-SBA-15 supported NiRu catalysts with a high surface area and ordered mesoporous structure. The monometallic Ni catalyst were prepared by the modified beta-cyclodextrin impregnation method by maintaining 1/50 mol of β-CD/Ni and bimetallic NiRu catalyst was prepared by maintaining constant loading of 8 wt% Ni with different wt%. loading of xwt% Ru (x = 2, 4, 6 and 8). The developed catalysts were characterized by XRD, N2 sorption, FT-IR, Py-IR, UV-DRS, HRSEM, HRTEM and XPS. The NH3-SCR efficiency of both monometallic (Ni) and bimetallic (NiRu) catalysts are discussed. Additionally, monometallic 8 wt% Ni and Ru supported catalysts have been also synthesized by the conventional impregnation method for purposes of comparison. The remarkable low-temperature NH3-SCR conversion (90.2% NO conversion at 300 °C) of 8Ru8Ni/Al-SBA-15 catalyst under excess oxygen condition (7%) and superior % promotion (87%) of ruthenium addition was demonstrated with excellent textural properties, stability and corroborated with the physicochemical properties of the catalyst.

Biochar supported manganese based catalyst for low-temperature selective catalytic reduction of nitric oxide

Biochar supported manganese based catalyst for low-temperature selective catalytic reduction of nitric oxide

Identification of Intrinsic Active Sites for the Selective Catalytic Reduction of Nitric Oxide on Metal-Free Carbon Catalysts via Selective Passivation

Although carbon catalysts have been commercialized for the low-temperature selective catalytic reduction (SCR) of NO with NH3 for decades, the nature of the active sites remains unclear. Herein we design a proof-of-concept study to directly evidence that the nucleophilic ketonic carbonyl groups are the intrinsic SCR active sites on metal-free carbon catalysts through ex situ and in situ selective passivation strategies. The turnover frequency of the ketonic carbonyl group is provided, and the structure–reactivity relationship is established for the first time. Density functional theory calculations combined with in situ spectroscopy reveal that the standard carbon-catalyzed SCR reaction involves a redox cycle of ketonic carbonyl/phenol (C═O/C–OH) pairs, during which the activation of NH3 is the rate-limiting step. This work identifies the intrinsic SCR active sites and advances the metal-free carbon catalysis. As a further step, the carbon catalysts demonstrate analogous mechanistic features with vanadyl catalysts, opening up the possibility to parallel the SCR mechanisms on metal-free carbon catalysts to those on transition-metal catalysts on the atomic scale.

A promising catalytic solution of NO reduction by CO using g-C3N4/TiO2: A DFT study

The direct catalytic reduction of nitric oxide (NO) by carbon monoxide (CO) to form harmless N2 and CO2 is an ideal strategy to simultaneously remove both these hazardous gases. To investigate the feasibility of using graphitic carbon nitride/titanium dioxide (g-C3N4/TiO2) to catalyze the NO reduction by CO, we systematically explore the effect of the interfacial coupling between g-C3N4 and TiO2 on the photo-induced carrier separation, the light absorption, and the surface reaction for the NO reduction by using density functional theory. The g-C3N4/TiO2 is predicted to have a better photocatalytic activity for NO reduction than g-C3N4, due to the enhanced light absorption intensity and the accelerated separation of the photo-excited electron-hole pairs. By comparing the reaction routes on g-C3N4/TiO2 and g-C3N4, the results indicate that the introduction of TiO2 can keep the surface reaction process intact with the NO dissociation (N2O formation) being the rate-determining (crucial) step. Moreover, TiO2 can facilitate the desorption of NO reduction products, avoiding the deactivation of g-C3N4. This work shows that the composition of TiO2 into g-C3N4 provides a promising catalyst in NO reduction by CO.

Catalytic Reduction of Nitric Oxide with Hydrogen Using Carbon-Supported d-Metal Catalysts

Technologies for the reduction of nitric oxide are well established and are a critical resource in reducing the emission of nitrogen oxides released during combustion. Herein, we demonstrate a more sustainable approach, utilising Earth-abundant metals supported on waste-derived carbon to facilitate this reaction. Selective catalytic reduction, whereby a reductant is used to convert NO over a catalytic bed into nitrogen, is regarded as the best available technology for NO reduction. Here, we have investigated the use of H2, which has the potential to be produced from sustainable resources, as the reductant. Three selected d-metals (copper, iron and manganese) were impregnated over palm kernel shell activated carbons via incipient wetness. The characteristics of the carbon support and the derivative catalysts were analysed to investigate structure-performance relationships. H2-SCR was performed in a fixed-bed reactor; the results showed that the supported-copper catalyst converted NO completely at temperatures of 250 °C and above. This is attributed to the high reducibility and acidity of the catalyst as demonstrated via temperature-programmed reduction, ammonia-temperature programmed desorption, Fourier-transform infra-red spectroscopy and nitric oxide adsorption–desorption experiments. It is concluded that the carbon-supported d-metal catalysts are viable for use in H2-SCR, thereby promoting a more sustainable approach to mitigating NOx emissions.Graphic Abstract

Alkali metal-resistant mechanism for selective catalytic reduction of nitric oxide over V2O5/HWO catalysts

A series of V2O5/HWO catalysts are prepared by hydrothermal and impregnation methods using different precursors, among which the V2O5/HWO-C catalyst exhibited the optimal NH3-SCR performance. Compared to oxalic acid (O) and water (W), commercial bacterial cellulose (C) as a precursor can firstly achieve a more controllable synthesis to form hexagonal WO3 (HWO) of V2O5/HWO-C catalyst. Various characterization (XRD, N2-BET, TEM, SEM, XPS, EDX mapping, and NH3/NO-TPD-MS) indicate that a higher specific surface area, abundant active oxygen and surface acidity result from the V2O5/HWO-C catalyst. The reason is that HWO-C has an excellent and smooth rod-shaped morphology, which promotes high dispersion of V2O5 on its surface. In situ IR results show that the SCR follows the Langmuir-Hinshelwood (L-H) mechanism, where absorbed NOx intermediate species are formed on the V2O5 and react with the NH4+ and NH3abs groups of V2O5 and HWO. After loading 1.75 wt% K+, the obtained K-V2O5/HWO-C catalyst exhibits effective resistance to K poisoning and SO2, and retains 78 % NOx conversion efficiency at 360 °C after 10 h, attributed to the effective capture of K+ (1.04 wt%) in HWO-C channels via a new pathway, although approximately 0.71 wt% K+ are located on HWO-C external surface with weak bonding to V2O5.

Coupling CO2 utilization and NO reduction in chemical looping manner by surface carbon

Carbon deposits typically enforce shutdowns of chemical reactors to periodically regenerate their active materials, a continuous challenge for industry and scientists alike. This downside can however be turned to benefit by utilizing carbon’s reductive power. Here, we integrate dry reforming of methane (DRM) and selective reduction of nitric oxide (SRN) in a novel chemical looping process, coupled by the periodic deposition and removal of surface carbon, which participates in both reactions. Carbon formed on the catalyst during dry reforming is subsequently used as reductant for nitric oxide (NO) conversion into N2, hence not only achieves stable CO2 utilization and NO reduction with cyclic catalyst regeneration, but also reduces the consumption of valuable energy resources demanded by NO removal. In contrast to co-feeding all reactants in a single step, the inherent temporal-spatial separation of reactions in chemical looping allows for complete conversion of carbon and NO, ensuring a higher exergetic efficiency (70 % more efficient). Also, adaptation of the relative duration of the DRM and SRN steps to the conditions of the CO2 and NO industrial point sources allows to account for the differences in supply of carbon and NO, offering an attractive strategy for selective catalytic reduction of NO with greenhouse gases rather than NH3.

A Monohydrosulfidodinitrosyldiiron Complex That Generates N2O as a Model for Flavodiiron Nitric Oxide Reductases: Reaction Mechanism and Electronic Structure.

Flavodiiron nitric oxide reductases (FNORs) protect microbes from nitrosative stress under anaerobic conditions by mediating the reduction of nitric oxide (NO) to nitrous oxide (N2O). The proposed mechanism for the catalytic reduction of NO by FNORs involves a dinitrosyldiiron intermediate with a [hs-{FeNO}7]2 formulation, which produces N2O and a diferric species. Moreover, both NO and hydrogen sulfide (H2S) have been implicated in several similar physiological functions in biology and are also known to cross paths in cell signaling. Here we report the synthesis, spectroscopic and theoretical characterization, and N2O production activity of an unprecedented monohydrosulfidodinitrosyldiiron compound, with a [(HS)hs-{FeNO}7/hs-{FeNO}7] formulation, that models the key dinitrosyl intermediate of FNORs. The generation of N2O from this unique compound follows a semireduced pathway, where one-electron reduction generates a reactive hs-{FeNO}8 center via the occupation of an Fe-NO antibonding orbital. In contrast to the well-known reactivity of H2S and NO, the coordinated hydrosulfide remains unreactive toward NO and acts only as a spectator ligand during the NO reduction process.

3D-printed monolithic catalyst of Mn-Ce-Fe/attapulgite for selective catalytic reduction of nitric oxide with ammonia at low temperature

An alternative method was taken to fabricate Mn-Ce-Fe/attapulgite (ATP) monolithic catalysts employing a direct ink writing (DIW) 3D printing technique for selective catalytic reduction reactions with ammonia (NH3-selective catalytic reduction (SCR)) at low-temperature. In this paper, the effects of different active components on powder catalysts were investigated. The results showed that Mn-Ce-Fe/ATP powder catalyst contained higher Mn4+ and Oads, and the catalyst included more reducible substances at low temperatures. The Mn-Ce-Fe/ATP powder catalyst exhibited excellent catalytic activity (NO conversion 90%, N2 selectivity 70%) over a wide temperature window range of 100–400 °C. Therefore, a series of Mn-Ce-Fe/ATP monolithic catalysts were prepared by 3D printing. The effects of porosity and space velocity on the performance of the catalysts were investigated. The result showed that the NO removal rates of Mn-Ce-Fe/ATP monolithic catalysts were 50% at 100 °C and 70% at 400 °C when the porosity was 42.82% and the gas hourly space velocity was 2000 h−1.

Mechanistic Insights into the Dicopper-Complex-Catalyzed Hydroxylation of Methane and Benzene Using Nitric Oxide: A DFT Study.

Although hydrocarbons are known to act as reductants for the catalytic reduction of nitric oxides (NOx) over copper-based catalysts, the reaction mechanism requires clarification. Herein, density functional theory (DFT) calculations were carried out to investigate the reduction mechanisms of NOx to dinitrogen coupled to the hydroxylation of methane or benzene using the dicopper complex reported by Zhang and co-workers [ J. Am. Chem. Soc. 2019, 141, 10159-10164]. The B3LYP functional was used to optimize the (μ-oxo)(μ-nitrosyl)dicopper complex in the quartet state and the (μ-η2:η2-NO2)dicopper complex in the doublet state, the latter of which was found to be the ground state. Then, we investigated the reactivities of the (μ-η2:η2-NO2)dicopper complex toward methane and benzene by considering the conversions of N2O to N2 in the presence and the absence of methane or benzene. In the presence of methane and benzene, the calculated activation energies were 27.0 and 21.0 kcal/mol, respectively, whereas that with N2O alone was prohibitively high (61.9 kcal/mol). Thus, the (μ-η2:η2-NO2)dicopper complex prefers the reactions with methane and benzene to that with N2O. The reaction of the (μ-η2:η2-NO2)dicopper complex with methane or benzene generated the (μ-nitrosyl)dicopper complex. The (μ-nitrosyl)dicopper complex then reacted with N2O to regenerate the (μ-η2:η2-NO2)dicopper complex and N2 with an activation barrier of 31.5 kcal/mol. The overall reactions for methane and benzene hydroxylation were calculated to be exothermic by 41.7 and 54.1 kcal/mol, respectively. These results suggest that the catalytic reduction of NOx using hydrocarbons is feasible at certain operating temperatures. Thus, our calculations provide new insights into the design of catalysts for NOx purification.

The enhanced catalytic activity of Cu/SAPO-34 by ion exchange method for selective catalytic reduction of nitric oxide

The Cu/SAPO-34 catalysts were prepared via the ion exchange process for achieving improved catalytic activity. Effects of various parameters including copper ions sources, copper ions loadings and ion exchange temperature on catalyst performance of the Cu/SAPO-34 catalysts were investigated. The results showed that the Cu/SAPO-34 catalysts roughly maintained similar cubic-like morphology and crystalline structure with the SAPO-34 catalyst, and achieved enhanced catalytic activity. Among various copper ion sources, the Cu/SAPO-34 catalyst using Cu(CH3COO)2 as copper ion source demonstrated high nitric oxide conversion rate. Increasing the copper ion loadings, the nitric oxide conversion rate of the Cu/SAPO-34 catalysts achieved significant improvement. However, when the copper ion loadings exceeded 0.01 mol, the nitric oxide conversion rate began to decline. The catalytic activity of the Cu/SAPO-34 also closely depended on ion exchange temperature. The nitric oxide conversion rate of the Cu/SAPO-34 catalyst showed a trend of first increasing and then decreasing with the improvement of ion exchange temperature, and the recommended ion exchange temperature was 60 °C. Hydrothermal aging treatment further confirmed the good stability of the Cu/SAPO-34 catalyst. Moreover, kinetic investigation was carried out, which was in agreement with the SCR results.

Preparation of Metal-Incorporated SAPO-34 catalysts and their Catalytic Performance in Selective Catalytic Reduction of Nitric Oxide

Abstract Metal-incorporated SAPO-34 catalysts were prepared by one-step hydrothermal method. Effects of various parameters including the types of metal ions, Cu2+ sources, structure directing agents (SDAs), hydrothermal temperature and time on catalyst activity of the metal-incorporated SAPO-34 catalysts were investigated. Three types of metal-incorporated SAPO-34 catalysts (Cu/SAPO-34, Fe/SAPO-34 and Mn/SAPO-34) were successfully obtained. Compared with Fe/SAPO-34 and Mn/SAPO-34 catalysts, Cu/SAPO-34 catalyst revealed complete cubic-like microstructure, wide active temperature window and high conversion rate in selective catalytic reduction (SCR) of nitric oxide. After defined Cu2+ was the favorable active site of SAPO-34 catalyst, the effects of four Cu2+ sources on SCR performance were further investigated, and found Cu2+ sources did not produce significant influence on nitric oxide conversion rates. SDAs determined the formation of Cu/SAPO-34 catalyst, and the Cu/SAPO-34 catalyst adopting tetraethylenepentamine (TEPA) as SDA could maintain higher crystal integrity and active sites, which were in favor of the SCR reaction. Moreover, hydrothermal temperature and time had great influences on the formation of Cu/SAPO-34 catalyst. When the hydrothermal temperature was higher than 150 oC and the hydrothermal time was longer than 3 days, the Cu/SAPO-34 catalyst with a cubic-like structure and high catalyst activity could be obtained.

Carbon deposition enhanced selective catalytic reduction of nitric oxide by a new catalytic process as well as increasing reducibility of catalyst

Carbon deposition usually hinders catalytic activity in one catalysis. In this work, carbon-deposition influence was investigated on selective catalytic reduction (SCR) of nitric oxide (NO) by a theoretical−experimental method. Density-functional-theory calculations showed that carbon deposition increased adsorption energy of NO on oxide. For example, adsorption energy on Fe2O3 increased from 1.70 to 5.27 eV. Carbon deposition increased activity by following processes: NO adsorption, NO dissociation, oxygen transmittance, CO-group formation, and N2/CO2 evolutions. Among these stages, CO-group formation was a key step. Based on these computational predictions, an experimental SCR was carried out for the verification. As a result, a carbon-deposited catalyst had a better SCR activity (20% higher) than the corresponding oxide catalyst. Characterizations showed that carbon deposition increased the amounts of medium/strong acidic sites as well as the reducibility of the catalytic center. The main result of this article helps to understand the interface behavior of carbon on a catalyst during SCR. Above results are also in favor of designing a more effective SCR reactor to ensure a more stable running.