Mechanism Of NO Oxidation Research Articles

Gas-phase oxidation of NO at high pressure relevant to sour gas compression purification process for oxy-fuel combustion flue gas

Abstract The removal of NO from oxy-fuel combustion is typically incorporated in sour gas compression purification process. This process involves the oxidation of NO to NO2 at a high pressure of 1–3 MPa, followed by absorption of NO2 by water. In this pressure range, the NO conversion rates calculated using the existing kinetic constants are often higher than those obtained experimentally. This study aimed to achieve the regression of kinetic parameters of NO oxidation based on the existing experimental results and theoretical models. Based on three existing NO oxidation mechanisms, first, the expressions for NO conversion against residence time were derived. By minimizing the mean-square errors of NO conversion ratio, the optimum kinetic rate constants were obtained. Without considering the reverse reaction for NO oxidation, similar mean-square errors for NO conversion ratio were calculated. Considering the reverse reaction for NO oxidation based on the termolecular reaction mechanism, the minimum mean-square error for NO conversion ratio was obtained. Thus, the optimum NO oxidation rate in the pressure range 0.1–3 MPa can be expressed as follows: − d NO / d t = d NO 2 / d t = 0.0026 NO 2 O 2 − 0.0034 NO 2 2 Detailed elementary reactions for N2/NO/NO2/O2 system were established to simulate the NO oxidation rate. A sensitivity analysis showed that the critical elementary reaction is 2NO + O2 ⇌ 2NO2. However, the simulated NO conversions at a high pressure of 10–30 bar are still higher than the experimental values and similar to those obtained from the models without considering the reverse reaction for NO oxidation.

Improving visible-light-driven photocatalytic NO oxidation over BiOBr nanoplates through tunable oxygen vacancies

In this work, a series of BiOBr nanoplates with oxygen vacancies (OVs) were synthesized by a solvothermal method using a water/ethylene glycol solution. The number of OVs and facets of BiOBr were tuned by changing the water/ethylene glycol ratio. Although the role of OVs in photocatalysis has been investigated, the underlying mechanisms of charge transfer and reactant activation remain unknown. To unravel the effect of OVs on the reactant activation and photocatalytic NO oxidation process, in situ diffuse reflectance infrared Fourier transform spectroscopy, so-called DRIFTS, and theoretical calculations were performed and their results combined. The photocatalytic efficiency of the as-prepared BiOBr was significantly increased by increasing the amount of OVs. The oxygen vacancies had several effects on the photocatalysts, including the introduction of intermediate energy levels that enhanced light absorption, promoted electron transfer, acted as active sites for catalytic reaction and the activation of oxygen molecules, and facilitated the conversion of the intermediate products to the final product, thus increasing the overall visible light photocatalysis efficiency. The present work provides new insights into the understanding of the role of OVs in photocatalysts and the mechanism of photocatalytic NO oxidation.

The activation of reactants and intermediates promotes the selective photocatalytic NO conversion on electron-localized Sr-intercalated g-C3N4

Photocatalysis technology has been widely adapted to address air pollution. However, the photocatalysis efficiency and selectivity should be optimized to achieve efficient and safe air purification. Take the g-C3N4 (CN) as a case study, in order to promote the photocatalytic performance and the selectivity of g-C3N4 in oxidizing NO into target products of NO2− and NO3−, the electron localization has been built in Sr-intercalated g-C3N4 (CN-Sr) to promote the activation of reactants and intermediates, as well as the charge separation and transfer. The intercalated Sr atom causes the uneven electron distribution on the plane of CN. The O2 could capture the localized excess electrons and be activated producing O2− radicals. The NO and the reaction intermediates could deplete the electrons more easily and be activated on the surface of CN-Sr. The activated species possess longer bond length and have higher reactivity during photocatalysis, making them easier to be destroyed by active radicals and transforming to target products of NO2− and NO3− rather than other toxic byproducts. With the pivotal effect of localized electrons in CN-Sr, the photocatalytic activity and selectivity can be simultaneously promoted. With the in situ DRIFTS investigation and theoretical calculation, the present work specified the transportation and transformation of photogenerated carriers and revealed the mechanism of photocatalytic NO oxidation. This work could provide a new approach to enhance the photocatalytic activity and selectivity for efficient and safe air pollution control.

In situ FT-IR investigation on the reaction mechanism of visible light photocatalytic NO oxidation with defective g-C3N4

The g-C3N4 with different structures was prepared by heat treatment using urea (CN-U) and thiourea (CN-T) as precursors under the same conditions. The microstructure and optical properties of the photocatalyst were analyzed with advanced tools. The results showed that the CN-U has a porous structure, a high specific surface area and a wide band gap in comparison with CN-T. The in situ FT-IR technique was used to monitor the adsorption and reaction process of visible photocatalytic NO oxidation on g-C3N4. The corresponding reaction mechanism was proposed based on the results of reaction intermediate observation and electron paramagnetic resonance (EPR) radical scavenging. It was revealed that (1) the presence of defective sites favored the adsorption of gas molecules and electronically compensated it leading to promoted formation of the final products; (2) the high separation efficiency of photogenerated electron-hole pairs enhanced the production of radicals during the photocatalytic reaction; (3) the hydroxyl radicals (OH) are not selective for the decomposition of pollutants, which are favorable to the complete oxidation of the reaction intermediates. The above three aspects are the main reasons for the CN-U possessing the efficient visible light photocatalytic activity. The present work could provide new insights and methods for understanding the mechanism of photocatalysis.

Quantum chemistry study on the oxidation of NO catalyzed by ZSM5 supported Mn/Co–Al/Ce

The detailed mechanism of NO oxidation catalyzed by ZSM5 supported Mn/Co–Al/Ce is investigated and revealed by Quantum Chemistry Calculation. A three-step catalytic mechanism for NO oxidation is proposed and studied. Theoretical results show that, the activate energies of reactions catalyzed by ZSM-5 supported Mn/Co (71.1[Formula: see text]kJ/mol/80.6[Formula: see text]kJ/mol) are much lower than that obtained from the direct NO oxidation. This indicates that the ZSM-5 supported Mn/Co has an obvious catalytic effect. When the active center Si is replaced by Al and Ce, the activation energies are further decreased to about 40[Formula: see text]kJ/mol. This indicates that the doping of Al and Ce can obviously improve the catalytic effect. The theoretical study illustrates that the catalysts for NO oxidation not only relate to the supported transition metal such as Co and Mn, but also highly relate to the activity centers such as Al and Ce.

2D BiOCl/Bi12O17Cl2 nanojunction: Enhanced visible light photocatalytic NO removal and in situ DRIFTS investigation

Novel two-dimensional (2D) BiOCl/Bi12O17Cl2 nanojunctions were fabricated by a facile one-pot in situ method at room temperature. The as-prepared samples were analyzed by XRD, SEM, TEM, HRTEM, UV–vis DRS, PL, ESR and BET-BJH measurement in detail. The photocatalytic performance of the samples was evaluated by removal of NO at ppb level under visible-light illumination. The result reveals that the BiOCl/Bi12O17Cl2 nanojunctions manifests conspicuously enhanced photocatalytic efficiency for NO removal. The facilitated performance can be ascribed to the well-matched band structure and relatively high specific surface area. In addition, the in situ diffuse reflectance infrared Fourier transform spectroscopy was applied to investigate the adsorption and photocatalytic NO oxidation processes. The reaction mechanism of photocatalytic NO oxidation was proposed based on the observed intermediates. The present work could pave a way to synthesize novel visible light photocatalysts at room temperature for environmental application.

In situ DRIFT investigation on the photocatalytic NO oxidation mechanism with thermally exfoliated porous g-C3N4nanosheets

A cell for FT-IR with anin situphoto-reactor was developed to investigate the photocatalytic NO oxidation mechanism on foliated porous g-C3N4nanosheets.

The active sites and mechanism of NO oxidation on modified activated carbon

The industrial-grade coconut shell activated carbon (AC) was modified by nitric acid (HNO3) and employed for NO oxidation. The effect of calculation temperature was investigated. It was demonstrated that the modified AC without calcination exhibited the highest catalytic activity. Subsequently, combined with BET, FTIR and TPD analysis, it suggested that the specific surface area of catalyst had no direct relation with catalytic activity and the nitro and C=O bonds were the active sites in favor of NO conversion. Finally, the mechanism was further discussed.

Plasmonic Bi metal as cocatalyst and photocatalyst: The case of Bi/(BiO)2CO3 and Bi particles

Semimetal bismuth with plasmonic properties has triggered increased interests. In this work, a facile strategy was developed to synthesize the Bi/(BiO)2CO3 (Bi-BOC) nanocomposites and Bi elemental photocatalysts. The Bi nanoparticles were produced via the insitu reduction of (BiO)2CO3 by NaBH4. The catalysts were utilized for the photocatalytic NO removal under visible light and UV illumination. Significantly, the photocatalytic capability of the Bi-BOC was highly enhanced with an unprecedented NO removal of 63.6%. The Bi metal demonstrated a direct plasmonic photocatalytic NO removal ratio of 53.6% under UV irradiation. The significantly enhanced photocatalytic capability of Bi-BOC can be ascribed to the synergistic effects of the SPR effect, enhanced visible-light-harvesting and the efficient electron–hole separation induced by Bi nanoparticles. The Bi nanoparticles can perform as a non-noble metal-based plasmonic cocatalyst for advancing photocatalytic ability. The mechanism of photocatalytic NO oxidation was proposed and compared under both visible light and UV illumination. Furthermore, the Bi-BOC photocatalysts showed good photochemical stability under repeated tests. This work could not only offer new insights into in-situ fine-tune reduction strategy for Bi-based photocatalysts, but also proves the potentials of utilizing low cost Bi cocatalysts as a substitute for noble metals to improve other photocatalysts.

Kinetic Modeling of C3H6 Inhibition on NO Oxidation over Pt Catalyst

Exhaust after treatment for lean burn and diesel engine is a complex catalytic system that consists of a number of catalytic units. Pt/Al2O3 is often used as a model Diesel Oxidation Catalyst (DOC) that plays an important role to facilitate oxidation of NO to NO2. In the present study, we proposed a detailed kinetic model of NO oxidation as well as low temperature C3H6 inhibition to simulate temperature-programmed reaction (TPR) data for NO oxidation over Pt/Al2O3. A steady-state microkinetic model based on Langmuir-Hinshelwood mechanism for NO oxidation was proposed. In addition, low temperature C3H6 inhibition was proposed as a result of site blocking as well as surface nitrite consumption. The model can explain the experimental data well over the studied temperature range.

The Low Temperature Interaction of NO + O2 with a Commercial Cu-CHA Catalyst: A Chemical Trapping Study

We investigate the interaction of NO + O2 with a Cu-CHA SCR catalyst in the low temperature region. To this purpose, a well validated experimental protocol, including a combination of transient response methods, ex situ FTIR spectroscopy and chemical trapping techniques, is applied. In particular, the combination of a Cu-CHA catalyst powder and a BaO/Al2O3 NOx trap in a physical mixture is used in an attempt to capture and stabilize reactive and possibly elusive NOx intermediates, which might go otherwise undetected. As a result, direct evidence for the formation of nitrites in the oxidative activation of NO over Cu-CHA is obtained. Identification of trapped nitrites intermediates has been confirmed in several ways, including: (i) their thermal decomposition to an equimolar mix of NO and NO2 during temperature programmed desorption, (ii) N2 formation during their reaction with NH3 at low temperature, (iii) ex situ IR analysis of the BaO/Al2O3 phase after discharge and separation from the mechanical mixture. On such a basis, a low temperature redox mechanism for NO oxidation and Standard SCR reaction over Cu-zeolite catalysts is proposed and discussed.

Ambient-temperature NO oxidation over amorphous CrOx-ZrO2 mixed oxide catalysts: Significant promoting effect of ZrO2

A series of novel CrOx-ZrO2 mixed oxide catalysts are prepared via a sol-gel method. Within a range of Cr/Zr atomic ratios, the mixed oxides maintain high surface area, homogeneous amorphous phases. As compared to CrOx-only catalysts formed using the same method, the addition of zirconia greatly enhances the catalytic performance for ambient-temperature, low-concentration NO oxidation. X-ray Photoelectron Spectroscopy (XPS) and Electron Paramagnetic Resonance (EPR) analyses indicate an electronic effect of ZrO2 addition to the oxidation state of Cr. That is, ZrO2 addition induces an increase in surface concentrations of Cr6+. Rapid deactivation of a pre-reduced catalyst, coupled with the fact that a deactivated catalyst contains lower concentrations of surface Cr6+, provide rather strong evidence for a Mars-van Krevelen NO oxidation mechanism. Such a mechanism is also consistent with in situ DRIFTS observations.

Macrokinetics of NO Oxidation by Vaporized H2O2 Association with Ultraviolet Light

The macrokinetics of NO oxidation by vaporized H2O2 association with ultraviolet light was evaluated. The effects of various influencing factors on the NO oxidation rate were investigated, such as the H2O2 concentration, the H2O2 pH, the reaction temperature, the UV energy density, the UV light wavelength, the coexistence of gases (SO2 and O2), and the radical scavenger tert-butanol. The NO oxidation was conformed to a pseudo-first-order kinetics pattern under the experimental conditions. The rate constants determined at 373–413 K fitted the Arrhenius equation well, yielding an apparent activation energy of 20.8 kJ/mol. The experimental results indicated that both UV power density and UV light wavelength exhibited significant influences on NO oxidation, and decreasing pH and increasing O2 concentration could enhance NO oxidation. A mechanism of NO oxidation was also proposed.

A first principles investigation of the NO oxidation mechanism on Pt/γ-Al2O3

We report first principle calculations about the NO oxidation mechanism on Ptn/γ-Al2O3(110) with an aim to improve the understanding of the catalytic activity and the catalytic process.

Mechanism of NO Photocatalytic Oxidation on g-C3-N4 Was Changed by Pd-QDs Modification. [Corrected

Quantum dot (QD) sensitization can increase the light absorption and electronic transmission of photocatalysts. However, limited studies have been conducted on the photocatalytic activity of photocatalysts after modification by noble metal QDs. In this study, we developed a simple method for fabricating Pd-QD-modified g-C3N4. Results showed that the modification of Pd-QDs can improve the NO photocatalytic oxidation activity of g-C3N4. Moreover, Pd-QD modification changed the NO oxidation mechanism from the synergistic action of h+ and O2− to the single action of ·OH. We found that the main reason for the mechanism change was that Pd-QD modification changed the molecular oxygen activation pathway from single-electron reduction to two-electron reduction. This study can not only develop a novel strategy for modifying Pd-QDs on the surface of photocatalysts, but also provides insight into the relationship between Pd-QD modification and the NO photocatalytic oxidation activity of semiconductor photocatalysts.

Immobilization of self-stabilized plasmonic Ag-AgI on mesoporous Al2O3 for efficient purification of industrial waste gas with indoor LED illumination

The immobilization of a photocatalyst on a proper support is pivotal for practical environmental applications. In this work, the plasmonic Ag-AgI with photoinduced self-stability was successfully immobilized on mesoporous Al2O3 by a wet impregnation-precipitation and in-situ photoreduction method. The immobilized Ag-AgI/Al2O3 was applied for photocatalytic removal of ppm-leveled NO under illumination of indoor LED tubes, which could obtain 100% NO removal in minutes at optimized conditions. The large dispersion of Ag-AgI on mesoporous Al2O3 was firmly enough to overwhelm the continuous air flowing, and the enlarged surface area of Ag-AgI could efficiently enhance NO removal efficiency than pure Ag-AgI powder. Most importantly, the plasmonic Ag-AgI/Al2O3 was stable in activity and can be used repeatedly without deactivation. On the basis of reactive species trapping and reaction intermediate monitoring, the responsible reactive species of photogenerated OH, O2−, H2O2 and h+ were confirmed and the possible reaction mechanism of photocatalytic NO oxidation by Ag-AgI/Al2O3 was revealed. The present work could provide new perspectives for promoting large-scale environmental applications of supported plasma photocatalysts.

DFT Analysis of NO Oxidation Intermediates on Undoped and Doped LaCoO3 Perovskite

Perovskites are of interest as low-cost replacements for Pt-based NO oxidation catalysts. While the mechanism of Pt-catalyzed NO oxidation is fairly well understood, such is not the case for the oxides. The perovskite LaCoO3 itself has been shown to have good NO oxidation activity, and Sr substitution improves NO oxidation rates and reduces NO2 inhibition. In this work, we report density functional theory (DFT) results for the adsorption of NOx (x = 1, 2) to the undoped and Sr doped (100) LaO and CoO2 terminated LaCoO3. Further, we used first-principles thermodynamic models to determine the most common surface species under NO oxidation conditions. Nitrates and adsorbed NO are most stable on the LaO and CoO2 terminations, respectively. We explored the relative free energies of surface and vacancy-mediated pathways for NO oxidation. The vacancy-mediated pathways suffer from energetically costly removal of surface oxygen, while the surface pathway is most feasible for NO oxidation to occur. The perovskite s...

Investigation of NO2 and NO interaction with an Fe-ZSM-5 catalyst by transient response methods and chemical trapping techniques

We investigate the interaction of both NO2 and NO with an Fe-ZSM-5 catalyst under oxidative conditions at low temperature. A combination of transient response methods, chemical trapping techniques, and ex situ IR spectroscopy is used to gain insight into the nature of the resulting surface intermediates, which are relevant to the mechanisms of NO oxidation to NO2 and of the standard NH3-SCR reactions. For this purpose, three different systems are comparatively studied in various transient runs: (i) a commercial Fe-ZSM-5 catalyst in powder form, (ii) a BaO/Al2O3 powder prepared in house, i.e., a well-known NOx trap, and (iii) a physical mixture of the two compounds. Specifically, the BaO/Al2O3–Fe-ZSM-5 mixture is used in an attempt to trap and stabilize reactive and possibly elusive NOx intermediates formed on Fe-ZSM-5, which might otherwise go undetected. Several important results are obtained, including (i) direct evidence of nitrite formation in the oxidative activation of NO over Fe-ZSM-5; (ii) exclusion of nitrates as primary reaction intermediates in the low-temperature standard SCR reaction; and (iii) evidence for the interaction between nitrites and nitrates adsorbed onto the two component phases of the physical mixture, possibly via the corresponding gas-phase species HONO and HNO3. On this basis, a low-temperature redox mechanism for NO oxidation over Fe-zeolite catalysts, which fully accounts for the new experimental observations, is proposed and discussed.

Mechanism of visible light photocatalytic NO(x) oxidation with plasmonic Bi cocatalyst-enhanced (BiO)2CO3 hierarchical microspheres.

Semimetal bismuth (Bi), as an emerging non-noble metal-based cocatalyst and plasmonic photocatalyst, has attracted significant attention. In this work, a one-pot solvent-controlled synthesis strategy was utilized for the in situ-deposition of plasmonic Bi nanoparticles onto the surfaces of (BiO)2CO3 microspheres (BOC-WE) using bismuth citrate, sodium carbonate, and ethylene glycol as precursors. The introduction of the Bi nanoparticles has a pivotal effect on the morphology, optical and photocatalytic performance of the pristine (BiO)2CO3. The results indicated that the Bi nanoparticles were generated on the surface of (BiO)2CO3 microspheres via the in situ reduction of Bi(3+) by ethylene glycol. The Bi-deposited (BiO)2CO3 microspheres were used for the photocatalytic purification of NOx in air under visible light irradiation. Significantly, the BOC-WE samples exhibited a drastically promoted photocatalytic performance with a NOx removal ratio (η) of 37.2%, superior to pristine (BiO)2CO3 (η = 19.1%), outperforming other well-known visible light photocatalysts, such as C-doped TiO2 (η = 21.8%), BiOBr (η = 21.3%), BiOI (η = 14.9%) and C3N4 (η = 25.5%). The conspicuously enhanced photocatalytic capability can be attributed to the synergistic effects of the surface plasmon resonance (SPR) effect, increased visible light absorption and the efficient separation of electron-hole pairs induced by the Bi nanoparticles. The Bi nanoparticles can act as a non-noble metal-based cocatalyst for strengthening photocatalytic performance, which is similar to the behavior of noble metals (Au, Ag) in enhancing photocatalysis. The mechanism of visible light photocatalytic NOx oxidation was investigated. DMPO-ESR spin-trapping results demonstrated that hydroxyl radicals were confirmed to be the main active species for NOx photo-oxidation. Due to the SPR effect of Bi, the BOC-WE could produce more hydroxyl radicals than BOC, which was responsible for the enhanced NO photo-oxidation ability. Moreover, the BOC-WE photocatalysts showed high photochemical stability under repeated irradiation. This work demonstrates the feasibility of utilizing low cost Bi cocatalysts as a substitute for noble metals to enhance the performance of other photocatalysts. This work could not only provide new insights into the in situ fabrication of Bi/semiconductor nanocomposites, but also pave a new way for the modification of photocatalysts with non-noble metals as cocatalysts to achieve an enhanced performance for environmental and energetic applications.

Water-assisted production of honeycomb-like g-C3N4 with ultralong carrier lifetime and outstanding photocatalytic activity.

Graphitic carbon nitride (g-C3N4) is a visible light photocatalyst, limited by low activity mainly caused by rapid recombination of charge carriers. In the present work, honeycomb-like g-C3N4 was synthesized via thermal condensation of urea with addition of water at 450 °C for 1 h. Prolonging the condensation time caused the morphology of g-C3N4 to change from a porous honeycomb structure to a velvet-like nanoarchitecture. Unlike in previous studies, the photocatalytic activity of g-C3N4 decreased with increasing surface area. The honeycomb-like g-C3N4 with a relatively low surface area showed highly enhanced photocatalytic activity with an NO removal ratio of 48%. The evolution of NO2 intermediate was dramatically inhibited over the honeycomb-like g-C3N4. The short and long lifetimes of the charge carriers for honeycomb-like g-C3N4 were unprecedentedly prolonged to 22.3 and 165.4 ns, respectively. As a result, the honeycomb-like g-C3N4 was highly efficient and stable in activity and could be used repeatedly. Addition of water had the following multiple positive effects on g-C3N4: (1) formation of the honeycomb structure, (2) promotion of charge separation and migration, (3) enlargement of the band gap, (4) increase in production yield, and (5) decrease in energy cost. These advantages make the present preparation method for highly efficient g-C3N4 extremely appealing for large-scale applications. The active species produced from g-C3N4 under illumination were confirmed using DMPO-ESR spin-trapping, the reaction intermediate was monitored, and the reaction mechanism of photocatalytic NO oxidation by g-C3N4 was revealed. This work could provide an attractive alternative method for mass-production of highly active g-C3N4-based photocatalysts for environmental and energetic applications.