Deep Oxidation of NO by a Hybrid System of Plasma-N-Type Semiconductors: High-Energy Electron-Activated "Pseudo Photocatalysis" Behavior

Dual-course dielectric barrier discharge with a novel hollow micro-holes electrode to efficiently mitigate NOx

The effect of a novel hollow annular micro-hole electrode on the DBD de-NOx performance was investigated. The experimental results show that the hollow electrode allows the feed gas to take full advantage of the redundant heat of the electrode, thus reducing the energy consumption of the system. Subsequently, the micro-hole structure can improve the uniformity of feed gas in the plasma channel and prolong the residence time of the feed gas in the plasma channel. The reactor can also raise the temperature of the feed gas and enhance the plasma electric field. The optimum NOx removal efficiency of about 82.6% is achieved at 16 annular micro-holes. Compared to the rod electrode reactor, the novel electrode reactor shows 19.7% reduction in energy consumption and 13.2% enhancement in de-NOx efficiency. The calculations of de-NOx mechanism show that the NO2 concentration decays significantly as the feed gas residence time increases, accompanied by a slight increase in N2O concentration. The NO2 concentration marginally increases while N2O concentration slightly decreases as the increase of feed gas temperature. DBD de-NOx presents the mode of accelerated reduction of NO, essential removal of NO2, and gradual consumption of N2O with the reduced electric field increases.

Synergistic catalysis induced by a multi-component system constructed by DBD plasma combined with α-Fe2O3/FeVO4/HCP and peroxymonosulfate for gatifloxacin removal

The dielectric barrier discharge (DBD) multi-component system containing plasma, α-Fe₂O₃/FeVO₄, and peroxymonosulfate (PMS) with high catalytic activity was successfully constructed. Thereinto, α-Fe₂O₃/FeVO₄ was loaded on the honeycomb ceramic plate (HCP) surface (α-Fe₂O₃/FeVO₄/HCP) and placed under the water surface below the discharge area. The catalytic activity was evaluated by the removal rate of gatifloxacin (GAT), and the DBD+α-Fe₂O₃/FeVO₄+PMS system exhibited the optimal catalytic activity. The enhanced catalytic activity can be attributed to the fact that the occurrence of synergistic catalysis that simultaneously includes plasma oxidation, photocatalysis, PMS oxidation, O₃ catalysis, and Fenton reaction. The effect of various initial degradation parameters including input power, PMS dosage, pH, etc. On GAT removal was investigated. DBD+α-Fe₂O₃/FeVO₄+PMS system has a significant increase in the concentration of H₂O₂ and O₃, and the role played in the multi-component system was analyzed. The identification and analysis of organic matters during GAT degradation were visualized with the help of 3D EEMs. HPLC-MS and theoretical calculations identified the major intermediates and further deduced the possible GAT degradation pathways. Additionally, the acute toxicity of the major intermediates was predicted by the QSAR model. Finally, the possible mechanisms of synergistic catalysis to enhance catalytic activity were discussed based on the characteristics of several advanced oxidation processes (AOPs) and the results of experimental and characterization. This work provides a feasible technical route and theoretical basis for wastewater treatment by plasma combined with other AOPs.

Multi-catalysis of glow discharge plasma coupled with FeS2 for synergistic removal of antibiotic

Fe-based composites improved the energy utilization efficiency of plasma for removing contaminants through multi-catalysis have received much attention. However, the energy efficiency and catalytic activity are compromised by the slow transformation from Fe (Ⅲ) to Fe (Ⅱ). Here, given the electron-donating ability of reducing sulfur species, as well as the acidic environment generated by FeS₂, single FeS₂ was introduced into the glow discharge plasma (GDP) reactor for the removal of tylosin (TYL). The results showed that a significant synergistic effect between FeS₂ and GDP improved the energy efficiency of plasma and the removal efficiency of TYL (99.7%). FeS₂ boosted the generation of radicals (·OH, ·O₂^-) and nonradicals (h⁺, e^-) rather than H₂O₂ and O₃, which played an important role in TYL abatement. Moreover, the electrons donating sulfur and iron species from FeS₂ can accelerate the conversion of Fe(III) to Fe(II), which was conducive to the generation of radicals. Besides, acid solution self-adjustment resulted from the oxidation of FeS₂ improved heterogeneous Fenton reaction, the oxidation potential of ·OH and adsorption of positive charged TYL. The plausible degradation pathways of TYL were proposed in GDP/FeS₂ system. In summary, enhanced removal of TYL was mainly attributed to the catalytic pathway altered by FeS₂ through high-energy electrons, photocatalysis, heterogeneous Fenton and O₃ catalysis in the GDP system simultaneously. The strategy of integrating GDP with FeS₂ proposed in this work is expected to offer a feasible and potential technique for organic wastewater treatment.

Switching on photocatalytic NO oxidation and proton reduction of NH2-MIL-125(Ti) by convenient linker defect engineering

Photocatalysis technology has been widely adopted to abate typical air pollutants. Nevertheless, developing photocatalysts aimed at improving photocatalytic efficiency is a challenge. Herein, the linker-defect NH₂-MIL-125(Ti) photocatalyst was synthesized through a convenient one-step heating-stirring method (just adjusting multiple temperatures) to firstly realize efficient photocatalytic performances of NO removal and hydrogen evolution. The optimal sample (named 65-NMIL) with a linker-defect content of 32.08% exhibited a NO removal ratio of 65.49%, which was 37.57% higher than that of pristine NH₂-MIL-125(Ti), and displayed better H₂-production activity. Through ESR, it was confirmed that 65-NMIL can generate more •O₂^- and •OH under visible light, and the radical trapping experiment further proved that •O₂^- played a more important role in photocatalytic activity. Moreover, the photocatalytic NO oxidation process was also monitored by in situ DRIFTS, it was found that the defective samples could promote the oxidation of NO and intermediates to the final product (NO₃^-). On the basis of the above-mentioned photocatalytic experimental results and characterization, a possible mechanism or pathway was proposed and illustrated. This work can provide a new strategy for the subsequent defect engineering for photocatalytic MOFs materials to further solve environmental and energy crises.

Activation and characterization of environmental catalysts in plasma-catalysis: Status and challenges

Plasma-catalysis has attracted great attentions in environmental/energy-related fields, but the synergetic mechanism still suffers intractable defects. Key issues are that what kind of catalysts are applicable for plasma system, how are they activated in plasma, and how to characterize them in plasma. This review systematically gives a comprehensive summarization of the selection of catalysts and its activation mechanism in plasma, based on the character of plasma, including physical effects containing the enhancement of discharge intensity and adsorption of reactants, and the utilization of plasma-generated active species such as·O, heat, O₃, ultraviolet light and e* . Focus is given to the illumination of the activation mechanisms of catalysts when placed in plasma zone. Subsequently, the novel characterization techniques for catalysts, which may associate properties to performance, are critically overviewed. The challenges and opportunities for the activation and characterizations of catalysts are proposed, and future perspectives are suggested about where the efforts should be made. It is expected that a bridge between catalysts design and character of plasma can be built to shed light on the synergetic mechanism for plasma-catalysis and design of new plasma-catalysis systems.

Promotion mechanism of -OH group intercalation for NOx purification on BiOI photocatalyst

Bismuth oxyiodide (BiOI) is a traditional layered oxide photocatalyst that performs in a wide visible-light absorption band, owing to its appropriate band structure. Nevertheless, its photocatalytic efficiency is immensely inhibited due to the serious recombination of photogenerated charge carriers. Herein, this great challenge is addressed via a new strategy of intralayer modification by -OH groups in BiOI, which leads to enhancement of the reactants' activation capacity to promote photocatalytic activity and generate more active species. Furthermore, analysis via a combination of experimental and theoretical methods revealed that the -OH group-functionalized samples reduce the energy barriers for conversion of the main intermediate (NO₂), which is easily transformed to NO₂^-, thus accelerating the oxidation of NO to the final product (NO₃^-). This study gives insight into NO oxidation, improving the photocatalytic efficiency, and mastering the photocatalysis reaction mechanism to curb air pollution.

Efficient visible light photocatalytic NO abatement over SrSn(OH)6 nanowires loaded with Ag/Ag2O cocatalyst

SrSn(OH)₆ (SSOH) possesses a high oxidation potential in the valence band (VB), which is suitable for photocatalytic oxidation removal of pollutants. However, the electrons in the VB of these catalysts are difficult to transition to the conduction band (CB) under visible light, which makes it difficult to utilize sunlight effectively. In this work, Ag/Ag₂O is loaded on the surface of SSOH nanowires, which stimulates the interfacial charge-transfer transition on SSOH. Compared with pure-phase SSOH, the NO abatement ratio of Ag/Ag₂O-SSOH under visible light irradiation is increased to 45.10%. The e^- in the VB of Ag₂O are excited into the CB under visible light, and are further transferred to the Ag to react with O₂ to produce superoxide radicals. The photo-excited e^- in the VB of SSOH enter into the VB of Ag₂O through interfacial charge-transfer transition to recombine with the photo-generated holes in the VB of Ag₂O, thereby leaving photo-generated holes in the VB of SSOH. The holes in the VB of SSOH have sufficient oxidizing ability to oxidize the adsorbed hydroxyl groups into hydroxyl radicals. This work provides a new perspective for photocatalytic removal of pollutants by wide band gap photocatalyst under visible light.

In-plasma-catalysis for NO x degradation by Ti3+ self-doped TiO2-x /γ-Al2O3 catalyst and nonthermal plasma

In an attempt to realize the efficient treatment of NO_x, a mixed catalyst of Ti³⁺ self-doped TiO_2−x and γ-Al₂O₃ was constructed by reducing commercial TiO₂. The degradation effect on NO_x was evaluated by introducing the mixed catalyst into a coaxial dual-dielectric barrier reactor. It was found that the synthesized TiO_2−x could achieve considerable degradation effects (84.84%, SIE = 401.27 J L⁻¹) in a plasma catalytic system under oxygen-rich conditions, which were better than those of TiO₂ (73.99%) or a single plasma degradation process (26.00%). The presence of Ti³⁺ and oxygen vacancies in TiO_2−x resulted in a relatively narrow band gap, which contributed to catalyzing deeply the oxidation of NO_x to NO₂⁻ and NO₃⁻ during the plasma-induced “pseudo-photocatalysis” process. Meanwhile, the TiO_2−x showed an improved discharge current and promoted discharge efficiency, explaining its significant activation effect in the reaction. Reduced TiO_2−x could achieve an impressive degradation effect in a long-time plasma-catalysis process, and still maintained its intrinsic crystal structure and morphology. This work provides a facile synthesis procedure for preparing Ti³⁺ self-doped TiO_2−x with practical and scalable production potential; moreover, the novel combination with plasma also provides new insights into the low-temperature degradation of NO_x.

TiO_2−x has a smaller forbidden band width, more abundant Ti³⁺ and oxygen vacancies, so as to obtain a better and more stable degradation effect of NO_x in plasma-catalysis process.

Neodymium oxide (Nd2O3) coupled tubular g-C3N4, an efficient dual-function catalyst for photocatalytic hydrogen production and NO removal

Graphitic carbon nitride (g-C₃N₄) has emerged as a most promising photocatalyst, non-toxicity and low density, but it is plagued by low activity due to the small specific surface area and poor quantum efficiency. Morphological engineering and coupling with other materials to form hybrids have proven to be effective strategies for enabling high photocatalytic performances. Here, neodymium oxide (Nd₂O₃) coupled tubular g-C₃N₄ composites had been facilely synthesized by a solvent evaporation and high-temperature calcination method to realize efficient photocatalytic activity of hydrogen production and NO removal. A series of characterizations, such as XRD, ESR, in-situ DRIFTS, etc., were used to analyze the physical and chemical properties of the bifunctional photocatalyst, which demonstrated that the composite material had more active sites and a faster electron transfer rate. The optimal sample (1 wt% Nd₂O₃/CN-T) had a H₂ generation rate of 4355.34 μmol·g^-1·h^-1, which was 9.46 times than that of original g-C₃N₄ obtained through heating melamine (CN-M). In addition, the NO removal rate was also 32.32% higher than that of original CN-M. On the basis of the above photocatalytic experimental results and characterizations, a possible mechanism or pathway was proposed and illustrated. This work could provide a feasible strategy to fabricate tubular g-C₃N₄-based composites with rare earth metal oxides (dual-factor regulation) to simultaneously enhance photocatalytic hydrogen production and NO removal efficiently (double application).

Mechanisms of Active Substances in a Dielectric Barrier Discharge Reactor: Species Determination, Interaction Analysis, and Contribution to Chlorobenzene Removal

Several typical active substances (^•NO, ^•NO₂, H₂O₂, O₃, ^•OH, and O₂^-•), directly or indirectly play dominant roles during dielectric barrier discharge (DBD) reaction. This study measured these active substances and removed them by using radical scavengers, such as catalase, superoxide dismutase, carboxy-PTIO (c-PTIO), tert-butanol (TBA), and MnO₂ in different reaction atmospheres (air, N₂, and O₂). The mechanism for chlorobenzene (CB) removal by plasma in air atmosphere was also investigated. The production of O═NOO^-• generated by ^•NO took around 75% of the total production of O═NOO^-•. Removing ^•NO increased the O₃ amount by about 80% likely because of the mutual inhibition between O₃ and reactive nitrogen species in or out of the discharge area. The quantitative comparison of ^•OH and H₂O₂ revealed that the formation of ^•OH was 3.06-4.65 times that of H₂O₂ in these reaction atmospheres. Calculation results showed that approximately 1.61% of H₂O was used for O₃ generation. Ionization patterns affected the form of solid deposits during the removal of CB in N₂ and O₂ atmospheres caused by Penning ionization and thermal radiation tendencies, respectively. Correlation analysis results suggested the macroscopic synergistic or inhibitory effects happened among these active substances. A zero-dimensional reaction kinetics model was adopted to analyze the reactions during the formation of active substances in DBD, and the results showed good consistency with experiments. The interactions of each active substance were clarified. Finally, a response surface method model was developed to predict CB removal by the DBD plasma process. Stepwise regression analysis results showed that CB removal was affected by the contents of different active substances in air, N₂ atmosphere, and O₂ atmosphere, respectively: O₂^-•, ^•OH, and O₃; H₂O₂, O═NOO^-•, and O₃; ^•OH and O₃.

Understanding of TiO2 catalysis mechanism in underwater pulsed discharge system: Charge carrier generation and interfacial charge-transfer processes

Compared with traditional photocatalysis system, TiO₂ charge carrier generation and interfacial charge-transfer process may be influenced by various chemical and physical effects in underwater pulsed discharge plasma system. Here, the role of high-energy electron, ozone in TiO₂ charge carrier generation and transfer process has been investigated using phenol as the probe molecule. The introduction of electron-trapping agent (KH₂PO₄) have an inhibiting effect on TiO₂ catalytic activity, indicating high-energy electrons played a significant role in TiO₂ catalytic process. EPR analysis showed that TiO₂ could be activated to initiate pairs of electron-hole by high-energy electrons from plasma, and the electrons on the conduction band (CB) could be trapped on the oxygen vacancies. XPS analysis showed that the Ti³⁺OH species formed during discharge process due to the capture of CB electrons by Ti⁴⁺OH groups located at the TiO₂ surface. The CB electrons transfer processes on TiO₂ surface was strongly dependent on the redox potential of electron acceptors, which adsorbed on the TiO₂ surface. The CB electrons can be transferred to dissolved O₃, resulting in more OH production. Meanwhile, the CB electron also transferred to benzoquinone adsorbed on TiO₂, resulting in accumulation of hydroquinone.

Efficient removal of antibiotic thiamphenicol by pulsed discharge plasma coupled with complex catalysis using graphene-WO3-Fe3O4 nanocomposites

Pulsed discharge plasma (PDP) induced complex catalysis for synergetic removal of thiamphenicol (TAP) was investigated using graphene-WO₃-Fe₃O₄ nanocomposites. The prepared samples were characterized systematically in view of the structure and morphology, chemical bonding state, optical property, electrochemical property and magnetic property. Based on characterization and TAP degradation, the catalytic performance followed: graphene-WO₃-Fe₃O₄>graphene-WO₃>WO₃, and the highest removal efficiency and kinetic constant could reached 99.3% and 0.070 min^-1, respectively. With increase of catalyst dosage, the removal efficiency firstly enhanced and then declined. Lower pH value was beneficial for TAP degradation. The prepared graphene-WO₃-Fe₃O₄ owed higher stability and lower dissolution rate of iron ion. The rGO-WO₃-Fe₃O₄ could decompose O₃ and H₂O₂ into more ·OH in PDP system. The degradation intermediates were characterized by fluorescence spectrograph, LC-MS and IC. Based on the detected intermediates and discrete Fourier transform (DFT) analysis, degradation pathway of TAP was proposed. Besides, the toxicity of intermediates was predicted. Finally, catalytic degradation mechanism of TAP by PDP with graphene-WO₃-Fe₃O₄ was summarized.

Non-Thermal Plasma Coupled with Catalyst for the Degradation of Water Pollutants: A Review

Non-thermal plasma is one of the most promising technologies used for the degradation of hazardous pollutants in wastewater. Recent studies evidenced that various operating parameters influence the yield of the Non-Thermal Plasma (NTP)-based processes. In particular, the presence of a catalyst, suitably placed in the NTP reactor, induces a significant increase in process performance with respect to NTP alone. For this purpose, several researchers have studied the ability of NTP coupled to catalysts for the removal of different kind of pollutants in aqueous solution. It is clear that it is still complicated to define an optimal condition that can be suitable for all types of contaminants as well as for the various types of catalysts used in this context. However, it was highlighted that the operational parameters play a fundamental role. However, it is often difficult to understand the effect that plasma can induce on the catalyst and on the production of the oxidizing species most responsible for the degradation of contaminants. For this reason, the aim of this review is to summarize catalytic formulations coupled with non-thermal plasma technology for water pollutants removal. In particular, the reactor configuration to be adopted when NTP was coupled with a catalyst was presented, as well as the position of the catalyst in the reactor and the role of the main oxidizing species. Furthermore, in this review, a comparison in terms of degradation and mineralization efficiency was made for the different cases studied.

Synergistic effects of crystal structure and oxygen vacancy on Bi2O3 polymorphs: intermediates activation, photocatalytic reaction efficiency, and conversion pathway

This work unraveled the synergistic effects of crystal structure and oxygen vacancy on the photocatalytic activity of Bi₂O₃ polymorphs at an atomic level for the first time. The artificial oxygen vacancy is introduced into α-Bi₂O₃ and β-Bi₂O₃ via a facile method to engineer the band structures and transportation of carriers and redox reaction for highly enhanced photocatalysis. After the optimization, the photocatalytic NO removal ratio on defective β-Bi₂O₃ was increased from 25.2% to 52.0% under visible light irradiation. On defective α-Bi₂O₃, the NO removal ratio is just increased from 7.3% to 20.1%. The difference in the activity enhancement is associated with the different structure of crystal phase and oxygen vacancy. The density functional theory (DFT) calculation and experimental results confirm that the oxygen vacancy in α-Bi₂O₃ and β-Bi₂O₃ could promote the activation of reactants and intermediate as active centers. The crystal structure and oxygen vacancy could synergistically regulate the electrons transfer pathway. On defective β-Bi₂O₃ with tunnel structure, the reactants activation and charge transfer were more efficient than that on α-Bi₂O₃ with zigzag-type configuration because the defect structures on the surface of α-Bi₂O₃ and β-Bi₂O₃ were different. Moreover, the in situ FT-IR revealed the mechanisms of photocatalytic NO oxidation. The photocatalytic NO conversion pathway on α-Bi₂O₃ and β-Bi₂O₃ can be tuned by the different surface defect structures. This work could provide a novel strategy to regulate the photocatalytic activity and conversion pathway via the synergistic effects of crystal structure and oxygen vacancy.

An atomic insight into BiOBr/La2Ti2O7 p–n heterojunctions: interfacial charge transfer pathway and photocatalysis mechanism

Mechanisms of the enhancement photocatalytic activity of p–n heterojunction BiOBr/La₂Ti₂O₇ and photocatalytic NO oxidation are proposed.

Light-Induced Generation and Regeneration of Oxygen Vacancies in BiSbO4 for Sustainable Visible Light Photocatalysis

Oxygen vacancy (OV)-containing semiconductor photocatalysts have been extensively studied and applied in environmental and energy fields, but there are a few studies concerning the mechanisms of inactivation and regeneration of OVs to prevent the catalysts from deactivation. In this paper, we put forward a novel in situ method to introduce the OVs into BiSbO₄ (BiSbO₄-OV) via UV-light-induced breaking down of Bi-O and Sb-O bonds. The formation of OVs could broaden the photoresponse range and improve the charge carrier separation as confirmed by density functional theory calculation and UV and photoluminescence spectroscopy. The unique electronic structure of OVs endowed BiSbO₄ with high visible light photocatalytic NO activity. It was significant to reveal that oxygen in the air could fill the OV sites during the photocatalytic reaction and the consumption of the OVs led to the direct deactivation of BiSbO₄-OV. By re-irradiation of the deactivated photocatalysts, BiSbO₄-OV could get back to its initial state, realizing the refreshment of OVs for sustainable photocatalysis. Additionally, the visible light photocatalytic NO conversion pathway on BiSbO₄-OV was uncovered via in situ diffuse reflectance infrared Fourier transform spectroscopy based on the identification of the reaction intermediates and products. The light-induced generation and regeneration of OVs could also be extended to other semiconductors for sustainable visible light photocatalysis.

Gas-Phase Photoelectrocatalysis for Breaking Down Nitric Oxide

Photoelectrocatalysis (PEC) produces high-efficiency electron-hole separation by applying a bias voltage between semiconductor-based electrodes to achieve high photocatalytic reaction rates. However, using PEC to treat polluted gas in a gas-phase reaction is difficult because of the lack of a conductive medium. Herein, we report an efficient PEC system to oxidize NO gas by using parallel photoactive composites (TiO₂ nanoribbons-carbon nanotubes) coated on stainless-steel mesh as photoanodes in a gas-phase chamber and Pt foil as the working electrode in a liquid-phase auxiliary cell. Carbon nanotubes (CNTs) were utilized as conductive scaffolds to enhance the interaction between TiO₂ and stainless-steel skeletons for accelerated photogenerated electron transfer. Such a PEC system exhibited super-high performance for the treatment of indoor NO gas (550 ppb) with high selectivity for nitrate under UV-light irradiation owing to the conductive, intertwined network structure of the photoanode, fast photocarrier separation, and longer photogenerated hole lifetime. The photogenerated holes were proven to be the most important active sites for directly driving PEC oxidation of indoor NO gas, even in the absence of water vapor. This work created an efficient PEC air-purification filter for treating indoor polluted air under ambient conditions.

Degradation of antibiotic chloramphenicol in water by pulsed discharge plasma combined with TiO2/WO3 composites: mechanism and degradation pathway

Pulsed discharge plasma (PDP) combined with TiO₂/WO₃ composites for chloramphenicol (CAP) degradation was investigated. The prepared TiO₂/WO₃ composites were characterized by scanning electron microscope, transmission electron microscope, nitrogen adsorption apparatus, zeta sizer, X-ray diffraction, Raman spectra, UV-Vis absorption spectroscopy, X-ray photoelectron spectroscopy, photocurrent and electrochemical impedance spectroscopy. The degradation performance showed that the addition of TiO₂/WO₃ composites significantly enhanced the removal efficiency of CAP in PDP system. At a peak voltage of 18 kV, the highest removal efficiency of CAP could reach 88.1% in PDP system with 4 wt% TiO₂/WO₃, which was 36.8% and 26.0% higher than that in sole PDP system and PDP/TiO₂ system, respectively. The TiO₂/WO₃ composites significantly accelerated interfacial charge transfer process compared to the pure TiO₂. Besides, the effect of catalyst dosage and peak voltage on CAP removal was evaluated. OH, O₃O₂^-, h⁺ and high-energy electrons contributed to CAP degradation in PDP-TiO₂/WO₃ system. Addition of TiO₂/WO₃ composites can decompose O₃ and produce more OH and H₂O₂. The degradation intermediates were measured by liquid chromatography-mass spectrometry (LC-MS) and ion chromatography (IC). The cycling degradation experiment showed that the TiO₂/WO₃ composites have good reusability as well as stability.

Metal Oxides as Catalysts and Adsorbents in Ozone Oxidation of NO x

NO _x represents an important group of pollutants that are formed in fuel combustion. As these pollutants cause significant environmental problems, removal of NO _x from exhaust gases is necessary. The present study investigated the influence of metal oxide powders on the removal of NO _x by oxidation with ozone. The aim of the study was to compare the catalytic effect of TiO₂, Al₂O₃, and Fe₂O₃ and to investigate the dependence of the effective rate constant on temperature. NO (400 ppm) and a variable concentration of ozone in a mixture of N₂ and O₂ was directed through the catalyst chamber and heated to 60-140 °C. The addition of metal oxides resulted in a significant increase in the efficiency of the oxidation of NO to N₂O₅. Fe₂O₃ had the largest effect with a maximum of an approximately 3-fold increase in the effective rate constant at 100 °C. At the same time Fe₂O₃ had the lowest NO _x adsorption capacity. In the case of all metal oxides, oxidation of NO to N₂O₅ caused an abrupt increase in adsorption of NO _x.