Catalytic Conversion Of NO Research Articles

Catalytic Conversion of NO and CO by Noble-Metal-Free Copper-Vanadium Oxide Cluster Anions CuVO3,4.

Herein, by using state-of-the-art mass spectrometry, we demonstrated experimentally that the bimetallic copper-vanadium oxide cluster anions CuVO3,4- can catalyze the reduction of NO by CO into N2O and CO2. Note that the catalysis of NO reduction by CO has been rarely established in the gas phase and noble-metal containing clusters were commonly emphasized. Benefiting from quantum-chemical calculations, the Cu-V synergistic effect that both metal atoms work energetically to favor NO adsorption, N-N coupling, and CO oxidation by facilitating electron transfer can be understood at a strictly molecular level. Theoretical results demonstrated that the precaptured NO molecule encourages the adsorption of the second NO by electron donation. This finding deepens our understanding on NO reduction that NO functions not only as a reactant but also as a promoter during the reactions. This discovery could be helpful to permeate the nature and mechanism of active sites on related copper-vanadium heterogeneous catalyst used in real-life NO reduction.

The intense charge migration and efficient photocatalytic NO removal of the S-scheme heterojunction composites Bi7O9I3-BiOBr

For purpose of efficient photocatalytic removal of NO molecules, both components of bismuth-rich Bi7O9I3 and BiOBr were typically selected to create binary heterojunction composites Bi7O9I3-BiOBr (BI-BBX) through a facile in-situ synthetic protocol. These composites underwent thorough characterization using a range of analytical methods and were subjected to photocatalytic removal of NO at ppb level under visible light for the first time. These produced composites showed enhanced photocatalytic behavior and the best candidate BI-BB0.2 demonstrated the highest removal efficiency among all tested samples. Density-functional theory (DFT) calculations revealed the intense charge migration at the interface. In addition, the composites surface exhibited a propensity to adsorb and activate O2, H2O, and NO molecules, thus promoting the catalytic conversion of NO to NO2– and further NO3– species. Entrapping experiments and electron spin resonance (ESR) results demonstrated the increased generation of oxygen-containing species O2–, 1O2, and ·OH, which was attributed to the rational creation of heterojunctions with similar crystal structures and efficient migration and separation of generated charge carriers in an S-scheme model. This study provides insight into the design and development of Bi7O9I3-based composites for effective dislodge of NO upon the visible-light irradiation.

Direct reduction of NO into N2 catalyzed by fullerene-supported rhodium clusters.

Catalytic conversion of NO has long been a focus of atmospheric pollution control and diesel vehicle exhaust treatment. Rhodium is one of the most effective metals for catalyzing NO reduction, and understanding the nature of the active sites and underlying mechanisms can help improve the design of Rh-based catalysts towards NO reduction. In this work, we investigated the detailed catalytic mechanisms for the direct reduction of NO to N2 by fullerene-supported rhodium clusters, C60Rh4+, with density functional theory calculations. We found that the presence of C60 facilitates the smooth reduction of NO into N2 and O2, as well as their subsequent desorption, recovering the catalyst C60Rh4+. Such a process fails to be completed by free Rh4+, emphasizing the critical importance of C60 support. We attribute the novel performance of C60Rh4+ to the electron sponge effect of C60, providing useful guidance for designing efficient catalysts for the direct reduction of NO.

Theoretical prediction of superatom WSi12-based catalysts for CO oxidation by N2O.

Catalytic conversion of N2O and CO into nonharmful gases is of great significance to reduce their adverse impact on the environment. The potential of the WSi12 superatom to serve as a new cluster catalyst for CO oxidation by N2O is examined for the first time. It is found that WSi12 prefers to adsorb the N2O molecule rather than the CO molecule, and the charge transfer from WSi12 to N2O results in the full activation of N2O into a physically absorbed N2 molecule and an activated oxygen atom that is attached to an edge of the hexagonal prism structure of WSi12. After the release of N2, the remaining oxygen atom can oxidize one CO molecule via overcoming a rate-limiting barrier of 28.19 kcal mol-1. By replacing the central W atom with Cr and Mo, the resulting MSi12 (M = Cr and Mo) superatoms exhibit catalytic performance for CO oxidation comparable to the parent WSi12. In particular, the catalytic ability of WSi12 for CO oxidation is well maintained when it is extended into tube-like WnSi6(n+1) (n = 2, 4, and 6) clusters with energy barriers of 25.63-29.50 kcal mol-1. Moreover, all these studied MSi12 (M = Cr, Mo, and W) and WnSi6(n+1) (n = 2, 4, and 6) species have high structural stability and can absorb sunlight to drive the catalytic process. This study not only opens a new door for the atomically precise design of new silicon-based nanoscale catalysts for various chemical reactions but also provides useful atomic-scale insights into the size effect of such catalysts in heterogeneous catalysis.

Catalytic conversion of NO and CO into N2 and CO2 by rhodium–aluminum oxides in the gas phase

The catalytic conversion of NO and CO into N2 and CO2 mediated by gas-phase rhodium-aluminum oxides (RhAlO0–3 and RhAl2O1–4) has been identified. The polarized Rh–Al bond in the reactive systems is required to catalyze NO reduction by CO.

NO catalytic performance analysis of gasoline engine tapered variable cell density carrier catalytic converter.

Improving the flow field uniformity of catalytic converter can promote the catalytic conversion of NO to NO2. Firstly, the physical and mathematical models of improved catalytic converter are established, and its accuracy is verified by experiments. Then, the NO catalytic performances of standard and improved catalytic converters are compared, and the influences of structural parameters on its performance are investigated. The results showed that: (1) The gas uniformity, pressure, drop and NO conversion rate of the improved catalytic converter are increased by 0.0643, 6.78%, and 7.0% respectively. (2) As the cell density combination is 700 cpsi/600 cpsi, NO conversion rate reaches the highest, 73.7%, and the gas uniformity is 0.9821. (3) When the tapered height is 20 mm, NO conversion rate reaches the highest, 72.4%, and the gas uniformity is 0.9744. (4) When the high cell density radius is 20 mm, NO conversion rate reaches the highest, 72.1%, and the gas uniformity is 0.9783. (5) When the tapered end face radius is 20 mm, NO conversion rate reaches the highest, 72.0%, and the gas uniformity is 0.9784. The results will provide a very important reference value for improving NO catalytic and reducing vehicle emission.

Computational investigation of N2O adsorption and dissociation on the silicon-embedded graphene catalyst: A density functional theory perspective

One of the encouraging processes to protect the environment is the catalytic conversion of N2O and other harmful greenhouse gases. Employing heteroatom dopants into the Graphene structure for this conversion is an attractive technique owing to its relatively low price and the very low destructive impacts. DFT was applied to explore fundamental and principal reactions of N2O adsorption and dissociation over the Silicon-embedded Graphene catalyst to contribute to the search for green catalysts in the conversion of toxic gases into less harmful ones. Forming a surface peroxy group O22−, N2O bond cleavage and oxygen atom transfer were theoretically investigated. It is found that the N2O molecule requires +0.52, +0.88 and + 0.4 eV of activation energies through mentioned three reactions, respectively, to adsorb and decompose to N2 and O2. The parallel, lying-atop-011 and flat were stable forms with adsorption energies of −0.20 (−4.65), −0.19 (−4.53) and −0.18 (−4.46) and −0.19 eV (−4.53 kcal/mol), respectively. The achieved outcomes reveal that Silicon-embedded Graphene has a high potential to be used as a more efficient and green catalyst for the catalytic conversion of the air polluting gases in comparison to the Selenium-doped Graphene, Fe+, Manganese-embedded Graphene and Magnesium oxide (MgO) catalysts.

Insights of the mechanisms for CO oxidation by N2O over M@Cu12 (M = Cu, Pt, Ru, Pd, Rh) core-shell clusters

Catalytic conversion of N2O and CO to the nonharmful gases can relieve many environmental problems caused by them. A density functional theory study is performed to investigate the CO oxidation by N2O over the M@Cu12 (M = Cu, Pt, Ru, Pd, Rh) core-shell clusters. The stability of clusters and the adsorption of N2O are enhanced with the doping of noble metals which act as the electron collectors according to the NBO analysis. Two mechanisms (the stepwise adsorption mechanism and the co-adsorption mechanism) with adsorption of N2O on metallic clusters by N end and O end are well-established. On these clusters, the highest activation energy for the N2O decomposition to yield N2 is predicted to be 7.7 kcal/mol. The average barrier for the subsequent CO oxidation is about 14 kcal/mol, which is the rate-limiting step for the overall reaction. Overall, this oxidation process is both thermodynamically and kinetically favorable on Cu-based core-shell clusters. By comparison, the alloy clusters exhibit superior catalytic activities compared to pure Cu cluster for the rate-limiting step.

Catalytic conversion of NO assisted by plasma over Mn-Ce/ZSM5-multi-walled carbon nanotubes composites: Investigation of acidity, activity and stability of catalyst in the synergic system

Mn-Ce/ZSM5-MWCNTs (MWCNTs, multi-walled carbon nanotubes) catalysts were synthesized via an impregnation method to study NO conversion and sulfur resistance with the assistance of plasma. The reaction proceeded excellently at ambient temperature by using bimetallic manganese-cerium as active component, exhibited superior nitric oxide removal efficiency (around 90%) and turnover frequency at relatively low input energy; XRD, BET, TPD, XPS and in-situ FTIR investigations showed that the highly oxidizing environment, interaction of physicochemical structure, improvement of both Lewis and Brønsted acid sites as well as acceleration of the Eley-Rideal (E-R) mechanism were responsible for this catalyst enhancement. The comparison of characterizations over fresh and spent MnCe catalyst along with mechanistic analysis revealed that valence variation of Mn and Ce cations not only provide oxygen vacancies, but more importantly, directly affect the formation and consumption of Lewis acid sites in whole reaction process. Subsequently, an increased number of Lewis acid sites coordinate NH3 molecules with Mn and Ce atoms, then contribute to ammonia adsorption and rob electrons directly from gaseous NO or nitrate anions as following Eley-Rideal pathway. At last, oxidizing radicals (O, O3) and cerium modification inhibit further speciation of ammonium and manganese sulfates species on the catalyst surface, thereby promoting low temperature catalytic activity and sulfur durability.

Plasma-assisted catalytic conversion of NO over Cu-Fe catalysts supported on ZSM-5 and carbon nanotubes at low temperature

A series of Cu-Fe catalysts with fixed metal content and variable ratios of ZSM-5 and CNTs were used to investigate NO removal in a plasma-catalyst system at low temperature. The catalysts were characterized by BET, XRD, TGA, XPS, H2-TPR and NH3-TPD. The results showed that the catalyst promoted the NO conversion rate from only plasma system, and 1Z3C catalyst exhibited the best NO conversion rate of 86.1% at 191.5 J/L. Composite support promoted the catalytic activity at low temperature, and greatly improved the Cu2+ and Fe3+ concentration on the 1Z3C catalyst, both Cu2+ and Fe3+ contributed greatly to catalytic reaction of NO conversion. Chemisorbed oxygen also acted as activated sites for NO conversion. When the input energy was 75 J/L, the NO conversion rate decreased from 70% to 41% with adding 500 ppm SO2, which decreased with increasing SO2 concentration, while generation rate of SO3 was less than 0.2% in the combination system. In the presence of SO2, copper sulfate species, ferric sulfate species and ammonium sulfates generated on the surface of catalyst, which consumed Lewis acid sites and blocked the active sites, decreasing NO conversion rate.

Synergistic photo-thermal catalytic NO purification of MnOx/g-C3N4: Enhanced performance and reaction mechanism

Both MnOx and g-C3N4 have been proved to be active in the catalytic oxidation of NO, and their individual mechanisms for catalytic NO conversion have also been investigated. However, the mechanism of photo-thermal catalysis of the MnOx/g-C3N4 composite remains unresolved. In this paper, MnOx/g-C3N4 catalysts with different molar ratios were synthesized by the precipitation approach at room temperature. The as-prepared catalysts exhibit excellent synergistic photo-thermal catalytic performance towards the purification of NO in air. The MnOx/g-C3N4 catalysts contain MnOx with different valence states on the surface of g-C3N4. The thermal catalytic reaction for NO oxidation on MnOx and the photo-thermal catalytic reaction on 1:5 MnOx/g-C3N4 were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The results show that light exerted a weak effect on NO oxidation over MnOx, and it exerted a positive synergistic effect on NO conversion over 1:5 MnOx/g-C3N4. A synergistic photo-thermal catalytic cycle of NO oxidation on MnOx/g-C3N4 is proposed. Specifically, photo-generated electrons (e−) are transferred to MnOx and participate in the synergistic photo-thermal reduction cycle (Mn4+→Mn3+→Mn2+). The reverse cycle (Mn2+→Mn3+→Mn4+) can regenerate the active oxygen vacancy sites and inject electrons into the g-C3N4 hole (h+). The active oxygen (O−) was generated in the redox cycles among manganese species (Mn4+/Mn3+/Mn2+) and could oxidize the intermediates (NOH and N2O2−) to final products (NO2− and NO3−). This paper can provide insightful guidance for the development of better catalysts for NOx purification.

CO-NO/O2 redox reactions over Cu substituted cobalt oxide spinels

Catalytic conversion of NO and CO over Cu substituted cobalt oxide spinels show excellent activity for CO-O2 and NO-CO reactions. Lower concentration of Cu in cobalt oxide spinel is having an enhancing effect on the catalytic conversion. Best activity among the tested catalyst was found for Co2.9Cu0.1O4 and complete conversion (100%) is observed at 93°C for CO oxidation by O2 and 209°C for NO reduction by CO. Prepared catalysts show promising activity compared to few of the precious metal based catalysts reported in the literature. The influence of moisture and oxygen on catalytic conversion has been studied.

Sensing catalytic conversion: Simultaneous DRIFT and impedance spectroscopy for in situ monitoring of NH3–SCR on zeolites

In order to meet the legislative emission requirements for NOx-containing exhaust gases, SCR catalysts, in particular zeolites, are used. To improve catalysts and the catalytic processes, an in-depth understanding of the reaction mechanisms is required as well as an analysis of the physicochemical properties of the SCR catalysts, preferably in real-time. Here, we introduce a setup combining impedance spectroscopy and infrared spectroscopy in diffuse reflection mode for in situ measurements on zeolites under SCR-related conditions. This setup allows for the first time to simultaneously monitor both, the proton conductivity of zeolites and the vibration modes of the molecules involved in the catalytic conversion of NO by NH3. We studied both, pure and Fe-promoted H-form zeolites, as sensors and model catalysts at the same time, and found out that weakly bound NH3 is dominating the proton conductivity of both zeolites in a temperature range below the desorption temperature of NH3. When a part of the weakly bound NH3 is consumed by the SCR reaction, proton conductivity and thus the sensing effect gets dominated by strongly bound NH3. This allows applying impedance spectroscopy to assess the degree of NH3 loading and the state of the SCR conversion process in zeolite catalyst.

NO sensitivity of perovskite-type electrode materials La0.6Ca0.4B′1−xB″xO3±δ (B′ = Mn, Cr; B″ = Ni, Fe, Co; x = 0, 0.1, …, 0.6) in mixed potential sensors

Perovskite-type electrode materials of the compositions La0.6Ca0.4B′1−xB″xO3±δ (B=Mn, Cr; B″=Ni, Co, Fe; x=0, …, 0.6) were investigated with regard to their NO sensitivity and the cross-sensitivity to NO2 and propylene in a potentiometric solid electrolyte measuring setup. The highest NO sensitivity was found for nickel and iron manganites with x=0.2 and 0.3. Dependencies on the NO gas concentration were found using impedance spectroscopy. For some materials, the catalytic conversion of NO, NO2, methane and propene was determined by thermal investigation in the temperature range of 25–800°C.

FTIR and density functional study of NO interaction with reduced ceria: Identification of N3− and NO2− as new intermediates in NO conversion

To design new deNOx materials and processes we need to know in details the mechanism of NO interaction with solid surfaces. Many NOx conversion catalysts contain ceria as a component and cerium changes its oxidation state in the course of redox catalytic processes. Here we show that the interaction between NO and reduced ceria leads to formation of azides (N3−, 2044–2042cm−1) and nitric oxide dianion (NO2−, 1010–980cm−1) with the simultaneous oxidation of Ce3+. These species are established for the first time after NO adsorption on solid surfaces and their relative concentrations strongly depend on the sample morphology. At ambient temperature N3− does not interact with NO or O2 alone but easily disappears in the co-presence of the two gases, thus demonstrating reactivity similar to that of isocyanates. In contrast, NO2− accepts one NO molecule and is converted into hyponitrite (bands in the 1030–970cm−1 region). At further stages of interaction, N2O, N2, nitrites and nitrates are formed. The present findings enrich the current opinions of catalytic NO conversion and impose some revisions.

Formation of N3(-) during interaction of NO with reduced ceria.

We show that the first stages of interaction between NO and reduced ceria comprise the formation of azides, N3(-), with simultaneous oxidation of Ce(3+) to Ce(4+). This finding imposes revision on some current views of catalytic NO conversion and may contribute to design of new deNOx materials and processes.

Catalytic reduction of NOx by CO over a Ni–Ga based oxide catalyst

A new low-cost Ni–Ga based oxide catalyst with an inverse spinel structure was developed for NO removal. Our results open up a new route for designing efficient catalysts, combining the strong gas capture properties of a specific element with high catalytic NO conversion properties, by constructing an appropriate element ligand environment.

NO oxidation by microporous zeolites: Isolating the impact of pore structure to predict NO conversion

Anthropogenic NOx emissions from stationary sources to the atmosphere continue to be of concern because of their adverse effects on public health and the environment. An alternative method to conventional selective catalytic reduction to reduce the amount of NOx emissions is oxidation of NO to more soluble NO2 upstream of an absorption device. Zeolites and activated carbon are effective NO oxidation catalysts, but their oxidation mechanisms are not fully understood. Here, zeolites were evaluated as NO oxidation catalysts experimentally and theoretically to identify preferred pore structures for NO conversion to NO2 and to show mechanistic similarities to activated carbon fiber-catalyzed NO oxidation. For the 17 zeolites tested here, steady-state NO conversion is not related to their chemical composition (i.e., Si, Al, or P). However, similar to carbonaceous catalysts, steady-state NO conversion mainly depend on the zeolite's physical properties, including the zeolite's cage size, pore width, and pore volume. Zeolites with maximum free sphere diameter (Dmax) between 4.70 and 6.45Å, or zeolites with maximum included sphere diameter (Di) between 5.71 and 7.37Å, derived from experimental results, are most effective for NO oxidation to NO2 regardless of the zeolites’ channel-to-cage ratio. A mechanism for NO oxidation proceeding through the [ONOONO] transition state (TS) is described here, challenging the reported mechanism that adsorbed C*–NO3 species are the precursors of NO2 formation. Void spaces with dimensions similar to or slightly larger (i.e., 7Å width) than the size of the TS (4.46Å×4.67Å×6.94Å, derived from TS calculations) are the most effective spaces for NO oxidation due to strengthened van der Waals interaction between the confined TS and pore walls of the zeolite. Criteria for screening effective zeolites for NO oxidation are proposed based on results for the 17 tested zeolites, and 82 of the 206 available zeolite codes are predicted to be effective with high steady-state NO conversion. Consistent physical geometries of zeolite pore sizes (Di=4.70–6.45Å or Dmax=5.71–7.37Å) and activated carbon fiber pore widths (5–7Å) provide convincing evidence that such pore sizes are needed to provide efficient steady-state NO conversion. These results may be extended to other categories of microporous materials to choose the most appropriate material for catalytic NO conversion to NO2.

Sustainable and Near Ambient DeNOx Under Lean Burn Conditions: A Revisit to NO Reduction on Virgin and Modified Pd(111) Surfaces

Catalytic conversion of NO in the presence of H2 and O2 has been studied on Pd(111) surfaces, by using a molecular beam instrument with mass spectrometry detection, as a function of temperature and reactants composition. N2 and H2O are the major products observed, along with NH3 and N2O minor products under all conditions studied. Particular attention has been paid to the influence of O2 addition toward NO dissociation. Although O2-rich compositions were found to inhibit the deNOx activity of the Pd catalyst, some enhancement in NO reduction to N2 was also observed up to a certain O2 content. The reason for this behavior was determined to be the effective consumption of the H2 in the mixture by the added O2 and O atoms from NO dissociation. NO was proven to compete favorably against O2 for the consumption of H2, especially ≤550 K, to produce N2 and H2O. Compared with other elementary reaction steps, a slow decay observed with the 2H + O → H2O step under SS beam oscillation conditions demonstrates its contribution to the rate-limiting nature of the overall reaction. Pd(111) surfaces modified with O atoms in the subsurface (Md-Pd(111)) induces steady-state NO reduction at near-ambient temperatures (325 K) and opens up a possibility to achieve room temperature emission control. A 50% increase in the reaction rates was observed at the reaction maximum on Md-Pd(111), as compared with virgin surfaces. Oxygen adsorption is severely limited below 400 K, and effective NO + H2 reaction occurs on Md-Pd(111) surfaces. Valence band photoemission with a UV light source (He I) under different oxygen pressures with APPES clearly identified the characteristics of the Md-Pd(111) surfaces and PdO. The electron-deficient or cationic nature of Md-Pd(111) surfaces enhances the NO dissociation and inhibits oxygen chemisorption ≤400 K under lean-burn conditions.

Fe-substituted 10Al2O3·2B2O3 as a multifunctional support for automotive Pd catalysts

Partial substitution of various transition metals for Al in 10Al2O3·2B2O3 (Al20B4O36, 10A2B) was studied to use as a thermostable support for Pd catalysts. Fe-substitution afforded the formation of solid solutions in the range of x≤1.5 in FexAl20−xB4O36 (Fe-10A2B) without impurities. The DFT calculations confirmed that Fe-substitution was most likely to occur on the 5-coordinated Al site and predicted a shorter Fe–O distances of 1.84–2.05Å compared to 1.94 and 2.12Å in Fe2O3, in accordance with Fe-K edge EXAFS results. Fe-substitution improved the catalytic NO conversion in a rich region (A/F<14.6), because CO–H2O and subsequent NO–H2 reactions were accelerated. The redox between Fe3+ and Fe2+ in Fe-10A2B generated the oxygen storage capacity (OSC), which yielded an efficient buffering effect on air-to-fuel (A/F) fluctuation in a simulated exhaust gas stream with lean/rich perturbation. These results suggest that Fe-10A2B is expected to be used as a multifunctional support material for Pd catalysts in automotive three-way catalysts.