High N2 Selectivity Research Articles

Exploration of nanoporous graphene membranes for the separation of N2 from CO2: a multi-scale computational study.

Density functional theory (DFT) calculations and molecular dynamic (MD) simulations were performed to investigate the capability of graphene membranes with H-passivated nanopores for the separation of N2/CO2 gas mixtures. We found that the graphene membrane, H-pore-13, with its appropriate pore size of 4.06 Å, can efficiently separate N2 from CO2. Different from the previously reported preferential permeation of CO2 over N2 resulting from size sieving, H-pore-13 can exhibit high N2 selectivity over CO2 with a N2 permeance of 10(5) GPU (gas permeation unit), and no CO2 was found to pass through the pore. It was further revealed that electrostatic sieving plays a cruical role in hindering the passage of CO2 molecules through H-pore-13.

ChemInform Abstract: Recent Advances in Automotive Catalysis for NOx Emission Control by Small‐Pore Microporous Materials

AbstractReview: 211 refs.

Highly Effective Direct Decomposition of Nitric Oxide by Microwave Catalysis over BaMeO3 (Me=Mn, Co, Fe) Mixed Oxides at Low Temperature under Excess Oxygen

AbstractThe direct catalytic decomposition of NO with high activity and N2 selectivity is a great challenge at low temperature under excess oxygen. Herein, we report the NO decomposition by microwave catalysis over BaMeO3 (Me=Mn, Co, Fe) mixed oxides for the first time at low temperature under excess oxygen, for which the BaCoO3 catalyst has an outstanding activity with a high NO conversion and N2 selectivity up to 99.8 % and 99.9 %, respectively, even at 250 °C. Comparatively, the best NO conversion is 93.7 % for BaMnO3 and only 64.1 % for BaFeO3 under microwave irradiation. H2 temperature‐programmed reduction, O2 temperature‐programmed desorption, and the microwave‐absorbing properties of the mixed oxides were characterized to illustrate possible reasons that cause such clear differences in the catalytic performance. Importantly, the apparent activation energies for BaMnO3, BaCoO3, and BaFeO3 are as low as 33.4, 13.7, and 46.7 kJ mol−1, respectively, which suggests a significant microwave catalytic effect.

Highly dispersed V2O5/TiO2 modified with transition metals (Cu, Fe, Mn, Co) as efficient catalysts for the selective reduction of NO with NH3

Different transition metals were used to modify V2O5-based catalysts (M-V, M = Cu, Fe, Mn, Co) on TiO2 via impregnation, for the selective reduction of NO with NH3. The introduced metals induced high dispersion in the vanadium species and the formation of vanadates on the TiO2 support, and increased the amount of surface acid sites and the strength of these acids. The strong acid sites might be responsible for the high N2 selectivity at higher temperatures. Among these catalysts, Cu-V/TiO2 showed the highest activity and N2 selectivity at 225–375 °C. The results of X-ray photoelectron spectroscopy, NH3-temperature-programmed desorption, and in-situ diffuse reflectance infrared Fourier transform spectroscopy suggested that the improved performance was probably due to more active surface oxygen species and increased strong surface acid sites. The outstanding activity, stability, and SO2/H2O durability of Cu-V/TiO2 make it a candidate to be a NOx removal catalyst for stationary flue gas.

Porous bimetallic Mn2Co1Ox catalysts prepared by a one-step combustion method for the low temperature selective catalytic reduction of NOx with NH3

The porous Mn2Co1Ox catalysts are prepared by a one-step combustion (CB) method for low temperature deNOx process. This catalyst displays an excellent NH3-SCR activity, achieving 100% NOx conversion at 150–300°C. In addition, the catalyst shows high N2 selectivity, wide operating temperature window, high stability, and high H2O and SO2 resistance. The crystalline phase of CoMn2O4, porous structure and large specific surface areaare beneficial for the deNOx activity. The catalyst exists abundant Mn4+, Co3+ and surface adsorbed oxygen species, which improve the catalyst's redox ability. Plenty of acid sites from the strong interaction of manganese and cobalt oxide enhance the catalyst performance.

Photocatalytic reduction of nitrate over chalcopyrite CuFe0.7Cr0.3S2 with high N2 selectivity

Photocatalytic reduction of nitrate (NO3−) is a green and potentially inexpensive technique for reducing NO3− pollution in ground water. TiO2-based photocatalysts have been studied extensively for this purpose. In the present study, the semiconducting catalyst CuFe0.7Cr0.3S2 was applied to NO3− reduction. Loading this catalyst with metal co-catalysts (Ru, Au, Cu, Ag, Pt, and Pd) greatly increased the rate of NO3− reduction and the N2 selectivity. In addition, there was a synergistic enhancement of the photocatalytic performance when the catalyst was loaded two co-catalysts. For example, the catalyst loaded with Pd and Au at mass fractions of 0.75% and 3%, respectively, could photocatalyze the complete reduction of NO3− in a 100 ppm N aqueous solution with 100% N2 selectivity in less than 5 h with UV irradiation. However, with an inner irradiation from a full-arc Xe lamp, the NO3− conversion rate reduced to 0.065 mg N/h, probably because of the low density of the photoexcited electrons. The results show the potential of metal sulfides for photocatalytic reduction of NO3−, and the possibility of use of visible light.

MnOx supported on Fe–Ti spinel: A novel Mn based low temperature SCR catalyst with a high N2 selectivity

In this work, a novel 10% Mn/Fe–Ti spinel catalyst with an excellent N2 selectivity was developed for the selective catalytic reduction (SCR) of NO with NH3 at low temperatures. The mechanism of NO reduction and N2O formation over Fe–Ti spinel, 10% Mn/Fe–Ti spinel and 5% Mn–10% Fe/TiO2 was investigated using in situ DRIFT study and the transient reaction study. Meanwhile, the reaction kinetic constant of the SCR reaction (i.e., N2 formation) through the Eley–Rideal mechanism, the reaction kinetic constant of the SCR reaction through the Langmuir–Hinshelwood mechanism and the reaction kinetic constant of the non selective catalytic reduction (NSCR) reaction (i.e., N2O formation) were obtained according to the steady state kinetic study. They all indicate that the NSCR reaction over 10% Mn/Fe–Ti spinel through the Eley–Rideal mechanism was cut off. NH2 adsorbed on 10% Mn/Fe–Ti spinel can be hardly oxidized to NH as NH2 mainly adsorbed on the support (i.e., Fe–Ti spinel) of 10% Mn/Fe–Ti spinel, which was far away from Mn4+ cations on 10% Mn/Fe–Ti spinel. Therefore, the NSCR reaction over 10% Mn/Fe–Ti spinel through the Eley–Rideal mechanism was suppressed. However, the regeneration of Fe3+ on Fe–Ti spinel was accelerated due to the rapid electron transfer between Mn4+ and Fe2+ on 10% Mn/Fe–Ti spinel resulting in a remarkable promotion on NH3 activation although NH3 adsorbed on 10% Mn/Fe–Ti spinel cannot be directly activated by Mn4+ on 10% Mn/Fe–Ti spinel. Therefore, the SCR reaction over Fe–Ti spinel was promoted remarkably after the load of MnOx. As a result, 10% Mn/Fe–Ti spinel showed an excellent SCR performance especially N2 selectivity at low temperatures, which was much better than 5% Mn-10% Fe/TiO2 with the same chemical composition.

A facile preparation of Ag2O/P25 photocatalyst for selective reduction of nitrate

Ag2O/P25 and Ag/P25 catalysts were synthesized and applied in the photocatalytic reduction of nitrate with formic acid as a hole scavenger. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM) analysis demonstrate that Ag2O or Ag nanoparticles were well distributed on the surface of P25. Under UV irradiation, 5% Ag2O/P25 showed a fast rate (0.95h−1, 6.3 times of P25), a high conversion (97.2%, 3.63 times of P25) and a high N2 selectivity (83.1%, 1.15 times of P25) for nitrate reduction. The improved separation of electron–hole pairs on Ag2O/P25 results in the high conversion of nitrate. Moreover, compared with the Ag/P25 catalyst, Ag2O/P25 exhibited a good reusability in nitrate reduction. The formed Ag–Ag2O structure [Ag(0) present on the surface of Ag2O] over P25 by partial photoreduction of deposited Ag2O contributed to the better stability of Ag2O/P25 in nitrate reduction. The present study provides a new sight into the use of Ag2O/P25 catalyst in the reduction of nitrate.

Effectively enhance catalytic performance by adjusting pH during the synthesis of active components over FeVO4/TiO2–WO3–SiO2 monolith catalysts

The effect of pH during co-precipitation on the structural and physicochemical properties of a FeVO4/TiO2–WO3–SiO2 catalyst was investigated by using XRD, SEM, HR-TEM, BET, TPD, TPR and XPS. The as-prepared catalysts were tested in an NH3-SCR reaction within a wide temperature range of 175–500°C. When the active component FeVO4 was synthesized at pH=4.5, the corresponding catalyst (FeVO4-4.5-C) showed the best catalytic activities with high resistance to H2O and SO2 poisoning under a gas hourly space velocity of 30,000h−1. It could even reach over 90% NOx conversions in a wide temperature range of 246–476°C with relatively high N2 selectivity in the presence of 10% H2O. Taking the structure performances into consideration, the FeVO4/TiO2–WO3–SiO2 may be defined as a kind of structure-sensitive catalyst. For such FeVO4-4.5-C, the SEM and TEM results showed that it displayed relatively uniform particles and crystal morphologies with the smallest average particle sizes. The NH3-TPD patterns showed that it had the largest amount of acid sites. The H2-TPR and XPS results indicated that the remarkably improved redox performance and deep surface-enrichment of FeVO4 played a key role in its enhanced catalytic performance. The surface-enrichment and the particle sizes effect induced, synergistically, a larger amount of oxygen vacancies. All of the above-mentioned account for the excellent catalytic performance. Summarizing these characterization results, it can be concluded that the pH values adopted during the synthesis can greatly affect the nano-structures and morphologies, which play a dominant role in the catalytic activity.

Low-temperature selective catalytic reduction of NO with NH3 over ordered mesoporous MnxCo3 − xO4 catalyst

The ordered mesoporous materials (MnO2, Co3O4 and MnCo2O4) were successfully synthesized by the nanocasting method and tested for the selective catalytic reduction (SCR) of NO with NH3. The MnCo2O4 catalyst had higher N2 selectivity, more extensive operating-temperature window, and high SO2 tolerance. The TEM results suggested that the special porous structure provided a larger surface area to adsorb and activate reaction gases. The H2-TPR and NH3-TPD results demonstrated that the MnCo2O4 catalyst possessed a more powerful reducibility and stronger acid strength.

Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials.

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

Microwave Irradiation Coupled with Physically Mixed MeOx (Me=Mn, Ni) and Cu‐ZSM‐5 Catalysts for the Direct Decomposition of Nitric Oxide under Excess Oxygen

AbstractThe direct catalytic decomposition of NO into N2 and O2 with high activity and N2 selectivity at low temperature under excess oxygen is a challenge. Herein, we report a new approach for the direct decomposition of NO into N2 and O2 by microwave catalysis over MeOx‐Cu‐ZSM‐5 (Me=Mn, Ni) under excess oxygen. We observed that the microwave direct catalytic decomposition of NO over MeOx‐Cu‐ZSM‐5 under excess oxygen is highly efficient, and the NO conversions are 94.3 % over MnO2‐Cu‐ZSM‐5 at 300 °C and 92.3 % over Ni2O3‐Cu‐ZSM‐5 at 350 °C. Meanwhile, the N2 selectivity remains more than 98 %. Importantly, the apparent activation energies of MnO2‐Cu‐ZSM‐5 and Ni2O3‐Cu‐ZSM‐5 are as low as 15.5 and 25.7 kJ mol−1, which suggests a significant microwave catalytic effect. Furthermore, microwave irradiation exhibits a microwave selective effect. The oxygen concentration has almost no influence on the activity of catalytic decomposition of NO over MeOx‐Cu‐ZSM‐5 under microwave irradiation.

Role of the support on the behavior of Ag-based catalysts for NH3 selective catalytic oxidation (NH3-SCO)

In this work four supports with different textural properties (Al2O3, SiO2, NaY and TiO2) are used to evaluate the formation of different silver species and their effect on the NH3-SCO performance. Characterization analyses have shown that the predominant Ag0 species and the oxidized Ag species (Ag+ and Agnδ+) are located on the four catalysts with different ratio of Ag0/oxidized Ag. Small Ag0 particle with high dispersion which is obtained on 10wt% Ag/Al2O3 (5nm, Ag0 ratio=97.7%) shows an excellent catalytic performance for SCO reaction at low temperatures (a complete NH3 conversion at 180°C and a high N2 selectivity of 89%). In addition, a certain amount of oxidized Ag species formed on the catalysts, especially for the NaY and SiO2 support, as well as the surface acidity of catalysts have also been found to play an important role in the enhanced N2 selectivity above 140°C by promoting iSCR reaction involving NO as an intermediate for N2 formation. A model to relate the silver species with the performance of NH3-SCO is proposed, which is very significant for the further optimization of the catalyst (e.g. choosing a suitable support) for NH3-SCO reaction and its practical application in the future.

A Remarkable Catalyst Combination to Widen the Operating Temperature Window of the Selective Catalytic Reduction of NO by NH3

AbstractOn the basis of the idea of combining high selective catalytic reduction (SCR) activity, high N2 selectivity, and broad operating temperature window, we developed a combined catalyst process that was found to be a convenient way to design a highly efficient SCR process for NO with NH3. The single catalysts of MnOx–CeO2/TiO2 and V2O5–WO3/TiO2 as well as combined catalysts with different configurations were fully characterized by XRD, temperature‐programmed reduction of H2, and temperature‐programmed desorption of NO/O2. In addition, they were tested for the oxidation of NO or NH3 as well as the SCR of NO with NH3. The correlations between combining configurations and SCR activity results were also revealed. Furthermore, fundamental evidences were given for the rational design of the process and its efficiency maximization (assembly order and catalytic bed volume). An adequate configuration, with MnOx–CeO2/TiO2 and V2O5–WO3/TiO2 active materials, ensures an excellent SCR performance (NO conversion >85 %, N2 yield >70 %, and decreased N2O production) over a wide operating temperature window (from 150 to 400 °C).

NOx selective catalytic reduction by ammonia over Cu-ETS-10 catalysts

Ion exchange method was used to fabricate Cu-ETS-10 titanosilicate catalysts, which possessed high activity, N2 selectivity and SO2 resistance for NOx selective catalytic reduction (SCR). N2 sorption measurements indicated that the microporous catalysts had high surface areas of 288–380 m2/g. The Cu content and speciation were investigated by inductively coupled plasma atomic emission spectrometry, H2 temperature-programmed reduction, and diffuse reflectance infrared Fourier transform spectroscopy. Various Cu species coexisted within the catalyst. Isolated Cu2+ species were the active sites for NH3-SCR, the number of which initially increased and then decreased with increasing Cu content. The catalytic activity of Cu-ETS-10 depended on the isolated Cu2+ species content.

Fabrication of Hierarchically Porous RuO2–CuO/Al–ZrO2 Composite as Highly Efficient Catalyst for Ammonia-Selective Catalytic Oxidation

A hierarchically porous RuO2–CuO/(Al–ZrO2) nanocomposite, with RuO2 and CuO nanocrystals being homogeneously dispersed in the hierarchically porous structure of Al-doped ZrO2 (Al–ZrO2), has been developed by a hydrothermal and wet impregnation method for efficient ammonia-selective catalytic oxidation (SCO) applications. The microstructures of the RuO2–CuO/Al–ZrO2 nanocomposites were characterized by XRD, TEM, FESEM, EDX elemental mapping, and N2 sorption. XPS analysis and H2-TPR results indicate that the hierarchically porous RuO2–CuO/Al–ZrO2 composites possess a large number of oxygen vacancies and surface catalytic active sites, which endows the composite with high catalytic activity and N2 selectivity for NH3 oxidation. NH3 complete oxidization has been achieved at 195 °C with 100% N2 selectivity over an obtained RuO2–CuO/Al–ZrO2 composite at RuO/CuO = 1:1 (weight rate). The high efficiency of hierarchically porous RuO2–CuO/Al–ZrO2 nanocomposites for ammonia SCO reaction has been attributed to the synergetic catalytic effects among the metal oxides, in which the porous Al–ZrO2 support promotes oxygen activation by the generation of oxygen vacancies due to the Al doping, and the ultrahigh catalytic activity of RuO2 is responsible for the active NH3 oxidation. Successively, CuO plays a role of NO intermediate conversion for enhanced N2 selectivity.

Rational Design of High-Performance DeNOx Catalysts Based on MnxCo3–xO4 Nanocages Derived from Metal–Organic Frameworks

Herein, we have rationally designed and originally developed a high-performance deNOx catalyst based on hollow porous MnxCo3–xO4 nanocages with a spinel structure thermally derived from nanocube-like metal–organic frameworks (Mn3[Co(CN)6]2·nH2O), which are synthesized via a self-assemble method. The as-prepared catalysts have been characterized systematically to elucidate their morphological structure and surface properties. As compared with conventional MnxCo3–xO4 nanoparticles, MnxCo3–xO4 nanocages possess a much better catalytic activity at low-temperature regions, higher N2 selectivity, more extensive operating-temperature window, higher stability, and SO2 tolerance. The feature of hollow and porous structures provides a larger surface area and more active sites to adsorb and activate reaction gases, resulting in the high catalytic activity. Moreover, the uniform distribution and strong interaction of manganese and cobalt oxide species not only enhance the catalytic cycle but also inhibit the formatio...

Novel approach for a cerium-based highly-efficient catalyst with excellent NH3-SCR performance

A highly-efficient NH3-SCR catalyst, CeO2/WO3–TiO2, was prepared by a novel stepwise precipitation approach. This catalyst showed excellent catalytic performance with superior low-temperature activity, high N2 selectivity and a broad operational temperature window.

Excellent Performance of One-Pot Synthesized Cu-SSZ-13 Catalyst for the Selective Catalytic Reduction of NOx with NH3

Cu-SSZ-13 samples prepared by a novel one-pot synthesis method achieved excellent NH3-SCR performance and high N2 selectivity from 150 to 550 °C after ion exchange treatments. The selected Cu3.8-SSZ-13 catalyst was highly resistant to large space velocity (800 000 h(-1)) and also maintained high NOx conversion in the presence of CO2, H2O, and C3H6 in the simulated diesel exhaust. Isolated Cu(2+) ions located in three different sites were responsible for its excellent NH3-SCR activity. Primary results suggest that the one-pot synthesized Cu-SSZ-13 catalyst is a promising candidate as an NH3-SCR catalyst for the NOx abatement from diesel vehicles.

Catalytic performance of acidic zirconium-based composite oxides monolithic catalyst on selective catalytic reduction of NOx with NH3

A series of monolithic acidic zirconium-based composite oxide catalyst (Ce–ZrTiSiWOx) were prepared by different introduction methods and loading contents of CeO2 for selective catalytic reduction (SCR) of NOx with ammonia. In this work, a acidic zirconium-based composite oxide (10wt.% CeO2/ZrTiSiWOx) catalyst prepared only by impregnation method presented good NH3-SCR activity and high N2 selectivity in a broad operation temperature window, comparative strong tolerance of gas hourly space velocities (GHSV), water, sulfur and hydrothermal aging. Compared with V2O5/TiSiWOx and Fe-ZSM-5 catalysts, the CeO2/ZrTiSiWOx catalyst showed a better NH3-SCR performance and higher thermal aged performance. More than 80% NOx could be removed in the temperature range of 277–545°C under GHSV of 30,000h−1 over the monolithic 10wt.% CeO2/ZrTiSiWOx catalyst. More surface atomic concentration of Ce, strong reducible ability and the maximum acid amount are important reasons for the high NH3-SCR activity of the CeO2/ZrTiSiWOx catalyst.