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The improvement of air quality in densely-populated urban regions constitutes an environmental challenge of increasing concern. In this respect, the abatement of NO emissions, primarily emanating from combustion processes associated with motor-vehicles, along with industrial/domestic combustion systems, represents one of the main problems. Here, three hydrochars from diverse organic residues were used as activated carbon precursors for their evaluation in the NO removal in two potential application scenarios. Hydrochars were physically activated at 800 °C with pure-CO 2 or diluted-O 2 . These materials were tested in a lab-scale biofilter at different conditions (NO concentration, temperature, relative humidity, NO-containing gas and carbon particle size) and in a larger-scale biofilter to evaluate the long-term NO removal capacity. Hydrochar-derived carbons present a relatively well-developed micro- and mesoporous structure, with BET areas of up to 421 m 2 /g, and a variety of oxygen surface functionalities (carboxylic, lactone, carbonyl and quinone groups), especially concerning CO 2 -activated carbons. These exhibited an excellent behaviour at low NO concentration (5 ppmv) between 25 and 75 °C with removal capacities of ≈97 % and > 82 %, respectively; and still good-performance (≈66 %) in a more concentrated gas (120 ppmv). Whilst, carbons obtained by diluted-O 2 activation from the same hydrochars, evidenced a higher removal capacity loss at high NO concentration. The O 2 presence in the gas stream was confirmed as a crucial factor in the NO elimination, since both co-adsorb on the carbon surface favouring NO oxidation to NO 2 . Besides, the humidity in the airstream diminished the NO removal capacity from 0.88 to 0.51 mg NO /g carbon , but still remained at 0.54 mg NO /g carbon , when the carbon (in pellet) was operated at larger-scale biofilter in 9-fold longer test under humid air. Therefore, this study highlights the potential of renewable carbons to serve as cost-effective component in urban biofilters, to mitigate NO emissions from exhaust gases in biomass boilers and urban semi-close areas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier B.V. All rights reserved.)

Solar-driven photocatalysis offers an environmentally friendly and sustainable approach for the removal of air pollutants such as nitric oxides without chemical addition. However, the low specific surface area and adsorption capacity of common photocatalysts restrict the surface reactions with NO at the ppb-level. In this study, imidazolium-based hyper-cross-linked polymer (IHP) was introduced to modify the surface of TiO 2 to construct a porous TiO 2 /IHP composite photocatalyst. The as-prepared composite with hierarchical porous structure achieves a larger specific surface area as 309 m 2 /g than that of TiO 2 (119 m 2 /g). Meanwhile, the wide light absorption range of the polymer has brought about the strong visible-light absorption of the TiO 2 /IHP composite. In consequence, the composite photocatalyst exhibits excellent performance toward NO oxidation at a low concentration of 600 ppb under visible-light irradiation, reaching a removal efficiency of 51.7%, while the generation of the toxic NO 2 intermediate was suppressed to less than 1 ppb. The enhanced NO adsorption and the suppressed NO 2 generation on the TiO 2 /IHP surface were confirmed by in situ monitoring technology. This work demonstrates that the construction of a porous structure is an effective approach for efficient NO adsorption and photocatalytic oxidation.

The Ba-CMK-3(x) (x was the Ba(NO 3 ) 2 :CMK-3 mass ratio and equals to 5, 10, and 15 wt%) samples were prepared by the incipient impregnation method, which were used for the adsorption of NO + O 2 at room temperature. The samples were characterized by the XRD, BET, TEM, TPD, TG, and DRIFTS techniques. The results showed that the CMK-3 and Ba-CMK-3(x) samples possessed an ordered two-dimensional hexagonal mesoporous structure, and Ba was uniformly dispersed on the surface of CMK-3. After Ba doping, the surface areas and pore size distributions of the Ba-CMK-3(x) samples were altered due to the synergistic effect of partial blocking of the channels by Ba and partial etching of the carbon materials by O 2 produced from Ba(NO 2 ) 3 decomposition at high temperatures. The sequence in NO adsorption capacity was Ba-CMK-3(10) (108.1 ± 0.55 mg/g) > Ba-CMK-3(15) (106.2 ± 0.72 mg/g) > Ba-CMK-3(5) (102.3 ± 1.33 mg/g) > CMK-3(88.8 ± 1.15 mg/g), with the Ba-CMK-3(10) sample showing the best (NO + O 2 ) adsorption performance. We proposed the two main adsorption pathways in the process of NO adsorption: (i) NO reacted with O 2 to form NO 2 , part of NO 2 were weakly adsorbed on the surface hydroxyl groups, part of NO 2 were adsorbed to form the nitrite and nitrate species, and the left NO 2 was disproportionated to the NO, NO 2 - , and NO 3 - species; and (ii) NO was directly oxidized to the NO 2 - species by the oxygen-containing functional groups in carbon, and then some of the NO 2 - species were transformed to the NO 3 - species directly or via disproportionation. The regeneration efficiencies of the Ba-CMK-3(x) samples were slightly inferior to that of the CMK-3 sample.

The alkali metal (M = Na, K, Rb, and Cs)-modified C-FDU-15 (M-C-FDU-15(x); x was the M/C-FDU-15 M ratio, and equal to 0.01-0.03) samples were prepared through an in situ process, and characterized by means of the TG, XRD, TEM, EDS, N 2 adsorption-desorption, O 2 -TPD, and CO 2 -TPD techniques. The (NO + O 2 ) adsorption mechanism was investigated using the (NO + O 2 )-TPD and DRIFTS techniques. The results show that the sequence of (NO + O 2 ) adsorption performance was Na-C-FDU-15(0.01) (104.1 mg/g) > K-C-FDU-15(0.01) (92.4 mg/g) > C-FDU-15 (76.2 mg/g) > Rb-C-FDU-15(0.01) (65.1 mg/g) > Cs-C-FDU-15(0.01) (62.3 mg/g). The alkali metal was uniformly distributed in C-FDU-15 and its doping enhanced the amount of the basic sites in the sample. Moreover, the optimal Na/C-FDU-15 M ratio was 0.02. (NO + O 2 ) were chemically adsorbed mainly in the forms of nitrite (NO 2 - ) and nitrate (NO 3 - ) on M-C-FDU-15(x). A more amount of NO was converted to nitrate than to nitrite. There were three key factors of enhancing the (NO + O 2 ) adsorption capacity of C-FDU-15 due to alkali metal doping: the first factor was the increasing of surface area and pore volume of the sample, the second one was the enhancement in amount of the active sites in the sample, and the third one was the smaller alkali metal ionic radius in the sample., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Cu@Pt nanoparticles (NPs) are experimentally regarded as improved catalysts for NO x storage/reduction, with higher activities and selectivities compared with pure Pt or Cu NPs, and with inverse Pt@Cu NPs. Here, a density functional theory-based study on such NP models with different sizes and shapes reveals that the observed enhanced stability of Cu@Pt compared with Pt@Cu NPs is due to energetic reasons. On both types of core@shell NPs, charge is transferred from Cu to Pt, strengthening the NP cohesion energy in Pt@Cu NPs, and spreading charge along the surface in Cu@Pt NPs. The negative surface Pt atoms in the latter diminish the NO bonding owing to an energetic rise of the Pt bands, as detected by the appliance of the d-band model, although other factors, such as atomic low coordination or the presence of an immediate subsurface Pt atom do as well. A charge density difference analysis discloses a donation/back-donation mechanism in the NO adsorption., (© 2020 Wiley-VCH GmbH.)

Surface architecting of the catalyst is a hopeful method to expand the surface property of the impetus material for upgrading their response towards the chemical reaction. In the present study, designing of the catalyst was carried out using specific transition metals to boost the simultaneous NO-CO conversion reaction catalytically. These metal oxide systems have been prepared using the combustion and wet impregnation method. Prepared oxides were characterized using XRD, BET, XPS, SEM, and TEM. Further, the surface phenomenon of the catalyst was monitored through H 2 -TPR, O 2 -TPO, NO-TPD, and CO-TPD studies. The highly remarkable activity was perceived by Pd-based modified manganese oxide-cobalt chromite system as compared with simple Pd-based manganese oxide and Pd-based cobalt chromite. The catalyst showed the highest activity for NO-CO redox reaction with T 100 at 170 °C. Also, good stability was observed with a runtime of 7 h.

The properties of Cu ions in dealuminated faujasite-type zeolites (Si/Al = 31) containing 1, 2, and 5 wt.% of Cu were investigated by IR spectroscopy with CO and NO as probe molecules. Cu was introduced by impregnation into zeolites in both protonic (HFAU) and sodium (NaFAU) forms of zeolite. Four kinds of Cu species were found: Cu + exch. , Cu + oxide , Cu 2+ exch. (square, planar, and square pyramidal), and Cu 2+ oxide (CuO). The proportions between these four kinds of Cu depended on the amount of Cu and on the form of zeolite to which Cu was introduced (HFAU or NaFAU). Zeolites with 1 wt.% of Cu introduced to HFAU (denoted as Cu(1)HFAU) contained only Cu + exch. , whereas other forms of Cu were present in zeolites of higher Cu contents. The concentration of Cu + exch. was determined by quantitative IR studies of CO adsorption. According to the IR results, some Cu ions were situated inside hexagonal prisms and/or cuboctahedra, and were inaccessible to adsorbed molecules. IR studies also evidenced that Cu ions in oxide forms-Cu + oxide and Cu 2+ oxide (CuO)-were better electron donors than Cu in exchange positions (Cu + exch. and Cu 2+ exch ).

H2 selective catalytic reduction (H2-SCR) has been proposed as a promising technology for controlling NOx emission because hydrogen is clean and does not emit greenhouse gases. We demonstrate that Pt doped into a nickel ferrite spinel structure can afford a high catalytic activity of H2-SCR. A superior NO conversion of 96% can be achieved by employing a novel NiFe1.95Pt0.05O4 spinel-type catalyst at 60 °C. This novel catalyst is different from traditional H2-SCR catalysts, which focus on the role of metallic Pt species and neglect the effect of oxidized Pt states in the reduction of NO. The obtained Raman and XPS spectra indicate that Pt in the spinel lattice has different valence states with Pt(2+) occupying the tetrahedral sites and Pt(4+) residing in the octahedral ones. These oxidation states of Pt enhance the back-donation process, and the lack of filling electrons of the 5d band causes Pt to more readily hybridize with the 5σ orbital of the NO molecule, especially for octahedral Pt(4+), which enhances the NO chemisorption on the Pt sites. We also performed DFT calculations to confirm the enhancement of adsorption of NO onto Pt sites when doped into the Ni-Fe spinel structure. The prepared Pt/Ni-Fe catalysts indicate that increasing the dispersity of Pt on the surfaces of the individual Ni-Fe spinel-type catalysts can efficiently promote the H2-SCR activity. Our demonstration provides new insight into designing advanced catalysts for H2-SCR.

This study aimed at investigating the reason of high catalytic activity for CeO2-WO3/TiO2 catalyst from the aspects of WO3 interaction with other species and the NO oxidation process. Analysis by X-ray diffractometry, photoluminescence spectra, diffuse reflectance UV-visible, X-ray photoelectron spectroscopy, density functional theory calculations, electron paramagnetic resonance spectroscopy, temperature-programmed-desorption of NO and in situ diffuse reflectance infrared transform spectroscopy showed that WO3 could interact with CeO2 to improve the electron gaining capability of CeO2 species. In addition, WO3 species acted as electron donating groups to transfer the electrons to CeO2 species. The two aspects enhanced the formation of reduced CeO2 species to improve the formation of superoxide ions. Furthermore, the Ce species were the active sites for the NO adsorption and the superoxide ions over the catalyst needed oxidizing the adsorbed NO to improve the NO oxidation. This process was responsible for the high catalytic activity of CeO2-WO3/TiO2 catalyst., (Copyright © 2015 Elsevier Inc. All rights reserved.)

In this paper, density functional theory (DFT) calculation was employed to study the adsorption of nitric oxide (NO) on the highly reactive anatase TiO2 (001) surface. For comparison, the adsorption of NO on the (101) surface was also considered. Different from the physical adsorption on the (101) surface, NO molecules are found to chemisorb on the TiO2 (001) surface. The twofold coordinate oxygen atoms (O2c) on the anatase (001) surface are the active sites. Where NO is oxidized into a nitrite species (NO2(-)) trapping efficiently on the surface, with one of the surface Ti5c-O2c bonds adjacent to the adsorption site broken. Our results, therefore, supply a theoretical guidance to remove NO pollutants using highly reactive anatase TiO2 (001) facets., (Copyright © 2014 Elsevier Inc. All rights reserved.)