S-scheme Sb2WO6/g-C3N4 photocatalysts with enhanced visible-light-induced photocatalytic NO oxidation performance

Normal photocatalysts cannot effectively remove low-concentration NO because of the high recombination rate of the photogenerated carriers.To overcome this problem,S-scheme composites have been developed to fabricate photocatalysts.Herein,a novel S-scheme Sb2WO6/g-C3N4 nanocomposite was fabricated by an ultrasound-assisted method,which exhibited excellent performance for photocatalytic ppb-level NO removal.Compared with the pure constituents of the nanocomposite,the as-prepared 15%-Sb2WO6/g-C3N4 photocatalyst could remove more than 68%continuous-flowing NO(initial concentration:400 ppb)under visible-light irradiation in 30 min.The findings of the trapping experiments confirmed that•O2^–and h+were the important active species in the NO oxidation reaction.Meanwhile,the transient photocurrent response and PL spectroscopy analyses proved that the unique S-scheme structure of the samples could enhance the charge separation efficiency.In situ DRIFTS revealed that the photocatalytic reaction pathway of NO removal over the Sb2WO6/g-C3N4 nanocomposite occurred via an oxygen-induced route.The present work proposes a new concept for fabricating efficient photocatalysts for photocatalytic ppb-level NO oxidation and provides deeper insights into the mechanism of photocatalytic NO oxidation.

Selective catalytic reduction of NO by C_2H_2 over Ce-Al_2O_3 catalyst with rate-determining step of NO oxidation

Ce-A12O3 catalysts prepared by co-precipitation are investigated both in NO oxidation by 02 and in selective catalytic reduction of NO by C2H2 （C2H2-SCR）. It is found that C2H2-SCR is initiated and controlled by NO oxidation to NO2 over A12O3. Ce loading on A12O3 is almost inactive for NO oxidation below 350℃, since NO2 strongly adsorbs on cerium oxide, leading to the active sites being blocked, which was characterized by temperature-programmed desorption of NO and NO2 and Fourier transform infrared spectroscopy after NO＋O2 coadsorption over the samples. However, in the case of C2H2-SCR, Ce loading on A1203 significantly improves the reaction by accelerating the NO oxidation step in the temperature range of 250-450℃, since the nitrate species produced by NO2 adsorption is an active intermediate required by C2H2-SCR.

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.

Facile synthesis of Bi_(12)O_(17)Br_2 and Bi_4O_5Br_2 nanosheets:In situ DRIFTS investigation of photocatalytic NO oxidation conversion pathway

Bi12O17Br2and Bi4O5Br2visible‐light driven photocatalysts,were respectively fabricated by hydrothermal and room‐temperature deposition methods with the use of BiBr3and NaOH as precursors.Both Bi12O17Br2and Bi4O5Br2were composed of irregular nanosheets.The Bi4O5Br2nanosheets exhibited high and stable visible‐light photocatalytic efficiency for ppb‐level NO removal.The performance of Bi4O5Br2was markedly higher than that of the Bi12O17Br2nanosheets.The hydroxyl radical(?OH)was determined to be the main reactive oxygen species for the photo‐degradation processes of both Bi12O17Br2and Bi4O5Br2.However,in situ diffuse reflectance infrared Fourier transform spectroscopy analysis revealed that Bi12O17Br2and Bi4O5Br2featured different conversion pathways for visible light driven photocatalytic NO oxidation.The excellent photocatalytic activity of Bi4O5Br2resulted from a high surface area and large pore volumes,which facilitated the transport of reactants and intermediate products,and provided more active sites for photochemical reaction.Furthermore,the Bi4O5Br2nanosheets produced more?OH and presented stronger valence band holeoxidation.In addition,the oxygen atoms of NO could insert into oxygen‐vacancies of Bi4O5Br2,whichprovided more active sites for the reaction.This work gives insight into the photocatalytic pollutant‐degradation mechanism of bismuth oxyhalide.

Enhanced visible photocatalytic activity of TiO_2 hollow boxes modified by methionine for RhB degradation and NO oxidation

Hierarchical TiO2 hollow nanoboxes(TiO2‐HNBs)assembled from TiO2 nanosheets(TiO2‐NSs)show improved photoreactivity when compared with the building blocks of discrete TiO2‐NSs.However,TiO2‐HNBs can only be excited by ultraviolet light.In this paper,visible‐light‐responsive N and S co‐doped TiO2‐HNBs were prepared by calcining the mixture of cubic TiOF2 and methionine(C5H11NO2S),a N‐and S‐containing biomacromolecule.The effect of calcination temperature on the structure and performance of the TiO2‐HNBs was systematically studied.It was found that methionine can prevent TiOF2‐to‐anatase TiO2 phase transformation.Both N and S elements are doped into the lattice of TiO2‐HNBs when the mixture of TiOF2 and methionine undergoes calcination at 400°C,which is responsible for the visible‐light response.When compared with that of pure 400°C‐calcined TiO2‐HNBs(T400),the photoreactivity of 400°C‐calcined methionine‐modified TiO2‐HNBs(TM400)improves 1.53 times in photocatalytic degradation of rhodamine‐B dye under visible irradiation(?>420 nm).The enhanced visible photoreactivity of methionine‐modified TiO2‐HNBs is also confirmed by photocatalytic oxidation of NO.The successful doping of N and S elements into the lattice of TiO2‐HNBs,resulting in the improved light‐harvesting ability and efficient separation of photo‐generated electron‐hole pairs,is responsible for the enhanced visible photocatalytic activity of methionine‐modified TiO2‐HNBs.The photoreactivity of methionine modified TiO2‐HNBs remains nearly unchanged even after being recycled five times,indicating its promising use in practical applications.

Core-shell MnCeO_(x)catalysts for NO oxidation and mild temperature diesel soot combustion

Environmental contamination such as soot particles and NO_(x)has aroused extensive attraction recently.However,the main challenge lies in the oxidation of soot at mild temperature with the assistance of NO_(x).Here,a series of core-shell MnCeO_x catalysts were successfully synthesized by hydrothermal method and employed for low-temperature catalytic oxidation of soot in the presence of NO_(x).X-ray diffraction(XRD),inductively coupled plasma-optical emission spectrometry(ICP-OES),energy-dispersive X-ray spectroscopy(EDS),transmission electron microscopy(TEM),N_(2)adsorptio n/deso rption,H_(2)-tempe rature programmed reduction(H_(2)-TPR),O_(2)-temperature programmed desorption(O_(2)-TPD),X-ray photoelectron spectroscopy(XPS)and Raman analyses were conducted for identifying the structure of prepared catalysts and investigating redox properties of the core-shell mixed oxide,which shows excellent performance for NO oxidation and the T_(50)about 350℃for soot oxidation.Besides,soot particle is oxided at 304℃in the presence of NO,while T_(50)augments up to 340℃in the absence of NO.In the NO_(x)assisted elimination cases of soot,NO_(2)generated from NO in emission gas is generally reckoned as a facilitator which significantly reduces ignition temperature of soot particles.Possible catalytic mechanism over core-shell MnCeO_(x)catalysts is also proposed in this paper.

Synergistic effect between MnO and CeO_2 in the physical mixture:Electronic interaction and NO oxidation activity

MnO and CeO2 powders were mechanically mixed by a spatula and by milling to obtain loose-contact and tight-contact mixed oxides,respectively.The monoxides and their physical mixtures were characterized by X-ray diffraction（XRD）,Brunauer-Emmett-Teller（BET）,X-ray photoelectron spectroscopy（XPS）,Raman,O2 temperature-programmed desorption（O2-TPD）,H2 temperature-programmed reduction（H2-TPR） and NO temperature-programmed oxidation（NO-TPO）.The MnOx-CeO2 solid solutions did not form without any calcination process.The oxidation state of manganese tended to increase while the ionic valence of cerium decreased in the mixed oxides,accompanied with the formation of oxygen vacancies.This long-ranged electronic interaction occured more significantly in the tight-contact mixture of MnO and CeO2.The formation of more Mn4＋and oxygen vacancies promoted the catalytic oxidation of NO in an oxygen-rich atmosphere.

Enhanced visible-light photocatalytic performance of a monolithic tungsten oxide/graphene oxide aerogel for nitric oxide oxidation

Photocatalysis is considered a promising technique for removal of pollutants from indoor air.However,the low selectivity and limited recyclability of photocatalysts in powder form currently limit their practical application.In this work,we reported the successful preparation of a monolithic tungsten oxide(WO3)/graphene oxide(GO)aerogel photocatalyst through a cost‐effective freeze‐drying method.GO not only acts as a macroscopic support,but also increases the catalyst surface area from 46 to 57 m2/g,enhances the light absorption in the visible‐light region,and raises the separation efficiency of photogenerated electron‐hole pairs.The Obtained WO3/GO aerogel exhibited an outstanding visible‐light photocatalytic degradation rate of nitric oxide of 51%,which was 3.3 times that of pristine WO3 powder.In addition,the aerogel displayed excellent selectivity,with a generation fraction of toxic nitrogen dioxide of as low as 0.5%.This work presents a facile synthesis route to fabricate a monolithic WO3/GO aerogel photocatalyst with great promise for air purification.

Effect of preparation method on MnO_x-CeO_2 catalysts for NO oxidation

The NO oxidation reaction was studied over MnOx-CeO2 catalysts prepared by co-precipitation, impregnation and mechanical mixing method, respectively. It was found that the co-precipitation was the most active and a 60% NO conversion was achieved at 250 oC. X-ray diffraction （XRD）, Brumauer-Emmett （BET）, H2-temperature programmed reduction （H2-TPR） and oxygen storage capacity （OSC） techniques were employed to characterize the physical and chemical properties of the catalysts. XRD results showed that amorphous MnOx or Mn-O-Ce solid solution existed in co-precipitation and impregnation prepared sample, while crystalline MnOx was found in mechanical mixing catalyst. A larger surface area was observed on co-precipitation prepared catalyst compared to those prepared by impregnation and mechanical mixing. The strong interaction between MnOx and CeO2 enhanced the reducibility of the oxides and increased the amount of Mn4＋ and activated oxygen, which are favorable for NO oxidation to NO2.

O_(x)ygen vacancies on the BiOCl surface promoted photocatalytic complete NO oxidation via superoxide radicals

One of the core issues in the photocatalytic oxidation of nitric oxide is the effective co nversion of NO into the final product(nitrate).More than just improving the visible light photocatalytic performance of BiOCl,we aim to inhibit the generation of toxic by-product NO_(2) during this process.In this study,we demonstrate that the oxygen vacancies(OVs)modulate its surface photogene rated carrier transfer to inflect the NO conversion pathway by a facile mixed solvent method to induce OVs on the surface of BiOCl.The photocatalytic NO removal efficiency under visible light increased from 5.6%to 36.4%.In addition,the production rate of NO_(2) is effectively controlled.The effects of OVs on the generation of reactive oxygen species,electronic transfer,optical properties,and photocatalytic NO oxidation are investigated by combining density functional theory(DFT)theoretical calculations,the in situ FTIR spectra and experimental characterization.The OVs on the surface of BiOCl speed the trapping and transfer of localized electrons to activate the O_(2),producing O_(2)·,which avoid NO_(2) formation,resulting in complete oxidation of NO(NO+O_(2)·→NO_(3)).These findings can serve as the basis for controlling and blocking the generation of highly toxic intermediates through regulating the reactive species during the NO oxidation.It also can help us to understand the role of OV on the BiOCl surface and application of photocatalytic technology for safe air purification.

Active manganese oxide on MnOx-CeO2 catalysts for low-temperature NO oxidation：Characterization and kinetics study

MnO_x-CeO_2 catalysts were synthesized to investigate the active sites for NO oxidation by varying the calcination temperature. XRD and TEM results showed that cubic CeO_2 and amorphous MnO_x existed in MnO_x-CeO_2 catalysts. High temperature calcination caused the sintering of amorphous MnO_x and transforming to bulk crystalline Mn_2O_3, H_2-TPR and XPS results suggested the valence of Mn in MnO_x-CeO_2 was higher than pure MnO_x, and decreased with the increasing calcination temperature, The turnover frequency（TOF） was calculated based on the initial reducibility according to H_2-TPR quantitation and kinetic study. The TOF results indicated that the initial reducibility of amorphous MnO_x with high valence manganese ions was equivalent to the active sites for NO oxidation. It can be inferred that the amorphous MnO_x plays a key role in low-temperature NO oxidation.

Construction of oxygen vacancy on Bi_(12)O_(17)Cl_(2)nanosheets by heat-treatment in H_(2)O vapor for photocatalytic NO oxidation

Oxygen-vacancies(OVs)play significant roles in semiconductor-based photocatalysis,such as elevating light absorption property,photogenerated carries separation efficiency,molecular activation,and photocatalytic activity.However,heat-treatment of semiconductors in dangerous H_(2)atmosphere is usually indispensable for OVs formation.In this work,C-doped Bi_(12)O_(17)C_(12)nanosheets were facially heat-treated in H_(2)O vapor(~2.3 vol%)mixed with Ar at 300℃to in-situ introduce OVs by the proposed reactions of C(s)+H_(2)O(g)→CO(g)+H_(2)(g)and H_(2)(g)+O_(Lattice)→H_(2)O(g)+OV.The formation of OVs,which was confirmed by electron paramagnetic resonance(EPR),can narrow the band gap,and enhance the photogenerated e~-/h~+separation efficiency on Bi_(12)O_(17)C_(12).Moreover,OVs-rich Bi_(12)O_(17)C_(12)nanosheets can facilitate molecular O_(2)activation and produce more reactive oxygen species(ROS),especially~1 O_(2),which greatly improve the NO to NO_(3)~-conversion efficiency with NO removal rate of~63%and NO_(3)~-production selectivity of~92.6%.The present work will bring new insights into the construction and roles of OVs in semiconductor-based photocatalysis.

Chloridion-induced dual tunable fabrication of oxygen-deficient Bi_(2)WO_(6) atomic layers for deep oxidation of NO

Engineering an efficient interface is a trustworthy strategy for designing advanced photocatalytic systems for solar energy conversion.Herein,oxygen-deficient Bi_(2)WO_(6)atomic layers without organic residues were successfully fabricated via a facile solvothermal strategy by the multifunctional regulatory mechanism of introduced chloridion.Both DFT calculations and speciation determination revealed that chloridion displayed a more pronounced effect in the controllable synthesis of oxygen-deficient Bi_(2)WO_(6)atomic layers without organic residues:ultrathinning and defect-engineering.This built-in multi-cooperative interface endowed Bi_(2)WO_(6)with intriguing photoelectrochemical properties,O_(2) activation ability,and ultrahigh activity in visible-light powered deep oxidation of NO.A reasonable photocatalytic mechanism was proposed based on in situ infrared spectroscopy analysis and theoretical calculations.We believe that this multi-cooperative interface engineering of oxygen-deficient Bi_(2)WO_(6)atomic layers without organic residues could provide new insights into the design of two-dimensional(2D)layered materials with efficient active sites and pave the way for efficient NO photooxidation systems.

Synergistic photo-thermal catalytic NO purification of MnO_x/g-C_3N_4: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.

A Bi/BiOI/(BiO)_2CO_3 heterostructure for enhanced photocatalytic NO removal under visible light

Narrow-band BiOI photocatalysts usually suffer from low photocatalysis efficiency under visible light exposure because of rapid charge recombination. In this work, to overcome this deficiency of photosensitive BiOI, oxygen vacancies, Bi particles, and Bi2O2CO3 were co-induced in BiOI via a facile in situ assembly method at room temperature using NaBH4 as the reducing agent. In the synthesized ternary Bi/BiOI/(BiO)2CO3, the oxygen vacancies, dual heterojunctions (i.e., Bi/BiOI and Bi- OI/(BiO)2CO3), and surface plasmon resonance effect of the Bi particles contributed to efficient electron-hole separation and an increase in charge carrier concentration, thus boosting the overall visible light photocatalysis efficiency. The as-prepared catalysts were applied for the removal of NO in concentrations of parts per billion from air in continuous air flow under visible light illumination. Bi/BiOI/(BiO)2CO3 exhibited a highly enhanced NO removal ratio of 50.7%, much higher than that of the pristine BiOI (1.2%). Density functional theory calculations and experimental results revealed that the Bi/BiOI/(BiO)2CO3 composites promoted the production of reactive oxygen species for photocatalytic NO oxidation. Thus, this work provides a new strategy to modify narrow-band semiconductors and explore other bismuth-containing heterostructured visible-light-driven photocatalysts.

Unlocking photocatalytic NO removal potential in an S-type UiO-66-NH_(2)/ZnS(en)Unlocking photocatalytic NO removal potential in an S-type UiO-66-NH_(2)/ZnS(en)_(0.5)heterostructureheterostructure

The contamination of nitric oxide presents a significant environmental challenge,necessitating the development of efficient photocatalysts for remediation.Conventional heterojunctions encounter obstacles such as large contact barriers,sluggish charge transport,and compromised redox capacity.Here,we introduce an innovative S-type heterostructure photocatalyst,UiO-66-NH_(2)/ZnS(en)0.5,designed specifically to overcome these challenges.The synthesis,employing a unique microwave solvothermal method,strategically aligns the lowest unoccupied molecular orbital of UiO-66-NH_(2)with the highest occupied molecular orbital of ZnS(en)0.5,fostering the formation of a stepped heterojunction.The resulting intimate interface contact generates a built-in electric field,facilitating charge separation and migration,as evidenced by time-resolved photoluminescence spectroscopy and photoelectrochemical tests.The abundant active sites in the porous UiO-66-NH_(2)counterpart provide adsorption and activation sites for nitrogen monoxide(NO)oxidation.Performance evaluation reveals exceptional photocatalytic NO removal,achieving 70%efficiency and 99%selectivity toward nitrates under simulated solar illumination.Evidence from X-ray photoelectron spectroscopy and trapping experiments supports the effectiveness of the S-type heterostructure,showcasing refined reactive oxygen species,particularly superoxide.Thus,this study introduces a new perspective on advanced NO oxidation and unlocks the potential of S-scheme heterojunctions to refine reactive oxygen species for NO remediation.

Review on the NO removal from flue gas by oxidation methods

Due to the increasingly strict emission standards of NOx on various industries,many traditional flue gas treatment methods have been gradually improved.Except for selective catalytic reduction(SCR)and selective non-catalytic reduction(SNCR)methods to remove NOx from flue gas,theoxidation method is paying more attention to NOx removal now because of the potential to simultaneously remove multiple pollutants from flue gas.This paper summarizes the efficiency,reaction conditions,effect factors,and reaction mechanism of NO oxidation from the aspects of liquid-phase oxidation,gas-phase oxidation,plasma technology,and catalytic oxidation.The effects of free radicals and active components of catalysts on NO oxidation and the combination of various oxidation methods are discussed in detail.The advantages and disadvantages of different oxidation methods are summarized,and the suggestions for future research on NO oxidation are put forward at the end.The review on the NO removal by oxidation methods can provide new ideas for future studies on the NO removal from flue gas.

Effects of calcium substitute in LaMnO_3 perovskites for NO catalytic oxidation

La1-x Cax MnO3 （x=0-0.3） perovskite-type oxides were synthesized by citrate sol-gel method. The physical and chemical properties were characterized by X-ray diffraction （XRD）, Brumauer-Emmett-Teller method （BET）, X-ray photoelectron spectroscopy （XPS）, NO＋O2 -TPD （temperature-programmed desorption）, activated oxygen evaluation and H2 -TPR （temperature-programmed reduction） technologies. The results showed that NO catalytic oxidation activity was significantly improved by Ca substitution, especially for lower temperature activity. The La0.9 Ca0.1 MnO 3 sample showed the maximum conversion of 82% at 300 oC. The monodentate nitrates played a crucial role for the formation of NO2 . The reducibility of Mn 4＋ ions and reactivity of activated oxygen were favorable for the catalytic performances of NO oxidation.

Comparative study of La(1-x)CexMnO(3+δ) perovskites and Mn-Ce mixed oxides for NO catalytic oxidation

A series of La1-xCexMnO3+δ(x=D,0.05,0.1,0.2,and 0.3)perovskites and Mn-Ce mixed oxides were prepared.Their physico-chemical properties were systematically characterized and the NO oxidation activities of the catalysts were investigated.The La0.9Ce0.1MnO3+δhas the best activity among all of the catalysts,with a maximum NO conversion of 85%at 300℃.The characterization results indicate that the doping of Ce improves the properties of the perovsidtes in terms of the specific surface area,the average valence state of Mn ions,the number of reactive oxygen species and the NOx desorption behaviors.The Mn-Ce mixed oxide calcined at 500℃shows a similar NO oxidation activity with La0.9Ce0.1MnO3+δ.However,the activity of the mixed oxide obtained at 750℃decreases a lot,which results from the loss of active sites and active oxygen species.

Surface hydrogen bond network spatially confined BiOCl oxygen vacancy for photocatalysis

Rational engineering of oxygen vacancy(VO) at atomic precision is the key to comprehensively understanding the oxygen chemistry of oxide materials for catalytic oxidations. Here, we demonstrate that VO can be spatially confined on the surface through a sophisticated surface hydrogen bond(HB) network.The HB network is constructed between a hydroxyl-rich Bi OCl surface and polyprotic phosphoric acid,which remarkably decreases the formation energy of surface VO by selectively weakening the metal–oxygen bonds in a short range. Thus, surface-confined VO enables us to unambiguously distinguish the intrafacial and suprafacial oxygen species associated with NO oxidation in two classical catalytic systems.Unlike randomly distributed bulk VO that benefits the thermocatalytic NO oxidation and lattice O diffusion by the dominant intrafacial mechanism, surface VOis demonstrated to favor the photocatalytic NO oxidation through a suprafacial scheme by energetically activating surface O2, which should be attributed to the spatial confinement nature of surface VO.