NHx Species Research Articles

Kinetic Modeling Analysis of Ar Addition to Atmospheric Pressure N2-H2 Plasma for Plasma-Assisted Catalytic Synthesis of NH3.

Zero-dimensional kinetic modeling of atmospheric pressure Ar-N2-H2 nonthermal plasma was carried out to gain mechanistic insights into plasma-assisted catalytic synthesis of ammonia. Ar dilution is a common technique for tailoring plasma discharge properties and has been shown to enhance NH3 formation when added to N2-H2 plasma. The kinetic model was developed for a coaxial dielectric barrier discharge quartz wool-packed bed reactor operating at near room temperature using a kHz-frequency plasma source. With 30% Ar mixed in a 1:1 N2-H2 plasma at 760 Torr, we find that NH3 production is dominated by Eley-Rideal (E-R) surface reactions, which heavily involve surface NHx species derived from N and H radicals in the gas phase, while the influence of excited N2 molecules is negligible. This is contrary to the commonly proposed mechanism that excited N2 molecules created by Penning excitation of N2 by Ar(4s) and Ar(4p) play a significant role in assisting NH3 formation. Our model shows that the enhanced NH3 formation upon Ar dilution is unlikely due to the interactions between Ar and H species, as excited Ar atoms have a weak effect on H radical formation through H2 dissociation compared to electrons. We find that excited Ar atoms contribute to 28% of the N radical production in the gas phase via N2 dissociation, while the rest are dominated by electron-impact dissociation. Furthermore, Ar species play a negligible role in the product NH3 dissociation. N2 conversion sensitivity analyses were carried out for electron number density (ne) and reduced electric field (E/N), and contributions from Ar to gas-phase N radical production were quantified. The model can provide guidance on potential reasons for observing enhanced NH3 formation upon Ar dilution in N2-H2 plasma beyond changes in the discharge characteristics.

Layered oxide precursor strategy for the synthesizing of titanium oxynitrides

Oxynitride is a mixture of mixed oxide and nitride anions. The creation of simple synthesis methods remains a challenge. Herein, we propose a synthetic method that uses the layered oxide Cs0.68Ti1.83O4 precursor to directly create oxynitride Ti2.85O4N without the need for ammonia. Using a topochemical reaction at 500 °C, a combination of solid Cs2CO3 and TiO2 was converted into the layered oxynitride Cs0.68+xTi1.83O4-xNx.‬‬‬‬‬‬ The crystal structure of Ti2.85O4N has been analyzed by Rietveld refinement with synchrotron radiation data. The UV–Vis spectra revealed that Ti2.85O4N has an absorption edge around 650 nm. Ti2.85O4N exhibits photocatalytic activity for hydrogen evolution utilizing methanol as a reversible electron donor in the visible range (λ > 420 nm) when promoted by platinum. In contrast, the intermediate oxynitride Cs0.68+xTi1.83O4−xNx with surface NHx species does not exhibit photocatalytic activity. These findings open new opportunities for novel titanium oxynitrides photocatalysts.‬‬‬‬‬‬‬

Mechanism of selective catalytic reduction on VOx/TiO2 catalyst by in situ ambient pressure X-ray photoelectron spectroscopy

Unveiling the relationship between NH3 adsorption/activation behaviors and SCR reaction mechanism is imperative for rational catalyst design, but is is still not established because of lacking methodology of in situ spectroscopies. Here, vanadia-based SCR catalysts were used to identify the electron transfer and dynamic evolutions of active sites in SCR reaction by in situ AP-XPS, Raman, and DRIFTS spectra. The activation of NH3 occurred at V5+O sites, and then they reduced to V4+OH species by proton-coupled electron transfer process at high temperature (350 °C). Reducibility of vanadium dominated the catalytic reaction and both lattice and surface oxygen can oxidize the V4+OH to fulfill redox cycles. Whereas, at low temperature (150 °C), the adsorptions of NH3 on Brønsted acid sites was the rate-determine step of NH3 activations. NO was stored as nitrate/nitrite with the help of surface chemisorbed oxygen. The NHx species reacted with adsorbed nitrite/nitrate to generate the NHxNOx intermediate that subsequently decomposed into N2 and H2O.

Theoretical Approach toward a Mild Condition Haber-Bosch Process on the Zeolite Catalyst with Confined Dual Active Sites.

The Haber-Bosch (H-B) process is today's dominant technology for ammonia production, but achieving a mild reaction condition is still challenging. Herein, we combined density functional theory (DFT) calculations and microkinetic modeling (MKM) to demonstrate the feasibility of conducting the H-B process under ambient conditions on a zeolite catalyst with confined dual active sites. Our designed dual Mo(II) cation-anchored ferrierite [2Mo(II)-FER] catalyst shows an energy barrier of only 0.58 eV for N≡N bond breaking due to the enhanced π-back-donation. Meanwhile, the three hydrogen sources (BH, FMH, and NMH) within 2Mo(II)-FER greatly enrich the hydrogenation mechanisms of NHx species, resulting in barriers of <1.1 eV for NHx (x = 0-2) hydrogenations. This dual-site catalyst properly decouples the N2 dissociation and NHx hydrogenation steps, which elegantly circumvents the linear scaling relation between the N2 dissociation barrier and the nitrogen binding energy. It is worth noting that our MKM results show 4 orders of magnitude higher reaction rates on 2Mo(II)-FER than the stepped sites of the FCC Ru catalyst at low temperatures, paving a solid basis to conduct the H-B process at low temperatures. We believe that our strategy will provide crucial guidance for synthesizing state-of-the-art zeolite catalysts to achieve the near-ambient condition H-B process and other chemical reactions in heterogeneous catalysis.

Degradation and chemical stability of graphitic carbon nitride during ultraviolet light irradiation

Graphitic carbon nitride (g-C3N4) was synthesized by pyrolysis of urea at 450 °C for 2 h. X-ray diffraction and transmission electron microscopy confirmed the formation of the stacked hexagonal layers and platelet-like structure of g-C3N4. The sample was irradiated by ultraviolet (UV, 370 nm) light for about 62 h and X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), Nitrogen adsorption–desorption isotherm, and photoluminescence spectroscopy (PL) were used to investigate the degradation. XPS spectra of C 1s electrons revealed four components associated with adventitious carbon, the –C≡N group, heptazine/triazine and carboxyl/carbonate species. Quantitative XPS analysis indicated an increase in the peak areas associated with the –C≡N group and carboxyl/carbonate species, accompanied by a decrease in the areas of the components associated with the heptazine/triazine and adventitious carbon. XPS spectra of N 1s were fitted using four components which were assigned to pyridinic nitrogen (C–N=C), nitrile group (N≡C), amine groups (NHx) and tertiary nitrogen (N-(C)3). The relative areas of peaks associated with the amine and nitrile groups have slightly increased after UV irradiation whereas the peak associated with pyridinic nitrogen decreased. FTIR spectra suggested a reduction in the C–N and C=N heterocycle bonds and slight variation in the C≡N bonds after UV irradiation. BET analysis revealed that the surface area and pore volume have decreased after irradiation. ToF-SIMS analysis indicate the formation of NOx, CNO, NHx, and C4H3NO2 species after irradiation. PL spectra of the g-C3N4 showed a broad blue-green emission which was fitted using three bands and ascribed to the δ*-LP, π-LP, and π-π* transitions within the bandgap. During UV irradiation, the g-C3N4 powder lost about 22% of its blue-green emission intensity after nearly 45 h and then remained stable for further irradiation time. This study confirmed the degradation of g-C3N4 under UV irradiation, which should be further studied for better integration of this material in photoexcited applications.

Plasma-assisted CO2 and N2 conversion to plant nutrient

Colossal research on CO2 and N2 conversion using non-thermal plasma (NTP) technology has been ongoing since many years. The primary focus is on CO and NH3 production through CO2 and N2 conversion, respectively, with high conversion efficiency and low energy consumption with or without catalysts. Although in the present study, we propose that the NTP can assist in converting CO2 and N2 to plant nutrients in the form of plasma-treated/activated water. We used a homemade streamer plasma device and produced plasma-activated water (PAW) using CO2 and N2 feed gas, CO2-activated water (CAW) and N2-activated water (NAW). Later, we used CAW and NAW to treat the radish seeds and evaluate the germination rate, germination percentage, and seeding growth. To understand the chemical changes in PAW after the NTP treatment, we performed a chemical analysis to detect NO2¯, NO3¯, NH4+, and H2O2 along with the PAW pH and temperature shift. Additionally, to understand the other species produced in the gas phase, we simulated chemical reactions using COMSOL Multiphysics® software. Our results show that NOx and NHx species are less produced in CAW than in NAW, but CO2-generated PAW offers a significantly more substantial effect on enhancing the germination rate and seeding growth than NAW. Therefore, we suggested that CO and H2O2 formed during CAW production trigger early germination and growth enhancement. Furthermore, the total plasma reactor energy consumption, NO3¯ and NH4+ selective production percentage, and N2 conversion percentage were calculated. To our best knowledge, this is the first study that uses plasma-assisted CO2 conversion as a nutrient for plant growth.

Catalytic gas nitriding of M50NiL steel by LaFeO3 pervoskite oxide under low-pressure conditions

To generalize the application of the LaFeO3 oxide in catalytic nitriding of steels, the M50NiL steel, a typical high-alloy bearing steel, was nitrided under low-pressure conditions. The catalytic effect of the LaFeO3 oxide on nitriding kinetics of M50NiL steel was evaluated and a new understanding of the catalytic mechanism was proposed. The microstructure, phase composition, surface morphologies and depth of nitrided layers were characterized by optical microscope, X-ray diffraction, scanning electron microscope and microhardness tester, respectively. The wear resistance of nitrided layers was evaluated by a pin-on-disk wear tester and the chemical bonds on nitrided surfaces were identified by X-ray photoelectron spectroscopy to analyze the process of nitriding reactions. The results show that the LaFeO3 catalytic nitriding was suitable for M50NiL steel. The LaFeO3 catalytic nitriding could dramatically improve the toughness of nitrided layer via the suppression of nitrides formation. The adsorption and decomposition of NHx species on the LaFeO3 oxide were alternatively occurred on La-O terminated surfaces and Fe-O terminated surfaces, resulting in the fluctuate nitrogen concentration and rapid diffusion of active nitrogen atoms. The nitriding kinetics greatly depended on the oxidation state of nitrided surfaces. Namely higher oxidation capacity corresponded to higher nitriding efficiency.

Plasma-catalytic synthesis of ammonia over Ru/BaTiO3-based bimetallic catalysts: Synergistic effect from dual-metal active sites

Plasma-catalytic synthesis of ammonia (NH3) was carried out using BaTiO3 supported Ru-M bimetallic catalysts (Ru-M/BaTiO3, M = Fe, Co and Ni) in a dielectric barrier discharge (DBD) reactor. The NH3 synthesis performance followed the order of Ru-Ni/BaTiO3 > Ru/BaTiO3 > Ru-Co/BaTiO3 > Ru-Fe/BaTiO3, with the highest NH3 concentration (3895 ppm) and energy yield (0.39 g kWh−1) achieved over Ru-Ni/BaTiO3 at 25 W and 10 W, respectively. To gain insights into the physio-chemical properties of the Ru-M/BaTiO3 catalysts, comprehensive catalyst characterizations were performed, including X-ray diffraction, N2 physisorption measurements, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), and temperature-programmed desorption of CO2 and N2 (CO2 and N2-TPD). The results indicated that the loading of Ni enhanced the basicity and N2 adsorption capacity of the catalyst, as well as the density of oxygen vacancy (OV) on the BaTiO3 surface, which facilitated the adsorption and activation of N2 on catalyst surface. These effects led to the enhanced NH3 synthesis, as excited N2 could be adsorbed on Ru-Ni/BaTiO3 from plasma region and stepwise hydrogenated to form NHx species and ultimately NH3.

Plasma-enabled sustainable ammonia synthesis at atmospheric pressure: The role of catalysts on synergistic effect

Ammonia (NH3) has received remarkable interest for the synthesis of many nitrogenous product and as storage hydrogen media. However, the synthesis of NH3 remains challenging due to the high pressure and temperature from the dissociation of the stable N≡N triple bond. Here, we report an environmentally friendly plasma-thermal catalytic process for the sustainable synthesis of NH3 over M/CeO2 and M/MgO catalysts (M = Ru, Fe) with dielectric barrier discharge (DBD) reactor at atmospheric pressure, which showed excellent activity and high temperature stability. The synergistic effect strongly depended on the support properties, that CeO2-supported catalysts were more obvious below 300 °C compared with the MgO-supported catalysts regardless of the active metals (Ru or Fe). The highest NH3 synthesis rate of 6.8 mmol g−1 h−1 was achieved over Ru/CeO2 at 400 °C under a ratio of N2:H2 = 1:3. This value was much higher compared with that using DBD reactor reported in the literature. The structure-activity relation of catalysts was characterized through X-ray photoelectron spectroscopy, temperature programmed reduction by hydrogen, Raman spectra and the in-situ diffuse reflectance infrared Fourier transform spectroscopy (Iin-situ DRIFTS). In addition to the gas-phase reaction mediated by plasma and the catalyst surface reaction, the increased NH3 concentration under plasma-catalysis condition can be attributed the synergistic effect between plasma and catalyst, where plasma irradiation promoted the desorption of NH3 and NHx species. These results are helpful for the design of highly efficient catalysts and the understanding for the role of plasma in plasma-enabled green chemistry.

Promoting effect exerted by WOx on Pt/ZrO2 catalysts during NOx reduction using H2 in the excess of O2

This work addresses the reduction of NOx by H2 under O2-rich conditions below 200 °C using Pt/W/ZrO2 catalysts with tungsten loadings from 0 to 14 wt%. Samples were thoroughly characterized by N2 physisorption, temperature-programmed desorption of CO, X-ray diffraction, laser Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as probe molecule. Catalytic investigations showed the highest efficiency for the sample with 5 wt% W providing NOx conversion of 94 % and N2O selectivity of 15 %. The effect of W promoter is mostly attributed to structural promotion by isolated WO6 entities stabilizing the ZrO2 surface and leading to a high number of active Pt sites. By contrast, negligible electronic interaction between the promoter and Pt sites appears. Moreover, DRIFTS studies suggest that, in addition to the predominate NOx reduction by H2 on Pt sites, a side reaction occurs including NHx species formed at the interface of Pt and WOx/ZrO2.

A rational construction of Pt@Cu-ZSM-5@CuOx catalyst with detached multi-functional Pt and Cu sites for high-performance acetonitrile decomposition

Pt@Cu-ZSM-5@CuOx catalyst was constructed for the catalytic oxidation of acetonitrile (AN), which achieved a complete mineralization rate and more than 95 % N2 selectivity within 300–450 °C. Characterization results revealed that the satisfactory synergistic action of hydrolysis and oxidation pathways was well achieved. The NOx release from CN oxidation reaction on out-layer CuOx species and NHx species over-oxidation over Pt sites could be eliminated by NH3 from CN hydrolysis reaction on Cu ions via internal selective catalytic reduction mechanism. The deep oxidation of carbon-containing intermediates (such as CO etc.) during CN decomposition over Cu species effectively occurred over Pt site. The encapsulation of Pt inner ZSM-5 zeolite not only strengthened low-temperature mineralization rate compared to Cu/ZSM-5 sample, but also inhibited NOx release caused by over-oxidation of AN at an elevated temperature with co-impregnation of Pt species over Cu/ZSM-5. This work provided a viable strategy for designing catalysts to effective abatement of NVOCs.

Ionic conductivity and disorder in calcium and barium nitrogen hydrogen phases.

Nitrogen-hydrogen based alkali and alkaline earth metal compounds have recently received a substantial amount of attention as co-catalysts for heterogeneous mild condition ammonia synthesis (MCAS). The incorporation of these materials has been shown to result in positive reaction orders with respect to H2, solving the issue of hydrogen poisoning, e.g., the occupation of the majority of transition metal (TM) active sites by H-adatoms due to the significantly faster kinetics of H2 dissociation as compared to N2. The mechanism that underlies this is thought to be the incorporation (sinking) of H-adatoms from the surface of TMs to the bulk of the N-H phases. Thus, the slower kinetics of N2 dissociation no longer inhibit ammonia synthesis, and improvements in the kinetics dissociation for TM can be realised without consideration for which specific gases are affected (e.g., the circumventing of scaling relations). The ability to transport H-adatoms from the surface of TM is therefore of fundamental importance to the properties of the N-H co-catalyst implying that the conductivity of these species towards H and N ions, and NHx species, is of utmost importance. As such, we investigate two N-H systems that can be prepared by reacting the respective hydrides with nitrogen resulting in nitride-hydride and imide forms for Ca and Ba, respectively. These have both been previously shown to promote ammonia synthesis and here we investigate their conductive properties, and discuss these systems in the context of activity and stability of the total system with specific focus on the rise of secondary anion species, and the presence of barium in the system.

Low-Temperature NOx Reduction by H2 on Mo-Promoted Pt/ZrO2 Catalysts in Lean Exhaust Gases

AbstractThis article aims to improve the low-temperature H2-deNOxperformance of the active Pt/ZrO2catalyst using MoOxas a promoter. For this purpose, a systematic series of Pt/ZrO2samples were prepared with a Pt content of 0.25 wt% and Mo loads from 0 to 10 wt%. The samples were physico-chemically characterized by means of powder X-ray diffraction, N2physisorption, temperature-programmed desorption of CO and NH3, Raman spectroscopy and diffuse reflectance infrared spectroscopy using NH3as probe molecule, while the H2-deNOxefficiency was investigated in a lean synthetic exhaust. The Pt/ZrO2catalyst with a Mo load of 3 wt% showed the best performance, including H2-deNOxbetween 80 °C and 150 °C, a maximum NOxconversion of 90% and N2selectivity up to 78%. Isolated MoOxspecies predominately present at Mo loads below 4 wt% were found to act as structural promoter by stabilizing the BET surface area, while also providing smaller Pt particles and more active Pt sites, respectively. By contrast, the aggregated Mo oxide moieties found at higher Mo loads exhibit a clearly weaker promotional effect. The structure–activity-selectivity correlations also suggest that the promoter additionally enables a SCR-related mechanistic pathway to be followed, including the spill-over of NHxspecies from the Pt sites to strong Lewis acid sites in the case of highly dispersed MoOxentities followed by reaction with NOx.

Hydrogen Balmer line broadening in Ar-N2-SiH4 DC glow discharge

Broadened hydrogen Balmer lines are observed in a DC glow discharge sustained in Ar–SiH4–N2 gas mixture. This broadening is due to the Doppler frequency shift. H atoms’ translational energy distribution is calculated in different positions between the two electrodes of the discharge along the interelectrode axis and in different gas mixtures. Three main components are observed at 0.6(±0.5) eV, 12(±3) eV, 62(±6) eV, 1(±1) eV, 28(±6) eV, and 105(±12) eV for Hα and Hβ, respectively. They correspond to hydrogen dissociation by electron impact producing H(n = 3, 4) atoms and dissociative recombination of the ion in the cathode fall or backscattered fast H atoms on the cathode surface. The density ratio nH2/nAr ranges from 44% to 13% or from 10% to 3% when a = nH/nH2 = 0.01 and 0.1, respectively, in function of the gas mixture. So, large hydrogen density could be due to hydrogen desorption from the wall and cathodes and/or to the gas recirculation within the reactor. A new low energy component is observed in the Hα line profile, when silane and nitrogen concentrations decrease and increase, respectively. It could also be due to the H atom desorbing from the reactor wall or NHx species dissociated by an electron impact within the reactor.

The reaction behaviors of acetonitrile and ethyl acetate simultaneous degradation over Cu–Ce/ZSM–5 catalyst

Aiming at the coexistence of pollutants in the actual industrial process, the catalytic oxidation of acetonitrile and ethyl acetate mixture on the Cu–Ce/ZSM–5 catalyst was studied. The mutual inhibition effect during catalytic removal of acetonitrile and ethyl acetate mixture was observed. Acetonitrile showed a more obvious inhibitory effect on the conversion of ethyl acetate, and the introduction of ethyl acetate reduced the N2 selectivity of acetonitrile oxidation. The simulation results demonstrated acetonitrile could be preferentially adsorbed on Cu center of the catalyst surface, leading to significant inhibition of the initial conversion of ethyl acetate. In addition, the coexistence of ethyl acetate would weaken the hydrolysis of acetonitrile to generate carboxylic acid and –NHx species. As such, no enough adsorbed NHx species could be used to reduce the NOx produced by excessive oxidation of acetonitrile, resulting in a decreased in N2 selectivity. This study could provide reference on the application of catalysts for cyanide–containing waste gas degradation.

Plasma-catalytic ammonia synthesis over BaTiO3 supported metal catalysts: Process optimization using response surface methodology

Non-thermal plasma (NTP) assisted ammonia (NH3) synthesis has been performed in a packed-bed dielectric barrier discharge (DBD) reactor. The presence of M/BaTiO3 (M = Fe, Co and Ni) catalysts in the DBD reactor significantly improved the NH3 concentration and energy efficiency with a sequence of Ni/BaTiO3 > Co/BaTiO3 > Fe/BaTiO3 > BaTiO3. Catalyst characterization, including N2 adsorption-desorption, X-ray diffraction analysis and temperature-programmed desorption of CO2, was performed to get insights into the relationships between the plasma-assisted NH3 synthesis process and M/BaTiO3 catalysts. The improved performance of plasma-assisted NH3 synthesis over Ni/BaTiO3 could be attributed to abundant basic sites and weaker metal-nitrogen (M-N) binding on its surfaces, which benefit the generation of NHx species and desorption of formed NH3. Response surface methodology (RSM) was used to assess the contribution of each independent process parameter and explore their mutual effects on the NH3 synthesis process systematically. The results of analysis of variance (AVONA) suggested that plasma discharge power and gas flow rate were the most significant factors in affecting the NH3 concentration and energy efficiency of the plasma-assisted NH3 synthesis process, respectively. Based on the proposed polynomial regression model, the maximum energy efficiency of 2.37 g kWh−1 was achieved at the predicted optimal operating conditions over Ni/BaTiO3.

Insights to sulfur-resistant mechanisms of reduced graphene oxide supported MnOx-CeOy catalysts for low-temperature NH3-SCR

The MnOx-CeOy/rGO catalysts were prepared by a facile hydrothermal method for selective catalytic reduction (SCR) of NO with NH3. Compared with MnOx-CeOy catalyst, the optimal MnOx-CeOy/rGO(0.1) catalyst not only shows an excellent low-temperature catalytic activity (60–180 °C), but also exhibits a better sulfur-resistant performance in the presence of 100 ppm SO2+5 vol% H2O at 160 °C. The in-situ DRIFTS results show that more NH4+ on Brønsted acid sites and NH3 on Lewis acid sites, as well as NH4HSO4 are detected on MnOx-CeOy catalyst during SCR reaction with SO2, while the adsorbed NHx species over MnOx-CeOy/rGO(0.1) catalyst can be quickly activated and consumed via Eley-Rideal (E-R) reaction pathway. The highly dispersed Ce sites can effectively protect more Mn active sites from sulfation, and the Mn sites with stronger acidity and abundant oxygen vacancy defects promote the oxidative dehydrogenation of NH3 to form NH2, thus enhancing the sulfur resistance of MnOx-CeOy/rGO(0.1) catalyst.

SO2-Tolerant Catalytic Reduction of NOx via Tailoring Electron Transfer between Surface Iron Sulfate and Subsurface Ceria.

Currently, SO2-induced catalyst deactivation from the sulfation of active sites turns to be an intractable issue for selective catalytic reduction (SCR) of NOx with NH3 at low temperatures. Herein, SO2-tolerant NOx reduction has been originally demonstrated via tailoring the electron transfer between surface iron sulfate and subsurface ceria. Engineered from the atomic layer deposition followed by the pre-sulfation method, the structure of surface iron sulfate and subsurface ceria was successfully constructed on CeO2/TiO2 catalysts, which delivered improved SO2 resistance for NOx reduction at 250 °C. It was demonstrated that the surface iron sulfate inhibited the sulfation of subsurface Ce species, while the electron transfer from the surface Fe species to the subsurface Ce species was well retained. Such an innovative structure of surface iron sulfate and subsurface ceria notably improved the reactivity of NHx species, thus endowing the catalysts with a high NOx reaction efficiency in the presence of SO2. This work unraveled the specific structure effect of surface iron sulfate and subsurface ceria on SO2-toleant NOx reduction and supplied a new point to design SO2-tolerant catalysts by modulating the unique electron transfer between surface sulfate species and subsurface oxides.

Constructing Crystalline g‐C3N4/g‐C3N4−xSx Isotype Heterostructure for Efficient Photocatalytic and Piezocatalytic Performances

Graphitic carbon nitride (g‐C3N4) is viewed as a promising visible‐light photocatalyst for industrialization due to its low processing temperature and high chemical stability. However, serious charge recombination caused by incomplete polymerization during direct calcination of nitrogen‐rich precursors significantly limits its photocatalytic performances. To boost charge separation, herein, we propose a rational strategy by constructing a crystalline g‐C3N4/g‐C3N4−xSx isotype heterostructure through the molten salt method. Theoretical calculation reveals that apparent charge‐transfer channels are formed between g‐C3N4 and S‐doped g‐C3N4 layers in the heterostructure. Owing to high crystallinity for decreasing charge recombination and isotype heterostructure for efficient charge transfer, the as‐prepared g‐C3N4/g‐C3N4−xSx showed remarkable photocatalytic performances with the hydrogen production rate elevated by up to 12.3 times of its singular components. Another novelty of this work is we investigated for the first time the piezocatalytic activity of crystalline g‐C3N4 by characterizing its performance for H2O2 generation and KMnO4 reduction. Strikingly, its superior piezocatalytic performance over components can be further improved by NaBH4 treatment, which is uncovered to enhance the asymmetric structure of crystalline g‐C3N4 by introducing extra cyano groups and removing partial NHx species in its tri‐s‐triazine layer structure. This work opens up new strategies for the design of highly efficient polymeric photocatalysts and highlights the piezocatalytic studies of g‐C3N4.

Nitrogen incorporation in graphene nanowalls via plasma processes: Experiments and simulations

The interest in doped and functionalized graphene nanomaterials for various applications is growing due to the development of new and simple production and treatment methods. Amongst the techniques used to treat graphene nanomaterials dry methods like plasmas or ion beams are of particular interest. In this work a low temperature plasma technique is used to incorporate nitrogen atoms into the carbon network of graphene sheets. In order to gain a better understanding of such processes material analysis techniques (NEXAFS and XPS) were combined with the results coming from molecular dynamics simulations and plasma know-how. The results show that the plasma post treatment of graphene nanowalls can be regarded as a balance between vacancy formation, functionalization, doping and crosslinking. Moreover, MD simulations provided insight into fundamental mechanisms like the formation of different bonds due to the interaction of the surface with different kind of species with variable kinetic energy. This can help to improve different types of doping/functionalization techniques using energetic species: the study reveals for example the role of N2+ species, the kinetics of vacancy formations depending on type and energy of the species, the formation of amines or graphitic nitrogen, and the role of impurities such as NHx species.