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1.
Modern Fe–N–C electrocatalysts are promising as alternatives to expensive Pt-based catalysts for oxygen reduction reactions (ORR). Although the activity of this type of electrocatalyst have been improved over the years, their durability and longevity need critical enhancements for practical applications in fuel cells. Typically, the incomplete oxygen reduction inevitably generates reactive oxygen species, including ·OH and HO2· radicals, which will fiercely attack the carbon support and directly damage active sites in Fe–N–C electrocatalysts. Herein, a durable and robust Fe–N–C@Ti3C2–TiO2 electrocatalyst for high-efficiency ORR is synthesized, in which Ti3C2–TiO2 could effectively scavenge ·OH radicals or decompose H2O2 molecules, and synergistically work with Fe–N–C catalysts to improve the durability. Consequently, the Fe–N–C@Ti3C2–TiO2 electrocatalyst shows prominent ORR performance in both alkaline and acidic electrolytes, low H2O2 yield, and long-term stability. This work provides great prospects for the design of highly stable ORR electrocatalysts by introducing radical scavengers as an active defense to proactively eliminate H2O2 and its radicals.  相似文献   

2.
Cu/CuCr2O4 catalysts were prepared by impregnation method at various calcination temperatures (300, 400, and 500 °C) and then reduced in H2 stream. The aggregated particles and decreasing surface area/pore volumes of the deactivated catalysts during HCOOH and CH3OH formations were also observed. Particularly, the EXAFS data showed that first shells of Cu atoms transforms from Cu–O to Cu–Cu after catalytic reactions, their bond distances and coordination numbers are quite different, respectively. It revealed that metallic Cu atoms are one of the important active species over catalyst surface at different reaction temperatures having many unoccupied binding sites for HCOOH and CH3OH formations. Additionally, the optimal calcination temperature for Cu/CuCr2O4 catalysts was demonstrated at 400 °C that attributed to its strongest acidity and basicity. The catalytic reactions in the duration of HCOOH and CH3OH preparation were proposed that were composed of HCOOH formation, CH3OH formation, and CH3OH decomposition happening at CuCr2O4, Cu, and CuO active sites, respectively. The highest CO2 conversion (14.6%), HCOOH selectivity/yield (87.8/12.8%), and TON/TOF values (4.19/0.84) were obtained at 140 °C and 30 bar in 5 h, respectively. Optimal rate constant (2.57 × 10?2 min?1) and activation energy (16.24 kJ mol?1) of HCOOH formation were evaluated by pseudo first-order model and Arrhenius equation, respectively.  相似文献   

3.
Two-dimensional measurements of primarily hydroperoxyl radicals (HO2) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H2/O2, CH4/O2, and CH4/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q1(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO2, but also smaller contributions from H2O2 and, for the methane flames, CH3O2. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO2, from which O atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H2O and H2. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO2 is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis.  相似文献   

4.
A fluffy mesoporous graphitic carbon nitride (g-CN) was synthesized and investigated, as the first metal-free polymeric semiconductor photocatalyst, for partial oxidation of methane to methanol at a mild condition (35°C) in the presence of hydrogen peroxide (H2O2). The addition of g-CN photocatalyst enhanced the methanol production with H2O2 by 10 times under visible light and this production was 3-folds that when WO3 was used. Besides, while attaining a comparable saturated production of CH3OH, the visible-light-driven reactions allowed a higher utilization ratio of H2O2 compared with those under the irradiation of simulated AM1.5G sunlight. More importantly, via electron spin resonance technique, scavenger experiments and isotope tracer investigations, as well as studies on the effects of reaction condition variations, the specific reaction mechanisms were revealed under both visible light and simulated full sunlight, respectively, which should shed some new insights on photocatalytic oxidation of CH4 to value-added chemicals.  相似文献   

5.
ZrO2-supported tungsten oxides were used for cyclic production of syngas and hydrogen by methane reforming (reduction) and water splitting (re-oxidation). The reduction characteristics of WO3 to WO2 and WO2 to W were examined at various temperatures (1073–1273 K) and reaction times. Significant portions of the tungsten oxides were also reduced by the produced H2 and CO. The extent of reduction by H2 varied greatly depending on temperature and WO3 content and also on the reduction of either WO3 or WO2, while that by CO was consistently low. When the overall degree of reduction became sufficiently high, methane decomposition started to proceed rapidly, resulting in considerable carbon deposition and H2 production. Consequently, the H2/(CO + CO2) ratio varied from around 1 to higher than 2. During the repeated cyclic operations with a proper reduction time at a given temperature, the syngas and hydrogen yields decreased gradually while the H2/(CO + CO2) ratio remained nearly constant and the carbon deposition was negligible.  相似文献   

6.
The advancement of highly-efficient, non-noble metal electrocatalysts with high active sites for oxygen evolution reaction (OER) continues to face severe challenges. In this work, a new 3D flower-like α-Co(OH)2 catalyst with high Co–NH2 active sites (referred to as α-Co(OH)2–NH2) is prepared. The Co–NH2 active sites are formed through the interactions between Co2+ and –NH2 groups and exhibit higher electrocatalytic performance for OER in alkaline mediums, with a lower overpotential (300 mV at 10 mA cm−2), a flatter Tafel slope (75 mV dec−1) and a longer stability. Moreover, water splitting catalyzed by α-Co(OH)2–NH2 delivers 1.62 V to reach a current density of 10 mA cm−2. The mechanism studies show that the OER electrocatalytic activity is closely related to the content of –NH2. Density functional theory (DFT) calculations unveil that –NH2 groups can facilitate the formation of OH1 on the α-Co(OH)2 surface and promote the desorption of O21 into O2(g).  相似文献   

7.
The hydrogen production through solar water splitting could be eco-friendly, environmental-benign and sustainable, alternative to fossil fuels energy carriers. In this work, the novel heterojunction WO3/fullerene/Ni3B/Ni(OH)2 composites were successfully fabricated through different methods. The Ni3B/Ni(OH)2 as a needle like nanostructures were integrated into the interfaces between WO3 and fullerene. The structural phase and morphology of prepared nanomaterials were significantly modified by incorporating the Ni3B/Ni(OH)2 as a co-catalyst. The results demonstrated that dopant element serves as the conductive electron bridges, rather than general cocatalyst, to accumulate electrons and inspire the H2 generation kinetics over pure WO3 photocatalyst. The significantly improved hydrogen rate of evolution up to 1578 μmol h−1 g−1 was found for hybrid photocatalyst [WO3/fullerene@ 1.5% Ni3B/Ni(OH)2] which was 9.6 times higher as compared to pure photocatalyst. The interfacial contact between co-catalyst and composite could play substantial role in effective separation and transference of charge-carriers for improved hydrogen evolution rate. Under the visible light illumination, the heterojunction nanostructures inspire rapid transfer of electrons from WO3 towards Ni3B/Ni(OH)2 to inhibit the fast recombination rate and prolong the WO3 charge-carrier's lifetime, thereby releasing more electrons with greater reducing power for hydrogen production. This work may open an avenue for the strategy and green preparation of robust photocatalysts for scalable applications in solar H2 evolution and sustainable environment.  相似文献   

8.
Recently, the copper phthalocyanine based 2D conjugated MOFs have been demonstrated that it can efficiently catalyze the oxygen reduction reaction (ORR) experimentally. Herein, the inherent properties and ORR mechanism of PcCu-O8-M (M = Fe, Co, Ni, Cu) containing two potential active sites (M–O4 and Cu–N4 sites) are investigated by density functional theory methods. The binding energy of oxygen-containing intermediates on PcCu-O8-M indicates that the binding strength of 1OOH, 1O, and 1OH at the M–O4 site is more moderate than that at the Cu–N4 site. For all studied sites, their potential-determining steps are the step of O2 → 1OOH except for the Ni–O4 site of PcCu-O8-Ni (1OH → H2O). Among all sites of PcCu-O8-M, the Fe–O4 and Co–O4 sites have excellent ORR activity with the overpotentials of 0.46 and 0.49 V, respectively, and the Cu–N4 site of PcCu-O8-Co possesses the relatively high ORR activity with the overpotential of 0.79 V. It can be concluded that compared with the Cu–N4 site, the M–O4 site plays major role for each PcCu-O8-M during the ORR process. Moreover, the charge analysis of PcCu-O8-Co suggests that the ORR activities of the Co–O4 and Cu–N4 sites are mainly originated from metal atoms (Co atom and Cu atom) as well as the O and N atoms (O and N-1) coordinated with metal atoms. In addition, PcCu-O8-Co has high poisoning-tolerance ability to NO, NH3, CO, SO2, and H2S.  相似文献   

9.
The density functional theory (DFT) calculations were performed to investigate the stability of the S-doped Fe–N2G electrocatalysts, as well as ORR mechanism and activity. The most stable configuration is the Fe–N2S1G because of forming a strong bond structure of Fe–S. In addition, the structures of Fe–N2S3G and Fe–N2S4G also exhibit the higher stability compared to the undoped Fe–N2G. According to the distinct charge difference on the surface, the O-contained intermediates would like to adsorb on the active sites of Fe–N2 complex active sites. The binding strength of OH on these different catalysts follows the increasing order of Fe–N2S4G < Fe–N2S3G < FeN2G < Fe–N2S1G < Fe–N2S5G < Fe–N2S2G < Fe–N2S6G < Fe–N2S7G, implying the opposite order of the catalytic activity. The calculations of the free energy diagrams show that all elementary reaction steps on Fe–N2S4G, Fe–N2S3G, FeN2G and Fe–N2S1G are downhill. Besides, the rate determining step (RDS) for these catalysts (excluded Fe–N2S4G) is the fourth reduction step (OH*+H++e→H2O+*). The study of the reaction mechanisms predicted that the direct 4-electron reduction process is the favorable ORR pathway, and the alternative reaction pathways containing the formation of OH* + OH* co-adsorbate also process without the formation of H2O2 for these catalysts. Particularly, Fe–N2S4G also exhibits the outstanding performance for H2O2 reduction. In general, since the higher stability and working potential for ORR, Fe–N2S4G is predicted to be the prior candidate site for ORR among S-doped FeN2G catalysts.  相似文献   

10.
First-principle density functional theory (DFT) calculations are performed to study the active sites in FeN4G electrocatalysts, as well as ORR activity and mechanism. The possible intermediates and transition states existing in the possible reaction paths from Langmuir-Hinschelwood (LH) mechanism are investigated. The results show that the associative pathways of OOH1 formation is prior to that of O21 dissociation. The condition of proton adsorbed on top N sites (T2) is more beneficial to the reduction of O-contained species adsorbed on top Fe site (T1) compared to the conditions of proton adsorbed on top C sites (T3). However, the dissociation of O21, OOH1 and H2O21 is more likely to occur on the paired T1-T3 sites, since their lower energy barriers compared to other paired sites. The most favorable four-electron reduction pathway follows the mechanisms: O21→ OOH1→ O1+H2O→ OH1+H2O→ 2H2O. The rate determining step for ORR on FeN4G is the reduction of O1 into OH1 (barrier, 0.47 eV). The most feasible pathway for ORR is downhill at a high electrode potential (0.76 V vs. SHE at pH = 0) according to the free energy diagram. Compared to the ideal catalyst, the adsorption energy of OOH1 on FeN4G is much lower in free energy, while those of OH1 and O1 are slightly higher. Additionally, the elementary reaction rate for OOH1→O1+H2O is much larger than that of OOH1→H2O2 based on the parameter of activation barrier. Therefore, the formation of H2O2 (l) is unfavorable on FeN4G catalysts.  相似文献   

11.
Ni-based/SBA-15 catalysts, were promoted by 3wt % of samaria (Sm2O3), Yttria (Y2O3) and Zirconia (ZrO2), by two-solvent impregnation method. The catalysts characterization was performed by N2 adsorption–desorption, X-ray Diffraction (XRD), X-ray Fluorescence (XRF), High Resolution Transmission Electron Microscopy (HRTEM), Field Emission Electron Scanning Microscopy (FESEM), Temperature Programmed Oxidation/Reduction (TPO/TPR) and NH3-Temperature Programmed Desorption (NH3-TPD) techniques. Then, evaluated by CO2/methane reforming.The CO2/methane reforming outcomes revealed that samaria-promoted catalyst showed excellent activity, stability and cock resistance, while yttria-promoted catalyst just illustrated good activity at high temperature and zirconia-promoted catalyst didn't show any modification in catalytic performance in comparison to Ni-based catalyst with no promoter. Samaria-promoted TEM and TPR analysis, indicated adding samaria improved the NiO particles interaction with SBA-15 support pores wall and NiO dispersion. The TPO analysis displayed that coke deposition in samaria-promoted sample after 12 h reaction is less than yttria-promoted during stream of 5 h. Also, it is suggested that for samaria containing catalyst, cock deposition occurred on the support. Therefore, nickel active sites were preserved for time on stream of 12 h, which is the main reason for samaria-promoted catalyst superior stability than other's.  相似文献   

12.
Combined density functional theory and grand canonical monte Carlo (GCMC) calculations were performed to study the electronic structures and hydrogen adsorption properties of the Zn-based metal-organic framework MOF-650. The benzene azulenedicarboxylate linkers of MOF-650 were substituted by B atoms, N atoms, and boronic acid B(OH)2 linkers, and the Zn atoms were substituted by Mg and Ca atoms. The calculated electronic densities of states (DOSs) of MOF-650 showed that introduction of B atoms reduces the band gap but damages the structure of MOF-650. Introduction of single N bonds cannot provide active electrons to attract H2 molecules. Thus, substitutions of B and N into MOF-650 are not suggested. B(OH)2 substitute in MOF-650 decreased its band gap, slightly improved its hydrogen storage ability and made H2 molecules more intensively distributed besides organic linkers. GCMC calculations were carried out by estimating the H2 storage amount of the pure and modified MOFs at 77 and 298 K and from 1 bar to 20 bar. B(OH)2 linker and Mg/Ca co-doped MOF-650 showed increased H2 adsorption by approximately 20 wt%. The adsorption of H2 around different bonds showed the order N–C < C = C < B–C < C–O < B–O.  相似文献   

13.
The adsorption of hydrogen (H2) on tiny titanium dioxide Tin(O2)n clusters where n = 1, 2 and 3 decorated a (5, 5) ultra-small boron nitride nanotube (BNNT) is studied theoretically using the density functional theory calculation. Tin(O2)n/BNNT is very stable and it can hold a large number of H2 molecules while maintaining its stability. That H2 adsorption on Tin(O2)n/BNNT/BNNT shifts the geometry of Tin(O2)n/BNNT as the number of H2 molecules adsorbed on its surface increased. For example, the bond between N–O increases while the bond between the H atoms in the H2 molecules shortens. Furthermore, the local density of states (LDOS), crystal orbital overlaps population (COOP), and charge distribution analysis all confirm that H2 formed a bond with TiO2/BNNT. Thus, we can conclude that Tin(O2)n/BNNT is a promising material for hydrogen storage.  相似文献   

14.
Activation of C–H, B–H as well N–H sigma bonds has been a subject of fundamental interest due to potential feedstock in chemical industry. The co-operative effect of the metal centers in a di-nuclear carbide cluster Ta2C4? for methane C–H activation and dissociation has recently been revealed, in which a molecule of hydrogen is evolved. Based on that, we have explored reaction pathways for ‘E?H’ sigma bond activation and dissociation processes of gaseous Ammonia (E: N) and Borane (E: B) using the stable form of the aforesaid complex at DFT levels. The geometries and energetics associated with the reactions are found to be method insensitive. The course of the reaction is initiated by a 1:1 precursor complex formed between the cluster and N (/B)H3. This complex formation is found to be more exothermic than that of its methane counterpart. In case of BH3, considerable lengthening of two B–H bonds in the first step is observed which implies that the two B–H bonds are activated simultaneously under the influence of Ta2C4?. But for NH3, N–H bond lengthening as well as activation is observed to be insignificant in this precursor complex. The chemical nature of the participating hydrogen atoms is inspected by NPA analysis (‘protic’ type in NH3 and ‘hydride’ type in BH3).The overall dehydrogenation procedures for both the molecules are found to be multi-step with high exothermicity. Highly negative net energy change in terms of Gibbs Free Energy (?31.03 kcal/mol for ammonia and ?31.36 kcal/mol for borane) as well as Enthalpy (?33.68 kcal/mol for ammonia and ?34.13 kcal/mol for borane), in forming a H2 molecule with stable Ta2C4N(/B)H? complex, with respect to the reactant pair govern the thermodynamic feasibility of the overall process. In a nutshell, this computational study provides a detail understanding of the activation and dissociation processes of the concerned gaseous molecules, which will be beneficial for further experimental studies.  相似文献   

15.
Aiming at fabricating high-activity and stable methane combustion catalysts in the dry/wet conditions, Co–Mn binary oxides were employed as promoters to Pd/Al2O3 system herein. The introduction of appropriate amount of manganese made Mn3+ maximally enter into the Co3O4 spinel structure, conducive to the conversion of Co3+ to Co2+ by Mn3+ and then the enhancement of lattice distortion. Therefore, abundant oxygen vacancies were produced, which enhanced the surface-concentrations of active Pd2+ and Oads species, together with the exchange of oxygen species. The resulting catalyst with a molar Mn/Co ratio of 0.20 performed superior low-temperature activity and durability. Moreover, the synergy of Mn and Co could accelerate the removal process of accumulated OH/H2O from the active sites, thereby promoting the regeneration of PdO and oxygen vacancies. This endowed the tailored catalyst with remarkable moisture-tolerance and hydrothermal stability, and inspiring enhanced activity (T90 = 350 °C) after removing water vapor.  相似文献   

16.
The interfacial reaction of highly active plutonium hydride in humid circumstance is of great interest in nuclear safe handling and storage, but it is poorly understood so far. In this paper, we first studied the O2 adsorption on (110) and (111) surfaces of PuH2 by first-principles DFT + U method. The results show that there are dissociative and non-dissociative adsorption of oxygen on the surfaces. We analyze the vibrational frequencies of non-dissociative oxygen adsorbed on the surfaces. It is found that the corresponding frequency of oxygen with bond length of 1.330–1.340 Å is 1094.8–1098.2 cm?1. The corresponding frequency of oxygen with bond length of 1.448–1.500 Å is 726.3–905.2 cm?1. It shows that non-dissociative oxygen could be considered as superoxide (O2?) or peroxide (O22?) species. In order to expound the atomistic evolution process of oxidized surface exposed to moist air or corrosive solution, the interactions between H2O molecules and the strongest oxygen adsorption structures were further explored. The results indicate that H2O molecules could dissociate into OH groups and H atoms, then they were captured to create Pu–O and H–O bonds. This work could provide new insights into the adsorption morphology of oxygen on hydride surface and the interaction between oxide/hydride interface and water.  相似文献   

17.
A methane steam reforming process for producing mainly hydrogen in an atmospheric-pressure microwave plasma reactor is demonstrated. Nano carbon powders, COx, C2H2, C2H4, and HCN were also formed. Intermediates such as OH, NH, CH, and active N2 were identified using optical emission spectroscopy. The selectivity of H2 was greater than 92.7% at inlet H2O/CH4 molar ratio (R) ≧ 0.5, and was higher than that obtained using methane plasmalysis because steam inhibited the formation of C2H2. The highest methane conversion was obtained at R = 1, reaching 91.6%, with the lowest specific energy consumption of H2 formation at [CH4]in = 5%, 1.0 kW, and 12 slpm. The plasma-assisted catalysis process, which packed Ni/Al2O3 catalysts in the discharge zone and supplied heat using hot effluents, was used to elevate the methane conversion and hydrogen selectivity. However, large amounts of 40–70 nm carbon powder, which is electrically conductive, were produced, resulting in rapid catalyst deactivation due to carbon being deposited on the surface and in the pores of catalysts.  相似文献   

18.
Unloaded and 0.25–1.0 wt% Pt-loaded WO3 nanoparticles were synthesized by hydrothermal method using sodium tungstate dihydrate and sodium chloride as precursors in an acidic condition and impregnated using platinum acetylacetonate. Pt-loaded WO3 films on an Al2O3 substrate with interdigitated Au electrodes were prepared by spin-coating technique. The response of WO3 sensors with different Pt-loading concentrations was tested towards 0.01–1.0 vol% of H2 in air as a function of operating temperature (200–350 °C). The 1.0 wt% Pt-loaded WO3 sensing film showed the highest response of ∼2.16 × 104 to 1.0 vol% H2 at 250 °C. Therefore, an operating temperature of 250 °C was optimal for H2 detection. The responses of 1.0 wt% Pt-loaded WO3 sensing film to other flammable gases, including C2H5OH, C2H4 and CO, were considerably less, demonstrating Pt-loaded WO3 sensing film to be highly selective to H2.  相似文献   

19.
Differences in the activity of Pd/WO3 and Pd/MoO3 (Pd loading 0.4–4 wt%) catalysts in competitive hydrogenations of the CC and CO groups in polyfunctional reagents have been studied as a function of two effects: (1) the in situ formation of hydrogen bronzes, HxWO3 and HxMoO3, and (2) the electronic interaction between the supports and the metallic Pd. The cinnamaldehyde (CAL), furfural (FU) and 5-hydroxymethylfurfural (HMF) were hydrogenated under mild reaction conditions. The formation of hydrogen bronzes in Pd/WO3 and physical mixture of Pd/WO3 with supporting WO3 oxide upon exposure to H2 was also studied using the gas flow-through microcalorimetry. In both Pd/MoO3 and Pd/WO3 catalysts, the electronic interactions contributed to the promotion of selectivity toward the CO hydrogenation in CAL and FU, yet in Pd/MoO3 this effect was much more pronounced. On the other hand, apart from increasing the overall reaction rate, the formation of hydrogen bronzes remarkably enhances the CC hydrogenation in CAL, as well as the decarbonylation of FU to furan and hydrogenolysis of C–OH in HMF to 5-methylfurfural. The bronze effects are significantly stronger in HxWO3, compared to HxMoO3, which may be related to higher H-species mobility and weaker H-bonding in the W–O–H (54 kJ/mol H2) than in the Mo–O–H (100 kJ/mol H2). This may also explain very high tendency of Pd/WO3 to furan ring hydrogenation in FU and HMF as well as almost selective (>98%) hydrogenation of furfuryl alcohol to tetrahydrofurfuryl alcohol.  相似文献   

20.
In order to simultaneously inhibit the Ni sintering and coke formation as well as investigate the effects of WO3 promoter on catalytic performance, the ordered mesoporous Ni–WO3/Al2O3 catalysts were synthesized by a facile one-pot evaporation-induced self-assembly method for CO methanation reaction to produce synthetic natural gas. Addition of WO3 species could significantly promote the catalytic activity due to the enhancement of the Ni reducibility and the increase of active centers, and the optimal N10W5/OMA catalyst with NiO of 10 wt% and WO3 of 5 wt% achieved the maximum CH4 yield 80% at 425 °C, 0.1 MPa and a weight hourly space velocity of 60000 mL g−1 h−1. Besides, the reference catalyst N10W5/OMA-Im prepared by the conventional co-impregnation method was also evaluated. Compared with N10W5/OMA, N10W5/OMA-Im showed lower catalytic activity due to the partial block of channels by Ni and WO3 nanoparticles, which reduced active centers and restrict the mass transfer during the reaction. In addition, the N10W5/OMA catalyst showed superior anti-sintering and anti-coking properties in a 425oC-100 h-lifetime test, mainly because of confinement effect of ordered mesoporous structure to anchor the Ni particle in the alumina matrix.  相似文献   

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