Nb Doping Research Articles

Stability and physical properties of the K, Na and Nb doped BiFeO3 lead-free system synthesized by the solid state route

A study of the structural, dielectric, and optical properties of the lead-free ceramic compound (1-x)BiFeO3–(x)(K0.5Na0.5)NbO3 [(1-x)BFO-(x)KNN] prepared by the solid-state reaction method is presented. The structural analysis was carried out by performing Rietveld refinement on the X-ray diffraction data showing a distorted rhombohedral structure of the R3c space group for all samples. The presence of all active Raman modes confirms this result. The effect of K, Na, and Nb incorporation in the BFO structure is clearly seen in the scanning electron microscopy micrographs of the samples, and energy dispersive x-ray spectroscopy (EDS) confirms the existence of all elements. Piezoelectric force microscopy analysis reveals a well-defined domain structure for the higher concentrations of KNN as well as ferroelectric switching behavior. The room temperature dielectric response increases with KNN doping. A small hump in the dielectric permittivity vs. temperature plots is observed near 350 °C and is ascribed to a para-antiferromagnetic phase transition at the Nèel temperature. An optical absorption study revealed that tuning of the bandgap energy is possible through controlled doping, as shown in the present case, where the bandgap varies from 2.16 eV for x = 0.0 to 1.93 eV for x = 0.08. The results of this investigation demonstrate that K, Na, and Nb doping is an effective way to enhance the dielectric and optical properties of BFO.

Systematic Study of Oxygen Evolution Reaction Activity and Stability of SnnSbmNblOx Ternary Oxide-Supported IrO2 Catalysts

Supporting IrO2 with conductive oxides has proven to be a practical way to increase the conductivity of the catalyst layer as well as decrease the anode IrO2 loading in proton exchange membrane electrolyzers. In this work, we proposed a high-throughput aerogel synthesis method to fabricate Sn–Sb–Nb ternary oxide supports. Their thermal stability, conductivity, and acidic stability were then systematically investigated; the results show that Nb addition decreases the ternary oxide’s conductivity by eliminating charge carriers. At the same time, Nb doping improves the thermal stability and increases the specific surface area of the ternary oxides; acidic stability is also increased with 5 at % Nb addition. IrO2 nanoparticles are deposited on selected oxide aerogels via the Adams fusion method to synthesize 50 wt % IrO2/SnnSbmNblOx catalysts. The catalytic performance and stability of catalysts with various supports were compared, revealing a boosted intrinsic activity than unsupported IrO2. The optimal ternary oxide support employed in this work was Sn80Sb15Nb5Ox. Its supported catalyst counterpart has a mass activity of 467 A g–1 at 1.6 V (vs reversible hydrogen electrode) and a Tafel slope of 43.43 ± 0.43 mv dec–1. Compared with other Nb doping amounts, 5 at % Nb catalyst dissolves Ir the least during the oxygen evolution reaction test, which we ascribed to Nb sacrifice. Moreover, the surface area of the supporting materials shows more remarkable influence on the resistance of the catalyst layer than their conductivity, which matters only when the supported catalysts have an approximate surface area. This finding also puts forward a strategy for the screening of supporting materials and provides valuable data for the design and prediction of supporting materials.

Metal-insulator transition tuned by valence variation of Nb dopants in Nb-doped VO2 films

High-valence cation doping was proved to be an effective method in reducing the high metal–insulator transition (MIT) temperature (θc). However, the mechanism of cation-dopants reducing θc is not quite clear and the reported data about the θc reduction induced by Nb dopants are dispersive. In this work, we prepared Nb-doped VO2 films by hydrothermal process and investigated the effects of Nb dopants on the MIT behavior. Valence variation of Nb dopants was observed in the Nb-doped VO2. The Nb5+ dopants elevate the valence band top to higher level than the Nb4+-containing sample. Correspondingly, the Nb5+ doping reduces θc in a higher rate of ∼21 ℃/at%Nb than the Nb4+-containing sample which has the decline rate of ∼18 ℃/at%Nb. The results imply Nb5+ dopants contributing excess electrons modify the band structure and consequently promoting the MIT transition. In addition, the Nb doping realizes the θc reduction almost without detriment to the optical properties of VO2, suggesting the promising applications in smart window and other optical switching devices.

Modulating the Stoichiometry and Lithium Content to Boost the Electrochemical Performance of Li2ZnTi3O8 via High Valence Nb5+ Dopants

AbstractA series of anode materials with the general formula of Li2ZnTi3O8 (LZTO) and Li2‐xZnTi3‐xNbxO8 (x = 0.2, 0.4, and 0.5) (LZTNO) are designed. Different amount of high valence Nb5+ dopants not only reduces the content of lithium in LZTO but also alters the intrinsic characteristics of the composites, leading to optimized electrochemical performances. XRD, SEM, TEM, and XPS results suggest that Nb dopants are introduced successfully, but large amount of Nb5+ dopants (x > 0.2) result in the formation of ZnNb2O6 and TiO2. Owing to the small particle size and the improved structural stability, LZTNO‐2 sample exhibits the best electrochemical performance, and it can deliver a charge/discharge capacity of 182.7/181.2 mAh g−1 at 1 A g−1 after 500 cycles, which is much higher than the value of LZTO (100.41/100.41 mAh g−1). The experiments suggest that the introduction of high valence dopants can effectively modulate the stoichiometry and lithium content of the anode materials, making the available vacant sites for subsequent intercalation of lithium obviously extended. Such a strategy is expected to be feasibly applied to other anode materials to enhance their specific capacity and electrochemical performances.

Enhancing the stability of the ω phase of zirconium alloys via local interlayer twists

The addition of solute provides an effective means to alter the stability of phases in metallic materials. This is particularly true for the metastable t\ensuremath{\omega} phase in Ti- and Zr-based alloys; however, the underlying mechanism remains inconclusive. In the present work, we show that the $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\beta}$ phase-transformation process can be hindered to stabilize the \ensuremath{\omega} phase over a wide temperature and pressure range. This is demonstrated by molecular dynamics simulations of Zr-Nb alloys. In particular, we show that Nb dopants lead to the formation of a defect structure in the Zr-\ensuremath{\omega} phase, i.e., there is a corotation of six atoms in the {0002} plane by 30 \ifmmode^\circ\else\textdegree\fi{} along the $c$ axis of the \ensuremath{\omega} lattice. We refer to this as a local interlayer twist (LIT). The LITs are energetically preferred within Nb-rich regions and can interlock \ensuremath{\omega} lattices, thus retarding the $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\beta}$ phase transformation. Furthermore, our density-functional theory calculations suggest that this mechanism should work in Zr- and Ti-based alloys with small solute atoms. The findings enrich our understanding of complex phase-transformation kinetics in alloys.

Tuning Nb Solubility, Electrical Properties, and Imprint through PbO Stoichiometry in PZT Films.

Lead zirconate titanate (PZT) films with high Nb concentrations (6-13 mol%) were grown by chemical solution deposition. In concentrations up to 8 mol% Nb, the films self-compensate the stoichiometry; single phase films were grown from precursor solutions with 10 mol% PbO excess. Higher Nb concentrations induced multi-phase films unless the amount of excess PbO in the precursor solution was reduced. Phase pure perovskite films were grown with 13 mol% excess Nb with the addition of 6 mol% PbO. Charge compensation was achieved by creating lead vacancies when decreasing excess PbO level; using Kroger-Vink notation, NbTi• are ionically compensated by VPb″ to maintain charge neutrality in heavily Nb-doped PZT films. With Nb doping, films showed suppressed {100} orientation, the Curie temperature decreased, and the maximum in the relative permittivity at the phase transition broadened. The dielectric and piezoelectric properties were dramatically degraded due to increased quantity of the non-polar pyrochlore phase in multi-phase films; εr reduced from 1360 ± 8 to 940 ± 6, and the remanent d33,f value decreased from 112 to 42 pm/V when increasing the Nb concentration from 6 to 13 mol%. Property deterioration was corrected by decreasing the PbO level to 6 mol%; phase pure perovskite films were attained. εr and the remanent d33,f increased to 1330 ± 9 and 106 ± 4 pm/V, respectively. There was no discernable difference in the level of self-imprint in phase pure PZT films with Nb doping. However, the magnitude of the internal field after thermal poling at 150 °C increased significantly; the level of imprint was 30 kV/cm and 11.5 kV/cm in phase pure 6 mol% and 13 mol% Nb-doped films, respectively. The absence of mobile VO••, coupled with the immobile VPb″ in 13 mol% Nb-doped PZT films, leads to lower internal field formation upon thermal poling. For 6 mol% Nb-doped PZT films, the internal field formation was primarily governed by (1) the alignment of (VPb″-VO•• )x and (2) the injection and subsequent electron trapping by Ti4+. For 13 mol% Nb-doped PZT films, hole migration between VPb″ controlled internal field formation upon thermal poling.

Single metal-doped silicon (Si59X; X = Nb, Mo, Y, Zr) nanostructured as nanosensors for N-Nitrosodimethylamine (NDMA) pollutant: Intuition from computational study

The present work examines the efficacy of metal-doped silicon (Si59X; X = Nb, Mo, Y, Zr) nanocages as an efficient nanosensors for N-Nitrosodimethylamine (NDMA). A suitable computational approach employing the exchange-correlation functional (PBE) along the Lanl2DZ basis set was utilized to this end to investigate the potential of silicon nanostructure, doped transition metal as a detector for NDMA. To comprehend the nature of the interaction between the nanostructure and adsorbate (NDMA), diverse computational descriptors including the adsorption energy of interaction, electronic attributes of the surfaces, sensing mechanism and recovery time of the modelled nanostructures were examined. The results show that doping all Si60 fullerene nanostructure with transition metals (Nb, Mo, Y, and Zr) enhanced the sensing attributes of the nanostructure towards NDMA. The average adsorption energy was calculated to be − 169.43 kcal/mol. The doped metals were also observed to influence the surface conductivity and binding affinity; the obtained adsorption energy was found to decrease with the doping of Zr metal whereas an increase was observed in the case of Nb doping. Other molecular descriptors also pointed out favourable results and affirmed the suitability of the surfaces to detect NDMA. Moreover, the recovery of NDMA from the surfaces was calculated to be in the milliseconds’ range and therefore suggests the recyclability of the surfaces. Finally, classical MD simulations affirmed the stabilities of the surfaces and disclosed that adsorption the adsorbate does not result in considerable deformation of the modelled surfaces. It is, therefore, concluded that the modelled surfaces could be further explored as potential candidates for the construction of amperometric nano-sensors.

Theoretical investigation on niobium doped LiFePO4 cathode material for high performance lithium-ion batteries

In this work, the first principle study has been conducted to investigate the properties of LiFePO4 (LFP) cathode material for the Li-ion batteries, and the effects of doping on their crystalline structures with Nb atoms was explored. The obtained results showed that doping of the LFP structure with Nb at the Fe site significantly improved the conductivity of LFP and decreased the band gap from 3.502 to 0.658 eV, which is consistent with the acquired density of state data. Moreover, energy formation results proved that the stability of the system after doping was ideal. The geometrical analysis of the structure showed that the Nb doping leads to widening of the Li passage channel, increased bond length, and reduction in covalent strength among atoms which in turn mitigated energy barrier. Furthermore, our calculation proved that Nb doping has no adverse effect on the theoretical voltage of LFP and, its value remained constant at about 3.1 V. Then, the nudged elastic band method was examined for the different pathways of Li ion diffusion in the LFP crystal. The finding represented that energy barriers of Li migration along all paths were decreased through Nb doping. In addition, the diffusion coefficient of Li-ion in the doped systems was improved by four orders of magnitude compared to undoped LFP. Moreover, the effective mass of electron theory and theoretical voltage of pristine LFP and Nb-doped LFP were evaluated. Finally, the electron localization function theory was implemented, for in-depth study and illustration of the difficulty of Li-ion diffusion and bond characteristics.

Niobium doped triazine based covalent organic frameworks for supercapacitor applications

Covalent organic frameworks (COFs) are gaining high importance in energy storage systems due to their uniform porosity and versatile functionality. The present work deals with the fabrication of triazine-based COF through Schiff base formation. The method involves the polycondensation reaction between melamine and terephthalaldehyde. In addition, the COF was also decorated with Niobium to generate Nb@COF. The morphological, elemental mapping, X-ray photoelectron spectroscopy, C13 NMR results show the effective doping of Nb to the COF and provide insights into the nature of bond formation. The obtained experimental XRD results are in good agreement with Materials Studio simulated results. COF and Nb@COF have been used for supercapacitor applications in a three-electrode system. Enhanced specific capacitance was observed in Nb@COF (367 F g−1) compared to pure COF (244 F g−1) at a scan rate of 2 mV s−1. The superior electrochemical performance in Nb@COF could be due to the increased porosity and interlayer spacing. COF and Nb@COF exhibited good stability towards charge and discharge and managed to retain 82 and 89 % specific capacitance, respectively even at 5000 cycles. COF and Nb@COF were assembled in a coin cell and fabricated an asymmetric supercapacitor device as negative and positive electrodes, respectively. The results obtained were satisfactory and shows a specific capacitance of 87 F g−1 at a scan rate of 2 mV s−1.The good stability, specific capacitance and reliability of Nb@COF indicate its potential application in energy storage devices.

Synergistic Strategy Using Doping and Polymeric Coating Enables High-Performance High-Nickel Layered Cathodes for Lithium-Ion Batteries

As one of the most promising cathode materials for lithium-ion batteries, nickel-rich layered oxide LiNi0.83Co0.11Mn0.06O2 (NCM83) has an inherent issue with rapid capacity decay caused by phase change and interface side reactions during the charge/discharge cycling. Herein, an effectively synergistic strategy for improving the structural stability and electrochemical performance of NCM83 cathodes has been proposed, combining surface polymeric coating with bulk doping by the high-temperature solid-phase method. The comprehensive results demonstrate that the niobium (Nb) element can be successfully bulk-doped into the crystal lattice, which could increase the layer spacing, thus stabilizing the crystal structure and minimizing the Li+/Ni2+ mixing in the as-prepared NCM83 cathode. Meanwhile, a small amount of Nb in the form of oxide and a layer of polyaniline (PANI) was coated on the NCM83 cathode’s surface. This not only can prevent electrolyte erosion and inhibit side reactions but also can effectively improve the transport coefficient of Li+ during the charge and discharge process. The optimized NCM83 cathode exhibited outstanding discharge-specific capacity (236.79 mAh g–1 at 0.2 C rate and 215.67 mAh g–1 at 2 C rate), stable cycling performance (capacity retention of 84.4% after 100 cycles at 2 C), and excellent rate performance (150.58 mAh g–1 at 10 C) by taking advantage of the synergistic effects. The synergistic strategy using Nb doping in combination with a surface polymeric coating can enhance the fundamental understanding of the high-nickel layered oxide cathodes for lithium-ion batteries.

A first principles study on physical properties of Nb-doped LiCoO2 for memristor (CM-3:IL02)

The electronic, magnetic, and vibrational properties of Nb-doped LiCoO2 were studied theoretically via ab initio calculations using density functional theory. We find that the Nb-doping induced sizable mid-gap states due to the strong Nb 4d – O 2p hybridization, which results in the shrinking of the gap between the valence band maximum and the bottom of the conduction band in LixCo1-yNbyO2. Moreover, the oxidation state of Co ions becomes a mixture of Co3+ and Co2+ instead of purely Co3+ in pristine LiCoO2, which indicates the existence of non-zero magnetic moment in the Nb-doped systems. The mixed oxidation states of Co ions are further confirmed by the X-ray absorption spectra calculated for the main three edges, O K-edge, Co K-edge, and Co L-edge, which shows that the Co LIII – edge is shifted to lower energy in Nb-doped LiCoO2. To investigate the structural properties, we calculate phonon spectra for both pristine and doped cases. We find that while the phonon dispersion of LiCoO2 does not have negative-frequency modes, the Nb-doped LiCoO2 has phonon modes with negative frequencies that can be removed by applying Hubbard correction, indicating the importance of the electron correlation upon Nb doping. We argue that the Nb-doping induced modulation of electron correlation could be employed to engineer the electrical transport properties, making LixCo1-yNbyO2 an ideal candidate for memristor.

Heterogeneous doping via charge carrier transport improves Photoelectrochemical H2O oxidative H2O2 synthesis

Photoelectrochemical (PEC) water splitting into H2O2 and H2 has attracted significant attention due to its low cost and sustainability. However, slow charge carrier transport and water oxidation kinetics limit the selectivity for H2O2 production and solar conversion efficiency. Herein, we propose a heterogeneous doping approach that combines surface gradient doping with bulk doping to improve the charge carrier transport in the photoelectrode. Inducing gradient Nb and homogeneous Mo doping into BiVO4 photoanode (G-Nb/Mo:BVO) can promote charge separation and carrier transfer, leading to the high selectivity for H2O2 generation and suppression of O2 production. Consequently, the heterogeneously doped G-Nb/Mo:BVO photoanode presents an average Faraday efficiency (FE) of over 80% for H2O2 production in a wide potential of 0.6–1.8 VRHE with the maximum FE of 83.7% at 1.2 VRHE under AM 1.5G illumination. More importantly, H2O2 production rate can reach 1.23 μmol min−1 cm−2 at 1.23 VRHE, representing the best H2O2 production rate reported for the photoelectrodes. Density functional theory calculations prove that Mo and Nb co-doping increases the reactivity of BiVO4 and improves 2e− water oxidation reaction activity and selectivity. This work demonstrates that heterogeneous doping provides a cost-effective strategy to break performance trade-offs by improving charge carrier separation and transport in light absorbers, and modulating the selectivity and activity of water oxidation reaction to generate H2O2, which can be extended to other photocatalytic reactions.

Study of structural, magnetic and electronic properties of a new off-stoichiometric series of full-Heusler alloy Co2Nb1+xZ1−x (Z = Sn, In, Ga): Ab initio approach

Using ab initio calculation, we investigate systematically the structural, electronic and magnetic properties of Co2Nb1+xZ1−x (x = 0, 0.25, 0.50). Here, Plane-Wave self-consistent field (PWscf) program implemented in open-source Quantum Espresso software package within generalized gradient approximation (GGA) - Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation functional has been used. Equilibrium lattice parameters in cubic austenitic phase of Co2Nb1+xZ1−x (x = 0, 0.25, 0.50) have been inquired. It has been found out that except Co2NbSn, all the compositions do not show martensitic structural transition. The microscopic origin for this has been associated with Jahn-Teller distortion of Co 3deg orbitals in close proximity of the Fermi level. The density of states for both tetragonal and 4 O orthorhombic structure in martensitic phase of Co2NbSn have been investigated. Moreover, from the density of states plots we have recognized the metallic character of Co2NbSn. However, Co2NbGa & Co2NbIn and all of the off-stoichiometric alloys have been anticipated to possess partial half-metallic character. Furthermore, we have examined the total magnetic moment and average atomic magnetic moments in off-stoichiometric Co2Nb1+xZ1−x in the cubic phase to understand the effect of Nb doping in stoichiometric compounds. Nb doping increases the total magnetic moment due to increased Nb-Co hybridization of d orbitals.

Positive correlation of Nb/Cr doping with dehydrogenation performance of ZrCo-based hydrides

It is well known that element substitution is vital to an improvement of hydrogen storage capacity degraded by hydrogen-induced disproportionation of zirconium-cobalt (ZrCo) alloy. Contrary to other studies, this work is devoted to investigating the hydrogen desorption behavior of Zr1-xNbxCo1-yCryH3 (x = 0–0.25 and y = 0–0.125) systems formed by Nb substitution and Nb/Cr co-substitution in the ZrCoH3 crystal using the first-principles calculations based on the density functional theory. Firstly, the preferred positions of Nb and Nb/Cr substitutions are determined through the minimum total energy. Lattice constants and the density of states are employed to characterize the bonding behavior between atoms. The thermodynamic properties of Zr1-xNbxCo1-yCryH3 are also assessed by the enthalpy of formation and equilibrium hydrogen pressure. It is shown that both the thermodynamic stability and dehydrogenation temperature of ZrCoH3 decrease by increasing the Nb content, which is attributed to the fact that the Zr–H and Co–H binding capabilities are weakened by the presence of Nb. In addition, there are weaker thermal stability, lower dehydrogenation temperature, smaller volume of 8e site, and comparable volume of the hydride after ZrCoH3 is co-substituted by Nb and Cr, thereby promoting the hydrogen release and enhancing the anti-disproportionation performance. Clearly, both Nb doping and Nb/Cr co-doping possess a positive effect on the hydrogen desorption ability of ZrCoH3. More importantly, it is discoverable that compared to other substitution systems, the dehydrogenation temperature is the least for the Zr0.75Nb0.25Co0.875Cr0.125 hydride, highlighting that Nb and Cr co-substitution is more beneficial to improving the dehydrogenation performance of ZrCo-based alloy.

Sol-Gel controlled dispersion of Nb within TiO2 to obtain water tolerant catalysts with tunable acid-base properties for dihydroxyacetone transformation

Nanostructured Nb doped TiO2 were prepared by a novel Sol-Gel method using new N‐methyldiethanolamine‐modified precursors of Ti (IV) and Nb(V). Co-hydrolysis of Ti(IV) and Nb(V) precursors in suitable ratio resulted in the introduction of atomically dispersed Nb cation, along with the formation of small Nb oxo clusters homogenously dispersed within the TiO2 matrix. Characterizations showed a progressive decrease of the crystallinity as well as an increase in the defects and the BET surface areas of the TiO2 nanoparticles with the Nb doping. Upon Nb doping, gas phase calorimetry of NH3 and CO2 adsorption, evidenced the acidity but also the basicity increase. The aqueous phase dihydroxyacetone conversion increased significantly over Nb/TiO2 as compared to undoped TiO2. However, upon increasing the Nb doping, the base-catalyzed pathway, which forms hexoses from DHA, became the prevailing mechanism at the expense of the usual acid-catalyzed pathway leading to formation of Pyruvaldehyde.

Controllable 2H/3R phase transition and conduction behavior change in MoSe2:Nb substitution by high pressure synthesis for promising thermoelectric conversion

Transition metal dichalcogenides (TMDs) are uniquely multifunctional materials with optical, electronic, and catalytic properties. Despite the advantages of low cost, low toxicity, and high abundance, the thermoelectric transport properties of MoSe2 were not extensively investigated. Meanwhile, MoSe2 bulk material with 3R phase was rarely reported compared to 2H phase. In this work, controllable phase transition from 2H to 3 R for MoSe2 bulk polycrystalline material was achieved with various Nb contents by a simple and feasible high-pressure method. The preferred orientation resulted in anisotropy of both electrical and thermal transport. The samples converted from n type for pristine sample to p type conduction after Nb doping. Meanwhile, the conduction type gradually changed from semiconductor to degenerated semiconductor. The electrical properties were distinctly improved by Nb doping systematically from the reduced bandgap and the enhanced carrier concentration and mobility. The lattice thermal conductivity was reduced by point defects and grain/phase boundaries generating from Nb doping. Maximum zT of 0.17 at 873 K was obtained for Nb0.04Mo0.96Se2, which is among the highest values for Te-free Mo dichalcogenides. The strategy of chemical doping and high-pressure synthesis provides an alternative route to achieve MoSe2 bulk materials with a controllable 2H/3R phase ratio for potential applications, which can be extended to other TMDs.

Influence of doping and thickness on domain avalanches in lead zirconate titanate thin films

In undoped lead zirconate titanate films of 1–2 μm thick, domain walls move in clusters with a correlation length of approximately 0.5–2 μm. Band excitation piezoresponse force microscopy mapping of the piezoelectric nonlinearity revealed that niobium (Nb) doping increases the average concentration or mobility of domain walls without changing the cluster area of correlated domain wall motion. In contrast, manganese (Mn) doping reduces the contribution of mobile domain walls to the dielectric and piezoelectric responses without changing the cluster area for correlated motion. In both Nb and Mn doped films, the cluster area increases and the cluster density drops as the film thickness increases from 250 to 1250 nm. This is evident in spatial maps generated from the analysis of irreversible to reversible ratios of the Rayleigh coefficients.

Characteristics of deactivation and thermal regeneration of Nb-doped V2O5–WO3/TiO2 catalyst for NH3–SCR reaction

This study investigated the effect of Nb doping into V2O5–WO3/TiO2 (VWT) catalyst for removing NOxvia the SCR (selective catalytic reduction) by NH3. The experimental results exhibited that Nb can improve the reactivity of the VWT catalyst at low temperatures. The addition of Nb also enhanced the tolerance to SO2 and H2O. The de-NOx efficiency of the V2O5–WO3–Nb2O5/TiO2 (VWNbT) catalyst was increased up to 12% over that of the VWT catalyst at 240 °C when the catalyst was poisoned for 24 h. The prepared catalysts were characterized by FT−IR, XRD, XPS, and N2 physisorption, elemental analysis. The results showed that the ammonium bisulfate (ABS) was less formed in the VWNbT than in the VWT. Moreover, evolved gas analysis was performed to examine the thermal decomposition behavior of the poisoned catalyst, and confirmed that the ABS deposited on the catalyst was sufficiently decomposed between about 300 and 400 °C. In particular, to most effectively recover the characteristics and activity of the catalysts, thermal treatment at a temperature of 400 °C is suitable.

Enhanced low-temperature activity and huimid-SO2 resistance of MnFe-based multi-oxide catalysts for the marine NH3-SCR reaction

Several MnFe-based multi-metal oxides were synthesized as NH3-SCR catalysts by a simple coprecipitation method for abating NOx of marine diesel exhausts. The Co and Nb-doped MnFeCeAl catalysts exhibit NOx conversion over 90% and N2 selectivity above 95% at 180–270 °C, especially the MnFeCeAlCo catalysts can inhibit nearly all sulfate species growth within 150 ppm humid-SO2 gases at 225 °C. The structural characterization results revealed that Co, Nb, Sm, and Sb doping can enhance interactions among different components and promote active component dispersion. Temperature programmed analysis indicated that the Co doping is not only more favorable for improving redox properties, but can also enhance the surface acidity, which are advantageous to improve the activity, N2 selectivity, and humid-SO2 resistance. Moreover, the XPS results implied that the binding energy shift or the valence variation of the Sm, Sb, Nb, and Co species on catalyst surfaces are favored to raise the atomic ratios of high-valent Mn species and surface adsorbed oxygen, which can promote the redox property significantly and further facilitate SCR activity. Accordingly, the excellent activity and humid-SO2 tolerance of the MnFeCeAlCo catalyst should attribute to its lower redox temperature, strong interaction between oxides, 47.3% surface Mn4+/Mn3+ species, and 71.8% adsorbed oxygen, which provide a method for improving the SCR performances of MnFe-based catalysts with humid SO2 resistance.

Enhancing thermoelectric performance of n-type AgBi3S5 through synergistically optimizing the effective mass and carrier mobility

AgBi3S5 is a new n-type thermoelectric material that is environmentally friendly and composed of elements of earth-abundant, non-toxic and high performance-cost ratio. This compound features an intrinsically low thermal conductivity derived from its complex monoclinic structure. However, the terrible electrical transport properties greatly limited the improvement of thermoelectric performance. Most previous studies considered that carrier concentration is the main reason for low electrical conductivity and focused on improving carrier concentration by aliovalent ion doping. In this work, we found that the critical parameter that restricts the electric transport performance of AgBi3S5 was the extremely low carrier mobility instead of the carrier concentration. According to the Pisarenko relationships and density functional theory calculations, Nb doping can sharpen the conduction band of AgBi3S5, which contributes to reducing the effective mass and improving the carrier mobility. With a further increase of the Nb doping content, the conduction band convergence can enlarge the effective mass and preserve the carrier mobility. Combined with the decrease in lattice thermal conductivity due to the intensive phone scattering, a maximum ZT value of ∼0.50 at 773 K was achieved in Ag0.97Nb0.03Bi3S5, which was ∼109.6% higher than that of pure AgBi3S5. This work will stimulate the new exploration of high-performance thermoelectric materials in ternary metal sulfides.