The advancement in experimental techniques, like the atom probe tomography and high resolution electron microscopy, is fueling interest in studying structural transformations of grain boundaries in ...metal and alloys to uncover correlations between mechanical properties and solute or impurity segregation to grain boundaries. Atomistic modeling is an important tool that can pinpoint the intricate dynamics of grain boundary phase transitions, but the lack of accurate interatomic potentials needed to simulate the complex dynamics of grain boundary structural transitions and identify different metastable phases has been the bottleneck. To this end, we use niobium as a model body centered cubic (BCC) metal and develop an interatomic potential to study grain boundary phase transitions. The potential for Nb is based on a generalization of the embedded atomic method potential and has sufficient flexibility to learn complex energy landscapes using a small set of training structures. We systematically test and validate the using data from ab initio density functional theory calculations and experiments. Using this potential, we calculate energies of multiple symmetric-tilt grain boundaries spanning a wide range of misorientation angles. Further, we explore different metastable structures of the Σ27(552) 11̄0 grain boundary and use molecular dynamic simulations to study the coexistence of metastable phases and grain boundary transitions at finite temperature.
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Abstract For the last 75 years several studies have been reporting on the physical properties of niobium oxides, but there is still many contradictory, inconsistent and insufficient information on ...these metal oxides. This review will begin by describing the niobium oxygen system and the different stoichiometric and non-stoichiometric phases, specifically Nb, NbO, NbO2, Nb2O5 and Nb2O5- delta . The crystalline phases and polymorphs of these materials are often inconsistently identified in different works and thus, a clarification of the nomenclature of the several niobium oxides polymorph and their crystalline structure is also presented. Due to their interesting physical properties, many applications of these materials have been suggested such as solid electrolytic capacitors, catalysis, photochromic devices, transparent conductive oxides or memristors, becoming obvious that a good understanding of niobium oxides physical properties and their control is essential and urgent. Additionally, a short review on different types of niobates, namely alkali niobates, columbite niobates and rare earth niobates and the relation of the properties of these materials with niobium oxides will be presented.
Hydrogen production through facile photocatalytic water splitting is regarded as a promising strategy to solve global energy problems. Transition‐metal carbides (MXenes) have recently drawn attention ...as potential co‐catalyst candidates for photocatalysts. Here, we report niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials (Nb2O5/C/Nb2C) as photocatalysts for hydrogen evolution from water splitting. The Nb2O5/C/Nb2C composites were synthesized by one‐step CO2 oxidation of Nb2CTx. Nb2O5 grew homogeneously on Nb2C after mild oxidation, during which some amorphous carbon was also formed. With an optimized oxidation time of 1.0 h, Nb2O5/C/Nb2C showed the highest hydrogen generation rate (7.81 μmol h−1 gcat−1), a value that was four times higher than that of pure Nb2O5. The enhanced performance of Nb2O5/C/Nb2C was attributed to intimate contact between Nb2O5 and conductive Nb2C and the separation of photogenerated charge carriers at the Nb2O5/Nb2C interface; the results presented herein show that transition‐metal carbide are promising co‐catalysts for photocatalytic hydrogen production.
Shedding light on carbides: Niobium pentoxide/carbon/niobium carbide hybrid materials (Nb2O5/C/Nb2C) are prepared as photocatalysts for hydrogen evolution. The Nb2O5/C/Nb2C sample with an oxidation time of 1.0 h shows a hydrogen generation rate that is four times higher than that of pure Nb2O5. The enhanced performance is attributed to the separation of photogenerated carriers and the formation of a Schottky junction.
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•A comprehensive review of biomass catalytic conversion over Nb-based catalysts.•Properties of various Nb-based catalysts are summarized.•Systematic reaction routes for biomass ...catalytic conversion are evaluated.•Advantages on the application of Nb-based catalysts are discussed.•Future research on bioenergy production over Nb-based catalysts is proposed.
Catalytic conversion of biomass to fuels and chemicals has attracted considerable attention in addressing the global challenges associated with energy and environmental crisis. Catalyst plays a crucial role in the biomass conversion process, and the characteristic of catalysts are continuously improved to meet the requirements. Niobium (Nb) based catalysts, generally in the form of niobic acid, niobium pentoxide (Nb2O5), niobium phosphate (NbOPO4), and their supporting materials, have been widely employed for bioenergy production. This review summarizes the special characteristics of Nb-based catalysts, and the application of these catalysts on the catalytic conversion of biomass, including (I) properties of Nb-based catalysts, (II) catalytic hydrolysis/dehydration for platform chemicals, (III) catalytic pyrolysis for hydrocarbons, (IV) catalytic esterification, (V) catalytic hydrogenation, and (VI) catalytic oxidation. The Nb compounds become an attractive candidate in the bioenergy field, and this review serves as a basis for future research on biomass catalytic conversion over Nb-based catalysts.
Catalysts with heteronuclear metal active sites may have high performance in the nitrogen reduction reaction (NRR), and the in-depth understanding of the reaction mechanisms is crucial for the design ...of related catalysts. In this work, the dissociative adsorption of N
on heteronuclear trimetallic MFe
and M
Fe (M=V, Nb, and Ta) clusters was studied with density functional theory calculations. For each cluster, two reaction paths were studied with N
initially on M and Fe atoms, respectively. Mayer bond order analysis provides more information on the activation of N-N bonds. M
Fe is generally more reactive than MFe
. The coordination mode of N
on three metal atoms can be end-on: end-on: side-on (EES) for both MFe
and M
Fe. In addition, a unique end-on: side-on: side-on (ESS) coordination mode was found for M
Fe, which leads to a higher degree of N-N bond activation. Nb
Fe has the highest reactivity towards N
when both the transfer of N
and the dissociation of N-N bonds are taken into account, while Ta-containing clusters have a superior ability to activate the N-N bond. These results indicate that it is possible to improve the performance of iron-based catalysts by doping with vanadium group metals.
Reported here is the first highly selective conversion of various waste plastics into C2 fuels under simulated natural environment conditions by a sequential photoinduced C−C cleavage and coupling ...pathway, where single‐use bags, disposable food containers, food wrap films, and their main components of polyethylene, polypropylene, and polyvinyl chloride can be photocatalytically transformed into CH3COOH without using sacrificial agents. As an example, polyethylene is photodegraded 100 % into CO2 within 40 h by single‐unit‐cell thick Nb2O5 layers, while the produced CO2 is further photoreduced to CH3COOH. Various methods and experiments disclose that O2 and .OH radicals trigger the oxidative C−C cleavage of polyethylene to form CO2, while other investigations show that the yielded CH3COOH stems from CO2 photoreduction by C−C coupling of .COOH intermediates. This two‐step plastic‐to‐fuel conversion may help to simultaneously address the white pollution crisis and harvest highly valuable multicarbon fuels in natural environments.
Waste conversion: Reported is the first highly selective conversion of various waste plastics into C2 fuels by a sequential photoinduced C−C bond cleavage and coupling pathway under simulated natural environment conditions. The first step is degradation of plastics into CO2, which is reduced in a second step to C2 fuels over the same photocatalyst.
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