Adsorption Of N2 Molecules Research Articles

In operando identification of the V4+-site-dependent nitrogen reduction reaction of VSx

VS2−x (V2+, V4+) and VS2 (V4+) are prepared to investigate the effect of valence states of vanadium on the catalytic activity. The adsorption of N2 molecules is exothermic on the V4+ site, but endothermic on the V2+ site. The spontaneous formation of *N2 at V4+ substantially boosts the ENRR.

Theoretical insights into the origin of promoter effect of alkali metals on Au-catalyzed nitrogen electroreduction

A deeper understanding on promotion effect of alkali metals on initial N2 electroreduction at Au(111)/H2O electrochemical interface is performed. The results show that presence of alkali metals can lead to spontaneous occurrence of initial N2 electroreduction. We speculate that proton in solvent is more facile to migrate into physically adsorbed N2 molecule due to incorporation of alkali metals. The promotion effect of alkali metals may originate from increased kinetic overpotential. It can be also attributed to the retarded HER, which can further increase selectivity of initial N2 electroreduction. Thus, origin of the promotion effects of alkali metals is revealed.

Adsorption States of N2/H2 Activated on Ru Nanoparticles Uncovered by Modulation-Excitation Infrared Spectroscopy and Density Functional Theory Calculations.

The adsorption states of N2 and H2 on MgO-supported Ru nanoparticles under conditions close to those of ammonia synthesis (AS; 1 atm, 250 °C) were uncovered by modulation-excitation infrared spectroscopy and density functional theory calculations using a nanoscale Ru particle model. The two most intense N2 adsorption peaks corresponded to the vertical chemisorption of N2 on the nanoparticle's top and bridge sites, while the remaining peaks were assigned to horizontally adsorbed N2 in view of the site heterogeneity of Ru nanoparticles. Long-term observations showed that vertically adsorbed N2 molecules gradually migrated from the top sites to the bridge sites. Compared to those adsorbed vertically, N2 molecules adsorbed horizontally exhibited a lower dipole moment, an increased N─N bond distance, and a decreased N─N bond order (i.e., were activated), which was ascribed to enhanced Ru-to-N charge transfer. H2 molecules were preferentially adsorbed horizontally on top sites and then rapidly dissociated to afford strongly surface-bound H atoms and thus block the active sites of Ru nanoparticles. Our results clarify the controversial adsorption/desorption behavior of N2 and H2 on AS catalysts and facilitate their further development.

Filling characteristics of the micropores in coals with different metamorphic degrees

To explore the occurrence state of coalbed methane (CBM) in coals of different metamorphic degrees and accurately predict CBM, we studied the micromechanism of coal and gas outbursts in a bid to avoid or reduce personnel injuries or property losses in such underground accidents. Low-temperature N2 adsorption experiments were conducted on three coal samples of different metamorphism degrees. We explored the critical filling characteristics of the packed N2 molecules in the coal. The Dubinin–Astakhov equation in the micropore filling theory and the density functional theory method were applied to obtain the critical filling pressure and critical filling aperture range for the N2 molecules to complete the micropore filling in the coal. The results showed that in the low-temperature nitrogen adsorption experiment, the pore structure of lignite was uniform, and the adsorption of N2 molecules was better. The ratio of nitrogen molecules in the adsorbed form was higher than those found in bituminous coal and anthracite. The initial filling of the micropores in coal started when the relative pressure was P/P0 ≈ 10−6–10−4 and completed when P/P0 ≈ 10−2. The order of completion was anthracite, bituminous coal, and lignite. The slit-type channel was more conducive to microhole filling than the ink bottle channel. The critical filling pore size of the coal samples with different levels of metamorphism was in the range of 1.7–2.2 nm. Finally, the critical filling pore size was divided into a primary filling stage with the range being 0.36–1.7 nm and a secondary filling stage with the range being 1.7–2.2 nm.

Computational screening of highly selective and active electrocatalytic nitrogen reduction on single-atom-embedded artificial holey SnN3 monolayers

Billowy interest during nitrogen reduction reaction (NRR) for single-atom catalysts (SACs) has been evoked by the discovery of single transition metal (TM) atom structures featured by TM-Nx coordinate sites as an excellent catalytic center. However, a great challenge of currently available SACs, far away from industrial requirement, is the low activity and poor selectivity. Therefore, in NRR, the first-principles high-throughput screening calculations were performed to evaluate the feasibility of a single TM atom (from Sc to Au) embedded an artificial holey defective SnN3 (d-SnN3) monolayer. Here, all TM atoms can be stably anchored on d-SnN3 (TM/d-SnN3), meanwhile, most of adsorbed N2 molecules can be favorably activated via the “σ donation - π* back-donation” interaction. Eventually, among 27 TM centers, V, Mo, Hf and Ta/d-SnN3 stand out because of extremely low limiting potential (-0.21, −0.40, −0.56 and −0.54 V, respectively), lower than majority of TM-based NRR catalysts and far below that of the Ru (0001) surface (0.98 V), indicative of fast kinetics and low energy cost of NRR. Moreover, their intrinsic characteristic, such as centralized spin-polarization on these TM atoms, high-efficient prohibition of the competitive hydrogen evolution reaction is responsible for high selectivity with theoretical faradic efficiency of 100%. Also, multiple-level descriptors including ΔG∗N, ICOHP, and Φ were used to make the source of NRR activity clear, realizing an efficient and quick prescreening among different candidates. Particularly, their excellent durability, kinetic stability and synthetic accessibility guarantee the feasibility in real experimental conditions.

Recyclable Composites of Mo/Fe Co-Doped ZnO1−x/Cu2O for Enhanced Photocatalytic Nitrogen Fixation

A layer of Cu2O crystal film and ZnO nanoparticles doped with bimetallic Mo and Fe were successively grown on the surface of a copper mesh, which was applied in a liquid membrane photoreaction apparatus for photocatalytic nitrogen reduction reaction (PNRR). The characterization results indicate that the excellent ammonia yield benefits mainly from the synergistic effect of bimetallic doping, moderate oxygen vacancies (OVs), and p-n heterojunction. Surface defects caused by in situ doping of Mo and Fe ions act as active centers to capture and migrate electrons from the catalyst to adsorbed N2 molecule. With the introduction of bimetallic ions, the formation of more mesoporous enlarges the specific surface area, causing abundant reaction sites on the surface of the catalyst, which enhances the adsorption and activation of dinitrogen molecules. The existence of oxygen vacancy reduces the band gap and enhances the absorption of visible light. Furthermore, the p-n heterostructure promotes the effective transfer and separation of photogenerated carriers. Compared with the original Cu2O or ZnO, the photocatalytic ammonia yield of Mo–Fe co-doped ZnO/Cu2O is increased by 6.6 and 7.3 times, respectively, reaching 119.42 μmol cm−2 h−1.

Density Functional Theory Study on the Effect of Isomorphic Substitution of FAU Molecular Sieve on N2 Adsorption Performance

Low pressure and anoxia are the main characteristics of the environment in the Tibetan Plateau, which means people living there have a large demand for oxygen to reduce the symptoms of altitude sickness. Pressure swing adsorption (PSA) is a competitive oxygen production technology in plateau areas, which relies on the molecular sieves for the separation of N2 and O2 in industry and portable medical equipment. The adsorption characteristics of the Faujasite-type (FAU) molecular sieves, as one kind of the most widely used adsorbents for O2 production, depend on the properties, amount, and distribution of the skeleton cations and atoms. In this paper, we explore the isomorphic substitution effect on the adsorption properties of N2 in FAU molecular sieves using the computational approaches based on the density functional theory (DFT). The structural analysis and adsorption energy calculated for the Zn, Ca, and Ga substitutions at the Si/Al skeleton sites in the β-cage structure, the basic unit of FAU molecular sieves, prove that the isomorphic substitution effect can strengthen the adsorption of N2. The Bader charge and density of states analysis validate the formation of electron-deficient holes near the Fermi level and hence strengthen the local polarity of the pore structure and enhance the adsorption of N2 molecules. The work about isomorphic substitution on the FAU molecular sieves might provide an insight into heteroatom isomorphic modification mechanisms and designing excellent air separation materials.

Achieving efficient N2 electrochemical reduction by stabilizing the N2H* intermediate with the frustrated Lewis pairs

Electrocatalytic nitrogen reduction reaction (eNRR) with sustainable energy under ambient conditions represents an attractive approach to producing ammonia, but the design of the-state-of-the-art electrocatalyst with high efficiency and selectivity still faces formidable challenges. In contrast to traditional eNRR catalyst design strategies focusing on N≡N triple bond activation, we herein theoretically proposed an alternative strategy to improve eNRR performance via stabilizing the N2H* intermediate using catalysts with the frustrated Lewis pairs (FLPs), i.e., transition metal (TM) atoms and boron (B) atom co-doped 2D black phosphorus (TM-B@BP). Our density functional theory (DFT) results reveal that the TM atom donates electrons to the adsorbed N2 molecule, while B atom provides empty orbital to stabilize the adsorption of N2H* intermediate. This framework successfully identifies five promising candidates (i.e., Ti-B@BP, V-B@BP, Cr-B@BP, Mn-B@BP and Fe-B@BP) with low theoretical limiting potentials (−0.60, −0.41, −0.45, −0.43 and −0.50 V, respectively) and high selectivity for eNRR. We believe that the intermediate stabilization strategy introduced in current work offers a new opportunity to achieve accelerated and cost-effective ammonia synthesis with electrocatalysis.

Taming the challenges of activity and selectivity in catalysts for electrochemical N2 fixation via single metal atom supported on WS2

Developing the novel types of electrocatalysts with intrinsic stability, high activity and selectivity is a significant mission to advance the sustainable conversion of naturally abundant N2 to NH3, but it remains a long-standing challenge. Herein, through theoretical screening approach, focusing on addressing this challenge, we rationally designed a series of WS2 monolayer decorated by single transition metal (TM) atom as TM@WS2 candidates and systematically explored their potentials for catalyzing N2 reduction by building a full profile of stability, activity and selectivity. Our results unveil that, among 26 candidates, Cr@WS2, Tc@WS2 and Os@WS2 finally stand out as the most promising candidates, on which the adsorbed N2 molecule can be efficiently converted into NH3 through the distal mechanism with the limiting potentials of −0.37, −0.27 and −0.33 V, respectively, and impressively, the competing HER process can be pronouncedly inhibited. In particular, the activity trend is constructed by using *NNH adsorption energy and φ, and ICOHP reveals the origin of catalytic activity toward NRR. Our investigation not only contributes to the development of the highly efficient electrocatalysts, but also provides insightful views on the optimal design of WS2-based catalysts for advanced NH3 synthesis.

Piezo-enhanced activation of dinitrogen for room temperature production of ammonia

The catalytic conversion of nitrogen to ammonia remains an energy-intensive process, demanding advanced concepts for nitrogen fixation. The major obstacle of nitrogen fixation lies in the intrinsically high bond energy (941 kJ mol−1) of the N≡N molecule and the absence of a permanent dipole in N2. This kinetic barrier is addressed in this study by an efficient piezo-enhanced gold catalysis as demonstrated by the room temperature reduction of dinitrogen into ammonia. Au nanostructures were immobilized on thin film piezoelectric support of potassium sodium niobate (K0.5Na0.5NbO3, KNN) by chemical vapor deposition of a new Au(III) precursor [Me2Au(PyTFP)(H2O)] 1 (PyTFP = (Z)-3,3,3-trifluoro-1-(pyridin-2-yl)-prop-1-en-2-olate) that exhibited high volatility (60 °C, 10–3 mbar) and clean decomposition mechanism to produce well adherent elemental gold films on KNN and Ti substrates. The gold-functionalized KNN films served as an efficient catalytic system for ammonia production with a Faradaic efficiency of 18.9% achieved upon ultrasonic actuation. Our results show that the spontaneous polarization of piezoelectric materials under external electrical fields augments the sluggish electron transfer kinetics by creating instant dipoles in adsorbed N2 molecules to deliver a piezo-enhanced catalytic system promising for sustained activation of dinitrogen molecules.

Adsorption of CH4, N2, CO2, and their mixture on montmorillonite with implications for enhanced hydrocarbon extraction by gas injection

Understanding the adsorption behaviors of CH4, N2, CO2, and their mixture in clay minerals is crucial to the approach of enhanced hydrocarbon recovery using gas injection with application in clay-rich gas-shale reservoirs. In this study, the adsorption measurements of CH4, N2, CO2, and their mixture on montmorillonite were conducted using a gravimetric technique. The underlying competitive adsorption mechanism of CH4, N2, and CO2 on montmorillonite was discussed using Henry's constant, adsorption selectivity, and thermodynamic variables. The adsorption selectivity of CO2 over N2 (αCO2/N2) was the highest, followed by the selectivity of CO2 over CH4 (αCO2/CH4) and that of CH4 over N2 (αCH4/N2). The αCO2/CH4 value was greater than one, confirming that CO2 can stimulate CH4 desorption through molecular swapping. Reducing the temperature increased the selectivity and the desorption of the pre-adsorbed gas was more easily triggered by the injection of the favorably adsorbing gas at lower temperatures. The uptake and affinity of CO2 were the highest, whereas N2 exhibited the smallest uptake and affinity. In addition, CO2 adsorption was more spontaneous than CH4 and N2 adsorption. Although N2 adsorption was less favorable than CH4 adsorption, injecting N2 could lower the spontaneity of CH4 adsorption by decreasing the CH4 partial pressure. The adsorbed CO2 molecules were the most ordered, and the freedom of adsorbed N2 molecules was the highest. The adsorption isotherm of the mixed gases was closer to that of the strong adsorption component. Thus, a strong adsorption gas plays a leading role in the adsorption of mixed gases.

Thermodynamic N2 Vapor Isotherms of Materials: Zeta Adsorption Isotherm Approach

For N2 vapor adsorbing on a nanopowder at 77 K, we use a multicomponent canonical ensemble to determine the conditions for equilibrium between a contacting vapor phase and the adsorbed molecular clusters. Provided the substrate variations (heterogeneities) do not cause a phase transition, such as wetting or capillary condensation, we show that the equilibrium conditions allow an expression for an adsorption isotherm to be formed. The isotherm expression contains four constants that are to be obtained empirically from mass-specific adsorption measurements. The specific surface area of a substrate is expressed in terms of the specific number of adsorption sites and the cross-sectional area of an adsorbed N2 molecule on materials. This procedure is investigated by using the respective specific surface areas of different samples of a given material to transform the data into an area-specific basis. We pooled data sets from multiple samples with possibly different specific surface areas to form a single data set. The thermodynamic (zeta adsorption) isotherm is formulated after a regression analysis is performed on the pooled data set. This isotherm is used with the Gibbs adsorption equation to obtain the solid surface energy of a material as a function of the amount adsorbed. Five materials are examined: silica, α-alumina, γ-alumina, carbon black, and graphitized carbon. Their respective solid surface energies are each shown to be a material property.

Exploring the N2 Adsorption and Activation Mechanisms over the 2H/1T Mixed-Phase Ultrathin Mo1-xWxS2 Nanosheets for Boosting N2 Photosynthesis.

Solar-driven conversion of nitrogen (N2) to ammonia (NH3) is highly appealing, yet in its infancy, the low photocatalytic efficiency and unclear adsorption and activation mechanisms of N2 are still issues to be addressed. In this study, ultrathin alloyed Mo1-xWxS2 nanosheets with tunable hexagonal (2H)/trigonal (1T) phase ratios were proposed to boost photoreduction N2 efficiency, while the mechanisms of N2 adsorption and activation were explored simultaneously. The alloyed Mo1-xWxS2 nanosheets for the 1T phase concentration of 33.6% and Mo/W = 0.68:0.32 were proven to reach about 111 μmol gcat-1 h-1 under visible light, which is 3.7 (or 3)-fold higher than that of pristine MoS2 (or WS2). With the aid of density functional theory calculations and in situ N2 adsorption X-ray absorption near-edge fine structure techniques, the adsorption and activation behaviors of N2 over the interface of Mo1-xWxS2 nanosheets were investigated during the N2 reduction process. The results show that the W doping causes a higher electron density state in W 5d orbitals, which can further polarize the adsorbed N2 molecules for adsorption and activation. This work provides a new insight into the adsorption and activation mechanisms for the NH3 synthesis.

A computational study of N2 adsorption on aromatic metal Mg16M;(M=Be, Mg, and Ca) nanoclusters

Metal nanoclusters have been considered as a new class of chemical sensors due to their unique electronic structures and the particular physicochemical properties. The interaction of N2 molecule with neutral and ionic magnesium nanoclusters Mg17q(q=0,±1), as well as neutral magnesium nanoclusters with the centrality of beryllium and calcium Mg16M (M=Be, Mg, and Ca) have been investigated using CAM-B3LYP/6-311+G(d) level of theory in the gas phase. The electronic properties of magnesium nanoclusters were significantly affected by the adsorption of N2 molecule. The NBO analysis revealed a charge transfer from the adsorbed N2 molecule to the nanocluster. Based on the adsorption energies and enthalpies, a thermodynamically favorable chemisorption process was predicted for the Mg16Ca–N2 complex. The negative value of the Gibbs free energy of Mg16Ca–N2 confirmed the spontaneous adsorption process. The estimated recovery time for Mg16Ca–N2 complex for 8-MR (0.089 s) and 4-MRs (0.075 s) illustrated a possible desorption process for N2 molecule from the surface of Mg16Ca. Our finding also revealed the Mg16Ca has the ability to use as a sensor for detection and absorption of N2 molecule.

Growth and thermal stability studies of layered GaTe single crystals in inert atmospheres

In this study, we investigated the thermal stability properties in inert atmospheres of GaTe single crystals fabricated by a temperature gradient technique. The obtained crystals possess a monoclinic layered structure with high crystalline quality. To explore the thermal stability of GaTe, the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed up to 1173 ​K in N2 and Ar atmospheres. GaTe crystals are thermally stable up to 700 ​K in Ar and stable up to 935 ​K in N2 due to the protective role of the physically adsorbed N2 molecules on GaTe surface. Annealing effect on the structural and optical properties of GaTe were examined after thermal treatment in N2 atmosphere at different temperatures; 573, 673, 773, and 873 ​K. Three prominent Raman modes at 97 ​cm−1 (Ag), 145 ​cm−1 (Ag), and 158 ​cm−1 (Bg) corresponding to the monoclinic GaTe phase almost remained, suggesting no structural damage at elevated temperatures. Especially, the photoluminescence (PL) intensity is improved as the annealing temperature increases and reaches a maximum at 673 ​K owing to the enhancement in the GaTe crystallinity. Above 873 ​K, we observed a significant reduction in the PL intensity, which is attributed to the vacancies induced by tellurium evaporation.

Elongated heterometal double-sites promote nitrogen reduction on two-dimensional MM′B7 monolayers

Long-distance heterometal M and M′ metals can further promote the activation and polarization of the intrinsic triple bond due to an eligible orbital matching and a stronger accepting–donating interaction to the side-on adsorbed N2 molecule.

Enhanced catalytic activity of MXene for nitrogen electoreduction reaction by carbon doping

The electrocatalytic performance of nitrogen reduction reaction (NRR) is seriously hindered by the lack of cost-effective electrocatalysts with high-efficiency and high-selectivity. In this work, the NRR catalytic activity of single carbon (C) atom embedded into two-dimensional (2D) transition metal carbides (M2CO2, M = Ti, Zr, Hf, Nb, Ta, Mo, and W) with oxygen vacancy was systematically evaluated by means of comprehensive density functional theory (DFT) computations. Our results revealed that the embedded single C atom possesses good durability due to its strong interaction with metal atoms around vacancy in these MXenes. Interestingly, through high-throughput screening, the single C atoms anchored on Nb2CO2, Mo2CO2, and W2CO2 nanosheets are identified as promise NRR catalysts with high-activity due to their low limiting potentials (–0.14 to –0.38 V) via a distal mechanism and outstanding selectivity again the competing hydrogen evolution reaction. Remarkably, the intrinsic activity of the C single atom supported by these MXenes mainly originates from the activation degree of the adsorbed N2 molecule, which is greatly dependent on the electron filling degree of pz orbital in C atom. Thus, by carefully choosing suitable substrates, the single C catalyst can be utilized as ideal NRR catalysts for NH3 synthesis.

Efficient photocatalytic nitrogen fixation to ammonia over bismuth monoxide quantum dots-modified defective ultrathin graphitic carbon nitride

The photocatalytic nitrogen reduction offers a promising strategy for ammonia (NH3) production, with the kinetics of the reaction depending on the transfer of photoinduced electrons to the adsorbed N2 molecules. However, the energetic electrons may flow into the defect states of the photocatalyst, thereby impeding nitrogen fixation. Herein, were report a novel heterojunction photocatalyst system for photocatalytic NH3 production, comprising BiO quantum dots (QDs) and nitrogen defective ultrathin carbon nitride (UCNx), which overcomes this limitation. Efficient spatial charge separation is achieved through nitrogen vacancies and the intimate heterojunction contact between the BiO QDs and UCNx. Benefitting from efficient charge separation, the adsorbed N2 on nitrogen vacancies and Bi2+ sites can effectively receive energetic electrons and in turn boost NH3 production. The optimal hybrid catalyst (BiCNx-5) shows excellent photocatalytic performance, with NH3 production rate of 576.11 μmol g−1 h−1 under visible light illumination (λ > 420 nm). This study provides a platform for the future design of effective photocatalysts for NH3 production.

Fe, V-co-doped C2N for electrocatalytic N2-to-NH3 conversion

Designing providential catalyst is the key to drive the electrochemical nitrogen reduction reactions (NRR), which is referring to multiple intermediates and products. By means of density functional theory (DFT) calculations, we studied heteronuclear bi-atom electrocatalyst (HBEC) for NRR. Our results revealed that compared to homonuclear bi-atom electrocatalyst (Fe2@C2N, V2@C2N), Fe, V-co-doped C2N ([email protected]2N) had a smaller limiting potential of −0.17 V and could accelerate N2-to-NH3 conversion through the enzymatic pathway of NRR. Importantly, NN bond length monotonically increases with increasing the Bader charges of adsorbed N2 molecule but decreases with increasing the Bader charge difference of two adsorbed N atoms. Additionally, the [email protected]2N could suppress the production of H2 by the preferential adsorption and reduction of N2 molecule. Thus, the as-designed HBEC may have the outstanding electrochemical NRR performance. This work opens a new perspective for NRR by HBECs under mild conditions.

Chemisorption-Induced and Plasmon-Promoted Photofixation of Nitrogen on Gold-Loaded Carbon Nitride Nanosheets.

Photocatalytic fixation of nitrogen is a promising method for green conversion of solar light, but has been substantially limited by inefficient activation of the nonpolar N≡N bond and the poor utilization of visible light. In this study, carbon nitride nanosheet composites with abundant nitrogen vacancies and strong plasmonic resonance absorption of visible light have been fabricated through the combination of hydrogen treatment and loading of Au nanoparticles. Ammonia yields of 184 μmol g-1 and 93 μmol g-1 are obtained without any sacrificial agent under full-light and visible-light irradiation, respectively. In particular, the visible-light activity is enhanced tenfold with the help of Au. Combining the experimental results and theoretical calculations, both the hydrogen treatment and Au loading help form nitrogen vacancies on the carbon nitride nanosheets, which promote N2 activation by enhancing the chemisorption. Furthermore, the Au loading further improves the nitrogen reduction efficiency through charging the excited hot electrons formed from the surface plasmonic resonance to the adsorbed N2 molecules.