N2 Temperature-programmed Desorption Research Articles

Constructing Mo5+ sites in molybdenum oxide by lattice stress for efficient ammonia synthesis

Molybdenum oxide (MoO3) is a promising catalyst for electrocatalytic nitrogen reduction (eNRR). However, Mo sites with saturated coordination are not favorable to stable N2 adsorption. Herein, we employed sodium ions as electron donors to interact with the interlayer atoms of MoO3, causing localized lattice stress on it and weakening the M-O bond, thus inducing the Mo5+ active sites (NaxMoO3). The N2 temperature-programmed desorption (N2-TPD) result of the as-prepared NaxMoO3 exhibits that introducing Mo5+ could significantly improve the N2 adsorption on the catalyst’s surface. The optimized NaxMoO3 has an NH3 yield of 41.3 μg h−1 mg−1 at ambient temperature and an FE of 21.4 %, which is in the relatively advanced position in the contemporaneous studies. Both experiments and density-functional theory (DFT) calculation demonstrated that the Mo5+ sites can effectively reduce the d-orbital energy level of Mo, which significantly enhances the interaction with N2.

Regulating Electronic Structure in Bi2 O3 Architectures by Ti Mediation: A Strategy for Dual Active Sites Synergistically Promoting Photocatalytic Nitrogen Hydrogenation.

Under mild conditions, nitrogen undergoes the associative pathways to be reduced with solar energy as the driving force for fixation, avoiding the high energy consumption when undergoing dissociation. Nevertheless, this process is hindered by the high hydrogenation energy barrier. Herein, Ti was introduced as hard acid into the δ-Bi2 O3 (Ti-Bi2 O3 ) lattice to tune its local electronic structure and optimize its photo-electrochemistry performance (reduced bandgap, increased conduction band maximum, and extended carrier lifetime). Heterokaryotic Ti-Bi dual-active sites in Ti-Bi2 O3 created a novel adsorption geometry of O-N2 interaction proved by density functional theory calculation and N2 temperature-programmed desorption. The synergistic effect of dual-active sites reduced the energy barrier of hydrogenation from 2.65 (Bi2 O3 ) to 2.13 eV (Ti-Bi2 O3 ), thanks to the highly overlapping orbitals with N2 . Results showed that 10 % Ti-doped Bi2 O3 exhibited an excellent ammonia production rate of 508.6 μmol gcat -1 h-1 in water and without sacrificial agent, which is 4.4 times higher than that of Bi2 O3 . In this work, bridging oxygen activation and synergistic hydrogenation for nitrogen with Ti-Bi dual active sites may unveil a corner of the hidden nitrogen reduction reaction mechanism and serves as a distinctive strategy for the design of nitrogen fixation photocatalysts.

Atomically Dispersed Manganese Lewis Acid Sites Catalyze Electrohydrogenation of Nitrogen to Ammonia

Atomically Dispersed Manganese Lewis Acid Sites Catalyze Electrohydrogenation of Nitrogen to Ammonia

Double-Phase Heterostructure within Fe-Doped Cu2–xS Quantum Dots with Boosted Electrocatalytic Nitrogen Reduction

Electrochemical nitrogen reduction reaction (NRR) is considered as one of the most promising methods for NH₃ synthesis under room temperature and ambient pressure. A grand challenge of NRR is the development of efficient electrocatalysts, for which the delicate nanostructuring of catalysts plays an important role. Herein, a series of Fe-doped Cu₂–ₓS quantum dots (QDs) are synthesized with multiple active sites and interface engineering, in which the double-phase heterostructure plays a key role for boosting NRR activity. The yield of NH₃ was obviously improved with the increase of Fe content from 0 to 3% but started to decrease with Fe from 3 to 9%. The optimized Fe₃%–Cu₂–ₓS QDs show an outstanding NH₃ yield of 26.4 μg h–¹ mg–¹cₐₜ at −0.7 V (vs the reversible hydrogen electrode), which is 5 times higher than that of Cu₂–ₓS QDs. More importantly, we observed that the highest NRR activity in Fe₃%–Cu₂–ₓS QDs was ascribed to the formation of an inherent double-phase heterostructure of Cu₂–ₓS/Cu₅FeS₄, whereas the complete conversion to single-phase Cu₅FeS₄ with increased Fe doping (9%) resulted in the activity decrease. Further, N₂ temperature-programmed desorption and electrochemical impedance spectra characterizations confirm the stronger chemical adsorption of N₂ and faster charge transfer in the Cu₂–ₓS/Cu₅FeS₄ QDs. A plausible mechanism was proposed for the double-phase Cu₂–ₓS/Cu₅FeS₄ heterostructure, where the interface provides efficient charge transfer and more active sites of Cu, Fe, and S for the synergetic adsorption and activation of N₂. Our work provides a simple strategy for the design of NRR electrocatalysts, which may also bring new inspiration for the preparation of the inherent double-phase heterostructure within other doped QDs.

Molecular chemisorption of N2 on IrO2(110).

We investigated adsorption of N2 on stoichiometric and O-rich IrO2(110) surfaces using temperature programmed desorption (TPD) experiments and density functional theory (DFT) calculations. TPD shows that N2 desorbs predominantly from the stoichiometric-IrO2(110) surface in a well-defined peak at 270 K for N2 coverages below about 0.5 ML and that a shoulder centered near 235 K develops in the N2 TPD traces as the coverage approaches saturation, indicating that adsorbed N2 molecules destabilize at high N2 coverages. Experiments of N2 adsorption onto O-rich IrO2(110) surfaces provide evidence that N2 adsorbs exclusively on the coordinatively unsaturated Ir atoms (Ircus) of the surface and that pre-adsorbed O-atoms ("on-top" oxygen) stabilize adsorbed N2 molecules, causing the main N2 TPD peak to shift toward higher temperature with increasing oxygen coverages. Consistent with prior results, our DFT calculations predict that an N2 molecule preferentially adsorbs into an upright configuration on an Ircus atom of the IrO2(110) surface and achieves a binding energy of about 100 kJ/mol. The computed binding energy agrees well with our experimental estimate of ∼90 kJ/mol for low N2 coverages on stoichiometric IrO2(110). The DFT calculations also quantitatively reproduce the observed stabilization of N2 by co-adsorption on-top O-atoms and predict the destabilization of N2 on IrO2(110) as the N2 adlayer becomes crowded at high coverages.

Zeolite-seed-directed Ru nanoparticles highly resistant against sintering for efficient nitrogen activation to ammonia

To stabilize Ru nanoparticles against sintering is an urgent problem in the utilization of Ru-based catalysts for NH3 synthesis. In the present study, we used Ru-containing ZSM-5 as seeds to crystallize ZSM-5, and the resulted Ru@ZSM-5 catalyst is highly resistant against Ru sintering. According to the results of diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and transmission electron microscopy (TEM) analyses, the average size of Ru nanoparticles is around 3.6 nm, which is smaller than that of Ru/ZSM-5-IWI prepared by incipient wetness impregnation. In NH3 synthesis (N2:H2 = 1:3) at 400 °C and 1 MPa, Ru@ZSM-5 displays a formation rate of 5.84 mmolNH3 gcat−1 h−1, which is much higher than that of Ru/ZSM-5-IWI (2.13 mmolNH3 gcat−1 h−1). According to the results of TEM, N2-temperature-programmed desorption (N2-TPD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) studies, it is deduced that the superior performance of Ru@ZSM-5 is attributable to the small particle size and the ample existence of metallic Ru0 sites. This method of zeolite encapsulation is a feasible way to stabilize Ru nanoparticles for NH3 synthesis.

A General Strategy to Glassy M-Te (M = Ru, Rh, Ir) Porous Nanorods for Efficient Electrochemical N2 Fixation.

Electrochemical conversion of nitrogen (N2 ) into value-added ammonia (NH3 ) is highly desirable yet formidably challenging due to the extreme inertness of the N2 molecule, which makes the development of a robust electrocatalyst prerequisite. Herein, a new class of bullet-like M-Te (M = Ru, Rh, Ir) glassy porous nanorods (PNRs) is reported as excellent electrocatalysts for N2 reduction reaction (NRR). The optimized IrTe4 PNRs present superior activity with the highest NH3 yield rate (51.1 µg h-1 mg-1 cat. ) and Faraday efficiency (15.3%), as well as long-term stability of up to 20 consecutive cycles, making them among the most active NRR electrocatalysts reported to date. Both the N2 temperature-programmed desorption and valence band X-ray photoelectron spectroscopy data show that the strong chemical adsorption of N2 is the key for enhancing the NRR and suppressing the hydrogen evolution reaction of IrTe4 PNRs. Density functional theory calculations comprehensively identify that the superior adsorption strength of IrTe4 adsorptions originates from the synergistic collaboration between electron-rich Ir and the highly electroactive surrounding Te atoms. The optimal adsorption of both N2 and H2 O in alkaline media guarantees the superior consecutive NRR process. This work opens a new avenue for designing high-performance NRR electrocatalysts based on glassy materials.

Template-free method for synthesizing sponge-like graphitic carbon nitride with a large surface area and outstanding nitrogen photofixation ability induced by nitrogen vacancies

Nitrogen fixation is the second most important chemical process in nature next to photosynthesis. Herein, we report a convenient template-free method for synthesizing sponge-like graphitic carbon nitride (g-C3N4) with a large surface area and outstanding nitrogen photofixation ability under visible light. X-ray diffraction (XRD), N2 adsorption, UV–vis spectroscopy, N2-Temperature programmed desorption (N2-TPD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and photocurrent measurements were used to characterize the prepared catalysts. The results indicate that solvent species (alcohols) play the most important role in the formation of the porous structure. Methanol-treated g-C3N4 exhibits the highest surface area and nitrogen photofixation ability, which are 15.2-fold and 27.2-fold greater, respectively, than that of untreated g-C3N4. Nitrogen vacancies could activate N2 and promote interfacial electron transfer, thereby significantly improving the nitrogen photofixation ability.

Ammonia decomposition by ruthenium nanoparticles loaded on inorganic electride C12A7:e−

The use of ammonia as a hydrogen carrier has received much attention due to its high hydrogen content and liquid state under mild conditions, which could lead to fuel cell applications. This study demonstrates facile ammonia decomposition on ruthenium nanoparticles loaded on inorganic electride, C12A7:e−. A high turnover frequency (∼12 s−1 at 400 °C) and low activation energy (64 kJ mol−1) for recombinative N2 desorption were obtained for Ru/C12A7:e−. N2-temperature programmed desorption (N2-TPD) and kinetic analyses indicate that the high catalytic performance is due to the low work function of chemically stable C12A7:e−, which enables electron injection to the antibonding orbital of the Ru–N bond formed transiently through the reaction by raising the Fermi level of Ru metal.

Reactive Growth of Nanoscale MgO Films by Mg Atom Deposition onto O2Multilayers

Nanometer thick layers of Mg metal vapor deposited onto a polycrystalline gold substrate at 22 K with and without O2 multilayers were examined after annealing. Auger electron spectroscopy and temperature-programmed desorption of N2 were used to determine the Mg oxidation state and the surface area of the deposits immediately after deposition at 22 K, and following annealing. Deposited on 20 layers of O2, Mg oxidizes and forms an MgO film having a high surface area (∼700 m2/g). The surface area decreases upon annealing, approaching that of a well-ordered MgO(100) surface by 500 K. On thinner O2 multilayers, both oxidized and metallic Mg are observed at 22 K. Mild thermal annealing (200−400 K) results in transport of the excess metallic Mg to, and alloy formation with, the underlying Au substrate. MgO films annealed to 1000 K display a narrow distribution of N2 binding sites, similar to well-ordered MgO(100). The film sublimes above 1000 K when heated.

The temperature-programmed desorption of N2 from a Ru/MgO catalyst used for ammonia synthesis

The temperature-programmed desorption (TPD) of N2 from a Ru/MgO catalyst used for ammonia synthesis was studied in a microreactor flow system operating at atmospheric pressure. Saturation with chemisorbed atomic nitrogen (N-*) was achieved by exposure to N2 at 573 K for 14 h and subsequent cooling in N2 to room temperature. With a heating rate of 5 K/min in He, a narrow and fairly symmetric N2 TPD peak at about 640 K results. From experiments with varying heating rates a preexponential factor Ades = 1.5×1010 molecules/(site s) and an activation energy Edes = 158 kJ/mol was derived assuming secondorder desorption. This rate constant of desorption is in good agreement with results obtained with a Ru(0001) single crystal surface in ultra-high vacuum (UHV). The rate of dissociative chemisorption was determined by varying the N2 exposure conditions. Determination of the coverage of N-* was based on the integration of the subsequently recorded N2 TPD traces yielding Aads = 2×10−6 (Pa s)−1 and Eads = 27 kJ/mol. The corresponding sticking coefficient of about 10−14 at 300 K is in agreement with the inertness of Ru(0001) in UHV towards dissociative chemisorption of N2. However, if the whole catalytic surface were in this state, then the resulting rate of N2 dissociation would be several orders of magnitude lower than the observed rate of NH3 formation. Hence only a small fraction of the total Rumetal surface area of Ru/MgO seems to be highly active dominating the rate of ammonia formation.

The dissociative adsorption of N2 on a multiply promoted iron catalyst used for ammonia synthesis: a temperature-programmed desorption study

The temperature-programmed desorption (TPD) of N2 from a multiply promoted iron catalyst used for ammonia synthesis has been studied in a microreactor system at atmospheric pressure. From TPD experiments with various heating rates a preexponential factorA = 2 × 109 molecules/site s and an activation energyE = 146 kJ/mol was derived assuming second-order desorption. The observed dependence of the TPD peak shapes on the heating rates indicated the influence of readsorption of N2 in agreement with the results obtained for various initial coverages. Simulating the N2 TPD curves using the model by Stoltze and Norskov revealed that the calculated TPD curves were not influenced by the molecular precursor to desorption. However, the calculated rate of readsorption was found to be overestimated at high coverage compared with the experimental results. A coverage-dependent net activation energy for dissociative chemisorption (E*) was introduced as the simplest assumption rendering the dissociative chemisorption of N2 activated at high coverage. The best fit of the experimental data yieldedE* = (−15+30θ) kJ/mol using only a single type of atomic nitrogen species. These findings are in satisfactory agreement with the parameters underlying the Stoltze-Norskov model for the kinetics of ammonia synthesis as well as with the data reported for Fe(111) single crystal surfaces.