Showing papers on "Chemisorption published in 2018"

Refining Defect States in W18O49 by Mo Doping: A Strategy for Tuning N2 Activation towards Solar-Driven Nitrogen Fixation.

Abstract: Photocatalysis may provide an intriguing approach to nitrogen fixation, which relies on the transfer of photoexcited electrons to the ultrastable N≡N bond. Upon N2 chemisorption at active sites (e.g., surface defects), the N2 molecules have yet to receive energetic electrons toward efficient activation and dissociation, often forming a bottleneck. Herein, we report that the bottleneck can be well tackled by refining the defect states in photocatalysts via doping. As a proof of concept, W18O49 ultrathin nanowires are employed as a model material for subtle Mo doping, in which the coordinatively unsaturated (CUS) metal atoms with oxygen defects serve as the sites for N2 chemisorption and electron transfer. The doped low-valence Mo species play multiple roles in facilitating N2 activation and dissociation by refining the defect states of W18O49: (1) polarizing the chemisorbed N2 molecules and facilitating the electron transfer from CUS sites to N2 adsorbates, which enables the N≡N bond to be more feasible fo...

Unravelling the electrochemical mechanisms for nitrogen fixation on single transition metal atoms embedded in defective graphitic carbon nitride

Abstract: Electrochemical reduction of nitrogen (N2), in which the conversion of N2 to ammonia (NH3) takes place under mild conditions, is of timely significance for paving a way toward technological applications in agriculture and the chemical industry. In this work, various single transition metal atoms anchored on graphitic carbon nitride (g-C3N4) with nitrogen vacancies (TM@NVs-g-C3N4), acting as electrocatalysts for N2 reduction, were systematically investigated by means of density functional theory (DFT) calculations. Most of the isolated metal atoms (Ti, V, Co, Ni, Zr, Mo, Ru and Pt) can be fixed by the nitrogen vacancies stably after performing the molecular dynamics simulation. For hexagonal close-packed and body centered cubic metal atoms, their N2 chemisorption activity decreases as the coordination number of the single atom rises. Nevertheless, the anchored cubic close-packed metal atom does not serve as a good site for N2 adsorption and activation even with a low-coordination number. Among all studied TM single atoms, the single Ti atom is found to be the most promising catalyst for its excellent N2 reduction performance with a potential-limiting step of 0.51 eV and a rate-determining barrier of 0.57 eV. Atomic level insights are provided to elucidate the electrochemical mechanisms for N2 reduction. The coordination number of the active center is accountable for the robust N2 reduction activity with high stability. Overall, this work exemplifies the in-depth investigations of different single TM atoms, including the coordination number and binding mode, which are essential to lay the groundwork for the advancement of single atom catalysis toward practical implementation.

Synthesis of polyacrylamide immobilized molybdenum disulfide (MoS2@PDA@PAM) composites via mussel-inspired chemistry and surface-initiated atom transfer radical polymerization for removal of copper (II) ions

Abstract: In present work, novel polyacrylamide immobilized molybdenum disulfide (MoS2@PDA@PAM) composites were synthesized via the mussel-inspired chemistry and surface initiated atom transfer radical polymerization (SI-ATRP). The as-prepared MoS2@PDA@PAM composites were characterized by energy dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Characterization results provide sufficient evidences for the successful functionalization of MoS2 with PAM. The products were used as adsorbents for removing of copper (II) ions. Results show the introduction of PAM onto MoS2 could enhance the adsorption capacity of MoS2@PDA@PAM towards copper (II) ions. The amount of adsorbed copper (II) ions by MoS2@PDA@PAM composites is 2.5 times that of pristine MoS2. The effects of various experimental factors on the adsorption process, including contact time, initial copper (II) ion concentrations, solution pH and temperature, were also studied in this work. The batch experiments show that the adsorption of copper (II) ions onto MoS2@PDA@PAM is dependent on time, pH and temperature. The optimum solution pH is observed at pH 7 and the increase of temperature is favorable for the adsorption of MoS2@PDA@PAM towards copper (II). Based on the experiment data, the adsorption kinetics, isotherms and thermodynamics were also investigated. The kinetics and isotherm studies show that pseudo-second-order kinetic and Freundlich isotherm models could well fit with the adsorption data. The thermodynamic results show that the adsorption of copper (II) ions on MoS2@PDA@PAM is a spontaneous and endothermic process. The adsorption process is mainly governed by the chemisorption involving the electrostatic interaction and/or chemical chelation between copper (II) ions and amino groups on the surface of MoS2@PDA@PAM. Taken together, it is proven that the PAM can be immobilized onto the MoS2 nanosheets via the mussel-inspired chemistry and SI-ATRP, and it can enhance the adsorption performance of MoS2@PDA@PAM composites, which might be used as adsorbents to remove heavy metal ions in real environment treatment.

Facilitated Oxygen Chemisorption in Heteroatom-Doped Carbon for Improved Oxygen Reaction Activity in All-Solid-State Zinc–Air Batteries

Abstract: Driven by the intensified demand for energy storage systems with high-power density and safety, all-solid-state zinc-air batteries have drawn extensive attention. However, the electrocatalyst active sites and the underlying mechanisms occurring in zinc-air batteries remain confusing due to the lack of in situ analytical techniques. In this work, the in situ observations, including X-ray diffraction and Raman spectroscopy, of a heteroatom-doped carbon air cathode are reported, in which the chemisorption of oxygen molecules and oxygen-containing intermediates on the carbon material can be facilitated by the electron deficiency caused by heteroatom doping, thus improving the oxygen reaction activity for zinc-air batteries. As expected, solid-state zinc-air batteries equipped with such air cathodes exhibit superior reversibility and durability. This work thus provides a profound understanding of the reaction principles of heteroatom-doped carbon materials in zinc-air batteries.

A DFT Study on the Adsorption of H₂S and SO₂ on Ni Doped MoS₂ Monolayer

Abstract: In this paper, a Ni-doped MoS₂ monolayer (Ni-MoS₂) has been proposed as a novel gas adsorbent to be used in SF₆-insulated equipment. Based on the first-principles calculation, the adsorption properties of Ni-MoS₂ to SO₂ and H₂S molecules, the main decomposition components of SF₆ under a partial discharge (PD) condition have been studied. The adsorption energy, charge transfer, and structural parameters have been analyzed to find the most stable gas-adsorbed Ni-MoS₂. Furthermore, the density of states (DOS), projected density of states (PDOS), and electron density difference were employed to explore the interaction mechanism between SO₂, H₂S, and the Ni-MoS₂ surface. It is found that the H₂S molecule and SO₂ molecule interact with the Ni-MoS₂ surface by strong adsorption energy. Therefore, we conclude that the interaction between these two kinds of gases and the Ni-MoS₂ monolayer belongs to chemisorption, and the Ni-MoS₂ monolayer might be a promising gas adsorbent for the fault recovery of SF₆-insulated equipment. Additionally, we have to point out that all of the conclusions only considered the final adsorption energy, the barrier in the transition state has not been analyzed in this paper.

Noble metal (Pt or Au)-doped monolayer MoS 2 as a promising adsorbent and gas-sensing material to SO 2 , SOF 2 and SO 2 F 2 : a DFT study

Abstract: We explored the adsorption of SO2, SOF2, and SO2F2 on Pt- or Au-doped MoS2 monolayer based on density functional theory. The adsorption energy, adsorption distance, charge transfer as well as density of states were discussed. SO2 and SOF2 exhibit strong chemical interactions with Pt-doped MoS2 based on large adsorption energy, charge transfer, and changes of electron orbitals in gas molecule. SO2 also shows obvious chemisorption on Au-doped MoS2 with apparent magnetism transfer from Au to gas molecules. The adsorption of SO2F2 on Pt–MoS2 and SOF2 on Au–MoS2 exhibits weaker chemical interactions and SO2F2 losses electrons when adsorbed on Pt-MoS2 which is different from other gas adsorption. The adsorption of SO2F2 on Au–MoS2 represents no obvious chemical interaction but physisorption. The gas-sensing properties are also evaluated based on DFT results. This work could provide prospects and application value for typical noble metal-doped MoS2 as gas-sensing materials.

Effects of dielectric barrier discharge plasma on the catalytic activity of Pt/CeO2 catalysts

Abstract: CeO2 nanorod was synthesized by a hydrothermal method and impregnated with Pt to synthesize Pt/CeO2 catalysts, which were modified by dielectric barrier discharge plasma. The modified CeO2 and Pt/CeO2 were characterized by XRD, STEM, N2 adsorption/desorption, CO pulse chemisorption, XPS, H2-TPR, O2-TPD and UV-Raman spectroscopy techniques. After the plasma treatment, the T90 value of (Pt/CeO2)-P catalyst decreased from 287 °C to 208 ℃ for toluene oxidation. This significantly changed activity of catalyst indicated that plasma has greatly impacted its performance. More and larger notches on surface and broken fragments were found from STEM analysis. In addition, smaller Pt particle size and higher dispersion of nanoparticles was found on (Pt/CeO2)-P, which was characterized by CO pulse chemisorption and TEM analysis. In addition, Pt/(CeO2-P) and (Pt/CeO2)-P possessed higher concentration of oxygen vacancies and Ce3+, which was observed by UV-Raman spectroscopy and XPS. Moreover, according to TPR results, the interaction between Pt and CeO2 was obviously strengthened, which led to a lower reduction temperature after plasma treatment. After plasma treatment, the (Pt/CeO2)-P presented the highest activity due to it possessing the highest TOFPt and TOFov values of 9.88 × 10−4 s−1 and 9.49 × 10−5 s−1, respectively, and lower activation energies of 63.8 kJ mol−1. Furthermore, the toluene conversion of (Pt/CeO2)-P without significantly decreasing for working at least 50 h and under 9.6 vol% water vapor.

H 2 Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H 2 Activation at the Metal-Support Interface.

Abstract: Water adsorbed at the metal–support interface (MSI) plays an important role in multiple reactions Due to its importance in CO preferential oxidation (PrOx), we examined H2 oxidation kinetics in the presence of water over Au/TiO2 and Au/Al2O3 catalysts, reaching the following mechanistic conclusions: (i) O2 activation follows a similar mechanism to that proposed in CO oxidation catalysis; (ii) weakly adsorbed H2O is a strong reaction inhibitor; (iii) fast H2 activation occurs at the MSI, and (iv) H2 activation kinetics are inconsistent with traditional dissociative H2 chemisorption on metals Density functional theory (DFT) calculations using a supported Au nanorod model suggest H2 activation proceeds through a heterolytic dissociation mechanism, resulting in a formal hydride residing on the Au and a proton bound to a surface TiOH group This potential mechanism was supported by infrared spectroscopy experiments during H2 adsorption on a deuterated Au/TiO2 surface, which showed rapid H–D scrambling with s

Adsorption and possible dissociation of glucose by the [BN fullerene-B 6 ] − magnetic nanocomposite. In silico studies

Abstract: The adsorption, activation and possible dissociation of the glucose molecule on the magnetic [BN fullerene-B6]− system is performed by means of density functional theory calculations. Three models of magnetic nanocomposites were inspected: i) pristine BN fullerene, BN fullerene functionalized with a magnetic B6 cluster which generates two structures: ii) pyramidal (P) and iii) triangular (T). Chemical interactions of glucose appear for all these cases; however, for the BNF:B6(T)—glucose system, the interaction generates an effect of dissociation on glucose, due to the magnetic effects, since it has high spin multiplicity. The latter nanocomposite shows electronic behavior like-conductor and like-semi-conductor for the P and T geometries, respectively. Intrinsic magnetism associated to values of 1.0 magneton bohr (µB) for the pyramidal and 5.0 µB for the triangular structure, high polarity, and low-chemical reactivity are found for these systems. These interesting properties make these functionalized fullerenes a good option for being used as nano-vehicles for drug delivery. These quantum descriptors remain invariant when the [BN]−fullerene and [BNF:B6 (P) or (T)]− nanocomposites are interacting with the glucose molecule. According to the determined adsorption energy, chemisorption regimes occur in both the phases: gas and aqueous medium.

Mechanism and Kinetics of Hydrogen Peroxide Decomposition on Platinum Nanocatalysts

Abstract: The decomposition of H2O2 to H2O and O2 catalyzed by platinum nanocatalysts controls the energy yield of several energy conversion technologies, such as hydrogen fuel cells. However, the reaction mechanism and rate-limiting step of this reaction have been unsolved for more than 100 years. We determined both the reaction mechanism and rate-limiting step by studying the effect of different reaction conditions, nanoparticle size, and surface composition on the rates of H2O2 decomposition by three platinum nanocatalysts with average particle sizes of 3, 11, and 22 nm. Rate models indicate that the reaction pathway of H2O2 decomposition is similar for all three nanocatalysts. Larger particle size correlates with lower activation energy and enhanced catalytic activity, explained by a smaller work function for larger platinum particles, which favors chemisorption of oxygen onto platinum to form Pt(O). Our experiments also showed that incorporation of oxygen at the nanocatalyst surface results in a faster reactio...

Hydrogen Oxidation on Ni-Based Electrocatalysts: The Effect of Metal Doping

Abstract: Carbon supported nanoparticles of monometallic Ni catalyst and binary Ni-Transition Metal (Ni-TM/C) electrocatalytic composites were synthesized via the chemical reduction method, where TM stands for the doping elements Fe, Co, and Cu. The chemical composition, structure and morphology of the Ni-TM/C materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS). The electrochemical properties towards hydrogen oxidation reaction in alkaline medium were studied using the rotating disc electrode and cycling voltammetry methods. A significant role of the TM dopants in the promotion of the hydrogen electrooxidation kinetics of the binary Ni-TM/C materials was revealed. A record-high in exchange current density value of 0.060 mA cm2Ni was measured for Ni3Fe1/C, whereas the monometallic Ni/C counterpart has only shown 0.039 mA cm2Ni. In order to predict the feasibility of the electrocatalysts for hydrogen chemisorption, density functional theory was applied to calculate the hydrogen binding energy and hydroxide binding energy values for bare Ni and Ni3TM1.

CO2 methanation over Fe- and Mn-promoted co-precipitated Ni-Al catalysts: Synthesis, characterization and catalysis study

Abstract: The methanation reaction of CO2 is in discussion to be a sustainable pathway to address future questions arising from limited primary energy feedstock and the accumulation of CO2 in the atmosphere. Therefore, the development of highly active and thermostable catalysts for this reaction is an indispensable matter of research. For this reason, an equimolar NiAlOx benchmark catalyst (44 wt.% Ni loading) was synthesized and modified by doping with Fe or Mn up to 10 wt.% of promoter by co-precipitation at constant pH 9. Their activity and stability performances in the CO2 methanation reaction were evaluated by comparing the conversion versus temperature characteristics before and after an aging period of 32 h at 500 °C. Material characterization studies comprising BET, XRD, in situ IR spectroscopy, XPS, H2 and CO2 chemisorption, and EPR/FMR contributed to derive structure-activity relationships and to obtain a deeper understanding of the catalytic behavior. Promotion with Mn led to a significant enhancement of the catalytic activity. This is assumed to be caused by a higher density of medium basic sites and an enhanced CO2 adsorption capacity on the activated catalyst related to interactions between Mn oxide species and the mixed oxide phase, in combination with a stabilization of the Ni surface area at moderate Mn loadings. Promotion with Fe increased the thermal stability of the catalyst, which is attributed to the formation of a Ni-Fe alloy during catalyst activation. For both phenomena, the optimum molar Ni to promoter ratio for co-precipitated catalysts was found to be around 5.

Alkali-metal-adsorbed g-GaN monolayer: ultralow work functions and optical properties.

Abstract: The electronic and optical properties of alkali-metal-adsorbed graphene-like gallium nitride (g-GaN) have been investigated using density functional theory. The results denote that alkali-metal-adsorbed g-GaN systems are stable compounds, with the most stable adsorption site being the center of the hexagonal ring. In addition, because of charge transfer from the alkali-metal atom to the host, the g-GaN layer shows clear n-type doping behavior. The adsorption of alkali metal atoms on g-GaN occurs via chemisorption. More importantly, the work function of g-GaN is substantially reduced following the adsorption of alkali-metal atoms. Specifically, the Cs-adsorbed g-GaN system shows an ultralow work function of 0.84 eV, which has great potential application in field-emission devices. In addition, the alkali-metal adsorption can lead to an increase in the static dielectric constant and extend the absorption spectrum of g-GaN.