Direct Dissociation Research Articles

Modeling the Atomic Layer Deposition of Metal Thin Films: Studying the Surface Reaction Mechanism By Density Functional Theory Simulations

Atomic layer deposition (ALD) is widely used in microelectronics and semiconductor industry to deposit thin films as part of device fabrication in nano- or subnano-dimensions. The key advantages of ALD are the conformality and precise thickness control at the atomic scale, which are difficult for physical or traditional chemical vapor deposition methods. The atomic scale understanding of ALD is vital and essential to design and optimize the deposition process, where density functional theory (DFT) calculations play an important role in providing detailed reaction mechanism, theoretical screening of suitable precursors and estimated growth-per-cycle (GPC).In this study, we show our recent works on DFT calculations of the growth mechanism study of Cobalt (Co) thin film by plasma-enhanced ALD1,2. We first addressed the surface reaction mechanism at the metal precursor pulse and plasma half-cycle on NHx-terminated Co surfaces, which corresponds to the steady growth for the PE-ALD. The adsorption and reactions of metal precursors (CoCp2) on NHx terminated metal surfaces were investigated with the inclusion of van der Waals corrections. The plausible reaction pathways include: precursor adsorption, hydrogen transfer, CpH formation and CpH desorption. The direct Cp dissociation mechanism is not considered on these NHx-terminated metal surfaces due to experimentally observed minimal C impurities at the deposited metal thin films, which indicates that most of the Cp ligand is removed completely. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method.The reactions at the initial stages on typical H:Si(100) surface are investigated to gain atomic insight on the effect of different substrates on the elimination of Cp ligands. Our work is important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.In addition, we present our recent works3, 4 on the thermal ALD of Co using reducing agent Zn(DMP)2 and thermal ALD of Fe4Zn9 thin film with diethyl zinc (DEZ). Our DFT results show that by selecting suitable reducing agent, we can control the deposited thin films to be Zn-free high quality Co thin film or desired intermetallic thin film with high Zn content.These DFT results are important to understand the reaction mechanism for ALD and provides new knowledge in depositing the thin films by adjusting the reducing agents.

Coal gasification slag-derived highly reactive silica for high modulus sodium silicate synthesis: Process and mechanism

Coal gasification slag (CGS) was used to synthesize highly reactive silica, which can be rapidly dissolved in dilute alkali solution at ambient temperature. High modulus sodium silicate was further synthesized by alkali dissolution at ambient temperature instead of the conventional high temperature melting method. During the acid activation process, the inert Si-O-M (M = Al, Ca, Fe) bonds are broken, resulting in the formation of Q3 and Q2 coordination structures with active Si-O-H bonds. The active Si-O-H bonds are further spontaneously condensed to generate highly reactive amorphous silica with a low Si-O-Si bond strength. Under the optimum reaction temperature of 25℃ and reaction time of 0.5 h, the leaching efficiency of Al, Ca and Fe are 86.20 %, 97.61 % and 97.83 % respectively, and the solubility of silica can reach 97.85 %. During the alkali dissolution of silica process, with the decrease of NaOH concentration and liquid–solid ratio, the breaking of Si-O-Si bridging oxygen bond and the formation of Si-O-Na non-bridging oxygen bond can be decreased, resulting in the direct dissociation of the Si-O groups into the liquid phase in the form of polysilicate, thus synthesizing high modulus sodium silicate. Under the optimum reaction temperature of 25℃ and reaction time of 0.5 h, the leaching efficiency of silica is 76.35 % and the modulus of sodium silicate can reach 3.28. The findings of this study provide valuable insights for enriching the theoretical basis of alkali solution-based silica dissolution and for improving the energy-intensive status of the silica industry.

In Situ Quantitative Study of Single-Molecule Photoreduction Activities and Kinetics on 1D-1D Heterostructure.

Understanding the underlying catalytic mechanisms with nanometer resolution is of critical importance to the rational design of 1D heterogeneous catalysts. However, a fundamental investigation of photocatalytic activities and kinetics at their individual sites is still challenging. Herein, in situ single-molecule fluorescence microscopy is employed to study the site-specific catalytic activities and dynamics on 1D-1D heterostructure for the first time. For carbon nanotube (CNT)/CdS nanorod composites, it is found that the CdS end with heterojunction exhibits the highest catalytic conversion rate constant of resazurin photoreduction, which is 30%, 7%, and 19% higher than those of the middle segment and the bare end of CdS, and the CNT end with heterojunction, respectively. A similar trend of adsorption abilities is observed in these structures. Such phenomena can be attributed to the different content of defects in these structures. Regarding the dissociation behaviors, the dissociation rate constants of all structures exhibit an opposite trend to those of adsorption and conversion. The direct and indirect dissociation are found to be predominant on CdS and CNT, respectively. Such investigation provides a deep insight into the understanding of site-specific properties on 1D heterogeneous catalysts and helps construct the "structure-dynamics" correlations at the nanoscale.

A direct complete dissociation mechanism for nitrate reaction to ammonia

Nitrate electroreduction (NO3ER) is essential to improve water quality and nitrogen cycle in nature. However, developing efficient catalysts remains a challenge due to the unclear reaction mechanism. Herein, taking transition metal dual-atom supported on hexa-nitrogen coordinated graphene as an example to explore its potential mechanism as the NO3ER electrocatalysts by using density functional theory (DFT) computations. The results revealed the NO3ER prefers to take place via a complete-dissociation pathway, in which the adsorbed *NO3 species was dissociated into (*O + *NO2), which eventually generated the NH3 product by alternating hydrogenation and dissociation. Remarkably, based on a two-step screening strategy, CrFe on hexa-nitrogen coordinated graphene was extracted from various dual atom catalysts with an ultra-low limiting potential (−0.12 V). In addition, the selectivity of CrFe on hexa-nitrogen coordinated graphene can overcome competing reactions, and thus it can be identified as an ideal electrocatalyst for NO3ER. Our findings not only contribute to a deeper understanding of the NO3ER mechanism on dual-atom catalysts, but also provide theoretical guidance for the exploration of efficient NO3ER catalysts.

Single-Atom Ru Alloyed with Ni Nanoparticles Boosts CO2 Methanation.

Designing catalysts to proceed with catalytic reactions along the desired reaction pathways, e.g., CO2 methanation, has received much attention but remains a huge challenge. This work reports one Ru1Ni single-atom alloy (SAA) catalyst (Ru1Ni/SiO2) prepared via a galvanic replacement reaction between RuCl3 and Ni nanoparticles (NPs) derived from the reduction of Ni phyllosilicate (Ni-ph). Ru1Ni/SiO2 achieved much improved selectivity toward hydrogenation of CO2 to CH4 and catalytic activity (Turnover frequency (TOF) value: 40.00×10-3s-1), much higher than those of Ni/SiO2 (TOF value: 4.40×10-3s-1) and most reported Ni-based catalysts (TOF value: 1.03×10-3-11.00×10-3s-1). Experimental studies verify that Ru single atoms are anchored onto the Ni NPs surface via the Ru1-Ni coordination accompanied by electron transfer from Ru1 to Ni. Both in situ experiments and theoretical calculations confirm that the interface sites of Ru1Ni-SAA are the intrinsic active sites, which promote the direct dissociation of CO2 and lower the energy barrier for the hydrogenation of CO* intermediate, thereby directing and enhancing the CO2 hydrogenation to CH4.

Exploring the effects of potassium-doping on the reactive oxygen species of CeO2 (110) for formaldehyde catalytic oxidation: A DFT study

The influence of K-doping on the reactive oxygen species and elementary reactions of HCHO catalytic oxidation has not been well understood. Herein, a density functional theory (DFT) study was conducted and found that the introduction of K-doping advantageously changed the electronic structures of both Ce and O atoms on the (110) crystal plane of CeO2. This change, in turn, facilitated the adsorption and activation of HCHO and O2 molecules, enhanced the mobility of lattice oxygen, and reduced the energy barrier associated with HCHO oxidation. The adsorbed oxygen species were generated through the direct dissociation of O2 on the CeO2 (110) surface. However, in the case of K-CeO2 (110), it was more probable for the adsorbed oxygen species to be produced by gaseous O2 filling the oxygen vacancy, followed by dissociation. Additionally, the incorporation of K-doping proved to be advantageous in facilitating the formation of hydroxyl groups by promoting the dissociation of water molecules. The inclusion of hydroxyl groups facilitated the HCHO adsorption and reduced the energy barrier for its subsequent oxidation. The findings provide new insights into the development of novel catalysts for HCHO removal with high performance through alkali metal modification.

Roaming Dynamics in Hydroxymethyl Hydroperoxide Decomposition Revealed by the Full-Dimensional Potential Energy Surface of the CH2OO + H2O Reaction.

The CH2OO + H2O reaction is an important atmospheric process that leads to the formation of formic acid (HCOOH) and water via the intermediate hydroxymethyl hydroperoxide (HOCH2OOH, HMHP). We investigated the intricacies of this process by employing quasiclassical trajectory calculations on an accurate, full-dimensional ab initio potential energy surface (PES). In addition to the direct mechanism via the transition state (TS), an interesting roaming mechanism was found to play the predominant role in producing H2O and HCOOH. This roaming pathway is featured as the near direct dissociation of HMHP into OH and hydroxymethoxy radical, followed by the retraction of OH and abstraction of the H atom, culminating in the formation of H2O. Due to the longer interaction time of the roaming mechanism, less product translational energy was released, but more internal energies of HCOOH were obtained, as compared with the direct TS mechanism. The enhanced yield of H2O and formic acid achieved through roaming dynamics underscores the significance of dynamics simulations based on an accurate full-dimensional PES. This work provides new insights into the dynamics of the CH2OO + H2O reaction and its implications for atmospheric chemistry.

How Can the PtPd‐Based High‐Entropy Alloy Triumphs Conventional Twc Catalyst During the NO Reduction? A Density Functional Theory Study

AbstractDensity functional theory is used to compare the catalytic performance of PtPdRhFeCo(100) high entropy alloy (HEA) three‐way catalyst (TWC) to the conventional Pt(100) in the NO reduction step during NH3 production that supplies to passive NH3‐SCR. Stronger adsorption of NO on the HEA(100) surface is beneficial to capture NO. During adsorption, the catalyst surface acts as an electron donor while the adsorbate is the acceptor on both HEA(100) and Pt(100) systems. Herein, the reaction mechanism of NO reduction can be classified into two steps: 1) NO activation and 2) product formation. During NO activation, direct NO dissociation is the preferable pathway on both HEA(100) and Pt(100) surfaces with the same Ea, whereas HNO and NOH pathways on HEA(100) are suppressed. For NH3, N2, and N2O production on HEA(100) is found to be more difficult than on Pt(100). However, the thermodynamic driving force of all reactions on HEA(100) is more spontaneous than on Pt(100). Also, the rate‐determining step on HEA(100) is found to be NH3 formation different from the Pt(100), while difficult H diffusion on HEA(100) is the key factor that reduces NH3 production.

Ni12P5 Confined in Mesoporous SiO2 with Near-Unity CO Selectivity and Enhanced Catalytic Activity for CO2 Hydrogenation.

CO2 hydrogenation via the reverse water gas shift (RWGS) reaction is a promising strategy for CO2 utilization while constructing Ni-based catalysts with high catalytic activity and perfect CO selectivity remains a great challenging. Here, we demonstrate that the product selectivity for CO2 hydrogenation can be significantly tuned from CH4 to CO by phosphating of SiO2-supported Ni catalysts due to the geometric effect. Interestingly, nickel phosphide catalysts with different crystalline phases (Ni12P5 and Ni2P) differ sharply in CO2 conversion, and Ni12P5 is remarkably more active. Furthermore, we developed a facile strategy to confine small Ni12P5 nanoparticles in mesoporous SiO2 channels (Ni12P5@SBA-15). Enhanced activity is exhibited on Ni12P5@SBA-15, ascribed to the highly effective confinement effect. The in situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculations unveil that catalytic CO2 hydrogenation follows a direct CO2 dissociation route with adsorbed CO as the key intermediate. Notably, strong multibonded CO (threefold and bridge-bonded CO) is feasibly formed on the Ni catalyst accounting for CH4 as the dominant product whereas only weak linearly bonded CO exists on nickel phosphide catalysts resulting in almost 100% CO selectivity. The present results indicate that Ni12P5@SBA-15 combining the geometric effect and the confinement effect can achieve near-unity CO selectivity and enhanced activity for CO2 hydrogenation.

Mechanisms of CH4 activation over oxygen-preadsorbed transition metals by ReaxFF and AIMD simulations.

The chemisorbed oxygen usually promotes the CH bond activation over less active metals like IB group metals but has no effect or even an inhibition effect over more active metals like Pd based on the static electronic structure study. However, the understanding in terms of dynamics knowledge is far from complete. In the present work, methane dissociation on the oxygen-preadsorbed transition metals including Au, Cu, Ni, Pt, and Pd is systemically studied by reactive force field (ReaxFF). The ReaxFF simulation results indicate that CH4 molecules mainly undergo the direct dissociation on Ni, Pt, and Pd surfaces, while undergo the oxygen-assisted dissociation on Au and Cu surfaces. Additionally, the ab initio molecular dynamics (AIMD) simulations with the umbrella sampling are employed to study the free-energy changes of CH4 dissociation, and the results further support the CH4 dissociation pathway during the ReaxFF simulations. The present results based on ReaxFF and AIMD will provide a deeper dynamic understanding of the effects of pre-adsorbed oxygen species on the CH bond activation compared to that of static DFT.

A Review of Theoretical Studies on Carbon Monoxide Hydrogenation via Fischer-Tropsch Synthesis over Transition Metals.

The increasing demand for clean fuels and sustainable products has attracted much interest in the development of active and selective catalysts for CO conversion to desirable products. This review maps the theoretical progress of the different facets of most commercial catalysts, including Co, Fe, Ni, Rh, and Ru. All relevant elementary steps involving CO dissociation and hydrogenation and their dependence on surface structure, surface coverage, temperature, and pressure are considered. The dominant Fischer-Tropsch synthesis mechanism is also explored, including the sensitivity to the structure of H-assisted CO dissociation and direct CO dissociation. Low-coordinated step sites are shown to enhance catalytic activity and suppress methane formation. The hydrogen adsorption and CO dissociation mechanisms are highly dependent on the surface coverage, in which hydrogen adsorption increases, and the CO insertion mechanism becomes more favorable at high coverages. It is revealed that the chain-growth probability and product selectivity are affected by the type of catalyst and its structure as well as the applied temperature and pressure.

Structure Sensitivity of CO2 Hydrogenation on Ni Revisited.

Despite the large number of studies on the catalytic hydrogenation of CO2 to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO2 hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2-12 nm), we found that the active sites responsible for the conversion of CO2 to CO are different from those for the subsequent hydrogenation of CO to CH4. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO2 hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO2 hydrogenation and demonstrated that direct dissociation of CO2 to CO is followed by the conversion of strongly bonded carbonyls to CH4. These findings provide essential insights into the much debated structure sensitivity of CO2 hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts.

H2O Derivatives Mediate CO Activation in Fischer-Tropsch Synthesis: A Review.

The process of Fischer-Tropsch synthesis is commonly described as a series of reactions in which CO and H2 are dissociated and adsorbed on the metals and then rearranged to produce hydrocarbons and H2O. However, CO dissociation adsorption is regarded as the initial stage of Fischer-Tropsch synthesis and an essential factor in the control of catalytic activity. Several pathways have been proposed to activate CO, namely direct CO dissociation, activation hydrogenation, and activation by insertion into growing chains. In addition, H2O is considered an important by-product of Fischer-Tropsch synthesis reactions and has been shown to play a key role in regulating the distribution of Fischer-Tropsch synthesis products. The presence of H2O may influence the reaction rate, the product distribution, and the deactivation rate. Focus on H2O molecules and H2O-derivatives (H*, OH* and O*) can assist CO activation hydrogenation on Fe- and Co-based catalysts. In this work, the intermediates (C*, O*, HCO*, COH*, COH*, CH*, etc.) and reaction pathways were analyzed, and the H2O and H2O derivatives (H*, OH* and O*) on Fe- and Co-based catalysts and their role in the Fischer-Tropsch synthesis reaction process were reviewed.

Interfaces in reinforced epoxy resins: from molecular scale understanding towards mechanical properties

ContextWe report on atomic level of detail analyses of polymer composite models featuring epoxy resin interfaces to silica, iron oxide, and cellulose layers. Using “reactive” molecular dynamics simulations to explore epoxy network formation, resin hardening is investigated in an unprejudiced manner. This allows the detailed characterization of salt-bridges and hydrogen bonds at the interfaces. Moreover, our sandwich-type composite systems are subjected to tensile testing along the interface normal. To elucidate the role of relaxation processes, we contrast (i) direct dissociation of the epoxy-metal oxide/cellulose contact layer, (ii) constant strain-rate molecular dynamics studies featuring (visco-)elastic deformation and bond rupture of the epoxy resin, and (iii) extrapolated relaxation dynamics mimicking quasi-static conditions. While the fracture mechanism is clearly identified as interface dissociation of the composite constituents, we still find damaging of the nearby polymer phase. The observed plastic deformation and local cavitation are rationalized from the comparably large stress required for the dissociation of salt-bridges, hydrogen bonds, and van der Waals contacts. Indeed, the delamination of the contact layers of epoxy resins with slabs of silica, magnetite, and cellulose call for a maximum stress of 33, 26, and 21 MPa, respectively, as compared to 84 MPa required for bulk epoxy yielding.MethodsMolecular dynamics simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code were augmented by a Monte Carlo–type procedure to probe epoxy bond formation (Macromolecules 53(22): 9698–9705). The underlying interaction models are split into conventional Generalized Amber Force Fields (GAFF) for non-reacting moieties and a recently developed reactive molecular mechanics potential enabling epoxy bond formation and cleavage (ACS Polymers Au 1(3): 165–174).

Photodissociation Dynamics of the [O2-H2O]+ Ionic Complex.

We present an experimental study on the photodissociation dynamics of [O2-H2O]+ in the 580-266 nm wavelength range using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces mass selected and internally cold ions for photodissociation. By detecting both the O2+ and H2O+ photofragments using the time-of-flight mass spectrometry and velocity map imaging techniques, branching ratios and total kinetic energy release distributions of the O2+ + H2O and H2O+ + O2 product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O2-H2O]+ are interpreted as (1) the O2(X3Σg-) + H2O+, O2(a1Δg) + H2O+, and O2(X3Σg-) + H2O+ channels are produced from direct dissociation of [O2-H2O]+ in its excited , , and states, respectively; (2) the O2+(X2Πg) + channel is produced from nonadiabatic relaxations of the excited , , and states to the ground state with subsequent dissociation. The latter nonadiabatic processes involve charge-transfer on the potential energy surfaces, and the charge-transfer probabilities are determined from experimental results. The dissociation energy of the ground state to the lowest dissociation limit is experimentally refined as D0 = 1.05 ± 0.05 eV. This work provides important information to understand the charge-transfer dynamics in the photochemistry of [O2-H2O]+ and in the ion-molecule reaction O2 + H2O+ → O2+ + H2O.

SnRK1 inhibits anthocyanin biosynthesis through both transcriptional regulation and direct phosphorylation and dissociation of the MYB/bHLH/TTG1 MBW complex.

Plants have evolved an extensive specialized secondary metabolism. The colorful flavonoid anthocyanins, for example, not only stimulate flower pollination and seed dispersal but also protect different tissues against high light, UV- and oxidative stress. Their biosynthesis is highly regulated by environmental and developmental cues and induced by high sucrose levels. Expression of the biosynthetic enzymes involved is controlled by a transcriptional MBW complex, comprising (R2R3) MYB- and bHLH-type transcription factors (TF) and the WD40 repeat protein TTG1. Anthocyanin biosynthesis is obviously useful but also carbon- and energy-intensive and non-vital. Consistently, the SnRK1 protein kinase, a metabolic sensor activated in carbon- and energy-depleting stress conditions, represses anthocyanin biosynthesis. Here we show that Arabidopsis SnRK1 represses MBW complex activity both at the transcriptional and post-translational level. In addition to repressing expression of the key transcription factor MYB75/PAP1, SnRK1 activity triggers MBW complex dissociation, associated with loss of target promoter binding, MYB75 protein degradation and nuclear export of TTG1. We also provide evidence for direct interaction with and phosphorylation of multiple MBW complex proteins. These results indicate that repression of expensive anthocyanin biosynthesis is an important strategy to save energy and redirect carbon flow to more essential processes for survival in metabolic stress conditions.

Photodissociation dynamics of methylamine in the blue edge of the A-band. I. The H-atom elimination channel.

The photodissociation dynamics of methylamine (CH3NH2) upon excitation in the blue edge of the first absorption A-band, in the 198-203nm range, are investigated by means of nanosecond pump-probe laser pulses and velocity map imaging combined with H(2S)-atom detection through resonance enhanced multiphoton ionization. The images and corresponding translational energy distributions for the H-atoms produced show three different contributions associated with three reaction pathways. The experimental results are complemented by high-level abinitio calculations. The potential energy curves computed as a function of the N-H and C-H bond distances allow us to draw a picture of the different mechanisms. Major dissociation occurs through N-H bond cleavage and it is triggered by an initial geometrical change, i.e., from a pyramidal configuration of the C-NH2 with respect to the N atom to a planar geometry. The molecule is then driven into a conical intersection (CI) seam where three outcomes can take place: first, threshold dissociation into the second dissociation limit, associated with the formation of CH3NH(Ã), is observed; second, direct dissociation after passage through the CI leading to the formation of ground state products; and third, internal conversion into the ground state well in advance to dissociation. While the two last pathways were previously reported at a variety of wavelengths in the 203-240nm range, the former had not been observed before to the best of our knowledge. The role of the CI and the presence of an exit barrier in the excited state, which modify the dynamics leading the two last mechanisms, are discussed considering the different excitation energies used.

The formation of O and H radicals in a pulsed discharge in atmospheric pressure helium with water vapour admixtures

The spatio-temporal distribution of O and H radicals in a 90 ns pulsed discharge, generated in a pin–pin geometry with a 2.2 mm gap, in He + H2O (0.1% and 0.25%), is studied both experimentally and by 1D fluid modelling. The density of O and H radicals as well as the effective lifetimes of their excited states are measured using picosecond resolution two-photon absorption laser induced fluorescence. Good agreement between experiments and modelling is obtained for the species densities. The density of O and H is found to be homogenous along the discharge axis. Even though the high voltage pulse is 90 ns long, the density of O peaks only about 1 μs after the end of the current pulse, reaching 2 × 1016 cm−3 at 0.1% H2O. It then remains nearly constant over 10 μs before decaying. Modelling indicates that the electron temperature (Te) in the centre of the vessel geometry ranges from 6 to 4 eV during the peak of discharge current, and after 90 ns, drops below 0.5 eV in about 50 ns. Consequently, during the discharge (<100 ns), O is predominantly produced by direct dissociation of O2 by electron impact, and in the early afterglow (from 100 ns to 1 μs) O is produced by dissociative recombination of O2 +. The main loss mechanism of O is initially electron impact ionisation and once T e has dropped, it becomes mainly Penning ionisation with He2* and He* as well as three-body recombination with O+ and He. On time scales of 100–200 μs, O is mainly lost by radial diffusion. The production of H shows a similar behaviour, reaching 0.45 × 1016 cm−3 at 1 μs, due to direct dissociation of H2O by electron impact (<100 ns) followed by electron–ion recombination processes (from 200 ns to 1.5 us). H is dominantly lost through Penning ionisation with He* and He2* and by electron impact ionisation, and by charge exchange with O+. Increasing concentrations of water vapour, from 0.1% to 0.25%, have little effect on the nature of the processes of H formation but trigger a stronger initial production of O, which is not currently reproduced satisfactorily by the modelling. What emerges from this study is that the built up of O and H densities in pulsed discharges continues after electron-impact dissociation processes with additional afterglow processes, not least through the dissociative recombination of O2 + and H2 +.

Enhancing Coke Resistance of Mg1–xNixAl2O4 Catalysts for Dry Reforming of Methane via a Doping‐Segregation Strategy

AbstractDry reforming of methane (DRM) is an important reaction to utilize two greenhouse gases (CO2 and CH4) simultaneously and produce syngas (CO and H2) for the manufacture of downstream high‐value chemicals. One of the major obstacles to preventing the commercialization of Ni‐based catalysts in DRM is the serious coking problem. This work develops a doping‐segregation method to manipulate the active metal structure and metal‐support interaction of Mg1–xNixAl2O4 catalysts to enhance their coke resistance for DRM. The Mg1–xNixAl2O4 catalysts prepared via the sol‐gel method exhibit outstanding coke resistance: the coke deposition rate of supported catalyst Ni/MgAl2O4 is 326 times higher than that of the Mg0.7Ni0.3Al2O4 catalyst at 600 °C. The Ni structure and metal‐support interaction of Mg1–xNixAl2O4 catalysts have been systematically studied using various characterization. Combined with the temperature‐programmed surface reaction (TPSR) experiments, the enhanced coke resistance of Mg1–xNixAl2O4 catalysts is attributed to smaller Ni particles, higher Ni dispersion, and stronger metal‐support interaction, which significantly inhibit the direct dissociation of CH4 and promote the oxidation of formed coke. The findings of this work provide insights into the correlation between the enhanced coke resistance and the Ni structure of Mg1–xNixAl2O4 catalysts synthesized via the doping‐segregation strategy using spinel as the precursor, leading to a stable catalyst design for many dry reforming reactions.

The CO oxidation on Mn atom embedded in N vacancy of C3N monolayer: A DFT-D study

In this study, the use of a single Mn atom anchored to a nitrogen vacancy in a C3N monolayer ([email protected]3N) for CO oxidation is investigated using first-principles calculations. The stability of [email protected]3N is confirmed through the analysis of binding energy, the movement of the Mn atom within the N vacancy, and ab initio molecular dynamics simulations at 400K for a period of 6 ps. O2 is found to have more favorable adsorption on [email protected]3N compared to CO, and the direct dissociation barrier for O2 is only 0.69 eV. The Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms are also examined on the [email protected]3N, and it is found that the ER mechanism was preferred due to its lower rate limiting barrier (0.57 eV compared to 0.91 eV for the LH mechanism). In addition, the oxidation of CO by the leftover O atom on the [email protected]3N is found to be facile, with a small barrier of 0.05 eV via the ER mechanism. These findings provide insight into the potential of the [email protected]3N as a single-atom catalyst for CO oxidation at low temperatures.