CO oxidation over BC3 nanosheet: a theoretical study

ABSTRACT

Metal-free catalysts have attracted more attention due to their highly active in catalytic oxidation reactions. The electronic structure and catalytic property of BC₃ sheet are investigated by using first-principles calculations. It is found that the BC₃ sheet as the active surface can effectively regulate the adsorptive stability of reactive gases. Besides, the possible reaction processes for CO oxidation on the BC₃ sheet are comparably analysed through different reaction mechanisms, which include the Eley–Rideal (ER), Langmuir–Hinshelwood (LH) and termolecular Eley–Rideal (TER). In the CO oxidation reactions, the decomposition of O₂ molecule as the starting state (0.40?eV) is an energetically more favourable process than those of other processes, the Eley–Rideal (ER) reactions (2O_ads+2CO→CO₂) are more prone to take place with lower energy barriers (3 sheet. These results provide an important guidance on exploring the highly efficiency metal-free catalyst for CO oxidation.     

Mechanistic study of the CO oxidation reaction on the CuO (111) surface during chemical looping combustion

Cu-based oxides oxygen carriers and catalysts are found to exhibit attractive activity for CO oxidation, but the dispute with respect to the reaction mechanism of CO and O₂ on the CuO surface still remains. This work reports the kinetic study of CO oxidation on the CuO (111) surface by considering the adsorption, reaction and desorption processes based on density functional theory calculations with dispersion correction (DFT-D). The Eley–Rideal (ER) CO oxidation mechanism was found to be more feasible than the Mars-van-Krevelen (MvK) and Langmuir–Hinshelwood (LH) mechanisms, which is quite different from previous knowledge. The energy barrier of ER, LH, and MvK mechanisms are 0.557, 0.965, and 0.999 eV respectively at 0 K. The energy barrier of CO reaction with the adsorbed O species on the surface is as low as 0.106 eV, which is much more active in reacting with CO molecules than the lattice O of CuO (111) surface (0.999 eV). A comparison with the catalytic activity of the perfect Cu₂O (111) surface shows that the ER mechanism dictates both the perfect Cu₂O (111) and the CuO (111) surface activity for CO oxidation. The activity of the perfect Cu₂O (111) surface is higher than that of the perfect CuO (111) surface at elevated temperatures. A micro-kinetic model of CO oxidation on the perfect CuO (111) surface is established by providing the rate constants of elementary reaction steps in the Arrhenius form, which could be helpful for the modeling work of CO catalytic oxidation.     

Structural,electronic and catalytic performance of single-atom Fe anchored 3Si-doped graphene

By density functional theory (DFT) calculations, it is found that the single-atom Fe anchored three Si modified defective graphene (3Si-graphene-Fe) exhibits the high stability, and this system is semiconducting property and has non-magnetic moment. Besides the most stable configurations, electronic structures and magnetic properties of adsorbed species (O₂, CO, 2CO and CO/O₂) on 3Si-graphene-Fe systems are comparably discussed. The adsorption of O₂ is more stable than that of CO molecule and the coadsorption of 2CO and CO/O₂ has the larger adsorption energy than that of the isolated one. The adsorbed O₂, CO and CO/O₂ can induce the change in magnetic properties of 3Si-graphene-Fe system, and the coadsorbed CO/O₂ on system exhibits the metallic property. Among the reaction mechanisms, the CO oxidation reactions through Eley–Rideal (ER) reactions have lower energy barriers (<0.5?eV) than those of the Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms, indicating that the ER reaction as starting step is an energetically favourable process. These results provide an important guidance on validating the catalytic activity of single atom on graphene-based materials.     

Theoretical study on the catalytic properties of single-atom catalyst stabilised on silicon-doped graphene sheets

ABSTRACT

The stable configurations, electronic structures and catalytic activities of single-atom metal catalyst anchored silicon-doped graphene sheets (3Si-graphene-M, M?=?Ni and Pd) are investigated by using density functional theory calculations. Firstly, the adsorption stability and electronic property of different gas reactants (O₂, CO, 2CO, CO/O₂) on 3Si-graphene-M substrates are comparably analysed. It is found that the coadsorption of O₂/CO or 2CO molecules is more stable than that of the isolated O₂ or CO molecule. Meanwhile, the adsorbed species on 3Si-graphene-Ni sheet are more stable than those on the 3Si-graphene-Pd sheet. Secondly, the possible CO oxidation reactions on the 3Si-graphene-M are investigated through Eley–Rideal (ER), Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms. Compared with the LH and TER mechanisms, the interaction between 2CO and O₂ molecules (O₂?+?CO → CO₃, CO₃?+?CO → 2CO₂) through ER reactions (< 0.2?eV) are an energetically more favourable. These results provide important reference for understanding the catalytic mechanism for CO oxidation on graphene-based catalyst.     

Theoretical study of stabilities,electronic, and catalytic performance of supported platinum on modified graphene

The geometry, electronic structure, and catalytic properties for CO oxidation of Pt atom supported on pri-graphene (PG), Haeckelite (H), and Stone–Wales-defect-graphene are investigated by density functional theory (DFT) calculations. In contrast to a Pt atom on PG, defective graphene, especially the Haeckelite, strongly stabilises the Pt atom and makes it more positive and thus the CO poisoning. At the same time the catalytic activities are as high as the pristine one. Langmuir–Hinshelwood mechanisms are favoured as the starting state and are followed by the Eley–Rideal reaction. The results indicate the benefit of Haeckelite as a substrate for the Pt atom and validate the reactivity of catalysts on the atomic scale with low cost and high activity.     

Microscopic insight into catalytic HCN removal over the CuO surface in chemical looping combustion

Hydrogen cyanide (HCN) is well-accepted as a main nitrogen-containing precursor from fuel nitrogen to nitrogen oxides. When using coal as fuel with a CuO-based oxygen carrier in chemical looping combustion (CLC), complex heterogeneous reactions exist among the system of HCN, O₂, NO, H₂O, and CuO particles. This work performs density functional theory (DFT) calculations to systematically probe the microscopic HCN heterogeneous reactions over the CuO particle surface. The results indicate that HCN is chemisorbed on the CuO surface, and the third dissociation step within the consecutive three-step HCN dissociations (HCN*→CN*→NCO*→N*) is the rate-determining step. Namely, the CN*/NCO* radicals can be deemed as an indicator of the performance of HCN removal due to their quite higher dissociation energies. With the existence of O₂, H₂O, and NO, the reaction mechanism of HCN conversion becomes extremely complex. Both DFT calculations and kinetic analyses determine that O₂, NO, and H₂O all significantly accelerate the consumption of CN*/NCO* radicals to produce various N-containing species (NOx or NH₃) to different extents. Finally, a skeletal reaction network in a system of O₂/NO/H₂O/HCN is concluded, which clearly elucidates that CuO exhibits excellent catalytic activity toward HCN removal.     

Features of low-temperature oxidation of isobutane in water vapor and carbon dioxide with increased density of reagents

The oxidation of isobutane at high density of reagents in a mixture of i-C₄H₁₀/O₂/H₂O and i-C₄H₁₀/O₂/CO₂ with oxygen deficiency (a molar ratio [O₂]0/[i-C₄H10]0 = 3.5–5.8) has been studied for the first time. The experiments were carried out in a tubular reactor under uniform heating (1 K/min) to 590 K. Data on the kinetics, auto-ignition temperature, and the products of isobutane conversion have been obtained. The auto-ignition was found to be a two-stage process and begin at a temperature of 510–522 K. The heat capacity of the reaction mixture suppressed the autoacceleration of the oxidation. Mass spectrometric analysis of the reactants revealed a difference in the mechanisms of isobutane conversion in water vapor and carbon dioxide. In water vapor, the oxidation is dominant and is realized with the participation of vibrationally excited O*₂ molecules, which appear mainly from resonant exchange with H₂O* molecules. In the CO₂ medium, the oxidation proceeds against the background of intense isobutane dissociation, initiated by the vibrational pumping of i-C₄H₁₀ molecules in their resonant excitation by CO*₂ molecules.     

Insight into the reaction mechanism of ethanol steam reforming catalysed by Co–Mo6S8

The production of hydrogen via steam reforming of ethanol (SRE) is favourable for the use of hydrogen as an alternative fuel. Co–Mo₆S₈ possesses high activity and stability for SRE to sustainably produce hydrogen. The competition among reaction pathways related to C–H, O–H, C–C, C–O cleavage and H₂ formation was studied. The adsorption and reaction of related intermediates in the ESR reaction pathway are described. The results indicated that the most feasible route for the decomposition of ethanol catalysed by Co–Mo₆S₈ is CH₃CH₂OH*→CH₃CH₂O*→CH₃CHO*→CH₂CHO*→CHCHO*→CHCO*→CH*+CO*. The CH* can be decomposed into C*+H*, and CO* can be oxidised via the redox mechanism of the water gas shift (WGS) reaction. Thus the final products are CO₂ and H₂. The present result may help people to design an SRE catalyst, which has the ability to break C–C to form CO and H₂, then CO react with H₂O in the WGS reaction generating CO₂ and H₂.     

Mechanism of heterogeneous mercury oxidation by HCl on V2O5(001) surface

The selective catalytic reduction (SCR) system for NO_X removal in coal-fired power plants has a promoting effect on the oxidation and removal of elemental mercury. In this study, basic mechanism of mercury oxidation by V₂O₅-based SCR catalyst is investigated via density functional theory method and the periodic slab models. Calculations are conducted to determine the adsorption energies and geometries of Hg⁰, HgCl, HgCl₂ and HCl on V₂O₅(001) surface, and to reveal the energy profile of oxidation reaction and the structures of relative transition states and intermediates. The results indicate that HCl can significantly promote Hg⁰ oxidation on V₂O₅(001) surface, by forming an intermediate HgCl-surface which is important for Hg⁰ oxidation. The Hg⁰ oxidation goes through Hg⁰ → HgCl → HgCl₂, and the two stages of the reaction follow Eley–Rideal mechanism and Langmuir-Hinshelwood mechanism, respectively. The formation of HgCl₂ is the rate-determining step due to its high energy barrier. Three detailed reaction pathways are obtained, and the related energy profiles and structures are analyzed in detail. The Hg⁰ oxidation reaction can take place through all three pathways even if differences exist in each other, while pathways I and II have relatively low energy barriers.     

Dynamic Monte Carlo simulation of the NO+H2 reaction on Pt(100): TPR spectra

The catalytic reduction of nitric oxide by hydrogen over a Pt surface is studied using a dynamic Monte Carlo (MC) method on a square lattice under low pressure conditions. Using a Langmuir–Hinshelwood reaction mechanism, a simplified model with only four adsorbed species (NO, H, O, and N) is constructed. The effect on the NO dissociation rate, the limiting step in the whole reaction, is inhibited by co-adsorbed NO and H₂ molecules and is enhanced both by the presence of empty sites and adsorbed N atoms at nearest neighbors. In these simulations, several experimental parameter values are included, such as: adsorption, desorption and diffusion of the reactants. The phenomenon is studied while varying the temperature over the 300–550 K range. The model reproduces well-observed TPD and TPR experimental results. For the whole NO+H₂ reaction, the phenomena of “surface explosion” is observed and can be explained as the result of the abrupt production of N₂ due to both the autocatalytic NO decomposition favored by the presence of vacant sites and the development of inhomogeneous fluctuations. MA simulations also allow a visualization of the spatial development of the surface explosion as heating proceeds.     

Selective CO oxidation in hydrogen-rich gas over CuO/CeO2 catalysts

A series of CuO/CeO₂ catalysts with different Cu-Ce compositions were synthesized by co-precipitation method and characterized by X-ray diffraction, H₂-TPR, CO-TPD, SEM and X-ray photoelectron spectroscopy (XPS) techniques. The effects of Cu-Ce composition and water vapor on the catalytic properties for the selective CO oxidation in the hydrogen-rich gas were investigated. The results indicated that CuO (10%)/CeO₂ catalyst remained the maximum CO conversion and selectivity at 140 and 160 °C, while the performance of CuO/CeO₂ catalysts deteriorated with the CuO molar ratio further increased. The interfacial CuO and CeO₂ interaction and synergistic effect enhanced the redox properties of CuO/CeO₂ catalyst and the highly dispersed copper species were proposed as the active sites for the selective CO oxidation. The blockage of catalytic active sites by absorbed water and the formation of CO-H₂O surface complexes reduced the activity of CuO (10%)/CeO₂ catalyst. The decreasing of surface lattice oxygen and absorbed oxygen species and the agglomeration of copper particles were the plausible interpretations for the deactivation of CuO (10%)/CeO₂ catalyst.     

In-situ study of the catalytic oxidation of CO on a Pt(1 1 0) surface using ambient pressure X-ray photoelectron spectroscopy

CO and O₂ co-adsorption and the catalytic oxidation of CO on a Pt(1 1 0) surface under various pressures of CO and O₂ (up to 250 mTorr) are studied using ambient pressure X-ray photoelectron spectroscopy (APXPS) and mass spectrometry. There is no surface oxide formation on Pt under our reaction conditions. CO oxidation in this pressure (<500 mTorr), O₂ to CO ratio (<10), and temperature (150 °C) regime is consistent with the Langmuir-Hinshelwood reaction mechanism. Our findings provide in-situ surface chemical composition data of the catalytic oxidation of CO on Pt(1 1 0) at total pressures below 1 Torr.     

The hydrolysis mechanism and kinetic analysis for COS hydrolysis: A DFT study

Density Functional Theory (DFT) was utilized to study the hydrolysis mechanism and kinetic analysis of carbonyl sulfide (COS). The structures of reactants (R), transition states (TS), intermediates (IM) and products (P) were analyzed and a conclusion reached that hydrolysis mechanism of COS occurs in two paths with One path as a C=S path and the other as a C=O path and all featuring potential for forming H₂S and CO₂. Function change analysis of COS hydrolysis indicated the rate-determining step of COS hydrolysis was the first elementary reaction as OH and H in H₂O attacked C=O and S=O in COS, respectively, with the two paths parallel and competitive and the C=S path more reactionary than the C=O path. Influence on each elementary reaction was also not consistent with reaction temperature increase. The study also included further investigation of the COS catalytic hydrolysis.     

Oxidation of hydrogen sulfide and corrosion of stainless steel in gas mixtures containing H<Subscript>2</Subscript>S,O<Subscript>2</Subscript>, H<Subscript>2</Subscript>O,and CO<Subscript>2</Subscript>

The composition of volatile and solid products of oxidation of hydrogen sulfide and stainless steel in gas mixtures containing H₂S, O₂, H₂O, and CO₂ has been determined using mass spectrometry, x-ray diffraction analysis, and scanning electron microscopy. It has been shown that holding an H₂S–O₂ mixture at 301 K results in prevailing formation of elemental sulfur and iron sulfides in the form of porous hygroscopic crust on the reactor wall surface. Formation of gas-phase sulfur causes self-acceleration of the oxidation of hydrogen sulfide; the resulting water triggers corrosion of the reactor wall. Heating of the resulting sulfur-sulfide crust in O₂ medium is accompanied by formation of SO₂ and heat release at T > 508 K. After heating of the H₂S–CO₂ mixture to 615 K, H₂ and COS were found in the volatile reactants; no noticeable corrosion of the reactor wall has been detected. It has been established that addition of O₂ to the H₂S–CO₂ mixture and its heating to 673 K leads to formation of ferrous sulfates. The mechanisms of the observed processes are discussed.     

In‐situ Raman spectroscopy study of Ru/TiO2 catalyst in the selective methanation of CO

Raman spectroscopic technique has been used to characterize a Ru/TiO₂ catalyst and to follow in situ their structural changes during the CO selective methanation reaction (S‐MET). For a better comprehension of the catalytic mechanism, the in‐situ Raman study of the catalysts activation (reduction) process, the isolated CO and CO₂ methanation reactions and the effect of the composition of the reactive stream (H₂O and CO₂ presence) have been carried out. Raman spectroscopy evidences that the catalyst is composed by islands of TiO₂–RuO₂ solid solutions, constituting Ru–TiO₂ interphases in the form of Ru_xTi_{1 − x}O₂ rutile type solid solutions. The activation procedure with H₂ at 300 °C promotes the reduction of the RuO₂–TiO₂ islands generating Ru^o–Ti³⁺ centers. The spectroscopic changes are in agreement with the strong increase in chemical reactivity as increasing the carbonaceous intermediates observed. The selective methanation of CO proceeds after their adsorption on these Ru^o–Ti³⁺ active centers and subsequent C―O dissociation throughout the formation of CH_x/C_nH_x/C_nH_xO/CH_x―CO species. These intermediates are transformed into CH₄ by a combination of hydrogenation reactions. The formation of carbonaceous species during the methanation of CO and CO₂ suggests that the CO presence is required to promote the CO₂ methanation. Similar carbonaceous species are detected when the selective CO methanation is carried out with water in the stream. However, the activation of the catalysts occurs at much lower temperatures, and the carbon oxidation is favored by the oxidative effect of water. Copyright © 2015 John Wiley & Sons, Ltd.     

Optical observation of oxidation and reduction of small supported copper particles

In situ and real-time optical absorption measurements of supported copper particles (4–10 nm) at wavelengths of 300 to 800 nm are carried out under H₂, CO, and O₂ respectively as ambient gases in the temperature range of 300 to 673 K. We observe a reversible change in the optical spectra caused by oxidation of copper and reduction of copper oxide. The data strongly indicate that the oxidation of small copper particles is composed of a fast process of Cu to CuO_x (x ≈ 0.67) and a slow process of CuO_x (x≈ 0.67) to CuO.     

Study of flowerlike CeO2 microspheres used as catalyst supports for CO oxidation reaction

Porous flowerlike CeO₂ microspheres were synthesized via a novel hydrothermal method and were used as supports for the oxidation of CO. After loaded with Au or CuO, it exhibited an excellent low-temperature catalytic activity toward CO oxidation reaction. Especially, for the Au-loaded flowerlike CeO₂ microsphere catalyst, CO gas started its conversion into CO₂ above 80% at room temperature. The possible reasons for the superior catalytic activity of flowerlike CeO₂ microsphere catalysts were discussed.     

The effect of oxygen–oxygen repulsion in the production of water from hydrogen–oxygen reaction on the surface: A computer simulation study

The formation of water from hydrogen–oxygen reaction on a metal surface is of immense importance due to the technological reasons. This reaction has been studied via a thermal mechanism on a Pt single crystal surface where the two molecules, H₂ and O₂, have been adsorbed dissociatively in atomic form. The reaction takes place between the adsorbed atoms through an intermediate OH radical. We have studied this reaction via a thermal (Langmuir–Hinshelwood mechanism) as well as a non-thermal mechanism (precursor mechanism) by the Monte Carlo computer simulations. In this study, we have applied a novel approach based upon the experimental observations that the dissociated oxygen atoms do not sit next to one another on a catalytic surface. Some interesting results like the shifting of the phase transition points, the broadening of the reaction window width and the elimination of the second-order phase transition in the non-thermal reaction mechanism are obtained by considering various possibilities of the reaction scheme. The phase diagrams as well as the snapshots of the surface covered with the reacting species are presented.     

H2S在氧化石墨烯表面的吸附和分解的理论研究

用周期性密度泛函方法对H₂S在氧化石墨烯(GO)上的吸附和分解进行了理论计算, 讨论了H₂S和GO上的羟基和环氧基团的反应过程．结果表明,反应过程是通过H₂S或-SH上的H转移使得GO的环氧基开环和羟基氢化,当GO相反面存在羟基时有助于环氧基团的开环和羟基氢化反应．H₂S在GO上吸附和分解到S原子的反应机理中引入了相应的中间态,计算两次脱氢过程能垒分别为3.2和10.4 kcal/mol,第二个H原子的转移是GO还原过程的速率决定步骤．结果还表明GO上的羟基和环氧基团有助于加强S原子和石墨烯的结合．     

Isomers of OCS and their reaction with H2O on singlet potential energy surface

The isomers of the carbonyl sulfide (OCS) molecule are investigated in detail at CCSD(T)/cc-pVTZ//MP2/6-311++G(2d,2p) level of theory. One cyclic isomer was identified along with three different linear minima of the OCS molecule. Three interconversion transition states were also located between cyclic and linear forms of OCS. Among these four isomers, the singlet potential energy surface (PES) for the molecule–molecule reaction between the three most energetically favoured isomers of OCS and H₂O has been explored theoretically at the CCSD(T)/cc-pVTZ//MP2/6-311++G(2d,2p) level. This singlet PES comprises of three paths. Path 1 is the reaction of linear OCS molecule with water producing the major product P1 (CO₂?+?H₂S), minor product P2 (S?+?HCOOH) and two isomers via 14 minima and 15 transition states. The Path 2 is an isomerization process in which cyclic isomer of OCS reacts with water molecule via another initial barrierless aduct producing five isomers of the OCS–H₂O system through five interconversion transition states. The reaction of linear COS isomer with water is shown in Path 3. This path produces the radicals SH and COOH from another COS–H₂O complex via a transition state. Among these three products, the product P1 is energetically most favoured. The overall exothermicity of the product channels for the formation of major product P1 on PES is calculated to be about 10.60?kcal/mol possessing initial high entrance barriers of 45.48 and 55.47?kcal/mol in two possible pathways. As the process is favoured thermodynamically but not kinetically, the reaction is expected to be very slow.