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.     

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.     

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.     

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.     

Interaction mechanism among CO,H2S and CuO oxygen carrier in chemical looping combustion: A density functional theory calculation study

When using coal-derived syngas or coal as fuel in chemical looping combustion (CLC), CO as a representative pyrolysis/gasification product and H₂S as the main sulfurous gas coexist in fuel reactor. Either CO or H₂S can absorb on the surface of CuO (the active component of Cu-based oxygen carriers), and reactions will occur among them. In this study, density functional theory (DFT) calculations are conducted to investigate the interaction among H₂S, CO, and CuO, including: the reaction between CO and H₂S over CuO particle, the influence of CO on the H₂S dissociation and further reaction process, and the impact of H₂S dissociation products on CO oxidation. Firstly, the co-adsorption results suggest that H₂S might directly react with CO to produce COS via the Eley–Rideal mechanism, while CO prefers to react with HS* or S* via the Langmuir–Hinshelwood mechanism. This means that the reaction mechanisms between CO and H₂S will change as the H₂S dissociation proceeds, which has already been forecasted by the co-adsorption energies and verified by all of potential Eley–Rideal and Langmuir–Hinshelwood reaction pathways. Then, the influence of CO on the H₂S dissociation process is examined, and it is noted that the presence of CO greatly limits the dissociation of H₂S due to the increased energy barrier of the rate-determining dehydrogenation step. Furthermore, the impact of H₂S dissociation products on CO oxidation by CuO is also investigated. The presence of H₂S and S* significantly supresses the CO oxidation activity, while the presence of HS* slightly promotes the CO oxidation activity. Finally, the complete interaction mechanisms among H₂S, CO, and CuO are concluded. It should be noted that COS will be inevitably produced via the Langmuir–Hinshelwood reaction between surface S* and CO*, which is prior to H₂O generation and subsequent sulfidation reaction.     

Tracer studies on the reaction path and kinetics of the Co oxidation over iridium

The reaction path and kinetics of the CO oxidation over polycrystalline indium were studied under low pressures by a transient isotope tracer method. Carbon monoxide is oxidized entirely through a Langmuir-Hinshelwood (LH) process, namely CO(a) + O(a) → CO₂. No evidence of an Eley-Rideal process, namely CO(g) + O(a) → CO₂, was observed. At constant temperatures the apparent rate constant of the LH process, i.e., the CO₂ formation rate divided by the product of the coverages of CO and oxygen, decreased rapidly with an increase in the total coverage. The kinetics was explained in terms of the boundary reaction model in which the oxidation took place predominantly outside or near the boundary of the CO(a) island.     

Role of adsorption complexes in catalytic acceleration of heterogeneous reactions

The universal equation that imposes restrictions on the kinetics of heterogeneous chemical reactions has been derived. A method of studying the participation of chemisorbed or physisorbed gas molecules in heterogeneous chemical reactions has been proposed. It has been shown that the heterogeneous reaction rate is independent of the degree of occupation of the catalyst surface by the reactants as long as the product is formed by chemisorbed complexes and physisorbed molecules. It has been found that the formation of CO₂ in the heterogeneous chemical reactions CO+ O → CO₂ and 2CO + O₂ → 2CO₂ is participated by physisorbed particles and chemisorbed complexes of oxygen atoms and CO molecules.     

The HO4H → O3 + H2O reaction catalysed by acidic,neutral and basic catalysts in the troposphere

A detailed effects of catalyst X (X?=?H₂O, (H₂O)₂, NH₃, NH₃···H₂O, H₂O···NH₃, HCOOH and H₂SO₄) on the HO₄H → O₃?+?H₂O reaction have been investigated by using quantum chemical calculations and canonical vibrational transition state theory with small curvature tunnelling. The calculated results show that (H₂O)₂-catalysed reactions much faster than H₂O-catalysed one because of the former bimolecular rate constant larger by 2.6–25.9 times than that of the latter one. In addition, the basic H₂O···NH₃ catalyst was found to be a better than the neutral catalyst of (H₂O)₂. However it is marginally less efficient than the acidic catalysts of HCOOH, and H₂SO₄. The effective rate constant (k'_t) in the presence of catalyst X have been assessed. It was found from k'_t that H₂O (at 100% RH) completely dominates over all other catalysts within the temperature range of 280–320?K at 0?km altitude. However, compared with the rate constant of HO₄H → H₂O?+?O₃ reaction, the k _eff values for H₂O catalysed reaction are smaller by 1–2 orders of magnitude, indicating that the catalytic effect of H₂O makes a negligible contribution to the gas phase reaction of HO₄H → O₃?+?H₂O.

Highlights

• A detailed effects of catalyst of H₂O, (H₂O)₂, NH₃, NH₃···H₂O, H₂O···NH₃, HCOOH and H₂SO₄ on the HO₄H → O₃?+?H₂O reaction has been performed.

• From energetic viewpoint, H₂SO₄ exerts the strongest catalytic role in HO₄H → O₃?+?H₂O reaction as compared with the other catalysts.

• At 0 km altitude H₂O (at 100% RH) completely dominates over all other catalysts within the temperature range of 280–320 K.

• HO₄H → H₂O?+?O₃ reaction with H₂O cannot be compete with the reaction without catalyst, due to the fact that the effective rate constants in the presence of H₂O are smaller.

     

Activation mechanisms of silver toward CO oxidation by tungsten carbide substrate: A density functional theory study

The development of efficient catalysts for low temperature CO oxidation is important to the application of fuel cells. In this work, we report that the Ag monolayer on WC (0001) surface (Ag_ML/WC) could effectively catalyze CO oxidation through the L-H mechanism (CO + O₂ → OOCO → CO₂ + O). The most sluggish reaction step is suggested to be CO + O → CO₂ with a barrier of 0.48 eV, which is 1.21 eV lower than the barrier of O₂ dissociation. The electronic structure and d-band center analyses demonstrate that the promoted activity may originate from the synergistic effect between Ag monolayer and WC. The present study is conducive to design new efficient and cost-effective catalysts for CO oxidation without using of the noble platinum.     

NO--CO--O2 Reaction on a Metal Catalytic Surface using Eley--Rideal Mechanism

Interactions among the reacting species NO, CO and O2 on metal catalytic surfaces are studied by means of Monte Carlo simulation using the Eley-Rideal （ER） mechanism. The study of this three-component system is important for understanding of the reaction kinetics by varying the relative ratios of the reactants. It is found that contrary to the conventional Langmuir-Hinshelwood （LFI） thermal mechanism in which two irreversible phase transitions are obtained between active states and poisoned states, a single phase transition is observed when the ER mechanism is combined with the LH mechanism. The phase diagrams of the surface coverage and the steady state production of CO2, N2 and N2O are evaluated as a function of the partial pressures of the reactants in the gas phase. The continuous production of CO2 starts as soon as the CO pressure is switched on and the second order phase transition at the first critical point is eliminated, which is in agreement with the experimental findings.     

DRIFTS study of different gas adsorption for CO selective oxidation on Cu-Zr-Ce-O catalysts

The adsorptions of different gases (CO, H₂ and O₂) in the hydrogen-rich gas on the co-precipitated Cu-Zr-Ce-O catalyst were discussed and the active sites were ascertained with infrared spectroscopy technique. It was shown that the adsorption strength of CO was stronger than that of O₂ or H₂. Hydrogen and CO were competitive adsorption and the coexistence H₂ and CO on the surface accelerated the rate of CO desorption. Adsorbed H₂ could convert into geminal OH groups on the ceria surface at high temperatures in the absence of oxygen, while it was easy to form surface hydroxyl groups at low temperatures and condensed to physical water with increasing desorption temperature in the existence of oxygen. The adsorption of CO₂ was strong and it could transform into thermal stable carbonate species even in the reaction conditions. The active sites of the Cu-Zr-Ce-O catalyst were Cu²⁺ and Cu⁺, mainly the latter. The oxygen defect sites could be formed on the Cu-Zr-Ce-O catalyst surface through dehydration and decarboxylation.     

Effect of (H2O)n (n = 1–3) clusters on H2O2 + HO → HO2 + H2O reaction in tropospheric conditions: competition between one-step and stepwise routes

ABSTRACT

Effects of (H₂O)_n (n?=?1–3) on the H₂O₂?+?HO?→?HO₂?+?H₂O reaction have been investigated by the reactions of H₂O₂L(H₂O)_n (n?=?1–3)?+?HO and H₂O₂?+?HOL(H₂O)_n (n?=?1–3) at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level of theory, coupled with rate constant calculations by using canonical variational transition state theory. Interestingly, for the former reactions, one-step process and stepwise mechanism are involved, where one-step processes occurring though cage-like hydrogen bonding network complexes and the transition states are favourable. Due to larger effective rate constants, these favourable processes are also favourable than the corresponding latter reactions. Meanwhile, the catalytic effect of (H₂O)_n (n?=?1–3) is mainly taken from water monomer, because the effective rate constant (k'(R_WM2)) of H₂O₂···H₂O?+?HO reaction is, respectively, larger by 3, 6–10 orders of magnitude than that of H₂O₂···(H₂O)₂?+?HO (k'(R_WD1)) and H₂O₂···(H₂O)₃?+?HO (k'(R_WT1)) reactions. Furthermore, the enhancement factor of water molecular (k'(R_WM2)/k_tot) is only 0.28% at 240?K, while at high temperature (such as at 425?K), the positive water vapour effect enhances up to 27.13%. This shows that at high temperatures the positive water effect is obvious under atmospheric conditions.     

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.     

电子束引起的残余含氧气体在镍(001)面上的吸附

当一束具有一定能量和强度的电子束轰击超高真空系统中残余的水汽、一氧化碳和二氧化碳时,将导致这些气体分子通过如下反应:H₂O→O_ad+H₂,CO₂→O_ad+CO,CO→O_ad+C_ad分解并共吸于镍表面。碳和氧的原子各自占据镍(001)面部份四重吸附位置,形成结构为p(2×2)或c(2×2)的许多独立的吸附畴,电子束轰击促进畴的成核、长大、连结和有序化。当氧和碳的原子占据了镍(001)面约一半的四重吸附位后,上述吸附反应将与导致氧和碳的脱附反应:C^*+O_ad→CO,O^*+C_ad→CO平衡,氧化镍与碳化镍开始成核。由于残余含氧气体中氧的含量超过碳,氧化镍成核占优势,使碳的吸附被排斥,已吸附的碳被排挤,形成电子束斑内氧高碳低、束斑外碳高氧低的“互补”分布。电子束轰击过程中碳的俄歇峰形的变化反映着碳原子与基底原子的不同结合状态。电子束的解离效应在吸附的初始阶段起重要作用,而其热效应对氧化镍的长大起重要作用。 关键词：     

中性金-钒-氧团簇氧化一氧化碳研究

本文实验制备了中性AuVO_(2～4)团簇并研究了其与CO和O_2的反应.实验结果表明,团簇AuVO_4和AuVO_3与CO反应主要生成吸附产物,而团簇AuO_2可与O_2反应生成AuVO_4.密度泛函理论计算结果表明,团簇AuVO_4和AuVO_3可在较高温度条件下氧化CO生成CO_2,从而形成催化氧化循环:AuVO_4+CO■AuVO_3+CO_2,AuVO_3+CO■AuVO_2+CO_2,AuVO_2+O_2→AuVO_4.     

Theory of oscillatory oxidation of carbon monoxide over platinum

It is shown that the finite rate of decay for temperature fluctuations of the catalytic surface in the Langmuir-Hinshelwood reaction: 2 CO + O₂ → 2 CO₂ over Pt can give rise to a self-sustained oscillatory regime. The waveforms and periods are in good qualitative agreement with experiments by Dauchot and Van Cakenberghe.     

Nitric oxide reduction by CO ON Rh(111): Temperature programmed reaction

The reaction of NO with CO on Rh(111) has been studied with temperature programmed reaction (TPR). Comparisons are made with the reaction of O₂ with CO and the reaction of NO with H₂. The rate-determining step for both CO oxidation reactions is CO(a) + O(a) → CO₂(g). Repulsive interactions between adsorbed CO and adsorbed nitrogen atoms lead to desorption of CO in a peak at 415 K which is in the temperature range where the reaction between CO(a) and O(a) produces CO₂(g). Thus the extent of reaction of CO(a) with NO(a) is less than that between CO(a) and O(a) due to the lower coverage of CO caused by adsorbed N atoms and NO. A similar repulsive interaction between NO(a) and H(a) suppresses the NO + H₂ reaction. CO + NO reaction behavior on Rh(111) is compared to that observed on Pt(111).     

Melting and subsolidus phase relations in the system K2CO3–MgCO3 at 3 GPa

ABSTRACT

As part of an investigation of carbonate systems under mantle pressures and temperatures, phase relations in the K₂CO₃–MgCO₃ system have been studied at 3?GPa and 800–1300°C. Subsolidus assemblages comprise the stability fields of K₂CO₃?+?K₂Mg(CO₃)₂ and K₂Mg(CO₃)₂?+?MgCO₃ with the transition boundary near 50?mol% K₂CO₃ in the system. The K₂CO₃–K₂Mg(CO₃)₂ eutectic is located at 840°C and 52?mol% K₂CO₃. The K₂CO₃ content in the melt coexisting with potassium carbonate increases to 85?mol% as temperature increases to 1050°C. K₂CO₃ remains solid up to 1250 and melts at 1300°C. K₂Mg(CO₃)₂ melts incongruently at 890°C to produce magnesite and a liquid containing 51?mol% K₂CO₃. As temperature increases to 1300°C, the K₂CO₃ content in the liquid coexisting with magnesite decreases to 27?mol%.     

The stoichiometry and kinetics of carbon combustion at low temperature: A surface complex approach

The stoichiometry and rate of carbon combustion at low temperature (673 K) were investigated. Oxidation and TPD experimental data provide quantification of gaseous products and stable surface complexes over a broad range of conversion. Our analysis distinguishes between surface complexes forming CO and CO₂ and has assumed a certain fraction of each complex type decomposes instantaneously upon formation, leaving the remainder on the surface as stable complexes, C(O) and C(O₂). This analysis suggests that a maximum of 25% of CO-complexes and 89% of CO₂-complexes are unstable upon formation. At low conversion, unstable complex formation is the dominant pathway for the CO product. As conversion increases, decomposition of stable CO-complexes eventually becomes the main source of CO. Formation of unstable CO₂-complexes is the dominant pathway for the CO₂ product at all times. The combustion rate is initially high due to a high availability of vacant active sites, decreases sharply as these sites are filled with stable complexes, and gradually increases as the stable complexes promote CO₂-complex formation, in turn, driving their decomposition. The dynamics of formation and decomposition of C(O) and C(O₂) dictates their ratio on the carbon surface at any moment, which may be measured by TPD. This work may help in developing new kinetic models of carbon combustion which can predict the stoichiometry as well as the rate.     

Further study of continuous-wave CO flame chemical laser of the CS2/O2/N2O type

This report describes an experimental examination of the output characteristics of the continuous-wave (cw) carbon monoxide flame chemical laser (FCL) of the CS₂/O₂/N₂O type in case of small CS₂/O₂ reactants ratios (tipically CS₂/O₂≦1/10). A linear burner which gives a homogeneous and stable flame was used during the experimental study. The measurements of temperature distribution in CS₂/O₂ as well as CS₂/O₂/N₂O flames show maximum temperatures of 1040 and 890 K, respectively. The addition of nitrous oxide (N₂O) leads to dramatically enhanced output laser power caused primarily by V?V transfer processes. A chemical efficiency, based on the reaction O+CS→CO^*+S, of 3% was achieved. The spectral composition of the CO FCL of the CS₂/O₂/N₂O type shows lasing in the region from 5.130 to 5.586 μm. Experimental results were obtained with a nondispersive optical cavity.