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.     

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).     

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.     

Effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions

The effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions was studied by single particle experiments and modelling. Experiments of pine and beech wood char conversion were carried out in a single particle combustor under conditions of 1473-1723 K, 0.0-10.5% O₂, and 25-42% H₂O. A comprehensive progressive char conversion model, including heterogeneous reactions (char oxidation and char gasification with CO₂ and H₂O), homogeneous reactions (CO oxidation, water-gas shift reaction, and H₂ oxidation) in the particle boundary layer, particle shrinkage, and external and internal heat and mass transfer, was developed. The modelling results are in good agreement with both experimental char conversion time and particle size evolution in the presence of oxygen, while larger deviations are found for the gasification experiments. The modelling results show that the char oxidation is limited by mass transfer, while the char gasification is controlled by both mass transfer and gasification kinetics at the investigated conditions. A sensitivity analysis shows that the CO oxidation in the boundary layer and the gasification kinetics influence significantly the char conversion time, while the water-gas shift reaction and H₂ oxidation have only a small effect. Analysis of the sensitive parameters on the char conversion process under a typical pulverized biomass combustion condition (4% O₂, 13% CO₂, 13% H₂O), shows that the char gasification reactions contribute significantly to char conversion, especially for millimeter-sized biomass char particles at high temperatures.     

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.     

Dilution effects analysis of opposed-jet H2/CO syngas diffusion flames

This paper reported the analysis of dilution effects on the opposed-jet H₂/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO₂, H₂O and N₂. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH₂, XCO, XCO₂, XH₂O and XN₂ with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H₂/CO = 1) at strain rate of 10 s^?1, the maximum flame temperature decreases the most by CO₂ dilution, followed by H₂O and N₂. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N₂ and the chemical effect of H₂O actually contribute the increase of flame temperature. However, the chemical effect of CO₂ and the radiation effect always decreases flame temperature. For flame extinction by adding diluents, CO₂ dilution favours flame extinction from all contributing effects, while thermal/diffusion effects of H₂O and N₂ extend the flammability. Therefore, extinction dilution percentage is the least for CO₂. The dilution effects on chemical kinetics are also examined. Due to the inert effect, the reaction rate of R84 (OH+H₂ = H+H₂O) is decreasing greatly with increasing dilution percentage while R99 (CO+OH→CO₂+H) is less affected. When the diluents participate chemically, reaction R99 is promoted and R84 is inhibited with H₂O addition, but the trend reverses with CO₂ dilution. Besides, the main chain-branching reaction of R38 (H+O₂→O+OH) is enhanced by the chemical effect of H₂O dilution, but suppressed by CO₂ dilution. Relatively, the influences of thermal/diffusion and radiation effects on the reaction kinetics are then small.     

The application of XPS to the determination of the kinetics of the co oxidation reaction over the Ir(111) surface

The coverages of adsorbed oxygen and CO on an Ir(111) surface have been determined using X-ray photoelectron spectroscopy (XPS) during the steady-state catalytic production of CO₂. Correlating the coverages of the reacting adsorbates with the rate of CO₂ production allows the kinetics of the CO oxidation reaction to be determined. The reaction is found to obey a Langmuir-Hinshelwood rate expression of the form , where R_CO2 is the rate of CO₂ production, k⁰ is the pre-exponential factor of the reaction rate coefficient, [CO] and [O] are the surface coverages of CO and oxygen, respectively, and E_a is the activation energy for the oxidation reaction. The activation energy for this catalytic oxidation reaction is found to be approximately 9 $\text{kcal}\text{mole}$.     

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.     

Kinetic studies on the CO oxidation over platinum by means of carbon 13 tracer

The kinetics of the CO oxidation over polycrystalline platinum was investigated with a transient isotope tracer method. Transient CO₂ production was generated by a dosage of various gas mixtures of ¹²CO and O₂ to the surface precovered by various ratios of ¹³CO to ¹²CO. The initial ratio of ¹³CO₂ to ¹²CO₂ in the CO₂ produced transiently equaled that of ¹³CO to ¹²CO in the CO preadsorbed for all the conditions studied; CO must be chemisorbed before being oxidized, i.e. a Langmuir-Hinshelwood mechanism is operative. The ratio of ¹³CO₂ to ¹²CO₂ in CO₂ decreased more rapid than the average ratio of ¹³CO(a) to ¹²CO(a) in CO(a). This apparent inhomogeneity in the reactivity of CO(a) was explained in terms of the boundary reaction model, in which the reaction took place predominantly outside or near the boundary of the CO(a) island.     

Ni(CO)4 formation on single Ni crystals: Reaction kinetics and observation of surface facetting induced by the reaction

We have studied the kinetics of the synthesis of nickel carbonyl (Ni + 4 CO → Ni(CO)₄), using single crystalline Ni surfaces of different crystallographic orientation. A dependence of the reaction rate on the crystallographic orientation of the surface has been observed. Scanning electron micrographs showed that a very sharp (111) facetting of the surface takes place during the reaction. A reaction mechanism for the Ni(CO)₄ formation, taking into account recent experimental data of the chemisorption of CO on Ni, is discussed, which may explain the kinetic results and the observed facetting.     

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.     

Analysis of Dissociation—Recombination Processes for the CO<Subscript>2</Subscript> Molecule with the Spin—Orbit Coupling Taken into Account

The dissociation CO₂(X¹Σ) + M → CO(X¹Σ) + O(³P) + M and recombination CO(X¹Σ) + O(³P) + M → CO₂(X¹Σ) + M processes are considered with the spin—orbit coupling taken into account in the ground and several excited states of the CO₂ molecule. Because of the specific features of mutual position of potential energy surfaces of the CO₂ molecule in the ground and several excited states and the large values of spin—orbit interaction matrix elements, which causes the quantum nonadiabatic transition of the molecule from one state to another, these processes become effectively spin-allowed and the rate constants for the nonadiabatic reactions have large values. The proposed dissociation and recombination mechanisms include reactions involving singlet—triplet crossings.     

The oxidation of CO on the Pt(100)-(5 × 20) surface

The kinetics of the CO oxidation reaction were examined on the Pt(100)-(5 × 20) surface under UHV conditions. The transient isothermal rate of CO₂ production was examined both for exposure of an oxygen-dosed surface to a beam of CO and for exposure of a CO-dosed surface to a beam of O₂. Langmuir-Hinshelwood kinetics were found to apply in both cases. For the reaction of CO with preadsorbed oxygen atoms, the reaction rate was dependent upon the square-root of the oxygen atom coverage, suggesting that oxygen atoms were adsorbed in islands on this surface. The oxidation of preadsorbed CO was observed only when the initial CO concentrations were less than 0.5 monolayer (c(2 × 2) structure), suggesting that the dissociative adsorption of oxygen required adjacent four-fold surface sites. The activation energy calculated for the reaction of CO with preadsorbed oxygen was 31.4 kcal/mol. This value was 30 kcal/mol greater than the activation energy measured for the reaction of O₂ with preadsorbed CO. Strong attractive interactions within the oxygen islands were at least partially responsible for this difference. The reaction kinetics in both cases changed dramatically below 300 K; this change is believed to be due to phase separation at the lower temperature.     

FC(O)O自由基与NO反应机理的理论研究

用密度泛函理论(DFT)的B3LYP方法,在6-311G(d,p)基组水平上研究了FC(O)O自由基与NO反应的微观机理,全参数优化了反应过程中各反应物、中间体、过渡态和产物的几何构型,在CCSD(T)水平上计算了它们的能量,振动分析结果证实了中间体和过渡态的真实性,从对FC(O)O与NO的反应机理的研究结果看,FC(O)O与NO反应为4条反应通道多步反应过程,其反应的主要通道是FC(O)O+NO→M3→TS6→M5→FNO+CO₂,其主要产物是自由基FNO和CO_{2关键词： 密度泛函理论 反应机理 反应通道}     

A transient study of the co-adsorption and reaction of CO and oxygen on Ir(110)

The heterogeneously catalyzed reaction of CO and O₂ to form CO₂ over Ir(110) has been studied through measurements of the transient kinetics of the various elementary reactions that may limit the steady state rate. Rate expressions for these elementary reactions — the desorption of CO, the oxidation of CO via the Langmuir-Hinshelwood mechanism, the adsorption of CO and the adsorption of oxygen — were developed using thermal desorption mass spectrometry. Several phenomena were observed: (1) the activation energies for CO desorption and CO oxidation depend markedly upon the composition of the adlayer; (2) diffusion in the adlayer may limit the rates of CO desorption and CO oxidation; (3) the formation of a surface oxide modifies these four rate processes; and (4) chemisorbed CO blocks sites for oxygen adsorption, but chemisorbed oxygen does not block sites for CO adsorption.     

On the bonding and reactivity of CO2 on metal surfaces

We present ab-initio valence-bond calculations on free and coordinated CO₂. By analogy to transition-metal complexes with coordinated CO₂ three different coordination geometries for the CO₂ molecule are considered: (a) pure carbon coordination, (b) pure oxygen coordination, (c) mixed carbon-oxygen coordination. It is shown that pure oxygen ((b) geometry) and mixed carbon-oxygen coordination ((c) geometry) are more favourable than pure carbon coordination. In all cases studied, the bonding between the CO₂ moiety and the metal atom is described best as a CO₂⁻ anion interacting with a Ni cation. On the basis of the theoretical calculations, many observed and expected features (some already known and some predicted) of the interaction of CO₂ and metal surfaces can be discussed. The electron transfer to the CO₂ moiety drives the observed bent geometry of the coordinated CO₂ molecule and is accompanied by an elongation of the CO bond distance with respect to the free molecule. The bond elongation leads to a drastic lowering of the asymmetric CO stretching frequency, and a change in the relative energy position of the photoelectron peaks. We also consider intermolecular interaction between the CO₂⁻ anion and surrounding neutral CO₂ molecules via “solvation” in analogy to results of recent gas-phase cluster experiments. On the basis of the deduced metal-CO₂ bonding scheme the reactivity of coordinated CO₂ is investigated. Three reaction channels are considered: (a) dissociation into CO and O⁻, (b) oxidation to CO₃⁻ and CO₃²⁻, (c) disproportionation of CO₂⁻ + CO₂ to CO₃⁻ and CO. On the basis of energetic considerations we argue that dissociation is likely to occur on transition-metal surfaces, while oxidation to carbonate species is more likely on noble metals due to the low binding energies for the dissociation products, namely oxygen and CO on these surfaces.     

Studies of high-pressure n-butane oxidation with CO2 dilution up to 100 atm using a supercritical-pressure jet-stirred reactor

A novel supercritical-pressure jet stirred reactor (SP-JSR) is developed to operate up to 200 atm. The SP-JSR provides a unique platform to conduct kinetic studies at low and intermediate temperatures at extreme pressures under uniform temperature distribution and a short flow residence time. n-Butane oxidations with varying levels of CO₂ dilutions at pressures of 10 and 100 atm and over a temperature range of 500-900 K were conducted using the SP-JSR. The experiment showed that at 100 atm, a weak NTC behavior is observed and the intermediate temperature oxidation is shifted to lower temperatures. Furthermore, the results showed that CO₂ addition at supercritical conditions slows down the fuel oxidation at intermediate temperature while has little effect on the low temperature oxidation. The Healy model under-predicts the NTC behavior and shows little sensitivity of the effect of CO₂ addition on the n-butane oxidation. Reaction pathway and sensitivity analyses exhibit that both the low and intermediate temperature chemistries are controlled by RO₂ consumption pathways. In addition, the reactions of CH₃CO (+ M) and CH₃CO + O₂ become important at 100 atm. The results also revealed that fuel oxidation kinetics is insensitive to the third body effect of CO₂. The kinetic effect of supercritical CO₂ addition may come from the reactions involving H₂O₂, CO, CH₂O, and CH₃CHO, especially for the reactions of CO₂ + H and CO₂ + OH.     

Catalytic Reaction Mechanism Testing

Experimental tests for determining the mechanism of catalytic reactions were suggested. Quantitative relations were obtained that allowed the mechanism of formation of product molecules in an arbitrary catalytic reaction to be determined by isothermic relaxation methods. The relations found were compared with the literature data on the 2CO + O₂ → 2CO₂ reaction on the (111) surface of Pd. The experimental data were shown to be insufficient for the unambiguous determination of the mechanism of this reaction. The results available corresponded to the participation of physically adsorbed CO molecules in the formation of CO₂. The temperature dependence of the reaction rate was determined by the transition of strongly adsorbed oxygen atoms into the mobile reactive state.     

Bond making and breaking on transition-metal surfaces: Theoretical projections based on bond-order conservation

Our analytic Morse-potential model of chemisorption based on bond-order conservation [Surface Sci. 150 (1985) L115; 163 (1985) L645, L730] has been used to calculate the heats of chemisorption of various diatomic AB and polyatomic AB_x species (coordinated via A) and to estimate the activation barriers for their dissociation and transformations. Examples include adspecies such as CH_x, NH_x, OH_x, and possible intermediates and elementary steps of reactions such as CO + O → CO₂, NO + N → N₂ + O, N₂ + H₂ → NH₃, H₂ + O₂ → H₂O, and CO + H₂ → CH₄. Both the qualitative projections and numerical estimates are in good agreement with experiment. In particular, it is shown that (1) the most reactive adspecies should be the most weakly bound, and (2) the recombination activation barrier should primarily depend on (and may even be close to) the heat of chemisorption of the weaker bound partner.     

Bistable mean-field kinetics of CO oxidation on Pt with oxide formation

The bistability of the kinetics of CO oxidation on Pt at sub-atmospheric pressures can be complicated by surface-oxide formation. We present a simple mean-field model making it possible to describe a transition from the low reactive reaction regime occurring via the conventional mechanism of CO oxidation at CO excess, to the high reactive regime including CO interaction with a fully developed surface-oxide overlayer at O₂ excess. In the latter case, the oxide is assumed to form islands, the CO₂ formation may run primarily on oxide, and in agreement with recent experiments the reaction rate may be several orders of magnitude lower than the CO adsorption rate on a bare metal surface.