Langmuir-Hinshelwood Reaction Research Articles

A theoretical simulation on the catalytic oxidation of CO on Pt/graphene

The catalytic oxidation of CO on Pt/X-graphene (X = "pri" for pristine- or "SV" for defective-graphene with a single vacancy) is investigated using the first-principles method based on density functional theory. In contrast to a Pt atom on pristine graphene, a vacancy defect in graphene strongly stabilizes a single Pt adatom and makes the Pt adatom more positively charged, which helps to weaken the CO adsorption and facilitates the O(2) adsorption, thus enhancing the activity for CO oxidation and alleviating the CO poisoning of the platinum catalysts. The CO oxidation reaction on Pt/SV-graphene has a low energy barrier (0.58 eV) by the Langmuir-Hinshelwood (LH) reaction (CO + O(2)→ OOCO → CO(2) + O(ads)) which is followed by the Eley-Rideal (ER) reaction with an energy barrier of 0.59 eV (CO + O(ads)→ CO(2)). The results validate the reactivity of catalysts on the atomic-scale and initiate a clue for fabricating carbon-based catalysts with low cost and high activity.

A Review of the Aqueous Aerosol Surface Chemistry in the Atmospheric Context

In this review the surface chemistry and properties of aqueous atmospheric aerosols are explored. Water plays a major role in scavenging pollutants. Reactions occur on thin water films in atmospheric aerosols. The study of the aerosol wa- ter surface is important to properly account for chemical transformations in the troposphere. The thermodynamics of adsorption of organic molecules and oxidant species on the aqueous surface and, the techniques employed to quantify the adsorption isotherms are summarized. Experimental techniques for elucidating the reactions on the water surface are described. Field and laboratory data for oxidation reactions of compounds at the air-water interface are summarized. The Langmuir-Hinshelwood reaction mechanism is useful in quantifying the reaction rate on the aqueous aerosol sur- face. A hypothesis for the large heterogeneous reaction rate on the water surface over the homogeneous bulk aqueous phase reaction is presented.

Reaction kinetics of dye decomposition processes monitored inside a photocatalytic microreactor

The photocatalytic decomposition processes of several kinds of dyes were monitored in real-time, in a TiO(2)-immobilized microcapillary. Their fluorescence spectra were measured directly from the UV-irradiated area. The photocatalytic reactions proceeded two orders of magnitude faster in the microcapillary than in a bulk reaction, and intermediate species were easily observed, due to their high concentrations compared with those of the reactants. Even for molecules that were not originally fluorescent, fluorescence was detected for the reactants or intermediate species of all the molecules studied. Photocatalytic reactions are typically analyzed in terms of pseudo-first-order or Langmuir-Hinshelwood reaction mechanisms, but it was ascertained that all of the dyes investigated in this study decomposed via a multi-step reaction such as a simple multi-step reaction, a self-catalytic reaction, and further, a more complicated reaction, depending on the molecular structure. These reactions were simulated using models based on the reaction kinetics, and reaction mechanisms were assigned to each type of dye. The fact that intermediate species (which are difficult to observe using conventional analytical methods) were successfully detected meant that mechanisms for different dyes could be further clarified.

Determination of the Specific Surface Area of Snow Using Ozonation of 1,1-Diphenylethylene

We measured the kinetics of ozonation reaction of 1,1-diphenylethylene (DPE) in artificial snow, produced by shock freezing of DPE aqueous solutions sprayed into liquid nitrogen. It was demonstrated that most of the reactant molecules are in direct (productive) contact with gaseous ozone, thus the technique produces snow with organic molecules largely ejected to the surface of snow grains. The kinetic data were used to evaluate the snow specific surface area (∼70 cm(2) g(-1)). This number is a measure of the availability of the molecules on the surface for chemical reaction with gaseous species. The experimental results were consistent with the Langmuir-Hinshelwood type reaction mechanism. DPE represents environmentally relevant compounds such as alkenes which can react with atmospheric ozone, and are relatively abundant in natural snow. For typical atmospheric ozone concentrations in polar areas (20 ppbv), we estimated that half-life of DPE on the surface of snow grains is ∼5 days at submonolayer coverages and -15 °C.

Catalytic formation of carbonyl sulfide during warm gas clean-up of simulated coal-derived fuel gas with Pd/γ-Al2O3 sorbents

Coal gasification processes, such as the Integrated Gasification Combined Cycle (IGCC), will increase in importance due to the expanding concern over CO2 emissions and global climate change. During the development of a Pd/γ-Al2O3 sorbent for warm (200°C) fuel gas cleanup, the catalytic formation of carbonyl sulfide (COS), was observed. This is attributed to a heterogeneous reaction involving fuel gas components (CO/CO2/H2/H2S/H2O) and Pd/γ-Al2O3. The concentration of COS increases 200-fold when exposed to the Pd/γ-Al2O3 sorbent. A Langmuir–Hinshelwood reaction mechanism is proposed and a kinetic model is developed based on experimental results. The effect of γ-Al2O3, a common catalyst for hydrolysis of COS, and H2O on the COS concentration is discussed.

Dynamic Monte Carlo simulation of the [formula omitted] 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 Catalytic Reduction at Quasi-Perfect Pt(100) Domains: A Universal Low-Temperature Pathway from Nitrite to N2

The highly selective conversion of nitrite to N(2) at a quasi-perfect Pt(100) electrode in alkaline media was investigated with a particular emphasis on its structure sensitivity and its mechanism. High-quality (100) terraces are required to optimize the catalytic activity and steer the selectivity to N(2): defects of any symmetry dramatically reduce the N(2) evolution at [(100) × (110)] and [(100) × (111)] surfaces. On the other hand, nitrite reduction proves to be an additional example of the unique intrinsic ability of (100) surfaces to catalyze reactions involving bond breaking and successive bond formation. In the present case, (100) is able to reduce nitrite to NH(2,ads), which in a certain potential window combines with NO(ads) to give N(2) in a Langmuir-Hinshelwood reaction. Our findings are similar to those for other processes generating N(2), from bacterial anoxic ammonia oxidation ("anammox") to the high-temperature NO + NH(3) reaction at Pt(100) crystals under ultra-high-vacuum conditions, thus suggesting that the combination of these two nitrogen-containing species is a universal (low-temperature) pathway to N(2). The advantages of this pathway over other N(2)-generating pathways are pointed out.

Heterogeneous Photocatalysis Applied to Indoor Building Material: Towards an Improved Indoor Air Quality

In the present article, kinetics of the photocatalytic oxidation (PCO) of nitric oxide (NO, as a typical air pollutant) is addressed. An extended Langmuir-Hinshelwood reaction rate model is proposed to describe the PCO of NO under indoor air conditions. The derived model incorporates the influence of the indoor air conditions in the process of the PCO. The good agreement between the predictions from the model and experimental results indicates the validity of the proposed model.

Influence of sulfation on iron titanate catalyst for the selective catalytic reduction of NOx with NH3

Iron titanate catalyst (FeTiOx) is a potential candidate for the substitution of conventional V2O5–WO3 (MoO3)/TiO2 and Fe/Cu-zeolite catalysts for the selective catalytic reduction (SCR) of NOx with NH3 because of its high SCR activity and N2 selectivity in the medium temperature range. Due to the presence of small amount of SO2 in typical diesel exhaust derived from combustion of sulfur-containing fuels, it is very important to investigate the influence of sulfation on SCR activity, catalyst structure and reaction mechanism. After sulfation under the SCR condition, the surface area and pore volume of FeTiOx catalyst decreased to a certain extent due to the formation of sulfate species. According to the characterizations of FeTiOx catalyst using X-ray diffraction, X-ray absorption fine structure spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy of SO2+O2 treatment, the sulfate species mainly formed on iron sites in a chelating bidentate conformation, resulting in the enhancement of Brønsted acidity and Lewis acid strength simultaneously. NH3 adsorption was greatly enhanced in the high temperature range, while NOx adsorption was severely inhibited due to the stronger acidity of sulfate species. The operation temperature window of the sulfated catalyst shifted ca. 50°C towards high temperature range accordingly. The reaction mechanism study shows that the Langmuir–Hinshelwood reaction pathway was cut off by the sulfation process, resulting in the activity loss at low temperatures; only Eley–Rideal reaction pathway between adsorbed NH3 species and gaseous or weakly adsorbed NO dominated in the SCR reaction, which made this catalyst resistant to SO2 poisoning at relatively high temperatures.

Critical review: Plasma-surface reactions and the spinning wall method

This article reviews methods for studying reactions of atoms and small molecules on substrates and chamber walls that are immersed in a plasma, a relatively unexplored, yet very important area of plasma science and technology. Emphasis is placed on the “spinning wall” technique. With this method, a cylindrical section of the wall of the plasma reactor is rotated, and the surface is periodically exposed to the plasma and then to a differentially pumped mass spectrometer, to an Auger electron spectrometer, and, optionally, to a beam of additional reactants or surface coatings. Reactants impinging on the surface can stick and react over time scales that are comparable to the substrate rotation period, which can be varied from ∼0.5 to 40 ms. Langmuir–Hinshelwood reaction probabilities can be derived from a measurement of the absolute desorption product yields as a function of the substrate rotation frequency. Auger electron spectroscopy allows the plasma-immersed surface to be monitored during plasma operation. This measurement is critical, since wall “conditioning” in the plasma changes the reaction probabilities. Mass spectrometer cracking patterns are used to identify simple desorption products such as Cl2, O2, ClO, and ClO2. Desorption products also produce a measurable pressure rise in the second differentially pumped chamber that can be used to obtain absolute desorption yields. The surface can also be coated with films that can be deposited by sputtering a target in the plasma or by evaporating material from a Knudsen cell in the differentially pumped wall chamber. Here, the authors review this new spinning wall technique in detail, describing both experimental issues and data analysis methods and interpretations. The authors have used the spinning wall method to study the recombination of Cl and O on plasma-conditioned anodized aluminum and stainless steel surfaces. In oxygen or chlorine plasmas, these surfaces become coated with a layer containing Si, Al, and O, due to slow erosion of the reactor materials, in addition to Cl in chlorine plasmas. Similar, low recombination probabilities were found for Cl and O on anodized Al versus stainless steel surfaces, consistent with the similar chemical composition of the layer that forms on these surfaces after long exposure to the plasma. In chlorine plasmas, weakly adsorbed Cl2 was found to inhibit Cl recombination, hence the Cl recombination probability decreases with increasing Cl2-to-Cl number density ratios in the plasma. In mixed Cl2/O2 plasmas, Cl and O recombine to form Cl2 and O2 with probabilities that are similar to those in pure chlorine or oxygen plasmas, but in addition, ClO and ClO2 form on the surface and desorb from the wall. These and other results, including the catalytic enhancement of O recombination by monolayer amounts of Cu, are reviewed.

Enhanced Photocatalytic Properties of SnO2 Nanocrystals with Decreased Size for ppb‐level Acetaldehyde Decomposition

AbstractUltrafine SnO2 nanocrystals are synthesized through a surfactant‐assisted solvothermal route in water–ethanol mixed media. SnO2 photocatalysts with sizes of about 4, 8, and 12 nm are prepared through controllable annealing of the as‐synthesized nanocrystals. Brunauer–Emmett–Teller (BET) specific surface areas of calcined SnO2 samples increase dramatically as their sizes decrease. Photocatalytic performances of the as‐obtained catalysts for decomposing ppb‐level (parts per billion) acetaldehyde are tested in a continuous glass‐plate reactor. Result shows that SnO2 photocatalysts of about 4 nm with a large BET surface area of 130 m2 g−1 exhibit the best photocatalytic oxidation properties, which is comparable to that of Degussa P25 TiO2. Analysis by using the unimolecular Langmuir–Hinshelwood reaction model indicates that the abilities of adsorption have well synergized with the photocatalytic reaction activities in the as‐obtained SnO2 samples. This synergism finally enhances the photocatalytic performance of the as‐prepared SnO2 photocatalysts.

DFT and kinetics study of O/O2 mixtures reacting over a graphite (0001) basal surface

Spin-polarized density functional calculations were used to investigate the interaction of atomic and molecular oxygen on the basal graphite surface at several atomic coverages. Two carbon layers were enough to represent the graphite surface. Oxygen atoms bind mainly over C–C bridge sites forming an epoxide-like structure with a two carbon puckering and with adsorption energies in the 0.95–1.28 eV range, depending on the atomic coverage. Molecular oxygen only shows a very weak physisorption. Atomic adsorption and diffusion along with atomic recombination via Eley–Rideal and Langmuir–Hinshelwood mechanisms were studied. All surfaces processes were activated with energy barriers that decreased for lower atomic coverages. Relaxation effects were non-negligible. A microkinetic model with six elementary surface processes was proposed to see the overall behaviour of several initial O/O2 mixtures flowing over a graphite surface at 300–1,000 K. Thermal rate constants were derived from Density Functional Theory data and standard Transition State Theory. A very low steady-state atomic coverage (θΟ < 0.5%) was predicted, and also very low atomic recombination coefficients were observed (γO < 5 × 10−4). The Eley–Rideal together with the adsorption and desorption processes was much more important than the Langmuir–Hinshelwood reaction.

Modeling Free-Radical Reactions, Produced by Hydrocarbon Cracking, with Asphaltenes

Radical adsorptions and reactions produced in oil hydrocarbon cracking with an asphaltene model molecule were carried out with CATIVIC and MOPAC programs. Density functional theory (DFT) calculations were also performed for comparison. Eley—Rideal (ER) and Langmuir-Hinshelwood (LH) reaction mechanisms, of H•, CH 3 •, and CH 3 CH 2 • radicals with adsorbed H•, were studied on asphaltene unsaturated fragments. Qualitative results indicated that it is possible that radical adsorptions on the asphaltene surface lead to bond activations (surface deformation). The following adsorption energy trend was found for the three different programs: H• > CH 3 • > CH 3 CH 2 •. Approximate reaction energy barriers were evaluated. The ER mechanism has smaller reaction barriers and higher exothermic reactions than the LH mechanism, which means that ER is energetically more feasible than LH. These results indicate that, at low temperatures and high pressure, the asphaltene surface may play an important role in the recombination of free radicals.

Studies of chlorine-oxygen plasmas and evidence for heterogeneous formation of ClO and ClO2

Plasma and surface diagnostics of Cl2/O2 mixed-gas inductively coupled plasmas are reported. Using trace rare gas optical emission spectroscopy and Langmuir probe analysis, electron temperatures (Te) and number densities for Cl atoms (nCl), electrons (ne), and positive ions were measured as a function of percent O2 in the feed gas and position in the plasma chamber. Adsorbates on and products desorbing from a rotating anodized aluminum substrate exposed to the plasma were detected with an Auger electron spectrometer and a quadrupole mass spectrometer. Te and ne increased with increasing percent O2 in the plasma, while nCl fell off with O2 addition in a manner reflecting simple dilution. Cl atom recombination probabilities (γCl) were measured and were found to be a nearly constant 0.036±0.007 over the range of Cl2/O2 mixing ratios and Cl coverage. Large yields of ClO and ClO2 were found to desorb from the surface during exposure to the plasma, ascribed predominantly to Langmuir–Hinshelwood reactions between adsorbed O and Cl.

Investigation of deep methane oxidation with the use of the improved flowing circulation method

The most correct method for studying the peculiarities of stationary kinetics of heterogeneous reactions is the flowing circulation method which has been substantially improved in recent years. The core of the introduced improvement lies in the direct determination of reaction velocity (catalytic activeness) at the given compound of the contact reaction mixture. The improved flowing circulation method was applied for the first time to study the deep oxidation of methane by molecular oxygen in order to obtain one-parameter dependences of catalytic reactivity on the temperature and concentration of reaction medium components and the solution of the inverse kinetic problem. Catalyst IK 12–72 compound, wt %: 16 ± 2 Cr2O3, 4 ± 0.6 MgO and the remaining Al2O3 has been studied. In the kinetic area, the apparent energy of reaction activation (110 kJ/mol) has been determined. On the basis of experiments and ideas on the mechanism of the Langmuir-Hinshelwood reaction of competitive absorption type, the equation well describing experimental data in the overall studied range of concentrations (average relative deviation 8%) was selected. The simplicity of the improved flowing circulation method allows for it to be recommended for the selection of the most active catalysts and analysis of dependences of stationary velocities of heterogeneous catalytic reactions on various process parameters.

Catalytic oxidation of CO on CuO x revisited: Impact of the surface state on the apparent kinetic parameters

Analysis of kinetic features of the copper oxides reduction by CO pulses as related to mechanism of CO catalytic oxidation by oxygen combined with monitoring the state of the surface by an electrochemical technique using a solid electrolyte—Pyrex glass and high resolution TEM data on the defect structure of CuO allowed us to suggest a partially “flexible” model of CuO surface. This model, with a due regard for the data of FTIR spectroscopy of adsorbed CO test molecules, assigns the most active surface sites able to coordinate highly reactive CO and O forms to clusters of Cu + cations located at outlets of extended defects (dislocations, twins). Variation of the number, size and structure of these clusters under the reaction medium effect allows explaining the difference between quasi-steady and true steady states of copper oxides in catalytic CO oxidation reaction as well as the difference between kinetic parameters of reaction estimated at quasi-steady and constant states of the surface following Boreskov's approach. Kinetic features of the reaction agree with the Langmuir–Hinshelwood reaction mechanism operating for clustered defect centers of CuO.

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 O 2 co-adsorption and the catalytic oxidation of CO on a Pt(1 1 0) surface under various pressures of CO and O 2 (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), O 2 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.

Hydrogen-induced mitigation of O on Ru(101̄0): a density-functional study

The reaction of hydrogen with oxygen adsorbed on an Ru(1010) surface has been studied by density-functional calculations and kinetic Monte Carlo simulations. In agreement with experiment, it has been found that molecular hydrogen does not react with adsorbed O. In contrast, the hydrogenation of oxygen by an atomic H beam occurs spontaneously and results in the formation of adsorbed OH molecules. Subsequent impinging H-atoms can either initiate the formation of water, which readily desorbs at room temperature thus removing the O from the surface, or lead to formation and desorption of H2. It is the latter channel that hinders, at 300 K, a complete removal of O from Ru capping layers on Si/Mo mirrors for extreme ultraviolet radiation. The estimated height of the barrier for the Langmuir-Hinshelwood reaction between adsorbed H and OH, 0.92 eV, and related position of the H2O peak in model desorption spectra (approximately 320 K) are consistent with recent experiments. The H2 desorption peak appears at higher temperature, approximately 350 K, so that in the range from 320 to 330 K adsorbed hydrogen atoms will react predominantly with OH. Hence, the present simulations predict that an efficient removal of the chemisorbed O from Ru capping layers can be achieved by heating the surface to 320-330 K in a molecular hydrogen atmosphere.

Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au(20) was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O(2) molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O(2) molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au(20) nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects.

Quantum chemical study of the reactivity of boron-doped graphite layers towards water formation

Density functional calculations were used to study some fundamental features of boron-doped graphite layers (C x B y ) and the boron influence on the mechanisms leading to the formation of water molecules on the C x B y graphite like layers. The Langmuir–Hinshelwood reactions leading to water formation on the graphite-like layers containing 12 at% boron take place with activation energies 3–5 times lower than on pure graphite ones. For the Eley–Rideal mechanism, the activation energies are always very low whether or not the graphite contains boron. Similar results were observed for 25 at% B doping. As a consequence, the oxygen and hydrogen can be more easily eliminated from the doped surfaces in the form of water molecules than from the corresponding pure ones. The C x B y layers with high boron content or having accumulations of boron, lose their planar structure. Two such parallel layers strongly interact through the boron atoms with the formation of a B–B bond and the displacement of the boron atoms into the inter-layer space. As a whole the system deviates from the graphite-like structure.