Model For Dissociative Adsorption Research Articles

An Expanded Model for the Pressure Effect in Metal Dusting of Mn-Containing Alloy 600 Based on Advanced Scale Characterization

Commercial alloy 600 was exposed to a metal dusting inducing gas mixture to investigate the effect of ambient and elevated pressures on the oxide scale formation behavior. The formed scales on this alloy were investigated via Raman spectroscopy, transmission electron microscopy (TEM), and atom probe tomography (APT) to reveal the differences in their morphology and chemical composition(s). Raman spectroscopy revealed graphite within the chromia scales as well as oxide grains with Mn–Cr spinel. High-pressure exposure promoted the presence of Mn within the oxide scale, and APT demonstrated these Mn-enriched areas dissolved up to 4 at.% C. This was consistent with the TEM results, which confirmed higher amounts of Mn in the scale formed under elevated pressure, as well as crystalline graphite in the scales for both conditions. The consequences for the scale breakdown, reflecting the end of the incubation time for metal dusting attack, are discussed in light of these results. A two-fragment dissociative adsorption model for CO, previously applied to catalysis reactions, is proposed when Mn is incorporated into the chromia scale.

Characteristics of Hydrogen Sensors Based on Thin Tin Dioxide Films Modified with Platinum, Palladium, Silver, and Yttrium

The dependence of the characteristics of hydrogen sensors based on thin Pt/Pd/SnO2:Sb, Ag, Y and Ag/SnO2:Sb, Ag, Y films obtained by the method of magnetron sputtering at a direct current on the humidity level of the gas mixture during operation in thermal cyclic mode is investigated. The analysis of the experimental data was carried out using the expressions obtained on the basis of the dissociative adsorption model of hydrogen and water molecules on the SnO2 surface, taking into account the presence of three types of adsorbed particles: O–, OH, OН–. The use of complex modifiers Pt/Pd, Ag on the surface and Ag + Y in the bulk of tin dioxide films allows to reduce the influence of humidity on the characteristics of sensors. The role of these modifying additives in the process of dissociative adsorption of hydrogen molecules on the surface of SnO2 films in humid environment is established.

Structural and Electronic Properties of Different Terminations for Quartz (001) Surfaces as Well as Water Molecule Adsorption on It: A First-Principles Study

Structural and electronic properties of Si termination, O-middle termination, and O-rich terminations of a quartz (001) surface as well as water molecule adsorption on it were simulated by means of density functional theory (DFT). Calculated results show that the O-middle termination exposing a single oxygen atom on the surface is the most stable model of quartz (001) surface, with the lowest surface energy at 1.969 J·m−2, followed by the O-rich termination and Si termination at 2.892 J·m−2 and 2.896 J·m−2, respectively. The surface properties of different terminations mainly depend on the surface-exposed silicon and oxygen atoms, as almost all the contributions to the Fermi level (EF) in density of states (DOS) are offered by the surface-exposed atoms, especially the O2p state. In the molecular adsorption model, H2O prefers to adsorb on the surface Si and O atoms, mainly via O1–H1 bond at 1.259 Å and Si1–Ow at 1.970 Å by Van der Waals force and weak hydrogen bond with an adsorption energy of −57.89 kJ·mol−1. In the dissociative adsorption model, the O-middle termination is hydroxylated after adsorption, generating two new Si–OH silanol groups on the surface and forming the OwH2···O4 hydrogen bond at a length of 2.690 Å, along with a large adsorption energy of −99.37 kJ·mol−1. These variations in the presence of H2O may have a great influence on the subsequent interfacial reactions on the quartz surface.

A new solution of Langmuir kinetic model for dissociative adsorption on solid surfaces

In this Letter, a new simple rate equation is derived to describe the rate of dissociative adsorption on solid surfaces. This rate equation is obtained according to Langmuir model for dissociative adsorption. This new equation is simple and easy to analyze. To analyze the accuracy of the new equation, several sets of hypothetical kinetic points were generated. The created points proved that results of the new equation were in a perfect agreement with our theoretical derivation. Analysis of experimental data confirms the results of the present study. Theoretical and experimental data confirmed the applicability of the new equation.

Solvability of a mathematical model of dissociative adsorption and associative desorption type

Abstract A mathematical model of dissociative adsorption and associative desorption for diatomic molecules is generalized. The model is described by a coupled system of parabolic and ordinary differential equations. The existence and uniqueness theorem of the classical solution is proved.

On a mathematical model of dissociative adsorption and associative desorption

A mathematical model of dissociative adsorption and associative desorption for diatomic molecules is considered. The model is described by a system of parabolic and ordinary differential equations. The existence and uniqueness theorem of the classical solution is proved.

The Y-Procedure methodology for the interpretation of transient kinetic data: Analysis of irreversible adsorption

This paper deals with the analysis of transient kinetic data obtained in the Thin-Zone (TZ) configuration of the Temporal Analysis of Products (TAP) reactor. The Y-procedure that reconstructs the reaction rate and gas composition in the catalytic zone with no assumptions on the reaction mechanism ( Yablonsky et al., 2007) was applied to irreversible adsorption with coverage-dependent kinetic coefficients. We have used the Y-Procedure analysis to develop a novel framework for the interpretation of non-steady-state kinetic data. This framework was illustrated by multiple numerical examples concerning irreversible adsorption, and by experimental data corresponding to oxygen adsorption on polycrystalline platinum. The total concentration of active sites was estimated as 6 × 10 − 7 mol / g cat . It was found that for the surface coverage up to 0.8, kinetics followed the dissociative adsorption model with an intrinsic adsorption constant of 1.12 × 10 6 m 3 / mol / s . Kinetic parameters estimated via the Y-procedure agreed well with the values obtained via the moment analysis. The methodology developed in this paper will guide the systematic application of the Y-Procedure analysis to more complex catalytic systems.

Low Temperature Formation of Iron Carbide from Iron Oxide with CO‐H2 Gas Mixtures

Commercially pure iron oxide in the form of fine particles were reacted with CO‐H2 gas mixtures at ambient pressure (≍ 0.8 atm) in the temperature range 573 to 1073K. Reduction to metallic iron was carried out with pure hydrogen. The rate of formation of iron carbide was measured by recording the weight change with a thermogravimetric apparatus. The results obtained indicate that for each gas phase composition a maximum rate was observed, at apparently the same temperature. These maxima in the rate occur at a lower temperature than those reported in previous investigations. A dissociative adsorption model, based on the reduction of CO with hydrogen was developed which qualitatively describes the observed results.

Computer modeling of dissociative gas adsorption on laser-roughened surfaces

A simple computer model of dissociative adsorption of diatomic molecules on a solid surface with laser-induced defects was proposed. The defects (ablation craters) were assumed to have either cubicoid or pyramidal shape, depending on the approximation level. Special attention was paid to the influence of a degree of structural disorder on the adsorptive properties of the surface. In particular, both equilibrium adsorption isotherms and temperature programmed desorption (TPD) spectra of non-interacting diatomic molecules from the surfaces subjected to a different number of laser pulses were simulated. The observed changes in the adsorptive properties of the surface were explained using simple geometric arguments linking the adsorption probability for a single molecule with the topography of the surface. For example, it was demonstrated that, for a sufficiently large number of laser pulses ( N), the adsorption probability scales with 1 / N , regardless of the assumed crater shape. The obtained results also indicate that, in general, the surface roughness greatly affects the TPD spectra while it has minor influence on the shape of the adsorption isotherms.

Predicting the activation energy of catalytic dissociation of the heteroatomic AB bond

Two analytical formalisms, adiabatic and diabatic ones, were developed for the description of catalytic dissociation of heteroatomic bond AB interacting with a metal surface in the adsorption processes. In the adiabatic formalism, the transition state was localized on a four-dimensional potential energy surface in classical approximation. This approach generalizes the previous three-dimensional model for dissociative adsorption of homonuclear molecules X 2 on metals surfaces, and it was used for studying the effect of non-parallel orientation to a surface of O 2 molecules in the adsorption precursor state. The second formalism takes into account a possible quantum character of vibrations along the chemical bond AB. The calculation of the activation energy in this approach is performed by the density matrix method. This approach is applied for studying catalytic dissociation of CO molecule on a Ni(1 1 1) surface. The calculated apparent activation energy for this reaction is compared with published data for this system.

First-principles study of the adsorption of PH3 on Ge(001) and Si(001) surfaces

Using a first-principles pseudopotential method we have compared the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(001)-(21) and Ge(001){(21) surfaces. We find that the dissociated state is energetically more favourable than the molecular state by 1.70(0.81) eV, whereas the latter is 0.58(0.25) eV more stable than the system composed of the free silicon(germanium) surface and PH3(g). The chemisorbed system is characterised by elongated Si{Si(Ge{Ge) dimers that are symmetric in the dissociative case and asymmetric in the molecular case and by the fact that the Si(Ge){PH2 as well as the PH3(ads) groups retain the pyramidal geometry of the phosphine molecule. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.

First-principles calculations of the adsorption and dissociation of PH3 on Si(0 0 1)-(2×1)

Using a first-principles pseudopotential method we have studied the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(001)-(2×1) surface. We have found that phosphine adsorbs molecularly on one side of the Si–Si dimer and, at temperatures around 110 K, fully dissociates into PH2+H, with each component attached to one side of the dimer. After dissociation, the surface is characterized by an elongated dimer, symmetric for the dissociated case and asymmetric for the molecular case. The H–P–H angles and H–P bond lengths for the dissociative case have exactly the same values as obtained for the PH3 molecule. However, for the molecular case, while the H–P bond lengths are the same as observed for the PH3 molecule, the H–P–H angles are ∼8% bigger. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.

Dissociative adsorption ofPH3on the Si(001) surface

Using a first-principles pseudopotential method we have studied the adsorption and dissociation of the common n-type dopant molecule ${\mathrm{PH}}_{3}$ on the Si(001) surface. We have found that for low phosphorus coverages $(\frac{1}{4}$ monolayer) phosphine adsorbs molecularly on one side of the Si-Si dimer and, at temperatures around 140 K, fully dissociates into ${\mathrm{PH}}_{2}$ and H, with each component attached to one side of the dimer. For higher phosphorus coverages $(\frac{1}{2}$ monolayer) the interaction between adjacent dimers plays a decisive role in the dissociation process. For both coverages, the surface is characterized by an elongated dimer, symmetric for the dissociated case and asymmetric for the molecular case. The H-P-H angles and H-P bond lengths for the dissociative case are very close to those obtained for the ${\mathrm{PH}}_{3}$ molecule. However, for the molecular case, while the H--P bond length is close to that observed for the ${\mathrm{PH}}_{3}$ molecule, the H-P-H angle is $\ensuremath{\sim}8%$ bigger. Available experimental scanning tunneling microscopy image results are reinterpreted using theoretical images for the model provided in this work. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental work.

Kinetic studies using temperature-scanning: the oxidation of carbon monoxide

We report on an experimental investigation of carbon monoxide oxidation over a 0.05% Pt/γ-Al 2O 3 catalyst using a temperature scanning reactor (TSR) to increase the rate of data acquisition. Temperature scanning (TS) allows us to complete an experimental study of the kinetics of this reaction on one catalyst, at one pressure and feed composition, in less than one working day of fully automated reactor operation. Real-time measurement of CO conversion was done using a quadruple mass spectrometer (MS). The kinetics of this reaction were quantified by using ∼12,300 conversion–rate–temperature ( X, r, T) triplets calculated from raw data obtained using our TS-PFR, and fitting those with two proposed mechanistic rate equations. Due to the large volume of data and the methods of interpretation used in temperature scanning, we will present our results not only in terms of the traditional curves in the conversion–space time ( X, τ) plane but also nitroduce the concept of three-dimensional kinetic surfaces, such as the ( X, r, T) surface. The experimental rate data were correlated using two mechanistic rate models: 1. the Langmuir–Hinshelwood dual site molecular adsorption model (MAM), and 2. the Langmuir–Hinshelwood dual site dissociative adsorption model (DAM). The models differ only in their view of the state of the adsorbed oxygen. The DAM model, which involves the reaction of oxygen atoms with carbon monoxide molecules, both adsorbed on the same type of site, was found to fit our experimental data over a broader range of reaction conditions and feed compositions. The MAM model, which presumes a reaction of carbon monoxide molecules with adsorbed molecular oxygen, can fit experimental rates well for individual feed ratios but not over the range of feed ratios used in our study. We therefore, propose that the rate-controlling step in this mechanism consists of a reaction between adsorbed carbon monoxide molecules and adsorbed oxygen atoms.

Ab-initio study of the adsorption of H 2S onto the Si(001) surface

The adsorption of H 2S on to the Si(001)-(1×2) surface is studied, based upon ab-initio pseudopotential calculations. We have used a dissociative adsorption model with adsorbed species (SH) − and H +. The sulphur atom is located along the dangling bond direction on one component of the Si dimer, the H atom in the (SH) − complex is bonded to the S atom, and H + is bonded along the dangling bond direction of the other dimer component. The adsorption of the molecule removes the buckling of the Si dimer. Our calculated bond lengths for Si–S, Si–H and S–H are 2.15, 1.53 and 1.41 Å, which are very close to the sum of their corresponding covalent radii. We find that the adsorption of H 2S passivates the Si(001) surface. The system is characterized by the presence of a single surface state just below the bulk valence band maximum.

Theoretical study of the GaAs(110)-(1×1)–H 2S surface

Ab initio calculations, based on pseudopotentials and the density functional theory, are performed for H 2S adsorption on the GaAs(110)-(1×1) surface to investigate atomic geometry, electronic states and bonding within the “dissociative adsorption model”. We find that the fundamental band gap is free of surface states, indicating that the adsorption of H 2S passivates the GaAs(110) surface, in agreement with recent experimental results. We also find two characteristic electronic states in the bulk stomach gap region, due to the bonding between As and H +, and between Ga and SH −. The energy location of these states is in good agreement with angle-resolved photoelectron emission data.

Ab initiostudy of atomic geometry, electronic states, and bonding for H2S adsorption on III-V semiconductor (110)-(1×1)surfaces

Ab initio calculations, based on pseudopotentials and the density-functional theory, have been made to investigate the equilibrium atomic geometry, electronic states, and bonding of the H${}_{2}$S molecule on the InP(110), GaAs(110), and GaP(110) surfaces within a dissociative adsorption model. In general, the adsorption of H${}_{2}$S on the three semiconductors shows similar behavior. The calculated average distances between the topmost InP, GaAs, and GaP layer and the sulfur atom are 1.87, 1.67, and 1.65 \AA{}, respectively. In all the cases studied here, the fundamental band gap is free of surface states, with an occupied surface state close to the valence-band maximum. The calculated electronic states are in agreement with reported angle-resolved photoemission data.

An experimental and theoretical study of the interaction of CH3OH and CH3SH with ZnO

The adsorption of CH3SH on ZnO surfaces has been studied by means of X-ray photoelectron and IR spectroscopies. The CH3SH uptake was measured as a function of exposure and temperature. The adsorption is mainly dissociative, even though there is evidence for adsorbed molecular species at room temperature on the ZnO(0001) polar surface. As a whole, experimental results are in agreement with the usual model of dissociative adsorption of Bronsted acids on oxides. Decomposition of adsorbed thiolate species to sulfide occurs to a significant extent above 523 K. The interaction of CH3SH/CH3S– with the Lewis-acid site available on the ZnO(0001) surface has also been theoretically studied by coupling the molecular cluster approach to density functional theory. Calculations relative to the adsorption of CH3OH/CH3O– have also been carried out for comparison. The bonding between the CH3O–/CH3S– species and the Lewis-acid site is confirmed to play a leading role in determining the actual relative acidity scale on ZnO. As far as the molecular structure of the adsorbed species is concerned, the C—S bond is perpendicular to the surface, while the O atom of the CH3O– fragment is not on top of the Lewis-acid site and a non-negligible bonding interaction between the methyl group and the surface is found. These data agree very well with IR measurements which show, for the C—H stretching frequencies, significantly different red shifts for the adsorbed species with respect to the free molecules. Comparison with results recently published by us concerning the adsorption of H2O, HCN and H2S on ZnO(0001) indicate that, according to titration displacement reactions, the theoretical scale of the relative acidity is HCN > H2S > CH3SH > H2O > CH3OH.

Modelling of non‐stationary ammonia synthesis kinetics over an iron catalyst: I. Deriving a model and estimating parameters from transient response data

AbstractThe transient reaction kinetic models of ammonia synthesis in a Berty reactor are identified and the parameters in the models are estimated from their transient responses. The basic model is founded on widely held views concerning the mechanism of ammonia formation on iron catalyst. The results obtained in this study show that a two‐site, dissociative adsorption model is acceptable while a competitive adsorption model is not.

Modelling of non‐stationary ammonia synthesis kinetics over an iron catalyst II. Modelling results of forced feed composition in periodic operation and for step‐changes

AbstractSimulation using the two‐site dissociative adsorption model given in Part 1 of this study fails to predict the improvement observed in the time‐average rate under cyclic operation even though it does recreate the observed steady‐state and transient response features following a step‐change in the feed composition. The improvement in the time‐average cycling rate is probably due to the diffusion of nitrogen into the bulk‐phase of the iron catalyst, which is a metastable nitrogen? Iron system under forced feed composition cycling. A two‐site adsorption model, modified to take account of the inventory of nitrogen in the bulk‐phase of the iron catalyst, is presented. This model predicts the improvements in ammonia production observed under periodic operation.