Steady-state Surface Coverage Research Articles

Methanol to hydrocarbons conversion: Why dienes and monoenes contribute differently to catalyst deactivation?

Dienes, formed at much lower concentrations than monoenes via alkylation of monoenes with formaldehyde (HCHO), have much higher propensity than monoenes to foment catalyst deactivation in methanol-to-hydrocarbons (MTH) conversion over zeolites via HCHO-mediated deactivation pathways. The mechanistic basis for why monoenes and dienes contribute differently to cause catalyst deactivation during MTH is investigated using independent kinetic studies of HCHO reactions with representative monoenes and dienes, i.e., propylene and 1,3-butadiene, over zeolites. Despite 3,6-dihydro-2H-pyran being the predominant product for both propylene/HCHO and 1,3-butadiene/HCHO reactions, a combination of transient and steady-state rate measurements and stoichiometric experiments reveals distinct persistent intermediates being involved in the addition of HCHO to different surface-bound alkyl species derived from propylene and butadiene. These persistent intermediates desorb only via HCHO- and water-mediated proton transfer steps and as such can be sequestered on the catalyst. A nonlinear mixed-effects kinetic model reveals that the intermediate formed during HCHO-diene reactions has a higher steady-state surface coverage and its desorption via HCHO- and water-mediated routes exhibit > 8× and > 3×, respectively, higher rate constants than those during HCHO-monoene reactions. These results evidence the distinct identity, surface coverage, and reactivities of surface-bound alkyl intermediates as critical factors in the much more pernicious effects of dienes compared to monoenes on catalyst lifetime during MTH.

Methane dry reforming on supported cobalt nanoparticles promoted by boron

Stable operations for catalytic dry reforming of methane (DRM) are essential for industrial applications. High stability for syngas production can be achieved via a kinetic balance between formation of carbon species and their removal by oxygen species on the metal surface, which clears the surface for further reaction steps. This study reports highly stable performance by a boron-doped cobalt catalyst as a non-noble-metal coking-free catalyst. Although the precise location of doped boron could not be identified experimentally because of its low concentration, density functional theory calculations suggested that interstitial boron (B) is most likely present in the subsurface region of cobalt (Co) surfaces. B-doping was shown both experimentally and computationally to increase the reactivity of Co catalysts toward both methane (CH4) and carbon dioxide (CO2). Moreover, B-doping was found to balance the amounts of surface C and O and maintain the reduced state of Co surfaces while in a steady-state. Nevertheless, a negative kinetic order with respect to CO2 partial pressure indicates that steady-state surface coverage of oxygen species originating from CO2 dissociation was prevalent on B-doped Co, consistent with the coking-free nature of the catalyst. This study introduces a promising Co–B catalyst design for controlling metal surface reactivity toward DRM and relevant catalytic reactions.

Altering the selectivity of galvanostatic CO2 reduction on Cu cathodes by periodic cyclic voltammetry and potentiostatic steps

Analysis of the product selectivity and potential of Cu cathodes during galvanostatic CO2 reduction is reported. Initially, it is found that clean Cu cathodes are more selective for hydrogen evolution, but as the Cu is slowly poisoned by CO2 reduction products (most likely carbon) the cathode potential becomes more negative, which in turn drives the formation of CH4, C2H4 and CO. As the accumulation of surface poisons continues, the selectivity towards CH4 and C2H4 begins to decrease due to the loss in neighbouring reaction sites that support the hydrogenation of COads by Hads. In an attempt to avoid these changes in product selectivity, periodic cyclic voltammetry and potentiostatic steps were used throughout extended periods of galvanostatic CO2 reduction. Contrary to previous literature, it is demonstrated that temporarily interrupting galvanostatic CO2 reduction with short periods at potentials between −0.5 and −0.1V vs Ag|AgCl suppresses the formation of CH4, CO and C2H4. It is proposed that this is due to the partial removal or oxidation of adsorbed CO2 reduction intermediates and that this “clean” cathode surface is more active for the hydrogen evolution reaction. However, when brief potentiostatic steps (84 and 200s) were conducted at more negative potentials (−1.2V vs Ag|AgCl), the CO2 reduction selectivity could be switched from CH4 to CO, and maintained for at least 2hours. This change in selectivity is proposed to be caused by an increase in the surface coverage of COads (at the expense of Hads) during the brief −1.2V steps, which then enables the Cu cathode to switch between multiple steady-state surface coverages when the cathodic current is re-applied.

The effects of hydrogen dilution, carbon monoxide poisoning for a Pt–Ru anode in a proton exchange membrane fuel cell

The effects of carbon monoxide (CO) poisoning and hydrogen dilution are investigated for a series of simulated reformate gases. A simple mathematical model with an internal air bleed term is proposed to understand the interaction between CO poisoning, hydrogen dilution, and the internal air bleed (IAB). According to the steady-state modeling results, the presence of nitrogen can be considered as a diluting agent for a relatively dilute H2–N2 mixture. For a CO–H2–N2 mixture, hydrogen dilution and CO poisoning “magnify” each other in their combined effect on the surface coverage of hydrogen. The beneficial effect of IAB was found to be affected by the competitive adsorption between CO and hydrogen. The steady-state surface coverage of CO is influenced by the adsorption and desorption of CO, the interaction between CO and the electro-oxidation of hydrogen, the interaction between IAB/hydrogen dilution/CO, the interaction between IAB/CO. However, the steady-state surface coverage of CO is largely determined by its own adsorption and desorption properties. Analytical solutions are also derived from the proposed mode for the transient surface coverage of hydrogen and CO. The mathematical treatment has been proved extremely effective for the understanding of the steady-state and transient behavior of the CO–H2 and CO–H2–N2 mixtures.

Structure sensitivity in oxide catalysis: First-principles kinetic Monte Carlo simulations for CO oxidation at RuO2(111).

We present a density-functional theory based kinetic Monte Carlo study of CO oxidation at the (111) facet of RuO2. We compare the detailed insight into elementary processes, steady-state surface coverages, and catalytic activity to equivalent published simulation data for the frequently studied RuO2(110) facet. Qualitative differences are identified in virtually every aspect ranging from binding energetics over lateral interactions to the interplay of elementary processes at the different active sites. Nevertheless, particularly at technologically relevant elevated temperatures, near-ambient pressures and near-stoichiometric feeds both facets exhibit almost identical catalytic activity. These findings challenge the traditional definition of structure sensitivity based on macroscopically observable turnover frequencies and prompt scrutiny of the applicability of structure sensitivity classifications developed for metals to oxide catalysis.

Interaction of Uranyl with Calcite in the Presence of EDTA

Adsorption of uranyl at the surface of calcite was investigated by using batch sorption experiments and synchrotron X-ray standing wave (XSW) measurements. Aqueous solutions containing 236U(VI) (4.5 x 10(-7) to 1.0 x 10(-4) M) and EDTA (5.0 x 10(-7) to 1.1 x 10(-4) M) were reacted for 90 s to 60 min with freshly cleaved calcite (104) surfaces and calcite powders. Surface exchange coefficients, sorption kinetics, and influence of powder surface area/solution volume (SA/V) ratio were investigated by alpha-counting of 236U. Powder sorption results at SA/V = 870 cm2/mL fit a Freundlich isotherm [log [U]surface (in monolayers) = log K + n log [U]aq (in moles/L)], where K = 1.9+/-0.5 and n = 0.9+/-0.1, consistent with uptake of U(VI) by a specific surface reaction where the availability of sorption sites is nonlimiting in the U concentration range measured. Measured U(VI) coverages along this isotherm, based on the calcite (104) surface Ca site density, ranged from 0.04% to 5.4% of a monolayer. Steady state surface coverages were obtained within 90 s. Sorption of U(VI) on calcite (104) single-crystal cleavage surfaces using identical solutions yielded higher coverages, because of increased step density induced by dissolution at the relatively low SA/V ratio (approximately 1) of these measurements. The crystallographic location of the sorbed U(VI) was examined with the synchrotron XSW technique. Measurements were performed at the Advanced Photon Source on fresh calcite (104) cleavage surfaces reacted for 90 s with U(VI) solutions. Coherent fractions for sorbed U ranged from 0.14 to 0.62, and the mean value of the U coherent position was 0.84+/-0.02. This position was independent of dissolved U(VI) concentration and corresponds to a distance between the U atom and the calcite (104) plane of 2.55+/-0.06 A. These results are consistent with U(VI) adsorption atthe calcite surface as an inner-sphere uranyl-carbonate surface complex bonded with the outer oxygen atom(s) of a single surface carbonate group. Steric considerations allow this observed U(VI) surface complex to occur both at step sites ((441)_ and (481)_) and on terrace areas adjacent to Ca vacancies.

Adsorption Kinetics of 1-Alkanethiols on Hydrogenated Ge(111)

We have investigated the liquid-phase self-assembly of 1-alkanethiols (HS(CH2)n-1CH3, n = 8, 16, and 18) on hydrogenated Ge(111), using attenuated total reflection Fourier transform infrared spectroscopy as well as water contact angle measurements. The infrared absorbance of C-H stretching modes of alkanethiolates on Ge, in conjunction with water contact angle measurements, demonstrates that the final packing density is a function of alkanethiol concentration in 2-propanol and its chain length. High concentration and long alkyl chain increase the steady-state surface coverage of alkanethiolates. A critical chain length exists between n = 8 and 16, above which the adsorption kinetics is comparable for all long alkyl chain 1-alkanethiols. The steady-state coverage of hexadecanethiolates, representing long-chain alkanethiolates, reaches a maximum at approximately 5.9 x 10(14) hexadecanethiolates/cm2 in 1 M solution. The characteristic time constant to reach a steady state also decreases with increasing chain length. This chain length dependence is attributed to the attractive chain-to-chain interaction in long-alkyl-chain self-assembled monolayers, which reduces the desorption-to-adsorption rate ratio (kd/ka). We also report the adsorption and desorption rate constants (ka and kd) of 1-hexadecanethiol on hydrogenated Ge(111) at room temperature. The alkanethiol adsorption is a two-step process following a first-order Langmuir isotherm: (1) fast adsorption with ka = 2.4 +/- 0.2 cm3/(mol s) and kd = (8.2 +/- 0.5) x 10(-6)(s-1); (2) slow adsorption with ka = 0.8 +/- 0.5 cm3/(mol s) and kd = (3 +/- 2) x 10(-6) s(-1).

Influence of phosphine on Ge/Si(001) island growth by chemical vapor deposition

When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge to form distinctive, three-dimensional islands. The shape of the islands is determined by the energies of the surface facets, facet edges, and interfaces. When phosphorus is added during chemical vapor deposition of Ge, the surface energies change, modifying the island shapes and sizes. Three different island shapes are found for doped layers, as for undoped layers; however, each doped island type is smaller than the corresponding undoped island type. The intermediate-size doped islands are of the same family as the undoped multifaceted “dome” structures, but are considerably smaller; they also have a narrow size distribution. The largest doped islands are related to the defective “superdomes” found for undoped islands, but are bounded by a smaller number of facets, creating pyramidal-shaped structures with their edges aligned along 〈110〉 directions. The distribution of Ge among the different island types depends on the phosphine partial pressure. Phosphorus appears to act as a mild surfactant, suppressing small islands at high PH3 partial pressures. Within the assumptions made, the segregation enthalpy is estimated to be −0.4 eV. Phosphine decreases the Ge deposition rate because of competitive adsorption; however, the steady-state surface coverage (as indicated by the Ge deposition rate) is not reached for thin layers.

Spectroscopic and Transient Kinetic Studies of Site Requirements in Iron-Catalyzed Fischer−Tropsch Synthesis

The structure, reduction/carburization, and catalytic performance of K- and Cu-promoted Fe2O3 during initial contact with synthesis gas were examined by combining kinetic analysis of the initial stages of Fischer−Tropsch synthesis (FTS) with X-ray absorption spectroscopy. Oxygen removal initially occurs without FTS reactions as Fe2O3 is reduced to inactive oxygen-deficient Fe2O3 species. Hydrocarbon synthesis reactions become detectable only as Fe3O4 forms and rapidly converts to FeCx. FTS reactions require only the incipient conversion of the surface layers to a dynamic and active surface phase, which consists of FeCx with steady-state surface coverages of vacancies, formed via oxygen and carbon removal during the formation of monomers, CO2, and H2O. Such surfaces tend to respond to changes in the contacting gas phase within turnover times by changing the relative surface concentration of carbon, oxygen, CO, and hydrogen. The catalytic behavior of these dynamic surfaces is largely independent of the carb...

Self-assembled monolayers in the context of epitaxial film growth

Self-assembled monolayers of amphiphilic surfactant molecules form spontaneously on solid surfaces by exposure to dilute solutions of the adsorbate molecules. We report observations of the growth and desorption/dissolution processes for monolayers of octadecylphosphonic acid (OPA, CH3(CH2)17PO(OH)2) on mica. We find that these monolayers form via a mechanism that includes nucleation, growth, coalescence, etc. of densely-packed submonolayer islands of the long-chain organic molecules. In situ AFM measurements allow a quantitative analysis of island nucleation and growth rates as well as determination of the island size distribution as a function of coverage. In the growth regime, the nucleation and growth rates have a power law behavior consistent with a simple point island model of 2D cluster growth. In the aggregation regime, the island size distributions are shown to scale with a single evolving length scale in accordance with the dynamical scaling approximation. The desorption/dissolution process proceeds by nucleation of holes in the monolayer that grow and percolate across the sample, leaving isolated islands that gradually decrease in size. The relative rates of hole growth and hole nucleation suggest that removing a molecule from the monolayer/hole boundary is about 5×104 times more likely than removing a molecule from within a continuous region of monolayer. The coverage kinetics during dissolution can be quantitatively described by a model that incorporates desorption from “hole” regions and diffusive solution-phase transport through a stagnant layer of finite thickness that can be altered by solvent flow or stirring. If the monolayer is brought into contact with a small enough volume of stagnant solvent, the surface coverage eventually stabilizes due to the buildup of adsorbate molecules in solution. Under these conditions of steady-state surface coverage, the local dynamical processes of island shrinkage, growth, and nucleation continue, eventually leading to a distinctive (decaying exponential) island size distribution characteristic of the system.

Predictions of Relationships between Catalytic and Solid Phase Properties by Kinetic Models and Their Validation

The paper illustrates that realistic predictions about steady-state coverages and stability of surface species can be obtained by kinetic analysis. As an example, oxidative coupling of methane to ethane over CaO/CeO2 catalysts is considered. For this reaction, it was earlier predicted by a kinetic model that the steady-state surface coverage with OH groups, oxygen, and vacant sites depends on the CaO content of the mixed oxide catalysts. Now, the surface of solid solutions of CaO in CeO2 (5, 10, and 20 mole% CaO), in which the incorporation of Ca into the CeO2 host lattice can be demonstrated by pronounced effects on the electrical conductivity, has been characterised by XPS, UPS, ISS, and IR techniques. It was found that bulk and surface properties are interrelated but do not change proportionally. The oxygen anion conductivity increases and a change from n- to p-type conductivity occurs with increasing CaO-content. The external surface of the mixed oxides is strongly enriched in CaO, which forms CaCO3 under ambient conditions, but the degree of surface enrichment exhibits a marked minimum at 10% CaO. As predicted by former kinetic analysis in the oxidation of methane, this sample is capable of forming surface OH groups of exceptional thermal stability.

On the CO tolerance of novel colloidal PdAu/carbon electrocatalysts

The synthesis and characterization (physical and electrochemical) of novel PdAu/Vulcan XC 72 electrocatalysts are described. The catalysts were prepared via deposition and activation of preformed bimetallic colloidal precursors. Physical characterization of the catalysts involved high-resolution transmission electron microscopy and powder X-ray diffraction. Electrochemically, the PdAu-catalysts were characterized by means of the thin-film rotating disk method and by differential electrochemical mass spectrometry. H 2, CO and CO/H 2 oxidations on PdAu/Vulcan electrodes with different bulk compositions were used as probe reactions to determine the CO tolerance of this catalyst system and its physical origin. We propose that the CO-tolerance mechanism at low overpotentials for PdAu/Vulcan electrodes is governed by both a lower CO steady-state surface coverage compared to PtRu/Vulcan due to a lower CO adsorption energy and finite CO oxidation rates, which both result in free surface sites for H 2 oxidation to take place. At elevated temperatures (60°C) this effect is more pronounced, providing more free active Pd sites for H 2 oxidation. CO/H 2 oxidation measurements (1000 and 250 ppm CO) showed the superior activity of PdAu compared with a state-of-the-art PtRu catalyst at technically relevant potentials, especially in Pd-rich alloys.

Transient kinetics and mechanism of oxygen adsorption over oxide catalysts from the TAP-reactor system

For elucidating the mechanism of oxygen adsorption and its effect on selectivity in the oxidative coupling of methane (OCM) transient experiments were carried out in the TAP-2 reactor by pulsing oxygen over Na2O/CaO catalysts at temperatures between 623 and 873 K. The response signals were fitted to three different models for oxygen adsorption. Model discrimination showed that only a reversible and dissociative adsorption via the molecular precursor provides a good description of the transient oxygen responses over all catalysts studied. Rate constants and activation energies of the elementary reaction steps of oxygen adsorption, desorption, dissociation and association were estimated. Doping CaO with sodium oxide influenced the ratio of kads/kdis determining the coverage of the catalyst surface with molecular and atomic oxygen. The steady-state surface coverages with molecular and atomic adsorbed oxygen species were simulated for different partial oxygen pressures (0.5–15 kPa) using the kinetic parameters from transient experiments. These results may, however, be affected by extrapolating the pressure from 10-4 Pa to 15 kPa. It was derived that an increase of C2 selectivity in OCM on Na2O/CaO can be ascribed to a decrease in the coverages of adsorbed molecular oxygen, which appears to be a plausible interpretation confirming previous findings of the dependence of C2 selectivity on oxygen partial pressure.

Electrophoretic Deposition of Polymer Chains on an Adsorbing Surface in(2+1)Dimensions: Conformational Anisotropy and Nonuniversal Coverage

Electrophoretic deposition of polymer chains flowing in a $(2+1)$-dimensional system is studied by computer simulations. Steady-state surface coverage $({\ensuremath{\theta}}_{j})$ is found to decay with the chain's length, i.e., ${\ensuremath{\theta}}_{j}\ensuremath{\sim}{L}_{c}^{\ensuremath{-}\ensuremath{\alpha}}$ with a nonuniversal exponent $\ensuremath{\alpha}\ensuremath{\simeq}0.0--0.9$ depending on the magnitude of driving field and temperature. Conformational crossover occurs for chains from a surface or wall to an adjacent bulk region with different scaling exponents for their longitudinal and transverse spread.

Kinetics and Mechanism of Thiophene Hydrodesulfurization over Carbon-Supported Transition Metal Sulfides

Results of a detailed kinetic study on the thiophene hydrodesulfurization reaction at atmospheric pressure over a set of carbon-supported transition metal sulfides, i.e., the sulfides of Co, Mo, Rh, and the mixed CoMo sulfide, are presented. It is found that (partially) hydrogenated thiophenes, i.e., 2,3-dihydrothiophene, 2,5-dihydrothiophene, and tetrahydrothiophene, are important intermediates in the reaction mechanism. The reaction orders of thiophene suggest that carbon–sulfur bond cleavage is rate limiting for most of the catalysts. The CoMo catalyst may have hydrogenative sulfur removal as the rate limiting step. This catalyst shows a strong decrease in apparent activation energy with temperature to be ascribed to a large change in steady state surface coverage by thiophene (or H2S) as a function of temperature. This is consistent with a strong interaction between catalyst and thiophene. The Rh catalyst most probably shows a phase transition leading to different kinetic parameters. A strong interaction between the metal sulfide and thiophene is important for a high HDS activity.

Scaling of Si and GaAs trench etch rates with aspect ratio, feature width, and substrate temperature

The scaling of etch rates with feature dimensions is an important issue in the fabrication of microelectronic and photonic devices. Because etch rates depend on circuit layouts and design rules, considerable effort is spent to modify processes each time changes in design are made. Knowing how etch rates scale with design parameters should accelerate the introduction of new designs into manufacturing while minimizing the cost of doing so. Recently it has been shown that etch rates for a variety of conditions scale with the depth/width or aspect ratio and not on width or depth alone. While various mechanisms might be responsible for such scaling, isolating one mechanism from another is not straight forward. Nonetheless, it is important to understand the underlying mechanisms so that differences in etch chemistry and etch reactor design can be accounted for when scaling plasma processes from one design or reactor to another. To assess the effects of etch chemistry alone, the trench etch rates of Si and GaAs are compared under constant plasma conditions. Substrate temperature is varied to further assess the relative importance of surface vs transport phenomena during the etching process. At higher temperatures, both Si and GaAs trench etch rates scale only with aspect ratio in an Ar/Cl2 electron cyclotron resonance plasma. The results are consistent with an ion-neutral synergy model based on Langmuir adsorption kinetics where the scaling can be explained by the aspect ratio dependence of the neutral reactant transport into the trench: charging effects, ion shadowing, and Knudsen transport are not consistent with the data. At −45 °C, the etch rate no longer scales with aspect ratio alone, but now also depends on trench width. The data at this lower temperature are well described by a model incorporating the deposition of an etch inhibiting layer. While the trench etch rates for GaAs and Si scale in similar ways with aspect ratio and trench width, the dependence on feature dimensions of the Si etch rate is much stronger than that for GaAs at all substrate temperatures because the steady-state surface coverage of Cl is smaller for Si. This results in a greater sensitivity to the incoming, aspect ratio dependent, neutral fluxes. The implications of the models on the goal of aspect ratio independent etching are also discussed.

The oxidation of CO over Ru(001) at high pressures: CO residence times and reaction mechanism

In order to elucidate the unusual catalytic behavior of ruthenium in the oxidation of CO, we have studied this reaction on a Ru(001) single crystal surface at high pressures with in-situ Fourier Transform-Infrared Reflection Absorption Spectroscopy (FT-IRAS). From the measurement of the steady-state surface coverages we are able to obtain estimates of mean residence times of the reactants. Under highly oxidizing conditions, where an O-(1×1) layer is present at the surface and the CO 2 formation rate exhibits a maximum, we find CO residence times in the order of 10 −12 s. This suggests that the oxidation of CO may occur by direct reaction between gas-phase CO and chemisorbed oxygen atoms.

Work function of metallic surfaces bombarded with cesium ions

Reduction of work function due to the build-up of cesium coverage on beryllium, molybdenum, and tungsten caused by cesium ion bombardment was investigated for incident energies between 5 eV and 1 keV. Surface characterization was performed as a function of incident ion energy and dose using Auger spectra and work function measurements. A steady state surface coverage is achieved in all cases for a dose of less than 10 16 ions/cm 2. The coverage is strongly dependent on target mass. Beryllium has nearly monolayer coverage at all beam energies, while above 150 eV molybdenum has less than 50% coverage and tungsten has less than 20% coverage. The minimum steady state work function at 60% coverage can be achieved on molybdenum and tungsten at 100 and 45 eV, respectively. The effects of hydrogen coadsorption are also examined. The development of the surface coverage occurs due to the effects of implantation, reflection, and sputtering. A Monte Carlo calculation shows that a surface coverage of cesium on beryllium inhibits sputtering, which should be true for other light elements.

Photo-assisted dissolution of colloidal iron oxides by thiol-containing compounds: 2. Comparison of lepidocrocite (γ,-FeOOH) and hematite (α-Fe 2O 3) dissolution

Comparison of the photo-assisted dissolution of lepidocrocite (γ-FeOOH) and hematite (α-Fe 2O 3) in the presence of a series of α-mercaptocarboxylic acids under a variety of conditions of reactant concentration, temperature, and light intensity provides considerable insight into the relative importance of reactions at the oxide—solution interface in determining the rate of oxide dissolution. For hematite, the rates of ligand adsorption to the oxide surface and electron transfer within surface-located precursor complexes appear comparable, with low steady-state surface coverage of the oxide surface by thiol ligand. Low apparent activation energies of dissolution and the dramatic effect of light in increasing dissolution rate suggest that for lepidocrocite the rate of ligand uptake is rapid followed by relatively slow rate-determining electron transfer. The use of photolysis to selectively alter the rate of the electron transfer step appears to be an effective way of examining the mechanism of complex photo-assisted surface reactions.

TG study on the reaction of γ-Al 2O 3 by CCl 4. Part I. Kinetic model for the chlorination process

Thermogravimetric (TG) measurements were carried out to study the chlorination of γ-alumina by gaseous CCl 4 in the temperature range 650–1123 K at 10 2–10 4 Pa CCl 4 partial pressure. Experimental results indicate that the reaction involves two main steps: the substitution of oxygen atoms on the surface by chlorine leading to a mass gain and the volatilization of the main gaseous product, aluminium chloride, with a mass loss of the sample. These processes take place simultaneously during the course of the reaction. A kinetic model is proposed to describe these two steps providing a simple method for linearization of the initial part of the isothermal TG curves. Temperature and partial pressure dependence of the reaction rate and of the steady-state surface coverage are discussed on the basis of the proposed kinetic model. Activation energies and reaction orders are calculated for these two steps.