Dissociative Adsorption Of Methane Research Articles

Synthesis of MWCNTs by chemical vapor deposition of methane using FeMo/MgO catalyst: role of hydrogen and kinetic study

This study aims to investigate the role of hydrogen on CNTs synthesis and kinetics of CNTs formation. The CNTs were synthesized by catalytic chemical vapor deposition of methane over FeMo/MgO catalyst. The experimental results revealed that hydrogen plays an important role in the structural changes of catalyst during the pre-reduction process. The catalyst structure fully transformed into metallic FeMo phases, resulting in an increased yield of 5 folds higher than those of the non-reduced catalyst. However, the slightly larger diameter and lower crystallinity ratio of CNTs was obtained. The hydrogen co-feeding during the synthesis can slightly increase the CNTs yield. After achieving the optimum amount of hydrogen addition, further increase in hydrogen would inhibit the methane decomposition, resulting in lower product yield. The hydrogenation of carbon to methane was proceeded in hydrogen co-feed process. However, the hydrogenation was non-selective to allotropes of carbon. Therefore, the addition of hydrogen would not benefit neither maintaining the catalyst stability nor improving the crystallinity of the CNT products. The kinetic model of CNTs formation, derived from the two types of active site of dissociative adsorption of methane, corresponded well to the experimental results. The rate of CNTs formation greatly increases with the partial pressure of methane but decreases when saturation is exceeded. The activation energy was found to be 13.22 kJ mol−1, showing the rate controlling step to be in the process of mass transfer.

Intrinsic kinetics of low temperature methane steam reforming on Ru/La-Al₂O₃ catalyst

This study focuses on investigating the intrinsic kinetics of methane steam reforming over a lanthanum promoted Ru/La-Al₂O₃ catalyst at low temperatures ranging from 475 to 550 °C and 140 kPa. To facilitate the research, high-throughput experimentation was employed using 16 integral fixed bed reactors. The steam reforming reaction was influenced by co-reactants and products. In this work, we proposed a reaction mechanism and developed a kinetic model to understand the underlying process. The findings indicate that methane dissociative adsorption is the rate-determining step, and CO is identified as the most abundant surface intermediate. The developed kinetic model not only provides the best fit to the experimental data but also statistically significant and thermodynamically consistent.

Self-sustained oscillations in oxidation of methane over palladium: Experimental study and mathematical modeling.

An experimental study of the catalytic oxidation of methane over Pd foil in a flow reactor revealed that regular temporal oscillations in the reaction rate can arise at atmospheric pressure under methane-rich conditions. CO, CO2, H2, and H2O were detected as products. The oscillations of partial pressures of products and reactants in the gas phase were accompanied by oscillations of the catalyst temperature. According to an operando x-ray diffraction and mass-spectrometry study, the oscillations originate due to spontaneous oxidation and reduction of palladium; the high active catalyst surface is represented by metallic palladium, and the transition to the low-active state is accompanied by the formation of the PdO phase. In addition, it was detected that carbon dissolves in near-surface layers of palladium to form the PdCx phase. To describe the oscillations in the oxidation of methane, a 17-step reaction mechanism and a corresponding kinetic model were developed. The mechanism considers direct dissociative adsorption of methane and oxygen, pyrolytic activation of methane, oxidation and reduction of palladium, and direct formation and desorption of CO, CO2, H2, and H2O. Numerical solutions from the mathematical model of the continuously stirred-tank reactor qualitatively reproduce experimentally observed oscillatory dynamics. We have also developed a model, which considers the reversible diffusion of adsorbed oxygen and carbon atoms into the Pd bulk that allows us to explain the long induction period preceding the appearance of the oscillations. Mathematical modeling shows that the concentrations of dissolved oxygen and carbon atoms also oscillate under reaction conditions.

Computational Study of Zn Single-Atom Catalysts on In2O3 Nanomaterials for Direct Synthesis of Acetic Acid from CH4 and CO2

The direct conversion of methane and carbon dioxide will be a promising way to alleviate the greenhouse gas effect and reduce the dependency on traditional fossil fuels. In this study, the catalytic transformation of methane and carbon dioxide to acetic acid with a 100% atomic economy effect over the zinc single-atom catalyst supported on In2O3 nanomaterial with surface oxygen vacancy (Zn1/In2O3–x) was investigated by using density functional theory calculations, including dissociative adsorption of methane, C–C bond coupling, acetate species protonation, and desorption of acetic acid. The results indicate that the Zn single atom is beneficial to the dissociation of methane and the stabilization of methyl by forming the Zn–CH3 species. Besides, carbon dioxide can be adsorbed on the surface of the catalyst and activated further owing to the presence of surface oxygen vacancy. The facile C–C coupling of co-adsorbed methyl and carbon dioxide is realized by the synergistic mode combining the Zn single atom and surface oxygen vacancy following the Langmuir–Hinshelwood mechanism, in which a Zn–C–C moiety is formed in the transition state. The subsequently formed acetate species is bonded with Zn strongly and undergoes a protonation process to form acetic acid which also interacts strongly with the Zn single atom, resulting in desorption of acetic acid being the rate-determining step of the whole reaction. The insights in this work would be useful to clarify the reaction mechanism of producing acetic acid from methane and carbon dioxide over the Zn1/In2O3–x single-atom catalyst, which could provide a sufficient way to design efficient catalysts for methane and carbon dioxide conversion.

Enhancing oxygen activation on high surface area Pd-SnO2 solid solution with isolated metal site catalysts for catalytic CH4 combustion

A facile strategy of redox hydrothermal is developed to prepare highly dispersed Pd on SnO2 support as insertion compound or alloy with high surface area. By means of this method, the interaction between Pd and SnO2-x can be strengthened and results in synergistic effect between Pd center and the surrounding sites of the SnO2-x, which induces consequence changes in the physicochemical properties of the catalysts and, as a consequence, their catalytic performance and stability. The as-synthesized 3%Pd-SnO2 catalyst exhibits a remarkable reactivity of 14.34 × 104 mmol∙gcat-1∙s−1 and good stability for catalytic methane combustion, due to the high surface density of Pdδ+ species and oxygen vacancies. This later provides more active oxygen species and also to stabilize the highly dispersed Pdδ+ species which are active for the dissociative adsorption of methane. With correlation of structure and reactivity, it is revealed that such highly dispersed Pd species work synergistically with oxygen vacancy of the support to promote CH4 oxidation. This synergistic interaction also prevents catalyst deactivation which is of high interest for the future development of new generation of combustion catalysts.

Mechanistic insights of methane conversion to ethylene over gallium oxide and gallium nitride using density functional theory

Density functional theory (DFT) was used to study the reaction mechanisms of direct non–oxidative methane (CH4) coupling to ethylene (C2H4) on gallium–based catalysts, GaN (11¯00) and Ga2O3 (001) surfaces. The adsorption energies of the reactants, the potential intermediates, and the products were calculated on both surfaces. For each of the catalysts three mechanistic pathways have been proposed and discussed. Reaction energies and activation energy barriers for the reactions were calculated and compared. The dissociative adsorption of methane on both surfaces is exothermic. By comparing the activation barrier of the rate-limiting step on both surfaces, the results showed that the CH bond cleavage of the CH3* species have the highest activation barrier for both catalysts. The results also illustrate that mechanism I is the most feasible pathway among the GaN (11¯00) and Ga2O3 (001) surfaces for the ethylene formation. In the case of Ga2O3 (001) H2O, CO, and CO2 are formed as products creating an oxygen vacancy on the Ga2O3 (001) surface.

Investigation of supported palladium catalysts for combustion of methane: The activation effect caused by SO2

Catalytic combustion of methane was studied over Ce0.5Zr0.5O2 and CeO2 supported Pd (Pd/CZ and Pd/Ce) catalysts in the presence of SO2. The effect of sulfation (air including 100 ppm SO2 for 6 h at 500 °C) on the sulfur-tolerant performance of the catalysts was also investigated. Results indicated that there is a considerable promotion effect for CH4 complete oxidation on Pd/CZ catalyst, while the activity of Pd/Ce catalyst was inevitably inhibited. Further, the superior sulfur-tolerant behavior of Pd/CZ was verified through a 50 h time-on-stream stability test. Mechanistic insights into the synergistic effect between the support and Pd were unveiled by Raman, H2-TPR, XPS, NH3-TPD, DRIFTS, HAADF-STEM, etc. It was deduced that the formation of sulfates at the Pd-support interface could promote the dissociative adsorption of methane on Pd/CZ catalyst. Meanwhile, the higher relativeacid strength andthe acidamountwere also decisive factors for the high sulfur-resistant performance. The enhanced acid strength and acid amount prevented the further formation of bulk-like sulfate species for the SO2 treated Pd/CZ catalyst, thus enhancing the sulfur-tolerant ability of the catalyst.

Free energy landscape of dissociative adsorption of methane on ideal and defected graphene from ab initio simulations

We study the dissociative adsorption of methane at the surface of graphene. Free energy profiles, which include activation energies for different steps of the reaction, are computed from constrained ab initio molecular dynamics. At 300 K, the reaction barriers are much lower than experimental bond dissociation energies of gaseous methane, strongly indicating that the graphene surface acts as a catalyst of methane decomposition. On the other hand, the barriers are still much higher than on the nickel surface. Methane dissociation therefore occurs at a higher rate on nickel than on graphene. This reaction is a prerequisite for graphene growth from a precursor gas. Thus, the growth of the first monolayer should be a fast and efficient process while subsequent layers grow at a diminished rate and in a more controllable manner. Defects may also influence reaction energetics. This is evident from our results, in which simple defects (Stone-Wales defect and nitrogen substitution) lead to different free energy landscapes at both dissociation and adsorption steps of the process.

Methane dissociative adsorption in catalytic steam reforming of methane over Pd/CeO2 in an electric field

To elucidate the reaction mechanism of catalytic steam reforming of methane in an electric field, operando–diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been used to investigate methane dissociative adsorption at low temperatures using CH4, H2O, and their isotopes. Operando-analyses demonstrated that methane was adsorbed and converted into CO and CO2 more when CD4 and/or D2O was used instead of CH4 and/or H2O. This observed “inverse” kinetic isotope effect is based on the difference of CHH vibrational energy level at the transition state. Furthermore, CO2-methanation, which is the reverse reaction of steam reforming of methane, was irreversible with H2O in an electric field. These results demonstrate that the proton collision promoted methane activation irreversibly, even at low temperatures in an electric field.

Dissociative adsorption of methane on the Cu and Zn doped (111) surface of CeO2

The development of economical heterogeneous catalysts for the activation of methane is a major challenge for the chemical industry. Screening potential candidates becomes more feasible using rational catalyst design to understand the activity of potential catalysts for CH4 activation. The focus of the present paper is the use of density functional theory to examine and elucidate the properties of doped CeO2. We dope with Cu and Zn transition metals having variable oxidation state (Cu), and a single oxidation state (Zn), and study the activation of methane. Zn is a divalent dopant and Cu can have a +1 or +2 oxidation state. Both Cu and Zn dopants have an oxidation state of +2 after incorporation into the CeO2 (111) surface; however a Hubbard +U correction (+U=7) on the Cu 3d states is required to maintain this oxidation state when the surface interacts with adsorbed species. Dissociation of methane is found to occur locally at the dopant cations, and is thermodynamically and kinetically more favorable on Zn-doped CeO2 than Cu-doped CeO2. The origins of this lie with the Zn(II) dopant moving towards a square pyramidal geometry in the sub surface layer which facilitates the formation of two-coordinated surface oxygen atoms, that are more beneficial for methane activation on a reducible oxide surface. These findings can aid in rational experimental catalyst design for further exploration in methane activation processes.

Theoretical Study of the Dissociative Adsorption of Methane on Ir(111): The Role of Steps and Surface Distortions at High Temperatures

In this work we revisit the dissociative adsorption of methane on Ir(111) through density functional theory calculations. We focus on the role of surface defects entailing undercoordinated Ir atoms (e.g., steps), and thermally induced distortions of a defect-free terrace. Though both factors provoke a significant activation of the CH3···H bond cleavage, our results indicate that the latter (surface distortions) is more likely responsible for the low activation energy derived from experiments at high-surface-temperature (Ts = 1000 K) and low impact energy molecules (Ei ≲ 0.15 eV). Still, since surface distortions are strongly attenuated when Ts decreases, dissociation on undercordinated Ir atoms could play a more important role for low surface temperatures. Hence, we provide useful information to guide new experiments intended to unravel the origin of the dominant dissociation pathway for low kinetic energy molecules.

Cation doping size effect for methane activation on alkaline earth metal doping of the CeO2 (111) surface

Alkaline earth metal doping of CeO2 improves the dissociative adsorption of methane with the behaviour showing a correlation to dopant ionic radius.

Mechanistic investigation of methane steam reforming over Ce-promoted Ni/SBA-15 catalyst

Methane steam reforming experiments were carried out at atmospheric pressure for temperatures between 873 and 1073 K and by varying the partial pressure of methane and steam to achieve S:C between 0.5 and 2.5. Mechanistic considerations for Methane steam reforming (MSR) were derived on the basis of Langmuir–Hinshelwood and Eley–Rideal reaction mechanisms based on single- and dual-site associative and dissociative adsorption of one or both reactants. However, discrimination of these models on statistical and thermodynamic grounds revealed that the model representing a single-site dissociative adsorption of methane and steam most adequately explained the data. However, the product formation rates from these experiments were reasonably captured by power-law model. The parameter estimates from the power-law model revealed an order of 0.94 with respect to methane and −0.16 for steam with activation energy of 49.8 kJ mol−1 for MSR. The negative order with respect to steam for methane consumption was likely due to steam inhibition.

Acid–Base Interaction and Its Role in Alkane Dissociative Chemisorption on Oxide Surfaces

We use density functional theory to examine methane dissociative adsorption on lanthana to produce H and CH3 bonded to the surface. Previous work suggested that the binding energy of a Lewis base and a Lewis acid to an oxide exceeds by far the value expected from adding the binding energies of the same compounds chemisorbed alone. This rule suggests that the fragments (H and CH3) formed by the dissociative adsorption of methane on an oxide will adsorb so that they could benefit from this strong acid–base interaction. To do this, one fragment must bind to oxygen and the other to a cation. Such a configuration should have lower energy than the one obtained by binding both fragments to oxygen, even though each fragment prefers to bind to oxygen when it binds alone. We suggest that this behavior is encountered in most systems.

State and properties of ion-exchanged cations in zeolites: 2. IR spectra and chemical activation of adsorbed methane

Methane adsorption by hydrogen and cationic forms of various zeolites has been investigated by diffuse reflectance IR spectroscopy. In methane adsorption on faujasites, protons, univalent alkali metal cations, and divalent alkaline-earth cations exert similar perturbation and polarization effects on the methane molecule. Adsorbed methane in high-silica zeolites is perturbed by alkaline-earth cations somewhat more strongly than in faujasites and still more strongly by divalent transition metal cations. In the latter case, as in hydrogen adsorption, which was considered in the previous report, the interaction between methane and the zinc form of zeolite ZSM-5 at elevated temperatures causes the heterolytic dissociative adsorption of methane with the formation of methyl fragments and acidic hydroxyl groups bound to zinc ions.

DFT study of reaction processes of methane combustion on PdO(1 0 0)

The complex reaction mechanism of methane combustion on the PdO(100) surface is investigated within the framework of density functional theory. Driving forces and activation energies for the dissociative adsorption of methane and for the successive dehydrogenation of adsorbed hydrocarbons are calculated. Energy barriers of some of the dehydrogenation reactions are comparable to the barrier for the dissociative adsorption of methane, contrary to what is often assumed. Moreover, we find that reaction barriers for the early formation of C–O bonds are much lower than those for the complete dehydrogenation of CH4. In particular, reaction of oxygen molecules from the gas phase with suitable configurations of adsorbed H and CH3 can efficiently produce water and CH2O as oxidation products. Along this reaction path, the highest barrier is indeed given by the first dehydrogenation reaction.

Methane Activation on In-Modified ZSM-5: The State of Indium in the Zeolite and Pathways of Methane Transformation to Surface Species

Methane activation pathways as well as methane involvement in the reaction of ethylene aromatization on In-modified H-ZSM-5 zeolite (In/H-ZSM-5) have been studied with solid-state NMR spectroscopy. The state of indium in In/H-ZSM-5 in dependence of the zeolite activation procedure, reductive or oxidative, has been analyzed with X-ray photoelectron spectroscopy (XPS). On the basis of 1H MAS NMR analysis of the evolution of the quantity of Brønsted acid sites (BAS) and XPS analysis of the state of indium in dependence of zeolite activation procedure, it has been inferred that indium exists in the form of either In+ or InO+ isolated cationic species in the zeolite. Methane interaction with different indium cationic species has been analyzed with 13C MAS NMR spectroscopy. In+ species has been concluded to be inactive, whereas InO+ species provides dissociative adsorption of methane to afford primarily the oxyindium–methyl species. The secondary products of oxyindium–methyl species transformation are oxyindium–methoxy, ethane, formate, and acetaldehyde species. Methane can be involved in the reaction of ethylene aromatization on In-modified zeolite H-ZSM-5. This involvement is provided by the reaction of the surface oxyindium–methoxy species with simple aromatic molecules formed from ethylene. Preliminarily, oxyindium–methoxy species are generated by the interaction of oxyindium–methyl species with InO+ cations.

Oxidative Dehydrogenation of Methane by Isolated Vanadium Oxide Clusters Supported on Au (111) and Ag (111) Surfaces

We use density functional theory, with the GGA-PBE functional, to investigate the ability of vanadium oxide clusters, supported on Ag or Au, to break the C–H bond in methane. We perform a thermodynamic analysis to show that the VO4 cluster is the most likely oxidant and then proceed to calculate the energy of the dissociative adsorption of methane and its activation energy. We explain some peculiar features of the reaction path and propose that they are general for alkane activation on oxides.

Methane oxidation over Pd and Pt studied by DFT and kinetic modeling

Methane dissociation and subsequent formation of water and carbon-dioxide over Pd and Pt are investigated with density functional theory calculations and microkinetic modeling. Adsorption energies for reaction intermediates and activation barriers for CH4 dissociation and water formation are calculated for the (111), (100), (211) and (321) facets. The dissociative adsorption of methane is found to be the rate determining step on all considered facets. The results show that Pt has higher catalytic activity than Pd and that the (100) surface is the most active facet at moderate temperatures for both Pd and Pt. At low temperatures, the reaction is limited, in particular on Pd(100), by poisoning of OH-groups.

Simplified kinetic model for steam reforming of ethanol on a Ni/Al2O3 catalyst

AbstractEthanol steam reforming is a promising method for hydrogen production. Kinetic studies are carried out over a nickel‐based catalyst from 200 to 600°C. A simplified Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic model was proposed with nine parameters, where the surface decomposition of methane is assumed as the rate determining step (RDS), and while all other reaction steps are set as reversible. It is postulated that the velocity of the backward reaction step of the dissociative methane adsorption is similar to all other equilibrium reactions proposed, while the forward reaction step is set as the RDS. In addition, the formation of adsorbed carbon species is excluded due to the fact that SRE experiments were performed with excess water, suggesting that OH species, excluding O species formation, stick better on active Ni surface than methane. In addition, a power law kinetic model was used for analysis; the activation energy was 31.8 kJ/mol, and the reaction order of ethanol pressure was 1.52. Experimental results were successfully demonstrated by both kinetic models. The purpose of this work was to develop a simplified mechanistic model to replace empirical models (power rate model) and other proposed kinetic models that are too complex for an applied kinetic process. It was found that the simplified LHHW kinetic model has a better fit than that of the power law model. The proposed kinetic model works well over a wide temperature range (200–600°C).