Chemisorption Of Methane Research Articles

How to simulate dissociative chemisorption of methane on metal surfaces.

The dissociation of methane is not only an important reaction step in catalytic processes, but also of fundamental interest. Dynamical effects during the dissociative chemisorption of methane on metal surfaces cause significant differences in computed reaction rates, compared to what is predicted by typical transition state theory (TST) models. It is clear that for a good understanding of the catalytic activation of methane dynamical simulations are required. In this paper, a general blueprint is provided for performing dynamical simulations of the dissociative chemisorption of methane on metal surfaces, by employing either the quasi-classical trajectory or ring polymer molecular dynamics approach. If the computational setup is constructed with great care-since results can be affected considerably by the setup - chemically accurate predictions are achievable. Although this paper concerns methane dissociation, the provided blueprint is, so far, applicable to the dissociative chemisorption of most molecules.

Deciphering the Factors Controlling Hydrogen and Methyl Spillover upon Methane Dissociation on Rh/Cu(111) Single‐Atom Alloy

AbstractSpillover of adsorbed species from one active site to another is a key step in heterogeneous catalysis. However, the factors controlling this step, particularly the spillover of polyatomic species, have rarely been studied. Herein, we investigate the spillover dynamics of H* and CH3* species on a single‐atom alloy surface (Rh/Cu(111)) upon the dissociative chemisorption of methane (CH4), using molecular dynamics that considers both surface phonons and electron‐hole pairs. These dynamical calculations are made possible by a high‐dimensional potential energy surface machine learned from density functional theory data. Our results provide compelling evidence that the H* and CH3* can spill over on the metal surface at experimental temperatures and reveal novel dynamical features involving an internal motion during diffusion for CH3*. Increasing surface temperature has a minor effect on promoting spillover, as geminate recombinative desorption becomes more prevalent. However, the poisoning of the active site can be mitigated by the frequent gaseous molecular collisions that occur under ambient pressure in real‐world catalysis, which transfer energy to the trapped adsorbates. Interestingly, the bulky CH3* exhibits a significant spillover advantage over the light H* due to its larger size, which facilitates energy acquisition. These insights help to advance our understanding of spillover in heterogeneous catalysis.

Deciphering the Factors Controlling Hydrogen and Methyl Spillover upon Methane Dissociation on Rh/Cu(111) Single-Atom Alloy.

Spillover of adsorbed species from one active site to another is a key step in heterogeneous catalysis. However, the factors controlling this step, particularly the spillover of polyatomic species, have rarely been studied. Herein, we investigate the spillover dynamics of H* and CH3* species on a single-atom alloy surface (Rh/Cu(111)) upon the dissociative chemisorption of methane (CH4), using molecular dynamics that considers both surface phonons and electron-hole pairs. These dynamical calculations are made possible by a high-dimensional potential energy surface machine learned from density functional theory data. Our results provide compelling evidence that the H* and CH3* can spill over on the metal surface at experimental temperatures and reveal novel dynamical features involving an internal motion during diffusion for CH3*. Increasing surface temperature has a minor effect on promoting spillover, as geminate recombinative desorption becomes more prevalent. However, the poisoning of the active site can be mitigated by the frequent gaseous molecular collisions that occur under ambient pressure in real-world catalysis, which transfer energy to the trapped adsorbates. Interestingly, the bulky CH3* exhibits a significant spillover advantage over the light H* due to its larger size, which facilitates energy acquisition. These insights help to advance our understanding of spillover in heterogeneous catalysis.

Accurate Reaction Probabilities for Translational Energies on Both Sides of the Barrier of Dissociative Chemisorption on Metal Surfaces.

Molecular dynamics simulations are essential for a better understanding of dissociative chemisorption on metal surfaces, which is often the rate-controlling step in heterogeneous and plasma catalysis. The workhorse quasi-classical trajectory approach ubiquitous in molecular dynamics is able to accurately predict reactivity only for high translational and low vibrational energies. In contrast, catalytically relevant conditions generally involve low translational and elevated vibrational energies. Existing quantum dynamics approaches are intractable or approximate as a result of the large number of degrees of freedom present in molecule-metal surface reactions. Here, we extend a ring polymer molecular dynamics approach to fully include, for the first time, the degrees of freedom of a moving metal surface. With this approach, experimental sticking probabilities for the dissociative chemisorption of methane on Pt(111) are reproduced for a large range of translational and vibrational energies by including nuclear quantum effects and employing full-dimensional simulations.

Oxygen vacancy self-doped single crystal-like TiO 2 nanotube arrays for efficient light-driven methane non-oxidative coupling

Photocatalytic non-oxidative coupling of methane (PNOCM) is a mild and cost-effective method for the production of multicarbon compounds. However, the separation of photogenerated charges and activation of methane are the main challenges for this reaction. Here, single crystal-like TiO₂ nanotubes (V_O-p-TNT) with oxygen vacancies (V_O) and preferential orientation were prepared and applied to PNOCM. The results demonstrate that the significantly enhanced photocatalytic performance is mainly related to the strong synergistic effect between preferential orientation and V_O. The preferential orientation of V_O-p-TNT along [001] direction reduces the formation of complex centers at grain boundaries as the form of interfacial states and potential barriers, which improves the separation and transport of photogenerated carriers. Meanwhile, V_O provides abundant coordination unsaturated sites for methane chemisorption and also acts as electron traps to hinder the recombination of electrons and holes, establishing an effective electron transfer channel between adsorbed CH₄ molecule and photocatalyst, thus weakening C–H bond. In addition, the introduction of V_O broadens the light absorption range. As a result, V_O-p-TNT exhibits excellent PNOCM performance and provides new insights into catalyst design for CH₄ conversion.

Upgrading of methane emissions via chemical looping over copper-zeolites: Experiments and modelling

The interest in methane direct conversion into methanol has increased in the last years. A copper-mordenite looping process, based on the separation of methanol formation (200 °C) and copper re-activation (450 °C), has been studied. Methane partial pressure (studied in the range 5–100 kPa) had a positive effect on methanol productivity (10.7 μmol/g zeolite at 100 kPa CH4). It was found that the adsorption time had some impact on the productivity; a minimum value of 60 min was needed to reach the equilibrium. Using the experimental results, the equilibrium and kinetics of methane chemisorption were modelled. The proposed model is based on the formation of two adsorbed species: methoxy and CO2-precursors.Many methane emissions and feedstocks are diluted and contain oxygen. Thus, methane adsorption has also been studied at aerobic conditions, to apply the looping conversion of methane into methanol to these sources. High oxygen concentrations were considered (2.5–16 mol%), resulting in a decrease in methanol productivity and the over-oxidation of most adsorbed CO2-precursors during the adsorption step. The model was updated for considering the over-oxidation to CO2.

Assessing density functionals for describing methane dissociative chemisorption on Pt(110)-(2×1) surface

In this work, we explore the suitability of several density functionals with the generalized gradient approximation (GGA) and beyond for describing the dissociative chemisorption of methane on the reconstructed Pt(110)-(2×1) surface. The bulk and surface structures of the metal, methane adsorption energy, and dissociation barrier are used to assess the functionals. A van der Waals corrected GGA functional (optPBE-vdW) and a meta-GGA functional with van der Waals correction (MS PBEl-rVV10) are selected for ab initio molecular dynamics calculations of the sticking probability. Our results suggest that the use of these two functionals may lead to a better agreement with existing experimental results, thus serving as a good starting point for future development of reliable machine-learned potential energy surfaces for the dissociation of methane on the Pt(110)-(2×1) surface.

Towards bridging the structure gap in heterogeneous catalysis: the impact of defects in dissociative chemisorption of methane on Ir surfaces

A quantitative understanding of the role played by defect sites in heterogeneous catalysis is of great importance in designing new and more effective catalysts. In this work, we report a detailed dynamic study of a key step in methane steam reforming under experimentally relevant conditions on a new high-dimensional potential energy surface determined from first principles data with the aid of machine learning, with which the interactions of CH4 with both the flat Ir(111) and stepped Ir(332) surfaces are described. In particular, we argue based on our simulations that the experimentally observed "negatively activated" dissociative chemisorption of methane on Ir surfaces could be due to a combined effect of defects and high substrate temperature, which lowers the reaction barrier relative to that on terraces. Furthermore, a model based on dynamic information of trapping and reaction channels is proposed, which allows a quantitative prediction of the initial sticking probability for different defect densities, thus helping to close the so-called structure gap in heterogeneous catalysis.

Direct and trapping-mediated pathways to dissociative chemisorption: CH4 dissociation on Ir(111) with step defects.

The indirect chemisorption of methane on a transition metal, where the incident molecule first traps onto the surface and then reacts from a physisorbed molecular state, has only been observed on Ir(111) and Ir(110) at very low collision energies. We use quantum scattering methods to describe the direct reaction of methane on Ir(111) at high energy and rate theory to examine the indirect pathway at low energy. Overall, we find good agreement with the experiment with respect to the variation of sticking with the incident energy, surface temperature, and vibrational state. Compared with methane dissociation on other metals, vibrational excitation is found to be less effective at promoting the reaction, while coupling to the lattice motion is unusually strong. We show how step defects, even at low concentrations, can contribute significantly to indirect chemisorption. We explore indirect chemisorption on Ni(111) and Pt(111) and find that the indirect path can be important when relaxed lattice barriers are sufficiently low and the temperature is sufficiently high so that the indirect sticking is within detection limits and the incident energy is low enough so that the trapping is large and the direct component is small.

Non-activated adsorption of methane on nickel surfaces induced by reduced work function

Adsorption of methane has been studied in ultrahigh vacuum environments on single crystalline Ni(100) as well as on Ni nano-particles on mica substrates. Experimental techniques have been X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and temperature programmed desorption spectroscopy (TPD). In accordance with previous observations only physisorption of CH4 is observed on pure nickel at temperatures below 130 K for exposures in the high vacuum regime. TPD in the temperature range from 200 to 300 K shows that chemisorption of methane takes place when the sample work function is lowered by about 0.7 eV by a surface treatment, which presumably exposes the surface to atomic hydrogen. The lowered work function enables charge transfer from the Ni conduction band to the methane affinity level which subsequent leads to dissociative chemisorption.

Quantum state and surface-site-resolved studies of methane chemisorption by vibrational spectroscopies.

-The combination of quantum state-specific reactant preparation by infrared laser pumping with surface-site-resolved detection of chemisorbed reaction products by Reflection Absorption Infrared Spectroscopy (RAIRS) enables highly detailed studies of molecule/surface reactivity. In this perspective, we review the methodologies developed for simultaneous quantumstate and surface-site-resolved reactivity measurements and their application towards the chemisorption of methane on stepped and kinked platinum surfaces. We demonstrate that RAIRS allows for surface-site-resolved detection of methane dissociation, which serves to measure surface-site-resolved product uptake curves, sticking probabilities, and dissociation barrier heights. For the dissociation of C-H stretch excited singly deuterated CH3D on a stepped Pt surface such as Pt(211), RAIRS was used to detect bond selectivity in methane chemisorption and to reveal how the bond-selective dissociation proceeds from the step to the terrace sites with increasing incident kinetic energy of the CH3D reactant. Extension to site-selective RAIRS detection of methane dissociation to other vicinal surfaces such as Pt(210), Pt(531), and Pt(110)-(2 × 1) is also presented.

Methane Chemisorption on Oxide-Supported Pt Single Atom.

Methane chemisorption has been recently demonstrated on the rutile IrO2 (110) surface. However, it remains unclear how the general requirements are for methane chemisorption or complexation with a single atom on an oxide surface. By exploring methane adsorption on Pt1 substitutionally doped on many rutile-type oxides using hybrid density functional theory, we show that the occupancy of the Pt dz2 orbital is the key to methane chemisorption. Pt single atom on the semiconducting or wide-gap oxides such as TiO2 and GeO2 strongly chemisorbs methane, because the empty Pt dz2 orbital is located in the gap and can effectively accept σ-electron donation from the methane C-H bond. In contrast, Pt single atom on metallic oxides such as IrO2 and RuO2 does not chemisorb methane, because the Pt dz2 orbital strongly mixes with the support-oxide electronic states and become more occupied, losing its ability to chemisorb methane. This study sheds further light on the impact of the interaction between a Pt single atom and the oxide support on methane adsorption.

Lean methane oxidation over Co3O4/Ce0.75Zr0.25 catalysts at low-temperature: Synergetic effect of catalysis and electric field

Electric field was introduced for ultra-lean methane oxidation over a series of Co3O4/Ce0.75Zr0.25 catalysts. Catalytic activities were significantly promoted by the electric field. It was found that the catalyst with 25% of Co loading showed the highest catalytic activity at 9 mA of electric currency, and the light-off temperature (T50) was as low as 240 °C. The effect of the electric field over the catalysts was examined from the structural and surface point of view (XRD, XRS, BET, H2-TPR). The obtained results demonstrated that the electric field facilitated the release of oxygen from lattice by promoting the reduction of Ce4+ to Ce3+, therefore reinforced the formation of Co3O4 on catalyst surface. Moreover, tetrahedral Co2+ in the Co3O4 spinel were oxidized to Co3+. Besides, electric field enhanced the reducing capacity of the Co3O4 with the newly formed tetrahedral Co3+ easily reduced at low-temperature conditions. The FTIR results revealed that the active sites of methane chemisorption located respectively at tetrahedral Co3+ with active O species from lattice in the electric field, and octahedral Co3+ with pre-adsorbed gaseous O2 formed at relatively higher-temperature in conventional catalytic process. The following dehydrogenation and oxidation processes were accelerated in the electric field with fast conversion rates of intermediates, namely methyl and formates species. Based on experimental results and comprehensive analysis, the mechanism of catalytic oxidation of methane over Co3O4/Ce0.75Zr0.25 catalysts with the assistance of electric field was proposed.

Joint Conversion of Methane with Pentane on Alumina–Platinum Catalysts Modified with Zirconium Oxide under Nonoxidizing Conditions

The physicochemical properties of alumina–platinum catalysts with supports prepared by mixing aluminum and zirconium hydroxides have been studied. The chemisorption and nonoxidative conversion of methane in the presence of the above systems have been investigated. The effect of the zirconium content on the activity and selectivity of the conversion of chemisorbed CH4 and n-C5H12 into aromatic hydrocarbons has been established.

Современные катализаторы и способы неокислительного превращения метана

В работе проведено обобщение имеющихся данных по современным способам неокислительной конверсии метана и катализаторам для представленных процессов. Рассмотрены закономерности превращения молекулы на цеолитных и металлоксидных системах (двустадийные процессы). Также представлены результаты исследований по совместному превращению метана и н-пентана. Показано влияние природы металла, типа носителя, а также условий проведения процесса на особенности хемосорбции метана и степень его дегидрирования. Изучена реакционная способность образующихся углеводородных фрагментов в реакции образования ароматических углеводородов с н-пентаном.

Mechanism of the catalytic oxidation of methane on Pt(1 1 1) surfaces in moist environment: A density functional theory study

Density functional theory studies were carried out to gain mechanistic understanding of the catalytic oxidation of methane on Pt surfaces, especially in moist environment. Transition-state theory was used to estimate the energy barriers for each of the elementary reactions involved in the catalytic oxidation process. The optimal adsorption geometry and the corresponding chemisorption energy were determined for all the species involved in each elementary step to elucidate the energetics of the pathways for methane adsorption and oxidation on Pt surfaces. As a first step, the scope of this study is limited to a Pt(1 1 1) surface. The elementary steps involve the dissociative chemisorption of methane on the Pt(1 1 1) surface, dehydrogenation reactions of adsorbed CHx species, and oxidation reactions of adsorbed reactive intermediates by adsorbed O and OH species. Microscopic reaction pathways and corresponding transition-state structures were identified. The results indicated that the primary reaction pathway is CH4* → CH3* → CH2* → CH* → HCO* → HCOO* → CO2*. In moist environment, the primary reaction pathway is CH4* → CH3* → CH2* → CH* + OH* → CHOH* → CO* + OH* → COOH* → CO2*. The rate-determining step for the reaction pathway from methane to adsorbed carbon dioxide is the dissociative chemisorption of methane due to its relatively high energy barrier. The presence of water in reactants can promote the catalytic oxidation process.

Hindered methane decomposition on a coke-resistant Ni-In/SiO2 dry reforming catalyst

Decomposition of methane has been investigated by mass spectrometry assisted pulse chemisorption experiments at 600 °C on freshly reduced and on carburized Ni/SiO2 and Ni-In/SiO2 dry reforming catalysts. The results showed complete methane decomposition with stoichiometric hydrogen production on freshly reduced Ni/SiO2, while strong hydrogen chemisorption and partial methane decomposition was observed on Ni-In/SiO2. Hydrogen production decreased on both carburized catalysts and dissociative methane chemisorption without hydrogen formation was observed on the bimetallic catalyst. This difference in methane activation might be one of the reasons for the absence of coke on the bimetallic catalyst during dry reforming of methane.

Mode-specific and bond-selective dissociative chemisorption of CHD3 and CH2D2 on Ni(111) revisited using a new potential energy surface

Dissociative chemisorption of methane on a nickel surface is a prototypical system for studying mode-specific chemistry in gas-surface reactions. We recently developed a fifteen-dimensional potential energy surface for this system which has proven to be chemically accurate in reproducing the measured absolute dissociative sticking probabilities of CHD3 in thermal conditions and with vibrational excitation on Ni(111) at high incident energies. Here, using this new potential energy surface, we explored mode specificity and bond selectivity for CHD3 and CH2D2 dissociative chemisorption at low incidence energies down to ~50 kJ/mol via a quasi-classical trajectory method. Our calculated dissociation probabilities are consistent with previous theoretical and experimental ones with an average shift in translational energy of ~8 kJ/mol. Our results very well reproduce the C–H/C–D branching ratio upon the C–H local mode excitation, which can be rationalized by the sudden vector projection model. Quantitatively, however, the calculated dissociative sticking probabilities are systematically larger than experimental ones, due presumably to the artificial zero point energy leakage into reaction coordinate. Further high-dimensional quantum dynamics calculations are necessary for acquiring a chemically accurate description of methane dissociative chemisorption at low incident energies.

New Data on the Ability of Alumina–Platinum Systems to Catalyze the Methane Aromatization Reaction under Nonoxidizing Conditions

The state and size of the metal on the surface of aluminum oxide and the acidic properties of the support depending on the concentration of supported platinum were studied in this work. The effect of the Pt content of the alumina–platinum catalyst on the activation (chemisorption) of methane was investigated, and the composition of hydrocarbon fragments formed in this case was calculated. The sample most active in a reaction of the joint conversion of methane with n-pentane, which was performed for the production of aromatic hydrocarbons under nonoxidizing conditions, was established. The effect of the temperature of n-pentane supply to the reaction atmosphere was studied for the 0.6%Pt/Al2O3 catalyst. The degree of enrichment of the resulting aromatic hydrocarbons and the quantity of incorporated methane activated on the catalyst surface were determined with the use of isotope mass spectrometry.

Coordinate systems and kinetic energy operators for multi-configurational time-dependent Hartree calculations studying reactions of methane

Accurate full-dimensional quantum state-resolved calculations studying reactions of methane currently rely on the multi-configurational time-dependent Hartree (MCTDH) approach and curvilinear coordinate systems. The coordinate systems used in MCTDH simulations of the reactive scattering of methane were based on a coordinate system and kinetic energy operator introduced by Evenhuis et al. (2007). In this coordinate system, the body-fixed frame is tied to the methyl fragment. Here it is shown that a revised definition of the methyl-fixed frame can reduce the correlation between the rotation of the methyl fragment and its internal motion. Furthermore, a new coordinate system which defines the body-fixed frame using the Jacobi vector associated with the dissociating bond and the symmetry axis of the methyl fragment is introduced. This coordinate system is designed for MCTDH simulations of the dissociative chemisorption of methane on surfaces.