Dissociative Chemisorption Of Methane Research Articles

Deciphering the Factors Controlling Hydrogen and Methyl Spillover 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 hasminor effect on promoting spillover, as geminate recombinative desorption becomes more prevalent. However, the poisoning of the active site can be mitigated byfrequent 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.

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.

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.

Pt–Ni Subsurface Alloy Catalysts: An Improved Performance toward CH4 Dissociation

Methane-dissociative chemisorption is the rate-determining step in the industrially important steam reforming and dry reforming reactions of methane. Widely used industrial catalysts containing Ni ...

Rotational and steric effects in water dissociative chemisorption on Ni(111).

Powerful laser techniques have recently enabled quantum-state resolved molecular beam experiments for investigating gas-surface reactions, which have unveiled intriguing vibrational, rotational, and also steric effects. For reactions involving polyatomic molecules, e.g., the dissociative chemisorption of methane and water, the rotational and related steric effects are far less understood despite a large body of theoretical work having been able to reproduce the observed vibrational mode specificity and related bond selectivity semi-quantitatively or even within chemical accuracy. Herein, we report a high dimensional quantum dynamics study of water dissociation on Ni(111) on a first-principles potential energy surface, focusing on the reactivities of D2O in various rotational quantum states with different spatial orientations. Through an accurate quantum mechanical description of this asymmetric top, remarkable dependence of the reactivity on the orientation is observed. This dependence is site specific and rotational state specific. These single site rotational and steric effects are partially justified by a sudden model on the basis of the overlap between the rotational wavefunctions and the angular potential near the transition state, but rotational steering also plays a significant role which complicates the dynamics. Although site averaging weakens the influence of initial rotational excitations and leads to minor effects to the reactivity, steric effects are predicted to be observable if the water molecule is selectively excited and aligned by a linearly polarized laser.

Electron-hole pair effects in methane dissociative chemisorption on Ni(111).

The dissociative chemisorption of methane on metal surfaces has attracted much attention in recent years as a prototype of gas-surface reactions in understanding the mode specific and bond selective chemistry. In this work, we systematically investigate the influence of electron-hole pair excitations on the dissociative chemisorption of CH4/CH3D/CHD3 on Ni(111). The energy dissipation induced by surface electron-hole pair excitations is modeled as a friction force introduced in the generalized Langevin equation, in which the independent atomic friction coefficients are determined within the local-density friction approximation. Quasi-classical trajectory calculations for CH4/CH3D/CHD3 have been carried out on a recently developed twelve-dimensional potential energy surface. Comparing the dissociation probabilities obtained with and without friction, our results clearly indicate that the electron-hole pair effects are generally small, both on absolute reactivity of each vibrational state and on the mode specificity and bond selectivity. Given similar observations in both water and methane dissociation processes, we conclude that electron-hole pair excitations would not play an important role as long as the reaction is direct and the interaction time between the molecule and metal electrons is relatively short.

Origin of Steric Effects in Methane Dissociative Chemisorption

Steric effects in chemical reactions, namely, the dependence of reactivity on reactant spatial orientation and/or alignment, provide valuable information on the anisotropic potential energy surface and access to the transition state. However, a theoretical interpretation of steric effect is often not straightforward because of convolution of the dynamics by many other factors. Here, we present a theoretical study on the dissociative chemisorption of methane using a quasi-classical trajectory method on a density functional theory based global potential energy surface and offer a theoretical interpretation of the interesting but surprising steric effect in this prototypical and industrially relevant gas–surface reaction.

Dissociative chemisorption of methane on Ni and Pt surfaces: mode-specific chemistry and the effects of lattice motion.

The dissociative chemisorption of methane on metal surfaces is of great practical and fundamental interest. Not only is it the rate-limiting step in the steam re-forming of natural gas, but also the reaction exhibits interesting mode-specific behavior and a strong dependence on the temperature of the metal. Electronic structure methods are used to explore this reaction on various Ni and Pt surfaces, with a focus on how the transition state is modified by motion of the metal lattice atoms. These results are used to construct models that explain the strong variation in reactivity with substrate temperature, shown to result primarily from changes in the dissociation barrier height with lattice motion. The dynamics of the dissociative chemisorption of CH4 on Ni and Pt is explored, using a fully quantum approach based on the reaction path Hamiltonian that includes all 15 molecular degrees of freedom and the effects of lattice motion. Agreement with experiment is good, and vibrational excitation of the molecule is shown to significantly enhance reactivity. The efficacy for this is examined in terms of the vibrationally nonadiabatic couplings, mode softening, mode symmetry, and energy localization in the reactive bond.

Dissociative chemisorption of methane on metal surfaces: tests of dynamical assumptions using quantum models and ab initio molecular dynamics.

The dissociative chemisorption of methane on metal surfaces is of great practical and fundamental importance. Not only is it the rate-limiting step in the steam reforming of natural gas, the reaction exhibits interesting mode-selective behavior and a strong dependence on the temperature of the metal. We present a quantum model for this reaction on Ni(100) and Ni(111) surfaces based on the reaction path Hamiltonian. The dissociative sticking probabilities computed using this model agree well with available experimental data with regard to variation with incident energy, substrate temperature, and the vibrational state of the incident molecule. We significantly expand the vibrational basis set relative to earlier studies, which allows reaction probabilities to be calculated for doubly excited initial vibrational states, though it does not lead to appreciable changes in the reaction probabilities for singly excited initial states. Sudden models used to treat the center of mass motion parallel to the surface are compared with results from ab initio molecular dynamics and found to be reasonable. Similar comparisons for molecular rotation suggest that our rotationally adiabatic model is incorrect, and that sudden behavior is closer to reality. Such a model is proposed and tested. A model for predicting mode-selective behavior is tested, with mixed results, though we find it is consistent with experimental studies of normal vs. total (kinetic) energy scaling. Models for energy transfer into lattice vibrations are also examined.

Ab Initio Molecular Dynamics Calculations versus Quantum-State-Resolved Experiments on CHD3 + Pt(111): New Insights into a Prototypical Gas-Surface Reaction.

The dissociative chemisorption of methane on metal surfaces is of fundamental and practical interest, being a rate-limiting step in the steam reforming process. The reaction is best modeled with quantum dynamics calculations, but these are currently not guaranteed to produce accurate results because they rely on potential energy surfaces based on untested density functionals and on untested dynamical approximations. To help overcome these limitations, here we present for the first time statistically accurate reaction probabilities obtained with ab initio molecular dynamics (AIMD) for a polyatomic gas-phase molecule reacting with a metal surface. Using a general purpose density functional, the AIMD reaction probabilities are in semiquantitative agreement with new quantum-state-resolved experiments on CHD3 + Pt(111). The comparison suggests the use of the sudden approximation for treating the rotations even though CHD3 has large rotational constants and yields an estimated reaction barrier of 0.9 eV for CH4 + Pt(111).

Methane dissociative chemisorption and detailed balance on Pt(111): Dynamical constraints and the modest influence of tunneling

A dynamically biased (d-) precursor mediated microcanonical trapping (PMMT) model of the activated dissociative chemisorption of methane on Pt(111) is applied to a wide range of dissociative sticking experiments, and, by detailed balance, to the methane product state distributions from the thermal associative desorption of adsorbed hydrogen with coadsorbed methyl radicals. Tunneling pathways were incorporated into the d-PMMT model to better replicate the translational energy distribution of the desorbing methane product from the laser induced thermal reaction of coadsorbed hydrogen and methyl radicals occurring near T(s) = 395 K. Although tunneling is predicted to be inconsequential to the thermal dissociative chemisorption of CH4 on Pt(111) at the high temperatures of catalytic interest, once the temperature drops to 395 K the tunneling fraction of the reactive thermal flux reaches 15%, and as temperatures drop below 275 K the tunneling fraction exceeds 50%. The d-PMMT model parameters of {E0 = 58.9 kJ/mol, s = 2, η(v) = 0.40} describe the apparent threshold energy for CH4/Pt(111) dissociative chemisorption, the number of surface oscillators involved in the precursor complex, and the efficacy of molecular vibrational energy to promote reaction, relative to translational energy directed along the surface normal. Molecular translations parallel to the surface and rotations are treated as spectator degrees of freedom. Transition state vibrational frequencies are derived from generalized gradient approximation-density functional theory electronic structure calculations. The d-PMMT model replicates the diverse range of experimental data available with good fidelity, including some new effusive molecular beam and ambient gas dissociative sticking measurements. Nevertheless, there are some indications that closer agreement between theory and experiments could be achieved if a surface efficacy less than one was introduced into the modeling as an additional dynamical constraint.

The dissociative chemisorption of methane on Ni(100) and Ni(111): Classical and quantum studies based on the reaction path Hamiltonian

Electronic structure methods based on density functional theory are used to construct a reaction path Hamiltonian for CH4 dissociation on the Ni(100) and Ni(111) surfaces. Both quantum and quasi-classical trajectory approaches are used to compute dissociative sticking probabilities, including all molecular degrees of freedom and the effects of lattice motion. Both approaches show a large enhancement in sticking when the incident molecule is vibrationally excited, and both can reproduce the mode specificity observed in experiments. However, the quasi-classical calculations significantly overestimate the ground state dissociative sticking at all energies, and the magnitude of the enhancement in sticking with vibrational excitation is much smaller than that computed using the quantum approach or observed in the experiments. The origin of this behavior is an unphysical flow of zero point energy from the nine normal vibrational modes into the reaction coordinate, giving large values for reaction at energies below the activation energy. Perturbative assumptions made in the quantum studies are shown to be accurate at all energies studied.

Mode and Bond Selectivities in Methane Dissociative Chemisorption: Quasi-Classical Trajectory Studies on Twelve-Dimensional Potential Energy Surface

The mode and bond selectivities in methane dissociative chemisorption on Ni(111) are studied using a quasi-classical trajectory (QCT) method on a twelve-dimensional global potential energy surface based on a large number of density functional theory points. The calculated reaction probabilities near and above the reaction barrier reproduced the general trends observed in experimental investigations of various vibrationally excited CH4, CHD3, and CH2D2 species on nickel surfaces. The mechanism of these mode and bond selectivities is analyzed using the recently proposed sudden vector projection model.

State-Resolved Reactivity of Methane (ν2 + ν4) on Ni(111)

Vibrational state-resolved experiments that probe methane dissociation on Ni(111) quantify the reactivity of CH4 excited to the ν2 + ν4 bending vibration. A comparison of these data and previous state-resolved measurements reveals that this bending vibration is significantly less reactive than the ν3 C-H stretching vibration in the same polyad of vibrations. Comparison with the 3ν4 bend overtone also suggests that the doubly degenerate bending state, ν2, is less effective than the triply degenerate bend (ν4) in promoting methane dissociative chemisorption on Ni(111). This observation of vibrational mode selectivity contradicts thermal statistical theories of gas-surface reactivity and provides direct experimental evidence of roles that different vibrational states can play in activating this gas-surface reaction.

Dissociative Chemisorption of Methane on Pt(110)-(1×2): Effects of Lattice Motion on Reactions at Step Edges

The dissociative chemisorption of methane on Pt(110)-(1×2) is examined, with a focus on how the reaction dynamics are modified by the motion of the lattice atoms. The barriers to dissociation are found to be lowest at the step edges. The relaxation of the lattice in the presence of the dissociating molecule is found to be far more complicated than on the smooth surfaces of Pt and Ni. The dissociative sticking probabilities are computed using a full-dimensional treatment based on the reaction path Hamiltonian that includes all 15 molecular degrees of freedom and the effects of lattice motion. The potential energy surface and all parameters in our model are computed from first principles. The effects of lattice motion are strong, but not significantly larger than for dissociation on smoother surfaces. Vibrational excitation of the molecule can significantly enhance reactivity, though this effect varies from mode to mode. Agreement with recent experiments with regard to the variation of reactivity with translational energy and substrate temperature is good.

Mode selectivity in methane dissociative chemisorption on Ni(111)

Dissociative chemisorption of CH4 on transition-metal surfaces, representing the rate-limiting step in methane steam reforming, has been shown experimentally to be strongly mode selective. To understand the mode selectivity, a twelve-dimensional global potential energy surface is developed for CH4 interacting with a rigid Ni(111) surface based on a large number of density functional theory points. The reaction dynamics is investigated using an eight-dimensional quantum model, which includes representatives of all four vibrational modes of methane. After correcting for surface effects, key experimental observations, including the mode selectivity, are well reproduced. These theoretical results, along with mechanistic analysis, provide insights into this industrially important heterogeneous reaction.

Molecular reaction dynamics across the phases: similarities and differences

This introduction to the 157th Faraday Discussion describes features of bimolecular reaction and photodissociation in gases and liquids and at interfaces. Two unifying ideas are the concepts of a transition state on a single potential energy surface and of a conical intersection between two surfaces, the former being important in bimolecular reactions and the latter often being important in photodissociation. State-resolved studies of the reactions of methane and its isotopologues with F, Cl, and Br illustrate many aspects of bimolecular reactions including the ability of excitation in vibrational modes to enhance or inhibit a reaction and to control the cleavage of selected bonds. There are clear parallels between those gas-phase reactions and the dissociative chemisorption of methane and its isotopologues on metal surfaces. Similarly, features such as relative reaction rates and energy disposal patterns observed in gas-phase reactions largely carry over into reactions in solution. New experiments comparing photolysis in the gas phase and in solution show that there are again many similarities in the processes in the two environments. Although the influence of the surroundings in those cases is subtle, there are situations in which the surroundings can produce a much larger effect on the photolysis by hindering the dissociation and initiating isomerization.

Computational Investigation of the Thermochemistry and Kinetics of Steam Methane Reforming Over a Multi-Faceted Nickel Catalyst

A microkinetic model of steam methane reforming over a multi-faceted nickel surface using plane-wave, periodic boundary condition density functional theory is presented. The multi-faceted model consists of a Ni(111) surface, a Ni(100) surface, and nickel step edge sites that are modeled as a Ni(211) surface. Flux and sensitivity analysis are combined to gain an increased understanding of the important reactions, intermediates, and surface facets in SMR. Statistical thermodynamics are applied to allow for the investigation of SMR under industrially-relevant conditions (e.g., temperatures in excess of 500 °C and pressures in excess of 1 bar). The most important surface reactions are found to occur at the under-coordinated step edge sites modeled using the Ni(211) surface as well as on the Ni(100) surface. The primary reforming pathway is predicted to be through C* + O* → CO* at high temperatures; however, hydrogen-mediated reactions such as C* + OH* → COH* and CH* + O* → CHO* are predicted to become more important at low temperatures. The rate-limiting reactions are predicted to be dissociative chemisorption of methane in addition to the aforementioned C–O addition reactions.

Methane dissociative chemisorption on Ru(0001) and comparison to metal nanocatalysts

Microcanonical unimolecular rate theory (MURT) was used to characterize methane dissociative chemisorption on Ru(0001). Simulations of supersonic molecular beam and thermal bulb-derived dissociative sticking coefficients indicated that the threshold energy for CH 4 dissociative chemisorption on Ru(0001) was E 0 = 59 kJ / mol and that two surface oscillators were active in the gas-surface collision complexes. MURT analysis of CH 4 supersonic beam experiments on several surfaces found that E 0 decreased from Ni(100) → Ru(0001) → Pt(111) → Ir(111). Although MURT simulations of CH 4 thermal dissociative sticking coefficients were in fairly good accord with thermal bulb experiments at mbar pressures, they were as much as 3–4 orders of magnitude higher than the apparent sticking coefficients derived from CH 4 decomposition or reforming rates on supported nanoscale metal catalysts. Consequently, the varied surface science studies on single crystals all strongly suggested that relatively few surface sites were turning over on the CH 4-reforming nanocatalysts.

Oxygen influence on the dissociative chemisorption of methane on nickel: A quantum chemical cluster model study

A quantum chemical investigation of methane activation on reduced and partially oxidized Ni(1 0 0) surfaces has been performed. For all surfaces, methane activation has been assumed to proceed by dissociative adsorption at an on-top site. The activation energy of methane adsorption on a reduced Ni 25 cluster was calculated at 108 kJ/mol, while for various Ni 25O clusters (modelling partially oxidized surfaces) the activation energies were significantly higher.