Chemisorption Of Methane Research Articles

Quantum state-resolved CH4 dissociation on Pt(111): coverage dependent barrier heights from experiment and density functional theory

The dissociative chemisorption of CH4 on Pt(111) was studied using quantum state-resolved methods at a surface temperature (T(s)) of 150 K where the nascent reaction products CH3(ads) and H(ads) are stable and accumulate on the surface. Most previous experimental studies of methane chemisorption on transition metal surfaces report only the initial sticking coefficients S0 on a clean surface. Reflection absorption infrared spectroscopy (RAIRS), used here for state resolved reactivity measurements, enables us to monitor the CH3(ads) uptake during molecular beam deposition as a function of incident translational energy (E(t)) and vibrational state (ν3 anti-symmetric C-H stretch of CH4) to obtain the initial sticking probability S0, the coverage dependence of the sticking probability S(θ) and the CH3(ads) saturation coverage θ(sat). We observe that both S0 and θ(sat) increase with increasing E(t) as well as upon ν3 excitation of the incident CH4 which indicates a coverage dependent dissociation barrier height for the dissociation of CH4 on Pt(111) at low surface temperature. This interpretation is supported by density functional calculations of barrier heights for dissociation, using large supercells containing one or more H and/or methyl adsorbates. We find a significant increase in the activation energies with coverage. These energies are used to construct simple models that reasonably reproduce the uptake data and the observed saturation coverages.

Mode-specificity and transition state-specific energy redistribution in the chemisorption of CH4 on Ni{100}

We have investigated methane (CH(4)) dissociative chemisorption on the Ni{100} surface by first-principles molecular dynamics (MD) simulations. Our results show that this reaction is mode-specific, with the ν(1) state being the most strongly coupled to efficient energy flow into the reaction coordinate when the molecule reaches the transition state. By performing MD simulations for two different transition state (TS) structures we provide evidence of TS structure-specific energy redistribution in methane chemisorption. Our results are compared with recently reported state-resolved measurement of methane adsorption probability on nickel surfaces, and we find that a strong correlation exists between the highest vibrational efficacy measured on Ni{100} for the ν(1) state and the calculated highest fractional vibrational energy content in this mode.

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.

Bond-selective energy redistribution in the chemisorption of CH3D and CD3H on Pt{1 1 0}-(1 × 2): A first-principles molecular dynamics study

We have investigated the chemisorption of CH3D and CD3H on Pt{110}-(1×2) by performing first-principles molecular dynamics simulations of the recombinative desorption of CH3D (from adsorbed methyl and deuterium) and of CD3H (from adsorbed trideuteromethyl and hydrogen). Vibrational analysis of the symmetry adapted internal coordinates of the desorbing molecules shows that excitation of the single C–D (C–H) bond in the parent molecule is strongly correlated with energy excess in the reaction coordinate. The results of the molecular dynamics simulations are consistent with observed mode- and bond-specific reactivity measurements for chemisorption of methane and its isotopomers on platinum and nickel surfaces.

Nanostructured supported palladium catalysts—Non-oxidative methane coupling

The Pd on α-Al 2O 3 catalysts with Pd particles in the low nanometer range have been prepared by a sonochemical reduction and a colloidal method, respectively. The two catalysts differ in their particle size, the widths of their particle size distributions and the amount of carbon incorporation in the Pd lattice. The adsorptive properties of the Pd/Al 2O 3 samples are different as a result of the different preparation methods. The methane adsorption capacity of that sample with smaller particles is lower than that of the catalyst with larger particles and the energy of activation is nearly doubled. DRIFTS and TPD results of CO adsorption supported by transmission electron microscopy data indicate that the PdSON catalyst with smaller and more homogeneous particles than PdCOL is highly dispersed which influences the coupling-hydrogenolysis process. The catalytic activity evidenced the formation of different adspecies during methane coupling and chemisorption on both catalysts. During the hydrogenation the carbon adspecies formed mainly methane at low adsorption temperatures. The significant amount of adsorbed methane at 773 K is governed by the highly active coordination unsaturated sites at the surface.

The dissociative chemisorption of methane on Ni(100): Reaction path description of mode-selective chemistry

We derive a model for the dissociative chemisorption of methane on a Ni(100) surface, based on the reaction path Hamiltonian, that includes all 15 molecular degrees of freedom within the harmonic approximation. The total wavefunction is expanded in the adiabatic vibrational states of the molecule, and close-coupled equations are derived for wave packets propagating on vibrationally adiabatic potential energy surfaces, with non-adiabatic couplings linking these states to each other. Vibrational excitation of an incident molecule is shown to significantly enhance the reactivity, if the molecule can undergo transitions to states of lower vibrational energy, with the excess energy converted into motion along the reaction path. Sudden models are used to average over surface impact site and lattice vibrations. Computed dissociative sticking probabilities are in good agreement with experiment, with respect to both magnitude and variation with energy. The ν(1) vibration is shown to have the largest efficacy for promoting reaction, due to its strong non-adiabatic coupling to the ground state, and a significant softening of the vibration at the transition state. Most of the reactivity at 475 K is shown to result from thermally assisted over-the-barrier processes, and not tunneling.

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.

Monodispersed and nanostructrured Ni/SiO 2 catalyst and its activity for non oxidative methane activation

In this paper, we prepared nickel nanoparticles for the non oxidative methane activation. A nickel acetate solution was impregnated on silica support and then reduced with NaBH 4 (NiB/SiO 2) exhibiting only small metallic Ni 0 particles of the order of 15 nm. Part of this solid was then oxidized under mild conditions, where Ni ions were re-structured and redistributed as a Ni-O x /SiO 2 oxide solid that after reduction with hydrogen favored the formation of smaller Ni particles. XPS spectra of the Ni-O x /SiO 2 sample before the reduction showed that NiO is the predominant species at the surface and after reduction it indicates the presence of small metallic nickel particles at the surface interacting with the SiO 2 support. XRD and DRIFTS results confirm the structural interaction of metal-support after the mild oxidation and TEM images indicate small particles of the order of 3.5 nm and surface defects. The non oxidative methane activation evidenced significant formation of methane coupling products over the Ni-O x /SiO 2 catalyst during methane chemisorption. Chemisorption of methane on the NiB/SiO 2 showed ethane formation only. The temperature programmed surface hydrogenation (TPSH) results suggested the formation of carbidic species at lower temperatures of adsorption.

The effect of support on methane activation over Pt catalysts in the presence of MoO3

It is reported the role of different supports (silica, alumina and zirconia) on the non-oxidative conversion of methane, through the isothermal two-step reaction, on Pt-loaded catalysts, with or without the presence of MoO3. The activated chemisorption of methane, followed by reaction of the carbon adspecies with hydrogen was conducted in pulse and continuous modes, in the temperature range of 473–673 K. It was observed by using pulse mode that evolutions of H2, CO and CO2 occurred simultaneously with the pulses of methane without the formation of higher hydrocarbons. The reactivity, regarding the CH4 conversion, with or without MoO3, follows the sequence Pt/Al2O3 ≈ Pt/ZrO2 > Pt/SiO2. Also the hydrogenation of carbon residues produced only methane. The presence of MoO3 had a positive influence concerning the reactivity of carbon residues, on zirconia and especially on alumina supported catalysts. In the flow mode operation, methane chemisorption is followed by the evolution of H2, plus C2, C2, C3=, and C3, depending on the nature of the support. The hydrogenation of carbon residues produced mainly methane, but ethane and propane were also detected specially in the presence of MoO3 which improves the mobility of CHX adspecies, favoring the CC nucleation.

Methane Oxidation Mechanism on Pt(111): A Cluster Model DFT Study

The electronic energy barriers of surface reactions pertaining to the mechanism of the electrooxidation of methane on Pt (111) were estimated with density functional theory calculations on a 10-atom Pt cluster, using both the B3LYP and PW91 functionals. Optimizations of initial and transition states were performed for elementary steps that involve the conversion of CH(4) to adsorbed CO at the Pt/vacuum interface. As a first approximation we do not include electrolyte effects in our model. The reactions include the dissociative chemisorption of CH(4) on Pt, dehydrogenation reactions of adsorbed intermediates (*CH(x) --> *CH(x-1) + *H and *CH(x)O --> *CH(x-1)O + *H), and oxygenation reactions of adsorbed CH(x) species (*CH(x) + *OH --> *CH(x)OH). Many pathways were investigated and it was found that the main reaction pathway is CH(4) --> *CH(3) --> *CH(2) --> *CH --> *CHOH --> *CHO --> *CO. Frequency analysis and transition-state theory were employed to show that the methane chemisorption elementary step is rate-limiting in the above mechanism. This conclusion is in agreement with published experimental electrochemical studies of methane oxidation on platinum catalysts that have shown the absence of an organic adlayer at electrode potentials that allow the oxidation of adsorbed CO. The mechanism of the electrooxidation of methane on Pt is discussed.

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.

Microcanonical unimolecular rate theory at surfaces. III. Thermal dissociative chemisorption of methane on Pt(111) and detailed balance

A local hot spot model of gas-surface reactivity is used to investigate the state-resolved dynamics of methane dissociative chemisorption on Pt(111) under thermal equilibrium conditions. Three Pt surface oscillators, and the molecular vibrations, rotations, and the translational energy directed along the surface normal are treated as active degrees of freedom in the 16-dimensional microcanonical kinetics. Several energy transfer models for coupling a local hot spot to the surrounding substrate are developed and evaluated within the context of a master equation kinetics approach. Bounds on the thermal dissociative sticking coefficient based on limiting energy transfer models are derived. The three-parameter physisorbed complex microcanonical unimolecular rate theory (PC-MURT) is shown to closely approximate the thermal sticking under any realistic energy transfer model. Assuming an apparent threshold energy for CH(4) dissociative chemisorption of E(0)=0.61 eV on clean Pt(111), the PC-MURT is used to predict angle-resolved yield, translational, vibrational, and rotational distributions for the reactive methane flux at thermal equilibrium at 500 K. By detailed balance, these same distributions should be observed for the methane product from methyl radical hydrogenation at 500 K in the zero coverage limit if the methyl radicals are not subject to side reactions. Given that methyl radical hydrogenation can only be experimentally observed when the CH(3) radicals are kinetically stabilized against decomposition by coadsorbed H, the PC-MURT was used to evaluate E(0) in the high coverage limit. A high coverage value of E(0)=2.3 eV adequately reproduced the experimentally observed methane angular and translational energy distributions from thermal hydrogenation of methyl radicals. Although rigorous application of detailed balance arguments to this reactive system cannot be made because thermal decomposition of the methyl radicals competes with hydrogenation, approximate applicability of detailed balance would argue for a strong coverage dependence of E(0) with H coverage--a dependence not seen for methyl radical hydrogenation on Ru(0001), but not yet experimentally explored on Pt(111).

Publisher's Note: Pathways for dissociative methane chemisorption on Pt{110}-(1×2) [Phys. Rev. B71, 113410 (2005)

Publisher's Note: Pathways for dissociative methane chemisorption on Pt{110}-(1×2) [Phys. Rev. B<b>71</b>, 113410 (2005)

Microcanonical unimolecular rate theory at surfaces. II. Vibrational state resolved dissociative chemisorption of methane on Ni(100).

A three-parameter microcanonical theory of gas-surface reactivity is used to investigate the dissociative chemisorption of methane impinging on a Ni(100) surface. Assuming an apparent threshold energy for dissociative chemisorption of E(0)=65 kJ/mol, contributions to the dissociative sticking coefficient from individual methane vibrational states are calculated: (i) as a function of molecular translational energy to model nonequilibrium molecular beam experiments and (ii) as a function of temperature to model thermal equilibrium mbar pressure bulb experiments. Under fairly typical molecular beam conditions (e.g., E(t)>/=25 kJ mol(-1), T(s)>/=475 K, T(n)</=400 K), sticking from methane in the ground vibrational state dominates the overall sticking. In contrast, under thermal equilibrium conditions at temperatures T>/=100 K the dissociative sticking is dominated by methane in vibrationally excited states, particularly those involving excitation of the nu(4) bending mode. Fractional energy uptakes f(j) defined as the fraction of the mean energy of the reacting gas-surface collision complexes that derives from specific degrees of freedom of the reactants (i.e., molecular translation, rotation, vibration, and surface) are calculated for thermal dissociative chemisorption. At 500 K, the fractional energy uptakes are calculated to be f(t)=14%, f(r)=21%, f(v)=40%, and f(s)=25%. Over the temperature range from 500 K to 1500 K relevant to thermal catalysis, the incident gas-phase molecules supply the preponderance of energy used to surmount the barrier to dissociative chemisorption, f(g)=f(t)+f(r)+f(v) approximately 75%, with the highest energy uptake always coming from the molecular vibrational degrees of freedom. The predictions of the statistical, mode-nonspecific microcanonical theory are compared to those of other dynamical theories and to recent experimental data.

Erratum to: “Role of translational and vibrational energy in the dissociative chemisorption of methane on Pd{1 1 0}” [Surf. Sci. 482–485 (2001) 171–176

Erratum to: “Role of translational and vibrational energy in the dissociative chemisorption of methane on Pd{1 1 0}” [Surf. Sci. 482–485 (2001) 171–176

Quantum State Resolved Studies of Gas/Surface Reaction Dynamics

Laser light is an invaluable tool for detailed experimental studies of chemical reactions. The ability to prepare reagent molecules in specific quantum states and to resolve the quantum state distribution of reaction products using laser radiation has been key to the field of chemical reaction dynamics, which studies chemistry at a fundamental microscopic level. Today, state-resolved experiments are common for unimolecular and bi-molecular reactions in the gas phase, but equivalent measurements for gas/surface reactions have only recently started to appear. This contribution reviews the methodology developed in our group at the EPFL for the quantum state resolved investigation of molecule/surface reactions and gives representative results on the dissociative chemisorption of methane on single crystal nickel surfaces.

Photoinduced Adsorption of Hydrogen and Methane on γ-Alumina. The Photoinduced Chesorluminescence (PhICL) Effect

Adsorption of hydrogen and methane on a preirradiated surface of gamma-Al2O3 produces an afterglow, which has been described as a photoinduced chesorluminescence (PhICL), whose spectral features identify with the intrinsic photoluminescence of alumina. The emission spectrum consists of at least four overlapping single emission bands. For methane adsorption, the PhICL phenomenon is seen only if the solid is preirradiated in the presence of oxygen. Emission decay kinetics of the PhICL effect for gamma-Al2O3 reveal two wavelength regimes: a short wavelength regime at lambda = 300-370 nm (decay time tau = 1.1 +/- 0.2 s; signal width = 2.8 s), and a longer wavelength regime at lambda = 380-700 nm (decay time tau = 2.1 +/- 0.1 s; signal width = 4.3 s). A model is proposed in which there exist two different emission centers and, thus, two different pathways for emission decay. In the first, emission originates with electron trapping by such deep energy traps as anion vacancies {e- + Va --> F+ + hv1} to yield electron F-type color centers, whereas in the second, emission originates from electron/trapped hole recombination {e- + Os*- --> Os2- + hv2}. The first common step of the pathways is homolytic dissociative chemisorption of hydrogen and methane upon interaction with surface-active hole centers Os*-, produced upon preirradiation of alumina, to give atomic hydrogen H* and methyl radicals CH3*. Thermoprogrammed desorption spectra of photoadsorbed or postsorbed oxygen show that adsorbed oxygen interacts with atomic hydrogen and methyl radicals. The products of thermodesorption were H2O for hydrogen and H2O, CO2, and CH3CH3 for methane. The Solonitsyn memory effect coefficient was also evaluated for oxygen photoadsorption.

Dissociative chemisorption of methane on Ni(100): Threshold energy from CH4(2ν3) eigenstate-resolved sticking measurements

A three-parameter microcanonical theory of gas-surface reactivity is used to model the dissociative sticking of vibrationally excited methane with two quanta of energy in the ν3 antisymmetric C–H stretch. An apparent threshold energy for C–H bond cleavage of CH4 incident on Ni(100) of 65 kJ/mol is found, in quantitative agreement with ab initio quantum chemistry calculations but 38 kJ/mol less than GGA-DFT calculations. Successful microcanonical analysis and prediction of recent thermal equilibrium and various nonequilibrium dissociative chemisorption experiments for methane on Ni(100) provide no evidence for mode-specific reactivity.

The dynamics of the dissociative adsorption of methane on Pt(533)

The influence of well-defined steps on the dynamics of the dissociative chemisorption of methane on Pt(533) has been investigated using molecular beam techniques. The initial dissociative chemisorption probability S0 has been determined as a function of incident energy Ei, angle of incidence θi, and surface temperature TS. For incident kinetic energies in the range 26&amp;lt;Ei(meV)&amp;lt;1450, the initial dissociation probability of CH4 on the Pt(533) surface is higher than on Pt(111), for all surface temperatures investigated. This enhancement in dissociation is associated with the additional direct sticking mediated by the step sites, with no evidence for any additional indirect dynamical channel to dissociation induced by the step sites in the range of energies studied. The Ei dependence can be separated into the contributions of the (111) terraces and the (100) steps. The latter exhibits an effective activation barrier for dissociation ≈300 meV lower than the (111) terraces. The angular dependence can also be interpreted as having two contributions, one associated with the (111) terraces, and the second associated with the steps. The angular dependence associated with the step sites is broader than the dependence expected for the (111) terraces, and has a maximum for incident trajectories with an angle between the angles corresponding to the normal directions of the (111) and (100) facets. An enhanced TS dependence is also observed on the Pt(533) surface over Pt(111). This is also associated with the influence of the step sites, and results either from the lower barrier to dissociation, or more likely a more effective coupling of the energy from the surface into the reaction coordinate.

Microcanonical unimolecular rate theory at surfaces. I. Dissociative chemisorption of methane on Pt(111)

A model of gas–surface reactivity is developed based on the ideas that (a) adsorbate chemistry is a local phenomenon, (b) the active system energy of an adsorbed molecule and a few immediately adjacent surface atoms suffices to fix microcanonical rate constants for surface kinetic processes such as desorption and dissociation, and (c) energy exchange between the local adsorbate–surface complexes and the surrounding substrate can be modeled via a Master equation to describe the system/heat reservoir coupling. The resulting microcanonical unimolecular rate theory (MURT) for analyzing and predicting both thermal equilibrium and nonequilibrium kinetics for surface reactions is applied to the dissociative chemisorption of methane on Pt(111). Energy exchange due to phonon-mediated energy transfer between the local adsorbate–surface complexes and the surface is explored and estimated to be insignificant for the reactive experimental conditions investigated here. Simulations of experimental molecular beam data indicate that the apparent threshold energy for CH4 dissociative chemisorption on Pt(111) is E0=0.61 eV (over a C–H stretch reaction coordinate), the local adsorbate–surface complex includes three surface oscillators, and the pooled energy from 16 active degrees of freedom is available to help surmount the dissociation barrier. For nonequilibrium molecular beam experiments, predictions are made for the initial methane dissociative sticking coefficient as a function of isotope, normal translational energy, molecular beam nozzle temperature, and surface temperature. MURT analysis of the thermal programmed desorption of CH4 physisorbed on Pt(111) finds the physisorption well depth is 0.16 eV. Thermal equilibrium dissociative sticking coefficients for methane on Pt(111) are predicted for the temperature range from 250–2000 K. Tolman relations for the activation energy under thermal equilibrium conditions and for a variety of “effective activation energies” under nonequilibrium conditions are derived. Expressions for the efficacy of sticking with respect to normal translational energy and vibrational energy are found. Fractional energy uptakes, fj, defined as the fraction of the mean energy of the complexes undergoing reaction that derives from the jth degrees of freedom of the reactants (e.g., molecular translation, vibration, etc.) are calculated for thermal equilibrium and nonequilibrium dissociative chemisorption. The fractional energy uptakes are found to vary with the relative availability of energy of different types under the specific experimental conditions. For thermal dissociative chemisorption at 500 K the fractional energy uptakes are predicted to be ft=13%, fr=18%, fv=33%, and fs=36%. For this equilibrium scenario relevant to catalysis, the incident gas molecules supply the preponderance of energy used to surmount the barrier to chemisorption, fg=ft+fv+fr=64%, but the surface contribution at fs=36% remains significant.