S. T. Ceyer

Dynamics of the activated dissociative chemisorption of CH4 and implication for the pressure gap in catalysis: A molecular beam–high resolution electron energy loss study

The dynamics of the activated dissociative chemisorption of CH4 on Ni(111) are studied by molecular beam techniques coupled with high‐resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule’s translational energy and with vibrational excitation. The dissociative chemisorption probability of CD4 exhibits the same trends with a large kinetic isotope effect. High‐resolution electron energy loss spectroscopy identifies the nascent products of the dissociative chemisorption event as an adsorbed methyl radical and a hydrogen atom. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier likely arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interactio...

The Chemistry of Bulk Hydrogen: Reaction of Hydrogen Embedded in Nickel with Adsorbed CH3

Studies in heterogeneous catalysis have long speculated on or have provided indirect evidence for the role of hydrogen embedded in the catalyst bulk as a primary reactant. This report describes experiments carried out under single-collision conditions that document the distinctive reactivity of hydrogen embedded in the bulk of the metal catalyst. Specifically, the bulk H atom is shown to be the reactive species in the hydrogenation of CH3 adsorbed on Ni(111) to form CH4, while the H atoms bound to the surface were unreactive. These results unambiguously demonstrate the importance of bulk species to heterogeneous catalytic chemistry.

New mechanisms for chemistry at surfaces.

It is becoming increasingly apparent that chemistry at surfaces, whether it be heterogeneous catalysis, semiconductors etching, or chemical vapor deposition, is controlled by much more than the nature and structure of the surface. Recent experiments that principally make use of molecular beam techniques have revealed that the energy at which an incident molecule collides with a surface can be the key factor in determining its reactivity with or on the surface. In addition, the collision energy of an incident particle has proven essential to the finding of new mechanisms for reaction or desorption of molecules at surfaces, collision-induced activation and collision-induced desorption. These phenomena are often responsible for the different surface chemistry observed under conditions of high reactant pressure, such as those present during a heterogeneous catalytic reaction, and of low pressure of reactants (< 10-4 torr), such as those present in an ultrahigh vacuum surface science experiment. This knowledge of the microscopic origins of the effect of pressure on the chemistry at surfaces has allowed the development of a scheme to bypass the high-pressure requirement. Reactions that are normally observed only at high reactant pressures, and which are the ones most often of practical importance, can now be carried out in low-pressure, ultrahigh vacuum environments.

Collision induced dissociative chemisorption of CH4 on Ni(111) by inert gas atoms: The mechanism for chemistry with a hammer

The dissociation of CH4 physisorbed on Ni(111) at 46 K is observed to be induced by the impact of incident inert gas atoms. The dynamics and mechanism of this new process, collision induced dissociative chemisorption, are studied by molecular beam techniques coupled with ultrahigh vacuum electron spectroscopies. The absolute cross section for collision induced dissociation is measured over a wide range of kinetic energies (28–109 kcal/mol) and incident angles of Ne, Ar, and Kr atom beams. The cross section displays a complex dependence on the energy of the impinging inert gas atom characteristic of neither total nor normal energy scaling. Quantitative reproduction of the complex dependence of the cross section on the Ar and Ne incident energy by a two‐step, dynamical model establishes the mechanism for collision induced dissociation. Collision induced dissociation occurs by the impulsive transfer of kinetic energy upon collision of Ar or Ne with CH4, followed by the translationally activated dissociative ...

The structure and chemistry of CH3 and CH radicals adsorbed on Ni(111)

A detailed analysis of the vibrational spectra of CH3, CH2D, and CD3 adsorbed on Ni(111) and the products of their reactions is presented. The synthesis of adsorbed methyl radicals from CH4, CH3D, or CD4 is effected by molecular beam techniques. The ability to measure these spectra by high‐resolution electron energy loss spectroscopy (HREELS) at higher resolution (35 cm−1) and higher sensitivity (5×106 counts/s) has allowed new features to be observed and a symmetry analysis to be carried out. It is concluded that the CH3 radical is adsorbed with C3v symmetry on a threefold hollow site. The symmetric C–H stretch mode of CH3 and the overtone of the antisymmetric deformation mode are observed to be in Fermi resonance. At temperatures above 150 K, CH3 dissociates to form adsorbed CH. Confirmation for the assignment to a CH species is found in the observation that the spectrum measured after thermal decomposition of CH2D is a superposition of those from the decomposition of CH3 and CD3. The adsorption site of...

Bridge/atop site conversion of CO on Ni(111): Determination of the binding energy difference

A rapid site exchange process is observed in the equilibrated chemisorbed layer of CO on Ni(111). Following adsorption at 298 K, the relative populations of CO adsorbed on atop sites and twofold bridge sites are monitored by the high resolution electron energy loss intensities of the respective CO vibrational modes as a function of surface temperature. Since equilibrium is established, the binding energy difference between the terminal and bridge adsorption sites is determined. The bridge site is more stable than the atop site by 0.94±0.15 kcal/mol at a coverage of 0.13. As the coverage is increased to 0.42, the difference in binding energies decreases to 0.44±0.07 kcal/mol. At saturation coverage, 0.5, the binding energy difference effectively becomes very large, resulting in CO occupation of the twofold bridge sites exclusively.

Collision‐induced desorption of physisorbed CH4 from Ni(111): Experiments and simulations

The desorption of CH4 physisorbed on Ni(111) is observed to be induced by collision with Ar atoms incident with energies less than 2 eV. The absolute cross section for collision‐induced desorption of CH4 in the low coverage limit of an isolated CH4 molecule and from a saturated CH4 monolayer is measured as a function of the kinetic energy and incident angle of the Ar beam. The dominant mechanism for collision‐induced desorption is determined to involve the direct collision of the incident Ar with the physisorbed CH4. Indirect, surface mediated desorption processes and multiple desorptions are found to be unimportant. Three‐dimensional, classical molecular dynamics simulations based upon a hard sphere/hard cube model of the direct collision mechanism show that the complicated dependence of the desorption cross section at low CH4 coverage on the Ar energy and incident angle is the result of two competing dynamical effects: the increase in the geometrical collision cross section and the decrease in the Ar ki...

Effect of translational energy on the molecular chemisorption of CO on Ni(111): Implications for the dynamics of the chemisorption process

The effect of translational energy on the molecular chemisorption of CO on a Ni(111) surface is used as a probe of the dynamics of the adsorption process. Initial adsorption probabilities, apparent saturation coverages, spatially resolved Auger coverage profiles, and high resolution electron energy loss spectra of CO deposited on the Ni surface from a supersonic molecular beam are measured as a function of translational energy. It is found that the initial adsorption process for CO molecules incident with energies less than 4 kcal/mol differs from that for molecules incident with higher energies. Molecules with kinetic energies below 4 kcal/mol adsorb with an initial adsorption probability of 0.85±0.04 and a high apparent saturation coverage. Molecules with translational energies between 7 to 30 kcal/mol have an initial adsorption probability of 0.46±0.03, and an apparent saturation coverage approximately half that of the low energy molecules. Since the CO packing density and the final chemisorption state...

The adsorption of CO and O2 on Ni(111) at 8 K

The adsorption of CO and O2 on Ni(111) at 8 K is studied by molecular beam techniques coupled with high resolution electron energy loss spectroscopy and work function measurements. The work functions are measured by a retarding potential technique using the HREEL spectrometer. The cryogenic crystal temperatures are attained by use of a liquid helium cryostat described in detail. The total and normal components of the translational energy of the incident molecules are lowered to 0.3 and 0.02 kcal mol−1, respectively. No evidence for a trapped, physisorbed CO molecule or a physisorbed molecular O2 species is observed in the limit of zero coverage.

The Mechanism for the Dissociation of Methane on a Nickel Catalyst

Abstract The mechanism for the dissociation of CH 4 on Ni(111) is studied by molecular beam techniques coupled with high resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH 4 increases exponentially with the normal component of the incident molecules translational energy and with vibrational excitation. Dissociation can also be induced by the impact of an Ar atom incident on a monolayer of CH 4 physisorbed on Ni(111). The nascent products of the dissociation are identified as an adsorbed methyl radical and a hydrogen atom. The chemistry and stability of these adsorbed methyl radicals have also been studied. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interaction between the carbon and the Ni surface. Tunneling is suggested as the final process in the C-H bond cleavage. The presence of this barrier to dissociative chemisorption and collision-induced dissociation of adsorbates present plausible explanations for the pressure gap in heterogeneous catalysis.