Photochemical Exploration of Reaction Dynamics at Catalytic Metal Surfaces:  From Ballistics to Statistics

Harrison, I.

doi:10.1021/ar9700926

Cited by 16 publications

(11 citation statements)

References 40 publications

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“…The reader might be understandably dissatisfied with our limited efforts to provide an overview and a context for this vast field. Fortunately, others have provided similar contributions to this one and the reader is referred to that additional review literature not otherwise cited below [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].…”

Section: Background and History Including Previous Reviewsmentioning

confidence: 85%

Energy transfer and chemical dynamics at solid surfaces: The special role of charge transfer

Wodtke

Matsiev

Auerbach

2008

Progress in Surface Science

167

161

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Molecular energy transfer processes at solid surfaces are profoundly important, influencing trapping, desorption, diffusion, and reactivity; in short, all of the elementary steps needed for surface chemistry to take place. In this paper we review recent progress in our understanding of energy transfer at surfaces with a particular emphasis on those phenomena, which are peculiar to solids with delocalized electronic structure, e.g. electronically nonadiabatic energy transfer. This area of study represents an area requiring significant extensions of our theoretical understanding, which is largely based on density functional theory. This review provides an overview of some of the experimental and theoretical tools presently being used in this field and a description of several illustrative examples of work that have helped to shape our understanding.

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Section: Background and History Including Previous Reviewsmentioning

confidence: 85%

Energy transfer and chemical dynamics at solid surfaces: The special role of charge transfer

Wodtke

Matsiev

Auerbach

2008

Progress in Surface Science

167

161

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“…46 However, quantitative agreement between theory and experiment was recovered when the threshold energy for CH 4 /Pt(111) dissociative chemisorption was allowed to vary from E 0 ϭ0.63 to 1.2 eV as the coverage of HϩCH 3 on the surface was varied from 0 in the molecular beam experiments to close to full coverage in the laser induced thermal reaction experiments. 47 Arguing along Evans-Polanyi lines, 48 that as the reaction products are destabilized so too is the transition state, the threshold energy to methane dissociation should increase when a clean surface is tempered by an increasing coverage of chemisorbed species. Nevertheless, a doubling of the threshold energy as the chemisorbed HϩCH 3 coverage increases towards saturation may represent an unrealistically rapid rate of increase.…”

Section: Dissociative Sticking Coefficientmentioning

confidence: 99%

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

Bukoski

Blumling

Harrison

2003

The Journal of Chemical Physics

131

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A model of gas-surface reactivity is developed based on the ideas that ͑a͒ adsorbate chemistry is a local phenomenon, ͑b͒ 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 ͑c͒ 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 ͑MURT͒ for analyzing and predicting both thermal equilibrium and nonequilibrium kinetics for surface reactions is applied to the dissociative chemisorption of methane on Pt͑111͒. 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 CH 4 dissociative chemisorption on Pt͑111͒ is E 0 ϭ0.61 eV ͑over a C-H stretch reaction coordinate͒, 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 CH 4 physisorbed on Pt͑111͒ finds the physisorption well depth is 0.16 eV. Thermal equilibrium dissociative sticking coefficients for methane on Pt͑111͒ 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, f j , 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 f t ϭ13%, f r ϭ18%, f v ϭ33%, and f s ϭ36%. For this equilibrium scenario relevant to catalysis, the incident gas molecules supply the preponderance of energy used to surmount the barrier to chemisorption, f g ϭ f t ϩ f v ϩ f r ϭ64%, but the surface contribution at f s ϭ36% remains significant.

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“…In thermal catalysis, about 75% of the energy required to surmount the barrier for dissociative chemisorption derives from the gas ͑i.e., f g ϭ f ϩ f r ϩ f t ϳ75%). 25,62,63 Unfortunately, there are side reactions on Ni surfaces and adsorbed CH 3 radicals thermally decompose upon heating in the zero coverage limit. The reactive molecular vibrational energy distribution shows the most differences from a thermal distribution at T* but this is not so surprising based on the large molecular vibrational quanta ͑e.g., min ϭ 4 ϭ1306 cm Ϫ1 or T v 4 ϭ1879 K) and hence relatively choppy interaction between f v (E v ;Tϭ500 K) and S (E ) as f v,R (E v ;Tϭ500 K) is calculated similarly to Eq.…”

Section: Vibrational State Resolved Dissociative Sticking From Anmentioning

confidence: 99%

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

Abbott

Bukoski

Harrison

2004

The Journal of Chemical Physics

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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)/=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.

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Photochemical Exploration of Reaction Dynamics at Catalytic Metal Surfaces: From Ballistics to Statistics

Cited by 16 publications

References 40 publications

Energy transfer and chemical dynamics at solid surfaces: The special role of charge transfer

Energy transfer and chemical dynamics at solid surfaces: The special role of charge transfer

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

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

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