No abstract

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 reactivity of CH(4) impinging on a Pt(111) surface was examined using a precursor-mediated microcanonical trapping model of dissociative chemisorption wherein the effects of rotational and vibrational energy could be explored. Dissociative sticking coefficients for a diverse range of non-equilibrium effusive beam, supersonic beam, and eigenstate-resolved experiments were simulated and an average relative discrepancy between theory and experiment of better than 50% was achieved by treating molecular rotations and translation parallel to the surface as spectator degrees of freedom, and introducing a dynamically-biased vibrational efficacy. The model parameters are {E(0) = 57.9 kJ mol(-1), s = 2, η(v) = 0.40} where E(0) is the apparent threshold energy for reaction, s is the number of surface oscillators participating in energy exchange within each gas-surface collision complex formed, and η(v) is the mean vibrational efficacy for reaction relative to normal translational energy which figures in the assembly of the active exchangeable energy which is available to surmount the activation barrier to dissociative chemisorption. GGA-DFT electronic structure calculations provided vibrational frequencies for the transition state for dissociative chemisorption. The asymmetry of the rotational state populations in supersonic and effusive molecular beam experiments allowed kinetic analysis to establish that taking rotation as a spectator degree of freedom is a good approximation. Surface phonons, rather than the incident molecules, are calculated to play the dominant role in supplying the energy required to overcome the activation barrier for dissociative chemisorption under the thermal equilibrium conditions relevant to high pressure catalysis. Over the temperature range 300 K ≤T≤ 1000 K, the thermal dissociative sticking coefficient is predicted to be well described by S(T) = S(0) exp(-E(a)/RT) where S(0) = 0.62 and E(a) = 62.6 kJ mol(-1).

Effusive molecular beam experiments were used to measure alkane dissociative sticking coefficients, S(T g ,T s ), for which the impinging gas temperature, T g , and surface temperature, T s , could be independently varied. The 400-1000 K temperature range examined should be relevant to heterogeneously catalyzed industrial processes such as the steam reforming of alkanes. Methane, ethane, and propane all showed increasing dissociative sticking as either T g or T s were increasedsindicative of an activated reaction mechanism. Effusive beam experiments with gas impinging along the surface normal and T g ) T s ) T determined S n (T), a close approximation and formal upper bound to the thermal dissociative sticking coefficient, S(T), appropriate to reaction with a thermal ambient gas. Activation energies determined from S n (T) for methane, ethane, and propane are E a ) 58, 43, and 34 kJ mol -1 , respectively. An Evans-Polanyi plot of E a for alkane dissociative chemisorption versus the alkane thermal desorption energy, E D , is linear with a slope of -0.94. Assuming that the alkane E D serves as a good approximation to the van der Waals stabilization of the chemisorbed alkyl radical product of dissociative chemisorption, the slope of the Evans-Polanyi plot indicates a late transition state for alkane dissociative chemisorption on Pt(111). A microcanonical unimolecular rate theory (MURT) model of dissociative chemisorption was used to analyze the effusive molecular beam experiments. Explicit accounting of the gas-surface energy transfer for the nonequilibrium experiments became increasingly important as the alkane size was increased. A simple exponential down model of the molecule/phonon collision step size distribution with a mean energy down parameter of R ) 350 cm -1 for ethane, and R ) 1400 cm -1 for propane, sufficed to provide a good description of the S n (T g ,T s ) data. The methane S n (T g ,T s ) values reported here for effusive molecular beams are roughly 2.5 times smaller than expectations based on MURT analysis of earlier, higher energy, supersonic molecular beam experiments. † Part of the "D. Wayne Goodman Festschrift".

Effusive molecular beam measurements of angle-resolved thermal dissociative sticking coefficients for CH4 impinging on a Pt(111) surface, at a temperature of 700 K, are reported and compared to theoretical predictions. The reactivity falls off steeply as the molecular angle of incidence increases away from the surface normal. Successful modeling of the thermal dissociative sticking behavior, consistent with existent CH4 supersonic molecular beam experiments involving rotationally cold molecules, required that rotation be treated as a spectator degree of freedom.