C−H Bond Activation of Light Alkanes on Pt(111): Dissociative Sticking Coefficients, Evans−Polanyi Relation, and Gas−Surface Energy Transfer

Abstract

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".

Graphene Growth on Pt(111) by Ethylene Chemical Vapor Deposition at Surface Temperatures near 1000 K

“…[72][73] Direct observation of graphene island diffusion into the bulk is observed to occur ≥ 1075 K, i.e. at temperatures higher than used during our CVD so only interaction between gas phase and surface can be taken into consideration.…”

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

“…Earlier ambient gas dissociative sticking coefficient measurements 55,56 found that as T s was reduced measured dissociative sticking coefficients eventually approached an asymptotic value of ∼10 -7 . This lower dissociative sticking coefficient limit was presumed to be due to small amounts (ca., 10 −5 %) of highly reactive carbon-containing impurities in the methane gas.…”

Communication: Angle-resolved thermal dissociative sticking of CH4 on Pt(111): Further indication that rotation is a spectator to the gas-surface reaction dynamics

“…16 Methane molecules prepared in a supersonic beam with nozzle temperature T N suffer negligible vibrational cooling, T v = T N , but substantial rotational cooling, T r = 0.1 T N , due to inter-molecular collisions during the supersonic expansion. 10 A dynamically biased precursor mediated microcanonical trapping (d-PMMT) model of gas-surface reactivity 15 was recently shown to reproduce a diverse variety of effusive, 17,18 supersonic, 10,13,19,20 and laser-pumped 13 supersonic molecular beam dissociative sticking experiments for the CH 4 /Pt(111) system with an average relative discrepancy (ARD = |S theory − S expt |/min (S theory , S expt ) ) between theory and experiment of 43%. The d-PMMT model assumed that molecular rotations and translational motion parallel to the surface were spectator degrees of freedom and that the efficacy of molecular vibrational energy, E v , relative to normal translational energy, E n = E t cos 2 ϑ, in promoting reaction was η v = 0.4.…”