Light Alkane Reactivity on Pt(111) Investigated Using Effusive Molecular Beam Methods

Dissociative sticking coefficients (DSCs) were measured for a wide range of normal alkanes on a Pt(111) metal surface using a heated effusive molecular beam technique. The effusive beam emits gas with a cosine angular distribution that allows measurement of angle-resolved DSCs (ar-DSCs), S(ϑ; Tg, Ts) where ϑ is the angle from the surface normal and the gas temperature (Tg) and surface temperature (Ts) are separately varied. Averaging measured thermal ar-DSCs, S(ϑ; T = Tg = Ts) over the cosine flux distribution appropriate to an ambient thermal gas, yields the thermal DSC, S(T), relevant to high pressure catalysis. DSCs along the direction of surface normal, Sn(Tg, Ts), were measured for a number of alkanes under thermal T = Tg = Ts, and non-equilibrium, Tg ≠ Ts, conditions.
A precursor-mediated microcanonical trapping (PMMT) model of gas-surface reactivity, employing RRKM rate constants, allowed for DSC prediction and evaluation of experimental values. A general version was utilized to handle statistical systems and several other versions of the PMMT model were implemented in order to account for dynamical effects as well as gas-surface energy transfer.
Measurements of angle-resolved DSCs provided both great insights into dynamics, as well as allowed for the measurement of S(T) DSCs. Angle-resolved DSCs were measured for CH4, C2H6, and C3H8 at Tg = Ts = 700 K on Pt(111). The distributions were initially sharp for CH4 and increased in broadness up to C3H8. Both CH4 and C2H6 behave dynamically on Pt(111), however, C3H8 seems to be the point at which alkane systems begin to behave statistically. The angle-resolved distributions were integrated to yield the thermal DSCs at T = 700 K, S(T = 700 K). In addition, ar-DSCs were measured for methane on Pt(111) and K/Pt(111) surfaces at T = 900 K and it was found that the angular distribution was sharper on the K/Pt(111) surface.
An Arrhenius analysis of Sn(T) yielded activation energies, Ea, for dissociation of alkanes on Pt(111). Results showed that Ea scales with the van der Waals attractions between the impinging alkane and the metal surface which is approximated by the energies for molecular desorption, Ed, of intact alkanes from the surface. In addition , it is believed that the van der Waals attractions stabilize the products of alkane dissociative chemisorption thus reducing the activation barrier for chemisorption.
Non-equilibrium DSCs can provide insight into reactivity as well as gas-surface energy transfer between the impinging alkane and the metal surface. We find that gas-surface energy transfer increases with the size of the alkane. The tendency of the gas-surface energy transfer is to drive the molecular degrees of freedom towards thermalization to the temperature of the surface. In the case where there is little energy transfer, we find that increasing either Ts or Tg increases the DSCs and the effect of varying one temperature is greatest when the other temperature is relatively low. Conversely, when there is high gas-surface energy transfer, the effect of varying Tg on DSCs is muted and in the limiting case, DSCs become solely dependent on Ts, such that ambient gas dosing experiments at any Ts become equivalent to thermal DSCs, S(T = Ts).