Temperature dependent adsorption dynamics of CH4 on alkane-covered Pt (111)

Supersonic molecular beam experiments were used to probe the temperature de pendence of adsorption of methane and ethane into adsorbed layers of methan e, ethane, propane, and n-butane on Pt(111). Because adsorption must procee d via a transient state from which either desorption or conversion to a bou nd state may occur, the adsorption probability is, in general, temperature dependent. As the surface temperature is lowered below 75 K increasing amou nts of methane adsorbs into an alkane-covered surface to form a mixed adlay er. Methane forms multilayers on methane, ethane, propane, and butane adsor bates below 35 K; ethane forms multilayers on the same adsorbates below 63 K. The condensation temperature of methane and ethane into multilayers is i ndependent of the type of preadsorbed alkane and occurs well below the norm al boiling or melting point of each species. At a given temperature between 75 K and 35 K the steady-state uptake of methane (or ethane between 63 K t o 120 K) into the first layer decreases with increasing carbon chain length of the preadsorbed alkane, reflecting the lesser availability of adsorptio n sites due to more extensive site blocking by the larger alkanes. The temp erature-dependent sticking probability and steady-state uptake indicate an extrinsic precursor mechanism to molecular adsorption on all the alkane-cov ered surfaces. For methane adsorption on ethane-saturated Pt(111), the diff erence in activation energies for desorption and migration to a binding sit e from the extrinsic precursor, E-d' - E-m', is 5.03 kJ/mol and the ratio o f preexponential factors, k(d)' ((0))/km ' ((0)), is 4.1 x 10(3).