| AVS 59th Annual International Symposium and Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS-TuM |
| Session: | Surface Reactivity of Oxides |
| Presenter: | J.R.V. Sellers, University of Washington |
| Authors: | C.T. Campbell, University of Washington J.R.V. Sellers, University of Washington |
| Correspondent: | Click to Email |
Adsorbed molecules are involved in many reactions on solid surface that are of great technological importance. As such, there has been tremendous effort worldwide to learn how to predict reaction rates and equilibrium constants for reactions involving adsorbed molecules. Theoretical calculation of both the rate constant and equilibrium constant for such reactions require knowing the standard entropy and enthalpy of the adsorbed molecule. While much effort has been devoted to measuring and calculating the enthalpies of well-defined adsorbates, few measurements of the entropies of adsorbates have been reported. We present here a new way to determine the standard entropies of adsorbed molecules (Sad0) on single crystal surfaces from temperature programmed desorption data, prove its accuracy by comparison to entropies directly measured using equilibrium adsorption isotherms on MgO(100), and apply it to published data to extract new entropies. Most importantly, when combined with reported entropies, we find that at high coverage they linearly track the entropy of the gas-phase molecule at the same temperature (T) as:
Sad0(T) = 0.70 Sgas0(T)–3.29R,
with a standard deviation of only 2R from 0 to 60R, where R is the gas constant. These entropies, which are ~2/3 of that for the gas, are huge compared to most theoretical predictions (e.g. the harmonic approximation commonly employed in combination with DFT estimates of reaction barriers). This relationship applies at temperatures where desorption rates are fast enough to perform EAI and TPD measurements (~10-3 to 100 monolayers/s). It provides an important tool to aid in estimating equilibrium constants and rate constants for reactions where these adsorbates are involved, as proven here for prefactors in the Arrhenius rate constant for desorption. The prefactor can be estimated as :
ν = (kBT/h) exp{0.30 Sgas0(T)/R + 3.29 - 9.31 ln[(m/mAr)(T/298K)]},
where m/mAr is the mass of the adsorbate relative to Ar. For longer adsorbed molecules where Sgas0(T) exceeds 60 K (e.g. linear alkanes with >11 carbons), their entropies remain a constant 20.7R below gas entropies, and ~10.4R below Trouton’s Rule for liquid entropies.
Work supported by NSF under CHE-1010287.