Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA.
J Am Chem Soc. 2012 Oct 31;134(43):18109-15. doi: 10.1021/ja3080117. Epub 2012 Oct 17.
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 and equilibrium constants for such reactions requires knowing the 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 (S(ad)(0)) on single crystal surfaces from temperature programmed desorption data, prove its accuracy by comparison to entropies measured by equilibrium methods, 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), such that S(ad)(0)(T) = 0.70 S(gas)(0)(T) - 3.3R (R = the gas constant), with a standard deviation of only 2R over a range of 50R. These entropies, which are ~2/3 of the gas, are huge compared to most theoretical predictions. This result can be extended to reliably predict prefactors in the Arrhenius rate constant for surface reactions involving such species, as proven here for desorption.
吸附分子参与许多在固体表面上的反应，这些反应具有重要的技术意义。因此，全球范围内都在努力学习如何预测涉及吸附分子的反应速率和平衡常数。对于涉及吸附分子的此类反应，理论计算速率和平衡常数都需要知道吸附分子的熵和焓。虽然已经投入了大量精力来测量和计算定义明确的吸附物的焓，但很少有报道测量吸附物的熵。我们在这里提出了一种从程序升温脱附数据确定单晶表面上吸附分子的标准熵 (S(ad)(0)) 的新方法，通过与平衡方法测量的熵进行比较来证明其准确性，并将其应用于已发表的数据以提取新的熵。最重要的是，当与报告的熵结合使用时，我们发现，在高覆盖率下，它们在线性上跟踪同一温度 (T) 下气相分子的熵，使得 S(ad)(0)(T) = 0.70 S(gas)(0)(T) - 3.3R (R = 气体常数)，标准偏差仅为 2R，覆盖范围为 50R。与大多数理论预测相比，这些熵是气相的~2/3，非常大。这一结果可扩展到可靠地预测涉及此类物种的表面反应的 Arrhenius 速率常数的前因子，这里已通过脱附进行了证明。
