Department of Chemistry and Biochemistry, and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, USA.
J Am Chem Soc. 2010 Feb 24;132(7):2202-7. doi: 10.1021/ja907061m.
The high adsorbate coverages that form on the surfaces of many heterogeneous catalysts under steady-state conditions can significantly lower the activation energies for reactions that involve the coupling of two adsorbed intermediates while increasing those which result in adsorbate bond-breaking reactions. The influence of the surface coverage on the kinetics of metal-catalyzed reactions is often ignored in theoretical and even in some ultrahigh vacuum experimental studies. Herein, first principle density functional theoretical calculations are combined with experimental surface titration studies carried out over well-defined Pd(111) surfaces to explicitly examine the influence of coverage on the acetoxylation of ethylene to form vinyl acetate over Pd. The activation energies calculated for elementary steps in the Samanos and Moiseev pathways for vinyl acetate synthesis carried out on acetate-saturated palladium surfaces reveal that the reaction proceeds via the Samanos mechanism which is consistent with experimental results carried out on acetate-saturated Pd(111) surfaces. The rate-limiting step involves a beta-hydride elimination from the adsorbed acetoxyethyl intermediate, which proceeds with an apparent calculated activation barrier of 53 kJ/mol which is in very good agreement with the experimental barrier of 55 +/- 4 kJ/mol determined from kinetic measurements.
在稳态条件下，许多多相催化剂表面形成的高吸附覆盖度可以显著降低涉及两个吸附中间体偶联的反应的活化能，同时增加导致吸附体键断裂反应的活化能。在理论甚至一些超高真空实验研究中，通常忽略表面覆盖率对金属催化反应动力学的影响。在此，通过对具有明确 Pd(111)表面的实验表面滴定研究，结合第一性原理密度泛函理论计算，明确考察了覆盖率对 Pd 上乙烯乙酰氧基化形成醋酸乙烯酯的影响。在醋酸盐饱和钯表面上进行的醋酸乙烯酯合成的 Samanos 和 Moiseev 途径的基本步骤的计算活化能表明，反应通过 Samanos 机制进行，这与在醋酸盐饱和 Pd(111)表面上进行的实验结果一致。限速步骤涉及从吸附的乙酰氧基乙基中间体中β-氢消除，其表观计算活化能垒为 53 kJ/mol，与从动力学测量确定的 55 +/- 4 kJ/mol 的实验能垒非常吻合。
