Lim Kok Hwa, Moskaleva Lyudmila V, Rösch Notker
Department Chemie, Theoretische Chemie, Technische Universität München, 85747 Garching, Germany.
Chemphyschem. 2006 Aug 11;7(8):1802-12. doi: 10.1002/cphc.200600262.
PdZn (1:1) alloy is assumed to be the active component of a promising catalyst for methanol steam reforming. Using density functional calculations on periodic supercell slab models, followed by atomistic thermodynamics modeling, we study the chemical composition of the surfaces PdZn(111) and, as a reference, Cu(111) in contact with water and hydrogen at conditions relevant to methanol steam reforming. For the two surfaces, we determine similar maximum adsorption energies for the dissociative adsorption of H(2), O(2), and the molecular adsorption of H(2)O. These reactions are calculated to be exothermic by about -40, -320, and -20 kJ mol(-1), respectively. Using a thermodynamic analysis based on theoretically predicted adsorption energies and vibrational frequencies, we determine the most favorable surface compositions for given pressure windows. However, surface energy plots alone cannot provide quantitative information on individual coverages in a system of coupled adsorption reactions. To overcome this limitation, we employ a kinetic model, from which equilibrium surface coverages of H, O, OH, and H(2)O are derived. We also discuss the sensitivity of our results and the ensuing conclusions with regard to the model surfaces employed and the inaccuracies of our computational method. Our kinetic model predicts surfaces of both materials, PdZn and Cu, to be essentially adsorbate-free already from very low values of the partial pressure of H(2). The model surfaces PdZn(111) and Cu(111) are predicted to be free of water-related adsorbates for a partial H(2) pressure greater than 10(-8) and 10(-5) atm, respectively.
PdZn（1:1）合金被认为是一种有前景的甲醇蒸汽重整催化剂的活性成分。通过对周期性超胞平板模型进行密度泛函计算，随后进行原子热力学建模，我们研究了与甲醇蒸汽重整相关条件下，PdZn(111)表面以及作为参考的Cu(111)表面与水和氢气接触时的化学成分。对于这两个表面，我们确定了H₂、O₂的解离吸附以及H₂O的分子吸附具有相似的最大吸附能。计算得出这些反应分别放热约-40、-320和-20 kJ·mol⁻¹。基于理论预测的吸附能和振动频率进行热力学分析，我们确定了给定压力范围内最有利的表面组成。然而，仅表面能图无法提供关于耦合吸附反应体系中单个覆盖度的定量信息。为克服这一限制，我们采用了一个动力学模型，从中推导出H、O、OH和H₂O的平衡表面覆盖度。我们还讨论了我们的结果以及由此得出的结论对于所采用的模型表面和计算方法不准确之处的敏感性。我们的动力学模型预测，从非常低的H₂分压值开始，PdZn和Cu这两种材料的表面基本上都没有吸附质。对于PdZn(111)和Cu(111)模型表面，预测当H₂分压分别大于10⁻⁸和10⁻⁵ atm时，表面不存在与水相关的吸附质。
