Wörz Anke S, Heiz Ueli, Cinquini Fabrizio, Pacchioni Gianfranco
Lehrstuhl für Physikalische Chemie 1, Technische Universität München, D-85747 Garching, Germany.
J Phys Chem B. 2005 Oct 6;109(39):18418-26. doi: 10.1021/jp054093o.
Au atoms have been deposited on oxidized and reduced TiO2 thin films grown on Mo(110). The gold binding sites and the occurrence of Au-TiO2 charge transfer were identified by measuring infrared spectra as a function of temperature and substrate preparation. The results have been interpreted by slab model DFT calculations. Au binds weakly to regular TiO2 sites (De < 0.5 eV) where it remains neutral, and diffuses easily even at low temperature until it gets trapped at strong binding sites such as oxygen vacancies (De = 1.7 eV). Here, a charge transfer from TiO2 to Au occurs. Au(delta-)CO complexes formed on oxygen vacancies easily lose CO (De = 0.4 eV), and the CO stretching frequency is red-shifted. On nondefective surfaces, CO adsorption induces a charge transfer from Au to TiO2 with formation of strongly bound Audelta+CO complexes (De = 2.4 eV); the corresponding CO frequency is blue-shifted with respect to free CO. We propose possible mechanisms to reconcile the observed CO desorption around 380 K with the unusually high stability of Au-CO complexes formed on regular sites predicted by the calculations. This implies: (a) diffusion of AuCO complexes above 150 K; (b) formation of gold dimers when the diffusing AuCO complex encounters a Au atom bound to an oxygen vacancy (reduced TiO2) or a second AuCO unit (oxidized TiO2); and (c) CO desorption from the resulting dimer, occurring around 350-400 K.
金原子已沉积在生长于Mo(110)上的氧化和还原TiO₂薄膜上。通过测量红外光谱随温度和衬底制备的变化，确定了金的结合位点以及Au-TiO₂电荷转移的发生情况。结果已通过平板模型密度泛函理论计算进行了解释。金与规则的TiO₂位点弱结合（De < 0.5 eV），在此处它保持中性，并且即使在低温下也容易扩散，直到它被困在诸如氧空位等强结合位点（De = 1.7 eV）。在此，发生了从TiO₂到金的电荷转移。在氧空位上形成的Au(δ-)CO络合物很容易失去CO（De = 0.4 eV），并且CO伸缩频率发生红移。在无缺陷表面上，CO吸附会导致电荷从金转移到TiO₂，并形成强结合的Auδ⁺CO络合物（De = 2.4 eV）；相应的CO频率相对于游离CO发生蓝移。我们提出了可能的机制，以协调在380 K左右观察到的CO脱附与计算预测的在规则位点上形成的Au-CO络合物异常高的稳定性。这意味着：(a) AuCO络合物在150 K以上扩散；(b) 当扩散的AuCO络合物遇到与氧空位（还原的TiO₂）结合的金原子或第二个AuCO单元（氧化的TiO₂）时形成金二聚体；以及(c) 从所得二聚体中发生CO脱附，发生在350 - 400 K左右。
