Barros Ricardo B, Garcia Ana Rosa, Ilharco Laura M
Centro de Química-Física Molecular, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal.
Chemphyschem. 2005 Jul 11;6(7):1299-306. doi: 10.1002/cphc.200400546.
The influence of oxygen precoverage on the bonding geometry of methoxide on Ru(001) was studied using the isotopically labeled molecule CHD2OH by reflection-absorption infrared spectroscopy (RAIRS). This molecule is an excellent model because the vibrational spectra of CHD2O- may be unambiguously correlated with the adsorption configuration. For Ru(001)--O layers with an effective oxygen coverage (theta0) between 0.25 and 0.6 ML (ML=monolayer), the influence of the oxygen precoverage was shown to vary with the initial methanol exposure. For an extremely low dose of [D2]methanol (0.01 L; L=Langmuir, 1 L=10(-6) torr s), at 90 K, no oxygen-coverage effects were detected on the geometry of [D2]methoxide: it adsorbs in an upright orientation (pseudo-C(3v) local symmetry), just as on clean Ru(001). An increase in the methanol exposure to 0.1 L, at the same temperature, results in the formation of a disordered layer of tilted methoxide: for theta(O)=0.25 ML, C(s)/C1 and intrinsic C1 configurations are present on the surface, whereas for theta(O)> or =0.5 ML, only the former species were identified. The thermal activation of these tilted layers to 105 K results in a lower coverage of upright methoxide for any oxygen precoverage, coadsorbed with decomposition products, as confirmed by the detection of adsorbed formaldehyde and, on the denser oxygen layer (theta(O)=0.6 ML), formate. The influence of the oxygen precoverage becomes determinant when annealing a [D2]methanol multilayer to 105 K: for theta(O)=0.25 ML, the RAIR spectrum correlates with a disordered layer of tilted methoxide and formaldehyde, whereas for theta(O)=0.6 ML upright methoxide, formate, and carbon monoxide were identified. On clean Ru(001), for methanol exposures > or =0.1 L, the C(3v) methoxide configuration was never attained upon thermal activation.
利用反射吸收红外光谱（RAIRS），通过同位素标记分子CHD2OH研究了氧预覆盖对甲醇盐在Ru(001)上键合几何结构的影响。该分子是一个优秀的模型，因为CHD2O-的振动光谱可以明确地与吸附构型相关联。对于有效氧覆盖率（θ0）在0.25至0.6单层（ML = 单层）之间的Ru(001)-O层，结果表明氧预覆盖的影响随初始甲醇暴露量而变化。对于极低剂量的[D2]甲醇（0.01 L；L = 朗缪尔，1 L = 10(-6) 托秒），在90 K时，未检测到氧覆盖率对[D2]甲醇盐几何结构的影响：它以直立取向吸附（伪C(3v)局部对称性），就像在清洁的Ru(001)上一样。在相同温度下，将甲醇暴露量增加到0.1 L，会导致形成倾斜甲醇盐的无序层：对于θ(O)=0.25 ML，表面存在C(s)/C1和本征C1构型，而对于θ(O)≥0.5 ML，仅鉴定出前一种物质。将这些倾斜层热激活至105 K会导致在任何氧预覆盖情况下，直立甲醇盐的覆盖率降低，并与分解产物共吸附，这通过检测吸附的甲醛以及在较致密的氧层（θ(O)=0.6 ML）上检测到甲酸盐得到证实。当将[D2]甲醇多层膜退火至105 K时，氧预覆盖的影响变得至关重要：对于θ(O)=0.25 ML，RAIR光谱与倾斜甲醇盐和甲醛的无序层相关，而对于θ(O)=0.6 ML，鉴定出直立甲醇盐、甲酸盐和一氧化碳。在清洁的Ru(001)上，对于甲醇暴露量≥0.1 L，热激活后从未达到C(3v)甲醇盐构型。
