Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, 8093 Zurich, Switzerland.
Phys Chem Chem Phys. 2009 Nov 21;11(43):10132-9. doi: 10.1039/b910535b. Epub 2009 Sep 16.
Adsorption and desorption of acid molecules (10(-4) M in cyclohexane) with different functional groups, namely salicylic acid (SA), benzoic acid (BA) and 2-methyl-2-hexenoic acid (MHA), on an Al(2)O(3) thin film was studied by in situ polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). We used a flow-through PM-IRRAS cell to induce adsorption and desorption processes by alternating acid concentration. Simultaneous but separate detection of liquid-phase and surface species was achieved in a time-resolved manner by PM-IRRAS providing insight into the adsorption-desorption behavior and their kinetics. The reliability and sensitivity of recorded surface and liquid-phase spectra were assured by comparative measurements using attenuated total reflection infrared spectroscopy (ATR-IRS) and density functional theory (DFT) calculations. All three acids irreversibly adsorbed on the basic alumina sites in a bridging mode. Remarkably, reversibly adsorbing surface species were only detected for SA, whereas BA and MHA adsorption were irreversible. To enhance the sensitivity and to obtain kinetic information on adsorption and desorption processes, PM-IRRAS was combined with modulation excitation spectroscopy (MES) applying periodic acid concentration changes. The MES experiments revealed distinct kinetic responses of liquid-phase SA and two different types of surface adsorbed SA species. One of the adsorbed SA was a dimer-like species loosely bound to the alumina surface and the other was probably strongly bound and interacting with other adsorbed SA molecules by hydrogen-bonding. The formation of the two surface species was induced by the presence of the hydroxyl group in SA which most likely enhances the intermolecular interaction via hydrogen-bonding near and on the alumina surface.
吸附和脱附酸分子（10（-4）在环己烷）与不同的官能团，即水杨酸（SA），苯甲酸（BA）和2 - 甲基-2 - 己烯酸（MHA），在Al（2）O（3）薄膜上通过原位偏振调制红外反射吸收光谱（PM-IRRAS）进行了研究。我们使用了一个流通式 PM-IRRAS 池，通过交替酸浓度来诱导吸附和解吸过程。PM-IRRAS 以时间分辨的方式同时但分别检测液相和表面物种，从而深入了解吸附-解吸行为及其动力学。通过使用衰减全反射红外光谱（ATR-IRS）和密度泛函理论（DFT）计算对记录的表面和液相光谱进行比较测量，确保了记录的表面和液相光谱的可靠性和灵敏度。所有三种酸以桥接模式不可逆地吸附在碱性氧化铝位上。值得注意的是，仅检测到 SA 以可逆方式吸附表面物种，而 BA 和 MHA 的吸附是不可逆的。为了提高灵敏度并获得吸附和解吸过程的动力学信息，PM-IRRAS 与调制激发光谱（MES）相结合，采用周期性酸浓度变化。MES 实验揭示了液相 SA 和两种不同类型的表面吸附 SA 物种的明显动力学响应。吸附的 SA 之一是一种类似二聚体的物质，松散地结合在氧化铝表面上，另一种可能是强烈结合的，通过氢键与其他吸附的 SA 分子相互作用。两种表面物种的形成是由 SA 中的羟基引起的，这很可能通过氢键增强了近氧化铝表面和氧化铝表面上的分子间相互作用。
