Acidic Water Monolayer on Ruthenium(0001)

The ubiquity of water in natural environments makes the interaction of water with solid surfaces an important subject of study in a wide variety of scientific disciplines and technologies. One of the most intensively investigated systems for the interaction of water with metal surfaces is water on Ru(0001), which has become a test system for our understanding of this scientific field. Numerous experimental and theoretical studies conducted during the past decade have greatly improved our understanding of the structure and dynamics of water adsorption on Ru(0001). These studies have reached the consensus that water adsorption leads to the formation of an intact molecularwater layer on the surface at low temperature (less than about 155 K). As the surface is heated, H2O partially dissociates to form amixedOH+H2O+H adsorption layer, in competition with desorption of H2O. On the other hand, D2O does not dissociate on the surface and desorbs intact due to a kinetic isotope effect. Despite the wealth of theoretical and experimental research on this system, there are still many open questions, in particular concerning the acid–base properties of adsorbed water. This information is fundamentally important to heterogeneous catalysis, corrosion, and electrochemistry because it determines the proton transfer and acid–base characteristics of the water–solid interface. Therefore, it is highly desirable to investigate these properties for adsorbed water using a systematic surface science approach; this could be one way to unravel the intricacies of the acid–base chemistry at water–solid interfaces. In the present work, we study the proton-transfer ability of water molecules adsorbed on a Ru(0001) surface by using surface spectroscopic measurements and ammonia adsorption experiments. The study shows that the first monolayer of water is much more acidic than bulk water, with the ability to spontaneously transfer a proton to an ammonia molecule. We prepared a water layer on Ru(0001) in ultrahigh vacuum (UHV) by the adsorption of H2O vapor at 140 K to a monolayer saturation coverage, a condition that is known to produce an intact molecular-water layer on the surface. Then, NH3 was adsorbed onto the H2O monolayer surface for a small coverage [0.04 ML; 1 ML= 1.14 10 moleculescm 2 corresponding to the monolayer density of water on Ru(0001)]. The NH3 molecules served as a probe for the surface acidity. Figure 1 shows the results of low-energy sputtering (LES) and reactive-ion scattering (RIS) measurements for the H2O monolayer before and after the adsorption of NH3. The RIS and LES methods measure neutral and ionic species, respectively, on the surface. For a layer of pure H2O, spectrum a in Figure 1 shows the RIS signal of CsH2O + (m/z= 151 amu/charge), which was produced by the pickup of surface H2O molecules by scattering Cs + projectiles. The peak of elastically scattered Cs ions appeared at m/z= 133. After NH3 adsorption (spectrum b in Figure 1), a CsNH3 + (m/ z= 150) signal appeared with a small intensity, indicating the presence of neutral NH3 adsorbates on the surface. In addition, LES signals appeared for NH4 + (m/z= 18) and NH4(H2O) + (m/z= 36), indicating the presence of NH4 + and its hydrated species. These ammonium signals indicated that protons were transferred from the water monolayer to NH3 adsorbates to form NH4 . In additional experiments, we observed that the ammonium signals exhibited the following features. First, the NH4 + and NH4(H2O) + signals did not appear when NH3 was adsorbed onto a multilayer ice film grown on Ru(0001). This result showed that the first water monolayer was the proton donor to NH3. Second, in order to check if the ammonium signals originated from preformed ions on the surface, we measured the appearance threshold of the NH4 + signal as a function of Cs impact energy for the water monolayer (where the NH4 + was formed by proton transfer) and for the multilayer ice film (where only neutral NH3 was present). The two surfaces showed well-distinguished characteristics for the threshold energy and intensity of NH4 + emission. The NH4 + signal from the water monolayer exhibited a lower threshold energy (20–25 eV) and stronger intensity than that from the multilayer film, which is characteristic for the low-energy sputtering of preformed NH4 + species. On the other hand, on the multilayer ice film, Figure 1. LES and RIS mass spectra of positive ions obtained from a) H2O monolayer formed on Ru(0001) at 140 K, and b) after NH3 adsorption (ca. 0.04 ML) on surface at 80 K. The LES and RIS measurements were conducted at 80 K with a Cs beam energy of 25 eV.