Experimental Characterization of Unimolecular Water Dissociative

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Experimental Characterization of Unimolecular Water Dissociative Adsorption on α‑Alumina Harald Kirsch,† Jonas Wirth,‡ Yujin Tong,† Martin Wolf,† Peter Saalfrank,‡ and R. Kramer Campen*,† †

Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany Institute of Chemistry, University of Potsdam, Karl-Liebknecht Straße 24-25, D-14476 Potsdam, Germany

‡

S Supporting Information *

ABSTRACT: α-Al2O3 surfaces are common in both engineered applications and the environment. Much prior work indicates that their properties, e.g., reactivity, polarity, and charge, change dramatically on interaction with water. Perhaps the simplest question that can be asked of α-Al2O3/water interaction is how a single water molecule interacts with the most stable α-Al2O3 surface: the αAl2O3(0001). Over the last 15 years, a series of theoretical studies have found that water dissociatively adsorbs on α-Al2O3(0001) through two channels. However, to our knowledge no experimental evidence of these dissociation pathways has appeared. By combining sample preparation via supersonic molecular beam dosing, sample characterization via coherent, surface specific vibrational spectroscopy and electronic structure theory, we report the first experimental observation of reaction products of each, theoretically predicted, dissociation channel. These results thus overcome a 15 year old experiment/theory disconnect and make possible a variety of intriguing experiments that promise to provide significant new insights into water/Al2O3 and water/oxide interaction more generally.

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INTRODUCTION Alumina surfaces are ubiquitous in heterogeneous catalysis, optics, and electronics and a useful model system for more complicated, environmentally abundant, alumino-silicate phases.1,2 Because their reactivity, polarity, and structure change dramatically on interaction with water, water/alumina chemistry has been studied, both experimentally and theoretically, for decades.3 While much significant insight has been attained, gaining a molecular level picture of this interaction has proven surprisingly challenging: e.g., the OH stretch spectral response of the α-Al2O3(0001)/liquid water interface as a function of pH differs dramatically when investigated by different groups.4−6 Two origins of this challenge seem possible: difficulty in reproducibly preparing samples that are clean and have a well-defined surface termination or difficulty in interpretation of the experimental observables characterizing interfacial liquid water.7 Sample preparation and study of water/alumina interaction under high or ultrahigh vacuum (UHV) in principal allows both these hurdles to be overcome by enabling more straightforward complementary characterization of surface structure and possible contamination as well as the controlled addition of small amounts of water. While probing the interaction of small amounts of water and alumina surfaces in vacuum is thus of practical importance, it is also of scientific interest in its own right. A variety of recent studies of nanoparticle catalysts on alumina supports have shown that nanoparticle structure, and hence catalyst reactivity, is strongly a function of alumina exposure to water.2,8,9 As such systems generally contain © 2014 American Chemical Society

alumina surfaces in the presence of the sort of limited amounts of water accessible in vacuum based experiments, clearly catalyst optimization would benefit from understanding exactly how water and these surfaces interact under these sorts of controlled conditions. Because α-Al2O3(0001) is the most stable of the common alumina surfaces, the great majority of prior work probing water/alumina interaction has focused on this crystal face.3,10−17 From an experimental perspective, the (1 × 1) Al-terminated surface of α-Al2O3(0001) has been shown to be the most stable in vacuum and the probability of water’s dissociative adsorption (i.e., water’s sticking coefficient) on it to be strongly pressure dependent (over 7 orders of magnitude in water pressure).3,13,15,16 Thermal desorption spectroscopy (TDS) measurements of samples prepared via exposure to mbar water pressures show a long, high-temperature tail consistent with water dissociative adsorption.11,14 From a theoretical perspective, the Al-terminated (1 × 1) αAl2O3(0001) surface has also been shown to be the most stable in vacuum.18 Following the pioneering work of Hass and co-workers 15 years ago,12 on exposure of the Al terminated alpha-Al2O3(0001) surface to water, a series of studies has clarified, using different model chemistries and different system sizes, that there are two dominant mechanisms of unimolecular water dissociative adsorption (see Figure 1).12,19−25 Despite Received: February 28, 2014 Revised: May 27, 2014 Published: May 28, 2014 13623

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Figure 1. (1 × 1) Al-terminated α-Al2O3(0001) surface with D2O adsorbed via the 1−2 (a) and 1−4 (b) dissociation channels. The 1−4′ configuration (c) was taken as a model for configurations within which adsorbed deuterons have diffused away from the adsorbed OD group. Aluminum atoms are shown as large gray, alumina surface oxygens as large red, water oxygens as small red, and deuterium atoms as small white balls.

aluminum terminated surface with water using a supersonic molecular beam source (MBS). Preparing samples using the MBS allows us to dramatically increase the number of dissociatively adsorbed molecules present at the surface while remaining in UHV (see Supporting Information for a detailed discussion of this point). For characterization, we employ vibrationally resonant sum frequency generation (VSF) spectroscopy to probe the OD stretch response of interfacial species. By comparing VSF spectra measured under different experimental geometries with calculated frequencies and geometries, we here identify the products of both theoretically predicted dissociation pathways: we experimentally characterize the theoretically predicted, but heretofore unobserved, fragments resulting from unimolecular water dissociation on αAl2O3(0001). We thus bridge an experiment/theory gap that is 15 years old and pave the way for comprehensive descriptions of water reactivity on this paradigmatic surface.

this fairly clear theoretical consensus, there has thus far been no experimental evidence for these predicted pathways. One possible origin of this lack of experimental evidence is that, in general, the amount of water molecules present in theoretical and experimental studies has differed. For practical reasons, most theoretical studies of water reaction (i.e., studies that account for breaking and formation of the OH bond in H2O) have been performed using density functional approaches applied to submonolayer water coverages and water fragments that interact only weakly with their neighbors. From an experimental perspective, because of water’s low sticking coefficient on α-Al2O3(0001), most studies have prepared samples at relatively high, i.e., mbar, water pressures before transfer to UHV for sample characterization. If the kinetics and thermodynamics of water/α-Al2O3(0001) interaction change as a function of water pressure (i.e., as a function of surface coverage or as a result of mixed water layers containing both dissociatively and molecularly adsorbed waters), it seems likely that computation and experiments have investigated different surface chemistries. In agreement with this expectation, prior computational studies have suggested that the transition state free energy (the reaction barrier) for unimolecular water dissociative adsorption on α-Al2O3(0001) may be lowered in the presence of an additional water molecule at the surface12 and this effect has been invoked to explain water’s experimentally observed pressure dependent sticking coefficient.3 A second possible origin of this experiment/theory disconnect lies in limitations of the observables extracted from experiment. As discussed above, we would like to understand the manner in which water dissociatively adsorbs on α-Al2O3(0001) or, equivalently, the manner in which this surface hydroxylates. Problematically, the great majority of the experimental tools thus far employed to study α-Al2O3(0001) in UHV do not allow direct interrogation of surface hydroxylation (as it is not generally possible to detect hydrogens in X-ray based techniques, while electron based techniques face problems with charging and dehydroxylation).13,16 While in principle conventional optical vibrational spectroscopy (e.g., reflection absorption infrared) could overcome this problem through probing of the OH stretch, to our knowledge there has thus far been no work along these lines, perhaps because of the low infrared reflectivity (and attendant weak signals) from the oxide surface.26 In this work, then, we study the interaction of heavy water (D2O) with the α-Al2O3(0001) surface in UHV. In contrast to previous studies, we prepare our samples by dosing the clean,

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METHODS Experimental Section. The experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10−10 mbar, a molecular beam source for sample dosing, a quadrupole mass spectrometer to allow in situ determination of surface coverage, and transparent optical windows to enable VSF measurements on samples within the chamber (see Supporting Information for further details of the sample set up). All samples interrogated in this study were α-Al2O3(0001) crystals, 10 × 15 × 0.5 mm3, and polished on one side to a roughness 100 cm−1 to still lower energies47,48 (see Supporting Information for further discussion of the offset in absolute vibrational frequencies). Part of the challenge here is that, while gas phase cluster measurements of various water-containing systems furnish excellent benchmarks for calculation of various molecular properties using atom centered basis sets,49,50 similar benchmark systems for water on oxide surfaces are more challenging to create. While beyond the scope of this article, in this sense we expect the measurements presented here to be a valuable target for the assessment and benchmarking of model chemistries for the calculation of water properties on oxide surfaces. Regardless of the deviation in absolute frequencies between the experiment and computation, as remarked above, and as has been extensively documented for molecular systems, differences between the frequencies of various normal modes seem to be well described. Focusing, therefore, on relative frequency differences between modes, the main discrepancy between experiment 1−2 and computation is that the ν̃1−4 surf − ν̃surf is slightly larger and 1−2 1−2 ν̃ads − ν̃surf slightly smaller in the experiment than in the computation. As described in the Methods section, the frequencies presented in Table 1 are calculated for a perfectly periodic quarter monolayer of adsorbates. Clearly this situation likely differs from that of the experiment. At (locally) higher surface coverages, dipole/multipole coupling between OD groups may influence the resulting calculated frequencies in the sense of shifting them by ≈20−30 cm−1 to lower frequencies (see Supporting Information for further details). In the absence of more detailed and experimentally accessible insight into the spatial relationship between 1−2, 1−4, and 1−4′ fragments, it seems plausible that such coupling may in part explain the observed experiment/computation difference.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information on the UHV and laser setups as well as the sample dosing protocol; results of control experiments that sharply constrain the amount of molecularly adsorbed D2O present in our samples; calculation of the dependence of the measured Ivsf on VIS and IR incident angles assuming interfacial ODs have a Gaussian orientational distribution and move rapidly on the time scale of the inverse line width; description of the detail of the anharmonic correction to calculated frequencies and further discussion of the deviation of calculated absolute frequencies from experiment; and calculation of the OD stretch frequencies and orientation of molecularly adsorbed D2O. This material is available free of charge via the Internet at http://pubs.acs.org.

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SUMMARY AND CONCLUSIONS The adsorption of D2O on the (1 × 1) Al-terminated αAl2O3(0001) surface was studied experimentally by VSF spectroscopy and computationally by plane-wave based density functional theory. Samples were prepared by dosing D2O using a molecular beam source. Calculation of anharmonically corrected normal-mode frequencies for a suite of optimized geometries has demonstrated that on the α-Al2O3(0001) surface, in the absence of molecularly adsorbed D2O, OD fragments with five unique (two are indistinguishable) frequencies should exist corresponding to the ODsurf and ODads fragments associated with the 1−2, 1−4, and 1−4′ dissociation channels. However, given OD fragment orientation, these five frequencies are not expected to be equally intense in our two accessible experimental geometries. VSF analysis of this surface shows five spectral features, three of which are apparent in geometry 1 employing the ppp polarization condition, while the other two are apparent only in geometry 2 employing ssp. The relative frequencies and the intensity of each mode under the two employed experimental conditions are consistent with computation and strongly suggest the assignment shown in Table 2. The dissociative adsorption of water on oxide surfaces drastically changes oxide surface structure and reactivity.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49/0 30 8413-5230. Fax: +49/0 30 8413-5206. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft for support of this work through Collaborative Research Center 1109 Understanding of Metal Oxide/Water Systems at the Molecular Scale: Structural Evolution, Interfaces and Dissolution. R.K.C. 13628

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thanks Tobias Kampfrath for careful reading of the manuscript and discussion of the results.

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