Quantum Nature of the Proton in Water-Hydroxyl Overlayers on Metal Surfaces

Xin-Zheng Li, Matthew I. J. Probert, Ali Alavi, and Angelos Michaelides

Phys. Rev. Lett. 104, 066102 – Published 11 February 2010

Abstract

Using ab initio path-integral molecular dynamics, we show that water-hydroxyl overlayers on transition metal surfaces exhibit surprisingly pronounced quantum nuclear effects. The metal substrates serve to reduce the classical proton transfer barriers within the overlayers and, in analogy to ice under high pressure, to shorten the corresponding intermolecular hydrogen bonds. Depending on the substrate and the intermolecular separations it imposes, the traditional distinction between covalent and hydrogen bonds is lost partially [e.g., on Pt(111) and Ru(0001)] or almost entirely [e.g., on Ni(111)]. We suggest that these systems provide an excellent platform on which to systematically explore the magnitude of quantum nuclear effects in hydrogen bonds.

Received 29 September 2009

DOI:https://doi.org/10.1103/PhysRevLett.104.066102

Authors & Affiliations

Xin-Zheng Li¹, Matthew I. J. Probert², Ali Alavi³, and Angelos Michaelides^1,*

¹London Centre for Nanotechnology and Department of Chemistry, University College London, London WC1E 6BT, United Kindgom
²Department of Physics, University of York, York YO10 5DD, United Kingdom
³Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom

^*angelos.michaelides@ucl.ac.uk

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Vol. 104, Iss. 6 — 12 February 2010

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Images

Figure 1
Structure of the $\sqrt{3} \times \sqrt{3} - R 30 °$ overlayer (with classical nuclei) that forms on metal surfaces. Side views when the proton is donated from water to hydroxyl (upper, labeled “short”) and from hydroxyl to water (lower, labeled “long”) are shown on the right. Classically, at the ground state, the short and long H bond lengths are $\sim 1.7$ and $\sim 2.1 Å$ on Pt, $\sim 1.6$ and $\sim 1.9 Å$ on Ru, and $\sim 1.4$ and $\sim 1.6 Å$ on Ni, respectively. The coordinate for proton transfer $δ$ is defined as $R_{O_{a} H} - R_{O_{b} H}$ , where $R_{O_{a} H}$ and $R_{O_{b} H}$ are the instantaneous O-H distances between $O_{a}$ and H and $O_{b}$ and H, respectively. For a proton equidistant from its two O neighbors $δ = 0$ and upon transfer from one O to another $δ$ changes sign.Reuse & Permissions
Figure 2
Selected structural properties of the classical and quantum overlayers. Probability distributions of O-H [(a), (c), (e)] and O-O distances [(b), (d), (f)] on Pt, Ru, and Ni at 160 K, as obtained from MD with classical nuclei (labeled classical, black solid lines) and PIMD [labeled quantum, (red) dashed lines]. On the bottom [(g)–(i)] snapshots for typical spatial configurations of the overlayer on Pt (left), Ru (middle), and Ni (right) obtained from PIMD (with every atom represented by 16 beads) are shown. On Pt and Ru at any given snapshot one proton is equally shared by two of the Os yielding an intermediate “ $H_{3} O_{2}$ ” complex. On Ni at any given snapshot several protons can simultaneously be shared between the oxygens.Reuse & Permissions
Figure 3
Free energy profiles ( $Δ F$ ) for the protons along the intermolecular axes on Pt (left), Ru (middle), and Ni (right) at 160 K, obtained from MD (labeled classical, black solid lines) and PIMD [labeled quantum, (red) dashed lines]. The reaction coordinate $δ$ is defined in Fig. 1. The free energy is calculated from $Δ F (δ) = - k_{B} T \ln [P (δ)]$ , where $P (δ)$ is the probability distribution of $δ$ and $k_{B}$ is the Boltzmann constant.Reuse & Permissions
Figure 4
Probability distributions in the MD [(a), (d), (g)] and PIMD [(b), (e), (h)] simulations as functions of $δ$ and $R_{OO}$ . $δ$ is the reaction coordinate as defined in Fig. 1. In panels (c), (f), and (i) probability distribution functions for only the “most active proton,” that is, the proton with smallest $δ$ at each time step, are shown. All MD and PIMD distribution functions have been symmetrized about $δ$ . For more details on the simulation lengths, see the supplementary information in Ref. 22.Reuse & Permissions

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