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Adsorption, Diffusion, and Reactions of Water

Understanding the interactions of water with a metal surface is fundamental in the study of electrochemistry and catalysis. A key question is how does water interact with the surface---substrate/adsorbate, or SA interactions---relative to its interaction with itself---adsorbate/adsorbate, or AA interactions. The extent of clustering of water on a metal surface indicates: (1) its availability for participation in surface reactions; and (2) the fraction of contiguous, open surface sites available for other reactions, whether or not they involve water.

We have used density functional theory (DFT) to examine water adsorption, diffusion, and reaction in an effort to understand how water participates in heterogeneous catalytic and electrocatalytic reactions.

Water Adsorption on Terrace, Step, and Kink Sites

On platinum, the adsorption energy of monomeric water (Fig. 1a) is 0.3 eV, about the strength of a hydrogen bond. Adsorption of additional water molecules leads to growth of water clusters, as water diffusion is facile on Pt. The adsorption energy of dimer, trimer, (Fig. 1b,c) and larger clusters is approximately 0.5 eV, more in line with the heat of sublimation of water. The increase in adsorption energy with cluster size is interpreted as the result of hydrogen bonding among the water molecules.

For clusters on steps (Fig. 1f,i), hydrogen bonding has little influence on adsorption energy. Instead, the stronger interactions of water with the steps is the dominating factor in adsorption energy. Although the adsorption energy is still about 0.5 eV, it is due to adsorption bonds formed between water and the metal atoms of the steps. These substrate/adsorbate bonds cause water to adopt orientations unfavorable to hydrogen bonding, so strong hydrogen bonding is not possible.

These results provide the following description of water adsorption on Pt. Beginning with a clean surface, the first water molecules adsorb as monomers that rapidly migrate along the surface until finding either a step or a kink at which they adsorb strongly with little further diffusion. The process continues until all step and kink sites are filled, beyond which adsorption occurs on the terrace until complete coverage of the surface. Desorption occurs in the reverse process. Water molecules desorb from the terrace first, followed by molecules that diffuse from kinks and steps to the terrace and then desorb from the terrace.

Diffusion

Diffusion of monomeric water from terrace to step to kink is shown for a type B step/kink in Fig. 2 (upper portion). Path D1 is for terrace diffusion, D6 for terrace-step, D7 for along the step, and D12 for step-kink. The energy landscape for the entire diffusion process, D1--D12, is shown in the lower portion of Fig. 2 (dashed line) along with the analogous process for the type A step (solid line). The largest diffusion barrier of 0.22--0.26 eV is for the step-kink path, D12 (D10 for the type A step). Once at the kink, water is strongly bound and---with a diffusion barrier of 0.44 eV---will not diffuse away from the kink. Other significant barriers for diffusion from terrace-to-kink are terrace-terrace (D1) at 0.20 eV, and step-step for the type A step (D5) at 0.22 eV.

The calculations also reveal the possibility of complex water molecule motion along the terrace-terrace path, shown in Fig. 3. In a simple translation, water is parrallel to the surface for the first portion of the reaction coordinate, with the hydrogens lifting up from surface parallel at the end of the trajectory. Two other paths with similar diffusion barriers are the roll, in which the hydrogens roll over the top of the oxygen atom, and the flip, in which the hydrogens pass below the oxygen atom. While the translation path has the lowest diffusion barrier, our results do not allow the other two to be excluded as possibilities.

Interconversion of HCO and COH Intermediates

Carbon monoxide is a common intermediate and, if it remains on the surface, a poison in heterogeneous catalytic and electrocatalytic reactions. In direct methanol fuel cells, for example, CH3OH successively dehydrogenates until forming CO, which poisons the surface. In developing CO-tolerant electrocatalysts, it is necessary to understand the mechanism for CO production.

Two intermediates of the CO formation reation have been proposed: formyl (HC=O) and hydroxymethylidyne (COH). Despite numerous studies, these species have been difficult to detect experimentally, due in large part to their transient nature. We have examined the adsorption and activation energies of HCO and COH with DFT to understand their stability with respect to CO and to each other.

Figure 4 shows the energy landscape (top) and configurations (bottom) of HCO, CO + H, and COH on a Pt(111) surface. These species were studied in the absence of coadsorbed water (clean) and with one water molecule. On the clean surface (green line), CO + H is more stable than either HCO (1.0 eV) or COH (0.48 eV). Formyl has a modest activation energy (0.33 eV) for reaction to CO + H, whereas COH has a larger barrier (1.02 eV). The CO + H complex has activation barriers of at least 1.33 eV, and so will not react to either HCO or COH.

The presence of one water molecule alters the HCO-COH surface chemistry substantially. All three complexes; HCO, CO + H, and COH are stabilized by water. More importantly, the CO + H + H2O complex has nearly the same adsorption energy as COH + H2O with essentially no activation barrier between them (orange line). Thus, there is rapid interconversion between the two complexes, and an experimental measurement would be unlikely to isolate the COH intermediate. Direct conversion from HCO to COH (blue line) has a moderate activation barrier.

Adsorption with a full water bilayer (not shown) changes the surface chemistry further. The COH intermediate is unstable and dissociates to CO + H. The HCO species is stable, however, and amenable to detection. However, there is no vibrational mode around 1700 cm-1 as would be expected for a carbonyl group.

The configurations in the lower portion of Fig. 4 show that formyl adsorbs at a bridge and interacts with one water molecule at an atop site. In the CO + H + H2O complex, CO adsorbs at a hollow site, and water interacts with a hydrogen atom at an atop site. The H2O-H group is similar to a H3O+ hydronium ion. In the COH + H2O complex, COH adsorbs at a hollow site, and its interaction with water is strong enough to displace water away from the surface.

These results are described in the following publications:

Árnadóttir, L., E. M. Stuve, and H. Jónsson, “Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks I. Configurations, energies, and hydrogen bonding,” Surface Science 604 (2010) 1978-1986.

Árnadóttir, L., E. M. Stuve, and H. Jónsson, “Adsorption of Water Monomer and Clusters on Platinum(111) Terrace and Related Steps and Kinks II. Surface Diffusion,” Surface Science, 606 (2012) 233–238.

Árnadóttir, L., E. M. Stuve, and H. Jónsson, “The effect of coadsorbed water on the stability, configuration and interconversion of formyl (HCO) and hydroxymethylidyne (COH) on platinum(111), Chemical Physics Letters , 541 (2012) 32-38.

Acknowledgements

This work was supported by the Office of Naval Research and the National Science Foundation.

A portion of the research was performed as part of an EMSL Scientific Grand Challenge project at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. PNNL is operated for the Department of Energy by Battelle.

Last revised: December 11, 2012



Terrace steps and kinks

Figure 1. Configuration of water monomer, dimer, and trimer on the Pt(111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces. All water configurations are in their lowest energy state. The solid lines show unit cells of the surface. The dashed lines show unit cells of the water layer. Type A surfaces have (111)-oriented steps. Type B surfaces have (100)-oriented steps.

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Diffusion energy landscape

Figure 2. (Top) Diffusion pathways for water monomer on a type B step and kink surface. (Bottom) Energy landscape for water monomer diffusion from terrace to step to kink sites of the type-A and type-B surfaces.

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Water translation, flip, and roll

Figure 3. Configuration of water monomer in translational, rolling, and flipping diffusion pathways. For clarity, the reaction coordinate is not at the same scale as the distance from the Pt surface.

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HCO-COH Interconversion

Figure 4. (Top) Energy landscape for HCO-COH interconversion through an intermediate of CO + H. The green curve is in the absence of water, and the blue and orange curves are with one water molecule. Values are in eV. (Bottom) Plan and elevation views of HCO, CO + H, and COH species with one water molecule. Blue is oxygen, yellow is hydrogen, and orange is carbon.


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