ARTICLE pubs.acs.org/JPCC
Energy of Molecularly Adsorbed Water on Clean Pt(111) and Pt(111) with Coadsorbed Oxygen by Calorimetry Wanda Lew,† Matthew C. Crowe,† Eric Karp, and Charles T. Campbell* Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States ABSTRACT: The heat of adsorption and sticking probability of D2O were measured on Pt(111) with and without preadsorbed oxygen adatoms as a function of D2O coverage using single-crystal adsorption calorimetry from 88 to 120 K. In this temperature range, water adsorbs molecularly on both surfaces, and, at multilayer coverages, forms amorphous solid water on Pt(111). The integral heat of adsorption at a D2O coverage of 0.5 ML on clean Pt(111) was found to be 51.3 ( 1.6 kJ/mol at 120 K, 4.1 kJ/mol larger than in the multilayer. Its change with temperature gives a heat capacity for the adlayer islands of 130 ( 83 J/(mol K). This heat of adsorption at 120 K provides a standard enthalpy of formation of adsorbed √ D √2O on Pt(111) of 301 kJ/mol at 120 K and a D2O coverage of 0.5 ML, which we attribute to water islands in the ( 37 37)R25.3° structure based on prior structural studies. The integral heat of molecular adsorption of D2O at 120 K on Pt(111) predosed with 0.25 ML of oxygen adatoms was ∼55.0 kJ/mol at 0.5 ML, 3.7 ( 0.5 kJ/mol larger than with no O present. The heat of adsorption in the multilayer regime at 120 K (when growing amorphous solid water) was found to be 47.2 ( 1.4 kJ/mol, consistent with previous measurements.
’ INTRODUCTION The wetting of solid surfaces by water is of critical importance in countless materials applications, and thus the structure of adsorbed water at solid surface and the strength of water solid bonding have been subjects of extensive study. The interaction of water with Pt surfaces is particularly important, due to the dominance of Pt electrodes in many electrochemical processes, notably fuel cells. The water/Pt(111) interface is one of the best studied and well understood water/solid interfaces1 36 and has served as a prototype for understanding adsorbed water layers and the influence of hydrogen bonding on surface selfassembly.5,37 Here, we report the first calorimetric measurements of the adsorption energy of water to make molecularly adsorbed water on clean Pt(111), on oxygen-dosed Pt(111), and on water multilayers on Pt(111). The activation energy for desorption of submonolayer water on clean Pt(111) has been determined previously by Daschbach et al.35 to be 54.2 ( 3 kJ/mol from an Arrhenius plot of the desorption rate. However, this does not necessarily equal the heat of adsorption measured here, because there may be some excess activation energy for water adsorption. The current results prove that any excess activation energy must be less than 8 kJ/mol. The energetics of molecularly adsorbed water on oxygen-dosed Pt(111) have not been probed previously by any means (because it dissociates to form adsorbed hydroxyl groups upon heating before it desorbs), so our measurements here provide the first experimental information on the energy of that system. Water coadsorbed with O atoms is interesting because it makes adsorbed hydroxyl groups above 130 K and because it is a prototype for studying hydrogen bonding in adlayers. Also, we present here the first adsorption energy measurements versus temperature for any adsorbed species on r 2011 American Chemical Society
any single crystal surface, which allows the first ever experimental estimate of the heat capacity for any adsorbate on any single crystal surface. Water adsorbs molecularly on clean Pt(111) at temperatures below 120 K and desorbs at 160 K.7,10 Water dissociation is not observed at these temperatures.7,10 Water forms crystalline ice multilayers above 135 K, while below 120 K it forms amorphous solid water (ASW) multilayers, which are hydrogen bonded without long-range order.11 13 At 135 K, He scattering and low energy electron diffraction (LEED) show that water forms √ √ islands with long-range ( √ 37 √ 37)R25.3° order on Pt(111), which are replaced by a ( 39 39)R16.1° structure as the also inmonolayer is completed.10,17,19,32,38 STM experiments √ dicate that the first monolayer exclusively forms the (√37 √ √37)R25.3° structure at 14018K, while at 130 K the ( 39 39)R16.1° structure forms. Between 130 and 140 K, these superstructures were able to transform into each other through adsorption or desorption, consistent with other experiments showing that water gains sufficient mobility to form 2D hydrogen-bonded structures with long-range periodicity at 130 K and above.17,19,20 The two structures have been the subject of considerable theoretical investigation, which have helped elucidate the structures.1 6,21 23,28 31,34 There are different possible √ arrangements of water with similar energies for the ( 39 √ 39)R16.1° structure evidenced by broad O H (or O D for deuterated water) stretching frequencies in RAIRS except with very small clusters of water.39 41 Different theoretical studies Received: February 17, 2011 Revised: March 26, 2011 Published: April 20, 2011 9164
dx.doi.org/10.1021/jp201608x | J. Phys. Chem. C 2011, 115, 9164–9170
The Journal of Physical Chemistry C √ find different arrangements of water in the 39 structure with very similar energies.4,19,22,23 This was rationalized as being due to the lack of a strong localized chemisorption interaction with Pt 19 The unit cell to constrain water to a particular Pt site geometry. √ √ 17 proposed by Glebov et al. for the ( 39 39)R16.1° structure has a coverage of 32 waters per 39 Pt atoms (0.82 ML),17 but more recently Nie et al.37 proposed a unit cell with 28 waters per 39 Pt atoms (or 0.72 ML) based on new STM images and DFT calculations with image simulations. To our knowledge, no direct experimental measurement of the absolute coverage in this adlayer has been reported. We show here sticking probability versus coverage measurements that imply a saturation coverage of 0.73 ( 0.09 ML. Recent STM images √ and DFT calculations also have led to a structural model for the 37 structure formed in islands at lower coverages.6 For reasons to be described, the experiments were done with D2O rather than H2O. To our knowledge, no one has reported discernible differences between D2O and H2O in the submonolayer adsorption behavior for the systems studied here.
’ EXPERIMENTAL SECTION Experiments were performed in a UHV chamber (base pressure
