Article pubs.acs.org/JPCC
Interaction between H2O and Preadsorbed D on the Stepped Pt(553) Surface Angela den Dunnen, Maria J. T. C. van der Niet, Marc T. M. Koper, and Ludo B. F. Juurlink* Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, The Netherlands ABSTRACT: We have studied the interaction of Pt(553) with varying coverages of preadsorbed D and H2O and compare results to those from the similar Pt(533) surface. We have used temperature programmed desorption spectroscopy in combination with isotopic labeling to monitor H−D exchange. Similar to results for Pt(533), on Pt(553) small amounts of D weaken the H2O−metal bonding at steps. Larger D coverages also weaken the stability of the water overlayer at the (111) terraces. However, in contrast to Pt(533), water wets the Pt(553) surface at all D-coverages and the surface does not become hydrophobic. We attribute the difference in the long-range ordering of water to the difference in bond energy of deuterium to (100) and (110) step sites. This also affects HOD formation by H−D exchange. The exchange increases with H2O and Dad coverage, although less than proportional. Availability of bare Pt sites for both adsorbates is key to obtaining high reactivity.
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INTRODUCTION The interaction between platinum surfaces and water is essential for our understanding of electrochemistry, heterogeneous catalysis, corrosion chemistry, and fuel cell catalysis. Therefore, the adsorption and desorption of water, with and without coadsorbates, has been studied in detail, mostly using Pt(111) as this surface is considered to be the least complex system to study.1,2 However, even Pt(111) shows significant complexity in adsorption and desorption phenomena.3−5 Under ultra high vacuum (UHV) conditions, it has been studied with a large variety of techniques.6−15 Real catalysts have low coordination and defect sites which are thought to be more active for catalytic reactions involving bond breaking and making.16 Stepped single crystal surfaces allow for the introduction of such defects in a controlled manner. Figure 1
To date, only a few studies have focused on the interaction of water with such platinum stepped surfaces.10,17−20 A scanning tunneling microscopy (STM) study, using low temperature annealing to create steps and large, single atom deep hexagonal islands on a Pt(111) surface, showed that water adsorbs preferentially at the upper side of steps. For the (100) step type, it forms molecular chains. However, on the upper edge of the (110) step type, very short chains or clusters seemed to appear. At the lower edge of both step types, ice patches grow more readily inside the hexagonal islands.10 A temperature programmed desorption (TPD) study focusing on water desorption from the stepped Pt(533) surface (also indicated as Pt[4(111) × (100)], see Figure 1b) revealed two submonolayer states prior to multilayer formation. The presence of steps caused an additional stabilization of water for submonolayer coverages in comparison to Pt(111). A desorption peak around 188 K (low water coverages) was attributed to water desorption from (100) step sites and a peak around 171 K to water desorption from (111) terrace sites. A third peak (water coverage > 1 ML), located around 148 K, is due to the desorption from additional water layers.21 A comparison with Pt(553) (Pt[4(111) × (110)], see Figure 1a) showed that water molecules bind more strongly to the (110) step sites than to the (100) step sites. A desorption temperature of 197 K on (110) steps was found compared to 188 K for (100) steps. This may be related to the different structures observed for water adsorbed to the upper edge of the different step types in the STM study. The peak temperatures of water desorption from terraces (171 K) and from the multilayer (∼150 K) are not affected by the step geometry.20
Figure 1. View at an angle normal to the step of the (a) Pt(553) and (b) Pt(533) surfaces.
shows two of such surfaces containing the same (111) terrace but different step types. The Pt(553) surface has a (110) step type, indicated by the rectangle that is characteristic of the (110) unit cell. It may also be considered a (111) step type, indicated by the triangle of atoms forming the basis of the (111) unit cell. On the other hand, the Pt(533) surface has a square (100) arrangement of atoms forming the step. © 2012 American Chemical Society
Received: February 28, 2012 Revised: August 9, 2012 Published: August 13, 2012 18706
dx.doi.org/10.1021/jp301939y | J. Phys. Chem. C 2012, 116, 18706−18712
The Journal of Physical Chemistry C
Article
Surface Preparation Laboratory, Zaandam, The Netherlands) was cleaned by repeated cycles of Ar+ bombardment (Messer, 5.0; 20 μA, 10 min), annealing between 850 and 1000 K in an oxygen atmosphere (Messer, 5.0; 2 × 10−8 mbar), and annealing at 1200 K. The crystal temperature can be controlled between 84 and 1200 K with the use of liquid nitrogen for cooling and radiative heating combined with electron bombardment for heating. Low energy electron diffraction (LEED) images taken after cleaning procedures show split spots in a hexagonal configuration. The ratio of spot row spacing to spot splitting is found to be ∼3.9,20 consistent with literature values for this surface.44 Water from a Millipore Milli-Q gradient A10 system (18.2 MΩ cm resistance) was deaerated in a glass container by multiple freeze−pump−thaw cycles and kept at a total pressure of 2.0 bar helium (Air Products, BIP Plus). A water bath (∼30 °C) was used to keep the vapor pressure of the water in the glass container constant. The container was connected to a home-built glass capillary-array doser located ∼1.5 cm from the sample. Water was dosed directly on the surface at Ts ≤ 110 K at a rate of ∼0.009 ML s−1 by measuring the pressure rise due to the codosed helium. To minimize hydrogen contamination from background adsorption, all filaments were switched off during D2 dosing (Lindegas, 2.8; background dosing, 2 × 10−7 mbar while cooling down the sample from 500 to 120 K (∼5 min)). This produced a full monolayer of Dad. To vary the amount of deuterium on the surface, we remove Dad from the surface by ramping the crystal to a set temperature at 1 K s−1 and cooling the crystal before recording a TPD spectrum. All reported pressures are uncorrected for ion gauge sensitivity. For the coadsorption experiments, deuterium was adsorbed first. After the pressure in the system had reached the base pressure, water was dosed on top of the deuterated surface. We performed two types of coadsorption experiments, i.e. a full monolayer of D with varying amounts of H2O (0−2.3 ML), and varying amounts of D (0−1 ML) with ∼1.3 ML of H2O. For TPD experiments, the heating rate was 1 K s−1. During heating m/e = 2 (H2), 3 (HD), 4 (D2), 18 (H2O), 19 (HDO), and 20 (D2O) were monitored with the QMS. We have verified that cracking in the QMS ionizer of HOD and D2O yields no significant contribution to the signal at m/e = 18 at low signal intensities in experiments for m/e = 19 and 20. Therefore, the signal at m/e = 18 results in our experiments only from H2O within experimental error. Similarly, the signal at m/e = 19 results only from HOD and the signal at m/e = 20 only from D2O. During data analysis m/e = 2 was not taken into account, since the signal is negligibly small and mainly due to D+ and not H2+. Initially, m/e = 28 (CO) and 32 (O2) were monitored as well, but no desorption was detected. All H2O and D2 coverages are calculated from the integrated TPD peak areas. Our exact definition of a monolayer (ML) H2O is given in the Results and Discussion section. We are not aware of an unambiguous means to determine the integral for 1 ML HOD desorbing from the surface. Therefore, we have used the integral for 1 ML H2O (desorbing from bare Pt(553)) as a reference in quantifying the amounts of H2O and HOD. We assume that the cracking ratio in the QMS and channeltron amplification are similar for both isotopes, because of the relatively small difference in mass/charge ratio. The H2O TPD spectra show an almost stepwise increase in their baseline. This is due to the high vacuum time constant of
The interaction between hydrogen and transition metal surfaces, e.g. platinum, is often studied in relation to hydrogenation reactions.20,22−36 Molecular hydrogen adsorbs dissociatively on both Pt(111)23,37 and stepped platinum surfaces.24,29,38 TPD spectra from Pt(533) show two peaks. A peak at 380 K is associated with recombinative desorption of Dad from (100) steps. A peak below 300 K is associated with recombinative desorption of Dad from (111) terraces.29 The TPD spectra of hydrogen (H or D) desorption from Pt(553) are significantly different. Instead of two peaks, three peaks are observed located at 292, 235−256, and 196−206 K. The two high-temperature peaks are associated with hydrogen desorption from (111)-type terrace sites, whereas the low temperature peak is associated with desorption from sites related to (110) steps.20 This is remarkable, as it implies that (110) steps, in contrast to (100) steps, yield a weaker H−Pt bond at the step. In electrochemical environments (e.g. hydrogen fuel cells and the reversible hydrogen electrode), water and hydrogen are both present on a platinum surface, making coadsorption studies of relevance.39−42 On deuterium precovered Pt(111), D2O desorption in the monolayer regime occurs at higher temperatures (175 K, compared to 170 K for D2O desorption from the bare Pt(111) surface), indicating that D2O on Pt(111) is stabilized by coadsorption with deuterium. For higher deuterium coverages, the stabilization becomes weaker and the D2O desorption peak shifts back to lower temperatures (172 K).42 Coadsorption TPD experiments with different hydrogen isotopes (H2O on top of Dad) showed exchange between deuterium atoms and water molecules.40 It remains unclear what the intermediate species is, but mixtures of hydrated species such as H5O2+, H7O3+, and H9O4+ have been suggested as potential candidates.43 In contrast to the stabilization observed on Pt(111), the stepped Pt(533) surface showed only one peak in the TPD spectrum when the surface had been totally precovered with Dad.21 This peak is located around 150 K even at submonolayer coverages. It was therefore suggested that small amounts of water form multilayers, indicating that the D-covered surface is hydrophobic. Experiments in which the deuterium precoverage was varied show that the peak of water desorption from step sites (188 K) disappears when deuterium saturates the step sites. When deuterium also precovers part of the terraces, the water desorption peak shifts gradually to lower temperatures. This was attributed to the formation of 3-dimensional amorphous solid water (ASW) clusters at the steps.21 Here, we study coadsorption of D and H2O on Pt(553). We use TPD in combination with isotopic labeling to investigate how preadsorbed D influences postadsorption of H2O and the H−D exchange between Dad and H2O. As the (110) rows of this surface lie along the step edge (not orthogonal to), making a smooth step, results may be expected to resemble those of the Pt(533) surface, which consists of four atom wide (111) terraces separated by square (100) steps. However, we will show that small differences in the step geometry can lead to large differences in adsorption and desorption of species and of the reactivity of the surface.20
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EXPERIMENTAL SECTION Experiments were performed in an UHV apparatus containing a LEED/Auger (LK Technologies, RVL 2000/8/R), a quadrupole mass spectrometer (QMS, Pfeiffer QME 200), and various leak valves. The base pressure of the system was 2 × 10−10 mbar during experiments. The Pt(553) crystal (cut and polished
