Comment on “High Pressure Adsorbate Structures Studied by Scanning Tunneling Microscopy: CO on Pt(111) in Equilibrium with the Gas Phase”

Abstract

In a previous Letter [1], Jensen et al. investigated the high-pressure (200-750 torr) response of the Pt(111) surface towards carbon monoxide using scanning tunneling microscopy (STM). A new hexagonal CO overlayer structure was found, quite different from the structures formed at similar coverages under low pressure and temperature conditions, and Jensen et al. concluded that their results illustrated the inequivalence between studies of surfaces in catalytic conditions of high pressure and surface science studies carried out in high vacuum.In this Comment, we present atomically resolved STM images obtained at room temperature with a novel highpressure STM described elsewhere [2]. Our results differ from those obtained by Jensen et al. in several respects: At 1 bar of CO, we detect two rotational domains of a hexagonal CO Moiré pattern with a periodicity of 11.8 6 0.4 Å A B C FIG. 1. (A) STM image ͑240 3 125͒ Å 2 of two rotational domains of the Moiré pattern formed at 1 bar CO. (B) Highresolution STM image ͑55 3 51͒ Å 2 of the CO overlayer obtained at 1 bar CO. (C) The ͑ p 19 3p 19͒ R23.4 ± -13 CO structure. One CO molecule in the unit cell has been fixed in the on-top position. If, alternatively, the CO molecule is fixed to the bridge or fcc position, the energy is raised by about 0.4 eV per unit cell (three-layer Pt slab). The unit cell of the pattern is indicated with the solid line. Dark balls represent CO molecules adsorbed in nearly on-top sites (see text). rotated C 24 6 2 ± with respect to the underlying substrate (see Fig. 1A). This alone invalidates the model proposed in [1] consisting of a nonrotated CO overlayer. Furthermore, we resolve the individual CO molecules of the overlayer structure and find that the Moiré pattern arises from a ͑ p 19 3 p 19͒R23.4 ± -13 CO unit cell, corresponding to a surface coverage of 0.68 ML (Fig. 1B). From an interplay between our STM results and density functional calculations, we arrive at the lowest-energy structural model shown in Fig. 1C with 13 CO molecules per unit cell. The brightest protrusions in Fig. 1B are associated with CO in on-top adsorption sites.At low pressure and temperature conditions (170 K), we moreover observe the exact same CO Moiré structure when the Pt(111) surface is saturated with CO. This result is consistent with earlier low-temperature LEED studies [3]. 170 K is thus a sufficiently high temperature to overcome the CO diffusion barrier and facilitate long-range ordering, and yet sufficiently low to form high-coverage saturation structures. (At 90 K we observe a kinetically hindered domain structure.)Comparing the high-pressure CO structural model presented in Fig. 1C with the c͑4 3 2͒-2CO roomtemperature UHV saturation structure (u 0.5 ML), a 47% on-top occupancy increase is found. In this way our STM results are in excellent agreement with a recent sum-frequency generation study by Rupprechter et al., who observed a ϳ40% on-top occupancy increase when the CO pressure was raised from low to high pressure [4].With respect to the so-called p...

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

“…4d), as derived from atomically resolved images. 52,53 The same structure was observed under certain electrochemical conditions. 56 Somorjai and coworkers 51 also observed a moiré pattern with a similar periodicity in a mixture of 200 hPa CO and 67 hPa O 2 at RT, but with the moiré pattern aligned to the substrate lattice.…”

“…4). [51][52][53][54][55] The study of Longwitz et al is the most extensive. 53 They report that the exact properties of the moiré pattern depended on p CO , within the studied range of 10 À6 -10 3 hPa.…”

High-Coverage Structures of Carbon Monoxide Adsorbed on Pt(111) Studied by High-Pressure Scanning Tunneling Microscopy

“…Nevertheless, a true pressure gap is not observed for the CO/ Pt(111) system since cooling of the sample to approximately 170 K leads to an increase in CO coverage and the formation of a hexagonal CO overlayer similar to that at 760 Torr. 10 The formation of moiré structures has been observed previously for a large variety of adsorbate systems. For example, moiré patterns for CO overlayers have been reported for the Ni(111), 23 Ag(111), 24 and graphite 25 substrates and, at a pressure of ∼760 Torr, also for the CO/Pt(111) system.…”

“…The results for the CO coverage are in good agreement with the previously determined CO coverages at 10 -8 Torr (θ ) 0.5) 3 and 760 Torr (θ ) 0.68). 10 The CO coverage given for 10 -8 Torr corresponds to the c(4 × 2) structure observed in low-energy electron diffraction (LEED). 3,22 Discussion From Figure 4, the coverage of CO on Pt(111) at room temperature is seen to vary continuously (and reversibly, as outlined above) over the pressure range from 10 -6 to 760 Torr.…”

Reactivity of a gas/metal/metal-oxide three-phase boundary: CO oxidation at the Pt(111)–c(4×2)-2CO/α-PtO2 phase boundary

“…The GGA-PW91 based phase diagram depicted in Fig. 2b shows all phases shifted to lower chemical potentials as a consequence of the stronger adsorbate binding [10] with this XC functional, which means that the CO saturation phase, [27,28], is predicted to occur even at the relatively high temperatures of interest here. Staying consistent with the use of the GGA-RPBE functional, we choose, however, to adapt the c(4 · 2)-2CO structure far from the oxide edge.…”