High-density electrosorbed carbon monoxide monolayers on Pt(111) under atmospheric pressure

Abstract

We report structure studies of high-density CO monolayers on Pt͑111͒ surfaces in equilibrium with gaseous CO near atmospheric pressure, using surface x-ray scattering ͑SXS͒ and scanning tunneling microscopy ͑STM͒. We were able to stabilize extremely well-ordered CO monolayers by emersion transfer from an electrochemical cell. We found the hexagonal close-packed ͑2 ϫ 2͒-3CO structure at room temperature in ϳ1 atm CO gas pressure. We also confirm the ͑ ͱ 19ϫ ͱ 19͒-13CO structure previously proposed in STM studies ͓Vestergaard et al., Phys. Rev. Lett. 88, 259601 ͑2002͔͒. Based on SXS and STM measurements, a tentative phase diagram is proposed. Detailed SXS structure studies of the ͑2 ϫ 2͒-3CO structure suggest a surface structure and substrate reconstruction less symmetric than previously studies suggested. We also find that ͑ ͱ 19ϫ ͱ 19͒-13CO structure induces substrate reconstructions including the dimerization of Pt bonded to bridging CO and the rotation of the Pt cluster with atop CO.

Electrosorbed carbon monoxide monolayers on Pt(111)

“…[2] because of our electrochemical adsorption and transfer procedure. We also took sequential images during the transition from (2 × 2)-3CO to ( √ 19 × √ 19)-13CO while flushing the cylinder with a constant flow of Ar to reduce the CO partial pressure in the cylinder [13] confirming the phase transition seen in SXS measurements.…”

“…This model allows only vertical relaxation of Pt and CO from their high symmetry positions and produces zero intensity at (1/2 0 0) and (0 1/2 0). We found substantial intensity [13] at these positions. This means that the gas-phase (2 × 2)-3CO structure on Rh(1 1 1) [29,30] must be different from the (2 × 2)-3CO structure on Pt(1 1 1).…”

“…Note that the intensity distribution of the first quadrant is replicated over three other quadrants demonstrating the reliability of the data set. We emphasize that our data analysis fit simultaneously [13] all the data, namely, integer CTRs, fractional-integer CTRs, and the in-plane fractional-integer reflections, to arrive our model described below.…”

“…However, we find that, regardless the absolute values of the Pt-CO bond lengths, the length differences, i.e., the sequence of relative heights, are statistically significant and always maintained the order, atop > hcp > fcc. We also considered the tilt (16 • ± 4 • ) of CO molecules as one way to reconciliate the laterally shifted CO positions and symmetric bonding [13].…”

“…These measurements were done in an electrochemical cell. We performed equivalent measurements of CTRs of the ( √ 19 × √ 19)-13CO structure in gas CO (not shown) [13]. In addition, we added GIXD measurements to include a comprehensive survey of in-plane fractional-integer reflections that are often very weak, probably too weak to be seen with a high background of water/membrane scattering.…”