STM Investigation of CO Ordering on Pt(111): From an Isolated Molecule to High-Coverage Superstructures

Abstract

Carbon monoxide (CO) adsorbed on Pt(111) has been extensively studied as a model catalyst. However, there remain some ambiguities in overlayer structures, particularly regarding bridge-site occupation. Here, we report real-space observations of CO on Pt(111) using scanning tunneling microscopy (STM) under ultrahigh vacuum at a cryogenic temperature, from a single CO adsorbed on the atop site to the gradual development of overlayer structures including atop-dominant (√3 × √3)R30°islands, c(4 × 2) domains, and 1 × 1 boundaries in c(4 × 2)-2CO domains. Bridge CO appear at the edge of (√3 × √3)R30°islands, significantly in the center of local c(√3 × 2)rect geometry which is equivalent to c(4 × 2)-2CO. In the c(4 × 2) domain including bridge vacancies, the height of the atop CO in the STM image is modulated according to the number of adjacent bridge CO, which implies the interadsorbate interaction between two different adsorption species. The real space observation presented here not only resolves ambiguities about overlayer structures but also describes an atop-bridge interadsorbate interaction.

A temperature dependent investigation of the adsorption of CO on Pt(111) using low-temperature single crystal adsorption calorimetry

“…1 indicate. The transition coverage of θ abs,trans = 0.365 ML found in this work is in good agreement with Ertl and Yang as they reported the transition between the hexagonal and the rectangular phase at 0.33 ML and 0.375 ML, respectively [4,7]. On the other hand Ertl describes a lower phase transition temperature for the hexagonal structure at 0.33 ML of 170 K compared to 270 K for the rectangular structure at 0.5 ML which is in contrast to our results indicating a higher temperature for the fluid-to-hexagonal structure transition.…”

“…Studying the heat of adsorption with surface coverage gives insights into the lateral adsorbate-adsorbate interactions. However, a temperature dependent examination of adsorption enthalpies provides interesting thermodynamic and structural properties of the chemisorbed adlayer like their heat capacity or entropy and coverage and temperature dependent structural changes, respectively [3,4].…”

“…Many surface techniques have been applied to learn more about the electronic structure [24][25][26][27], the vibrational modes [28][29][30][31][32][33][34], adlayer structures [4,30,32,35] and surface kinetics [19,23,30,36] of that rather simple adsorbate-substrate system. Moreover, a lot of theoretical work has been done including Monte Carlo simulations combined with lattice gas models and density functional theory (DFT) calculations [28,29,37,38].…”

Weakly-bound hydrogen on defected Pt(111)

“…The large difference in the sticking probabilities between H 2 and CO on Pt(111) is apparent in the uptake curves shown in the insets. The absolute saturation coverage of CO on Pt(111) has been reported to be in the 0.48-0.68 range [52][53][54][55][56][57][58] and as high as 1.0 under a high-pressure (0.1 Torr) CO ambient [3]. Compressed adlayers of CO beyond a 0.5 ML coverage are known to form under a high-pressure CO ambient [3,58].…”

“…Thus, our roughened surface with a defect density of~12% would have approximately 8-atom wide terraces separated by (111)-or (100)-type steps on average. Since the local step coverages at the saturation of H and CO are not expected to exceed those on the (111) terrace, i.e.,~1 H [10,15,16,25,28,49,68] and~0.5 CO [3,[51][52][53][54][55][56][57][58][59][60][61][62]69,70] per platinum atom, respectively, the total H and CO uptakes would not increase significantly if all H atoms and CO molecules adsorb on the topmost surface sites only. In spite of this consideration, the saturation H and CO uptakes by the defected Pt(111) surface are larger by~1.48 and~1.1 times, respectively, those on the smooth surface (see the data in Figs.…”