Monitoring of CO Binding Sites on Stepped Pt Single Crystal Electrodes in Alkaline Solutions by in Situ FTIR Spectroscopy

Abstract

The site geometry preference of CO binding on stepped Pt single crystals in alkaline solution was investigated by in situ FTIR spectroscopy. The surfaces of the Pt single crystals consisted of different width (111) terraces, interrupted by ( 110) or (100) monoatomic steps.Experiments carried out with CO adsorbed exclusively on the top of the steps revealed that only linearly bonded CO formed on the (110) steps, while two CO binding geometries (linear and bridge) were observed on the (100) steps. On one hand, for CO adsorbed only on the steps, the positions of the bands corresponding to linearly bonded CO were similar, regardless of the density of steps, suggesting the existence of an interaction between CO ads only along the line of the steps. On the other hand, for full CO coverage, the CO stretching frequencies and the geometry of bound CO were sensitive to the width of the (111) terraces and the step orientations. Consequently, the CO binding sites favored linearly bonded CO for surfaces consisting of shorter (111) terraces and (110) steps. Bridge-bonded CO was favored on surfaces consisting of shorter (111) terraces interrupted by (100) steps. In order to understand the origin of the preference of CO binding sites, the results were compared to the corresponding behavior in acid media, which revealed that in addition to the effect inherent to the Pt surface, the charge on the metal side in an aqueous environment should be taken into consideration.The analysis suggested that the CO adlayers formed at full coverage in acidic and alkaline media had different structures. On the other hand, the structure of the layer of CO adsorbed only at steps was independent of pH.

CO organization at ambient pressure on stepped Pt surfaces: first principles modeling accelerated by neural networks

“…Our calculated results agree well with the experimental data from Farias et al under electrochemical conditions where Pt(322) and Pt(311) surfaces, that present a (100) step, show a large amount of bridge site CO on their terraces, while Pt(332) and Pt(331) surfaces, with a (111) step, show a very large majority of top site CO. 32 They also agree with the UHV experiments of Tränkenschuh et al in the case of Pt(553)). 24 The kink surface (Pt(643)) shows a different type of arrangement where CO aligns parallel to the step edge.…”

“…On Pt(335) at 0.5 ML coverage, EELS also shows that all edge sites are occupied with top CO, while on the terrace half of the sites are occupied with a 2:1 top-to-bridge ratio. 26 In situ FTIR studies in solution and electrochemical conditions indicates that the preferred binding mode of CO on the terrace of stepped surfaces with short terraces depends on the type of step: surfaces with (100) steps (Pt(322) and Pt(311)) show a large amount of bridge site CO on their terraces, while those with (111) steps (Pt(332) and Pt(331)) mostly provide top site CO. 32 This suggests an interesting mechanism, of unknown origin to our knowledge, by which the type of step can control the binding site of CO on the terraces.…”

“…In situ FTIR studies under solution and electrochemical conditions indicate that the preferred binding mode of CO on the terrace of stepped surfaces with short terraces depends on the type of step: surfaces with (100) steps (Pt(322) and Pt(311)) show a large amount of bridge site CO on their terraces, while those with (111) steps (Pt(332) and Pt(331)) mostly provide top site CO. 32 This suggests an interesting mechanism, of unknown origin to our knowledge, by which the type of step can control the binding site of CO on the terraces. Near-field microscopies, such as scanning tunneling microscopy, provide images with molecular resolution, but the detailed interpretation of the images can be challenging, and time resolution is usually limited.…”

“…As proposed by the experiments, the step edge is totally covered by on-top CO molecules. 24,26,32 CO adsorption results from the competition between stabilizing CO-surface interactions and destabilizing CO-CO lateral interactions that increase with CO coverage.…”

Role of dissolved CO in the solution on the origin of CO pre-oxidation on Pt(1 1 1)-Type electrodes

“…Moreover, it was observed that the position of the COads band in the in situ FTIR spectra is insensitive to the step density, suggesting that inter-molecular coupling is restricted to COads species along a line of steps [41]. For comparison, Figure 3 To determine the effect of the CO traces on the stripping voltammogram, the experiments in Figure 4 were repeated using a purging time for elimination of the nonadsorbed CO of 10 minutes (Figure 5).…”