AmericanChemicalSociety.com

The catalyst (Pt) sub-monolayers (SML) with various morphology and average nanocluster size were produced by galvanic displacement of Cu underpotentially deposited (UPD) monolayers (ML) on Au(111) substrate.  The UPD ML of Cu/Au(111) was formed from 0.01 M Cu²⁺ + 0.1 M HClO₄ solution. The galvanic displacement of Cu UPD ML by Pt was carried out from 10^-3 PtCl₆^2- + 0.1 M HClO₄ solution. The sets of experiments were carried out to find the Cu UPD ML adsorption isotherm so that precise coverage of Cu UPD ML on Au(111) is known for a given potential of its formation. The coverage of displaced Cu UPD ML on Au(111) has been varied from 0 to 100%, and corresponding coverage of the Pt SML deposit was investigated by ex-situ scanning tunneling microscope (STM). In all experiments, more than 95% of Pt deposit was found to be 2-dimensional, indicating a true ML catalyst configuration. The custom image processing algorithm based on Otsu's method for threshold determination has been developed in our lab to process numerous STM images from each deposition experiment. The sets of experiments were performed with different coverage of Cu UPD ML displaced by Pt and these data are used to determine the average and mean Pt cluster size as a function of the coverage of displaced Cu UPD ML, Figure 1. The Pt clusters with minimum size were found in experiments where coverage of displaced Cu UPM ML was ~66%. In addition, from the same type of STM image analysis, the data on Pt coverage as a function of displaced Cu UPD ML coverage were obtained as well, Figure 2. The linear of the data Pt coverage vs. Cu UPD ML converge shows the slope of 0.18. Knowing that full Cu UPD ML on Au(111) from 0.01 M Cu²⁺ + 0.1 M HClO₄ solution has packing density of 0.75 with respect to Au(111) substrate, the slope of 0.18 in Figure 2 indicates that oxidation state of Cu in the displacement reaction is +1. This is somewhat unexpected result, but our analysis proves that +1 oxidation state of Cu is stabilized by presence of Cl^- ions in the displacing solution. The plausible galvanic displacement reaction in investigated system is shown below. The stoichiometry of galvanic displacement reaction indicates that 4 Cu UPD adatoms are displaced by one Pt⁴⁺ ion (PtCl₆^2-):

Cu⁰_UPD/Au(111)+PtCl₆^2-+2Cl^-=Pt⁰/Au(111)+4Cl₂^-

The combination of results in Figure 1 and Figure 2 allows us too find correlation between the Pt cluster size and Pt coverage of Au(111).

The study of Pt SML activity towards H₂ oxidation reaction as a function of the Pt cluster size has been done using rotating disk electrode setup and the results were analyzed using Levich Kotecky formalism. The well characterized Pt SML deposit morphology provided the opportunity to normalize the current data/per Pt atom, and to correlate the activity of the Pt SML as a function of the mean Pt cluster size. The results are shown in Figure 3. For the first time it is shown that in ML catalyst configuration, the size of the cluster is affecting the catalyst activity. In our case, it is shown that smaller Pt clusters are less active for hydrogen oxidation. This results fall in agreement with our preliminary DFT calculation of the Pt d-band center as a function of the Pt cluster size.

Future efforts of our work will focus on producing more uniform Pt SML deposit using galvanic displacement deposition through the 2D supramolecular temples. We intend to study the activity of such well organized and defined Pt deposits for other catalytic reactions and derive the general conclusions about the importance of the size effects for monolayer catalyst design.