Carbon Monoxide Promoted Deposition of Ordered Pt Adlayer on Au(111) and Its Electrocatalytic Properties

Abstract

In situ scanning tunneling microscopy (STM) was used to examine the microstructures of platinum (Pt) deposited on ordered Au(111) substrate in aqueous Na 2 PtCl 6 solution saturated with carbon monoxide (CO) with and without potential control at room temperature. PtCl 6 2− complexes were reduced to Pt atoms by CO molecules and deposited on Au(111) in a hexagonal array without potential control. The adsorption of CO molecules first mobilized and rearranged Pt adatoms on Au(111) from disordered Pt aggregates to 0.9 nm wide tortuous stripes, and then to patches of ordered Pt adatoms at 0.1 V (versus reversible hydrogen electrode). Electrodeposition of Pt in the presence of CO ceased with a monatomic Pt film capped with CO molecules arranged in a (2 × 2) array at 0.1 V. The CO monolayer adsorbed on Pt/Au(111) was stripped off at 0.96 V in 0.1 M H 2 SO 4 , as compared with 0.83 V observed with Pt(111). The E 1/2 potential for oxygen reduction reaction (ORR) observed with the Pt-modified Au(111) electrode was 0.81 V in sulfuric acid, which is 0.16 V more positive than that of Pt(111). The microstructures of the as-prepared Pt monolayer on Au(111) changed with potential cycling between 0.05 and 1.0 V. Platinum (Pt) metal is active in catalyzing a number of electrochemical reactions, such as hydrogen evolution and oxidation, 1 oxidation of methanol, 2 and oxygen reduction reaction. 3,4 Since these electrochemical reactions all occur at the interface of the Pt electrode, it is important to learn the electrochemical characteristics of ultrathin Pt film, which can lead to cost-effective uses of Pt. The preparation and characterization of thin Pt films have been studied in the last decade and has been reviewed. 5 Substrates for Pt deposition range from noble transition metal substrates (Au, Ru, and so on), 6,7 to semiconductors (silicon 8 and TiO 2 9 ), carbon materials (graphite, 10,11 carbon nanotubes 12,13 ), and polymers (polyaniline, 14 Nafion 15 ). These Pt-modified electrodes yield new uses in electrocatalysis and electroanalysis.Of close relevance to this study -Pt deposited on gold substrate -has been examined in vacuum and electrochemical environments. 6,16-20 Scanning tunneling microscopy (STM) has been used to study Pt electrodeposition on Au(111) and Au(100) ordered electrodes, which reveals that Pt nucleates preferentially in steps, and develops into 3D clusters and cauliflower type clusters on these substrates, 21 while X-ray diffraction shows epitaxial deposition of Pt on Au(111) electrode with Pt adatoms occupying face-center cubic (fcc) three-fold sites. 18,22 Smooth Pt monolayers have been prepared by galvanic replacement of a predeposited Cu layer 19,20 or by electrodeposition in the presence of carbon monoxide, whose adsorption on Pt deposit hindered further Pt reduction. 23 As for preparing multilayer Pt films, a self-terminating electrodeposition process in K 2 PtCl 4 -NaCl electrolyte has been devised, which allows control over morphology and film thickness. 24 We employed in situ STM and infrared spe...

Au(111)-Supported Pt Monolayer as the Most Active Electrocatalyst toward Hydrogen Oxidation and Evolution Reactions in Sulfuric Acid

“…23 Recently, we found that carbon monoxide can serve as a reductant for PtCl 6 2− , which yields a CO-capped Pt monolayer on Au(111) without potential control. 24 We substantiate the study of an ordered Pt monolayer on Au(111) by showing that the CO adlayer can be removed to expose the Pt deposit without affecting the ordered state of the Pt monolayer. This supported Pt monolayer has Pt adatoms arranged in atomic structures different from those of bulk Pt(111) electrode, which enables tailoring unique electrocatalytic properties.…”

“…The use of 0.1 M H 2 SO 4 saturated with hydrogen gas as the electrolyte can prevent the Pt film from oxidation during the electrode transfer process. This is different from the process reported in the previous work . The first rinsing was used to remove the extra PtCl 6 2– on the as-prepared electrode, and then the CO was removed by the following rinsing.…”

“…The ordered Au(111) electrode was coated with a thin Pt film by being dipped in a Pt dosing solution made of Na 2 PtCl 6 (25 μM) and Millipore water saturated with carbon monoxide gas, as previously reported. 24 After being removed from the solution, the Au(111) electrode was rinsed with COsaturated Millipore water and subsequently with H 2 -saturated Millipore water, and then quickly transferred into an electrochemical or STM cell. The use of 0.1 M H 2 SO 4 saturated with hydrogen gas as the electrolyte can prevent the Pt film from oxidation during the electrode transfer process.…”

“…The Au(111) electrode (area = 0.0432 cm 2 ) was a bead made with the end of a gold wire, and its preparation and pretreatment followed the conventional method. The ordered Au(111) electrode was coated with a thin Pt film by being dipped in a Pt dosing solution made of Na 2 PtCl 6 (25 μM) and Millipore water saturated with carbon monoxide gas, as previously reported . After being removed from the solution, the Au(111) electrode was rinsed with CO-saturated Millipore water and subsequently with H 2 -saturated Millipore water, and then quickly transferred into an electrochemical or STM cell.…”

“…Yet an epitaxial electrodeposition of Pt on Au(111) is inferred from the X-ray diffraction experiments. , Meanwhile, a Pt film is produced when Pt 2+ replaces a predeposited Cu monolayer on a support. , When electrodepositing Pt from a CO-saturated PtCl 6 2– solution, the resultant Pt film is capped by CO molecules, which restricts its thickness to only one Pt atom . Recently, we found that carbon monoxide can serve as a reductant for PtCl 6 2– , which yields a CO-capped Pt monolayer on Au(111) without potential control . We substantiate the study of an ordered Pt monolayer on Au(111) by showing that the CO adlayer can be removed to expose the Pt deposit without affecting the ordered state of the Pt monolayer.…”