Hydrogenation of Ru(1,5-cyclooctadiene)(η³-C₃H₅)₂over Black Platinum. A Low-Temperature Reactive Deposition of Submonolayer Quantities of Ruthenium Atoms on Platinum with Real Time Control over Surface Stoichiometry

Abstract

Black Pt effected the hydrogenation of Ru(COD)(η 3 -C 3 H 5 ) 2 (1; COD is 1,5-cyclooctadiene) by dihydrogen gas (pressure ∼1 atm) at -10°C in hexanes. The hydrogenation resulted in adsorption of Ru adatoms by the surface of Pt with concomitant formation of propane, cyclooctane, and small amounts of cis-bicyclo[3.3.0]octane and n-octane. Compound 1 did not react with dihydrogen gas under these conditions in the absence of Pt. The total amount of cyclooctane, bicyclo[3.3.0]octane, and n-octane in solution was equal to the amount of 1 consumed at all stages of the hydrogenation. The lifetimes of the organic fragments on the surface were short on the time scale of the hydrogenation. It was therefore possible to observe in real time both the stoichiometry and the activity of the evolving Ru-Pt surface by monitoring the concentrations of either 1 or the C 8 product hydrocarbons in solution.There was a kinetic burst during the initial stages of the hydrogenation that ended after deposition of 0.2-0.5 equiv of Ru versus Pt surface (Pt surface is an active site on black Pt). The rate decreased after the burst and then increased as more Ru was deposited on Pt to reach a maximum, constant rate after deposition of 1.5-1.8 equiv of Ru. Cyclic voltammograms recorded in 1.0 M H 2 SO 4 of the bare surface, of the surface after adsorption of a monolayer of carbon monoxide, and of the surface in the presence of methanol showed that coverage of Pt by Ru was essentially complete after the maximum, constant rate was achieved during hydrogenation of 1. The surface area of a Ru surface resulting from hydrogenation of 2.7 equiv of 1 was 67% that of the original Pt surface according to the charge associated with oxidation of an adsorbed monolayer of carbon monoxide. Anodic stripping of Ru showed that the total of C 8 hydrocarbon products in solution after hydrogenation of 1 equaled the amount of Ru deposited on Pt. A catalyst surface resulting from hydrogenation of 0.11 equiv of 1 was up to ∼14 times more active than bare Pt for the potentiodynamic oxidation of methanol ([MeOH] ) 1.0 M, [H 2 SO 4 ] ) 0.5 M, 40°C, sweep rate 5 mV/s). A catalyst surface resulting from hydrogenation of 0.33 equiv of 1 oxidized methanol potentiostatically at 0.158 V (vs SCE, [MeOH] ) 0.5 M, [H 2 SO 4 ] ) 0.5 M, 25°C) for 45 min with ∼13 times the activity of Pt under the same conditions. A catalyst surface resulting from deposition of 0.8 equiv of Ru oxidized methanol potentiostatically at 0.256 V (vs SCE) under the above conditions for a total of 1.5 h with negligible dissolution of Ru into the electrolyte.

Deposition of Ru Adatoms on Pt Using Organometallic Chemistry: Catalysts for Electrooxidation of MeOH and Adsorbed Carbon Monoxide

“…We used blacked Pt gauzes for this investigation. The specific surface areas of the gauzes were determined from the Coulombic charge in the cathodic hydride region of cyclic voltammograms recorded in 1.0 M H 2 SO 4 under argon. , The lower limits to the surface roughness factors of these gauzes ranged from 250 to 500. These lower limits were determined by comparing the specific surface area of the shiny Pt gauze before blacking to its specific surface area after blacking .…”

“…Procedures. Detailed procedures for the preparation, electrochemical characterization, and manipulation of the blacked Pt gauzes; for carrying out and interrupting hydrogenations of 1 ; for potentiodynamic and potentiostatic oxidations of methanol; and for the adsorption and electrooxidation of CO are given in the previous publication . These procedures are also briefly described as appropriate in this report.…”

“…With a previous publication, we reported that black Pt effects the hydrogenation of (COD)Ru(η 3 -C 3 H 5 ) 2 ( 1 , COD is 1,5-cyclooctadiene) in hexanes at p (H 2 ) ∼ 1 atm and T = −25 to 25 °C. The hydrogenation generates cyclooctane (COA, major C 8 product), octahydropentalene (OHP, <1% of COA), propane, and Ru adatoms that are adsorbed by the surface of Pt (eq 1) The hydrogenation allows for real-time control over the equivalents of Ru adatoms deposited on Pt, and it provides a convenient method to generate Pt surfaces with known amounts of adsorbed Ru adatoms.…”

Role of Hydrous Ruthenium Oxide in Pt−Ru Direct Methanol Fuel Cell Anode Electrocatalysts: The Importance of Mixed Electron/Proton Conductivity

“…Although Pt−Ru is the preferred anode catalyst for methanol oxidation, improvements in its catalytic activity for this reaction are necessary in order to develop commercial DMFCs. Recent approaches taken to improve and understand the activity of these bimetallic catalysts include using Pt−Ru organometallic clusters as precursors to well-defined carbon-supported Pt−Ru compositions and structures, − depositing adatoms of Ru on Pt blacks , and single-crystal Pt, − and developing ternary and quaternary compositions of Pt and Ru with other transition metals …”

Adsorption and Decomposition of Ru3(CO)9(CH3CN)3 at Platinum Surfaces: An X-ray Photoelectron Spectroscopy and Fourier Transform-Infrared Spectroscopy Study

“…To obtain a good manufacturing process for these catalysts, a number of methods has been suggested. For example, adsorption of ruthenium adatoms on platinum black, from a Ru complex solution, by hydrogenation has been proposed . The starting material of noble metals to be deposited on a support could be their metal carbonyls and/or metal carbonyl derivatives.…”