J. M. Jin, W. F. Lin and P. A. Christensen Exploring the possibility of COads oxidation at Ru(0001) and Ru/pc-Pt electrodes at open circuit, and the potential role of active O species at Ru, using in-situ FTIR spectroscopy J. M. Jin, W. F. Lin and P. A. Christensen School of Chemical Engineering and Advanced Materials, University of Newcastle upon Tyne,

Winshields Crag on the Roman Wall, GOC

Direct/Indirect Methanol Fuel Cell Why Ru? Direct/Indirect Methanol Fuel Cell CH3OH + Pt(s) Pt-CH2OH + H+ + e- Pt-CH2OH + Pt(s) Pt2CHOH + H+ + e- Pt-CH2OH + Pt(s) Pt3COH + H+ + e- Pt3COH Pt-CO + 2Pt(s) + H+ + e- Pt(s) + H2O PtOH + H+ + e- Pt-CO + PtOH 2Pt(s) + CO2 + H+ Pt-CO + RuOH Pt(s) + Ru(s) + CO2 + H+ CO L

Community content with Langmuir-Hinshelwood inspired Bifunctionality mechanism of Watanabe and Motoo (1975): Pt-CO + Ru-OH* Pt + Ru + CO2 + H+ + e- ie “CO-tolerant anode” Surface electrochemistry of Ru not really investigated until recently, despite importance in direct and indirect methanol fuel cell electrocatalysis.

However: Watanabe and Motoo also postulated: Pt-CO + Ru-O* Pt + Ru + CO2 Pt-CO + 2RuOH Pt + 2Ru + CO2 + H2O And: No-one, until recently, considered CO surface mobility.

Summary of electrochemical data

Remember- Ru just supplies ‘active oxygen’…..

Cyclic voltammogram of the Ru(0001) in 0.1 M HClO4. Sweep rate 50mV/s. Lin et al, J. Phys Chem. B 104 (2000) 6642 Lin et al, J. Phys. Chem. B 104 (2000) 12002

At the gas-solid interface, Ru(0001) is inactive towards the oxidation of adsorbed CO: the Ru-O bond of the (2 x 2)-O layer formed at low oxygen coverage is too strong, and the Ru-O bond of the (1 x 1)-O layer formed at high coverages remains strong, and the coverage is such that CO is prevented from adsorbing anyway. It is not clear what the oxygen-containing layer at Ru(0001) is when immersed in aqueous electrolyte, whether –O or –OH. The (2 x 2)-O(H) is still inactive, but the (1 x 1)-O(H) layer, formed at potentials > 200 mV, is active, reaction with COads taking place at the perimeter of the (1 x 1)-O(H) domains. We have also shown that the frequency of the COads is an effective probe of the structure of the surface.

Ru(1 x 1)-O(H) active towards oxidation of COads What is the mechanism? RuOH + Ru-CO 2Ru + CO2 + H+ + e- RuO + Ru-CO 2Ru + CO2 2RuOH + Ru-CO 3Ru + CO2 + H2O Watanabe and Motoo, 1975

In-situ FTIR studies on COads oxidation at Ru(0001) at OCP

The in-situ FTIR experiments were carried out in N2-sat The in-situ FTIR experiments were carried out in N2-sat. 0.1M HClO4 unless otherwise stated. Experiments at 20 C and 55 C, only 20 C data presented here.

Electrode preparation In Newcastle, the Ru(0001) single crystal electrode (Fritz-Haber Institute) was polished with 0.015 mm alumina, washed thoroughly with millipore water and ultra-sonicated in millipore water for a few minutes prior to transfer to the cell. The polycrystalline Pt electrode was ‘top hat’–shaped with an area of 0.64 cm2 exposed to the electrolyte. The Ruad/Pt electrode was prepared by the spontaneous deposition of Ru onto the pc-Pt electrode from a solution of 0.2M RuCl3/0.1M HClO4 for 1 minute; the coverage of Ru was determined according to the method of Watanabe and Motoo and found to be c. 0.3.

Solution sparged with N2 15 min CO2 formation at open circuit as a function of time. CO pre-adsorbed on the Ru(0001) at -200 mV in 0.1 M HClO4. Background at -200 mV, then open circuit. Solution sparged with N2 2340 10 s

COL COH COads Spectra at open circuit as a function of time. Reference spectrum collected at 1100 mV. 100 co-added and averaged scans at 8 cm-1; 16s.

O O C C COL COH

? 50s Plot of the IR band intensity of the COL at open circuit as a function of time. (CO2 intensity is also shown).

RuCOL dn/dE = 30 cm-1/V Plot of the frequency of the COL feature at open circuit as a function of time.

//bm 550 mV: At 50 C, CO2 evolution commenced once OCP rose above 125 mV Open circuit potential estimated from the frequency of the COL feature as a function of time.

The induction time is required for OCP to increase to +200 mV in order to initiate the growth of the active (1 x 1)-O phase which is required before oxidation of COads can take place

Ru(0001) Very little change in OCP (see next slide) from 500 mV CO2 intensity as a function of time on holding the COads/Ru(0001) electrode at the //bm potential of +550 mV for 50s prior to switching to open circuit.

Plot of the frequency of the COL band as a function of time OCP changes little, rate of COads oxidation unchanged on switching to OCP Plot of the frequency of the COL band as a function of time

Conclusions The oxidation of adsorbed CO to CO2 takes place at OCP at Ru(0001) in 0.1M HClO4 via a chemical process. (First observation of this!) If the potential of the Ru(0001) electrode is stepped to the equilibrium OCP value and held, and then the circuit is broken, the rate of evolution of CO2 remains unchanged. The oxidation of COads only takes place once the OCP rises into the region where the active (1x1)-O oxide is formed. The rate of oxidation of the COads to CO2 is highly dependent upon the temperature, anion, etc.

COads oxidation at Ru/pc-Pt at OCP

qRu = 0.3 In-situ FTIR spectra taken from the Ru/pc-Pt electrode immersed in 0.1M HClO4 at open circuit after CO adsorption at -200 mV vs Ag/AgCl. Ref. Pot. +700 mV. Friedrich et. al., J. Electroanal. Chem., 524 (2002) 261 CO on Ru/Pt(111) Ru Pt Polycrystalline Pt!

COL and CO2 intensities, and OCP in 0.1M HClO4 Ru/pc-Pt COL and CO2 intensities, and OCP in 0.1M HClO4

Re-orientation of off-normal COL The initial rapid rise in the OCP is not observed in the absence of adsorbed CO. There is marked rise in the intensity of the COL bands (c. 16% in the case of the COL,Ru). Re-orientation of off-normal COL

cff Hayden * Possible COads stripping routes

The work of Koper et al, Far. Diss., 121 (2002) 285 Corrugation potential for CO adsorption on the Ru-modified Pt(111) surface and the different diffusion hops considered in the model, from perodic Density Functional Theory calculations. No diffusion is allowed from the Ru to the Pt because of the very high activation barrier involved.

One of the ‘Two-zone models’ for CO oxidation at Ru/Pt(111) OH + CO Zone 2 CO CO Ru Ru Ru Ru Pt Pt Pt* Pt Pt Pt Pt Pt* Pt Pt Pt One of the ‘Two-zone models’ for CO oxidation at Ru/Pt(111)

COL and CO2 intensities, and OCP in 0.1M HClO4 Ru/pc-Pt Free interchange of COads between Ru & Pt sites Onset of (1 x 1)-O COL and CO2 intensities, and OCP in 0.1M HClO4

COL frequency and OCP in 0.1M HClO4 Ru/pc-Pt COL frequency and OCP in 0.1M HClO4

CO2 frequency and OCP in 0.1M HClO4 Ru/pc-Pt Markedly different to COL Ru data!! CO2 frequency and OCP in 0.1M HClO4

Conclusions Under open circuit conditions: COL,Ru and COL,Pt can be oxidised by O-containing adlayer on Ru to CO2. Free exchange of COL between the Ru and Pt sites. Ru-CO + Pt* ↔ Ru + Pt*-CO Pt-CO + Pt* ↔ Pt + Pt*-CO

The initial potential-determining species is a small amount of adsorbed hydrogen. During the rapid stripping of this small amount of adsorbed H, the OCP rises dramatically and the COL species on the Pt and Ru re-orientate to the vertical. Once the hydrogen is stripped, oxygen is chemisorbed at the Ru edges of the Ru islands adjacent to the Pt* sites and possibly also at the Pt* sites. The adsorbed oxygen drives up the potential, causing the formation of the relatively inactive (2 x 2)-O(H) layer at potentials up to 200 mV. COL species on the Pt and Ru are oxidised through diffusion to the Pt* sites and reaction with adjacent Ru-OH.

E > 200 mV active (1 x 1)-OH layer forms leading to more rapid oxidation of COL ON THE Ru. Initially, the formation of the (1 x 1)-OH compresses the COL left on the Ru, leading to increased local coverage. Eventually, these domains break up. Over longer time scales, all the COL are oxidized, the Ru sites are fully covered by the (1 x 1)-O(H) layer and the potential rises to its equilibrium value.

See: J. M. Jin, W. F. Lin and P. A. Christensen, J. Electroanal. Chem., 563 (2004) 71. P. A. Christensen, J-M. Jin, W-F. Lin, and A. Hamnett, J. Phys. Chem. B, 108 (2004) 3391