P.N. Ross

Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review

In this review we selectively summarize recent progress, primarily from our laboratory, in the development of the oxygen reduction reaction (ORR) catalysis on well-defined surfaces. The focus is on two type of metallic surfaces: platinum single crystals and bimetallic surfaces based on platinum. The single crystal results provide insight into the effects of the platinum structure on the kinetics of the ORR, and create a fundamental link between the specific activity of Pt (rate per unit area) and particle size (for various particle shapes). The results show that the structure sensitive kinetics of the ORR arise primarily due to structure sensitive adsorption of anions. In the absence of specific adsorption, such as in Nafion polymer electrolyte, no particle size effect is expected. The knowledge of the electrocatalysis of the ORR on model bimetallic surfaces on Pt-Ni and Pt-Co bulk alloys was used to resolve the enhanced ORR kinetics on supported Pt-Ni and Pt-Co catalysts. Finally, we show that the ORR on platinum modified with pseudomorphic Pd metal film in alkaline solution is the best catalysts ever used in O2 reduction. For both bimetallic systems, we demonstrated that the ability to make a controlled and well characterized arrangement of two elements in the electrode surface region presage a new era of advances in the ORR electrocatalysis.

Methanol electrooxidation on supported Pt and PtRu catalysts in acid and alkaline solutions

The kinetics of methanol oxidation on supported 47.5 wt.% Pt and 54 wt.% PtRu (with nominal Pt:Ru ratios of 2:3) catalysts are measured in 0.5 M H2SO4 and 0.1 NaOH at 295 and 333 K using thin-film rotating disk electrode (RDE) method. It was found that the activity of Pt and PtRu for methanol oxidation is a strong function of pH of solution and temperature. The kinetics are much higher in alkaline than in acid solution; at 333 K, a factor of 30 for Pt and a factor of 20 for Pt2Ru3 at 0.5 V. The pH effect is attributed to the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. The activity of Pt and Pt2Ru3 catalysts at 333 K is higher (a factor of 5) than at 295 K. Irrespective of pH, only negligible differences in the kinetics are observed between Pt and on high Ru content Pt alloys, presumably owing to a slow rate of methanol dehydrogenation on the Ru-rich surface and insufficient number of Pt sites required for dissociative chemisorption of methanol.

On the reaction pathway for methanol and carbon monoxide electrooxidation on Pt-Sn alloy versus Pt-Ru alloy surfaces

It has been observed in this work and previous studies that Pt3Sn alloy surfaces are very effective catalysts for CO electrooxidation, but not for methanol electrooxidation. Since COads is postulated to be an intermediate in methanol electrooxidation on Pt alloy surfaces, the relative inactivity of Pt3Sn for methanol oxidation appears paradoxical. We present an explanation for this apparent contradiction in terms of a unique state of COads on this surface, which is not the same state of COads as occurs on either PtRu or pure Pt surfaces. It is also not a state of COads which is produced by methanol dehydrogenation. The state is unique in the sense that a significant fraction of COads is oxidized at a much lower (<400 mV) potential than the rest of the COads, a phenomenon that does not occur on any other Pt and Pt-alloy surfaces examined in the same way. This CO state is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by COads at low coverage.

Electrooxidation of CO and H2/CO Mixtures on Pt(111) in Acid Solutions

Electrocatalysis of CO oxidation and the interfacial structure of the CO adlayer (COad) on the Pt(111) surface in 0.5 M H2SO4 were examined by using the rotating disk electrode method in combination with in situ surface X-ray diffraction measurements. The results presented here elucidate the roles played by two different forms of COad:  one which is oxidized at lower overpotentials, in the so-called preoxidation region, we characterize as a weakly adsorbed state (COad.w), and a strongly adsorbed state (COad,s) which is oxidized at higher overpotentials. The COad,w state forms at saturation coverage by adsorption at E < 0.15 V and assumes a compressed p(2 × 2) structure containing 3 CO molecules in the unit cell (0.75 CO/Pt). We propose that oxidative removal of COad,w is accompanied by simultaneous relaxation of the CO adlayer, and that the remaining COad (≈0.6 CO/Pt) assumes a new bonding state which we identify as COad,s. The COad,s state is present in a structure lacking long-range order. Despite the r...

Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Abstract Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution–oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H2SO4 containing Cl−. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl− (Clad), and it occurs as a 3.5e− reduction via solution phase peroxide formation. In the hydrogen adsorption (Hupd) potential region, the orr is even more inhibited, and corresponds only to a 2 e− reduction at the negative potential limit where the electrode is covered by one monolayer of Hupd and some (unknown) amount of Clad. On the Pt(111)Clad surface, the orr is inhibited relatively little (in addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl− free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl−, since Clad does not affect platinum sites required for the breaking of the HH bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Clad is present on the surface even near 0 V.

Oxygen electrocatalysis in alkaline electrolyte: Pt(hkl) Au(hkl) and the effect of Pd-modification

Abstract The kinetics of the oxygen reduction reaction (ORR) was studied in alkaline electrolyte at 293–333 K on bare and Pd modified Pt(hkl) and Au(hkl) surfaces. The rotating ring-disk electrode technique was used to study the ORR with solution phase peroxide detected at the ring electrode. Pd modification was either by electrodeposition (Pt) or by vapor deposition in vacuum (Au). The surface concentration of Pd was determined in vacuum using low energy ion scattering. In agreement to the structure sensitivity found at room temperature previously, on the bare Au(hkl) surfaces the ORR was found to be strongly structure sensitive in the temperature range from 293 to 333 K, with order of activity being (100)≫(110)>(111). The structure sensitivity for Pt(hkl) is much less and varies in the nearly the opposite order (111)>(100)>(110). The peroxide intermediate pathway is clearly operative on Au(hkl) surfaces. At elevated temperature, significantly smaller amounts of peroxide are formed. The kinetics of the ORR were significantly enhanced by modification of both Pt(hkl) and Au(hkl) surfaces with Pd. The catalytic effect is most pronounced on the surfaces that are less active surfaces in the unmodified state, with enhancement at least an order of magnitude faster kinetics. Pd modification of the Au(hkl) surfaces, therefore, significantly reduces the structure sensitivity of the ORR. Even on the highly active Pt(111) surface the kinetics can be improved by a factor of approximately two to four due to Pd modification. The catalytic enhancement can be achieved with as little as 18 at.% Pd in the Au(hkl) surface.

Investigation of Enhanced CO Tolerance in Proton Exchange Membrane Fuel Cells by Carbon Supported PtMo Alloy Catalyst

E-TEK, Incorporated, Natick, Massachusetts 01760, USAWe report a two- to threefold enhancement of CO tolerance in a proton exchange membrane (PEM) fuel cell, exhibited by carbonsupported nanocrystalline PtMo/C as compared to the current state of the art PtRu/C electrocatalysts. The bulk of these nanocrys-tals were comprised of Pt alloyed with Mo in the ratio 8.7:1.3 as shown by both X-ray diffraction and in situ extended X-ray ab sorp-tion fine structure measurements. Rotating disk electrode measurements and cyclic voltammetry in a PEM fuel cell indicate theonset of CO oxidation at potentials as low as 0.1 V. Further, the oxidation of CO exhibits two distinct peaks, indicating redox b ehav-ior involving oxyhydroxides of Mo. This is supported by in situ X-ray absorption near edge structure measurements at the Mo Kedge.© 1999 The Electrochemical Society. S1099-0062(98)08-029-8. All rights reserved.Manuscript submitted August 10, 1998; revised manuscript received September 28, 1998. Available electronically October 30, 1998.

Temperature dependent surface electrochemistry on Pt single crystals in alkaline electrolytes. Part 2: The hydrogen evolution/oxidation reaction

The hydrogen evolution reaction (her) and the hydrogen oxidation reaction (hor) are studied on Pt(111), Pt(100), and Pt(110) single crystal surfaces in 0.1 M KOH over the temperature range 275–333 K. The results demonstrated that the kinetics of the her/hor are structure sensitive processes, with Pt(110) being about ten times more active than either of the atomically ‘flatter’ (100) or (111) faces at 275 K. At higher temperatures, however, the value of the exchange current density differs by less than a factor of two between Pt(110) and Pt(111). The difference in activity with crystal face is attributed to the structure sensitive adsorption of underpotentially deposited hydrogen (Hupd) and hydroxyl species (OHad) and the effect these species have on the formation of the electroactive intermediate, Hopd, whose physical state is uncertain. It is proposed that in the vicinity of the Nernst potential Hupd and OHad may have two modes of action on the kinetics of the her/hor: a blocking effect from competition for the same sites with molecular H2 and Hopd, and an energetic effect altering the adsorption energy of the reactive intermediate. The significant differences of the her/hor kinetics in alkaline versus acid electrolyte are suggested to arise mainly due to the presence of OHad even close to the reversible potential of the her/hor in alkaline electrolytes.

FTIR characterization of PEO + LiN(CF3SO2)2 electrolytes

Abstract FTIR transmission spectra of thin films of polyethylene oxide (PEO) + lithium trifluoromethanesulfonimide (LiN(CF3SO2)2, LiTFSI) mixtures have been obtained for ethylene oxide Li ratios from 64:1 to 2:1. The phase information from these spectra is compared with the reported phase diagrams based on thermal measurements. The infrared spectrum of LiTFSI is assigned by analogy with those of related compounds. Using the spectral subtraction technique, the effects of Li+ solvation on the PEO matrix and of ion pairing and aggregate formation on the TFSI anion are revealed. An explanation is offered for the variation in ionic conductivity for compositions within the “crystallinity gap”.

Using “Tender” X-ray Ambient Pressure X-Ray Photoelectron Spectroscopy as A Direct Probe of Solid-Liquid Interface

We report a new method to probe the solid-liquid interface through the use of a thin liquid layer on a solid surface. An ambient pressure XPS (AP-XPS) endstation that is capable of detecting high kinetic energy photoelectrons (7 keV) at a pressure up to 110 Torr has been constructed and commissioned. Additionally, we have deployed a “dip & pull” method to create a stable nanometers-thick aqueous electrolyte on platinum working electrode surface. Combining the newly constructed AP-XPS system, “dip & pull” approach, with a “tender” X-ray synchrotron source (2 keV–7 keV), we are able to access the interface between liquid and solid dense phases with photoelectrons and directly probe important phenomena occurring at the narrow solid-liquid interface region in an electrochemical system. Using this approach, we have performed electrochemical oxidation of the Pt electrode at an oxygen evolution reaction (OER) potential. Under this potential, we observe the formation of both Pt2+ and Pt4+ interfacial species on the Pt working electrode in situ. We believe this thin-film approach and the use of “tender” AP-XPS highlighted in this study is an innovative new approach to probe this key solid-liquid interface region of electrochemistry.