An NMR Investigation of CO Tolerance in a Pt/Ru Fuel Cell Catalyst

Abstract

We report the first combined application of solid-state electrochemical NMR (EC NMR), cyclic voltammetry (CV), and potentiostatic current generation to investigate the topic of the ruthenium promotion of MeOH electro-oxidation over nanoscale platinum catalysts. The CV and EC NMR results give evidence for two types of CO: CO on essentially pure Pt and CO on Pt/Ru islands. There is no NMR evidence for rapid exchange between the two CO populations. CO molecules on the primarily Pt domains behave much like CO on pure Pt, with there being little effect of Ru on the Knight shift or on Korringa relaxation. In sharp contrast, COs on Pt/Ru have highly shifted (13)C NMR resonances, much weaker Korringa relaxation, and, at higher temperatures, they undergo thermally activated surface diffusion. For CO on Pt, the correlation observed between the 2pi* Fermi level local density of states and the steady-state current suggests a role for Ru in weakening the Pt-CO bond, thereby increasing the CO oxidation rate (current). The combined EC NMR/electrochemistry approach thus provides new insights into the promotion of CO tolerance in Pt/Ru fuel cell catalysts, in addition to providing a novel route to investigating promotion in heterogeneous catalysis in general.

An NMR Determination of CO Diffusion on Platinum Electrocatalysts

“…9 The latter topic is also of interest in the context of fuel cell catalysis, but there has been no direct experimental determination of D CO in an electrochemical environment due to problems associated with the presence of the electrolyte. 10 Fortunately, however, NMR methods are not plagued by these problems, [11][12][13] and in this paper, we report the first direct determination of the diffusion constants of CO on Pt in a liquid electrochemical environment, together with the activation energy for diffusion, using the techniques of electrochemical NMR (EC-NMR) 11-14 and selective spin inversion NMR. 7 To determine diffusion constants, we used the "S-shape" pulse sequence developed by Becerra et al 7 The S-shape pulse sequence (Figure 1) exploits the fact that CO molecules adsorbed on a Pt nanoparticle can have different 13 C resonance frequencies, depending on the angle of CO's unique molecular axis with respect to the external magnetic field.…”

Shape-controlled synthesis of Pt-Ir nanocubes with preferential (100) orientation and their unusual enhanced electrocatalytic activities

“…Shape-controlled synthesis of Pt nanocubes, which are enclosed by (100) surfaces, has therefore received great attention as high-performance electrocatalysts [16,25,26]. More excitingly, incorporation of other metals into the Pt NPs to form bimetallic Pt NPs has been proved to be a promising method for further enhancing the catalytic properties due to the bifunctional mechanism [27] and the electronic effect [28,29]. For instance, Xu et al [30] demonstrated that Pt 80 Cu 20 nanocube catalyst was about ten times more durable than its Pt nanocube counterpart for formic acid oxidation.…”

A Pt3Sn/C Electrocatalyst Used as the Cathode and Anode in a Single Direct Ethanol Fuel Cell

“…Using Pt/C ETEK, the diffusion limit mass current density for the ORR was 20 % smaller than that using the same electrocatalyst without ethanol. These results can be explained because the strength of adsorption of both CO (Tong, Kim et al, 2002;Alayoglu, Nilekar et al, 2008;Park, Lee et al, 2008) and O 2 (Min, Cho et al, 2000;Aricò, Shukla et al, 2001;Shukla, Neergat et al, 2001;Lima, Lizcano-Valbuena et al, 2006) is reduced in Pt 3 Sn alloy materials, thereby facilitating a release of the catalytic sites and increase of the kinetic reaction.…”

“…Its high performance is because the electronic interactions between Pt and the alloyed metals modify the lattice parameters of platinum, leading to a change in the electron density of the material by shrinking of the "d" band of platinum (electronic effect) (Tong, Kim et al, 2002;Alayoglu, Nilekar et al, 2008;Park, Lee et al, 2008).…”