Elsevier

Electrochimica Acta

Volume 135, 20 July 2014, Pages 249-254
Electrochimica Acta

Anodic Oxidation of COads Derived from Methanol on Pt Electrocatalysts Linked to the Bonding Type and Adsorption Site

https://doi.org/10.1016/j.electacta.2014.05.006Get rights and content

Abstract

A newly formulated four-component modified Butler-Volmer model has been developed to evaluate global oxidation kinetic parameters for the various types of carbon monoxide adsorbates (COads) on a nanoparticle Pt surface determined by the type of bonding as well as the local structure of the adsorption site. Partial coverages of COads were prepared by potentiostatic adsorption of methanol followed by potentiostatic partial oxidation at various elevated potentials and for various durations. Anodic linear sweep voltammetry was then performed, and the COads oxidation peaks were fitted with the model to analyze the kinetics. According to the model, preferential oxidation with respect to COads bonding and Pt substrate structure can be achieved dependent upon the potential and extent of oxidation. Partial oxidation at 450 mV vs. RHE for 60 min. resulted in a majority population of linearly bonded COads on cubic-packed Pt sites; whereas partial oxidation at 650 mV vs. RHE for 220 sec. resulted in a majority population of bridged-bonded COads on close-packed Pt sites.

Introduction

Since the inception of low-temperature methanol fuel cells, intermediate species such as carbon monoxide formed during the oxidation of methanol have plagued the fuel cell's efficiency and lifetime. This is due to the strong adsorption of CO on the electrode surface, blocking the electrocatalytic sites from further oxidation of fuel. Consequently, methanol oxidation along with CO adsorption and oxidation on electrocatalysts such as Pt surfaces has been an intensely researched field for several decades [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Many studies have sought to gain enough understanding of this system to find CO-tolerant catalysts or to engineer a Pt surface structure or morphology with a lower propensity toward CO adsorption [13], [14], [15], [16], [17]. In fact, several recent studies have found that Pt surface structural features such as particular crystallographic faces, steps, edges and defects are significant factors in its activity for methanol and COads oxidation [18], [19], [20], [21], [22], [23], [24], [25], [26], [27].

In addition to the surface-dependent properties of the Pt substrate, another equally important factor in CO oxidation is the bonding characteristics of the adsorbed CO. It has been well established that CO may bond to Pt in one of three configurations: as a linear species, a bridged species or as a reduced carbonyl, which is associated with one, two or three Pt sites, respectively [28]. Previous studies involving advanced NMR techniques have found that the majority of the adsorbed CO is either in the linear or bridged configurations, and the bridged adsorbate is much more mobile on the Pt surface than its linear counterpart [28], [29], [30], [31], [32].

In previously published work it was reported that adsorption at 250 mV vs. RHE produces mostly linear COads species. During oxidation, conversion from linear to bridged species occurs at the sites that have been vacated by the linear COads oxidation. This transition can be monitored by calculating the Neps value, which is defined as the number of electrons passed per Pt adsorption site during the COads oxidation. This number for oxidized COads species is thus 1 for bridged, 2 for linear and 3 for reduced carbonyl [28]. In a later study, the characteristic two-peak hydrogen region was correlated with the types of adsorption sites of the Pt. The electrocatalytic electrodes were made with Pt nanoparticles, and the types of surface sites were grouped into two categories based on the degree of atomic packing. The close-packed surface sites were assigned as the weakly bound (WB) sites, and the cubic-packed surface sites were assigned as strongly bound (SB) sites. The H desorption peaks at lower and higher potentials were correlated with Hads located on the WB and SB sites, respectively [31]. Furthermore, it was found that the COads linear sweep oxidation peak could be fitted with a two-peak modified Butler-Volmer model. These two peaks were subsequently correlated with the site-specific Hads desorption current peaks: peak 1 at lower potential and peak 2 at higher potential with SB and WB site COads oxidation, respectively, as illustrated in Fig. 1 [33]. However, the values of the kinetic parameters associated with each peak were allowed to vary to obtain an adequate fit to the data, which suggests that the complex oxidation process may not be fully described by a two-peak model.

In recent years, a number of groups have formulated various models to better understand the oxidation kinetics at work [34], [35], [36], [37], [38], [39], [40]. Of particular note, Sethuraman et al. found that surface populations of WB COads species dominated at lower temperatures and SB CO adsorbates dominated at higher temperatures in thermally-driven adsorption/oxidation experiments.

The work presented here reports the continued study of methanol-derived COads by investigating the voltammetric oxidation kinetics of partial coverages of COads obtained by potentiostatic adsorption and oxidation at moderate and elevated potentials and for various durations. The limitations of the two-peak model reported previously [33] are discussed, and a four-peak Butler-Volmer model has been developed, which specifies both the adsorption site and the type of bonding of the adsorbed species. Global kinetic parameters are calculated using this model, and the conditions whereby preferential oxidation of COads based on bonding type and adsorption site are also identified.

Section snippets

Experimental Apparatus and Procedures

Linear sweep voltammetry (LS), cyclic voltammetry (CV) and chronoamperometry (CA) were performed using a special apparatus that allowed for continuous liquid flow from isolated electrolyte reservoirs as illustrated in Fig. 2b. A modified 50 mL 3-neck flask was used as the working electrode compartment. Separate compartments housed a Hg/HgSO4 reference electrode (Koslow Scientific) and a platinum grid counter electrode. The compartments were connected with Teflon tubing, and a capillary tube was

Two-Peak Model Analysis

Partial coverage populations of COads were prepared by partial oxidation of the saturated surface at 450 mV or 650 mV vs. RHE for various durations. Two techniques were utilized in analyzing the following linear sweep voltammetric curves to compare the changing WB/SB ratio of COads species, the results of which are reported in Fig. 3. For the 450 mV partial oxidation, both methods exhibit a decreased WB/SB ratio with increased oxidation duration at this low potential (see Fig. 3a). This result

Conclusion

The newly developed four-peak modified Butler-Volmer model has been utilized to determine global kinetic parameters for the four types of adsorbed CO on nanoparticle Pt surfaces: (1) linearly bound CO on SB sites, (2) linearly bound CO on WB sites, (3) bridged bound CO on SB sites and (4) bridged bound CO on WB sites. The fitting results further demonstrated that potential-dependent preferential oxidation is exhibited. In agreement with the results from the two-peak model, the four-peak model

Acknowledgements

This work has been supported in part by an NSF graduate research fellowship and by the U.S. Army Research Laboratory and the U.S. Army Research Office under contract/grant number 48713CH.

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