Particle shape control using pulse electrodeposition: Methanol oxidation as a probe reaction on Pt dendrites and cubes

Abstract

Shape controlled Pt particles were synthesized onto tungsten monocarbide (WC) substrates using a pulse electrodeposition method. The particle shape was strongly influenced by the deposition potential, with cubic particles formed using 0.14 V vs. NHE (normal hydrogen electrode) and dendritic particles formed at 0.04 V vs. NHE. The crystalline orientation and active surface area of the Pt particles were estimated using Cu stripping voltammetry. Finally, cyclic voltammetry and chronoamperometry were used to determine the methanol electrooxidation activity, which revealed that the dendritic Pt showed much higher electrochemical activity than the cubic particles. These results demonstrated the possibility of more effectively utilizing Pt electrocatalysts by controlling the shape of Pt particles.

Introduction

In catalyst synthesis, the structure of the particle is often overlooked. Rather, the focus is often on producing catalysts that are the smallest size to improve the ratio of exposed surface area per precious metal loading while keeping precious metal loadings as low as possible. For some reactions that have been shown to be structure sensitive, another way to optimize the catalyst is to synthesize metal particles with specific shapes. For example, Pt nanoparticles have been shown to effectively oxidize methanol, but their morphology and surface crystalline orientation have been found to play a considerable role in their effectiveness [1]. The methanol oxidation reaction is a structure sensitive reaction, and certain crystal planes have been shown to exhibit higher activity than others [2], [3]. Therefore, a method to produce Pt particles with shape control can be useful in optimizing the methanol oxidation reaction activity.

Colloidal methods using organic ligand stabilizers are one of the most commonly used methods in making shape controlled particles [4], [5], [6], [7], [8]. However, the organic ligand shells can be difficult to remove and can inhibit catalytic activity [9]. In contrast, electrochemical deposition is a fast, simple method that can produce three-dimensional particles directly onto a substrate or support. It is a one-step technique, requires no additional purification step, and its implementation is low in cost with commercially available technology [10], [11], [12]. It also has the added benefit of being applicable on a number of support geometries, as the deposition process is not a line-of-sight method. Furthermore, the deposition potential has been shown to be a convenient parameter for controlling the structure of the deposited metal [13]. Electrodeposition has been used to demonstrate the production of metallic particles of various shapes on a number of substrates. For example, the electrodeposition of chromium particles was demonstrated on Si, making spherical particles at low deposition potentials and hexagonal microrods at higher potentials [14]. Another example is the synthesis of tetrahexahedral Pd nanocrystals with high Miller index facets using a pulse electrodeposition method [15].

For most electrocatalytic application the chosen support can have a large influence on the effective use of the catalyst. This is especially true for supporting electrodeposited catalyst particles, since the nature of the catalyst can strongly affect particle nucleation [16]. In this work, we explore the use of tungsten monocarbide (WC) as a supporting material. Because of its similar bulk electronic properties to Pt, WC has been identified as a promising substrate to support Pt catalysts, exhibiting improved performance compared to carbon substrates [17], [18], [19], [20], [21]. It is inexpensive, electrically conductive, and durable in acidic environments [20], [21]. WC has previously been demonstrated to be a good support for Pt for several types of electrochemical reactions. For example, one monolayer of Pt on a WC substrate was found to have comparable activity for the hydrogen evolution reaction as bulk Pt [19]. The monolayer bimetallic system maximizes the use of the Pt catalyst by replacing the bulk of the catalyst with WC, leaving one monolayer of Pt to act as the catalyst for the reaction. Furthermore, it has been observed that Pt on a WC support maintains more of its electrochemically active surface area after rigorous potential cycling than Pt supported on carbon [17]. This suggests that Pt on a WC support is more resistant to catalyst degradation issues commonly seen on commercial Pt/C catalysts such as particle agglomeration and dissolution [17].

The aim of this work is to demonstrate the ease of pulse electrodeposition as a method to produce a variety of particles shapes and confirm the effect that this can have on the activity of methanol oxidation, a structure sensitive reaction. Here, pulse electrodeposition is utilized to synthesize Pt particles that are round, cubic, or dendritic, depending on the deposition potential. The preferred crystal orientations of the particles are confirmed with Cu stripping voltammetry and correlated with methanol oxidation activity to demonstrate the effects of Pt crystal planes on the electrooxidation activity.