Electrocatalytic Enhancement Effects at Platinized Nanoporous Substrates: Oxidation of Ethanol at PtRu Nanoparticles Dispersed over Rh-Containing ZrO2 Support

Nanostructured platinum is so far the best-known and the most active catalyst for the activation of small organic molecules via their specific adsorption and interfacial dissociation However, the ability of platinum to break effectively the C–C bonds is largely limited by side poisoning effects originating from the strong adsorption of CO_ads on Pt surfaces at low (below 60 or even 80 ^oC). Several two-carbon containing molecules have been reported as undesirable intermediates during ethanol oxidation at polycrystalline Pt, most of them requiring relatively high potentials for complete oxidation.

It is commonly accepted that the highest efficiency for electrooxidation of ethanol was achieved at carbon-supported PtSn (PtSn/C) catalysts and, more recently, very promising result were obtained with PtRh type systems. Despite the relatively good performance of the PtSn/C based anodes, the overall efficiency is still poor when compared to oxidation of hydrogen (at Pt) or even methanol (at PtRu). The main products remain acetaldehyde (2-electron oxidation) and acetic acid (4-electron oxidation) thus further modification of existing catalysts is necessary to obtain higher efficiency. There is a need to remove not only the poisoning CO adsorbates but also to activate the ethanol molecule and to break C-C bonds effectively. More recently, modification of the platinum-based surface is achieved by adding the second additive namely by introducing the second alloying metal.

We propose a hybrid electrocatalytic material for oxidation of ethanol utilized PtRu nanoparticles dispersed over thin-films of rhodium-containing zirconia. The system is particularly active when it has been deposited onto such platinized nanoporous substrate as silica-nanosphere-layered solid-assembly. Morphologies of the resulting catalytic surfaces were characterized here using electrochemical techniques as well as transmission and scanning electron microscopies. The enhancement of activity of PtRu nanoparticles (in 0.5 mol dm^-3 H₂SO₄  at room temperature in the presence of ethanol) has been evident from the comparative electrochemical (voltammetric, chronoamperometric) experiments performed with use of Rh-containing and Rh-free ZrO₂ films deposited onto platinized nanoporous plates. It is apparent from additional diagnostic chronoamperometric experiments (executed at inert glassy carbon electrode substrates) that electrocatalytic currents measured at potentials as low as 0.3 V (vs. RHE) are significantly larger upon application of zirconia and, particularly, Rh-containing zirconia supports for dispersed PtRu nanoparticles. The results are also consistent with the strong enhancement effect originating from the use of platinized silica-nanosphere-layered solid-assemblies. The overall activation effect may have origin in the existence of large population of hydroxyl groups (from zirconia) in the vicinity of platinum (from PtRu alloy nanoparticles) capable of inducing oxidative removal of passivating CO adsorbates. The presence of rhodium within zirconia seems to not only enhance conductivity but also to interact specifically with the oxide and/or dispersed catalytic centers. By analogy to titania, zirconia may influence activity of ruthenium oxo species in the oxidative removal of passivating (e.g. CO) intermediates. High surface area platinized nanoporous substrates enlarge the overall catalytic active area and, most likely, provide reactive sites for C-C bond breaking in the ethanol molecule.