(Invited) Development of Multifunctional Nanoscale Reactors for Electrocatalytic Oxidation of Dimethyl Ether Fuel
Authors: Rutkowska Iwona A. , Rytelewska Beata , Kulesza Pawel 
Published: August 2024

There has been growing interest in utilizing small (simple) organic molecules, as alternative fuels to hydrogen, for electrochemical energy conversion in fuel cells. Dimethyl ether (DME) is one of the possible choices in this respect. But electrooxidation of DME is rather slow at ambient conditions. Obviously, there is a need to develop novel electrocatalytic materials.
Bimetallic PtSn catalysts were found to be almost as active as PtRu for methanol oxidation, and PtSn was even more efficient toward the electrooxidation of ethanol where breaking of C-C bond is required. While PtRu forms a single-phase homogeneous (intermetallic) alloy in which Ru is largely metallic, tin does not maintain its metallic state in PtSn (heterogenous alloy) and is transformed readily to such oxygenated species as Sn oxides or hydroxides. During operation of PtSn, the Pt component retains its mostly metallic character and act independently as Pt0 catalyst. By analogy to Ru, the Sn “bifunctional agent” tends to activate the water dissociative adsorption and increases population of chemisorbed hydroxyl groups to diminish the Pt-poisoning effect through enhanced oxidation of CO-type intermediates. Nevertheless, the DME oxidation on both PtRu or PtSn is still kinetically less favorable, relative to the systems’ performance toward methanol. While Sn, and particularly Ru alloyed atoms activate readily the water dissociative adsorption on Pt and permit the oxidative stripping of the poisoning CO-type adsorbates, the feasibility of the enhancement of the of the DME adsorption and activation through alteration of the electronic structure of Pt catalytic sites has not been fully explained. By analogy to the ethanol oxidation, Pt sites in the heterogeneous PtSn alloy are active toward dissociation of the DME molecule but the Sn additive is less efficient than Ru (from intermetallic PtRu) in the oxidative removal of poisoning adsorbates. But PtRu is probably the most active during oxidation of methanol, and this small organic molecule is also the DME-oxidation-reaction intermediate. Nevertheless, in comparison to bare Pt, electrooxidation of DME starts at PtRu and PtSn at less positive potentials.
We pursue a concept of utilization of multicomponent hybrid electrocatalytic systems or nanoreactors utilizing zirconia oxide matrices for supporting and activation of primarily PtSn nanoparticles supported onto Vulcan XC-72 carbon black carriers. Electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys supported onto ZrO2 matrices has been significantly enhanced toward electrooxidation of DME in acid medium (0.5 mol dm-3 H2SO4) by decorating PtSn/V with bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence (even below 0.45 V vs. RHE) of reactive ruthenium oxo/ hydroxo groups on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In this respect, the Ru-oxo species seem to support activity of Sn forming with Pt the PtSn heterogenous alloy. The PtSn/V admixed with PtRu decorated ZrO2 exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles decorated zirconia oxide seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol).

Acknowledgement

Authors acknowledge financial support of the National Science Center (Poland) under Opus Project 2018/29/B/ST5/02627.