Probing the surface sensitivity of dimethyl ether oxidation on epitaxially-grown PtRh(1 0 0) alloys: Insights into the challenge of improving on Pt(1 0 0)

Abstract

Dimethyl ether (DME) has attracted significant interest as a potential alternative energy carrier for fuel cells. Its electrochemical oxidation is uniquely promoted at the Pt(1 0 0) surface, which features the double-bridge ensemble required for CO bond breaking to occur. To investigate the DME oxidation activity of bimetallic (1 0 0) surfaces in sulfuric acid, pulsed laser deposition (PLD) was employed to grow thin-film PtRh alloys on a well-ordered MgO (1 0 0) substrate. This technique ensured the epitaxial growth of the metal layer along the [0 0 1] plane in a cube-on-cube fashion, as highlighted by X-ray diffraction pole figures. The entire composition range was explored: Rh and Rh-rich alloys display a very poor DME oxidation activity, restricted to the first voltammetric cycle; this confirmed that the presence of an excess of Rh at the surface severely impairs DME oxidation. On the other hand, Pt-rich alloys are able to perform steady-state DME oxidation. Their actual (electrochemically-accessible) surface Rh content can also be fine-tuned by repeated cycling into the oxide region; however, this inevitably involves the introduction of defects. Hence, two useful lessons can be learned about the electrocatalysis of DME oxidation at bimetallic (1 0 0) surfaces: the intensity of DME oxidation is primarily controlled by the quality and quantity of Pt(1 0 0) surface domains, while the presence of surface and sub-surface Rh only induces a minor shift of the oxidation wave to earlier potentials. Finally, we also showed that an anticipation of the CO stripping potential does not translate into any significant enhancement of DME oxidation, in line with earlier reports indicating that CO oxidation is not the rate-determining step of DME oxidation.