CO₂ and Formate Pathway of Methanol Electrooxidation at Rhodium Electrodes in Alkaline Media: An In Situ Electrochemical Attenuated Total Refection Surface-Enhanced Infrared Absorption Spectroscopy and Infrared Reflection Absorption Spectroscopy Investigation

Abstract

Rh catalysts exhibit unexpected high activity for the methanol oxidation reaction (MOR) in alkaline conditions, making them potential anodic catalysts for direct methanol fuel cells (DMFCs). Nevertheless, the MOR mechanism on Rh electrodes has not been clarified thus far, which impedes the development of high-efficiency Rh-based MOR catalysts. To investigate it, a combination of in situ electrochemical techniques called attenuated total refection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and infrared reflection absorption spectroscopy (IRAS) is used. Cyclic voltammograms of MOR at Rh electrodes show considerable activity in alkaline media rather than acidic media, although the real-time ATR-SEIRA spectral results demonstrate that methanol can rarely self-decompose on Rh at open-circuit conditions. Meanwhile, in combination of ATR-SEIRAS and IRAS results, CO2 and formate are thought to be MOR products, suggesting a dual-pathway mechanism (“CO2 pathway” and “formate pathway”). Specifically, COad species, which are the major intermediates in the CO2 pathway, can produce at lower potentials and be oxidized into CO2 at a potential of 0.5–0.75 V. Concurrently, the formate can be produced from 0.5 V and diffuse into the bulk electrolyte to become one of the MOR products, while the further electrochemical conversion of formate to CO2 is essentially negligible. More directly, the apparent selectivity (r) of the CO2 pathway is estimated to reach ca. 0.63 at 0.9 V, confirming the potential-dependent selectivity of MOR at Rh surfaces. This study might provide fresh insights into the design and fabrication of effective Rh-based catalysts for MOR.

From Ethylene Glycol to Glycolic Acid: Electrocatalytic Conversion on Pt-Group Metal Surfaces

“…Whereas, the detection of CO 2 (and/or Carbonate) products still implies the existence of CO (and/or COH x ) pathway of EGOR at Pt-group metals, , and we indeed detect the CO ad species during EGOR at the Pt electrode by the in situ ATR-SEIRAS measurement (Figure S5), it could be highly related to the poor catalysis stability of EGOR on Pt electrode as shown in Figure S4b. On the other hand, the CO 2 product could partially come from the formate oxidation as described before. ,, …”

“…On the other hand, the electrooxidation of glycolate seems to produce undetectable formate species (see HPLC results in Figure S12), but we indeed observe the formate and CO 2 products on four electrodes during EGOR (Figure ). Therefore, it could be speculated that formate could come from the direct C–C bond scission of EG on Pt and Pd electrode as described before. , Meanwhile, as mentioned above, the CO 2 products might mainly come from the further oxidation of CO ad /COH x species and intermediate species which were formed by EG or GA. ,, …”

“…Therefore, it could be speculated that formate could come from the direct C−C bond scission of EG on Pt and Pd electrode as described before. 29,30 Meanwhile, as mentioned above, the CO 2 products might mainly come from the further oxidation of CO ad /COH x species and intermediate species which were formed by EG or GA. 15,47,49 On the above results, the EGOR pathway on the Pt-group electrodes can be put forward and summarized in Figure 7. First, EG adsorption on the surface generates the intermediate of glyoxal or ethanol aldehyde (undetectable in this work) according to the previous work.…”

“…On the other hand, the CO 2 product could partially come from the formate oxidation as described before. 15,46,47 To better identify the EGOR products at Pt, Pd, Rh, and Ir surfaces, ex situ HPLC was employed to qualitatively and quantitatively measure the residual electrolyte collected at each potential. As shown in Figure S6, the EGOR process generates glycolate, oxalate, and formate on Pt and Pd electrodes, while glycolate and oxalate can be produced at Rh and Ir electrodes.…”