As in the case of most small organic molecules, the electro-oxidation of methanol to CO 2 is believed to proceed through a so-called dual pathway mechanism. The direct pathway proceeds via reactive intermediates such as formaldehyde or formic acid, whereas the indirect pathway occurs in parallel, and proceeds via the formation of adsorbed carbon monoxide (CO ad ). Despite the extensive literature on the electro-oxidation of methanol, no study to date distinguished the production of CO 2 from direct and indirect pathways. Working under, far-from-equilibrium, oscillatory conditions, we were able to decouple, for the first time, the direct and indirect pathways that lead to CO 2 during the oscillatory electro-oxidation of methanol on platinum. The CO 2 production was followed by differential electrochemical mass spectrometry and the individual contributions of parallel pathways were identified by a combination of experiments and numerical simulations. We believe that our report opens some perspectives, particularly as a methodology to be used to identify the role played by surface modifiers in the relative weight of both pathways-a key issue to the effective development of catalysts for low temperature fuel cells.

We report the experimental study of the impact of anion adsorption on the two parallel pathways of CO2 formation during the electro-oxidation of methanol on platinum. The effect of nature of the supporting electrolyte (HClO4 and H2SO4) was investigated at two methanol concentrations. Voltammetric profiles and oscillatory time series of the electrode potential were registered in conjunction with the evolution of the production of carbon dioxide and methylformate, as measured by means of on line differential electrochemical mass spectrometry (DEMS). In all conditions studied, the production of CO2 was higher in the presence of HClO4 rather than that in H2SO4. Importantly, the inhibition caused by anion adsorption was generally more pronounced in the direct pathway, i.e., the non-COad pathway. Furthermore, we have noted an additional peak commonly observed at high potentials during the oscillatory electro-oxidation of small organic molecules can be generally attributed to the oxidative removal of COad. Altogether, the effect of anion adsorption is discussed in connection with both the oscillatory kinetics and the dynamics of adsorbed species, contributing thus to a comprehensive physical-chemistry description of the surface processes.

a b s t r a c tThe ethanol electro-oxidation reaction was studied on carbon-supported Pt, Rh, and on Pt overlayers deposited on Rh nanoparticles. The synthesized electrocatalysts were characterized by TEM and XRD. The reaction products were monitored by on-line DEMS experiments. Potentiodynamic curves showed higher overall reaction rate for Pt/C when compared to that for Rh/C. However, on-line DEMS measurements revealed higher average current efficiencies for complete ethanol electro-oxidation to CO 2 on Rh/C. The average current efficiencies for CO 2 formation increased with temperature and with the decrease in the ethanol concentration. The total amount of CO 2 , on the other hand, was slightly affected by the temperature and ethanol concentration. Additionally, the CO 2 signal was observed only in the positive-going scan, none being observed in the negative-going scan, evidencing that the C-C bond breaking occurs only at lower potentials. Thus, the formation of CO 2 mainly resulted from oxidative removal of adsorbed CO and CH x,ad species generated at the lower potentials, instead of the electrochemical oxidation of bulk ethanol molecules. The acetaldehyde mass signal, however, was greatly favored after increasing the ethanol concentration from 0.01 to 0.1 mol L À1 , on both electrocatalysts, indicating that it is the major reaction product. For the Pt/Rh/C-based electrocatalysts, the Faradaic current and the conversion efficiency for CO 2 formation was increased by adjusting the amount of Pt on the surface of the Rh/C nanoparticles. The higher conversion efficiency for CO 2 formation on the Pt 1 Rh/C material was ascribed to its faster and more extensive ethanol deprotonation on the Pt-Rh sites, producing adsorbed intermediates in which the C-C bond cleavage is facilitated.

The study of complex reaction under oscillatory conditions has been proven to be useful in uncovering features that are hidden under close to equilibrium regime. In particular, for the electro-oxidation of small organic molecules on platinum and platinum-based surfaces, such investigations have provided valuable mechanistic information, otherwise unavailable under nonoscillatory conditions. We present here the dynamics of production of volatile species along the oscillatory electro-oxidation of formic acid, methanol, and ethanol on platinum, as measured by online differential electrochemical mass spectrometry (DEMS). Besides the presentation of previously unreported DEMS results on the oscillatory dynamics of such systems, we introduce the use of multivariate linear regression to compare the estimated total faradaic current with the one comprising the production of volatile species, namely: carbon dioxide for formic acid, carbon dioxide and methylformate for methanol, and carbon dioxide and acetaldehyde for ethanol. The introduced analysis provided the best combination of the DEMS ion currents to represent the total faradaic current, or, equivalently, the maximum possible faradaic contribution of the volatile products for the global current. The mismatch between estimated total current and the one obtained by the best combination of partial currents of volatile products was found to be small for formic acid, 4 and 5 times bigger for ethanol and methanol, respectively, evidencing the increasing role played by partially oxidized, soluble species in each case. These results were discussed in connection with the mechanistic aspects of each system. Moreover, we have defined some descriptors to account for the production of volatile species, and discussed dynamics in terms of sample and populational covariances

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One of the most important current topics in the renewable and sustainable energy scenario is the CO2 electro‐reduction reaction (CO2RR), which is an alternative and important route for its conversion into various high value‐added chemicals, therefore making up a CO2 recycling process. Despite its importance and the works already developed in this field, many challenges still need to be overcome for CO2RR to reach high values of efficiency and selectivity. This is even more challenging considering that this reaction occurs with the transfer of several electrons, making the investigation and elucidation of the reaction mechanism a real need. Thus, several characterization techniques have been employed, and specially, the on‐line Electrochemical Mass Spectrometry (EC‐MS) technique emerges as a powerful tool, thus making possible to improve the understanding of reaction pathways, through the identification of products and intermediaries, and allowing the screening of electrocatalyst potentials for CO2RR. Herein, we present the evolution of adaptations of general electrochemical cell designs for the study of the CO2RR.

In this paper we present results on the electro-oxidation of ethanol on unsupported (carbon free) platinum nanoparticles, considering the effects of the alcohol concentration. The case of the so-called dual pathway mechanism during the electro-oxidation of ethanol showed to be influenced by the surface coverage of adsorbed carbon monoxide (CO ad ) at unsupported platinum. The influences of adsorbed intermediates were followed by in situ infrared spectroscopy (FTIR) and by electrochemical experiments. Unsupported platinum showed that the reaction leads to the formation of CO 2 and acetic acid as main products at low concentrations of ethanol (0.01 to 0.1 mol L −1 ). At least in this case of 0.01 mol L −1 ethanol, most formation of CO 2 occurred via CO ad (indirect pathway). At higher concentration of ethanol, however, most CO 2 was formed via a reactive intermediate such as acetaldehyde (direct pathway). In addition, in this higher concentration of ethanol, the acetic acid was produced via formation of adsorbed acetaldehyde (via acetate) at higher overpotentials. In case of the acetic acid formation, a dual pathway was identified during the electro-oxidation of ethanol at low alcohol concentrations, whereas a parallel pathway occurred without the formation of adsorbed acetate intermediates at low overpotentials.