Formation of methylformate during methanol oxidation revisited: The mechanism

Anodic Oxidation of Methanol to Formaldehyde Synergizing with a Br-/Br2 Redox-Mediated Chemical Route to Produce Methyl Formate

Methyl formate (MF) is one of the most important chemical commodities, which has a wide range of applications. Due to environmental friendliness, mild reaction conditions, and easy operations, electrosynthesis of MF has garnered increasing attention in recent years. In this work, we reported an electrosynthesis route toward MF in a halide-containing methanol solution. The thorough mechanistic investigations point out that electrosynthesis of MF is accomplished by instant reaction between aldehyde from anodic methanol oxidation, and methoxy bromide (CH3OBr) that is in-situ generated by reaction of Br2 from anodic oxidation of Br- with methoxide (CH3O-) from cathodic reduction of methanol. This method features high atomic economy only producing valuable MF and hydrogen gas, and shows distinct advantages compared to the reported MF electrosynthesis methods. Even at 200 mA/cm2, the faradaic efficiency (FE) of MF remains consistently around 60 % at the anode while a 100 % FE hydrogen gas is produced at the cathode.

Adsorbed Enolate as the Precursor for the C-C Bond Splitting during Ethanol Electrooxidation on Pt

Ethanol is a promising alternative fuel to methanol for direct alcohol fuel cells. However, the complete electrooxidation of ethanol to CO₂ involves 12 electrons and C-C bond splitting so that the detailed mechanism of ethanol decomposition/oxidation remains elusive. In this work, a spectroscopic platform, combining SEIRA spectroscopy with DEMS, and isotopic labeling were employed to study ethanol electrooxidation on Pt under well-defined electrolyte flow conditions. Time- and potential-dependent SEIRA spectra and mass spectrometric signals of volatile species were simultaneously obtained. For the first time, adsorbed enolate was identified with SEIRA spectroscopy as the precursor for C-C bond splitting during ethanol oxidation on Pt. The C-C bond rupture of adsorbed enolate led to the formation of CO and CH_x ad-species. Adsorbed enolate can also be further oxidized to adsorbed ketene at higher potentials or reduced to vinyl/vinylidene ad-species in the hydrogen region. CH_x and vinyl/vinylidene ad-species can be reductively desorbed only at potentials below 0.2 and 0.1 V, respectively, or oxidized to CO₂ only at potentials above 0.8 V, and thus they poison Pt surfaces. These new mechanistic insights will help provide design criteria for higher-performing and more durable electrocatalysts for direct ethanol fuel cells.

Integration of multiple advantages into one catalyst: non-CO pathway of methanol oxidation electrocatalysis on surface Ir-modulated PtFeIr jagged nanowires

Developing highly active methanol oxidation electrocatalysts with superior anti-CO poisoning capability remains a grand challenge. Herein, a simple strategy was employed to prepare distinctive PtFeIr jagged nanowires with Ir located at the shell and Pt/Fe located at the core. The Pt₆₄Fe₂₀Ir₁₆ jagged nanowire possesses an optimal mass activity of 2.13 A mg_Pt^-1 and specific activity of 4.25 mA cm^-2, giving the catalyst a great edge over PtFe jagged nanowire (1.63 A mg_Pt^-1 and 3.75 mA cm^-2) and Pt/C (0.38 A mg_Pt^-1 and 0.76 mA cm^-2). The in-situ Fourier transform infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS) unravel the origin of extraordinary CO tolerance in terms of key reaction intermediates in the non-CO pathway. Density functional theory (DFT) calculations add to the body of evidence that the surface Ir incorporation transforms the selectivity from CO pathway to non-CO pathway. Meanwhile, the presence of Ir serves to optimize surface electronic structure with weakened CO binding strength. We believe this work will advance the understanding of methanol oxidation catalytic mechanism and provide some insight into structural design of efficient electrocatalysts.

Toward Electrocatalytic Methanol Oxidation Reaction: Longstanding Debates and Emerging Catalysts

The study of direct methanol fuel cells (DMFCs) has lasted around 70 years, since the first investigation in the early 1950s. Though enormous effort has been devoted in this field, it is still far from commercialization. The methanol oxidation reaction (MOR), as a semi-reaction of DMFCs, is the bottleneck reaction that restricts the overall performance of DMFCs. To date, there has been intense debate on the complex six-electron reaction, but barely any reviews have systematically discussed this topic. To this end, the controversies and progress regarding the electrocatalytic mechanisms, performance evaluations as well as the design science toward MOR electrocatalysts are summarized. This review also provides a comprehensive introduction on the recent development of emerging MOR electrocatalysts with a focus on the innovation of the alloy, core-shell structure, heterostructure, and single-atom catalysts. Finally, perspectives on the future outlook toward study of the mechanisms and design of electrocatalysts are provided.

The pH and Potential Dependence of Pb-Catalyzed Electrochemical CO2 Reduction to Methyl Formate in a Dual Methanol/Water Electrolyte

The conversion of waste CO₂ to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of non-aqueous solvents like methanol allows for higher CO₂ availability and novel products. In this work, the electrochemistry of CO₂ reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO₂ in methanol), formic acid, and formate was critically dependent on the catholyte pH, with higher pH conditions leading to formate and low pH favoring methyl formate. The potential dependence of the product distribution in acidic catholyte was also investigated, with a faradaic efficiency for methyl formate as high as 75 % measured at -2.0 V vs. Ag/AgCl.

Effects of the Interfacial Structure on the Methanol Oxidation on Platinum Single Crystal Electrodes

Methanol oxidation has been studied on low index platinum single crystal electrodes using methanol solutions with different pH (1–5) in the absence of specific adsorption. The goal is to determine the role of the interfacial structure in the reaction. The comparison between the voltammetric profiles obtained in the presence and absence of methanol indicates that methanol oxidation is only taking place when the surface is partially covered by adsorbed OH. Thus, on the Pt(111) electrode, the onset for the direct oxidation of methanol and the adsorption of OH coincide. In this case, the adsorbed OH species are not a mere spectator, because the obtained results for the reaction order for methanol and the proton concentrations indicate that OH adsorbed species are involved in the reaction mechanism. On the other hand, the dehydrogenation step to yield adsorbed CO on the Pt(100) surface coincides with the onset of OH adsorption on this electrode. It is proposed that adsorbed OH collaborates in the dehydrogenation step during methanol oxidation, facilitating either the adsorption of the methanol in the right configuration or the cleavage of the C—H bond.

Probing the surface fine structure through electrochemical oscillations

In the course of (electro)catalytic reactions, reversible and irreversible changes, namely the formation of adsorbed poisons, catalyst degradation, surface roughening, etc., take place at distinct time-scales. Reading the transformations on the catalyst surface from the measurement of the reaction rates is greatly desirable but generally not feasible. Herein, we study the effect of random surface defects on Pt(100) electrodes toward the electro-oxidation of methanol in acidic media. The surface defects are gently generated in situ and their relative magnitudes are reproducibly controlled. The system was characterized under conventional conditions and investigated under an oscillatory regime. Oscillatory patterns were selected according to the presence of surface defects, and a continuous transition from large amplitude/low frequency oscillations (type L) on smooth surfaces to small amplitude/high frequency oscillations (type S) on disordered surfaces was observed. Importantly, self-organized potential oscillations were found to be much more sensitive to the surface structure than conventional electrochemical signatures or even other in situ characterization methods. As a consequence, we proved the possibility of following the surface fine structure in situ and in a non-invasive manner by monitoring the temporal evolution of oscillatory patterns. From a mechanistic point of view, we describe the role played by surface defects and of the adsorbed and partially oxidized, dissolved species on the oscillations of type S and L.

An exchangeable-tip scanning probe instrument for the analysis of combinatorial libraries of electrocatalysts

A combined scanning differential electrochemical mass spectrometer (SDEMS)-scanning electrochemical microscope (SECM) apparatus is described. The SDEMS is used to detect and spatially resolve volatile electrochemically generated species at the surface of a substrate electrode. The SECM can electrochemically probe the reactivity of the surface and also offers a convenient means of leveling the sample. It is possible to switch between these two different scanning tips and techniques without moving the sample and while maintaining potential control of the substrate electrode. A procedure for calibration of the SDEMS tip-substrate separation, based upon the transit time of electrogenerated species from the substrate to the tip is also described. This instrument can be used in the characterization of combinatorial libraries of direct alcohol fuel cell anode catalysts. The apparatus was used to analyze the products of methanol oxidation at a Pt substrate, with the SDEMS detecting carbon dioxide and methyl formate, and a PtPb-modified Pt SECM tip used for the selective detection of formic acid. As an example system, the electrocatalytic methanol oxidation activity of a sputter-deposited binary PtRu composition spread in acidic media was analyzed using the SDEMS. These results are compared with those obtained from a pH-sensitive fluorescence assay.

The dual pathway in action: decoupling parallel routes for CO2 production during the oscillatory electro-oxidation of methanol

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.