The electroactive species of aliphatic aldehydes during their electrocatalytic oxidation at gold electrodes Christoph Bondue and Kristina Tschulik Ruhr-Universität Bochum, Germany In many cases the selective oxidation of an aldehyde functional group to a carboxylic acid is an essential step in deriving value-added compounds from biomass [1] . It would be beneficial to develop a process that can achieve this step electrochemically, because this would allow us to directly utilize renewable electricity and to eliminate kinetically sluggish oxygen evolution as a counter reaction to hydrogen evolution during water electrolysis. It is widely assumed that the diolate is the electrochemically active species of aldehyde oxidation [2] . The latter forms in equilibrium when the carbonyl functional group reacts with OH - . Typically, this equilibrium lies far on the side of the aldehyde. Hence, even at high pH values only a small fraction of the aldehyde resides as diolate in the electrolyte. It is, therefore, believed that electrochemical aldehyde oxidation proceeds only in very alkaline electrolytes with high current densities. However, these conditions also lead to the rapid degradation of the reactant via aldol condensation and to the deactivation of the electrode by the decomposition products. Here, I present a study on the electrochemical oxidation of aliphatic aldehydes at gold electrode, which is a model reaction for the electrochemical upgrading of biomass derived aldehydes. Employing the Rotating Disc Electrode technique, we find for gold electrodes that the current due to aldehyde oxidation reaches diffusion limitation even in electrolytes of pH 12. At this pH the equilibrium concertation of the diolate is too low to account for the high currents observed experimentally. Accordingly, not only the diolate, but also other aldehyde species that reside in the aqueous electrolyte must be electroactive as well. Accordingly, there is no need for high pH-values to achieve the electrochemical conversion of aldehydes. However, OH - is still a reactant during aldehyde oxidation: this is evident from the fact that the current due to aldehyde oxidation becomes diffusion limited in OH - when the aldehyde concentration is increased beyond a certain threshold value. However, high reaction rates can be achieved in buffered electrolytes of mild alkalinity, thus avoiding electrolyte decomposition of the reactant. Furthermore, using Differential Electrochemical Mass Spectrometry, we confirm [3] that electrochemical aldehyde oxidation proceeds via hydrogen evolution. That is, the formyl hydrogen is not released as protons after cleavage of the C-H bond with the carbonyl carbon but evolved as H 2 . Based on this observation we propose a reaction mechanism in which the gold surface acts as a heterogeneous electro-catalyst. References 1. Werpy, T, and Petersen, G. Top Value Added Chemicals from Biomass: Volume I -- Results of Screening for Potential Candidates from Sugars and Synthesis Gas. United States: N. p., 2004. Web. doi:10.2172/15008859. 2. W.J. Bover, P. Zuman, J. Electrochem. Soc. 122 (1975) 368–377. 3. N.A. Anastasijevic, H. Baltruschat, J. Heitbaum, Electrochimica Acta 38 (1993) 1067–1072.
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