Electrocatalysis of Ethanol on a Pd Electrode in Alkaline Media: An in Situ Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy Study

Abstract

In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy in conjunction with H–D isotope replacement is used to investigate the dissociation and oxidation of CH3CH2OH on a Pd electrode in 0.1 M NaOH, with a focus on identifying the chemical nature of the pivotal intermediate in the so-called dual-pathway (C1 and C2) reaction mechanism. Real-time spectroelectrochemical measurements reveal a band at ∼1625 cm–1 showing up prior to the multiply bonded COad band. CH3CD2OH and D2O are used to exclude the spectral interference with this band from interfacial acetaldehyde and H2O, respectively, confirming for the first time that the ∼1625 cm–1 band is due to the adsorbed acetyl on the Pd electrode in alkaline media. The spectral results suggest that the as-adsorbed acetyl (CH3COad) is oxidized to acetate from approximately −0.4 V as the potential moves positively to conclude the C2 pathway. Alternatively, in the C1 pathway, the CH3COad is decomposed to α-COad and β-CH x species on the Pd electrode at potentials more negative than approximately −0.1 V; the α-COad species is oxidized to CO2 at potentials more positive than approximately −0.3 V, while the β-CH x species may be first converted to COad at approximately −0.1 V and further oxidized to CO2 at more positive potentials.

Computational screening for selective catalysts: Cleaving the C C bond during ethanol electro-oxidation reaction

“…14 In line with other experimental studies, [15][16][17]10,11,7,18,19 we found that a 5 potential improvement in the selectivity lies in the competition between two routes represented schematically in Figure 1.…”

Recent Advances on Electro-Oxidation of Ethanol on Pt- and Pd-Based Catalysts: From Reaction Mechanisms to Catalytic Materials

“…Adsorbed CO, C1 and C2 hydrocarbon residues have been identified as the major adsorbed intermediates on Pt-or Pd-based catalysts, while acetaldehyde and acetic acid have been detected as the main by-products using techniques such as infrared spectroscopy [13][14][15][16][17][18][19][20][21][22][23][24][25][26], online DEMS [27][28][29][30][31][32][33][34][35][36][37], ion chromatography [56,57], and liquid chromatography [14]. However, EOR has been shown to occur via a series of complex reactions involving a number of sequential and parallel reaction steps, thus resulting in more than 40 possible volatile and adsorbed intermediates or oxidative derivatives [43].…”

“…However, EOR has been shown to occur via a series of complex reactions involving a number of sequential and parallel reaction steps, thus resulting in more than 40 possible volatile and adsorbed intermediates or oxidative derivatives [43]. Previous studies agree that CO is a dominant adsorbed species formed during EOR, however, they disagree on details such as the adsorbed state of other intermediates and on the question of the rate limiting steps: the adsorption of intermediate or the cleavage of C-C bond or the formation of OH or oxides [20,23,26,31,38,45,47,54]. In the following we summarize EOR mechanisms developed in the last two decades from experimental as well as DFT calculation studies and discuss the unsolved issues in understanding the EOR mechanism.…”

“…To address these two issues, many efforts have been made to combine traditional electrochemical methods (cyclic voltammetry, chronoamperometry, rotating disc electrodes, etc.) with other physicochemical methods, such as in situ FTIR [9,[13][14][15][16][17][18][19][20][21][22][23][24], broadband sum-frequency generation (BB-SFG) spectroscopy [58], DEMS [16,[27][28][29][30][31][32][33][34][35][36], HPLC [56,57], GC [14], electrochemical quartz crystal microbalance (EQCM) [59] and in situ NMR [60] to probe the adsorbed intermediates and/or quantify the reaction products and by-products [56]. We devote the following section to summarize and discuss how these techniques were used for product quantification as well as the clarification of intermediates, starting with FTIR studies and followed by mass spectrometric results.…”

“…A detailed understanding of the reaction mechanism and in particular of the rate-limiting step(s) in EOR under continuous reaction conditions is of critical importance for the design of highly active catalysts [11,12]. Although numerous experimental studies using Fourier transform infrared spectroscopy (FTIR) [13][14][15][16][17][18][19][20][21][22][23][24][25][26] or differential electrochemical mass spectrometry (DEMS) [27][28][29][30][31][32][33][34][35][36][37], as well as theoretical studies [38][39][40][41][42][43][44][45] have been conducted to understand the EOR process, a detailed mechanism of EOR remains unclear or even contradictory. Nevertheless, a so-called dual-pathway (C1 and C2) mechanism has been largely agreed upon: the C1 pathway proceeds via adsorbed carbon monoxide (COads) intermediate to form CO2 (or carbonate in alkaline solutions) by delivering 12 electrons, and the C2 pathway mainly leads to the formation of acetic acid (or acetate in alkaline solutions) by delivering four electrons and/or acetaldehyde by delivering two electrons.…”