Methanol electrochemical conversion to formaldehyde over bulk metal and supported catalysts
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The electrochemical oxidation of 1.0 M CH3OH in 0.1 M H2SO4 over different types of platinum-ruthenium (PtRu) materials was investigated. Focus was on the determination of formaldehyde (H2CO) produced as a function of Ru content in arc-melted bulk alloys and nanometer-scale catalyst materials. A sensitive fluorescence assay for formaldehyde, which had a detection limit down to 100 nM H2CO, was employed. The reaction potentials, reported in volts versus the reversible hydrogen electrode reference (VRHE), were in the range of 0.5 VRHE to 0.8 VRHE. The lower potentials approach voltages that have technological relevance to fuel cells. Electrolysis was performed on 50 ƒÝL samples for a period of 180 s. Based on the coulometry, the reactant depletion in the cell is below 1%. In experiments with bulk PtRu alloys, three samples with respective Ru mole fraction (XRu) of 0.1, 0.3 and 0.9 were employed. Reactions were also carried out on bulk polycrystalline Pt for reference. Compared to Pt, the H2CO yields were lower for the oxidation of methanol over PtRu. Among the PtRu alloys, the H2CO yields decreased with increasing XRu, except at the lowest potential studied (0.5 VRHE), where the formaldehyde yield was lowest for the sample with XRu = 0.3. The finding is consistent with XRu = 0.3 being the most active PtRu composition for methanol electrochemical oxidation at ambient temperature. The results are attributed to lower reactivity of methanol on the electrode with XRu = 0.9 at 0.5 VRHE due to due to blocking of initial dissociative chemisorption steps by inhibiting oxides present at Ru sites. Compared to bulk electrodes, methanol oxidation over nanometer-scale catalyst resulted in H2CO yields below 10 % under the conditions studied. The following catalyst materials were used on gold electrode: Pt-Black, C/Pt, 10 wt % on Vulcan XC-72R carbon, PtRu black with XRu = 0.5, PtRu catalyst prepared via a sonochemical (SC) method with XRu = 0.1, 0.25 and 0.5. High current densities were achieved during sample electrolysis. The results indicate the nanometer-scale catalyst converts methanol to more complete oxidation products. The findings are consistent with earlier studies and are attributed to readsorption and complete oxidation of H2CO within multiple catalyst layers, leading to lower H2CO yields. Similar to results for smooth, bulk electrodes, the H2CO yield was significantly higher for methanol oxidation over the lowest Ru content nanometer-scale catalyst (XRu= 0.1) and approached the response for Pt black. This project also advanced the design of the small volume electrolysis cell employed for the thesis research by incorporating a machinable MACOR glass ceramic disk window, which resists oxygen permeation. The cell response was characterized by observing the reversible, diffusion controlled waves for hexamine ruthenium trichloride (Ru (NH3)6Cl3) in cyclic voltammetry measurements.