(Invited) Electrosynthesis of Fuels Directly from CO2

Introduction

Carbon dioxide (CO₂) is a readily available industrial byproduct that can be used to produce useful chemical compounds.  Electrochemical reduction is a highly effective method for converting CO₂ to CO, organic acids, low molecular weight hydrocarbons and alcohols.  Product distribution from the reduction of CO₂ is highly dependent on the composition and preparation of the catalysts and reduction potential, as such, multiple reduction pathways are observed depending on the catalysts material and the preparation methods.

Copper-based catalysts have been found to produce higher molecular weight hydrocarbons, such as ethylene, methanol and ethanol.  Here we demonstrate a high surface area Cu catalysts that has a preferential selectivity towards the formation of C2 and C3 species.  This paper will also discuss our attempts to understand the mechanistic pathway using in-situ vibrational spectroscopy under reaction conditions.

Experimental

A copper/aluminum alloy was used as the starting material for producing a high surface area copper catalyst.  The aluminum was removed through an etching procedure in strong base. The resulting nanoporous copper was crushed and mixed with Nafion, a binding agent, then casted over a copper foil substrate.  Carbon dioxide electroreduction was performed by placing the catalyst in a specially designed flow cell. A bubbler in the cell was used to deliver CO₂ to saturate the electrolyte, 0.1 M KHCO₃, at a rate of 10 mL/min. The catalyst acts as the working electrode in this electrochemical cell. The product distribution from the electroreduction process was collected at a series of potentials. Gaseous products were identified and quantified using gas chromatography coupled with both mass spectrometry and a thermal conductivity detector.  Liquid products were analyzed using nuclear magnetic resonance.

                The surface morphology and composition of the catalyst was characterized using scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS).  The SEM images show a rough surface that retains a porous structure before and after experiments. XPS studies incorporated depth profiling, and show that ruthenium from the surface of the material migrates to the bulk after electrolysis.