62nd Annual Report on Research 2017

Abstract: In the past year, we focused on studying Cu nanowires (NWs) as an efficient catalyst for selective CO/CO₂ reduction to C₂H₄ in 0.1 M KHCO₃ at room temperature. We prepared micrometer long Cu NWs of 25 and 50 nm diameters and studied their electrocatalysis for CO reduction. The 50 nm NWs showed better selectivity than the 25 nm ones towards C₂H₄ (+ C₂H₆) with a total Faradaic efficiency (FE) reaching ~60% at −1.2 V (vs RHE) and no CH₄ was detected over the tested potential range. When studied for CO₂ reduction, the 50 nm NWs were selective to C₂H₄ (FE ~30%) and C₂H₆ was not detected. Mixing 8 nm Au NPs with 50 nm Cu NWs (Cu/Au = 5/1 mass ratio) led to a drastic change to catalytic selectivity for the CO₂ reduction, yielding a series of products including CO, methane, ethylene, formate and aldehyde with the total FE reaching 80% at -0.9 V.

Report:

Copper (Cu) is known to catalyze the electrochemical reduction of CO₂ to hydrocarbons. Earlier studies on single crystal Cu electrodes show that the Cu (100) surface tends to catalyze the reaction to ethylene (C₂H₄) with the reaction Faradaic efficiency (FE) reaching 40.7%, while the Cu (111) surface promotes the formation of methane (CH₄) with the FE of 50.5%. However, in the common experimental conditions, C₂H₄ and CH₄ always co-exist. To further increase the selectivity of C₂H₄ over CH₄, we choose to study Cu nanowires (NWs) as the catalyst due to the presence of the five-twinned structure bounded by five (100) planes on the side.

We prepared the Cu NWs by the reduction of CuCl in a heated oleylamine solution. In the process, oleylamine served both as a reducing agent (to reduce CuCl to Cu) and a NW stabilizer. We obtained the 25 nm wide Cu NWs by heating the reaction mixture at 190°C for 0.5 h and separated 50 nm wide Cu NWs at 200°C for 1.5 h. These Cu NWs are in micrometer long and an average of 25 nm or 50 nm wide. X-ray diffraction (XRD) pattern of the representative 50 nm wide Cu NWs shows that the Cu NWs has a face-centered cubic structure.

To test their electrocatalytic properties of the Cu NWs, we deposited these NWs on the carbon support (Ketjen EC300J) at a mass ratio of 1:1. We immersed the carbon-supported NWs in butylamine under a N₂ atmosphere at room temperature for two days to remove the oleylamine surfactant and then washed the supported NWs with ethanol and distilled water, and dried at 80°C in the vacuum oven overnight, giving C-Cu catalysts (25 nm Cu NW and 50 nm Cu NW catalysts were denoted as C-NW-25 and C-NW-50, respectively.). We then prepared the carbon ink by mixing C-Cu, polyvinylidene fluoride (PVDF) and a few drops of N-methyl-2-pyrrolidone (NMP). We pasted the ink onto the carbon paper and dried it under vacuum. The dried pasted carbon paper served as a working electrode.

We first performed the electrolysis of CO in a conventional H-cell (separated by Nafion 212) filled with 0.1 M KHCO₃ solution (pH=8.3) by bubbling CO at room temperature. Based on the LSV curve, we set the reduction potentials from -0.7 V to -1.6 V. The gaseous products analyzed by gas chromatography (GC) contained C₂H₄, C₂H₆ and H₂ with net total FE at 100%±3%. There was no liquid product that could be detected by ¹H NMR. The C-NW-50 has the maximum C₂H₄ + C₂H₆ FE of 60 % at -1.2 V. The C-NW-25 had similar selectivity but less active than the C-NW-50 due likely to the small size induced enhancement of hydrogen evolution reaction. For both 25 nm and 50 nm Cu NWs, C₂H₄ + C₂H₆ was the dominant hydrocarbon product and no CH₄ was detected at the potentials applied. This supports that the strong selectivity arises from the intrinsic property of Cu NWs and is potential-independent. When CO was replaced by CO₂, the reduction still yielded C₂ products (C₂H₄ + C₂H₆) but with lower FE’s. In this reduction process, CO was detected in trace amount (FE below 0.5%). We are working on a manuscript and will submit this part of the work for a publication soon.

We further modified the 50 nm Cu NWs with 1,4-bipyridine and coupled 8 nm Au nanoparticles (NPs) to the NWs via bipyridine with Cu/Au mass ratio at 5/1. As 8 nm Au NPs have been proved to be selective for CO₂ conversion to CO, we expected that the composite might further enhance the CO₂ conversion to hydrocarbons via CO. To our surprise, we saw a dramatic change to the reduction chemistry. The reduction led to the formation of a series of products including CO, methane, ethylene, formate and aldehyde with the total FE reaching 80% at -0.9 V. We are now working to improve selectivity of this Cu-Au composite catalyst.

In a word, our past-year study has led to two interesting catalyst systems for CO/CO₂ reduction: the 50 nm wide Cu NWs that are selective for C₂H₄ + C₂H₆ formation (FE 60%) and Cu-Au NW-NP composite system for the formation of CO, methane, ethylene, formate and aldehyde with the total FE reaching 80%.