Solvent Effect on Electrochemical CO2 Reduction Reaction on Nanostructured Copper Electrodes.

The electrochemical reduction of CO2 (CO2RR) is a sustainable alternative for producing fuels and chemicals, although the production of highly desired hydrocarbons is still a challenge due to the higher overpotential requirement in combination with the competitive hydrogen evolution reaction (HER). Tailoring the electrolyte composition is a possible strategy to favor the CO2RR over the HER. In this work we studied the solvent effect on the CO2RR on a nanostructured Cu electrode in acetonitrile solvent with different amounts of water. Similar to what has been observed for aqueous media, our online gas chromatography results showed that CO2RR in acetonitrile solvent is also structure-dependent, since nanocube-covered copper (CuNC) was the only surface (in comparison to polycrystalline Cu) capable of producing a detectable amount of ethylene (10% FE), provided there is enough water present in the electrolyte (>500 mM). In situ Fourier Transform Infrared (FTIR) spectroscopy showed that in acetonitrile solvent the presence of CO2 strongly inhibits HER by driving away water from the interface. CO is by far the main product of CO2RR in acetonitrile (>85% Faradaic efficiency), but adsorbed CO is not detected. This suggests that in acetonitrile media CO adsorption is inhibited compared to aqueous media. Remarkably, the addition of water to acetonitrile has little quantitative and almost no qualitative effect on the activity and selectivity of the CO2RR. This indicates that water is not strongly involved in the rate-determining step of the CO2RR in acetonitrile. Only at the highest water concentrations and at the CuNC surface, the CO coverage becomes high enough that a small amount of C2+ product is formed.

Mechanistic Insights into the Formation of Hydroxyacetone, Acetone, and 1,2-Propanediol from Electrochemical CO2 Reduction on Copper.

Studies focused on the mechanism of CO2 electroreduction (CO2RR) aim to open up opportunities to optimize reaction parameters toward selective synthesis of desired products. However, the reaction pathways for C3 compound syntheses, especially for minor compounds, remain incompletely understood. In this study, we investigated the formation pathway for hydroxyacetone, acetone, and 1,2-propanediol through CO(2)RR, which are minor products that required long electrolysis times to be detected. Our proposed reaction mechanism is based on a systematic investigation of the reduction of several functional groups on a Cu electrode, including aldehydes, ketones, ketonealdehydes, hydroxyls, hydroxycarbonyls, and hydroxydicarbonyls, as well as the coupling between CO and C2-dicarbonyl (glyoxal) or C2-hydroxycarbonyl (glycolaldehyde). This study allowed us to derive the fundamental principles of the reduction of functional groups on Cu electrodes. Our findings suggest that the formation of ethanol does not follow the glyoxal pathway, as previously suggested but instead likely occurs via the coupling of CH3* and CO. For the C3 compounds, our results suggest that 1,2-propanediol and acetone follow the hydroxyacetone pathway during CO2RR. Hydroxyacetone is likely formed through the coupling of CO and a C2-hydroxycarbonyl intermediate, such as a glycolaldehyde-like compound, as confirmed by adding glycolaldehyde to the CO(2)-saturated solution. This finding is consistent with CO2RR product distribution, as glycolaldehyde formation during CO2RR is limited, which, in turn, limits hydroxyacetone production. Our study contributes to a better understanding of the reaction mechanism for hydroxyacetone, acetone, and 1,2-propanediol synthesis from CO2RR and gives insights into these interesting compounds that may be formed electrochemically.

Competition of CO and Acetaldehyde Adsorption and Reduction on Copper Electrodes and Its Impact on n-Propanol Formation.

Selective synthesis of n-propanol from electrocatalytic CO2/CO reduction on copper remains challenging and the impact of the local interfacial effects on the production of n-propanol is not yet fully understood. Here, we investigate the competition between CO and acetaldehyde adsorption and reduction on copper electrodes and how it affects the n-propanol formation. We show that n-propanol formation can be effectively enhanced by modulating the CO partial pressure or acetaldehyde concentration in solution. Upon successive additions of acetaldehyde in CO-saturated phosphate buffer electrolytes, n-propanol formation was increased. Oppositely, n-propanol formation was the most active at lower CO flow rates in a 50 mM acetaldehyde phosphate buffer electrolyte. In a conventional carbon monoxide reduction reaction (CORR) test in KOH, we show that, in the absence of acetaldehyde in solution, an optimum ratio of n-propanol/ethylene formation is found at intermediate CO partial pressure. From these observations, we can assume that the highest n-propanol formation rate from CO2RR is reached when a suitable ratio of CO and acetaldehyde intermediates is adsorbed. An optimum ratio was also found for n-propanol/ethanol formation but with a clear decrease in the formation rate for ethanol at this optimum, while the n-propanol formation rate was the highest. As this trend was not observed for ethylene formation, this finding suggests that adsorbed methylcarbonyl (adsorbed dehydrogenated acetaldehyde) is an intermediate for the formation of ethanol and n-propanol but not for ethylene. Finally, this work may explain why it is challenging to reach high faradaic efficiencies for n-propanol, as CO and the intermediates for n-propanol synthesis (like adsorbed methylcarbonyl) compete for active sites on the surface, where CO adsorption is favored.

Combining active phase and support optimization in MnO2-Au nanoflowers: Enabling high activities towards green oxidations.

Among the several classes of chemical reactions, the green oxidation of organic compounds has emerged as an important topic in nanocatalysis. Nonetheless, examples of truly green oxidations remain scarce due to the low activity and selectivity of reported catalysts. In this paper, we present an approach based on the optimization of both the support material and the active phase to achieve superior catalytic performances towards green oxidations. Specifically, our catalysts consisted of ultrasmall Au NPs deposited onto MnO2 nanoflowers. They displayed hierarchical morphology, large specific surface areas, ultrasmall and uniform Au NPs sizes, no agglomeration, strong metal-support interactions, oxygen vacancies, and Auδ+ species at their surface. These features led to improved performances towards the green oxidations of CO, benzene, toluene, o-xylene, glucose, and fructose relative to the pristine MnO2 nanoflowers, commercial MnO2 decorated with Au NPs, and other reported catalysts. We believe that the catalytic activities, stabilities, and mild/green reaction conditions described herein for both gas and liquid phase oxidations due to the optimization of both the support and active phase may inspire the development of novel catalytic systems for a wealth of sustainable transformations.

Catalytic Properties of AgPt Nanoshells as a Function of Size: Larger Outer Diameters Lead to Improved Performances.

We report herein a systematic investigation on the effect of the size of silver (Ag) nanoparticles employed as starting materials over the morphological features and catalytic performances of AgPt nanoshells produced by a combination of galvanic replacement between Ag and PtCl6(2-) and PtCl6(2-) reduction by hydroquinone. More specifically, we focused on Ag nanoparticles of four different sizes as starting materials, and found that the outer diameter, shell thickness, and the number of Pt surface atoms of the produced nanoshells increased with the size of the starting Ag nanoparticles. The produced AgPt nanoshells were supported into SiO2, and the catalytic performances of the AgPt/SiO2 nanocatalysts toward the gas-phase oxidation of benzene, toluene, and o-xylene (BTX oxidation) followed the order: AgPt 163 nm/SiO2 > AgPt 133 nm/SiO2 > AgPt 105 nm/SiO2 > AgPt 95 nm/SiO2. Interestingly, bigger AgPt nanoshell sizes lead to better catalytic performances in contrast to the intuitive prediction that particles having larger outer diameters tend to present poorer catalytic activities due to their lower surface to volume ratios as compared to smaller particles. This is in agreement with the H2 chemisorption results, and can be assigned to the increase in the Pt surface area with size due to the presence of smaller NPs islands at the surface of the nanoshells having larger outer diameters. This result indicates that, in addition to the overall diameters, the optimization of the surface morphology may play an important role over the optimization of catalytic activities in metal-based nanocatalysts, which can be even more pronounced that the size effect. Our data demonstrate that the control over surface morphology play a very important role relative to the effect of size to the optimization of catalytic performances in catalysts based on noble-metal nanostructures.

Controlling Size, Morphology, and Surface Composition of AgAu Nanodendrites in 15 s for Improved Environmental Catalysis under Low Metal Loadings.

In this work, a simple but powerful method for controlling the size and surface morphology of AgAu nanodendrites is presented. Control of the number of Ag nanoparticle seeds is found to provide a fast and effective route by which to manipulate the size and morphology of nanoparticles produced via a combined galvanic replacement and reduction reaction. A lower number of Ag nanoparticle seeds leads to larger nanodendrites with the particles' outer diameter being tunable in the range of 45-148 nm. The size and surface morphology of the nanodendrites was found to directly affect their catalytic activity. Specifically, we report on the activity of these AgAu nanodendrites in catalyzing the gas-phase oxidation of benzene, toluene and o-xylene, which is an important reaction for the removal of these toxic compounds from fuels and for environmental remediation. All produced nanodendrite particles were found to be catalytically active, even at low temperatures and low metal loadings. Surprisingly, the largest nanodendrites provided the greatest percent conversion efficiencies.