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.


S1. Electroreduction of aldehydes
Aldehydes can be easily reduced to the corresponding alcohol in a two-electron transfer step. Furthermore, aldehyde groups are quite reactive and the pH must be controlled to prevent misinterpretation due to purely chemical reactions such as aldol condensation 1 . Therefore, to better understand the electroreduction of aldehydes on a Cu electrode, a strongly buffered neutral electrolyte must be used to prevent dimerization reactions in alkaline solutions or non-electrochemical hydrogenation to alcohols in strongly acidic media 2 . Here, we investigated how monoaldehydes and dialdehydes behave under reducing potentials on a Cu electrode. To avoid purely chemical reactions during the electroreduction, the reduction was carried out in potassium phosphate buffer (0.1 M, pH 7). For monoaldehydes, a mixture of formaldehyde, acetaldehyde, and propionaldehyde (15 mM each) was reduced between -0.7 V to -1.1 V until 15 C of charge transferred was reached. The results are shown in Figure S1. The liquid products identified were only their corresponding alcohol: methanol, ethanol, and 1-propanol. For gaseous products, H 2 was the only compound detected due to the hydrogen evolution, a parallel reaction that cannot be avoided in a water-based electrolyte. Importantly, methane, ethane or propane were not detected in any test, indicating that the oxygen atom in the carbonyl is not easily removed from the aldehyde to form a hydrocarbon molecule at neutral pH on Cu electrode. C-C coupling between the aldehydes did not take place since no C 4+ compounds (such as crotonaldehyde) were observed in these tests and, therefore, aldol condensation was prevented by using the buffer electrolyte. An optimum to alcohols formation was found at -0.8 V. Potentials more negative than -0.8 V resulted in higher FE to H 2 , which was likely promoted by the phosphate anions that act as proton donor 3 . There is a positive correlation between the carbon chain length of the aldehyde molecule and its conversion and faradaic efficiency (FE) (i.e. propionaldehyde > acetaldehyde > formaldehyde). To investigate whether the mixture of formaldehyde, acetaldehyde, and propionaldehyde is somehow inhibiting the conversion of formaldehyde, the reduction of each aldehyde was carried out individually. The same trend for aldehyde conversion was observed in all potentials: propionaldehyde > acetaldehyde > formaldehyde. Propionaldehyde is more reactive than formaldehyde and acetaldehyde because the longer chain leads to a higher electron density on the carbonyl group, turning it more reactive 4 .
The electroreduction of formaldehyde, acetaldehyde, and propionaldehyde on a Cu electrode was also investigated in a CO 2 -saturated KHCO 3 electrolyte (0.1 M, pH 6.8) to determine if the observed trend in the buffer electrolyte was consistent with the conventional CO 2 reduction conditions. The same trend was observed for all three aldehydes (Fig. S2). Lower conversions for aldehydes were detected due to the competition with CO 2 RR on Cu surface 5 . As observed in phosphate electrolyte, no ethane or propane was detected between -0.7 V and -1.1 V. Formation of methane was found to be lower than 1% and similar to that observed when CO 2 was reduced in the absence of aldehydes in solution. Therefore, the detected methane was likely formed through CO 2 reduction only. Based on the results obtained from the reduction of monoaldehydes, which showed that the oxygen atom in the carbonyl group is not removed and the only product formed is the corresponding alcohol, it is reasonable to expect that the reduction of dialdehydes would result in the formation of the corresponding hydroxyaldehyde and diol products. To check this hypothesis, we investigated the reduction of glyoxal under the same conditions as above; the results are shown in Figure S3. The main products formed were glycolaldehyde and ethylene glycol, which are the expected hydroxyaldehyde and diol products, respectively. We observed that the formation of ethylene glycol increased with the overpotential, as the second aldehyde group in the glycolaldehyde molecule was further reduced to its respective alcohol. However, at potentials more negative than -0.8V, acetaldehyde and ethanol were also detected. This suggests that, unlike monoaldehydes, one of the oxygen atoms from the carbonyl group in the dialdehyde molecule could be removed to form the monoaldehyde and subsequently the monoalcohol compounds. This can be explained by the presence of carbonyl group adjacent to the hydroxyl group, which makes the later susceptible to further reduction. Further results on the reduction of hydroxyaldehydes and hydroxyketones are provided in section S5. We did not observe the formation of hydrocarbons such as ethane, which confirms that only one oxygen atom from the carbonyl group in the dialdehyde can be removed to form a monoaldehyde while the second carbonyl or alcohol group remains intact. The reduction of mono-and dialdehydes on a Cu electrode is useful in helping to understand the reaction mechanism of C 2+ compounds from CO 2 RR, which will be discussed in the results of the main text. In general, based on the results presented here, if an aldehyde is considered an intermediate, we can consider that a monoaldehyde is only electrochemically reduced to its corresponding alcohol, and a dialdehyde can be reduced to its respective hydroxyaldehyde and diol but can also be further reduced to its monoaldehyde and monoalcohol.

S2. Electroreduction of alcohols
Unlike aldehydes, alcohols are more stable final products of CO 2 RR and CORR. This is supported by the fact that ethanol and 1-propanol can achieve higher faradaic efficiencies from CO 2 or CORR on Cu-based electrodes 6 . Only H 2 was identified as product in all cases. To confirm this trend in a conventional CO 2 RR conditions, the alcohols were also reduced in CO 2sautrated KHCO 3 (0.1M, pH 6.8) electrolyte, and the concentration before and after did not change (for methanol and 2-propanol) or slightly increased (for ethanol and 1propanol) after 15C was transferred due to the formation of these alcohols from

S3. Electroreduction of ketones
Acetone is the simplest and smallest ketone molecule, and like aldehydes, it can undergo aldol condensation in alkaline conditions 1 . Therefore, in order to investigate its electroreduction, the control of pH is an important parameter to prevent dimerization. To this end, 50 mM of acetone was reduced in a 0.1 M potassium phosphate buffer at pH 7. Surprisingly, acetone was almost inactive, with less than 1% conversion observed at all potentials (Fig.S4a). 2-propanol was the only observed product, and propane was not found in the detection limit of the gas-chromatograph.
The only gas identified in the reaction was H 2 . To confirm this trend under conventional CO 2 RR conditions, we also carried out the reaction in CO 2 -saturated KHCO 3 electrolyte and obtained the same results. Acetone reduction is commonly reported on a Pt electrode in acidic media and 2-propanol and propane are commonly detected as outcomes [11][12][13][14] . The pH has an important role for the carbonyl reduction 4 . To check the reactivity of acetone on Cu in acidic media, 50 mM acetone was reduced in 0.1M HClO 4 . A higher concentration of 2-propanol was observed in comparison to neutral pH ( Fig.S4b) but propane was not detected. Therefore, on Cu, as observed for aldehydes, the carbonyl group can only be reduced to hydroxyl (but relatively small conversion) but not to hydrocarbons. Differently from Pt, propane was not observed, showing that besides the pH, the nature of the metal is also an important parameter. It is worth noting that the position of the carbonyl plays a significant role.
Propionaldehyde, a C 3 compound with the carbonyl located on the first carbon (aldehyde group), exhibits high reactivity even at neutral pH. However, under the same conditions, acetone, a C 3 compound with the carbonyl on the second carbon (ketone group), shows minimal reactivity. Even at more acidic electrolyte, acetone conversion is smaller than propionaldehyde in neutral pH. The reduction of these two molecules, mixed in a buffer solution (25 mM each), shows that 1-propanol is formed while not even traces of 2-propanol are detected (Fig. S5). Thus, for the CO 2 RR mechanism, the position of the carbonyl is an essential factor, where aldehydes will reduce easier than ketones. This is because the aldehyde functional group (CHO) is a more reactive functional group than the ketone functional group (C=O) as generally found in organic chemistry. This is due to a combination of steric hinderance and electronic effects 4 .
The aldehyde functional group is more electrophilic than the ketone functional group, meaning it is more likely to attract nucleophiles and undergo reactions such as nucleophilic addition or reduction 4 . Therefore, from the results we have shown here, the carbonyl reactivity depends on its position and an aldehyde would preferably react over ketone when both are mixed in the same solution. These results will be important to guide the reaction pathway for CO 2 RR towards C 3 compounds.

S4. Electroreduction of ketonealdehyde
We have shown in the previous section that the reactivity of the carbonyl group is influenced by its position in the carbon chain, as exemplified by comparing the reduction of acetone and propionaldehyde. In this section, we explore a similar trend by examining the behavior of these two functional groups when both ketone and carbonyl groups are present in the same molecule. To investigate this, we used methylglyoxal as a model molecule to better understand the behavior of a ketone group and an aldehyde group when reduced on a Cu electrode. The results obtained from methylglyoxal reduction are presented in Figure S6.  To summarize, considering the results presented in sections S3 and S4, it is evident that the carbonyl in the first carbon (aldehyde group) exhibits higher reactivity than the carbonyl in the second carbon (ketone group), but the presence of an adjacent carbonyl enhances the reactivity of the ketone group.

S5. Electroreduction of hydroxycarbonyl
In sections S1 and S4, we demonstrated that the reduction of a dialdehyde or a ketonealdehyde produces hydroxycarbonyls, including hydroxyaldehydes like glycolaldehyde and 2-hydroxypropanal, or hydroxyketones such as hydroxyacetone, which can be further reduced to their respective diols. Additionally, we found that one of the oxygen atoms could be removed from hydroxycarbonyls resulting in the formation of their respective aldehyde or ketone, as observed in the reduction of glyoxal, which led to the formation of acetaldehyde and ethanol at higher overpotentials, and in the reduction of methylglyoxal, which led to the formation of propionaldehyde, 1-propanol, and acetone. In this section, we systematically investigate the behavior of the hydroxyl group in the presence of a carbonyl group adjacent to it. To achieve this, we investigated the reduction of glycolaldehyde (Fig.S7a), hydroxyacetone (Fig.S7b), 2-hydroxypropanal (Fig.S7c), 3hydroxypropanal (Fig.S7d), glyceraldehyde (Fig.S7e), and dihydroxyacetone (Fig.S7f), which allows us to assess the impact of the position and number of hydroxyl groups in relation to the carbonyl group in the carbon chain. Specifically, we will investigate what happens with the hydroxyl group: 1) when the hydroxyl is placed in the first carbon while the carbonyl is adjacent (glycolaldehyde and hydroxyacetone); 2) when the hydroxyl is in the second carbon while the carbonyl is adjacent (2-hydroxypropanal); 3) when the hydroxyl is in the third carbon and the carbonyl in the first carbon (3hydroxypropanal); 4) when two hydroxyls are placed in the second and third carbon while the carbonyl is placed in the first one (glyceraldehyde); and 5) when two hydroxyls are placed in the first and third carbon while the carbonyl is placed in the second one (dihydroxyacetone). Our results indicate that reduction of the carbonyl group is the most favored reaction at lower overpotentials (-0.7 V and -0.8 V) in all cases, except for dihydroxyacetone. Diols and triols were the primary products observed, including ethylene glycol, 1,2-propanediol, 1,3-propanediol and glycerol. At potentials more negative than -0.8 V, it is more evident that the hydroxyl group adjacent to the carbonyl group can be removed: the hydroxyl from glycolaldehyde was removed to form acetaldehyde (Fig.S7a); similarly, hydroxyl group was removed from hydroxyacetone and 2-hydroxypropanal to form acetone (Fig.S7b) and propionaldehyde (Fig.S7c), respectively. The phenomenon of removing hydroxyl groups next to the carbonyl is confirmed by the reduction of dihydroxyacetone. One of the hydroxyls was selectively removed to form hydroxyacetone (Fig.S7f), but acetone was also detected at more negative potentials, indicating that both hydroxyl groups adjacent to the carbonyl group can be removed. However, if the hydroxyl group is not adjacent to the carbonyl group, it remains unchanged. When 3-hydroxypropanal was reduced, only 1,3-propanediol was formed (Fig.S7d) while no propionaldehyde was identified. 3-hydroxypropanal was formed in the reduction of glyceraldehyde (Fig.S7e), but 2-hydroxypropanal was not detected. Our group has recently investigated the reduction of dihydroxyacetone and hydroxyacetone on Pd electrodes 15,16 . Acetone is preferably formed over 1,2propanediol at pH > 3. When glyceraldehyde was reduced, 1,3-propanediol was the preferred product. In other words, hydroxycarbonyl molecules undergo preferably dehydroxylation over carbonyl reduction to alcohols on Pd electrodes while the opposite trend is observed for Cu electrodes. These results show that the nature of the electrode is also an important parameter in the reaction pathway towards a specific product.
With the results shown in the sections S4 and S5, we can conclude that a hydroxyl group can be cleaved in from the carbon chain on Cu electrode and at neutral pH when the hydroxyl is adjacent to a carbonyl group. Figure S8 shows 1 H-NMR spectrum for a standard solution of 1,2-propanediol (black line) and the liquid sample from CO (2) RR showed in Figure 4a in the main text.

Note 1. 1 H-NMR analysis
The signals observed for a standard solution of 1,2-propanediol centered at 3.

Note 2. CORR with interval addition of glycolaldehyde in 0.1M KOH electrolyte
In order to enhance the formation of dehydrogenated glycolaldehyde and increase the production of C 3 minor products, CORR was carried out in 0.1M KOH with interval addition of glycolaldehyde to the electrolyte and the results are shown Figure S9. The following strategy was used: 1) the copper electrode was immersed in a solution containing 1M glycolaldehyde for 1 minute; 2) the electrode was then air-