Effect of the electrolytes
Evaluation of the photocatalytic activity of the CuO for CO2 photoreduction in aqueous solutions of Na2C2O4, KBrO3, and NaOH, as well as in pure water, was performed under UV irradiation (Fig. 3). Four blank condition tests were conducted in order to obtain baselines, with irradiation in the absence of the catalyst (see Supplementary Information).
Analysis of the gas samples indicated that only CH4 was formed when the CO2 photoreduction was carried out in water or in sodium hydroxide solution. Increasing formation of CH4 was observed during 24 h under continuous irradiation (Fig. 3a), and the results indicated that water was more effective than aqueous sodium hydroxide solution for the reduction of CO2 to CH4. This was probably related to the isoelectric point of CuO (Fig. 1d), which was at pH ≈ 8.8. Considering that the NaOH solution had pH ≈ 9-10, this indicated that the CuO surface charge was negative under this condition, with electron migration to the surface being less probable and CO2 adsorption not being favored. This was because at higher pH, the solubility of CO2 increases (forming CO32−), hence influencing the adsorption process and interfering in the CO2 photoreduction23. However, the specie prevailing in equilibrium in our system (using other electrolytes) is HCO3−, which was assumed to be the main reactant since all reactions occurred at pH ranging from 7 to 9 in non-saline medium (in this range, at least 80% of total dissolved carbon is HCO3−)24.
In the first step of the photocatalytic process, CO2 adsorbed on the CuO catalyst surface reacted with electrons to produce carbon dioxide radicals (CO2•−), which then reacted with H+ to form surface CO and C, ultimately producing CH4 10,25:
The importance of the participation of water splitting by the holes in the formation of certain products such as CH4 can be elucidated by the addition of species that inject electrons preferentially into the semiconductor. Sodium oxalate, for example, can be used26, since it reacts directly with the holes, as represented by Equation 2, so H+ generation is suppressed, favoring only the CO formation reaction (Equation 3) (Fig. 4b)27,28. However, when the reaction was carried out in aqueous KBrO3 solution, only O2 was detected, as shown in Fig. 3c. Sodium oxalate is consumed in the reaction that generates electrons, as shown in Equations 2 and 3.
In the case of O2 evolution, BrO3− acts as an electron scavenger, hence suppressing any CO2 reduction. It is therefore expected that this compound will be reduced in the same way, forming Br−. The participation of electrons in the photoreduction process was related to the ability to reduce the CO2 present in the reaction medium to the CO2•− radical29. The addition of KBrO3 at low concentrations impaired formation of the CO2•−radical, due to its high capacity to capture electrons. On the other hand, it hindered recombination by generating more holes for the reaction with water molecules, hence damaging the photoreduction process (Fig. 4c).
It can be seen in Fig. 5 that the amount of CO2 present in the headspace remained practically constant throughout the reaction (24 hours). The small oscillations observed are attributed to the displacement caused by the system in search of a chemical equilibrium between the CO2 dissolved in the liquid phase and that in the gas phase. The CO2 dissolved is consumed during the photoreaction reaction and to maintain the CO2 saturated medium, the gaseous CO2 moves into the liquid. It is worth mentioning that the long-term CO2 level was around 149-151 μmol.L−1.g−1 for all samples, indicating that despite the small variation observed, CO2 concentration could be considered constant over long periods of reaction.