Green carbon dots made from pomegranate peel reshaped TiO2-driven CO2 photoreduction, shifting product selectivity from oxygenated compounds to CO and, with copper modification, toward more reduced hydrocarbons.
Study: The effect of green carbon Dots/Titanium dioxide for photocatalytic reduction of CO2. Image Credit: rsooll / Shutterstock
In a recent research article published in the journal Scientific Reports, researchers investigated photocatalytic CO2 reduction using nano-titanium dioxide, green carbon dots derived from pomegranate peel, a TiO2/CD composite, and a copper-modified TiO2/CD material.
CO2 Photocatalysis Overview
The relentless rise in industrialization has caused excessive carbon dioxide (CO2) emissions, disrupting the natural carbon balance and accelerating global warming. Photocatalytic reduction of CO2 offers a promising pathway to mitigate CO2 emissions by converting it into valuable chemicals and fuels using light energy.
Among photocatalysts, titanium dioxide (TiO2) is widely appreciated for its stability, strong oxidative properties, and low cost. However, its wide bandgap confines absorption to the ultraviolet (UV) region, limiting its photocatalytic efficiency. Carbon dots (CDs), nanoscale zero-dimensional carbon-based materials derived from pomegranate peel waste, exhibit excellent optical and electronic properties, including high photostability and tunable photoluminescence.
Integrating CDs with TiO2 nanoparticles aims to enhance light absorption, improve charge separation, and suppress electron-hole recombination, thus potentially improving photocatalytic CO2 reduction performance and altering product selectivity.
Synthesis and Characterization Techniques
Nano titanium dioxide (nano TiO2) was synthesized through a hydrothermal method. Characterization involved Fourier-transform infrared (FTIR) spectroscopy to identify functional groups and transmission electron microscopy (TEM) to visualize carbon-dot morphology and size at the nanoscale. TEM images revealed spherical carbon dots ranging from 0.85 to 1.5 nm, with some agglomeration, typical of nanoparticles in dried samples.
Photocatalytic reduction experiments were conducted in a three-neck Pyrex glass batch reactor under UV-C irradiation from a 254 nm, 20 W lamp. CO2 gas was bubbled through aqueous catalyst suspensions to saturation before irradiation. Product analyses monitored oxygenated species (esters, oxo groups), carbon monoxide (CO), and alcohols over time to assess product distribution and selectivity.
Catalytic Performance and Mechanisms
Pure carbon dots demonstrated high selectivity towards oxygenated products, with oxo group formation reaching 97.73% at 120 minutes of UV exposure, while alcohol declined to 0.29% and CO and ester products remained negligible. Increasing the amount of CDs raised the alcohol fraction to 5.3%, while esters reached about 80%, suggesting that CDs enhance electron transport and active-site availability, thereby favoring changes in product distribution.
Nano TiO2 photocatalysis exhibited pH-dependent product distribution. In alkaline conditions, esters were favored, whereas in acidic conditions, CO formation was favored. This behavior arises from surface-chemistry changes: acidic media are proposed to promote formate dissociation to CO, while alkaline conditions favor esterification. Photocatalytic mechanisms involve intermediates such as percarbonates and carboxylate ester complexes, which are influenced by light intensity and solution pH.
The authors proposed that the TiO2/CDs composite leveraged CDs as photosensitizers that absorb UV photons, exciting electrons that transfer to the TiO2 conduction band, thereby improving charge separation and suppressing electron-hole recombination. Tetraethyl orthosilicate (TEOS) acted as an electron donor and hole scavenger, while also facilitating CO2 capture and the formation of carbonate ester complexes involved in the proposed reduction pathway.
This composite system strongly favored CO formation, with CO accounting for approximately 99.9% of the reported products after 90 minutes. Intermediates, such as carboxylic acid complexes, were proposed to facilitate stepwise reduction to CO, highlighting the authors' proposed synergistic interactions among TiO2, CDs, and TEOS.
Incorporating copper into TiO2/CDs (Cu-CDs/TiO2) significantly altered the photocatalytic pathway. Copper sites were proposed to function as electron-accumulation centers that activate reactants and promote the dissociation of water or hydrogen into active hydrogen (H•) species.
These hydrogen atoms were proposed to participate in hydrogenation reactions that transformed intermediates into more reduced hydrocarbons, such as alkanes, diminishing CO accumulation by converting it further along reaction pathways. CO accounted for about 55% of the products after 2 hours, which the authors interpreted as evidence that it behaved as an intermediate, while formic acid was reported as a primary product at around 63% in the proposed pathway.
The nanoscale architecture of the composites was designed to facilitate electron transfer, limit charge recombination, and increase the number of accessible active sites. TEM characterization revealed approximately spherical CDs measuring 0.85-1.50 nm, with some agglomeration.
FTIR spectra revealed oxygen-containing surface functional groups on the CDs and spectral shifts consistent with interactions between CDs and TiO2. The authors attributed these interactions primarily to physical adsorption rather than chemical bonding. The synergistic interaction among CDs, TiO2, TEOS, and Cu at the nanoscale was proposed to support multiple reaction mechanisms, enabling tunable selectivity from oxygenated products to CO and hydrocarbons by varying catalyst composition and operating conditions.
Photocatalyst Efficacy Summary
This research demonstrates the potential of nanoengineered TiO2-based photocatalysts modified with green carbon dots and copper-containing components for selective photocatalytic reduction of CO2 under ultraviolet light.
Carbon dots derived from pomegranate peel altered photocatalytic product selectivity and were associated with electron-transfer behavior that the authors proposed could reduce charge recombination, favoring oxygenated products such as esters and alcohols. TiO2’s photocatalytic behavior is highly sensitive to pH, with ester formation dominant under alkaline conditions and CO production under acidic conditions.
The TiO2/CDs composite system promotes near-complete, selective CO generation via the proposed combination of efficient electron transfer and TEOS-assisted hole scavenging. Introduction of copper into the composite was proposed to modify the reaction pathway by promoting the hydrogenation of intermediates and favoring more reduced products, such as alkanes.
This nanoscale synergy provides a versatile approach for tailoring CO2 photoreduction pathways and product selectivity. Because the experiments were conducted in an aqueous batch system under UV-C irradiation, practical performance under solar or continuous-flow conditions remains to be established. Future work should focus on optimizing temperature, catalyst loading, and detailed kinetics to fully harness the nano-enabled photocatalytic system's capabilities for sustainable carbon capture and conversion.