Solar-Driven Photocatalytic Trichloroethylene Mineralization with High CO₂ Selectivity

Abstract

Sunlight-driven photocatalytic oxidation, using ambient air as the oxidant, offers a sustainable route for removing chlorinated volatile organic compounds (VOCs) such as trichloroethylene (TCE). Yet, practical implementation is hindered by low reaction rates and undesirable product selectivity. Here, we present a material design strategy that overcomes both challenges. Size-controlled anatase TiO₂ nanocrystals with truncated bipyramid (nanoTBP) morphology achieve exceptional photocatalytic activity for TCE degradation, comparable to rates reported for energy-intensive thermal catalysis, with unoptimized CO₂/CO product ratios. Decorating as-synthesized TiO₂ with Pt nanoparticles significantly enhances the CO₂ selectivity but significantly diminishes the overall reaction rates. To resolve this rate–selectivity trade-off, we integrate Pt/TiO₂ with pristine TiO₂ into an optimized composite photocatalyst, unlocking a tandem pathway that enables both rapid degradation and complete mineralization of TCE to CO₂ under solar irradiation. Taken together, this work establishes a scalable, solar-enabled, materials-based strategy for complete gas-phase mineralization of TCE, with broader implications for novel and sustainable VOC treatment processes.

Graphical Abstract

Trichloroethylene (TCE), a prevalent chlorinated volatile organic compound, is extensively utilized across various industries, resulting in considerable direct emissions into the environment.^1,2 Due to its toxicity and relative stability,^3–7 TCE pollution in ecosystems is a global environmental challenge. Chemical treatment of TCE can be broadly classified into two categories: reductive and oxidative methods. While reductive methods offer the advantage of transforming TCE into benign and useful chemical products such as ethylene, they are often hampered by insufficient conversion rates, formation of toxic partially dechlorinated byproducts such as vinyl chloride, and a requirement for aqueous media.^8–15 Catalytic oxidation is a more suitable approach for treating TCE in the gas phase.¹⁶ Conventional thermal catalysis can oxidize TCE at practically useful rates of 50 to 100 μmol·mg_cat⁻¹·h⁻¹.^16,17 However, effective mineralization demands elevated temperature (400–500 °C),^16,18–21 which can result in substantial energy consumption and unwanted byproducts such as Cl₂, dioxins, and phosgene (Figure 1a).^16,17,22,23

Figure 1.

Schematic illustration of TCE abatement methods based on (a) thermal catalysis, (b) photocatalysis with conventional TiO₂-based materials, and (c) tandem photocatalysis with Pt/TiO₂+TiO₂ developed in this work.

To overcome such limitations, researchers have explored photocatalytic oxidation, using solar irradiation as the only or major energy input, as an alternative method for TCE mineralization in the gas phase.^24,25 TiO₂-based materials are considered promising photocatalysts for this reaction due to low cost, exceptional chemical stability, and strong photooxidizing properties enabled by a wide band gap.^24–26 Despite these advantages, their photocatalytic performance remains relatively low in terms of TCE mineralization rates, product distribution/selectivity, and solar spectrum utilization (Figure 1b).^26,27 Recent optimization efforts have targeted both intrinsic activity of TiO₂ photocatalysts and reactor design.²⁴ For example, Puddu et al. demonstrated that optimizing morphological and physicochemical properties through a modified sol–gel synthesis can significantly improve photocatalytic activity, achieving a TCE mineralization rate of 2.8 μmol·mg_cat⁻¹·h⁻¹,²⁴ whereas Den et al. achieved an enhanced TCE degradation rate of 1.36 μmol·mg_cat⁻¹·h⁻¹ with commercially available P-25 TiO₂ through careful reactor engineering.² Despite these advancements, there is significant room to enhance TiO₂’s photocatalytic activity for TCE oxidation, improve its absorption in the visible spectrum to make better use of sunlight,^22,28 and shift process activity toward full mineralization to CO₂, minimizing unwanted byproducts such as CO.^24,28,29

In this work, we synthesize anatase TiO₂ nanocrystals with an average size of ~10 nm for the solar-driven degradation of TCE in the gas phase. With desirable particle sizes, crystallinity, and exposed facets, these TiO₂ nano truncated bipyramids (nanoTBPs) demonstrate remarkable TCE mineralization performance. Under Xe lamp illumination at the power density of 1.375 W·cm⁻² (default illumination condition), materials achieve a high TCE degradation rate of ~77 μmol·mg_cat⁻¹·h⁻¹ rivaling thermal catalytic reaction rates without requiring harsh operational conditions. To control product selectivity (i.e., CO vs CO₂), we incorporate Pt nanoparticles on the surface of TiO₂ nanoTBPs, enabling the utilization of the visible spectrum while simultaneously enhancing complete oxidation to CO₂. Despite these benefits, Pt decoration substantially diminishes the photocatalytic activity of TiO₂ nanoTBPs, reducing the TCE degradation rate to less than 1.8 μmol·mg_cat⁻¹·h⁻¹. To overcome the inherent trade-off between TCE degradation activity and CO₂ selectivity, we implement a tandem catalysis strategy employing a composite catalyst of both unmodified and Pt-decorated TiO₂ nanoTBPs (Figure 1c). This approach enables complete mineralization of TCE to CO₂ at ~76 μmol·mg_cat⁻¹·h⁻¹, representing a significant advancement in the sustainable treatment of gas-phase TCE compared to current oxidative technologies.

The TiO₂ nanocrystals were synthesized via a modified fluoride-assisted hydrothermal process,³⁰ and X-ray diffraction (XRD) reveals that they are in the anatase phase (Figure S1). Scanning electron microscopy (SEM) shows that these nanocrystals are truncated bipyramids with an average size of ~10 nm (Figure 2a). Transmission electron microscopy (TEM) further confirms the size and crystallinity of the material (Figure 2b). We evaluated the photocatalytic performance of the TiO₂ nanoTBPs for TCE degradation in a 20 mL reaction vial filled with air containing 2000 ppm of TCE under the default illumination condition (Xe lamp at 1.375 W·cm⁻² intensity). Complete TCE degradation is achieved within the initial 10 min with a CO₂ selectivity of ~76% (Figure 2c). Prolonged illumination progressively enhances oxidation of CO to CO₂, reaching a CO₂ concentration of ~4000 ppm with a selectivity of 97% after 120 min (Figure S2). Control experiments show that this reaction cannot occur without TiO₂ nanoTBPs (Figure S3) or illumination (Figure S4). Additionally, when the UV portion is filtered from the incident light, the TiO₂ nanoTBPs exhibit negligible TCE degradation (Figure S5) despite experiencing a similar equilibrium surface temperature (Figure S6), ruling out a photothermal pathway. These results collectively establish that the TCE mineralization process in this case mainly proceeds via a photocatalytic pathway.

Figure 2.

(a) SEM image of TiO₂ nanoTBPs. (b) High-resolution TEM image of TiO₂ nanoTBPs. The inset is a TEM image taken at a lower magnification. (c) Catalytic performance (TCE concentration and carbon distribution in the gas phase over time) of TiO₂ nanoTBPs for TCE oxidation. (d) Comparison of photocatalytic TCE degradation rates catalyzed by commercial R-812 TiO₂, no-shape-control TiO₂, and TiO₂ nanoTBPs (initial TCE concentration 9000 ppm). (e) Comparison of TCE degradation rate and operating temperature between this work and studies in the literature (reaction parameters are given in Table S1).

To benchmark catalytic performance, we compared nanoTBPs with two reference TiO₂ materials: commercial R-812 (mostly rutile, Figure S7) and laboratory-synthesized anatase (no shape control, Figure S8) nanoparticles. To evaluate maximum photocatalytic TCE decomposition rates, we evaluated an elevated initial TCE concentration of ~9000 ppm and shortened the reaction time to 1 min. For this, we observe a conversion rate of ~77 μmol·mg_cat⁻¹·h⁻¹ for TiO₂ nanoTBPs (Figure S9). In contrast, both commercial R-812 and no-shape-control TiO₂ show much lower photocatalytic activities, achieving maximum rates of ~0.5 μmol·mg_cat⁻¹·h⁻¹ and ~9 μmol·mg_cat⁻¹·h⁻¹, respectively (Figures 2d, S10, and S11). The exceptional photocatalytic activity of the TiO₂ nanoTBPs also exceeds that of reported photocatalysts for TCE degradation and rivals that of thermal catalysts operating at much higher temperatures (Figure 2e).^{18,19,21,24,26,28} Given that all three TiO₂-based materials reach comparable equilibrium surface temperatures under identical illumination conditions (Figure S12), we attribute the superior activity of the nanoTBPs to their crystalline phase and distinctive shape. Electrochemical impedance spectroscopy measurements reveal that nanoTBPs possess the smallest charge transfer resistance under illumination among the three TiO₂ materials (Figure S13). Indeed, previous work has shown that the truncated bipyramid structure and its exposed high-energy (001) facets are particularly effective for charge separation and active for multiple photocatalytic reactions.^30–33 These properties of nanoTBPs also enhance their ability to further oxidize CO to CO₂. With an initial TCE concentration of 2000 ppm, both commercial and no-shape-control TiO₂ exhibit substantially lower CO₂ selectivity (68% and 83%, respectively) compared to the 97% achieved by nanoTBPs for a 120 min reaction (Figures 2c, S14, and S15).

Despite the exceptional activity of the TiO₂ nanoTBPs for photocatalytic TCE decomposition, the suboptimal CO₂/CO ratio is a weakness in gas-phase treatment applications as CO is toxic (Figure 2c).^24,28 To address this limitation, we chose to incorporate Pt into the catalyst structure as Pt has been identified as one of the most efficient catalysts for CO oxidation to CO₂.^34,35 We used photodeposition to decorate the TiO₂ nanoTBPs with Pt. SEM coupled with energy-dispersive X-ray spectroscopy (EDS), TEM, XRD, and X-ray photoelectron spectroscopy (XPS) characterizations collectively confirm successful Pt incorporation without altering the overall morphology or crystallinity of the nanoTBPs (Figures 3a–c and S16–S18). The fluoride residue on the surface of TiO₂ nanoTBPs, which still exists after Pt deposition, should not affect the photocatalytic performance, as demonstrated in our control experiment (Figures S19–S21). Evaluated under identical conditions with 2000 ppm of TCE, the resulting Pt/TiO₂ photocatalyst shows an increased CO₂ selectivity of 90% for the first 10 min of reaction (Figure 3d), compared to 72% by unmodified TiO₂ nanoTBPs (Figure 2c). Despite this improvement in selectivity, the reaction rate decreases with Pt modification, failing to completely remove 2000 ppm of TCE within 10 min (Figures 3d and S22). The degradation rate is only ~1.8 μmol·mg_cat⁻¹·h⁻¹ when tested with an initial TCE concentration of 9000 ppm (Figure S23). Note that the equilibrium surface temperature under illumination increases from 135 to 208 °C upon Pt modification as a result of improved absorption in the visible and near-IR regions (Figures S24 and S25). The contrast between increased temperature and decreased reaction rate again indicates that TCE decomposition in this case is not a thermocatalytic but photocatalytic process. A similar phenomenon is observed for no-shape-control TiO₂ nanoparticles where Pt modification increases the CO₂/CO ratio from 2 to 6 (Figures S15 and S26) while decreasing the maximum TCE abatement rate from ~9 to ~1.5 μmol·mg_cat⁻¹·h⁻¹ (Figures S11 and S27). This behavior reflects the dual effect of Pt incorporation: while it effectively promotes CO-to-CO₂ conversion, it simultaneously compromises the photocatalytic activity of the TiO₂ support by occupying reactive sites.³⁶

Figure 3.

(a) SEM image of Pt/TiO₂. The inset is EDS mapping of the same area. (b) High-resolution TEM image of Pt/TiO₂. The inset is a TEM image at a lower magnification. (c) XRD pattern of Pt/TiO₂. (d) Catalytic performance (TCE concentration and carbon distribution in the gas phase over time) of Pt/TiO₂.

To achieve both fast TCE decomposition and high selectivity to benign CO₂, we combined pristine and Pt-decorated TiO₂ nanoTBPs into a composite photocatalyst (Figure 4a) and optimized the ratio of Pt/TiO₂ to TiO₂ for the reaction performance (Figure 4b). The best photocatalyst, comprising 1.5 mg of TiO₂ and 0.5 mg of Pt/TiO₂, completely mineralizes 2000 ppm of TCE within 10 min with a CO₂ selectivity of 95% (Figure 4c). Extending the reaction time to 120 min leads to full mineralization of TCE to CO₂, yielding a CO₂ concentration of ~4000 ppm (Figure S28). Such high photocatalytic activity and selectivity can be maintained without notable decay through at least eight reaction cycles (Figures 4d and S29). When the starting TCE concentration is increased to 9000 ppm, a high TCE degradation rate of ~76 μmol·mg_cat⁻¹·h⁻¹ is achieved by the Pt/TiO₂+TiO₂ photocatalyst within the first minute of reaction (Figure S30), comparable to that of the unmodified TiO₂ nanoTBPs. To more accurately compare with previous literature results, we carried out TCE oxidation with an initial concentration of 6000 ppm under LED-based UV illumination (maximum irradiance, 144 μW·cm⁻²; wavelength, 365 nm). For these reactions, the Pt/TiO₂+TiO₂ photocatalyst decomposes ~95% of the TCE within 1 min, with an observed reaction rate of 82.9 μmol·mg_cat⁻¹·h⁻¹ and 75% selectivity to CO₂ (Figure S31). This performance substantially surpasses that of previously reported photocatalysts under similar reaction conditions (see comparative studies tabulated in Table S1).

Figure 4.

(a) Schematic illustration of Pt/TiO₂+TiO₂ for the tandem catalysis of photochemical TCE oxidation to CO₂. (b) Comparison of TCE degradation percentage and the CO₂/CO ratio after 10 min of reaction with an initial TCE concentration of 2000 ppm across Pt/TiO₂+TiO₂ composite catalysts with varied compositions (total catalyst mass is 2 mg). (c) Catalytic performance (TCE concentration and carbon distribution in the gas phase over time) of Pt/TiO₂+TiO₂. (d) Catalytic performance (TCE degradation percentage and CO₂ selectivity) of Pt/TiO₂+TiO₂ over eight reaction cycles (initial TCE concentration of 2000 ppm, reaction time of 10 min).

The excellent catalytic performance of the composite toward TCE mineralization is a direct result of the activities of TiO₂ for TCE decomposition and Pt/TiO₂ for CO oxidation operating in tandem. To gain more insight into the enhanced selectivity for CO₂ enabled by Pt modification, we carried out a series of control experiments. We found that the elevated surface temperature is not a major contributor. At the same temperature of ~210 °C but without illumination, the Pt/TiO₂+TiO₂ composite can only oxidize ~900 ppm of the initial ~6000 ppm of CO in 15 min (Figure S32), whereas with illumination all the initial 6000 ppm of CO is converted to CO₂ within 5 min (Figure S33a). When the UV part of the spectrum is filtered, the catalyst still oxidizes ~3000 ppm of CO in 5 min (Figure S33b). This suggests that the Pt/TiO₂+TiO₂ catalyst effectively uses both UV and visible light for photocatalytic TCE mineralization to CO₂.

It is reported in the literature that upon UV irradiation, photogenerated electrons in TiO₂ react with molecular oxygen to produce superoxide radicals (·O₂⁻), while photogenerated holes oxidize adsorbed water to generate hydroxyl radicals (·OH). These radicals are widely recognized as the primary active species initiating TCE oxidation.^37–41 Following the initiation, several chlorinated radical intermediates form, which subsequently give rise to two key reaction intermediates: dichloroacetyl chloride (major) and phosgene (minor). Dichloroacetyl chloride undergoes further oxidation by ·O₂⁻ or ·OH to yield CO, CO₂, and additional phosgene.^39,40 In the presence of surface hydroxyl groups on TiO₂, phosgene can rapidly hydrolyze to form HCl and a surface-bound carbonyl species bridging two adjacent Ti atoms.⁴² Phosgene can also undergo photodecomposition on Pt surfaces (even under visible light) to generate CO.⁴³ The CO produced from both dichloroacetyl chloride oxidation and phosgene decomposition pathways is subsequently photo-oxidized to CO₂, a process that can be substantially accelerated by Pt nanoparticles on TiO₂ surfaces.^36,43

To demonstrate the applicability of the Pt/TiO₂+TiO₂ composite photocatalyst for more practical treatment of TCE in air, we carried out photocatalytic TCE decomposition reactions under natural sunlight, with the assistance of a cost-effective Fresnel lens to focus sunlight (Figure 5a).^44–46 From the areas of the lens and focused light spot, we estimate the intensity of the focused light to be ~1.5 W·cm⁻², assuming that solar illumination is 100 mW·cm⁻² and the lens has 90% optical transmission. Consistent with this estimate, the equilibrium surface temperature of the photocatalyst was measured to be ~230 °C. After 10 min of reaction, the Pt/TiO₂+TiO₂ composite achieves not only complete degradation of ~4000 ppm of TCE but also 96% CO₂ selectivity (Figure 5b). Extending the reaction time to 30 min further increases the CO₂ selectivity to 99%, achieving almost complete mineralization of the TCE reactant to benign products.

Figure 5.

TCE photooxidation (initial TCE concentration ≈ 4000 ppm) catalyzed by Pt/TiO₂+TiO₂ under natural sunlight focused by a Fresnel lens. (a) Photograph of the experimental setup. (b) Catalytic performance (TCE concentration and carbon distribution in the gas phase over time).

In conclusion, we demonstrate that the crystal phase, size, and morphology of TiO₂ nanoparticles are critical parameters governing their photocatalytic activity toward gas-phase TCE degradation. While Pt surface decoration critically enhances selectivity toward CO₂, it comes at the expense of the overall reaction rate. By integrating Pt/TiO₂ with bare TiO₂ into an optimized composite photocatalyst system, we effectively overcome this rate–selectivity trade-off, achieving record-high TCE degradation activity coupled with near-complete CO₂ selectivity via tandem catalytic pathway. Collectively, these results establish a highly efficient, material-based strategy for TCE treatment and broaden the design space for the sustainable management of other chlorinated volatile organic compounds.

Supplementary Material

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.5c06386.

Experimental details; additional structural and property characterization results of catalyst materials; additional reaction study results; comparison of reaction conditions and performance (PDF)