High-Throughput Electrospinning System| Synergistic photothermal catalysis overelectrospun Ti-0-Cu dual-redox sites.toward high-efficiency toluene oxidationunder solar illumination

Associate Professor Hu Zhun from Xi'an Jiaotong University and Professor Ye Qing from Beijing University of Technology: Dual-Active-Site Electrospun Fiber Catalysts Enable Efficient Photothermal Synergistic Degradation of Toluene

With rapid industrialization and economic development, annual emissions of volatile organic compounds (VOCs) continue to rise. Photothermal synergistic catalytic oxidation technology has demonstrated promising application value in VOC treatment due to its high efficiency and energy-saving potential. Using electrospinning technology, precise control of dopant concentration in precursor solutions and spinning parameters enables fine-tuning of metal doping levels and dispersion, ensuring uniform distribution of active sites on catalyst surfaces. However, the intrinsic mechanism of how these active sites synergistically integrate thermal and photocatalytic processes remains a core question requiring further clarification.

Recently, Associate Professor Hu Zhun from Xi'an Jiaotong University and Professor Ye Qing from Beijing University of Technology published their latest research titled "Synergistic photothermal catalysis over electrospun Ti-O-Cu dual-redox sites: toward high-efficiency toluene oxidation under solar illumination" in *Chemical Engineering Journal*. The researchers prepared TiO₂ catalysts with different Cu doping levels (xCu/TiO₂, x = 1 wt%, 5 wt%, 10 wt%, 15 wt%) through electrospinning, demonstrating highly efficient photothermal synergistic catalytic oxidation activity for toluene. The construction of Ti⁴⁺-O-Cu⁺ and Ti⁴⁺-O-Cu²⁺ dual-active sites effectively integrated the characteristics of thermal and photocatalytic processes. In-situ DRIFTS results showed that toluene was first oxidized to benzaldehyde and benzoic acid before finally converting to CO₂ and H₂O. Benzaldehyde and benzoic acid were key active intermediates in the oxidation pathway, with benzoic acid oxidation being the rate-determining step. This study provides valuable insights into the mechanism of photothermal catalytic oxidation and offers innovative strategies for the rational design of high-performance photothermal catalysts.

In this study, TiO₂ catalysts with different Cu doping levels (xCu/TiO₂, x = 1 wt%, 5 wt%, 10 wt%, 15 wt%) were prepared via electrospinning. SEM images showed that all catalysts exhibited fibrous morphology, with fiber diameter increasing with Cu doping content. EDS mapping confirmed highly dispersed elements in the prepared catalysts (Figure 1). XRD spectra indicated that all samples contained anatase and rutile crystal structures (Figure 2A). The 15Cu/TiO₂ sample displayed additional CuO crystal structures. Moreover, the 2θ angle of the anatase (101) crystal plane shifted to lower angles with increasing Cu content, suggesting Cu ion incorporation into the TiO₂ lattice, causing lattice distortion. Raman spectroscopy further confirmed the coexistence of anatase and rutile phases in the nanofibers (Figure 2B). Additionally, no distinct CuO peaks were observed in the Raman spectra, possibly due to peak overlap with TiO₂ or high dispersion of Cu in the catalyst. Notably, peak intensities decreased with increasing Cu content, indicating successful incorporation of Cu ions into the TiO₂ framework and the formation of Ti-O-Cu structures.

Figure 1: SEM images of prepared catalysts (inset: diameter distribution) and EDS mapping.

Figure 2: XRD patterns, Raman spectra, N₂ adsorption-desorption isotherms, and pore size distribution of prepared catalysts.  

As shown in Figures 3A and 3B, 5Cu/TiO₂ exhibited the best thermal and photothermal catalytic activity for toluene oxidation. Figures 3C and 3D suggested that thermal energy might enhance electron-hole pair excitation and separation, thereby strengthening the photopromotion effect. The photopromotion effect increased with Cu doping content. Arrhenius plots indicated that activation energy was not the key factor affecting reaction activity. Reaction rates depend not only on activation energy but also on the pre-exponential factor, which is determined by the probability of reactant molecules colliding with active sites and undergoing reactions. Thus, the catalytic activity of xCu/TiO₂ was related to the nature and quantity of surface active sites.

Figure 3: Performance evaluation of TiO₂ with different Cu doping levels.  

As shown in Figure 4A, Cu doping had minimal impact on the surface structure of TiO₂. However, changes in Cu valence states on the surface indicated that Cu doping altered the local microenvironment, leading to the formation of new active sites: Ti⁴⁺-O-Cu²⁺ and Ti⁴⁺-O-Cu⁺ (Figure 5B). Figure 4D displayed the relationship between toluene conversion in thermal catalysis at 250 °C and surface Cu⁺ content. A strong linear correlation was observed between toluene conversion and Cu⁺ content, demonstrating that Ti⁴⁺-O-Cu⁺ sites on the catalyst surface played a crucial role in enhancing catalytic performance. Figure 4E showed the relationship between the total photopromotion effect (k) and surface Cu²⁺ content. A good linear correlation also existed between k values and Cu²⁺ content. The k values increased with surface Cu²⁺ content, indicating that exposure of Ti⁴⁺-O-Cu²⁺ sites contributed to enhanced photopromotion.

Figure 4: XPS spectra of prepared xCu/TiO₂ catalysts and quantification of active sites.  

Figure 5: Reaction mechanism of thermal and photothermal catalytic oxidation of toluene on xCu/TiO₂. 

Finally, in-situ infrared experiments under dark and illuminated conditions with oxygen switching were conducted to investigate the role of oxygen species in the photothermal catalytic oxidation of toluene. The reaction mechanism for toluene catalytic oxidation was as follows: toluene → benzaldehyde → benzoic acid → CO₂. The results showed that benzaldehyde and benzoic acid were key active intermediates in the oxidation pathway, with benzoic acid oxidation being the rate-determining step. Adsorbed oxygen dominated the initial oxidation of toluene to benzaldehyde, while lattice oxygen governed the further oxidation of benzaldehyde. These findings provide theoretical guidance for catalyst design and understanding photothermal synergistic effects.

Paper link: https://doi.org/10.1016/j.cej.2025.164109