Structural Effects of Metal Single-Atom Catalysts for Enhanced Photocatalytic Degradation of Gemfibrozil

Abstract

The development of efficient catalysts is a highly necessary but challenging task within the field of environmental water remediation. Single-atom catalysts are promising nanomaterials within this respect, but in-depth studies encompassing this class of catalysts remain elusive. In this work, we systematically study the degradation of gemfibrozil, a persistent pollutant, on a series of carbon nitride photocatalysts, investigating both the effect of (i) catalyst textural properties and (ii) metal single atoms on the contaminant degradation. Tests in the absence of the catalyst result in negligible degradation rates, confirming the stability of the contaminant when dispersed in water. Then, photocatalytic tests at optimal pH, solvent, and wavelength reveal a correlation between the support surface area and the degradation. This points to the role of carbon nitride surface nanostructure on gemfibrozil degradation. In particular, the use of silver on mesoporous carbon nitride single-atom catalyst (Ag@mpgC₃N₄) leads to an unprecedented degradation of gemfibrozil (>90% within 60 min). The possible degradation intermediates and products were identified by mass spectrometry and were inert by cytotoxicity evaluation. We anticipate that, with further refinement and customization, the carbon nitride catalysts reported herein may find broad applications for light-driven degradation of other contaminants of emerging concern.

Figure 1

Illustration depicting the catalyst synthesis via microwave-assisted impregnation (a) and copolymerization (b), for the case of copper-based materials. The routes herein also apply to other metals.

Figure 2

Photographs of the PhotoCube photoreactor (top, courtesy of ThalesNano). This compact system is the first of its kind offering seven wavelengths that can be used (even simultaneously) for a diverse set of batch and flow photochemical reactions. General sequence of the steps performed in each photocatalytic experiment (bottom).

Figure 3

X-ray diffraction patterns (a). N₂ physisorption isotherms (b). Cu 2p (c), Ag 3d (d), and N 1s X-ray photoelectron spectroscopy data of different catalysts (e). Extended X-ray fine structure spectroscopy data of Cu@mpgC₃N₄-CP (f). The color codes in (a) apply to (b)-(e) as well.

Figure 4

Micrographs of Cu@mpgC₃N₄-CP. SEM images at different magnification (a,b); high-resolution transmission electron microscopy of the same catalysts, highlighting the absence of metal clusters (c), and a higher magnification portion (d), evidencing the single-atom nature of the catalysts (circled).

Figure 5

Photocatalytic degradation of gemfibrozil. Effect of the surface area for metal-free and Cu-based materials (a), evaluation of the degradation rate using several C₃N₄ catalysts (b), stability test over five reaction cycles (c), and cytotoxicity evaluation of the photochemically treated solution (d). Reaction conditions for all the catalytic tests: C_{0(Gemfibrozil)} = 10 mg L^–1, t = 60 min, catalyst amount 0.006% mol (corresponding to 15 mg for metal-free samples), room temperature, λ = 457 nm, and solvent = water.

Figure 6

Proposed mechanism for the degradation of gemfibrozil over single-atom catalysts and, in blue, products obtained from the decomposition and detected via mass spectrometry (a), mechanism of visible light-mediated catalyst activation (b), and ultraviolet photoelectron spectroscopy of metal-free, copper, and silver-loaded catalysts, with the values of the valence band edge (c).